GABA RECEPTOR-A WELL ESTABLISHED OLD TARGET Review Article

Abstract

Gamma-Amino butyric acid (GABA) is quantitatively one of the most important neurotransmitters in the central nervous system that targets the ionophoric GABAA and GABAC receptors and the metabotropic GABAB receptors. Of these, it is the GABAA receptor family which has been the most widely studied since this family is the site of action of a number of clinically important drugs, including benzodiazepines (BZs), barbiturates, and anaesthetics. Since the predominant action of GABA on neurons is inhibitory, activation of GABA receptors, and especially of GABAA receptors, causes an anticonvulsive effect. GABAA receptors can be activated either directly by GABA or GABA-agonists, or indirectly by allosteric modulation of these receptors. Since receptor subtypes exhibit a different regional distribution in the central nervous system, the development of subtype-selective GABAA

Full text

GABA RECEPTOR - A WELL ESTABLISHED OLD TARGET
Review Article
HEMANT U. CHIKHALE*, ELAHE KHAMISADEH, AMIT G. NERKAR, AND SANJAY D. SAWANT
*
Received: 5 Nov 2011, Revised and Accepted: 26 Dec 2011
Department of Pharmaceutical and Medicinal Chemistry, STES’s Smt. Kashibai Navale College of Pharmacy, Kondhwa (Bk), Pune, M.S. India.
ABSTRACT
Gamma-Amino butyric acid (GABA) is quantitatively one of the most important neurotransmitters in the central nervous system that targets the
ionophoric GABAA and GABAC receptors and the metabotropic GABAB receptors. Of these, it is the GABAA receptor family which has been the most
widely studied since this family is the site of action of a number of clinically important drugs, including benzodiazepines (BZs), barbiturates, and
anaesthetics. Since the predominant action of GABA on neurons is inhibitory, activation of GABA receptors, and especially of GABAA receptors,
causes an anticonvulsive effect. GABAA receptors can be activated either directly by GABA or GABA-agonists, or indirectly by allosteric modulation
of these receptors. Since receptor subtypes exhibit a different regional distribution in the central nervous system, the development of subtype-
selective GABAA
Keywords: GABA, Agonist, Anticonvulsa nts
receptor agonists result in anticonvulsants with fewer side effects. This review describes all types of GABA receptor and its
importance in medicinal chemistry.
INTRODUCTION
Several amino acids are found in high concentrations in brain, and
some have been established as neurotransmitters. l-Glutamic acid
(glutamate) is the major neurotransmitter for fast excitatory
synaptic transmission, where as gama amino butyric acid (GABA) is
the major neurotransmitter for fast inhibitory synaptic transmission.
GABA was shown to fulfill the criteria for establishment as a
neurotransmitter. It is synthesized by a specific enzyme, l-glutamic
acid decarboxylase (GAD), in one step from l-glutamate. Thus, in
addition to its role in protein synthesis, in cofactors such as folic acid
and in hormones such as thyrotropin-releasing hormone, and its
action as a neurotransmitter itself, glutamate must be available in
certain nerve endings for biosynthesis of GABA. Much of the
glutamate and GABA used as neurotransmitter is derived from glial
storage pools of glutamine.1 GABA is the principal inhibitory
neurotransmitter in the mammalian brain. It mediates fast synaptic
inhibition by interaction with the GABAA receptor. GABAA is most
studied amongst all types of GABA receptor GABAA receptors are
ligand gated ion channels that are modulated by a large number of
clinically relevant drugs such as benzodiazepines (BZs),
barbiturates, neurosteroids, and anesthetics. They are assembled
from individual subunits forming a pentameric structure. Nineteen
isoforms of mammalian GABAA receptor subunits have been cloned
α1–6, β1–3 , γ1–3 , δ, ε, p, ρ1–3 , and θ. The major receptor subtype of the
GABAA receptor in adults consists of α1, β2, and γ 2 subunits, and the
most likely stoichiometry is two α subunits, two β subunits, and one
γ subunit. The subunit composition of GABAA receptors influences
the effects of modulators. The therapeutically useful properties of
benzodiazepines (anxiolytic, anticonvulsant, sedative, and muscle
relaxant effects) may result from actions on different GABAA
receptor subtypes. Studies of mice deficient in particular a subunits
suggest that the α1-GABA A subunit is responsible for the sedative
properties of benzodiazepines, while the α2-GABA A subunit is
responsible for the anxiolytic properties The δ subunit has been
shown to confer significantly increased sensitivity to ethanol at
GABAA receptors.2
Fig. 1: It shows Main features of the GABA receptor
Chemistry of GABA: It is a small achiral molecule with
molecular weight of 103 g/mol and high water solubility. At 250C
one gram of water can dissolve 1.3 grams of GABA. Such a
hydrophilic (Log P= -2.13, PSA= 63.3 (A0) cannot cross blood
International Journal of Pharmacy and Pharmaceutical Sciences
ISSN- 0975-1491 Vol 4, Suppl 1, 2012
A
Ac
ca
ad
de
em
mi
ic
c
S
Sc
ci
ie
en
nc
ce
es
s

