5-HT1A Autoreceptors Research Articles

Elevated central monoamine receptor mRNA in rats bred for high endurance capacity: Implications for central fatigue

Although alteration to peripheral systems at the skeletal muscle level can contribute to one's ability to sustain endurance capacity, neural circuits regulating fatigue may also play a critical role. Previous studies demonstrated that increasing brain serotonin (5-HT) release is sufficient to hasten the onset of exercise-induced fatigue, while manipulations that increase brain dopamine (DA) release can delay the onset of fatigue. These results suggest that individual differences in endurance capacity could be due to factors capable of influencing the activity of 5-HT and DA systems. We evaluated possible differences in central fatigue pathways between two contrasting rat groups selectively bred for high (HCR) or low (LCR) capacity running. Using quantitative in situ hybridization, we measured messenger RNA (mRNA) levels of tryptophan hydroxylase (TPH), 5-HT transporter (5-HTT), 5-HT1A and 5-HT1B autoreceptors, dopamine receptor-D2 (DR-D2) autoreceptors and postsynaptic receptors, and dopamine receptor-D1 (DR-D1) postsynaptic receptors, in discrete brain regions of HCR and LCR. HCR expressed higher levels of 5-HT1B autoreceptor mRNA in the raphe nuclei relative to LCR, but similar levels of TPH, 5-HTT, and 5-HT1A mRNA in these areas. Surprisingly, HCR expressed higher levels of DR-D2 autoreceptor mRNA in the midbrain, while simultaneously expressing greater DR-D2 postsynaptic mRNA in the striatum compared to LCR. There were no differences in DR-D1 mRNA levels in the striatum or cortex between groups. These data suggest that central serotonergic and dopaminergic systems may be involved in the mechanisms by which HCR have delayed onset of exercise-induced fatigue compared to LCR.

Differential role of 5-HT1A and 5-HT1B receptors on the antinociceptive and antidepressant effect of tramadol in mice

Tramadol, (1RS,2RS)-2-[(dimethylamine)-methyl]-1-(3-methoxyphenyl)-cyclohexanol hydrochloride, is an atypical analgesic which binds weakly to ì-opioid receptors and enhances the extra-neuronal concentration of noradrenaline and serotonin by interference with both the uptake and release mechanisms. The present study was undertaken to evaluate the potential role of 5-HT1A and 5-HT1B receptors on the analgesic and antidepressant-like effect of tramadol. The effect of either a selective 5-HT1A receptor antagonist (WAY 100635; N-2-[4-(2-methoxyphenyl-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexane carboxamide; 0.2-0.8, 8 mg/kg) or a selective 5-HT1B receptor antagonist (SB 216641; N-[3-(3-dimethylamino) ethoxy-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-carboxamide; 0.2-0.8, 8 mg/kg) was investigated in mice in combination with tramadol by means of the hot-plate test, a phasic nociceptive model, and the forced swimming test, a paradigm aimed at screening potential antidepressants. The results showed that WAY 100635 enhanced the antinociceptive effect and produced a large decrease in the antidepressant-like effect of tramadol. In contrast, SB 216641 did not significantly modify either the analgesic or the antidepressant-like effects of tramadol. These findings suggest that 5-HT1A receptors modulate the analgesic and the antidepressant-like effects of tramadol in differing ways. The results suggest the involvement of the 5-HT1A autoreceptors from the raphe nuclei and spinal 5-HT1A receptors in the antinociceptive effect. In contrast, the 5-HT1A receptors located in the forebrain may be responsible for the blockade of the antidepressant-like effect of tramadol. 5-HT1B receptors seem not to modify these effects in the models investigated.

