Opioids and Opiates: Pharmacology, Abuse, and Addiction

Abstract

This chapter presents an overview of the opioid system consisting of a wide
variety of endogenous and exogenous ligands, as well as several receptor types,
which activate complex intracellular signaling pathways. The chapter begins with
a brief history of opiate use, opioid receptor discovery, and the identification of
main mechanisms of action. It also describes the effects and pharmacological
properties of main opioid receptor ligands (agonists, partial agonists, antagonists,
agonist-antagonists, and inverse agonists), emphasizing clinically relevant compounds.
The structure of opioid receptors is reviewed along with a description of
receptor types and putative receptor subtypes, G-protein receptor signaling,
opioid receptor regulation, and β-arrestin-dependent signaling. Furthermore,
there is a section concerning splice variants, single-nucleotide polymorphisms
of the μ-opioid receptor, and receptor dimerization. Other mechanisms that
modulate ligand–opioid receptor complexes are presented, such as biased
agonism, the action of positive and negative allosteric modulators, and epigenetic
factors. The final sections are dedicated to the definition and neurobiology of
opioid misuse, dependence and tolerance, and an overlook of some of the main
challenges posed by opioids as medications and misused drugs.

Full text

Opioids and Opiates: Pharmacology, Abuse,
and Addiction
Silvia L. Cruz and Vinicio Granados-Soto
Contents
Brief History .. . . .................................................................................. 2
Terminology ................................................................................... 4
Opioid Effects ..................................................................................... 5
Effects on the Nervous System .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 5
Effects on the Immune System .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Opioid Receptor Ligands .. . ...................................................................... 8
Endogenous Opioid Peptides .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Morphine and Its Derivatives .. ............................................................... 9
Clinically Relevant Opioids ................................................................... 14
Absorption, Distribution, Metabolism, and Excretion .. . .................................... 16
Opioid Receptors .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . 17
Structure ....................................................................................... 17
Receptor Types and Subtypes ................................................................. 18
Cellular Signaling .. . . . ........................................................................ 21
Additional Regulatory Mechanisms of Opioid Receptors ................................... 25
Addiction, Physical Dependence, and Tolerance ................................................ 26
Outlook ............................................................................................ 31
References .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 32
Abstract
This chapter presents an overview of the opioid system consisting of a wide
variety of endogenous and exogenous ligands, as well as several receptor types,
which activate complex intracellular signaling pathways. The chapter begins with
a brief history of opiate use, opioid receptor discovery, and the identification of
main mechanisms of action. It also describes the effects and pharmacological
properties of main opioid receptor ligands (agonists, partial agonists, antagonists,
S.L. Cruz (*)•V. Granados-Soto
Departamento de Farmacobiología, Cinvestav, Mexico City, Mexico
e-mail: cruz_farma@yahoo.com
#Springer Science+Business Media New York 2015
D.W. Pfaff, N.D. Volkow (eds.), Neuroscience in the 21st Century,
DOI 10.1007/978-1-4614-6434-1_156-1
1

agonist-antagonists, and inverse agonists), emphasizing clinically relevant com-
pounds. The structure of opioid receptors is reviewed along with a description of
receptor types and putative receptor subtypes, G-protein receptor signaling,
opioid receptor regulation, and β-arrestin-dependent signaling. Furthermore,
there is a section concerning splice variants, single-nucleotide polymorphisms
of the μ-opioid receptor, and receptor dimerization. Other mechanisms that
modulate ligand–opioid receptor complexes are presented, such as biased
agonism, the action of positive and negative allosteric modulators, and epigenetic
factors. The final sections are dedicated to the definition and neurobiology of
opioid misuse, dependence and tolerance, and an overlook of some of the main
challenges posed by opioids as medications and misused drugs.
Keywords
Opioids •Absorption and distribution •Addiction •Physical dependence and
tolerance •Agonist–antagonists •Agonists •Effects •History •Inverse agonists •
Metabolism and excretion •Receptor ligands •Receptors
Brief History
Opium (from a Greek word that means “juice”) is the milky exudate of unripe seed
capsules of the poppy plant Papaver somniferum. There is evidence that ancient
civilizations in Mesopotamia and Egypt were familiar with opium’s effects. The
Ebers Papyrus describes a mixture of opium and other compounds that was used for
quieting crying children in Egypt. The first undisputed mention to poppy juice is
attributed to the Greek philosopher Theophrastus in the third century BC. In the
medieval period, Avicenna of Persia described opium as the most powerful of the
stupefacients in his “Canon of Medicine.”Avicenna not only mentioned the hyp-
notic, antitussive, and gastrointestinal effects of opium; he also recognized its
potentially poisonous effects. In the seventeenth century, Thomas Sydenham pro-
moted the use of an opium product known as “laudanum”(from the Latin word
laudare, meaning “to praise”) for treating dysentery and other conditions. Laudanum
became very popular in different cultures and was considered a panacea; however,
opium preparations were variable in composition, and while some batches were
clearly effective, others produced excessive narcosis or no effect.
In 1803, the German pharmacist Friedrich W. Sert€
urner isolated the active
compound of opium and called it morphium, after Morpheus, the Greek god of
dreams. A few years later, Gay-Lussac changed its name to morphine. Codeine,
another naturally occurring opium alkaloid, was isolated in 1832, but it did not come
into clinical use until 1880. Heroin (diacetylmorphine) was synthetized from mor-
phine as a more potent analgesic and antitussive and commercialized by the end of
the nineteenth century. Physicians were not aware of the high addiction liability of
heroin until some years later, when the number of subjects dependent to heroin was
too high to be ignored. In an attempt to limit opiate use to medical purposes, in 1914
2 S.L. Cruz and V. Granados-Soto

the Harrison Narcotics Act was passed in the United States requiring physicians to
report their opiate prescriptions.
Although isolated from opium in the early 1800s, the structure of morphine was
not identified until 1925. In the following decades, the search for effective analgesics
with less negative side effects led to the active synthesis and screening of many
chemically related drugs. It was in 1973 when Candace Pert and Solomon Snyder
demonstrated the existence of stereospecific opiate binding sites in the nervous
system. In the following years, endogenous opioids and their precursor were iden-
tified. The variety of ligands with different structures and pharmacological profiles in
diverse experimental preparations led to the proposal of the existence of receptor
subtypes by 1967, but the identification of opioid receptors as biochemical entities
took place in the early years of the 1990s (Booth 1997; Snyder and Pasternak 2012).
The last decade has seen a dramatic increase in the research related to the sites and
mechanisms of action of opioids, along with an epidemic use of prescription drugs
and overdose deaths. Some milestones in opioid history are summarized in Table 1.
Today, as throughout history, the use of opiates for human well-being with little or no
negative effects remains a significant challenge. This chapter aims to present a brief
review of the main ligands, receptors, and effectors involved in acute and chronic
opioid effects.
OHHO
NCH3
1
2
36
5
4
8
7
9
10
11
12 13
14
15
16
O
17
Agonists
Codeine = R3: OCH3
Heroin = R3: OCOCH3, R6: OCOCH3
Antagonist
Nalorphine = R17: CH2CH=CH2
(R17)
(R6)(R3)
MORPHINE
Agonists
Oxymorphone = R3: OH, R17: CH3
Oxycodone = R3: OCH3, R17: CH3
Antagonists
Naloxone = R3: OH, R17:CH2CH=CH2
Naltrexone = R3: OH, R17:CH2
O
R3
NR17
O
OH
HO
NCH3
H
LEVORPHANOL
Fig. 1 Chemical structure of morphine and selected morphine-like drugs. The phenanthrene
skeleton is depicted in blue, the furan ring in green, and the piperidine ring in red
Opioids and Opiates: Pharmacology, Abuse, and Addiction 3

Terminology
The term opiate was first used to refer to substances found in the poppy plant but was
later expanded to include compounds directly derived or synthesized from thebaine
(a natural opium alkaloid) and its derivatives, with or without synthetic modifica-
tions. In this sense, morphine, heroin, codeine, and naloxone are correctly consid-
ered opiates, but completely synthetic compounds such as fentanyl are not. Opioid is
a broader term that includes endogenous or exogenous substances that bind to one or
more types of opioid receptors. Endogenous opioids are naturally occurring ligands
Table 1 Timeline of opioid history and research
Date Events
c. 3000 BC Opium poppy is used and cultivated in ancient cultures
c. 1300 BC There is evidence of poppy fields and opium trade in Egypt
1020 Avicenna teaches that opium is “the most powerful of stupefacients”
1680 Thomas Sydenham introduces laudanum, a compound of opium, wine, and
herbs, as a remedy for numerous ailments. The drink rapidly becomes very
popular
1803 Friedrich Sert€
urner isolates the active compound of opium and names it
“morphium”(morphine)
1827 E. Merck and Co. begins commercial manufacturing of morphine
1839–1841 The British send warships to China in response to China’s decision to suppress
the opium traffic. The First Opium War begins. In 1841, China is forced to pay an
indemnity and to cede Hong Kong to Britain
1843 The hypodermic syringe is introduced and, with it, a new and more efficient route
of morphine administration
1856 The Second Opium War. China is defeated and forced to legalize opium
importation
1898 The Bayer Company introduces heroin (diacetylmorphine) for medical use
1903 Heroin addiction rises to alarming rates
1914 The Harrison Narcotics Act is passed. Opium can be sold only with prescription
1925 Morphine’s chemical structure is identified
1950s–1960s Clinical and preclinical characterization of different opiate compounds leads to
proposals and models of opioid receptors. In 1967, Billy Martin suggests the
existence of more than one opiate receptor
1972 Methadone, first synthetized for use as analgesic in the Second World War, is
approved by the Food and Drug Administration (FDA) for use in treating opiate
addiction
1973 Opioid receptors are identified and characterized in binding assays
1975 Identification of endogenous opioids
1976–1981 Demonstration of mu, delta, and kappa opioid receptors
1992–1993 Cloning of delta, mu, and kappa opioid receptors
1994 Cloning of the nociceptin/orphanin FQ receptor
2002 The FDA approves buprenorphine products for use in opiate addiction treatment
2010s Extensive characterization of biased ligands, allosteric modulators of opioid
receptors, single-nucleotide polymorphisms (SNPs), and opioid receptor dimers
4 S.L. Cruz and V. Granados-Soto