Chikhale et al.
Int J Pharm Pharm Sci, Vol 4, Suppl 1, 61-66
62
brain barrier. It is produced in the brain by decorboxylation of L-
glutamic acid by the enzyme glutamic acid decarboxylase (GAD,
EC 4.1.1.15). It is a neutral amino acid with pk 1= 4.23 and pK2=
10.43.
Many drugs preferentially block the Na
2
MECHANISM OF ACTION
Prolongation of sodium channel inactivation
+ Channels that remain open
due to repetitive neuronal firing i.e. they block the use-dependent or
voltage dependent Na+ channels. The higher the frequency of firing
the greater is the block. When as neuron fires, the Na+
Inhibition of T-type Calcium current
channel
passes through its active-inactive and resting phases In antiepileptic
drugs the duration of inactivated phase and its delay its reversion to
the resting phase. This reduces their chances of becoming available
for activation again. Example of drug are phenytoin, carbamazepine,
Lamotrigine, Lidocain etc.
Ethosuximide is a major drug used for the treatment of absence
seizures. It inhibits the low threshold Ca+2 currents carried by T-type
Ca+2 channels. T-type Ca+2 current are responsible for generation of
the thalamic cortical in petit mal attack inhibition or reduction of the
low threshold T-type Ca+2 channel therefore, could account for the
seizure specific therapeutic action of ethosuximide. Example of other
drug is Valproate, Zonisamide etc.
3, 4
Fig. 2: It shows Schematic GABA synapse. Diagram showing the main features of the GABA synapse. Transporters are indicated by oval
symbols, receptors and ion channels by rectangular symbols. A: Transporters: GAT-1, GAT-3, plasma membrane GABA transporters;
VGAT, vesicular GABA transporter. B: Receptors: GABA-A, ionotropic GABA receptor; GABA-B, G-protein–coupled GABA receptor;
KAINATE, presynaptic kainate receptor; MGLUR, metabotropic glutamate receptor. C: Ion channels: GIRK2, G-D: Enzymes: GABA-T, GABA
transaminase; GAD, glutamic acid decarboxylase; GS, glutamate synthetase.
CLASSIFICATION OF GABA-RECEPTOR
The major type of receptor for the inhibitory neuro transmitter
gamma amino butyric acid (GABA), called the GABA A receptor.
Molecular cloning of these polypeptide reveals that they show
20-40% identity with each other, and 10-20% identity with
polypeptides of nicotinic acetylcholine receptors and strychnine-
sensitive glycine receptor. There are two major types of GABA
receptors: 1) The ionotropic GABA A and 2) The metabotropic
GABAB receptors. GABA A receptor belongs to the ligand gated
ion channel super family. It is a heteropentamer, with all of its
five subunits contributing to the pore formation. Eight subunit
isoforms have been cloned: α, β, γ, δ, ε, π, θ and ρ.1 the native
GABAA receptor, in most cases, consists of 2α, 2β and 1γ
subunits
GABA
5
A
The GABA
Receptor
A receptors are widely distributed within the mammalian
CNS and exhibit a differential topographical distribution. Systematic
modification of the natural agonist demonstrated that GABA A
receptors can be activated by a number of compounds such as
muscimol, isoguvacine, 3- aminopropane sulphonic acid, piperidine-
4- sulphonic acid and 4,5,6,7-tetrahydro-[5,4-c]- pyridin-3-ol, many
of which were subsequently used as radioligands. At equilibrium the
binding of GABA A agonists is heterogeneous with a high affinity
component (K values of 10-20 nM) and one or more low-affinity
sites with dissociation constants in the range of 100 nM to 1μM. The
GABAA receptor is analogous to the well-characterized nicotinic
acetylcholine receptor in that it is a ligand-gated ion channel with a
binding site for the natural activator, GABA, and intrinsic to the
receptor is the functional response, the chloride ion channel An
important advance in the biochemical investigations of the GABAA
receptor was the realization that the anxiolytic benzodiazepine
drugs act by a facilitation of GABAergic neurotransmission 6, 7
GABA
B
GABA also activates metabotropic GABA receptors, which are widely
distributed within the central nervous system and also in peripheral
autonomic terminals. Their activation causes an inhibition of both
basal and forskolin stimulated adenylate cyclase activity together
with a decrease in Ca
Receptor
+2 and an increase in K+ conductance in
neuronal membranes. The receptors are activated by baclofen, used
in the treatment of spasticity, (+)-baclofen being active isomer.
There is evidence that GABA receptor agonists may be useful in the
treatment of pain and to reduce the craving for drugs of addiction.
There is limited information on the therapeutic potential of GABA
receptor antagonists but there is support for the idea that they may