Immunocytochemical evidence for the existence of substance P receptor (NK1) in serotonin neurons of rat and mouse dorsal raphe nucleus

In addition to its neurotransmitter/modulator role in pain perception, substance P (SP) is involved in a regulation of mood, as antagonists of its neurokinin-1 receptor (NK1r) have been found to have antidepressant-like effects in humans. In rodents, treatment with NK1r antagonists has been shown to increase the firing of dorsal raphe nucleus (DRN) serotonin (5-hydroxytryptamine, 5-HT) neurons and to induce a desensitization of their 5-HT1A autoreceptors, suggesting local interactions between the SP and 5-HT systems. To search for the presence of NK1r on 5-HT neurons of the DRN, we used light and electron microscopic immunocytochemistry, as well as confocal microscopy, after single- and double-labelling of NK1r and of the biosynthetic enzyme of 5-HT, tryptophan hydroxylase (TpOH). A significant number of 5-HT (TpOH-positive) cell bodies and dendrites endowed with NK1r were thus demonstrated in the caudal part of rat and mouse DRN. As visualized by electron microscopy after gold immunolabelling, NK1r was mostly cytoplasmic in 5-HT neurons, while predominating on the plasma membrane in the case of TpOH-negative dendrites. The proportion of NK1r observed on the plasma membrane of 5-HT neurons was, however, slightly higher in mouse than rat. Thus, in both rat and mouse DRN, a subpopulation of 5-HT neurons is endowed with NK1r receptors and may be directly involved in the antidepressant-like effects of NK1r antagonists. These 5-HT neurons represent a new element in the neuronal circuitry currently proposed to account for the role of SP in mood regulation.

Augmentation Strategies to Improve Treatment of Major Depression

The monoamine hypothesis has dominated almost forty years the psychopathology of depression. Yet this has not lead to antidepressants that are significantly more efficacious than the early tri- and tetracyclics from which they have evolved. Alternative hypotheses such as those involving adult neurogenesis or components of the hypothalamic-pituitaryadrenal- axis are either too premature or have not lead to drugs with improved antidepressant activity. Antidepressants may not be perfect, both in terms of efficacy and side effects, however their performance may be improved by making use of so-called augmentation strategies. Augmentation strategies that have potential to hasten and/or improve the therapeutic effect of antidepressants, in particular serotonin reuptake inhibitors, can have different forms. They can be aimed at reducing comorbid symptoms, such as anxiety, or they may be directed to processes that counteract the effect of serotonin reuptake inhibitors. Examples of the latter are those involving 5-HT1A and 5-HT1B autoreceptors, 5-HT 2C receptors and the availability of tryptophan. Another option could be exploring neuropeptide/serotonin interactions. The various augmentation strategies are reviewed in the context of literature data regarding neurobiology and pharmacotherapy of major depression.

Reduction in repeated ethanol-withdrawal-induced anxiety-like behavior by site-selective injections of 5-HT1A and 5-HT2C ligands

Anxiety-like behavior resulting from repeated withdrawals from chronic ethanol diets is counteracted by systemic administration of a 5-HT2C receptor antagonist or a 5-HT1A receptor partial agonist. This study investigated whether prior treatment with these agents into the amygdala, dorsal raphe nucleus, nucleus accumbens, or paraventricular nucleus during early withdrawals would ameliorate the social interaction deficits observed after a subsequent withdrawal. Sprague-Dawley rats were exposed to three cycles of 5 days of forced ethanol diet (4.5%, w/v), with 2 days of control diet after the first and second cycles. Drugs were administered into one of four brain sites 4 h after removal of ethanol on the first and 2nd cycles but not the third. The social interaction test was performed 5 h after removal of ethanol on the third cycle. Drugs tested included SB-243213, a 5-HT2C receptor inverse agonist; buspirone, a 5-HT1A receptor partial agonist; and Ro 60 1075, a 5-HT2C receptor agonist. Only SB-243213 (at 3 microg, but not at 1 and 0.3 microg) counteracted the social interaction deficits after injections into the amygdala, while buspirone (at 0.3 and 1 microg but not at 0.1 microg) reduced deficits only when given into the dorsal raphe nucleus. In contrast, the 5-HT2C receptor agonist, Ro 60 1075, accentuated the behavioral deficit after two weekly injections into the amygdala. These results are consistent with the involvement of 5-HT2C receptors in the amygdala and 5-HT1A autoreceptors in the dorsal raphe nucleus in repeated ethanol withdrawal-induced sensitization of anxiety-like behavior.