for opioid receptors. Opiates are also referred to as narcotic analgesics, but the term
narcotic (a drug that causes sleep) is frequently used as synonym with abused drug,
and it will not be used here despite its historical relevance (Kreek 2007).
Opioid Effects
Effects on the Nervous System
Morphine and its derivatives relieve pain by activating opioid receptors, particularly
the μ-opioid receptor. Analgesia, euphoria, respiratory depression, cough suppres-
sion, pupillary constriction, nausea, and vomiting are the most prominent central
nervous system (CNS) effects of opioids. Analgesia is produced through inhibition
of ascending pathways carrying pain information gathered from primary sensory
neurons and activation of descending pain control systems through the rostral
ventromedial medulla down into the dorsal horn of the spinal cord. Opioids produce
respiratory depression by reducing the brain stem respiratory centers’responsiveness
to increased carbon dioxide levels. Respiratory arrest is nearly always the cause of
death produced by heroin and prescription drug overdose. Cough reflex is inhibited
by a direct neurodepressant effect on the medulla. Nausea and vomiting are produced
by activation of the chemoreceptor trigger zone for emesis in the medulla. Constric-
tion of the pupil (miosis) is due to stimulation of the parasympathetic nerve inner-
vating the pupil. This is an indirect effect of opioids through the inhibition of
GABAergic interneurons that regulate the parasympathetic outflow. With high
doses of agonists, miosis is marked (pinpoint pupils) and is considered a pathogno-
monic sign of opioid intoxication.
Opioids mildly reduce peripheral resistance and blood pressure. Intravenous
administration of different opioid agonists increases the incidence of these adverse
effects. The risk of severe orthostatic hypotension is higher in individuals who have
lost blood.
Opioids are useful antidiarrheal drugs because they delay gastric emptying, slow
intestinal transit time, and produce spam of the anal sphincter. Opioid receptor
agonists also induce an increase in biliary tract pressure that may result in biliary
spasm or colic, especially in the sphincter of Oddi.
Several opioids release histamine, which may cause facial itching, sweating,
flushing, and warmth of the face, neck, and upper thorax. The actions of opioids
on neuroendocrine function are complex, but in general, they can decrease the level
of hormones under the control of the hypothalamus-pituitary-adrenal axis, including
sex hormones. Kappa agonists inhibit the release of antidiuretic hormone causing
diuresis (Trescot et al. 2008).
Opioids are invaluable agents for the relief of pain. They can be indicated for
preoperative medication and support of anesthesia and as adjunctive therapy in
patients with dyspnea associated with pulmonary edema secondary to acute left
ventricular dysfunction. Adverse acute effects include dizziness, constipation, weak-
ness, and mental clouding, among others. Respiratory depression is the main hazard
Opioids and Opiates: Pharmacology, Abuse, and Addiction 5

associated with opiate use. Also, repeated use can lead to tolerance, physical
dependence, and addiction (Inturrisi 2002).
Effects on the Immune System
A complex interaction exists between the nervous and the immune system. Rela-
tively recent studies have shown that morphine, particularly in high doses, may be
immunosuppressive. This could be due to direct and indirect actions. Opioids have
immunomodulatory effects acting on both classical naloxone-sensitive opioid recep-
tors and non-naloxone-sensitive receptors expressed in cells involved in host defense
and immunity. Opioid receptors have been described in practically all types of
immune cells, including B and T lymphocytes, macrophages, granulocytes, and
monocytes. These receptors have similar biochemical and pharmacological charac-
teristics and are encoded by the same genes as neuronal opioid receptors. Moreover,
activation of these receptors triggers the same signaling pathways as in neuronal
cells and modulates immune cell proliferation, chemotaxis cytotoxicity, cytokine
and chemokine receptor expression, cytokine synthesis, and secretion in vitro (Sharp
2006).
One of the most important actions described for opiates is that they have anti-
inflammatory properties. These “non-analgesic”effects of opiates have been
described in a number of different peripheral immune cells that control the release
of cytokines and chemokines responsible for inflammation. The targets of opioids on
immune cells include transcription factors and specific kinases, as well as modula-
tion of cell functions, such as intracellular viral replication, calcium release from
intracellular stores, cAMP synthesis, respiratory burst activity, and other effector
responses. There are relatively few studies comparing the effects of chronic opioid
administration on the immunological system, but some evidence suggests that
significant variations among μ-opioid receptor agonists exist (Molina-Martínez
et al. 2014).
Opiate effects have been reported in mice lacking classical opioid receptors (triple
knockout mice). In those animals, opiates produce hyperalgesia. Microglia cells play
an important role in inflammatory responses that contribute to reduce analgesia and
seem to participate in tolerance development. For these reasons, several researchers
have studied the effects of opioids on microglial cells, particularly on TLR4 recep-
tors. TLR4 is a toll-like receptor activated by lipopolysaccharide from Gram-
negative bacteria. In vitro and in silico results strongly suggest that opioids interact
directly with the TLR4 receptor complex, particularly with its associated protein
MD-2, modulating cytokine production in a classical opioid receptor-independent
fashion (Hutchinson et al. 2011). The consequences of this interaction on cytokine
production or macrophage effector functions have not been fully addressed, but are
of interest due to the association of chronic opiate use and increased susceptibility to
infections.
6 S.L. Cruz and V. Granados-Soto

Opioid Receptor Ligands
Endogenous Opioid Peptides
Opioid analgesics activate an endogenous modulating system comprised of endog-
enous opioid peptides and their receptors. Several distinct families of endogenous
opioid peptides have been identified: enkephalins, endorphins, dynorphins, and
endomorphins (Table 2). In addition, nociceptin is an endogenous-related peptide
that binds to the nonclassical nociceptin opioid peptide (NOP) receptor.
The enkephalins (meaning “in the brain”) were the first natural ligands for opioid
receptors identified in pig brains’extracts. They are two pentapeptides differing only
in the last amino acid: met-enkephalin (Tyr-Gly-Gly-Phe-Met) and leu-enkephalin
(Tyr-Gly-Gly-Phe-Leu). Enkephalins are widely distributed in the body with high
concentrations in the brain, gastrointestinal tract, and the adrenal medulla. They bind
to both μand δreceptors and are derived from the larger precursor proenkephalin.
Proenkephalin contains multiple copies of met-enkephalin and a single copy of
leu-enkephalin.
Pro-opiomelanocortin (POMC) is the precursor protein not only of the potent
opioid peptide β-endorphin but also of several non-opioid peptides including
α-melanocyte-stimulating hormone (α-MSH), adrenocorticotropic hormone
(ACTH), and β-lipotropin (β-LPH). POMC is found in high concentrations in the
pituitary gland, which releases a variety of hormones in response to releasing factors.
In particular, stress produces corticotropin-releasing factor (CRH) in the hypothal-
amus, and CRH causes an increase in ACTH and β-endorphin production in the
anterior pituitary. This effect has been associated with stress-induced analgesia.
Prodynorphin, the precursor of dynorphins, has three leu-enkephalin core opioid
sequences, and differential processing can lead to various opioid peptides.
Prodynorphin products, dynorphin A, dynorphin B, and neoendorphin, are abundant
in neurons throughout the brain and spinal cord.
Nociceptin, usually referred to as nociceptin/orphanin NQ, is a neuropeptide of
17 amino acids derived from its precursor pronociceptin. Nociceptin is involved in
the transmission of pain and other functions, including the neuroendocrine stress
response (Kreek 2007).
Endomorphins are tetrapeptides with analgesic effect, high affinity, and selectiv-
ity for the μ-opioid receptor. The mechanism for the endogenous synthesis of
endomorphins has not yet been identified.
Endogenous peptide precursors are widely distributed in the CNS. They are
synthesized in the nucleus and transported to the nerve terminal where they are
cleaved by specific processing enzymes that recognize double basic amino acid
sequences positioned before and after the opioid peptide. Each of the opioid pre-
cursors contains multiple active peptides, which can be modified differentially in
different brain areas. They also often coexist with other neurotransmitters or neuro-
peptides. Processing of these peptides is altered by physiological demands, and the
Opioids and Opiates: Pharmacology, Abuse, and Addiction 7