Chikhale et al.
Int J Pharm Pharm Sci, Vol 4, Suppl 1, 61-66
63
prove valuable in the treatment of absence epilepsy and as cognition
enhancers.
In addition to the GABA
6, 7
GABAc Receptor
A receptors there is a distinct class of ligand
gated ion channels that are activated by GABA; referred to as the
GABAc receptor. The natural agonist GABA is about an order of
magnitude more potent at the GABA A receptors than at the most
common of the GABAc receptors. The GABAc receptors are activated
by cis-aminocrotonic acid (CACA), which is not recognized by either
the GABAA or GABAB receptors, suggesting that they recognize the
partially folded conformation of GABA. GABAc receptors are not
blocked by bicuculline and do not recognize the benzodiazepines,
barbiturates or the neuroactive steroids but, like GABAA receptors
are blocked by picrotoxin, while 1,2,5,6- tetrahydropyridine-4-yl
methyl phosphinic acid appears to inhibit GABAc receptors
selectively. Pharmacologically they are thus quite distinct. However,
molecular cloning studies have revealed that this pharmacological
profile is remarkably similar to that exhibited by the subunits when
expressed ectopically. Two homologous subunits, 1 and 2, have been
identified in human’s and these can be expressed as homomers or
heteromers, but do not co-assemble with any of the GABAA receptor
subunits.
6, 7
Fig. 3: It shows GABA A
H2N CO2H
GABA
N
H
CO2HCO2HNH2
Isoguvacine Gabapentin
O
N
H2N
OH
HN N
O
OH
Gaboxadol
Muscimol
CO2HH2N
CACA
receptor.
Fig. 4: It shows various GABAA
[
receptors Agonist

Chikhale et al.
Int J Pharm Pharm Sci, Vol 4, Suppl 1, 61-66
64
Fig. 5: It shows GABA B receptor is heterodimer of GABAB1 and GABA B2 subunits
Fig. 6: It shows various GABA A receptor antagonist
Fig. 7: It shows some representative flavonoids that have shown to influence benzodiazepine binding to brain membranes (3’7-
dihyroxyisoflavan, amentoflavone, apigenin, 6-Methylapigenin, and Oroxylin-A), to act at GABAA receptor as positive modulators
(hispidulin) or negative modulators (amentoflavone, apigenin), or at GABAC receptors as negative modulators (apigenin). In addition,
apigenin and (-)-epigallocatechin gallate have been found to have a novel second order modulatory action on the first order modulation
of GABA A receptors by diazepam

Chikhale et al.
Int J Pharm Pharm Sci, Vol 4, Suppl 1, 61-66
65
Table 1: Table shows Comparative Pharmacology of GABA receptor
Compound GABAa GABAb GABAc
GABA
Muscimol
Isoguvacine
(R)-Baclofen
Bicuculli ne
Picrotoxin
CACA
Agonist
Agonist
Agonist
Inactive
Antagonist
Antagonist
Inactive
Agonist
Inactive
Inactive
Agonist
Inactive
Inactive
Inactive
Agonist
Partial agonist
Antagonist
Inactive
Inactive
Antagonist
Partial agonist
GABA MODULATORS
Bezodiazepine Site
Benzodiazepines are widely used drugs exerting sedative, anxiolytic,
muscle relaxant, and anticonvulsant effects by acting through
specific high affinity binding sites on some GABAA receptors.
Benzodiazepines are widely used drugs, which bind to GABAA
receptors with high affinity. These molecules are divided in positive
and negative allosteric modulators and antagonists. Mutagenesis
studies identified the cleft between α and γ subunits as the binding
pocket for benzodiazepines α1H101 was identified as the target of
photo affinity labeling by [3H] flunitrazepam and α1Y209 as the
target of [3H] Ro15-4513. Pharmacophore modeling attempted to
describe the shape of the binding pocket. The benzodiazepine
binding site is constituted of six loops from A to F. The important
residue a1H101 forming part of loop A has previously been shown to
molecularly interact with diazepam and imidazobenzodiazepine
derivatives.8, 9 benzodiazepines enhance the postsynaptic actions of
GABA by binding to benzodiazepine receptors which are allosteric
modulatory binding sites on GABA (A) receptors. Conversely, there
are compounds which bind to the same benzodiazepine receptors,
but reduce the postsynaptic actions of GABA. These compounds
cause convulsions and are called "inverse agonists" of the
benzodiazepine receptors.10
Fig. 8: It shows various sites present at GABA receptor for ligand attachment