Effects of OPC-14523, a combined sigma and 5-HT1a ligand, on pre- and post-synaptic 5-HT1a receptors

OPC-14523 (OPC) is a novel compound with high affinity for sigma and 5-HT1A receptors that shows 'antidepressant-like' effects in animal models of depression. We have previously demonstrated that OPC produces an increase in 5-HT neurotransmission and a decreased response of 5-HT neurons to the acute administration of paroxetine in the DRN, an effect that appears to be mediated by OPC's 5-HT1A receptor affinity. The current study sets out to investigate more specifically the effects of OPC on 5-HT1A pre- and post-synaptic receptors, to assess whether it acts as an agonist or antagonist. Using an electrophysiological model of in vivo extracellular recordings in anaesthetized rats, the effects of OPC was assessed on pre-synaptic DRN 5-HT1A autoreceptors and post-synaptically on hippocampal 5-HT1A receptors of CA3 pyramidal neurons. OPC applied by microiontophoresis, produced a significant decrease in the firing activity of 5-HT neurons of the DRN and of quisqualate-activated CA3 pyramidal neurons of the dorsal hippocampus. The effects of OPC on 5-HT1A receptors were significantly reduced by the co-application of the 5-HT1A antagonist WAY-100635. In addition, the effects of OPC were not blocked by the injection of the sigma antagonists NE-100 or haloperidol. Therefore, OPC is acting as an agonist on both pre- and post-synaptic 5-HT1A receptors. The current findings combined with previous data on OPC suggest a pharmacological profile that warrants further investigation.

Basic Mechanisms of Augmentation of Antidepressant Effects with Thyroid Hormone

The thyroid hormone triiodothyronine (T3) has been used both to augment and accelerate the clinical effects of antidepressants, particularly the tricyclics. More recent work indicates that it may have similar actions with regard to the SSRIs. Two main mechanisms have been put forward to explain its antidepressant actions, (a) an action at the nuclear level involving stimulation of gene transcription, (b) an action at the cell membrane level involving potentiation of neurotransmission. In particular, there is considerable evidence for potentiation by T3 of the actions of the neurotransmitter 5-HT or serotonin. This evidence, which is mainly based on in vivo microdialysis studies, is reviewed, and evidence based on human and animal neuroendocrine studies considered. The effects of T3, alone and together with the SSRI fluoxetine, on mRNA levels for the 5-HT1A and 5-HT1B autoreceptors, which mediate serotonergic neurotransmission by feedback actions at the levels of cell firing(somatodendritic 5-HT1A autoreceptors) and neurotransmitter release (nerve terminal 5-HT1B autoreceptors) were also determined. Administration of a combination of fluoxetine and T3 induced reductions in the transcription of these autoreceptors, which may explain the clinical potentiating effects of this combination, and thus link the nuclear and neurotransmitter hypotheses of T3 action.

Microdialysis Approach to Study Serotonin Outflow in Mice Following Selective Serotonin Reuptake Inhibitors and Substance P (Neurokinin 1) Receptor Antagonist Administration: A Review

Classical antidepressant drugs such as Selective Serotonin Reuptake Inhibitors (SSRIs) display several disadvantages, e.g., the onset of action (2 to 3 weeks) to start clinical benefits is too long, and a significant proportion of patients do not respond to this monotherapy. Several strategies have been proposed to overcome these problems, notably the use of potentiating agents, which combined with SSRIs, augment or accelerate their established antidepressant activity. Recent clinical trials proposed that compounds with dual action on both central serotonin (5-HT) and noradrenaline (NA) systems would have a faster action than SSRIs alone. Preclinical electrophysiological and neurochemical studies demonstrated that the putative new class of antidepressants, substance P (neurokinin 1) NK1 receptor antagonists, enhance brain monoaminergic neurotransmissions by reducing the sensitivity of 5-HT1A autoreceptors in the Dorsal Raphe Nucleus, and possibly alpha2 autoreceptors in the Locus Coeruleus. However, in several clinical studies, a similar delay of therapeutic effects has been reported with NK1 receptor antagonists and SSRIs. Recently intracerebral in vivo microdialysis studies were performed to examine the effects of genetic or pharmacological blockade of Substance P (SP)/ NK1 neurotransmission on SSRIs-induced increases in extracellular 5-HT levels in awake, freely moving mice. New evidences suggest that the combination of a NK1 receptor antagonist with a SSRI should benefit to depressed patients. This review describes our current knowledge of the role of SP and its preferred NK1 receptors mainly in the modulation of brain serotonergic activity.

