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Contrasting effects of chronic paroxetine on 5HT1A control of dorsal raphe cell firing and 5HT release
Abstract
To test the role of 5-HT1A receptors in the action of antidepressants, we investigated the effect of chronic paroxetine (10 mg/kg, p.o. for 21 days) on functional assays of 5-HT1A sensitivity. We constructed cumulative concentration response curves to the selective 5-MT1A agonist (+)-8-OH-DPAT on both extracellular recordings of 5-HT neurones and electrically stimulated 5-HT release in dorsal raphe brain slices. Chronic paroxetine desensitized the 5-HT1A receptors controlling firing, with an increase in ECS, from 10.7nM to 46.2nlM 8-OH-DPAT. Chronic paroxetine did not, however, desensitize the 5-HT,, receptors controlling 5-HT release but increased the 8-OH-DPAT E-max from 54.9% to 79.2% inhibition of 5-HT release. These data suggest that there are either two distinct populations of 5-HT1A receptors or separate second messenger systems, one controlling 5-HT release and another influencing firing. Furthermore chronic paroxetine treatment can differentially modulate these different populations. NeuroReport 9: 2535-2538 (C) 1998 Rapid Science Ltd.
... Chronic administration of selective serotonin reuptake inhibitors (SSRIs) has been shown to desensitize 5-hydroxytryptamine 1A (5-HT 1A ) autoreceptors on serotonergic neurones in the dorsal raphe nucleus (DRN) Le Poul et al. 1995;Davidson and Stamford, 1998] and this allows levels of synaptic 5-HT in the forebrain to rise [Dawson et al. 2000;Gardier et al. 1996] where it can act on a range of 5-HT receptors, particularly postsynaptic 5-HT 1A receptors, which has been argued to be critical for antidepressant response ]. Corticosteroids also exert major effects on the expression of postsynaptic 5-HT 1A receptors [Herman et al. 1989b]. ...
Depression affects a significant proportion of the population, with 1-year and lifetime prevalence of 3-5% and 10-30% respectively. Full remission is achieved in only a third of patients following treatment with first-line antidepressant. There is a need for novel treatments for treatment-resistant depression (TRD). Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis has been described in patients with depression. There is persistent rise in the levels of cortisol (end product of the HPA axis) and impairment of the negative feedback inhibition mechanism of the HPA axis. Dysregulation of the HPA axis has been found to be linked to nonresponse to antidepressants and relapse following successful treatment. The efficacy of pharmacological agents that intervene with the mechanisms involved in dysregulation of cortisol synthesis and release are being explored in depression, particularly in TRD. Studies have been carried out with these drugs as augmenting agents for antidepressants or as monotherapy. The strongest evidence has come from studies using metyrapone, a cortisol synthesis inhibitor, and this has been described in detail in this review. The most robust evidence for its antidepressant efficacy in depression comes from a double-blind, randomized, placebo-controlled study of augmentation of serotonergic antidepressants with metyrapone. A 3-week augmentation of serotonergic antidepressants with 1 g metyrapone daily was shown to be superior to placebo in reducing the Montgomery-Asberg Depression Rating Scale by 50%, 5 weeks following initiation of treatment. The mechanism of the antidepressant action of metyrapone is not clear but the evidence for various potential mechanisms is discussed.
... Many investigations have described functional desensitization of 5-HT1A receptors after chronic SERT inhibition, but to varying degrees across brain regions Lucki, 1994, 1997;Cremers et al., 2000;Bosker et al., 2001;Rossi et al., 2008). Davidson and Stamford (1998) compared serotonin release and uptake and neuronal firing rates in the DRN of rats treated with water or paroxetine for 21 days. Paroxetinetreated rats had significantly higher serotonin release rates but exhibited no differences in firing rate. ...
Serotonin modulates a variety of processes throughout the brain, but it is perhaps best known for its involvement in the etiology and treatment of depressive disorders. Microdialysis studies have provided a clear picture of how ambient serotonin levels fluctuate with regard to behavioral states and pharmacological manipulation, and anatomical and electrophysiological studies describe the location and activity of serotonin and its targets. However, few techniques combine the temporal resolution, spatial precision, and chemical selectivity to directly evaluate serotonin release and uptake. Fast-scan cyclic voltammetry (FSCV) is an electrochemical method that can detect minute changes in neurotransmitter concentration on the same temporal and spatial dimensions as extrasynaptic neurotransmission. Subsecond measurements both in vivo and in brain slice preparations enable us to tease apart the processes of release and uptake. These studies have particularly highlighted the significance of regulatory mechanisms to proper functioning of the serotonin system. This article will review the findings of FSCV investigations of serotonergic neurotransmission and discuss this technique's potential in future studies of the serotonin system.
... Non-5-HT neurons within the dmDR and the MR did not exhibit any significant hyperpolarization. The 5-HT 1A receptor is altered by behavioral stress, stress hormones and antidepressant treatment (Bermack et al., 2004;Blier and Ward, 2003;Davidson and Stamford, 1998;Greenwood et al., 2005;Jolas et al., 1994;Laaris et al., 1995;Laaris et al., 1997;Lanfumey et al., 1999;Le Poul et al., 1999). Additionally, recent studies have demonstrated that the 5-HT 1A autoreceptor, not the heteroreceptor, is important for stress resilience and the etiology of mood disorders (Liu et al., 2010;Richardson-Jones et al., 2011). ...
The median (MR) and dorsal raphe (DR) nuclei contain the majority of the 5-hydroxytryptamine (5-HT, serotonin) neurons that project to limbic forebrain regions, are important in regulating homeostatic functions and are implicated in the etiology and treatment of mood disorders and schizophrenia. The primary synaptic inputs within and to the raphe are glutamatergic and GABAergic. The DR is divided into three subfields, i.e., ventromedial (vmDR), lateral wings (lwDR) and dorsomedial (dmDR). Our previous work shows that cell characteristics of 5-HT neurons and the magnitude of the 5-HT(1A) and 5-HT(1B) receptor-mediated responses in the vmDR and MR are not the same. We extend these observations to examine the electrophysiological properties across all four raphe subfields in both 5-HT and non-5-HT neurons. The neurochemical topography of glutamatergic and GABAergic cell bodies and nerve terminals were identified using immunohistochemistry and the morphology of the 5-HT neurons was measured. Although 5-HT neurons possessed similar physiological properties, important differences existed between subfields. Non-5-HT neurons were indistinguishable from 5-HT neurons. GABA neurons were distributed throughout the raphe, usually in areas devoid of 5-HT neurons. Although GABAergic synaptic innervation was dense throughout the raphe (immunohistochemical analysis of the GABA transporters GAT1 and GAT3), their distributions differed. Glutamate neurons, as defined by vGlut3 anti-bodies, were intermixed and co-localized with 5-HT neurons within all raphe subfields. Finally, the dendritic arbor of the 5-HT neurons was distinct between subfields. Previous studies regard 5-HT neurons as a homogenous population. Our data support a model of the raphe as an area composed of functionally distinct subpopulations of 5-HT and non-5-HT neurons, in part delineated by subfield. Understanding the interaction of the cell properties of the neurons in concert with their morphology, local distribution of GABA and glutamate neurons and their synaptic input, reveals a more complicated and heterogeneous raphe. These results provide an important foundation for understanding how specific subfields modulate behavior and for defining which aspects of the circuitry are altered during the etiology of psychological disorders.
... Interestingly, a difference seems to exist between individual SSRIs in their ability to desensitize 5-HT 1A autoreceptors and to induce delayed ejaculation. Paroxetine and fluoxetine seem to desensitize 5-HT 1A autoreceptors [21,24,[54][55][56][57] more strongly than citalopram and fluvoxamine [10, 58,59]. Likewise, paroxetine and fluoxetine are much more prone to delay ejaculation than citalopram and fluvoxamine in humans and rats, despite their comparable ability to elevate 5-HT levels [1-3, 41,44,49]. ...