Table 2 Amino acid sequence of some endogenous opioid peptides
Peptide Amino acid sequence
[Leu]-enkephalin Tyr-Gly-Gly-Phe-Leu
[Met]-enkephalin Tyr-Gly-Gly-Phe-Met
Endomorphin-1 Tyr-Pro-Trp-Phe
Endomorphin-2 Tyr-Pro-Phe-Phe
Heptapeptide Tyr-Gly-Gly-Phe-Met-Arg-Phe
α-Neoendorphin Tyr-Gly-Gly-Phe-Leu-Arg-Lys-Tyr-Pro-Lys
Dynorphin B Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Gln-Phe-Lys-Val-Val-Th
r
Dynorphin A Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln
Nociceptin Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Se-Ala-Arg-Lys-Leu-Ala-Asn-Gln
β-Endorphin Tyr-Gly-Gly-Phe-Met-Thr-Ser-Glu-Lys-Ser-Gln-Thr-Pro-Leu-Val-Thr-Leu-
Phe-Lys-Asn-Ala-Ile-Ile-Lys-Asn-Ala-Tyr-Lys-Lys-Gly-Glu
8 S.L. Cruz and V. Granados-Soto

final product produced by and stored within a given neuron depends not only on the
precursor, but also on the enzymes available to process the precursor in certain ways.
Endogenous opioid peptides play a regulatory role in the response of the organism to
physiological and environmental demands. They are involved in the response to
pain, regulation of the hypothalamic-pituitary-adrenal axis, and other neuroendo-
crine functions critical for survival.
Regarding the affinity for receptor subtypes, met-enkephalin, leu-enkephalin, and
β-endorphin bind to both μand δreceptors; endomorphins are selective for μ
receptors and dynorphin A for κreceptors. The amino acid sequences of main
endogenous opioid peptides are shown in Table 2.
Morphine and Its Derivatives
Chemical Classes
The structure of morphine and some derivatives are shown in Fig. 1. Distinctive
features of morphine include a phenanthrene skeleton (three fused benzene rings; in
blue), a furan ring (composed of one oxygen and four carbon atoms; in green), and a
piperidine ring with an N-methyl group and a quaternary carbon (in red). In addition,
two hydroxyl groups are attached to carbons at positions 3 and 6 (C3 and C6).
Several natural and semisynthetic compounds have similar structures with substitu-
ents in the nitrogen atom, one or both hydroxyl groups. Replacing the N-methyl with
larger radical groups such as allyl, cyclopropyl, or cyclobutyl results in compounds
with opioid-receptor antagonist properties. This is the reason why the names of
several antagonists begin with Nal-, like nalorphine (N-allyl morphine), the first
opioid-receptor antagonist synthesized. Other morphine derivatives have in common
that they have oxygen in C6, a single bond between C7 and C8, and a hydroxyl
group added to C14 (Fig. 1). Because of the oxygen in C6, their names finish with
–one (like ketone: C=O). Some examples of these drugs are oxymorphone,
oxycodone, naloxone, and naltrexone. The lack of the furan ring gives rise to
other group of synthetic compounds known as morphinans,of which levorphanol,
aμ-opioid receptor agonist with longer duration of action than morphine, is an
example (Fig. 1). The D-isomer of levorphanol, dextrorphan, is not active as an
analgesic, but has inhibitory effects at NMDA receptors.
Benzomorphan analogs are compounds with a three-ring simplified structure
(Fig. 2). Some examples are ketocyclazocine, pentazocine, cyclazocine, and
bremazocine. These agents differ in receptor selectivity (they have high affinity for
κreceptors) and in their ability to produce analgesia without causing some of the side
effects common to morphine and other classical opioids. Phenylpiperidine drugs
have a phenolic group and the piperidine ring as common structural components.
Examples of this group are the synthetic and potent derivatives meperidine (also
known as pethidine) and fentanyl. Other compounds such as etorphine and
buprenorphine have six rings instead of five. Etorphine is several thousand times
more potent than morphine and is used for immobilizing large animals.
Buprenorphine has long-lasting effects and is used for facilitating recovery from
Opioids and Opiates: Pharmacology, Abuse, and Addiction 9

opiate use disorders. Methadone and dextropropoxyphene pertain to a group of
drugs that bear little apparent chemical resemblance to morphine, but have the
basic structural features necessary to activate opiate receptors (a phenolic ring, a
protonated hydrogen, and a hydrophobic domain) and produce similar pharmaco-
logical and behavioral effects (Trescot et al. 2008). The structures of some of these
agents are shown in Fig. 2.
Intensive research and active chemical synthesis have produced a wide variety of
opioid receptor ligands with differences in receptor selectivity, pharmacokinetic
parameters (absorption, distribution, duration of effects, metabolism, etc.), and
pharmacological effects (potency, efficacy, agonist, or antagonist actions). Some
characteristics of the most relevant compounds are summarized in the following
sections.
Agonists, Antagonists, Agonist-Antagonists, and Inverse Agonists
Agonists bind to physiological receptors mimicking the effects of the endogenous
opioid peptides. A drug that binds to the same site than the endogenous agonists is an
orthosteric agonist; if it binds to a different region, it is an allosteric agonist.
Antagonists are compounds that block, reduce, or counteract the action of an agonist.
They can do it by direct competition with the agonist for the same site on the receptor
(competitive antagonists) or by indirect or functional antagonism. Partial agonists
Ketocyclazocine
N
N
HO
CH3
CH3
CH2
ON
COOCH2CH3
CH3
Meperidine
O
HO
NCH2
O
O
CH3
CH3CH3
CH3
CH3OH
Buprenorphine
CH2CH3
NCH3
CH3
CH3
O
Methadone
Fig. 2 Chemical structure of ketocyclazocine, meperidine, buprenorphine, and methadone
10 S.L. Cruz and V. Granados-Soto

produce lower responses than full agonists even at doses high enough to occupy all
of the receptors available in a tissue. Many G-protein-coupled receptors (GPCRs)
show varying degrees of basal or constitutive activity, which means that they can
activate G proteins even when not occupied by agonists. This constitutive activity is
usually minimal, but can be increased under specific circumstances. Interestingly,
when certain ligands bind these constitutively active receptors (R*), they produce
effects that are opposite to pure agonists. Such ligands are called inverse agonists,
and their effects can be observed only when constitutive activity is evident (Burdford
et al. 2015). It is worth noting that partial agonists and inverse agonists that interact
with a full agonist will behave as competitive antagonists. Finally, due to the
existence of several receptor subtypes, some drugs can produce an agonist effect at
one receptor and an antagonist effect at another; they are called mixed agonist-
antagonists.
Agonists may differ in affinity, efficacy, and potency. The affinity of opioid
receptor ligands is usually very high in the nanomolar or subnanomolar range. The
potency of a drug is the amount of drug needed to produce a determined effect and
can be estimated by the EC
50
(concentration needed to produce 50 % of the maximal
response) from a concentration-response curve. Fentanyl, for instance, is more
potent than morphine because it produces analgesia with a hundredth of morphine’s
active dose. Potency is a function of the affinity and efficacy of a compound. A
potent drug binds and activates receptor–effector complexes at low concentrations.
Efficacy refers to the ability of a drug to produce a cellular response after binding
to the receptor. Low-efficacy agonists need to occupy a larger fraction of the
available receptors to produce their effects than agonists with high efficacy. This
property cannot be directly determined from the maximal response obtained in a
dose–response curve because efficacy is dependent not only on the drug (intrinsic
efficacy) but also on the tissue, and a full agonist in one tissue might not be a full
agonist in another. Moreover, the efficacy of a substance might be difficult to
establish due to specific experimental limitations (e.g., when a cutoff value is
established to prevent tissue damage in experiments where the response to a noxious
stimulus is evaluated). Among the variables related to the tissue that can affect the
efficacy of a drug, we can mention the presence of receptor subtypes, how many of
these receptors are in a constitutively active state, if they are desensitized, internal-
ized, etc., as well as the quantity and type of specific enzymes and ion channels
available. A related concept is intrinsic activity,defined as the maximal response that
a substance can produce in a specific tissue. It is a relative value with respect to the
maximal response achievable by a drug considered as a full agonist with an intrinsic
activity of one (Kelly 2013).
In summary, in order to determine whether a ligand behaves as a full agonist or a
partial agonist, it is necessary to consider various factors such as the affinity for
specific receptor subtypes, the transduction system under investigation, the history
of previous opiate exposure, and the tissue/experimental preparation where the drug
is tested. As to antagonists, several drugs block the effects of opioid receptor
agonists most of the time, but can behave as inverse agonists when there is an
overexpression of R*. Examples of this are naloxone, naltrexone, and ICI 174864.
Opioids and Opiates: Pharmacology, Abuse, and Addiction 11

Clinically Relevant Opioids
Agonists
Morphine-like agonists share with morphine desirable and undesirable pharmaco-
logic effects; therefore, all of them are analgesics and can produce some or all of the
adverse effects associated with μ-opioid receptor stimulation. Despite this, different
drugs vary in aspects that need to be considered when it comes to choosing a specific
analgesic, including their pharmacokinetic properties (time to peak effect, duration
of actions, metabolism, etc.), relative potency, oral to parenteral efficacy ratio, and
drug–drug interactions. Table 3summarizes some characteristics of clinically rele-
vant opioids.
Heroin (diacetylmorphine) is more potent and crosses the blood–brain barrier
more easily than morphine. Consequently, heroin achieves higher concentrations in
the CNS when it is smoked or i.v. injected, producing an intense high. In some
countries like the United Kingdom, heroin can be used clinically, but it is mainly
misused and sold illegally throughout the world. The risk of adverse effects associ-
ated with heroin use is elevated due to its high potency and efficacy.
Codeine and hydrocodone are compounds commonly prescribed to suppress
cough at doses lower than those needed to produce analgesia. Both agents are also
used for the relief of mild to moderate pain and are marketed alone or combined with
acetaminophen or aspirin. Because they are μopioid receptor agonists, they can
produce euphoria, respiratory depression, and dependence. When a tablet containing
hydrocodone or codeine is crushed, dissolved, and injected, it produces significant
psychoactive effects and increases the risk of opioid overdose along with toxicity
caused by non-opioid compounds.
Oxycodone is another morphine-like agent available in continuous-release prep-
arations for the management of moderate to severe pain. Hydromorphone, a short-
acting potent opioid drug, can be used as a substitute to morphine in parenteral and
oral administration. Oxymorphone is also an effective opioid, which has higher
analgesic potency than morphine when given by the oral route. It can also be used
in suppository form. Tramadol is a codeine analog usually employed for postoper-
ative pain. It has complex cellular effects because it acts not only as a weak μ-opioid
receptor agonist but also as a norepinephrine and serotonin reuptake inhibitor.
Tramadol effects last approximately 6 h.
Among phenylpiperidine derivatives, meperidine is a strong analgesic with less
constipation effects than morphine but similar dependence liability. Its metabolite
normeperidine is an excitatory agent, the accumulation of which can produce
delirium, hyperreflexia, and seizures, especially in people with kidney dysfunction
or who misuse meperidine at high doses.
Fentanyl and its congeners, sufentanil, alfentanil, and remifentanil, are short-
acting μ-opioid receptor agonists. These drugs are highly lipid soluble and cross the
blood–brain barrier easily. Fentanyl and sufentanil are used parenterally during and
after surgery because of their rapid analgesic effect. Fentanyl is also available in skin
patches for prolonged drug delivery and in lollipops for the short treatment of
surgical pain in children (Inturrisi 2002; Trescot et al. 2008).
12 S.L. Cruz and V. Granados-Soto