Blockade of 5-HT1A Receptors by (±)-Pindolol Potentiates Cortical 5-HT Outflow, but not Antidepressant-Like Activity of Paroxetine: Microdialysis and Behavioral Approaches in 5-HT1A Receptor Knockout Mice

Selective serotonin reuptake inhibitors like paroxetine (Prx) often requires 4-6 weeks to achieve clinical benefits in depressed patients. Pindolol shortens this delay and it has been suggested that this effect is mediated by somatodendritic 5-hydroxytryptamine (5-HT) 1A autoreceptors. However clinical data on the beneficial effects of pindolol are conflicting. To study the effects of (+/-)-pindolol-paroxetine administration, we used genetical and pharmacological approaches in 5-HT1A knockout mice (5-HT1A-/-). Two assays, in vivo intracerebral microdialysis in awake mice and the forced swimming test (FST), were used to assess the antidepressant-like effects of this drug combination. Basal levels of extracellular serotonin, 5-HT ([5-HT]ext) in the frontal cortex (FCX) and the dorsal raphe nucleus (DRN) did not differ between the two strains of mice, suggesting a lack of tonic control of 5-HT1A autoreceptors on nerve terminal 5-HT release. Prx (1 and 4 mg/kg) dose-dependently increased cortical [5-HT]ext in both genotypes, but the effects were greater in mutants. The selective 5-HT1A receptor antagonist, WAY-100635 (0.5 mg/kg), or (+/-)-pindolol (5 and 10 mg/kg) potentiated the effects of Prx (4 mg/kg) on cortical [5-HT]ext in 5-HT1A+/+, but not in 5-HT1A-/- mice. Similar responses were obtained following local intra-raphe perfusion by reverse microdialysis of either WAY-100635 or (+/-)-pindolol (100 microM each). In the FST, Prx administration dose-dependently decreased the immobility time in both strains of mice, but the response was much greater in 5HT1A-/- mice. In contrast, (+/-)-pindolol blocked Prx-induced decreases in the immobility time while WAY-100635 had no effect in both genotypes. These findings using 5-HT1A-/- mice confirm that (+/-)-pindolol behaves as an antagonist of 5-HT1A autoreceptor in mice, but its blockade of paroxetine-induced antidepressant-like effects in the FST may be due to its binding to other neurotransmitter receptors.

Glucocorticoid Receptor-Dependent Desensitization of 5-HT1A Autoreceptors by Sleep Deprivation: Studies in GR-i Transgenic Mice

Sleep deprivation for one night induces mood improvement in depressed patients, an action that probably involves the serotonergic (5-HT) system. In animals, sleep deprivation and pharmacologic treatment with antidepressants exert similar effects on 5-HT neurotransmission, notably functional desensitization of 5-HT1A autoreceptors located on 5-HT neurons in the dorsal raphe nucleus (DRN). However, in stressful conditions, corticosterone can also induce a desensitization of these autoreceptors. To investigate the mechanisms of this adaptation during sleep deprivation and the possible involvement of corticosterone, we studied the effects of an 18-hour sleep deprivation, by forced locomotion, on 5-HT1A receptor-mediated firing response of DRN 5-HT neurons in transgenic mice with impaired glucocorticoid-receptor expression (GR-i) and in wild-type animals. We also examined the effects of chronic treatment with the antidepressant drug fluoxetine in the same paradigm. In both wild-type and GR-i mice, the 18-hour sleep deprivation or fluoxetine treatment had no effect on the spontaneous firing of 5-HT neurons recorded under anesthesia. However, sleep deprivation decreased the potency of the 5-HT1A agonist 8-OH-DPAT to inhibit 5-HT neuronal firing in wild-type mice, whereas it had no effect in GR-i animals. Conversely, after chronic fluoxetine treatment, the induced reduction of this 5-HT1A autoreceptor-driven response was of larger amplitude in GR-i than in wild-type mice. These data suggest that glucocorticoid-receptor activation by corticosterone participates in the antidepressant-like adaptive changes in 5-HT1A autoreceptors in sleep-deprived mice. On the other hand, GR-i animals exhibited enhanced 5-HT1A autoreceptor desensitization induced by fluoxetine, in line with data in other animal models of depression.