Introduction. Selective serotonin reuptake inhibitors (SSRIs) differ in the severity of induced ejaculation delay. Various studies indicate that oxytocin is involved in sexual behavior. Aim. To review and evaluate the involvement of oxytocin in SSRI-induced ejaculation delay. Main Outcome Measures. Oxytocine release, 5-hydroxytryptamine (5-HT) neurotransmission, and desensitization of 5-HT 1A receptors. Methods. A review and critical analysis of animal studies investigating the interaction of serotonergic and oxyto-cinergic neurotransmission in relation to the ejaculation process. Results. Although acute treatment with the SSRIs fluoxetine and paroxetine immediately causes increased serotonin levels, delayed ejaculation does not occur. The increased serotonin levels induce oxytocin release via activation of 5-HT 1A receptors, and this might compensate for the inhibitory actions of serotonin on sexual behavior. Chronic treatment with fluoxetine and paroxetine desensitizes 5-HT 1A receptors on oxytocin neurons, and that might in part determine the onset of delayed ejaculation. Desensitization of 5-HT 1A receptors is less strong following chronic treatment with the SSRIs fluvoxamine or citalopram, which may attenuate the degree of delayed ejaculation. Conclusions. Preliminary data suggest that the severity of chronic SSRI treatment-induced delayed ejaculation and the differences between the various SSRIs in inducing ejaculation delay is related to gradual desensitization of 5-HT 1A receptors on oxytocin neurons. de Jong TR, Veening JG, Olivier B, and Waldinger MD. Oxytocin involvement in SSRI-induced delayed ejaculation: A review of animal studies. J Sex Med 2007;4:14–28.
... The reports on the effects of chronic treatment with paroxetine on the sensitivity of 5-HT 1A receptors have been conflicting. Davidson and Stamford (1998) have found a desensitization of the 5-HT 1A receptor-mediated control of the firing rate of the 5-HT neurons, but the sensitization of the 5-HT 1A receptor-mediated control of 5- HT synthesis following chronic treatment (21 days, p.o.) with paroxetine. Le Poul et al. (1995) reported a desensitization of 5- HT 1A receptors controlling the firing rate of 5-HT neurons after only 3 days of paroxetine treatment (5 mg/kg, i.p.), and the desensitization had been progressing for up to 21 days of treatment. ...
It has been proposed that the desensitization of 5-HT(1A) (5-hydroxytryptamine; serotonin) receptors following chronic therapy with selective serotonin reuptake inhibitors (SSRIs) is necessary for their therapeutic efficacy. Stimulation of the 5-HT(1A) receptors decreases serotonin (5-HT) synthesis and release, but it is not clear if the receptors are fully desensitized following chronic SSRI treatment. The main objective of this study was evaluation of ability of 5-HT(1A) receptors to modulate 5-HT synthesis after 14-day paroxetine treatment. 5-HT(1A) receptor sensitivity following chronic administration of the SSRI paroxetine was assessed by the ability of an acute challenge with the 5-HT(1A) agonist, flesinoxan, to modulate 5-HT synthesis in the rat brain. The rates of 5-HT synthesis were measured using the alpha-[(14)C]methyl-l-tryptophan autoradiographic method. The rats were treated for 2 weeks with paroxetine (10mg/(kgday), s.c., delivered by osmotic minipump). After this treatment, the rats received an acute challenge with flesinoxan (5mg/kg, i.p.), while the control rats were injected with the vehicle. Forty minutes following the flesinoxan injection, the tracer, alpha-[(14)C]methyl-l-tryptophan, was injected over 2min. 5-HT synthesis rates were calculated from autoradiographically measured tissue tracer concentrations and plasma time-activity curves. The results demonstrated that the acute flesinoxan challenge produced a significant decrease in 5-HT synthesis rates throughout the rat brain. The greatest decrease was observed in the ventral hippocampus, somatosensory cortex and the ascending serotonergic cell bodies. In comparison with data reported on an acute challenge with flesinoxan in naïve rats (rats without any other treatment), the results presented here suggest a greater effect of flesinoxan on synthesis reduction in rats chronically treated with paroxetine. The results also suggest that the 5-HT receptors were not fully desensitized by paroxetine treatment, and that the stimulation of 5-HT(1A) receptors with an agonist is still capable of reducing 5-HT synthesis.
... Thus Cryan et al. (1999), using the olfactory bulbectomy-induced behavioural syndrome, showed no potentially faster onset of antidepressant action by 5-HT 1A receptor antagonist. These discrepancies can be explained by the site of the 5-HT 1A autoreceptor action, Hervas & Artigas (1998) having demonstrated that WAY 100,635 potentiates the eect of uoxetine more in the frontal cortex than in the dorsal hippocampus or by the hypothesis of Davidson & Stamford (1998), which postulates that there are two populations of 5-HT 1A receptors one of which controls the 5-HT release and the other the 5-HT ®ring. ...
The benefit of antidepressant treatment in human neuropathic pain is now well documented, but the effect is limited and slow to appear. It has been demonstrated that the association of a 5-HT1A antagonist and a serotoninergic antidepressant reduced the delay of action and increases the thymoanaleptic effect of the drug.
The purpose of this work was to evaluate the combination of an antidepressant and a 5-HT1A antagonist in animal models of chronic neuropathic pain. We studied the antinociceptive effect of the co-administration of clomipramine and a 5-HT1A antagonist (WAY 100,635) in a pain test applied in normal rats and in two models of neurogenic sustained pain (mononeuropathic and diabetic rats).
The results show an increase in the antinociceptive effect of acutely injected clomipramine due to WAY 100,635 in these models, which is majored when the two drugs are repeatedly injected. The 5-HT1A antagonist reduced the delay of onset and increased the maximal antinociceptive effect of clomipramine.
These new findings argue for using the combination of an antidepressant and a 5-HT1A antagonist in human neuropathic pain therapy.
British Journal of Pharmacology (2001) 132, 1118–1126; doi:10.1038/sj.bjp.0703897
... However, pharmacological studies indicate that SSRIs acutely inhibit 5-HT release by increasing activation of somatodendritic 5-HT 1A autoreceptors in the dorsal raphe nucleus (DRN) and that this effect limits their ability to increase synaptic 5-HT in the forebrain (Gartside et al., 1995; Hjorth and Auerbach, 1996). On chronic administration, SSRIs have been shown to desensitise 5-HT 1A autoreceptors (De Montigny et al., 1990; Le Poul et al., 1995; Davidson and Stamford, 1998). This desensitisation, which allows levels of synaptic 5-HT in forebrain to rise (Gardier et al., 1996; Dawson et al., 2000), shows some temporal correspondence with the onset of antidepressant efficacy (Gardier et al., 1996; Le Poul et al., 1995) and hence is accepted to be an important feature of the response to chronic administration of SSRIs (see Blier et al., 1987, Blier, 2001 Artigas et al., 2001). ...
Both glucocorticoids and selective serotonin reuptake inhibitors (SSRIs) alter aspects of 5-HT function including somatodendritic 5-HT1A autoreceptor sensitivity. Many depressed patients prescribed SSRIs have pre-existing flattened diurnal gluococorticoid rhythm. In these patients, interactions between flattened glucocorticoid rhythm and chronic SSRIs, which impact on the SSRI's ability to elevate forebrain 5-HT, may alter clinical efficacy. To address this issue rats underwent implantation of slow-release corticosterone (75 mg pellet s.c.) (to flatten the glucocorticoid rhythm) or sham surgery, and injection of fluoxetine (10 mg/kg/day i.p., 12 days) or vehicle. Using microdialysis in the frontal cortex we found that (21 h after the last injection) extracellular 5-HT was elevated in fluoxetine- or corticosterone-treated animals, but not in those treated with corticosterone plus fluoxetine. In fluoxetine-treated animals, blockade of terminal reuptake by local perfusion of fluoxetine increased 5-HT to the same level as it did in controls, suggesting normal terminal 5-HT release after chronic fluoxetine. However, 5-HT levels following local reuptake blockade in both the corticosterone and corticosterone plus fluoxetine groups were lower than controls, suggesting a corticosterone-induced decrease in terminal release. Finally in fluoxetine, corticosterone, and corticosterone plus fluoxetine groups, there was marked 5-HT1A receptor desensitization, evidenced by attenuation of the decrease in 5-HT release following systemic fluoxetine injection. The data indicate that, despite desensitization of 5-HT1A autoreceptors, concurrent flattened glucocorticoid rhythm compromises the ability of SSRIs to elevate forebrain 5-HT. These findings suggest a potential mechanism for the reduced antidepressant efficacy of SSRIs in those patients with pre-existing glucocorticoid abnormalities.
... However, we matched control and suicide subjects for these variables and did not find significant correlations of either of these parameters with binding measures. Another confounding factor may be the use of drugs or medications that may desensitize 5-HT 1A receptors or alter 5-HT 1A receptor density and their G-protein coupling (Le Poul et al., 1995;Blier and De Montigny, 1994;Davidson and Stamford, 1998;Stockmeier et al., 1998;Rossi et al., 2006). In the present study, no subjects were taking antidepressants at the time of death, therefore the observed differences are not likely attributable to medication. ...