Table 3 Clinically relevant opiates
Drug Use
Administration
route/dose
a
Pharmacokinetic aspects Comments μκ δ
Morphine Analgesic
Antitussive
p.o.: 15–30 mg;
i.m., s.c.:5mg
t
1/2
:2–3.5 h
Active metabolite: M6G.
First-pass metabolism
Morphine is the prototypical
μpreferring agonist
+++ + +
Heroin Analgesic
Misused drug
i.v., i.m.:5mg
i.v.: variable
t
1/2
: 0.5 h
First-pass metabolism
Morphine prodrug. More potent
than morphine
Deadly dose: 25 mg (i.v.)
+++ ++ +
+
Hydrocodone Analgesic Antitussive p.o.: 5–7.5 mg t
1/2
:2–4.5 h +++ +
Oxycodone Analgesic p.o.: 5–30 mg t
1/2
: 2.5–3 h Frequently combined with
non-opioids
+++
Fentanyl Analgesic
Preanesthetic
medication
p.o., s.l.:
100–800 μg
t
1/2
:2–4 h Available as patches and
lollipops
+++
Methadone Analgesic
Assisted maintenance
Tx
p.o., parenteral:
2.5 mg
p.o.: 20–40 mg
t
1/2
:20–37 h
Good absorption
NMDA receptor antagonist
activity
+++
Buprenorphine Analgesic
Assisted maintenance
Tx
i.v., i.m.: 0.4 mg
s.l.: 2–8mg
t
1/2
: variable
i.v.: 3h
s.l.: 30 h
Partial (P) μ-receptor agonist P 
Pentazocine Analgesic
Preanesthetic
medication
i.m.: 30–50 mg
i.v.: 20 mg
t
1/2
:2–3 h Agonist–antagonist - ++ +
Butorphanol Analgesic i.v.: 0.5–2mg
i.m.: 1–4mg
t
1/2
: 4.5–5.6 h Agonist–antagonist P +++
Nalbuphine Analgesic
Pre- and postoperative
medication
i.v., i.m.:
10–20 mg
t
1/2
: 5 h Agonist–antagonist  ++
(continued)
Opioids and Opiates: Pharmacology, Abuse, and Addiction 13

Table 3 (continued)
Drug Use
Administration
route/dose
a
Pharmacokinetic aspects Comments μκ δ
Meperidine Analgesic i.v., i.m.:
50–100 mg
t
1/2
:2–4h
First-pass metabolism
It has a toxic metabolite
α
2B
-adrenoceptor agonist
activity
++
Naloxone Opiate overdose p.o.: 0.4–2mg t
1/2
:2–4 h Receptor antagonist - - - - - -
-
Naltrexone Assisted maintenance
Tx Opiate overdose
p.o.: 50 mg t
1/2
:4–12 h Receptor antagonist - - - - - -
-
a
Doses are variable, depending on the intensity of pain and history of opioid use
14 S.L. Cruz and V. Granados-Soto

Diphenoxylate is another meperidine congener commercially available in com-
bination with atropine for the treatment of diarrhea. Loperamide is used alone for the
same purpose. Both compounds are effective per oral route, have low water solu-
bility, and cannot be misused intravenously because they lack central effects.
Methadone has a much longer plasma half-life than morphine (24 h vs. 2 h) and is
effective when administered by the oral route (the estimated oral to parental potency
ratio is 1:2 for methadone as compared to 1:6 for morphine). Despite its slow
elimination, methadone’s analgesic effects last only about 4–8 h. This can lead to
drug accumulation with repeated dosing. As a long-acting analgesic, methadone can
be used for the treatment of chronic pain. In addition, methadone is an orally active
substitute in patients with dependence to heroin.
Buprenorphine is a partial μagonist useful in treating postoperative pain and in
assisted maintenance treatment for people with opiate use disorders. Buprenorphine
has less misuse liability than morphine-like drugs, but similarly to the mixed
agonist–antagonists, it may precipitate withdrawal in patients who have been receiv-
ing μ-opioid receptor agonists for prolonged periods. Buprenorphine discontinuation
can also result in a withdrawal syndrome that is generally less severe than that
observed after stopping morphine use. Buprenorphine has a very slow rate of
dissociation from its receptor, and if it produces respiratory depression, relatively
large doses of naloxone are needed to reverse it. There are several commercially
available presentations of buprenorphine: parenteral for the relief of moderate to
severe pain, transdermal for chronic pain management, and sublingual for the
treatment of opiate dependence (Inturrisi 2002; Kreek 2007).
Agonist–Antagonists
Pentazocine,butorphanol, and nalbuphine are prototypical agonist–antagonists with
a lower potential than morphine-like drugs for causing respiratory depression. They
produce analgesia in nontolerant patients but may precipitate withdrawal in patients
who have been using morphine-like drugs.
Oral pentazocine belongs to the benzomorphan class of opioids; it is a κ- and
δ-opioid receptor agonist as well as a weak μ-opioid receptor antagonist. Oral
pentazocine is useful for the relief of mild to moderate pain and is marketed in
combination with naloxone. Parenterally, pentazocine acts as a potent analgesic
drug. Due to its antagonist properties, pentazocine can partially antagonize the
respiratory depression and analgesic effects of morphine.
Butorphanol is a morphinan-type analgesic with high affinity for κ-opioid recep-
tors and partial μ-agonist effects. As it occurs with κagonists, butorphanol may
produce unpleasant effects in some patients (dysphoria, nightmares, anxiety, etc.).
Butorphanol is available for use by injection and as a nasal spray. There is more risk
of misuse with the spray formulation.
Nalbuphine is primarily a κagonist but also a partial μantagonist. Due to this
mixed profile, nalbuphine is a potent analgesic in opioid-naïve subjects, but may
produce withdrawal symptoms when administered to patients who have been taking
other opioids.
Opioids and Opiates: Pharmacology, Abuse, and Addiction 15

Antagonists
Naloxone has high affinity for the same receptor sites where the endogenous opioid
peptides and morphine bind, but has no discernable effect in opioid-free subjects.
Naloxone competes with agonists and serves as an effective antidote to overcome the
respiratory suppression caused by heroin and other morphine-like analgesics. When
administered to an opiate-dependent person, naloxone can also precipitate a with-
drawal syndrome. Naloxone is effective only parenterally, has a short half-life, and is
rapidly redistributed after injection. These characteristics limit its clinical
application.
Naltrexone has a similar pharmacological profile than naloxone, but with a much
more prolonged action. Because it is a competitive antagonist, naltrexone attenuates
or blocks the subjective effects of intravenously administered opiates. It is used to
block the rewarding effects of heroin in detoxified opiate users in case they relapse. It
has also proved to be effective for the treatment of alcohol use (Inturrisi 2002;
Alexander et al. 2013).
Absorption, Distribution, Metabolism, and Excretion
Morphine is slowly absorbed from the digestive system because it is a base, and as
such, most of its molecules are ionized in acidic pHs. Also, morphine undergoes
significant first-pass metabolism by enzymes in the digestive system, and only a
small fraction can get to the brain when administered by the oral route. Opiate
analgesics are frequently given orally because their low absorption can be an
advantage to maintain stable drug levels in the blood. Other drugs, including
nalorphine and naloxone, are poorly absorbed from the digestive system and are
thus administered parenterally. Buprenorphine also undergoes substantial hepatic
first-pass metabolism, and therefore sublingual, rather than oral, administration is
used. The more lipophilic compounds are more rapidly absorbed. An example is
methadone because despite important between-subjects variability, up to 90 % is
absorbed after oral administration. Other agents with greater lipid solubility such as
fentanyl can be absorbed through the skin.
When opiates are consumed for their subjective effects, parenteral routes are
preferred. Heroin, for example, can be smoked, taken intranasally, or used intrave-
nously. All three-administration routes result in rapid absorption and distribution.
Because heroin is a highly lipid-soluble molecule, it enters the brain quickly and in
high concentrations. After absorption, most opiates are concentrated in the lungs,
liver, and spleen, and a large percentage is bound to blood proteins. Morphine is
eliminated rapidly, and very low concentrations remain in the body 24 h after
administration. Opiates cross the placental barrier and can induce a dependent
state in the infant born to an opiate-dependent mother (Jones et al. 2010).
Heroin is a prodrug (i.e., a drug that is administered in a biologically inactive
form and is biotransformed into an active metabolite) that needs to be converted first
into 6-monoacetylmorphine (6-MAM) and then into morphine to produce its effects.
16 S.L. Cruz and V. Granados-Soto