Chikhale et al.
Int J Pharm Pharm Sci, Vol 4, Suppl 1, 61-66
66
N
N
Cl
O
H
3
C
Diazepam
N
N
N
F
Cl
H
3
C
Midazolam
N
N
N
NCH
3
O
CH
3
CN
Zaleplon
N
H
N
CO
2
Pr
β-CCP
Fig. 9: It shows various benzodiazepine ligands
Other allosteric site
The barbiturates also produce many of their effects by interaction
with the GABA receptors of the mammalian CNS. Like the
benzodiazepines, they shift the GABA concentration-response curve
to the left but unlike the agonist benzodiazepines, the barbiturates
also increase the maximum response. They clearly interact with a
distinct allosteric site; the barbiturates augment the GABA mediated
current by increasing the average channel open time but have little
effect on channel opening frequency.
Whereas the benzodiazepines require the presence of a subunit
within the GABA receptor oligomer to exert their effects this is not
the case for the barbiturates. It is also clear that the barbiturates,
at high concentrations, are able to open GABA receptor channels
directly, which also distinguishes them from the benzodiazepines.
In addition to allosteric sites for the benzodiazepines and
barbiturates, the GABA receptors also exhibit high affinity
recognition sites for certain steroids. The observation that
alphaxalone, the synthetic steroid general anaesthetic, was able to
cause stereoselective potentiation of GABA receptor mediated
responses in cuneate nucleus slices from rat brain was
subsequently confirmed in voltage clamp studies conducted in
both neuronal and adrenomedullary chromaffin cells. Although the
majority of studies have focused on the GABA receptor it is clear
that certain anaesthetics, such as ketamine, nitrous oxide and
xenon do not produce their effects through this receptor but
probably by inhibition of the N- methyl-D -aspartate receptor. It is
also clear that many of the anaesthetics interact with other ligand
gated ion channels, in addition to the NMDA receptor, with
pronounced effects being seen on the neuronal nicotinic
acetylcholine receptors, particularly those containing the α 4
subunit, and the 5-HT receptor.
O
O
H
HO
Alphaxolone
H
3
CO
HO H
Allopregnanolone
HO
F
Cl F
FF
F
H
Enflurane
OH
Propofol
HN N
OH
OO
Pentobarbitone

Chikhale et al.
Int J Pharm Pharm Sci, Vol 4, Suppl 1, 61-66
67
Fig. 10: It shows selected steroids, anesthetics and barbiturate ligands
CONCLUSION
The various agonist and antagonist that were use in both
neuroimaging and molecular biology gave us fresh insights into the
role of the GABAA receptor in disease. The latest data confirm the
importance of the GABAergic system in the pathogenesis of disease
states and help to explain the important role of drugs that bind to
GABAA
1. Davis KL, Charney D, Coyle JT, and Nemeroff C. Editor.
Neuropsychopharmacology: The Fifth Generation of Progress.
American College of Neuropsychopharmacology. 2002.
receptor. A clear understanding of the mechanism
underlying dependence will also enable us to develop treatment
regimes for using current drugs which will optimize benefits and
minimizes any unwanted effects. Thus GABA receptor is a well
established old target.
REFERENCE
2. Johnston ARJ, Hanrahan JR, Chebib M, Duke RK, and Mewett
KN. Advances in Pharmacology: Modulation of ionotropic GABA
receptor by natural product of plant origin. Elsevier; 2006;
54:285-316.
3. Stephenson FA. Understanding the GABAA receptor: a
chemically gated ion channel Biochem. J. 1988; 249: 21-32.
4. Barar FSK, Essential of Pharmacotherapeutics. 1st ed. New
Delhi, S.Chand & Company Ltd.1985, 94.
5. Nikalje APG et.al: GABA Modulating Agent: A brief review.
Asian J.biol.sci 2011; 4(3): 201-220.
6. Olsen RW, Tobin AJ. Molecular biology of GABAa receptor. The
FASEB Journal, 1990; 4:1469-1480.
7. Gohar O. GABAA
8. Martin IL and Susan MJD. GABA Receptors Reviews No. 20; 2002.
receptor family, modulator no.18; 2004.
9. Tan RK, Baur R, Gonthier A, Goeldnerb M, Sigel E. Two
neighboring residues of loop A of the α1 subunit point towards
the benzodiazepine binding site of GABAA
10. Helmer SC, Pedersen, and McNamara. γ-amino butyric acid and
benzodiazepine receptor in kindling model of epilepsy: A
quantitative radiohistochemical study. J of Neuroscience. 1985;
5(10): 2696-2701.
receptors. FEBS
Letters, 2007; 581: 4718–4722.
11. Borole SP, Oswal RJ, Antre RV, Kshirsagar SV, Bagul YR.
Evaluation of anti-epileptic activity of Cuscuta reflexa Roxb.
RJPBCS 2011; 2(1): 657-663.