Serotonergic dysfunction is present in mood disorders and suicide. Brainstem 5-HT1A somatodendritic autoreceptors regulate serotonin neuron firing but studies of autoreceptor binding in the dorsal raphe nucleus (DRN) in depressed suicides report conflicting results. We sought to determine: (1) the anatomical distribution of 5-HT1A receptor binding in the DRN in depressed suicides and psychiatrically normal controls; and (2) whether sex differences in 5-HT1A binding in the DRN contribute to differences between depressed suicides and controls. Previously collected quantitative receptor autoradiograms of [3H]8-hydroxy-2-(di-n-propyl)aminotetralin (3H-8-OH-DPAT) in postmortem tissue sections containing the DRN from drug-free suicide victims (n=10) and matched controls (n=10) were analyzed. Less total receptor binding (fmol/mg tissuexmm3) was observed in the entire DRN in depressed suicides compared with controls (p<0.05). Group differences along the rostrocaudal extent of the DRN were observed for cross-sectional 5-HT(1A) binding (fmol/mg tissue) and receptor binding (fmol/mgxmm3, p<0.05). Cross-sectional 5-HT1A DRN binding in depressed suicides compared with controls was higher rostrally and lower caudally. The differences between depressed suicides and controls were present in males and females, although females had more binding than males. Less autoreceptor binding in the DRN of depressed suicides may represent a homeostatic response to less serotonin release, increasing serotonin neuron firing. More autoreceptor binding in rostral DRN might contribute to deficient serotonin release in ventromedial prefrontal cortex by lower neuronal firing.
La disfunción serotoninérgica está presente en los trastornos del humor y el suicidio. Los autorreceptores somatodrendríticos 5-HT1A del tronco cerebral regulan la descarga de neuronas serotoninérgicas, pero los estudios efectuados sobre la unión a los autorreceptores en el núcleo dorsal del rafe (NDR) en muestras de tejidos obtenidas en víctimas de suicidio deprimidas describen resultados conflictivos. Tratamos de determinar: 1) la distribución anatómica de la unión al receptor 5-HT1A en el NDR de muestras de tejidos obtenidas de víctimas deprimidas de suicidio y de individuos de control sanos, desde un punto de vista psiquiátrico, fallecidos por causas naturales, y 2) si las diferencias de sexo en la unión a 5-HT1A en el NDR contribuyen a las diferencias observadas en las muestras entre víctimas deprimidas de suicidio y los de control. Se analizaron las autorradiografías cuantitativas del receptor de [3H]8-hidroxi-2-(di-n-propil) aminotetralina ([3H]8-OH-DPAT) en las secciones de tejido post mórtem, obtenidas previamente, que contenían el NDR de víctimas de suicidio libres de fármacos/drogas (n=10) y de controles emparejados (n=10). En el NDR íntegro de los suicidas deprimidos, comparado con los controles, se observó una menor unión total al receptor (fmol/mg de tejido×mm3) (p<0,05). Se observaron diferencias de grupo a lo largo de la extensión rostrocaudal del NDR para la unión transversal a 5-HT1A (fmol/mg de tejido) y la unión al receptor (fmol/mg×mm3, p<0,05). Comparado con controles, en el NDR de suicidas deprimidos la unión transversal a 5-HT1A fue mayor rostralmente y menor, caudalmente. Las diferencias entre suicidas deprimidos y controles estuvieron presentes en hombres y mujeres, aunque en estas se detectó una mayor unión que en hombres. La menor unión al autorreceptor en NDR de suicidas deprimidos podría representar una respuesta homeostática a una menor liberación de serotonina, con un incremento de la descarga de las neuronas serotoninérgicas. La mayor unión al autorreceptor en el NDR rostral podría contribuir a una liberación deficiente de serotonina en la corteza prefrontal ventromedial mediante una menor descarga neuronal.
(-)-Pindolol, which possesses significant affinity for 5-HT1A, 5-HT1B, and β1/2-adrenergic receptors (AR)s, dose-dependently increased extracellular levels of dopamine (DA) and noradrenaline (NAD) versus 5-HT, in dialysates of the frontal cortex (FCX), but not accumbens and striatum, of freely-moving rats. In distinction, the preferential β1-AR antagonist, betaxolol, and the preferential β2-AR antagonist, ICI118,551, did not increase basal levels of DA, NAD, or 5-HT. Further, they both dose-dependently and markedly blunted the influence of (-)-pindolol upon DA and NAD levels. The selective -HT1A receptor antagonist, WAY100,635, slightly attenuated the (-)-pindolol-induced increase in DA and NAD levels, while the selective 5-HT1B antagonist, SB224,289, was ineffective. These data suggest that (-)-pindolol facilitates frontocortical dopaminergic (and adrenergic) transmission primarily by activation of β1/2-ARs and, to a lesser degree, by stimulation of 5-HT1A receptors, whereas 5-HT1B receptors are not involved. (-)-Pindolol potentiated the increase in FCX levels of 5-HT elicited by the 5-HT reuptake inhibitors, fluoxetine and duloxetine, and also enhanced their ability to elevate FCX levels of DA—though not of NAD. In contrast to (-)-pindolol, betaxolol and ICI118,551 did not affect the actions of fluoxetine, whereas both WAY100,635 and SB224,289 potentiated the increase in levels of 5-HT—but not DA or NAD levels—elicited by fluoxetine. In conclusion, (-)-pindolol modulates, both alone and together with 5-HT reuptake inhibitors, dopaminergic, adrenergic, and serotonergic transmission in the FCX via a complex pattern of actions at β1/2-ARs, 5-HT1A, and 5-HT1B receptors. These findings have important implications for clinical studies of the influence of (-)-pindolol upon the actions of antidepressant agents.
It is generally accepted that about two thirds of patients treated for depression respond only after several weeks (2 to 8 weeks) whilst a third do not respond at all. A depressed patient's response to a treatment is defined by at least 50% reduction of the symptoms evaluated on a standard instrument (i.e., Hamilton Depression Rating Scale). Thus, a response to an antidepressant treatment cannot be considered as a remission. Remission can take many months to occur. It is then crucial to find new targets for antidepressants development or co-administration strategies in order to reduce the long delay in onset of action and improve the efficiency of current treatments. According to their mechanism of action, current antidepressants induce an increase in serotonin and/or noradrenaline neurotransmission by increasing the monoamine extracellular level available in the synaptic cleft. It is then highly possible that the antidepressant effect depends on the synaptic receptor(s) activated. In the case of serotonergic compounds, 14 subtypes of receptors could be stimulated. In this short review, we focus on the impact of 5-HT1B receptor activation in the mediation of antidepressant-like effect. Drug Dev. Res. 65:141–146, 2005. © 2005 Wiley-Liss, Inc.
This study reports the effect of chronic paroxetine (10 mg/kg p.o., 21 days) on 5-HT1B and 5-HT1D autoreceptors controlling stimulated 5-HT efflux in slices of rat dorsal raphe nucleus. Electrically evoked 5-HT (10 pulses, 200 Hz, 0.1 ms, 10 mA) was measured using fast cyclic voltammetry. 5-HT efflux was inhibited by CP 93129 (10 nM–10 μM) and by sumatriptan (1 nM–1 μM) agonists at 5-HT1B and 5-HT1D receptors, respectively. Chronic paroxetine did not, initially, appear to alter the sensitivity of the 5-HT1B autoreceptors to CP 93129. However, when constructed in the presence of WAY 100635 (10 nM) the selective and silent 5-HT1A antagonist, there was a significant (P<0.001) rightward shift of the CP 93129 concentration-response curve in the paroxetine-treated rats but not in the controls, implying a desensitisation of the 5-HT1B autoreceptor by paroxetine. Chronic paroxetine did not affect the sumatriptan concentration-response curve, even with WAY 100635 present, implying that there was no (de)sensitisation of the 5-HT1D autoreceptor. These data suggest that chronic paroxetine treatment may desensitise 5-HT1B autoreceptors in the dorsal raphe nucleus but that this effect is unmasked only when the dominant 5-HT1A autoreceptor control is antagonised.