A certain amount of codeine is also transformed into morphine by a reaction
catalyzed by the hepatic enzyme CYP2D6. Important variations in the efficiency
of this enzyme exist, which can account for differences in the effects of codeine
among different populations. Hydrocodone and tramadol are also prodrugs that are
converted to active forms by CYP450 isoenzymes.
Morphine is metabolized by conjugation with glucuronic acid in the liver. The
resulting metabolites are morphine-3-glucuronide (M3G) and morphine-6-glucuro-
nide (M6G). M3G has little affinity for opioid receptors, but M6G is active and more
potent than morphine itself. About 10 % of morphine is excreted in the urine
unchanged. Its metabolites are eliminated in the urine and, a small proportion, in
the feces.
There are important changes in the duration of action between opiates. Short half-
life compounds (2–4 h) include morphine, codeine, oxycodone, hydromorphone,
meperidine, fentanyl, and naloxone, among others. Compared to other analgesics,
methadone has an extremely long half-life of approximately 24 h. This is due to the
fact that methadone binds extensively to blood proteins and is not available for
metabolism (Inturrisi 2002; Trescot et al. 2008). Other drugs with long half-life
values are levorphanol and propoxyphene (Table 3).
Knowledge of pharmacokinetic properties of opiates can be used to produce
useful drug combinations. A good example is a sublingual combination of
buprenorphine and naloxone for opioid maintenance therapy. When the pill is
administered sublingually, buprenorphine is absorbed, but naloxone is not. However,
the presence of naloxone serves to discourage the illicit use of this combination as it
precipitates drug withdrawal if the pill is dissolved and injected.
Opiate rotation is a common practice in pain relief. It involves substituting the
opiate analgesic drug a patient is receiving with another one in order to improve pain
management and/or to limiting adverse effects. Interindividual differences in sensi-
tivity to specific compounds may be due to variations in drug-metabolizing enzyme
activities, relative selectivity for opioid receptors, the presence of non-synonymous
single-nucleotide polymorphisms of the μ-opioid receptor, and previous history of
opiate use, among other factors. The following sections present an overview of
opioid receptor subtypes and cellular effects produced by different opioid receptor
ligands (Inturrisi 2002).
Opioid Receptors
Structure
Opioid peptide (OP) receptors are members of the class A (rhodopsin) family of
seven transmembrane GPCRs. As all members of the family, opioid receptors have
an extracellular amino terminus with glycosylation sites, seven membrane-spanning
segments linked with three loops on either side of the membrane, and an intracellular
carboxyl-terminal domain (C terminus) (Alexander et al. 2013). Opioid receptors
Opioids and Opiates: Pharmacology, Abuse, and Addiction 17

also have a highly conserved pair of Cys residues in the first and second extracellular
domains, which form a receptor-stabilizing disulfide bond. The intracellular regions,
particularly the C terminus, have important phosphorylation sites involved in the
receptor desensitization, internalization, and resensitization processes (Fig. 3).
Receptor Types and Subtypes
There are three opioid receptor types with close structural homologies: μ(or MOP),
δ(or DOP), and κ(or KOP). The fourth member of the family is the NOP (for
nociceptin opioid peptide receptor), which is considered an opioid-related receptor
due to its distinct pharmacology (Alexander et al. 2013; Cox et al. 2015). Four genes
designated as Oprm1 (or MOR1), Oprd1 (or DOR1), Oprk1 (or KOR1), and Oprl1
(or ORL1) codify for each receptor type. Each gene has a single regulatory pathway
and a specific expression pattern.
Opioid receptors are widely distributed throughout the peripheral nervous system
and CNS, mediating the diverse effects of opioid receptor agonists. All four receptor
types are expressed in the spinal cord on overlapping populations of neurons. In the
brain, μ,κ, and NOP receptors are found throughout the cortex, midbrain, and
hindbrain; in contrast, δ-opioid receptors have a more focal distribution throughout
the limbic and prelimbic brain regions (Inturrisi 2002). Specific ligands and
COOH
NH2
C
C
Phosphorylation sites
Regions
associated with cell
signaling
Glycosilation sites
Region associated
with receptor
constitutive activity
Fig. 3 Schematic representation of the μ-opioid receptor depicting some relevant structural and
functional characteristics. Yellow circles represent the two conserved cysteine residues characteristic
of the class A family of G-protein-coupled receptors
18 S.L. Cruz and V. Granados-Soto

Table 4 Opioid receptors, distribution, ligands, and functions
Receptor Distribution Agonists Antagonists
Physiological
functions
μ
MOR
MOP
(OP
3
)
Thalamus
Caudate
putamen
Neocortex
Nucleus
accumbens
Amygdala
Dorsal horn of
the spinal cord
Periaqueductal
gray
Brain stem
Morphine
Fentanyl
Sufentanil
DAMGO
Buprenorphine
Nalbuphine
Codeine
Levorphanol
Methadone
Meperidine
Endogenous:
Endomorphin-1
Endomorphin-2
β-Endorphin
Enkephalins
Naloxone
Naltrexone
Nalmefene
β-Funaltrexamine
Nalorphine
CTAP
CTOP
Analgesia, mood,
respiratory and
cardiovascular
functions,
gastrointestinal
(GI) motility,
feeding, locomotor
activity,
thermoregulation,
hormone secretion,
immune functions
δ
DOR
DOP
(OP
1
)
Similar
distribution
than μwith
high density in
olfactory areas
Also present in
thalamus and
hypothalamus
D-Ala-
deltorphin I and
II
DPDPE
SNC80
Endogenous:
Enkephalins
β-Endorphin
Naltrindole
Naltriben
Naltrexone
Naloxone
Analgesia,
gastrointestinal
motility, mood and
behavior, olfaction,
cardiovascular
regulation
κ
KOR
KOP
(OP
2
)
Cerebral cortex
Amygdala
Hypothalamus
Pituitary
Ketocyclazocine
Bremazocine
U-50488
U-69593
Butorphanol
Salvinorin A
Endogenous:
Dynorphin A
Dynorphin B
Nor-
binaltorphimine
GNTI
Nalmefene
Naloxone
Naltrexone
Buprenorphine
Regulation of
nociception, diuresis,
feeding,
neuroendocrine and
immune system
functions
Nociceptin
NOR
NOP
(OP
4
)
Cortex,
olfactory
nucleus
Ventral
forebrain
Hippocampus
Hypothalamus
Amygdala
Ventral
tegmental area
Rostral
ventromedial
medulla
Locus
coeruleus
Dorsal horn of
the spinal cord
Ro64-6198
N/OFQ-(1–13)
Endogenous:
Nociceptin/
orphanin FQ
SB612111
J-113397
UFP-101
Regulation of
nociception,
autonomic control of
physiological
processes
Opioids and Opiates: Pharmacology, Abuse, and Addiction 19

physiological functions have been identified for each opioid receptor type, some of
which are summarized in Table 4.
Pharmacological evidence has suggested the existence of several other putative
receptor subtypes, but no gene has been cloned for any of them. Recent evidence
suggests that this receptor heterogeneity may result from mechanisms that include
alternative splicing, single-nucleotide polymorphisms of opioid receptor genes, and
receptor dimerization.
Alternative mRNA Splicing
As is the case with most genes in eukaryotic cells, genes coding for opioid receptors
comprise introns and exons. After transcription, introns are removed from the
pre-mRNA by a process called splicing. Sometimes an exon can be either included
within or excluded from the final transcripts. Alternative splicing occurs when the
mRNAs produced from a single gene have different exon composition that will
eventually produce different proteins. Cloning studies have identified several splice
mRNA variants of the μ-opioid receptor, which basically differ at the tip of the
intracellular C terminus. The biological relevance of these splice variants is unclear;
however, differences in the C-terminal tails would be expected to affect the interac-
tion of the receptor with components of signaling, regulatory, and recycling path-
ways. Among the several splice mRNA variants described for the Oprm1 or MOR1
gene, only three (MOR1, MOR1A, and MOR1B) are widely distributed throughout
the brain. There is only one study reporting the presence of a splice variant of the
δ-opioid receptor in mouse brain; however, three splice mRNA variants have been
described for the κ-opioid receptor.
Polymorphisms
Non-synonymous single-nucleotide polymorphisms (SNPs) produce amino acid
changes, which have the potential to affect ligands’binding to their receptor site
and alter the conformation of the ligand–receptor complex and/or the complex’s
efficacy to G-protein coupling and signaling. Particularly, SNPs in the μ-opioid
receptor (Oprm1) gene can alter the analgesic responses to opiates in humans
(Knapman and Connor 2015; Nielsen et al. 2015). Currently, more than 100 poly-
morphisms in the Oprm1 gene have been identified. The effects of all these poly-
morphisms on opioid-induced analgesia have not been assessed. However, one of
the best-studied SNPs is the A118G, which removes one glycosylation site in the
N-terminal extracellular domain. This SNP reduces signaling efficacy and expres-
sion of the μ-opioid receptor and has been associated with differences in morphine
use by patients. Other SNPs affecting the third transmembrane segment of the
μ-opioid receptor can alter cell signaling. Potentially relevant SNPs are those coding
for the R260H- and R265H-mutant receptors, because these variants show reduced
basal activity of the receptors when expressed in several experimental preparations
(Knapman and Connor 2015; Nielsen et al. 2015). Although it is not sufficiently
studied yet, it is likely that several SNPs in opioid receptors can modify surface
receptor expression, receptor trafficking and signaling, as well as alterations to
ligand effects, giving rise to pharmacologically different receptor subtypes.
20 S.L. Cruz and V. Granados-Soto