The aim of the present study was to determine whether alterations in 5-hydroxytryptamine (5-HT)(1A) receptors would be found in knockout mice lacking the serotonin transporter (5-HTT). Hypothermic and neuroendocrine responses to the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) were used to examine the function of 5-HT(1A) receptors. Initial studies evaluated the dose-response and time course of 8-OH-DPAT-induced hypothermia and hormone secretion in normal CD-1 mice (the background strain of the 5-HTT knockout mice). 8-OH-DPAT dose-dependently produced hypothermic responses that peaked at 20 min postinjection. 8-OH-DPAT-induced hypothermia was blocked by the 5-HT(1A) antagonist WAY-100635. 8-OH-DPAT dose-dependently increased the concentrations of plasma oxytocin, corticotropin, and corticosterone. In the 5-HTT knockout (-/-) mice, the hypothermic response to 8-OH-DPAT (0.1 mg/kg s.c.) was completely abolished. Furthermore, 5-HTT-/- mice had significantly attenuated plasma oxytocin and corticosterone responses to 8-OH-DPAT. No significant changes in the hypothermic or hormonal responses to 8-OH-DPAT were observed in heterozygous (5-HTT+/-) mice. [(3)H]8-OH-DPAT- and [(125)I]MPPI [4-(2'-methoxyphenyl)-1-[2'-[N-(2"-pyridinyl)-iodobenzamido]ethyl] pip erazine]-binding sites in the hypothalamus and [(125)I]MPPI-binding sites in the dorsal raphe were significantly decreased in 5-HTT-/- mice. The results indicate that lack of the 5-HTT is associated with a functional desensitization of 5-HT(1A) receptor responses to 8-OH-DPAT, which may be a consequence, at least in part, of the decrease in density of 5-HT(1A) receptors in the hypothalamus and dorsal raphe of 5-HTT-/- mice.
Monoamine receptors are membrane-bound receptors that are coupled to G-proteins. Upon stimulation by agonists, they initiate a cascade of intracellular events that guide biochemical reactions of the cell. In the central nervous system, they undergo diverse regulatory processes, among which are receptor desensitization, internalization into the cell, and downregulation. These processes vary among different types of monoamine receptors. alpha 2-Adrenoceptors are often downregulated by agonists, and beta-adrenoceptors are internalized rapidly. Others, such as serotonin1A-receptors, are controlled tightly by steroid hormones. Expression of these receptors is reduced by the "stress hormones" glucocorticoids, whereas gonadal hormones such as testosterone can counterbalance the glucocorticoid effects. Because of this, the pattern of monoamine receptors in certain brain regions undergoes dynamic changes when there are elevated concentrations of agonists or when the hormonal milieu changes. Stress is a physiological situation accompanied by the high activity of brain monoaminergic systems and dramatic changes in peripheral hormones. Resulting alterations in monoamine receptors are considered to be in part responsible for changes in the behavior of an individual.
Presynaptic receptor theory has been expanded to encompass the regulation of the firing rate of serotonergic neurons through negative feedback mediated by the somadendritic release of transmitter. This has encouraged hypotheses as to the mechanisms of action of several classes of antidepressants and anxiolytics. One conspicuous example is the attribution of the clinical efficacy of 5-HT uptake inhibitors, such as fluoxetine and paroxetine, to desensitization of somadendritic 5-HT autoreceptors. An examination of the available evidence, mainly observations made with agonists, antagonists, monoamine oxidase inhibitors and uptake blockers, taken along with the theoretical expectations for a negative feedback loop, and the operational characteristics of inactivation pathways, indicates that negative feedback does not function at somadendritic sites to set firing rate or transmitter density, and suggests that the process may not function at all physiologically. The attribution of the effectiveness of neuroactive drugs to desensitization of raphe 5-HT inhibitory receptors, or to other interactions with feedback, is highly speculative and unlikely.
The gradual recovery of activity of serotonergic neurons following an initial inhibition has been hypothesized to play an important role in the delayed onset of efficacy of selective serotonin reuptake inhibitors. This study explored the clinical relevance of this hypothesis by examining the effects of different doses and routes of administration of fluoxetine on the recovery of activity of serotonergic neurons over the course of a 21-day exposure. Single-unit, extracellular recordings of serotonergic neurons were made in the dorsal raphe nucleus of anesthetized male rats. Acute i.v., s.c. and i.p. administration of fluoxetine inhibited the activity of serotonergic neurons. With chronic administration of fluoxetine, at clinically relevant doses, the activity of serotonergic neurons gradually recovered to baseline levels over the course of 14-21 days. The dose of fluoxetine (5, 10 or 20 mg/kg per day) did not make a significant difference in the time course of the recovery of activity of serotonergic neurons. A significant, non-parallel shift in the dose-response curve of serotonergic neurons to the serotonin-1A (5-HT1A) agonist 8-OH-DPAT occurred over the 21 days of treatment with fluoxetine, indicating a desensitization of the 5-HT1A receptor during this period. The recovery of firing did not correlate with either plasma or cerebrospinal fluid levels of fluoxetine or norfluoxetine. These results indicate that, similar to the effects of dose on the speed of onset of the clinical effects of SSRIs, increasing the dose of fluoxetine does not hasten the recovery of firing of serotonergic neurons during chronic administration. These results support the hypothesis that desensitization of the 5-HT1A receptor and consequent recovery of firing of 5-HT cells in the dorsal raphe nucleus plays a role in the delayed therapeutic onset of fluoxetine.
The present study investigated the possibility of multiple 5-HT(1) autoreceptor subtypes in the rostral raphé nuclei. Slices (350 microm) of rat dorsal or median raphé nucleus (DRN/MRN) were taken from male Wistar rats and superfused with artificial cerebrospinal fluid at 32 degrees C. Fast cyclic voltammetry at carbon fibre microelectrodes was used to monitor serotonin (5-HT) release following local electrical stimulation. In both DRN and MRN, 5-HT release on short trains was reduced by the selective 5-HT(1A) agonist 8-OH-DPAT (1 microM), an effect blocked by the selective 5-HT(1A) antagonist WAY 100635 (0.1 microM) but not by SB 216641 (0.05 and 0.2 microM) or BRL 15572 (0.5 microM), selective antagonists at the 5-HT(1B) and 5-HT(1D) receptors respectively. The selective 5-HT(1B) agonist CP 93129 (0.3 microM) also reduced 5-HT release in both nuclei. Its effect was blocked by SB 216641 but not by WAY 100635 or BRL 15572. The 5-HT(1D/1B) agonist sumatriptan (0.5 microM) decreased 5-HT release in both DRN and MRN. In DRN, the effect of sumatriptan was blocked by BRL 15572 but not by WAY 100635 or SB 216641. In MRN, the effect of sumatriptan was not blocked by any of the above antagonists. BRL 15572 increased 5-HT release on long stimulations in DRN and MRN while WAY 100635 had no effect. SB 216641 increased 5-HT release in MRN but not DRN. WAY 100635 potentiated the effect of SB 216641 in DRN but not MRN. The data suggest that 5-HT release in DRN is controlled by 5-HT(1A), 5-HT(1B) and 5-HT(1D) autoreceptors. 5-HT release in MRN is controlled by 5-HT(1A) and 5-HT(1B) autoreceptors and another, as yet unidentified mechanism.
The atypical analgesic tramadol has strong structural similarities to the antidepressant venlafaxine and is a mixed noradrenaline (NA) and serotonin (5-HT) uptake inhibitor. Because tramadol has been found active in the forced swim test, a common predictor of antidepressant efficacy, we therefore examined the effects of chronic tramadol on various pre- and post-synaptic monoamine measures. Male Wistar rats (150-200 g) received tramadol (20 mg/kg i.p.) or vehicle for 21 days and were sacrificed 24 h after the last dose. Quantitative autoradiography revealed that specific frontocortical [3H]dihydroalprenolol and [3H]ketanserin binding was lower in the chronic tramadol group than controls (beta: 37+/-8 and 217+/-56 fmol/mg; 5-HT2A: 23+/-3 and 44+/-7 fmol/mg, respectively, p < 0.05). Chronic tramadol had no effect on the magnitude of electrically stimulated noradrenaline (NA) efflux or uptake in locus coeruleus (LC) slices. Although dexmedetomidine (10 nM) decreased LC NA efflux equally (by approximately 60%) in chronic tramadol and vehicle groups, desipramine (50 nM) increased LC NA efflux more in vehicle (to 164+/-7%) than tramadol-treated rats (144+/-6%; p < 0.05). Chronic tramadol had no effect on dorsal raphé (DRN) or median raphé (MRN) 5-HT efflux. However, 5-HT uptake in tramadol-treated rats was slower (p < 0.05) in MRN and nearly so (p = 0.055) in DRN. The selective 5-HT1A agonist 8-OH-DPAT reduced 5-HT efflux in both DRN and MRN. Its effect in DRN was greater in rats given chronic tramadol than in vehicle controls (54+/-2 versus 32+/-6% reduction in 5-HT efflux, respectively). In conclusion, we suggest that tramadol has many of the pre- and postsynaptic neurochemical features of a conventional antidepressant, as might be predicted from its pharmacology.