Interestingly, SNPs are also present in the genes that codify for the opioid precursors
as well as in drug transporters involved in the absorption and metabolism of opioids.
Dimerization
Many GCPRs exist as dimers. Heteromeric dimers refer to macromolecular com-
plexes composed of at least two functional receptor units that show specific bio-
chemical properties different from those of the individual components. Numerous
studies in heterologous systems have reported that the opioid receptors
form homomeric dimers, as well as heteromeric dimers (μ/δ,μ/κ,μ/NOP, δ/κ,δ/
NOP, κ/NOP). Heteromeric dimers may be formed with non-opioid receptors, such
as those to cannabinoids, chemokines, or glutamate (e.g., δ/CB
1
,δ/CXCR4, δ/
CXCR2, μ/NMDA) (Massotte 2015). In this case, activation of a single receptor is
sufficient to initiate G-protein signaling.
Several receptor pairs have been co-localized in the same neurons in vivo,
particularly in the dorsal root ganglia (DRG) of the spinal cord. This is the case of
μ- and δ-opioid receptors, as well as adrenergic and opioid receptors (μ/α
2C
,μ/α
2A
,
δ/α
2A
,δ/α
2C
). A finding worth mentioning is that μ-opioid receptor expression is
reduced in DRG in knockout mice for the δ-opioid receptor. Moreover, methadone
and the synthetic peptide DAMGO (both μ-opioid receptor agonists) internalize not
only μ-opioid receptors, but also μ/δdimers. In addition, some antagonists of the
κ-opioid receptor act as allosteric modulators of the responses to δagonists in δ/κ
heteromeric dimers (Massotte 2015).
Some examples of opioid receptor dimerization which could account for different
pharmacological profiles observed with specific agonists in various tissues are
exemplified by the fact that the δ/δor μ/μreceptor dimers are coupled to and
signal via G
αi/o
proteins; however, δ/μheteromers can also be associated with G
z
and/or β-arrestin 2. Also, DAMGO stimulates Ca
2+
-mediated signaling instead of
G
αi/o-
mediated signaling in cells expressing the δ/μheteromers.
Cellular Signaling
G-Protein-Dependent Signaling
Acute activation of the different opioid receptor types produces similar intracellular
effects. Opioid receptors are predominantly coupled to the pertussis toxin-sensitive
guanosine triphosphate (GTP)-binding proteins G
i
/G
o
. After activation, the G pro-
tein’sα
i
subunit inhibits the adenylyl cyclase (AC) enzyme, reducing cAMP pro-
duction and the activity of protein kinase A (PKA), which in turn affects other cell
processes, including gene expression. On the other hand, the G protein’sβγ subunit
(which works as a single entity) increases K
+
conductance in G-protein-gated inward
rectifying potassium channels (GIRKs) causing hyperpolarization. It also binds to
the C terminus of Ca
2+
channels (N-type and P/Q-type) decreasing Ca
2+
entrance
(Fig. 4). A reduction in the amount of Ca
2+
that enters the cell during an action
potential proportionately reduces the amount of neurotransmitters released
(Waldhoer et al. 2004). This effect has been observed in several in vitro preparations
Opioids and Opiates: Pharmacology, Abuse, and Addiction 21

as well as in neurons of the locus coeruleus, ventral tegmental area, hippocampus,
and DRG, suggesting that ion channels are one of the most important targets for
opioid receptor activation.
Several reports have provided evidence that G protein’sβγ subunit also mediates
the stimulation of mitogen-activated kinase (MAPK) cascades by opioid receptors.
The MAPK pathways comprise the extracellular signal-regulated kinases (ERKs),
Jun N-terminal kinases (JNKs), and p38 kinases. Among them, the effect of opioids
on ERKs is the best understood. It is worth mentioning that MAPK activation occurs
not only through this G-protein-dependent mechanism, but also in a β-arrestin-
dependent manner (see below). The time course of the G-protein-dependent mech-
anism is rapid, while that depending on β-arrestin is slower.
Under certain circumstances, opioid agonists can couple to Gs. This occurs at
very low doses and is usually masked by the more pronounced inhibitory actions of
opioids. Very low doses of antagonists are enough to block opioid excitatory effects
(Waldhoer et al. 2004).
Phosphorylation, Desensitization, and Opioid Receptor Internalization
Opioid receptors are subject to a variety of regulatory processes that include phos-
phorylation, desensitization, internalization, and downregulation (Williams et al.
2013). Mu-opioid receptors have more than 15 serine, threonine, and tyrosine
residues, which are accessible to kinases (enzymes that add phosphate groups to
target molecules and change their biological activity). Phosphorylation is carried out
by various kinases, including serine/threonine protein kinases, G-protein-coupled
OHHO
NCH3
O
K+
Ca2+
AC
ATP
cAMP
-
Hyperpolarization PKA activation
CREB
Protein synthesis
Fig. 4 Opioid receptors are predominantly coupled to GTP-binding proteins G
i
/G
o
. When acti-
vated, the G protein’sαsubunit inhibits adenylyl cyclase (AC), decreasing cyclic adenosine
monophosphate (cAMP) levels and the activity of protein kinase A (PKA). This results in reduced
phosphorylation of the transcription factor CREB (cAMP response element binding protein),
thereby altering gene expression. The G protein’sβγ subunit increases K
+
conductance and
decreases Ca
2+
entrance causing hyperpolarization. βγ subunit also mediates the stimulation of
mitogen-activated kinase (MAPK) cascades
22 S.L. Cruz and V. Granados-Soto

receptor kinases (GRKs), and/or kinases activated by second messengers (such as
PKC and Ca
2+
/calmodulin-dependent protein kinase II (CAMKII)). The specific
sites where phosphorylation takes place vary depending on the particular kinases
involved. Some examples are shown in Table 5.
Recent studies suggest that phosphorylation in two amino acid clusters (between
residues 375 and 379) in the μ-opioid receptor are pivotal for acute desensitization.
Interestingly, different opioid receptor agonists may induce diverse degrees of
phosphorylation. It seems that this may depend on individual receptors achieving a
critical number of phosphorylated residues in a specific region of the carboxyl-
terminal domain.
Several research groups have shown that when endogenous opioids bind to
μ-opioid receptors, they are rapidly internalized by endocytosis into clathrin-coated
pits. There, the peptide and receptor dissociate, and the receptor promptly returns to
the cell surface to interact with other ligands. Both μand δreceptors can be
internalized in response to exogenous agonists. However, there are differences
between μand δagonists, as well as among μ-opioid receptor ligands in their ability
to induce receptor internalization; for example, methadone and fentanyl induce
endocytosis, but morphine does not. In general, phosphorylation of μ,δ,κ, and
probably NOP receptors promotes binding of regulatory proteins called β-arrestins,
which prevent the receptors from further coupling to G proteins. These processes
have different time courses. Phosphorylation takes place in about 1–2 min, whereas
β-arrestin 2 recruitment and rapid desensitization take about 3–5 min. The receptors
bound to β-arrestins are concentrated in clathrin-coated pits, which then undergo
endocytosis into the early endosomes. Here, the receptor may follow two pathways,
resensitization or endocytosis (Fig. 5). Mu-opioid receptors continue to be trafficked
in endosomes, they are dephosphorylated, the ligand comes off the receptors, and
then the receptors are represented in the cell membrane. In contrast, δreceptors are
trafficked to lysosomes where they usually are degraded. In general, receptor binding
of a ligand, without endocytosis and resensitization, contributes to downregulation
of that receptor. It is important to underline that both the desensitization and the
internalization processes are determined in part by the specific ligand bound to
opioid receptors, as high-intrinsic activity μagonists are more efficient to induce
receptor phosphorylation than low-intrinsic activity agonists. Interestingly,
Table 5 Kinases and phosphorylation sites of opioid receptors to produce acute desensitization
Receptor Kinase Phosphorylation site
β-arrestin
recruited
μGRK2, CaMKII, Tyrosine
kinase, PKCα, PKCε, and
PKCζ
Ser363, Ser370, Ser375,
Tyr106, Tyr166, Ser383,
Ser394
β-Arrestin 2
δGRK2 Ser358, Ser363 β-Arrestin1/2
κGRK3/GRK5 Ser369 β-Arrestin 2
NOP GRK3 Ser363 β-Arrestin 3
GRK G-protein-coupled receptor kinase, CaMKII Ca
2+
/calmodulin-dependent protein kinase, PKC
protein kinase C
Opioids and Opiates: Pharmacology, Abuse, and Addiction 23

internalization also depends on the kinases involved in the process of phosphoryla-
tion (Williams et al. 2013).
b-Arrestin-Dependent Signaling
As previously mentioned, opioid receptor-signaling pathways involve not only G
i/o
protein activation, inhibition of cAMP formation, and subsequent regulation of PKA
activity, but also activation of MAPKs and other enzymatic effectors. These signals
are transmitted from the ligand–receptor complex to the nucleus through the cyto-
plasm by several protein–protein interactions. Studies over the past years have
shown that β-arrestins, acting as scaffolding proteins, can switch the coupling of
opioid receptors from the acute regulation of ion channels’conductance to a different
mode of signaling involving MAPK cascade activation associated to long-term
changes in cellular function, such as cell proliferation, differentiation, and synaptic
plasticity.
Various opioid-receptor agonists activate kinase cascades, which include mem-
bers of the MAPK family and phospholipase D (PLD) to produce receptor desensi-
tization. High-intrinsic activity μagonists (DAMGO, β-endorphin, methadone, or
fentanyl), but not low-intrinsic activity μagonists (morphine, buprenorphine, or
Agonist binds
to the receptor
Receptor
endocytosis
-Arrestin
binding
Receptor
phosphorylation
Receptor
recycling Receptor
degradation
G protein
activation
G-protein-dependent
signaling
p
Early
Endosome
Lysosome
-Arrestin-dependent
si
g
nalin
g
G protein
GRKs-
-Arr
-Arr
-Arr
Fig. 5 Signaling through μ-opioid receptor differs between agonists. Agonist binding activates G-
protein-dependent signaling. G-protein-coupled receptor kinases (GRKs) phosphorylate the recep-
tor. The phosphorylated ligand–receptor complex binds to β-arrestins, which prevent further
coupling to G proteins. The receptors bound to β-arrestins undergo endocytosis into the early
endosomes. Once internalized, the receptor can be resensitized and replaced in the cell membrane
for further activation or trafficked to lysosomes for degradation. Several agonist–receptor com-
plexes that promote internalization favor β-arrestin-dependent activation of MAPK cascades
24 S.L. Cruz and V. Granados-Soto