Previous reports from our laboratory have provided evidence that acute, i.e., concurrent, treatment with selective serotonin reuptake inhibitors (SSRIs) augments the stimulus effects of indoleamine and phenethylamine hallucinogens in the rat. In the present investigation, the acute effects of fluoxetine and citalopram on stimulus control induced by (-)-2,5-dimethoxy-4-methylamphetamine (DOM) were compared with those following subchronic, i.e., 10-day treatment with the SSRIs. Stimulus control was established using DOM (0.56 mg/kg; 75-min pretreatment time) in a group of 11 rats. A two-lever, fixed ratio 10, positively reinforced task with saline controls was employed. The effects of a range of doses of DOM when given alone were compared with those following both acute and subchronic pretreatment with fluoxetine and citalopram in combination with DOM. It was found that acute administration of fluoxetine and citalopram potentiated the stimulus effects of DOM. Furthermore, it was observed that the degree of potentiation was not diminished by treatment with either fluoxetine or citalopram for a period of 10 days. It is concluded that whatever adaptive changes may take place in response to a 10-day period of treatment with either citalopram or fluoxetine, these adaptations are independent of the mechanisms responsible for the potentiation of the stimulus effects of DOM by the SSRIs.
Chronic treatment with selective serotonin reuptake inhibitors (SSRIs) can delay ejaculation in humans, but the extent of this effect differs between SSRIs. The involvement of 5-HT1A receptors is likely, since 5-HT1A receptor agonists accelerate ejaculation and chronic SSRI treatment is thought to desensitize 5-HT1A receptors.
This study was conducted to examine the effects of chronic pretreatment with the SSRIs fluvoxamine and paroxetine on the facilitation of ejaculation induced by the 5-HT1A receptor agonist 8-OH-DPAT.
Sexually experienced Wistar rats with normal ejaculatory behavior were treated for 22 days with vehicle, fluvoxamine (30 mg/kg/day), or paroxetine (10 or 20 mg/kg/day, p.o.). On day 22, rats received a challenge with saline or 8-OH-DPAT (0.4 mg/kg, s.c.). Sexual behavior was tested on days 1, 8, 15, and 22 of the SSRI-treatment.
Treatment with both doses of paroxetine, but not fluvoxamine, delayed ejaculation. 8-OH-DPAT strongly accelerated ejaculation under vehicle conditions. Pretreatment with paroxetine reduced the effects of 8-OH-DPAT on ejaculation in a dose-dependent manner and more strongly than fluvoxamine.
SSRIs affect 5-HT1A receptors involved in ejaculation. The degree to which this occurs, with paroxetine exerting a stronger effect than fluvoxamine, might determine the extent of SSRI-induced delayed ejaculation.
Unlabelled:
SSRIs are postulated to modulate motor behavior. A single dose of selective serotoninergic reuptake inhibitors (SSRIs) like fluoxetine, paroxetine, or fluvoxamine, has been shown to improve motor performance and efficiency of information processing for simple sensorimotor tasks in healthy subjects. At a cortical level, a single dose of SSRI was shown to induce a hyperactivation of the primary sensorimotor cortex (S1M1) involved in the movement (Loubinoux, I., Boulanouar, K., Ranjeva, J. P., Carel, C., Berry, I., Rascol, O., Celsis, P., and Chollet, F., 1999. Cerebral functional magnetic resonance imaging activation modulated by a single dose of the monoamine neurotransmission enhancers fluoxetine and fenozolone during hand sensorimotor tasks. J. Cereb. Blood Flow Metab. 19 1365--1375, Loubinoux, I., Pariente, J., Boulanouar, K., Carel, C., Manelfe, C., Rascol, O., Celsis, P., and Chollet, F., 2002. A Single Dose of Serotonin Neurotransmission Agonist Paroxetine Enhances Motor Output. A double-blind, placebo-controlled, fMRI study in healthy subjects. NeuroImage 15 26--36). Since SSRIs are usually given for several weeks, we assessed the behavioral and cerebral effects of a one-month chronic administration of paroxetine on a larger group. In a double-blind, placebo controlled and crossover study, 19 subjects received daily 20 mg paroxetine or placebo, respectively, over a period of 30 days separated by a wash-out period of 3 months. After each period, the subjects underwent an fMRI (active or passive movement, dexterity task, sensory discrimination task) and a behavioral evaluation. Concurrently, a TMS (transcranial magnetic stimulation) study was conducted (Gerdelat-Mas, A., Loubinoux, I., Tombari, D., Rascol, O., Chollet, F., Simonetta-Moreau, M., 2005. Chronic administration of selective serotonin re-uptake inhibitor (SSRI) paroxetine modulates human motor cortex excitability in healthy subjects. NeuroImage 27,314--322).
Results:
On the one hand, paroxetine improved motor performances at the finger tapping test (P=0.02) without affecting choice reaction time, strength and dexterity significantly. Subjects were also faster in processing the spatial incongruency between a stimulus and the motor response (P=0.04). In order to differentiate behavioral components, a principal component analysis was performed on all motor tests, and several characteristics were differentiated: strength, speed, skill, attention, and motor response coding. Paroxetine would improve the efficiency of motor response coding (MANOVA on the factors; factor 3, P=0.01). On the other hand, the chronic administration induced a significant hypoactivation of S1M1 whatever the task: motor or sensory, simple or complex (random effect analysis, P<0.05). The hypoactivation correlated with the improvement of performances at the finger tapping test (P<0.05) suggesting more efficiency in cerebral motor processing.
Conclusions:
Our results showed a clear modulation of sensory and motor cerebral activation after a chronic paroxetine administration. An improvement in both behavior and cerebral efficiency was suggested. It could be hypothesized that monoamines, by an unspecific effect, may tune the response of pyramidal neurons to optimize performances.
Chronic treatment with the selective serotonin reuptake inhibitor paroxetine impairs the functioning of 5-HT(1A) receptors involved in ejaculation. This could underlie the development of delayed ejaculation often reported by men treated with paroxetine. The neurobiological substrate linking the effects of selective serotonin reuptake inhibitor-treatment and 5-HT(1A) receptor activation with ejaculation was investigated. Male Wistar rats that were pretreated with paroxetine (20 mg/kg/day p.o.) or vehicle for 22 days and had received an additional injection with the 5-HT(1A) receptor agonist 8-OH-DPAT ((+/-)-8-hydroxy-2-(di-n-propyl-amino)tetralin; 0.4 mg/kg s.c.) or saline on day 22, 30 min prior to a sexual behavior test, were perfused 1 h after the sexual behavior test. Brains were processed for Fos-, and oxytocin immunohistochemistry. The drug treatments markedly changed both sexual behavior and the pattern and number of Fos-immunoreactive cells in the brain. Chronic pretreatment with paroxetine caused delayed ejaculation. Acute injection with 8-OH-DPAT facilitated ejaculation in vehicle-pretreated rats, notably evident in a strongly reduced intromission frequency, whereas 8-OH-DPAT had no effects in paroxetine-pretreated rats. Chronic treatment with paroxetine reduced Fos-immunoreactivity in the locus coeruleus, and prevented the increase in Fos-immunoreactive neurons induced by 8-OH-DPAT in the oxytocinergic magnocellular part of the paraventricular nucleus as well as in the locus coeruleus. Since oxytocin and noradrenalin facilitate ejaculation, the alterations in Fos-IR in these areas could connect selective serotonin reuptake inhibitor treatment and 5-HT(1A) receptor activation to ejaculation. Chronic paroxetine treatment and 8-OH-DPAT changed c-fos expression in a number of other brain areas, indicating that Fos-immunohistochemistry is a useful tool to find locations where selective serotonin reuptake inhibitors and 8-OH-DPAT exert their effects.
Selective serotonin reuptake inhibitors (SSRIs) differ in the severity of induced ejaculation delay. Various studies indicate that oxytocin is involved in sexual behavior.
To review and evaluate the involvement of oxytocin in SSRI-induced ejaculation delay.
Oxytocine release, 5-hydroxytryptamine (5-HT) neurotransmission, and desensitization of 5-HT(1A) receptors.
A review and critical analysis of animal studies investigating the interaction of serotonergic and oxytocinergic neurotransmission in relation to the ejaculation process.
Although acute treatment with the SSRIs fluoxetine and paroxetine immediately causes increased serotonin levels, delayed ejaculation does not occur. The increased serotonin levels induce oxytocin release via activation of 5-HT(1A) receptors, and this might compensate for the inhibitory actions of serotonin on sexual behavior. Chronic treatment with fluoxetine and paroxetine desensitizes 5-HT(1A) receptors on oxytocin neurons, and that might in part determine the onset of delayed ejaculation. Desensitization of 5-HT(1A) receptors is less strong following chronic treatment with the SSRIs fluvoxamine or citalopram, which may attenuate the degree of delayed ejaculation.
Preliminary data suggest that the severity of chronic SSRI treatment-induced delayed ejaculation and the differences between the various SSRIs in inducing ejaculation delay is related to gradual desensitization of 5-HT(1A) receptors on oxytocin neurons.
Disturbances in the serotonergic system are implicated in many central nervous system disorders. The serotonin transporter (SERT) regulates the serotonin homeostasis in the synapse. We recently developed a rat which lacks the serotonin transporter (SERT(-/-)). It is likely that adaptive changes take place at the level of pre- and postsynaptic 5-HT receptors. Because autonomic responses are often used to measure 5-HT(1A) receptor function, we analysed these responses by examining the effects of a 5-HT(1A) receptor agonist and antagonist under in vivo conditions in the SERT(-/-) rat. Moreover, we studied the effect of a mild stressor on the body temperature (stress-induced hyperthermia) because of the known involvement of 5-HT(1A) receptors in this phenomenon. Results show that core body temperature did not differ between genotypes under basal, non-stressed conditions. Compared to SERT(+/+) rats, stress-induced hyperthermia was reduced in SERT(-/-) rats. The 5-HT(1A) receptor agonist [R(+)-N-(2[4-(2,3-dihydro-2-2-hydroxy-methyl-1,4-benzodioxin-5-yl)-1-piperazininyl]ethyl)-4-fluorobenzoamide HCl (flesinoxan) reduced stress-induced hyperthermia in both genotypes. The flesinoxan-induced hypothermia in SERT(+/+) rats was blocked by the 5-HT(1A) receptor antagonist [N-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-N-(2-pyridinyl) cyclohexane carboxamide 3HCl (WAY100635). Moreover, WAY100635-induced hyperthermia in SERT(-/-), but not in SERT(+/+) rats. In SERT(-/-) rats, WAY100635 completely blocked the flesinoxan-induced reduction of stress-induced hyperthermia. Interestingly, flesinoxan-induced hypothermia was absent in SERT(-/-) rats. It is concluded that the SERT knockout rat reveals that 5-HT(1A) receptors modulating stress-induced hyperthermia belong to a population of receptors that differs from that involved in hypothermia.
The present study examined the distribution of [3H]8-OH-DPAT-labeled 5-HT1A receptors and their degree of coupling to G proteins in the hypothalamus and several other brain regions. In addition, we also investigated the effects of repeated injections of fluoxetine on the density and G protein coupling of 5-HT1A receptors in hypothalamic nuclei and other brain regions using autoradiography. Male rats received daily injections of either fluoxetine (10 mg/kg, ip) for 3, 7, 14 and 22 days, or saline for 22 days. 5-HT1A receptors were labeled by 2 nM [3H]8-hydroxy-2-(dipropylamino)tetralin ([3H]8-OH-DPAT) in the absence or presence of guanylylimidodiphosphate (Gpp(NH)p, 10−5 M) to determine the percentage of 5-HT1A receptors coupled to G proteins. -HT1A receptor densities ranged from 7 to 63 fmol/mg tissue equivalent among hypothalamic nuclei. Similarly, the degree of G protein coupling to 5-HT1A receptors varied markedly among hypothalamic nuclei (from 14% to 61%) and among other brain regions (from 17% to 85%). Fluoxetine did not alter the density or the degree of coupling of 5-HT1A receptors in any brain regions. These data indicate marked regional differences in the degree of G protein-coupled 5-HT1A receptors and suggest that fluoxetine-induced desensitization of hypothalamic 5-HT1A receptors is not mediated by changes in receptor density or G protein coupling.
The pathophysiology of major affective illness is poorly understood. However, several lines of preclinical and clinical evidence indicate that an enhancement of 5-HT-mediated neurotransmission might underlie the therapeutic effect of most antidepressant treatments. This net effect would, however, be obtained via different mechanisms. A better understanding of the neurobiological basis for the delayed onset of action of antidepressant treatments has led to the elaboration of strategies that could accelerate the antidepressant response. These strategies are discussed in this article by Pierre Blier and Claude de Montigny.
1. Changes in extracellular concentrations of 5-hydroxytryptamine elicited by electrical stimulation in rat brain slices containing the dorsal raphe nucleus and the suprachiasmatic nucleus were monitored with fast cyclic voltammetry. 2. Using pseudo single pulse stimulation (5 pulses applied at 100 Hz) we have shown that the release of 5-hydroxytryptamine in the dorsal raphe and the suprachiasmatic nucleus can be regulated by autoreceptors in both brain regions. 3. In the suprachiasmatic nucleus, 5-carboxamidotryptamine, RU24969, 1-(m-trifluoromethylphenyl) piperazine and sumatriptan caused a concentration-dependent inhibition of stimulated 5-hydroxytryptamine overflow in the range 1 x 10(-9) M to 3 x 10(-6) M. The actions of 5-carboxamidotryptamine and RU24969 were reversed competitively by methiothepin (10(-8) M to 10(-6) M); Schild plots revealed pKB values of 7.9 and 8.1. By contrast, ipsaparone and 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) are not effective 5-hydroxytryptamine autoreceptor agonists in the suprachiasmatic nucleus. 4. Isamoltane (10(-6) M), the putative 5-HT1B receptor antagonist, blocked the responses to RU24969 (10(-6) M) and 1-(m-trifluoromethylphenyl)piperazine (10(-6) M) in the suprachiasmatic nucleus. 5. In the dorsal raphe nucleus, 8-OH-DPAT, ipsapirone, RU24969, 5-carboxamidotryptamine, and sumatriptan (all 1 x 10(-8) M to 3 x 10(-6) M) produced a concentration-dependent reduction in the stimulated release of 5-hydroxytryptamine. The maximum effect observed was less than that seen in the suprachiasmatic nucleus.(ABSTRACT TRUNCATED AT 250 WORDS)
Extracellular recordings were made from serotonergic neurons of the rat dorsal raphe nucleus in a slice preparation. In the presence of phenylephrine (3 microM) to restore the pacemaker activity of otherwise silent serotonergic neurons, superfusion with the 5-hydroxytryptamine1A agonist ipsapirone depressed the firing of these neurons with an IC50 of approximately 50 nM. Complete inhibition was achieved with 100-300 nM of the drug. Concomitant superfusion with the 5-hydroxytryptamine1A antagonists spiperone (100 nM) or propranolol (10 microM) markedly reduced the inhibitory effect of ipsapirone (100 nM). Superfusion with K+ channel blockers such as apamin (50-100 nM), charybdotoxin (100 nM) or Ba2+ (1 mM) did not induce any changes in the electrical activity of serotonergic neurons. However, 4-aminopyridine (0.1-1 mM) disrupted the regularity of their discharge without affecting the mean firing rate. The ipsapirone-induced inhibition was unchanged by apamin and charybdotoxin, but was markedly reduced by Ba2+ and 4-aminopyridine. Thus the IC50 of ipsapirone was shifted to approximately 150 nM in the presence of 1 mM of 4-aminopyridine. These results indicate that, in serotonergic neurons within the dorsal raphe nucleus, the K+ channel opened through the stimulation of 5-hydroxytryptamine1A autoreceptors is 4-aminopyridine-sensitive.
Fast cyclic voltammetry at a carbon fibre microelectrode was used to monitor the time course of 5-hydroxytryptamine (5-HT) overflow in slices of rat dorsal raphe (DRN) and suprachiasmatic nuclei (SCN), incubated in a brain slice chamber for over 6 h. 5-HT overflow was detected in response to electrical brain stimulation in both regions. Voltammetric evidence showed that the released substance was identical to exogenously applied 5-HT. Overflow was reversibly abolished when Ca2+ was removed from the incubating medium or when TTX was added. Ro4-1284, a reserpine like agent, irreversibly abolished 5-HT overflow from both nuclei. The 5-HT uptake blockers, citalopram, clomipramine, fenfluramine and fluvoxamine dose dependently increased overflow and slowed the rate of removal of 5-HT from the extracellular space in both regions. Benztropine had no effect on overflow in the DRN and SCN whereas it caused a significant increase in dopamine overflow in slices of caudate putamen (CPu). Xylamine had no effect on 5-HT overflow in the DRN and SCN. This evidence indicates that the release of endogenous 5-HT can be measured reliably for long periods and that FCV can be used in brain slices for quantitative studies of 5-HT release and uptake.