oxycodone), stimulate phospholipase PLD2. Because PLD activation mediates
μ-opioid receptor internalization, endocytosis is a function not only of the receptor
but also of the agonist bound to it.
Activation of MAPKs is also dependent on the agonist–receptor complex; for
example, fentanyl, but not morphine, activates ERK1/ERK2 in a β-arrestin-depen-
dent manner. The mechanisms of ERK1/ERK2 to produce desensitization are
unknown; however, it has been proposed that these kinases phosphorylate
μ-opioid receptors at sites not occupied by Gα
i
subunits, thus preventing receptor–-
effector coupling. Recently, it has been demonstrated that p38 MAPK activation
facilitates μ-opioid receptor internalization by enhancing the function of endocytic
machinery (Raheal et al. 2011). Activation of MAPKs by κ- and δ-opioid receptors
has also been documented. There is evidence that the ERK pathway plays an
important role in the cellular and molecular mechanisms underlying drug
dependence.
Additional Regulatory Mechanisms of Opioid Receptors
Biased Agonism
Biased agonism refers to the ability of different agonists to differentially activate
signaling cascades or regulatory events, including differences in receptor trafficking.
This implies the formation of different protein complexes activated by the ligand
binding to the receptor, which in turn triggers different effects. Biased agonism has
led to the hypothesis that specific drugs catch the receptors with the precise confor-
mation that elicits specific downstream events. Different agonists have bias for
G-protein interaction versus the β-arrestin 2 recruitment pathway. For instance,
some morphine metabolites, including M6G, show lower potencies for G-protein
activation, but higher potencies and efficacies for β-arrestin 2 recruitment than
morphine. DAMGO recruits β-arrestin 1 and β-arrestin 2, while morphine only
recruits β-arrestin 2. Furthermore, it has been described that β-arrestin 2 knockout
mice exhibit strong analgesia with reduced respiratory and gastrointestinal side
effects (Kelly 2013). This highlights the importance of identifying biased agonists
to select only some desirable effects mediated by the μ-opioid receptor
Allosteric Regulation
As mentioned above, certain ligands can bind to sites on opioid receptors that are
separate (allosteric) from the orthosteric site, while orthosteric ligands bind to the
same site on the receptor that recognizes an endogenous agonist. Opioid receptors,
like other GPCRs, exist in at least two conformations: constitutively inactive (R) and
constitutively active (R*). Orthosteric and allosteric ligands binding to their respec-
tive binding sites in the opioid receptor can stabilize one receptor state at the expense
of the other.
Some molecules may behave as positive or negative allosteric modulators of
opioid receptors. For example, positive allosteric modulators (or PAMs) of the
μ-opioid receptor have little agonist activity on their own, but enhance the affinity,
Opioids and Opiates: Pharmacology, Abuse, and Addiction 25

potency, and maximal response of endogenous μ-opioid receptor agonists (Burford
et al. 2015). Negative allosteric modulators (NAMs) have no intrinsic agonist
efficacy; however, they bind to the receptor and inhibit the binding affinity and/or
efficacy of orthosteric agonists.
It has been known, since 1973, that sodium is a NAM because of its capacity to
decrease the binding of agonists to μ-opioid receptors. Interestingly, it is now
recognized that sodium inhibits about 65 % the agonist binding and signaling in
μ-and δ-opioid receptors, but only 20 % in κ-opioid receptors. Furthermore, other
cations such as potassium and lithium also reduce agonist binding to the δ-opioid
receptors. Contrariwise, manganese restores full agonist binding in the presence of
sodium. It is believed that sodium affects the equilibrium between R and R* by
binding to an aspartate residue in transmembrane helix 2, modulating the binding of
endogenous orthosteric ligands.
The research of the effects of PAMs for the opioid receptors is just starting.
However, it is likely that PAMs may avoid receptor downregulation and other
compensatory mechanisms, which are triggered by sustained opioid-receptor acti-
vation produced by orthosteric agonists. Based on this, it has been speculated that
PAMs could preserve the activity of the endogenous opioid peptides and produce
less tolerance and dependence than exogenous orthosteric agonists. It is also likely
that low doses of PAMs combined with clinical opiates could provide therapeutic
benefit with fewer side effects.
LIGANDS
Endogenous
Exogenous
Orthosteric
Allosteric
Function:
Agonists
Antagonists
Partial agonists
Agonist-
antagonists
Inverse agonists
RECEPTORS
Types: , , , NOP
Splice mRNA
variants
Phosphorylation
Desensitization
Internalization
Dimerization
R/R*
Polymorphisms
Variable in different brain areas,
tissues, experimental preparations,
etc.
EFFECTORS
Proteins kinases:
GRKs
PKC, PKA
CaMK
-arrestins
Exposure time
Fig. 6 The outcome of opioid receptor activation depends on the ligand, receptor type, receptor
state, and regulatory proteins available in a given tissue or experimental preparation. Opioid
exposure time and history of previous opioid also determine the resulting effect
26 S.L. Cruz and V. Granados-Soto

Epigenetic Regulation
All opioid receptors are subjected to epigenetic regulation because their coding
genes are rich in CpG islands, which can be highly methylated. DNA methylation
in cytosine nucleotides suppresses gene transcription. Several transcription factors
regulate the activity of the opioid receptor gene Oprm1, among which the best
studied is CREB (cAMP response element binding protein), but there are others.
Also, the Oprm1 gene is repressed by various transcription factors (Oct-1 or
octamer-1, among others). Therefore, DNA methylation can profoundly modify
μ-opioid receptor expression. Moreover, the Oprm1 gene can also be modulated
by microRNA (miRNA). Epigenetic regulation of coding genes for δ-, κ-, and NOP
receptors has also been described. Interestingly, the opioid peptide precursors
(proenkephalin, POMC, prodynorphin, and pronociceptin genes) are also epigenet-
ically regulated by DNA methylation and histone methylation (Muñoa et al. 2015).
Epigenetic regulation along with heteromerization, allosteric modulation, and
biased agonism combined with various expression patterns and differing selectivity
for opioid receptor subtypes may produce an enormous diversity in the physiologic
processes related to activation of opioid receptors (Fig. 6).
Addiction, Physical Dependence, and Tolerance
So far, we have discussed opioids mostly as clinical useful drugs, but they are
equally important as misused and addictive psychoactive substances. Some epide-
miological data illustrate this point. According to the more recent World Drug
Report, there are approximately 32.4 million opiate users worldwide. In North
America, the prevalence is 3.8 %. Additionally, opium poppy cultivation reached
historically high levels in 2014 since the late 1930s, and the number of opiate-related
deaths has reached the highest level in a decade and continues to rise in the United
States. Addiction (drug dependence), physical dependence, tolerance development,
and overdose deaths are the main adverse effects associated with opiate use.
Opioid misuse (abuse) can occur with prescription medications and illegal drugs.
Behaviors involved in misuse of prescription opioids include injecting oral formu-
lations, obtaining drugs from nonmedical sources, forging prescriptions, frequent
prescription refills, and escalating doses without medical authorization. A relatively
few number of opioid users develop an addiction. This condition is characterized by
a maladaptive pattern of drug use with impaired control over use, compulsive self-
administration, continued use despite negative consequences, and drug craving.
Although related, addiction is not synonymous with tolerance or physical depen-
dence (Kreek 2007). Tolerance is a state in which there is a decreased response to
doses that were once effective. As a result, doses are escalated in an attempt to
achieve the same effects that the user experimented initially. In several instances, full
recovery is not possible, and increasing the dose only enhances the probability of
experiencing adverse effects. Tolerance does not develop equally to all of opioid
actions and depends on the amount of drug used and the interval between doses; the
Opioids and Opiates: Pharmacology, Abuse, and Addiction 27

more frequent the administrations, the easier for tolerance to develop. The first
effects to decrease are euphoria and analgesia, followed by nausea and vomiting.
Tolerance to low gastrointestinal motility and pupil constriction, on the other hand, is
slow and incomplete. As a consequence, constipation may be a persistent problem in
patients undergoing chronic pain treatment, and pinpoint pupils can be observed
even in very chronic opiate users.
Compulsive use is often, but not always, accompanied by physical dependence.
When a subject is physically dependent to opiates, sudden discontinuation results in
a very uncomfortable withdrawal syndrome. Some of the early symptoms are similar
to flu and include myalgia, joint pain, lacrimation, rhinorrhea, sneezing, fatigue,
dysphoria, fever, and piloerection (gooseflesh). Other symptoms are sweating,
nausea, vomiting, continuous yawning, diarrhea, stomach cramps, and increased
blood pressure, heart rate, and temperature. Involuntary movements, particularly of
the feet, can occur, which is the reason why withdrawal is also called “kicking the
habit.”Once the initial stage of withdrawal is past (usually within a week), several
symptoms that include craving for the drug and altered response to stress may persist
for weeks and months after the last dose (Kreek 2007).
Withdrawal symptoms tend to be opposite to those produced by the agent that
induced the dependent state. For instance, the analgesia and miosis characteristic of
opioids are replaced by hyperalgesia and mydriasis during abstinence. Although
very uncomfortable, withdrawal symptoms are usually not life threatening, except to
the fetus of a mother dependent on opiates. This is why pregnant heroin users should
receive assisted maintenance programs with buprenorphine or methadone. The signs
of neonatal abstinence syndrome include irritability, tremors, high-pitched crying,
and feeding problems. There are also hyperactive reflexes, diarrhea, excessive
sucking, increased muscle tone, sneezing, yawning, vomiting, fever, restlessness,
short periods of sleep, and slow body weight gain. Treatment of newborns depends
on the drug used during pregnancy and the infant’s overall health and may include
dehydration management, special care and feeding programs, or medications (Jones
et al. 2010).
Pharmacokinetic variables are important in the magnitude and duration of the
withdrawal syndrome. If a competitive antagonist such as naloxone is administered,
it rapidly displaces the opiate agonist from its receptor, and a more intense and
shorter-lasting abstinence response is precipitated than when the opiate is
discontinued. Also, the signs and symptoms may be different if the drug is short
acting, like heroin, or long acting, like methadone. After abrupt cessation of heroin,
there is a rapid onset of withdrawal symptoms (within 6–12 h after the last dose) that
reach maximum intensity in the next two days and decline afterwards. The long-
acting drug methadone and buprenorphine (a partial agonist, which slowly dissoci-
ates from μ-opioid receptors) can be used to manage withdrawal signs following
chronic heroin use or in patients who have received opiates for chronic pain
treatment.
It is possible to be physically dependent on a drug without being addicted to
it. For example, patients who take opioids for pain control can become tolerant and
physically dependent on the drug but that does not mean that they would
28 S.L. Cruz and V. Granados-Soto

compulsively seek out the drug when it is no longer needed for analgesia. In such
cases, the clinical approach could be to gradually taper the dose instead of abrupt
discontinuation to diminish the discomfort associated with withdrawal. The opposite
is also true, a patient who has been recently detoxified from heroin may no longer be
experiencing withdrawal symptoms but will continue to crave for the subjective
effects of the opiate and may relapse to active misuse despite being aware of its
negative consequences.
Although physical dependence, tolerance, and addiction are difficult to separate,
there is evidence that different CNS regions are involved in these processes; for
example, the mesolimbic system plays a crucial role in the case of self-
administration, and the locus coeruleus and periaqueductal gray, in physical depen-
dence and withdrawal.
The role of the mesolimbic system in opiate addiction has been extensively
studied. As mentioned, opiates hyperpolarize cells via activation of G
i/o
proteins
reducing neurotransmitter release. However, opiates like all addictive drugs
increase the levels of dopamine in the mesolimbic system, which is a key detector
of rewarding stimuli. This is an indirect effect, through inhibition of GABAergic
neurons that exert a tonic inhibition of dopaminergic neurons in the ventral
tegmental area (VTA). The inhibition of an inhibitory effect results in an increase
in the amount of dopamine released from the VTA to the striatum and the frontal
cortex.
The role of the locus coeruleus (the major noradrenergic nucleus in the brain) in
physical dependence is also well described. Acutely, opiate agonists inhibit AC
activity decreasing the conversion of ATP into cAMP, but this effect diminishes with
time due to compensatory cellular adaptive changes, which include increased
expression of certain types of AC, PKA, and the transcription factor CREB. Mor-
phine withdrawal and opioid receptor blockade with antagonists produce an even
higher increase in cAMP levels, a phenomenon described as cAMP overshoot or
superactivation of adenylyl cyclase (Zhang et al. 2013). Increased cAMP activates
PKA and CREB-dependent gene transcription. This effect has been observed in the
locus coeruleus, periaqueductal gray, and several cell culture systems. The AC
pathway hyperactivation is associated with many of the withdrawal signs such as
nausea, vomiting, cramps, sweating, and increased blood pressure and heart rate. In
fact, administration of the α
2
adrenergic agonist clonidine, which inhibits AC
activity, can alleviate most of these effects (Kreek 2007).
Generally speaking, tolerance and dependence develop because the inhibitory
μ-opioid receptor mechanisms of action become less important, and their excitatory
effects become more pronounced. It is well known that potassium channels and
calcium channels play an important role in the neurodepressant effects of opioid
agonists. After morphine chronic treatment, potassium channel gating changes
markedly, decreasing its open probability. Furthermore, there is a reduction in the
effectiveness of opioid agonists to inhibit calcium channels during tolerance. Par-
ticularly, neurons from chronic morphine-treated mice show a significant reduction
in P/Q-type and L-type-mediated Ca
2+
currents. Accordingly, blockade of these
channels reduces morphine tolerance.
Opioids and Opiates: Pharmacology, Abuse, and Addiction 29

Several other mechanisms have been shown to play important roles in the
development of tolerance, including opioid receptor desensitization and trafficking
(Williams et al. 2013). It has been shown that high-intrinsic activity compounds
(e.g., DAMGO, fentanyl, etorphine, and methadone), but not morphine, induce
μ-opioid receptor desensitization and internalization. The apparent lack of mor-
phine’sefficacy to internalize the receptor may be due to its inability to induce
receptor phosphorylation by GRK2. Endocytosis seems to be an effective way to
rapidly stop G-protein-dependent signaling and allow the receptors to resensitize; as
when endocytosis does not take place, other longer-lasting intracellular adaptations
occur to regain homeostasis. In fact, it has been proposed that measurement of
relative agonist signaling versus endocytosis (RAVE) could be an index to predict
tolerance liability. Accordingly, the ability of morphine to induce changes in intra-
cellular signaling, along with its poor ability to internalize receptors, would confer
this agonist a high RAVE value and thus a high potential to induce tolerance.
Contrariwise, DAMGO and etorphine, which combine high-intrinsic activity with
high internalization potency, would lead to a low RAVE value and a low potential for
tolerance development.
Another hypothesis proposes that repeated exposure to various opioid receptor
agonists induces an overexpression of R*. When this happens, subtle alterations in
the structure and/or conformation of opioid receptors can occur that reveal negative
intrinsic activity of ligands previously shown to possess only neutral antagonist
properties. According to some authors, in naïve, untreated systems, there are very
low concentrations of R*. With repeated agonist administration, more receptors
would be in the active state, whereupon an agonist will have minimal or no further
effect, while an inverse agonist (such as naloxone) would be very effective (changes
from R* to R will be large) (Williams et al. 2013). Such receptor adaptations to
prolonged agonist exposure have been reported for μ- and δ-opioid receptors and
might contribute to tolerance and withdrawal.
Functional and gene expression studies suggest that several proteins, sites, and
systems are involved in the in vivo adaptations to chronic opiate exposure. Partic-
ularly, chronic morphine administration produces neuroplastic changes involving the
upregulation of pronociceptive systems. How these changes are started is unknown.
However, there is evidence that activation of the rostral ventromedial medulla
(RVM) plays an important role in this process. For instance, the sustained adminis-
tration of morphine increases the proportion of “on-cells”(cells that allow nocicep-
tive stimulus to be transmitted) and decreases the proportion of neutral cells recorded
in the RVM of rats. These data, along with evidence demonstrating that cholecys-
tokinin (CCK) activates on-cells in the RVM, point to this peptide as an important
agent in the development of opiate analgesic tolerance. In support to this point,
sustained morphine administration induces upregulation of CCK mRNA and CCK
release in the spinal cord, and coadministration of morphine with CCK antiserum or
CCK
2
receptor antagonists reduces or prevents tolerance development.
Preclinical data have shown that repeated intrathecal injection of μ-opioid recep-
tor agonists for several days produces tactile allodynia and antinociceptive tolerance
along with elevated dynorphin content in the spinal cord. Dynorphin promotes
30 S.L. Cruz and V. Granados-Soto

presynaptic release of excitatory neurotransmitters (calcitonin gene-related peptide
(CGRP), substance P, aspartate, and glutamate) in tolerant rats. There is also sound
evidence that the activation of the glutamate NMDA receptor subtype plays an
important role in opiate tolerance, since competitive and noncompetitive NMDA
receptor antagonists or antisense oligodeoxynucleotides retard the development of
tolerance to morphine.
A growing body of evidence suggests that the δ-opioid receptor is important for
the development of tolerance to morphine (Stockton and Devi 2012; Fujita et al.
2015). For example, concurrent administration of δ-opioid receptor antagonists with
morphine, or antisense oligodeoxynucleotides directed against the δ-opioid receptor,
partially blocks the development of tolerance to morphine (Fujita et al. 2015). This
could be due to the formation of μ/δheteromers, which switch to the β-arrestin
2-dependent signaling cascade, but more studies are needed to clarify the cellular
mechanisms underlying these effects.
Chronic morphine treatment has also been associated with activation of
microglial and astrocytic cells, which may lead to spinal release of prostaglandins,
CGRP, and pro-inflammatory cytokines, all of which would contribute to opiate
tolerance.
It is worth mentioning that physical dependence is not directly related to toler-
ance. For example, concomitant treatment of morphine with CCK antagonists pre-
vents the development of tolerance to morphine-induced analgesia but does not
modify the occurrence of physical dependence. Similar results have been reported
using PKC or PKA inhibitors in mice. Moreover, morphine produces physical
dependence with little tolerance development in β-arrestin knockout mice.
Outlook
We have now a more comprehensive understanding of the complexity of opioid
effects which depend, at least, on (a) the specific ligand used; (b) the quality,
quantity, and state of the receptor mediating the response; (c) the enzymatic proteins
and ion channels available in a given tissue; (d) the time of opioid exposure; and
(e) the frequency of opioid administration (Fig. 6).
As the field of opioid research continues to evolve, there will be more science-
based elements, which will help identify agonists or opioid analgesic combinations
with the desired effects of morphine, but devoid of significant adverse effects.
Promising findings will derive from the identification of positive allosteric modula-
tors and biased ligands for the μ-opioid receptor, as well as a deeper understanding in
the differences of opioid effects in populations with various SNPs of the human
μ-opioid receptor. Ironically, at the same time there is a growing countermovement
aimed to develop more addictive compounds. Among the new psychoactive sub-
stances synthesized in underground laboratories, there are several opiates based on
the fentanyl molecule. Of particular concern is the compound MPTP (N-methyl-4-
phenyl-1, 2, 3, 6-tetrahydropyridine), which can be a byproduct of attempted
meperidine synthesis. MPTP containing powder can be sold as “synthetic heroin,”
Opioids and Opiates: Pharmacology, Abuse, and Addiction 31

and aside from the negative effects associated with opiate misuse, it can induce
irreversible parkinsonism due to a neurotoxic effect in the substantia nigra. Another
threat comes from attempts to synthetize potent psychoactive opiates from commer-
cially available compounds as in the case of “krokodyl”(impure desomorphine
synthetized from codeine). In this way, what have been relatively safe marketed
presentations can become precursors for homemade contaminated drugs.
Adequate pain management, particularly in chronic patients, continues to be a
significant challenge, which requires education programs for the effective use of
opioids and appropriate public policies. According to a recent report from the United
Nations, the vast majority of morphine is used by a low percentage of the population
living in very few countries (the United States, Canada, Western Europe, Australia,
and New Zealand), and there are still over 5.5 billion people with limited or no
access to opiate medications for proper pain relief treatment.
On the other hand, we are facing an epidemic of drug prescription misuse where
opiates are available, with an alarming increase of overdose deaths. This underlies
the need to join efforts from researchers and prevention and treatment professionals
to address this problem.
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