Release of [3H]serotonin elicited by electrical stimulation from rat hypothalamus and its modulation through autoreceptors has been studied after chronic administration of two antidepressants, citalopram, specific inhibitor of serotonin uptake, and milnacipran (previously called midalcipran, F 2207) which blocks the uptake of serotonin and noradrenaline to the same extent. The amount of [3H]serotonin released by electrical stimulation was enhanced and the inhibitory effect of the agonist, LSD (lysergic acid diethylamide), reduced in rats treated with citalopram at 10 and 50 mg/kg per day for 21 days. These effects existed at both doses but were accentuated at the higher dose. Thus a chronic treatment with citalopram provokes a down-regulation of the serotonergic autoreceptor, allowing an increase of serotonin neurotransmission. After 21 days treatment with milnacipran, at 50 mg/kg per day, none of the parameters studied were modified. These data suggest that down-regulation of the serotonin autoreceptor is not a universally applicable hypothesis to explain the action of all antidepressants acting on the uptake of serotonin.
5-HT-containing terminals possess autoreceptors which modulate the release of 5-HT into the synaptic cleft. Tritiated imipramine ([3H]IMI), and more specifically [3H]citalopram and [3H]paroxetine, bind to a site associated with the 5-HT reuptake carrier on the 5-HT terminals. The function of terminal 5-HT autoreceptors is decreased following long-term treatment with the 5-HT reuptake blocker citalopram. The present study was undertaken to determine whether an increased synaptic availability of 5-HT or, the occupation of the [3H]IMI site, were responsible for this modification. Unitary extracellular recordings were obtained from CA3 dorsal hippocampus pyramidal neurons under chloral hydrate anesthesia in rats treated daily with fluoxetine (10 mg/kg/day X 14 days), a selective 5-HT reuptake blocker, or clorgyline (1 mg/kg/day X 21 days), an inhibitor of type A monoamine oxidase. The function of the terminal 5-HT autoreceptors was assessed by comparing the effectiveness of the electrical stimulation of the ascending 5-HT pathway on the firing activity of hippocampus pyramidal neurons prior to, and following, the administration of methiothepin, an antagonist of the terminal 5-HT autoreceptor, and, by determining the ratio of effectiveness of 0.8 Hz (S1) and 5 Hz (S2) stimulations. Long-term administration of fluoxetine or clorgyline both increased the efficacy of the stimulation of the 5-HT pathway. However, the enhancing effect of methiothepin on the efficacy of the stimulation was attenuated by the fluoxetine, but not by the clorgyline, treatment.(ABSTRACT TRUNCATED AT 250 WORDS)
A direct comparison was made of the effects of serotonin 5-HT1A and 5-HT1B selective compounds on the spontaneous firing rate of dorsal raphe serotoninergic neurons in chloral-hydrate-anesthetized rats. Following intravenous administration, the 5-HT1A selective compounds ipsapirone (TVX Q 7821) and LY 165163 potently inhibited single-unit activity in a dose-dependent manner whereas the 5-HT1B selective compounds, m-chlorophenylpiperazine (mCPP) and trifluoromethylphenylpiperazine (TFMPP), displayed only weak or irregular actions. Low microiontophoretic currents of ipsapirone and LY 165163 were also effective in suppressing spontaneous firing; dose-response relationships for the 5-HT1A compounds were indistinguishable from that of 5-HT itself. In contrast, dorsal raphe neurons were only weakly responsive to microiontophoretic application of mCPP and TFMPP; dose-response relationships for the 5-HT1B compounds were significantly displaced from that of 5-HT. In intracellular studies, ipsapirone and LY 165163, when added to the media bathing brain slices, mimicked the actions of 5-HT in hyperpolarizing dorsal raphe cell membranes and decreasing input resistance; however, the maximal effects of the 5-HT1A compounds on these membrane properties exceeded those of 5-HT. In summary, dorsal raphe 5-HT neurons appear highly responsive to 5-HT1A, but not to 5-HT1B compounds; these findings are discussed with regard to the 5-HT receptor subtypes as candidates for the somatodendritic autoreceptor of dorsal raphe neurons.
Serotonin (5-HT) has direct and specific effects on the activity of spinal cord motoneurons. The 5-HT1A receptor has been shown to mediate motoneuron responses in spinal reflex pathways using the highly selective 5-HT1A receptor agonist 8-OH-DPAT. We have developed an antipeptide antibody that recognizes a specific region (the second external loop) of the 5-HT1A receptor. This 5-HT1A receptor antibody labels populations of neurons and glia in the primate cervical spinal cord. The highest receptor density is present in the superficial lamina of the dorsal horn, around the central canal, and on the axon hillock of large ventral horn motoneurons. The cellular labeling pattern on motoneurons shows a single, densely stained, tapering process emanating from the perikaryon. A more diffuse label is also present throughout the soma. Dendritic labeling was not apparent. These results suggest that post-synaptic 5-HT1A receptors may be involved in modulating spinal motoneuron activity at the key site of action potential initiation, the axon hillock.
Electrically stimulated 5-hydroxytryptamine (5-HT) release was monitored in slices of rat dorsal raphé nucleus (DRN) by fast cyclic voltammetry. Pseudo-single pulse stimulations (5 pulses at 100 Hz) were used to enable the effect of various receptor agonists to be seen without competition from endogenously released transmitter. The selective 5-HT1A receptor agonist, (+)-8-OH-DPAT (1.0 microM) decreased stimulated 5-HT release to 31 +/- 3% of controls. This decrease was inhibited by the 5-HT1A receptor antagonists, (+)-WAY-100135 (1.0 microM) and WAY-100635 (0.1 microM) but not by the 5-HT1D/B antagonist, GR127935 (0.05 microM). The selective 5-HT1B receptor agonist, CP-93129 (0.3 microM) decreased stimulated 5-HT release to 61 +/- 4% of control. This effect was antagonized by the 5-HT1B receptor antagonist, isamoltane (0.5 microM) but not by (+)-WAY-100135. The 5-HT1D agonist, sumatriptan (0.5 microM) decreased stimulated 5-HT release to 52 +/- 2% of controls. This decrease was blocked by GR-127935 but not by WAY-100635. These results suggest that 5-HT release in the rat DRN is under the control of 5-HT1A, 5-HT1B and 5-HT1D autoreceptors.
Conflicting reports in the literature over heterogeneity (West et al. , 1990) or homogeneity (Arrang et al. , 1990) of histamine H 3 ‐receptor binding sites may be attributed to the use of different incubation conditions. In the present study we have investigated the extent to which the binding of H 3 ‐receptor ligands to rat cerebral cortical membranes can be modified by both sodium ions and guanine nucleotides.
The H 3 ‐selective antagonist, thioperamide, discriminated between two specific binding sites for [ 3 H]‐Nα‐methylhistamine (IC 50 1 = 2.75 ± 0.87 n m , IQ 50 2 = 101.6 ± 12.0 n m , % site 1 = 24 ± 2%) in 50 m m Tris HCl buffer, but showed homogeneity of binding in 50 m m Na/K phosphate buffer.
Sodium ions markedly altered the binding characteristics of thioperamide (i.e. heterogeneity was lost and IC 50 value shifted towards the high affinity site). The competition curves for a second H 3 ‐antagonist, clobenpropit and the H 3 ‐agonist N α ‐methylhistamine however, were unaltered in the presence of sodium ions.
Guanylnucleotides displaced only 60% of specific [ ³ H]‐N α ‐methylhistamine binding and modulated thioperamide binding in the same way as sodium ions.
These data suggest that the H 3 ‐receptor can exist in different conformations for which thioperamide, but not N α ‐methylhistamine and clobenpropit, show differential affinity.
The potential nature of these sites, and the implications of this apparent receptor heterogeneity for H 3 ‐receptor antagonism by thioperamide, are discussed.
Electrophysiological studies suggest that 5-HT autoreceptor desensitization may be responsible for the delayed clinical efficacy of some antidepressant drugs, such as selective 5-HT reuptake inhibitors (SSRI) and certain MAO inhibitors (MAOI). In the present study we have used in vivo microdialysis to test this hypothesis. Rats were treated for 2 weeks with the antidepressant SSRI citalopram (5 mg/kg, s.c., b.i.d.). After 24 hr withdrawal, dialysis probes were implanted in the dorsal hippocampus (DH) and the frontal cortex (FCx). The rats then received as acute challenge, a 5-HT1A autoreceptor-active dose of the reference 5-HT1A agonist 8-OH-DPAT (0.025 mg/kg s.c.). The 8-OH-DPAT-induced changes in dialysate 5-HT from the DH and the FCx were monitored and taken as an index of autoreceptor sensitivity. Chronic citalopram and control animals responded similarly to 8-OH-DPAT with a drop of 5-HT of about 50-65%; no significant difference between the chronic citalopram and control groups were obtained, either in the DH or in the FCx. These data suggest that cell body 5-HT1A autoreceptors do not desensitize in response to repeated administration with antidepressant SSRI drugs such as citalopram.
Changes in the extracellular concentration of 5-HT evoked by electrical stimulation of brain slices containing either dorsal raphe nucleus (DRN) or suprachiasmatic nucleus (SCN) from rats treated for 21 days with fluoxetine (5 mg/kg; i.p.) or water were monitored using fast cyclic voltammetry (FCV). Stimulated 5-HT overflow was enhanced significantly in both brain regions after 21 days treatment with fluoxetine but there was no change in the half time for re-uptake (t1/2). Concentration response curves for inhibition of electrically stimulated 5-HT overflow by 8-OH-DPAT (5-HT1a receptor agonist) or RU24969 (5-HT1b receptor agonist) in the DRN or SCN respectively were obtained in slices prepared from both groups of animals. There was a significant shift to the right in the dose-response curve for RU24969 in the SCN in fluoxetine treated animals but a shift to the left for the dose-response curve for 8-OH-DPAT in the DRN. These data suggest that down regulation of the 5-HT1b autoreceptors occurs in an axon terminal region (SCN) but that there is a sensitisation of 5-HT1a autoreceptor mechanisms controlling 5-HT overflow in the DRN.
Previous studies [Meller et al. (1990) Mol. Pharmacol., 37:231-237] have shown that a large receptor reserve exists for the inhibition of serotonin synthesis in rat cortex and hippocampus by the 5-HT1A agonist 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), whereas little or no reserve exists for the lower efficacy agonists ipsapirone and BMY 7378. The current studies were undertaken to determine if the above drugs exhibit similar relative efficacies and receptor reserves in an electrophysiological model of 5-HT1A receptor activation, i.e., the inhibition of dorsal raphe cell firing. Intravenous dose-response curves were constructed in untreated control rats, or in rats which received an injection of the irreversible receptor inactivator N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 6 mg/kg, s.c.) 24 hours before recording. All three drugs fully inhibited dorsal raphe cell firing in control rats (ED50's: 1.5 micrograms/kg, 8-OH-DPAT; 30.0 micrograms/kg, ipsapirone; 17.5 micrograms/kg, BMY 7378). However, unlike effects on serotonin synthesis, EEDQ treatments caused no depression of the maximal inhibitory response for any of the agonists, although all dose-response curves were shifted to the right (ED50's: 10.1 micrograms/kg, 6.7-fold shift, 8-OH-DPAT; 139.9 micrograms/kg, 4.7-fold shift, ipsapirone; 53.8 micrograms/kg, 3.1-fold shift, BMY 7378). Although the order of agonist efficacies was similar for both inhibition of serotonin synthesis and dorsal raphe cell firing (8-OH-DPAT > ipsapirone > BMY 7378), a large (> 50%) receptor reserve was estimated for all three drugs in this electrophysiological system.(ABSTRACT TRUNCATED AT 250 WORDS)
We used whole cell current- and voltage-clamp recording in neonatal rat brain stem slices to characterize firing properties and effects of serotonin (5-HT) on neurons (n = 225) in raphe pallidus (RPa) and raphe obscurus (ROb). Of a sample of 51 Lucifer yellow-filled neurons recovered after immunohistochemical processing to detect tryptophan hydroxylase (TPH), 34 were found to be TPH immunoreactive (i.e., serotonergic). Serotonergic neurons had long-duration action potentials and fired spontaneously at low frequency (approximately 1 Hz) in a pattern that was often irregular; at higher firing frequencies the discharge became more regular. These neurons displayed spike frequency adaptation, with maximal steady-state firing rates of < 4 Hz. The overwhelming majority of identified serotonergic neurons was hyperpolarized by bath-applied 5-HT (94%; n = 32 of 34); conversely, most cells in this sample that were hyperpolarized by 5-HT were serotonergic (78%; n= 32 of 41). TPH-immunonegative neurons were separated into two populations. One group had properties that were indistinguishable from those of serotonergic caudal raphe neurons. The other group was truly distinct; those neurons had more hyperpolarized resting membrane potentials, were not spontaneously active, had shorter-duration action potentials, and were depolarized by 5-HT. Caudal raphe neurons responded to 5-HT (1-5 microM) with membrane hyperpolarization in current clamp (-13.4 +/- 1.1 mV, mean +/- SE) or with outward current in voltage clamp (16.0 +/- 1.4 pA). The current induced by 5-HT was inwardly rectifying and associated with an increase in peak conductance and was highly selective for K+. It was completely blocked by 0.2 mM Ba2+ but not by glibenclamide, an inhibitor of ATP-sensitive K+ channels. Effects of 5-HT were dose dependent, with an EC50 of 0.1-0.3 microM. The 5-HT1A agonist 8-OH-DPAT mimicked, and the 5-HT1A antagonists (+)WAY 10,0135 and NAN 190 blocked, effects of 5-HT. The 5-HT2A/C antagonist ketanserin did not inhibit the effects of 5-HT. Fewer 5-HT-responsive neurons were encountered in slices exposed acutely to pertussis toxin (approximately 13%) than in adjacent control slices not exposed to pertussis toxin (approximately 85%). In addition, in neurons recorded with pipettes containing GTP gamma S (0.1 mM), 5-HT induced an inwardly rectifying current that did not reverse on washing. In many cells recorded with GTP gamma S, a current developed in the absence of agonist that had properties identical to those of the 5-HT-sensitive current; when followed for extended periods, the agonist-independent GTP gamma S-induced conductance desensitized, returning toward control levels with a time constant of approximately 18 min. Together these results indicate that serotonergic neurons of ROb and RPa are spontaneously active in a neonatal rat brain stem slice preparation and that hyperpolarization of those neurons by 5-HT1A receptor stimulation is due to pertussis toxin-sensitive G protein-mediated activation of an inwardly rectifying K+ conductance. In addition, we identified a group of nonserotonergic medullary raphe neurons that had distinct electrophysiological properties and that was depolarized by 5-HT.
The present study was undertaken to compare the properties of the [3H]8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) binding site in the dorsal raphe nucleus with the hippocampal 5-HT1A receptor. In both tissues inclusion of 1 mM Mg2+ enhanced specific [3H]8-OH-DPAT binding, while 1 mM GTP decreased radioligand binding. [3H]8-OH-DPAT appears to bind to a single population of binding sites in both the hippocampus and the dorsal raphe nucleus, although the K(d) for the radioligand at the dorsal raphe site was five times that observed at the hippocampal 5-HT1A receptor. Similarly, although 5-HT and selective 5-HT1A receptor ligands displayed high affinity for the [3H]8-OH-DPAT binding site in the dorsal raphe nucleus, the affinity at the dorsal raphe site was less than that observed at the hippocampal 5-HT1A receptor. 8-OH-DPAT inhibited forskolin-stimulated adenylyl cyclase activity in the hippocampus, but did not alter enzyme activity in the dorsal raphe nucleus. Conversely, 8-OH-DPAT inhibited the accumulation of [3H]inositol phosphates in the dorsal raphe nucleus, but not in the hippocampus. An inhibition of phosphoinositide hydrolysis in the dorsal raphe nucleus also was found with the putative 5-HT1A receptor selective ligands, flesinoxan and gepirone. However, addition of another putative 5HT1A receptor selective ligand, buspirone, did not alter the generation of [3H]inositol phosphates, but blocked the inhibitory effect of 8-OH-DPAT on phosphoinositide hydrolysis. These studies demonstrate that the 8-OH-DPAT binding site in the dorsal raphe nucleus displays a binding profile which is similar to the hippocampal 5-HT1A receptor, but unlike this 5-HT1A receptor the binding site in the dorsal raphe nucleus is negatively coupled to phosphoinositide turnover.