Reduced density of functional 5-HT1A receptors in the brain, medulla and spinal cord of monoamine oxidase-A knockout mouse neonates

Abnormally high brain 5-HT levels in monoamine oxidase-A knockout (MAO-A KO) mouse neonates raise the question of whether the distribution and density of the 5-HT1A receptors (5-HT1AR) expressed in the brain by postnatal day P7 are affected and, if so, whether the 5-HT1A autoreceptors in the dorsal raphe are modified in the same way as the postsynaptic 5-HT1AR present in raphe target structures. [3H]8-OH-DPAT binding and quantitative autoradiography were performed to answer these questions. Binding specificity was first confirmed in adult wild-type mice and rat brain sections. 5-HT1AR binding was then analyzed in four MAO-A mutant vs. five wild-type neonatal brains, from olfactory bulb to cervical cord. Among 12 structures expressing postsynaptic 5-HT1AR in wild-type neonates, the highest densities involved the retrosplenial cortex, entorhinal cortex, and septum (52-46 fmol/mg tissue); low densities occurred in the hippocampus and spinal cord (24 fmol/mg tissue); in addition, the raphe autoreceptor density was only 20 fmol/mg tissue. In mutants, the distribution of postsynaptic 5-HT1AR was unchanged, but an overall decrease in density occurred (-32% to -63%); the raphe autoreceptors decreased in mutants by at least -79%. Data are discussed with reference to the ectopic 5-HT uptake and accumulation reported to occur during the first 10 postnatal days in wild-type and MAO-A KO mice. As previously suggested to explain the raphe autoreceptor loss in 2-month-old MAO-A KO mice, the overall 5-HT1AR down-regulation in mutant pups probably results from extracellular 5-HT excess in both raphe and target structures. The greater the 5-HT excess, the more the functional receptor density decreases.

Potentiation of the resetting effects of light on circadian rhythms of hamsters using serotonin and neuropeptide Y receptor antagonists

Circadian rhythms are entrained by light/dark cycles. In hamsters, the effects of light on circadian rhythms can be modulated by serotonergic input to the suprachiasmatic nucleus from the raphe nuclei and by neuropeptide Y containing afferents to the suprachiasmatic nucleus from the intergeniculate leaflet in the thalamus. In this study we measured effects of compounds acting on serotonergic 1A and neuropeptide Y Y5 receptors to determine if combined serotonergic-neuropeptide Y inhibition could synergistically potentiate effects of light on rhythms. We used mixed serotonergic agonist/antagonists BMY 7378 or NAN-190 as well as a neuropeptide Y Y5 antagonist CP-760,542. Both BMY 7378 and NAN-190 are thought to block serotonin release via acting as agonists at the 5-hydroxytryptamine 1A (5-HT1A) autoreceptors on cells in the raphe, and also block response of target cells by acting as antagonists at post-synaptic 5-HT1A receptors, for example, in the suprachiasmatic nuclei or the intergeniculate leaflet. Replicating prior work, we found that pretreatment with either drug alone increased the phase shift to light at circadian time 19. The combined effect of BMY 7378 and CP-760,542 given prior to light at circadian time 19 was to further potentiate the subsequent phase shift in wheel-running rhythms (the phase shift was 317% of controls; light alone: 1.35 h phase shift vs. BMY 7378, CP-760,542, and light: 4.27 h phase shift). Combined treatment with NAN-190 and CP-760,542 produced a light-induced phase shift 576% of controls (phase shift to light alone: 1.23 h vs. NAN-190, CP-760,542, and light: 7.1 h phase shift). These results suggest that the resetting effects of light on circadian rhythms can be greatly potentiated in hamsters by using pharmacological treatments that block both serotonergic and neuropeptide Y afferents to the suprachiasmatic nuclei.

5-HT1a receptor antagonists block perforant path-dentate LTP induced in novel, but not familiar, environments

Numerous studies suggest roles for monoamines in modulating long-term potentiation (LTP). Previously, we reported that both induction and maintenance of perforant path-dentate gyrus LTP is enhanced when induced while animals explore novel environments. Here we investigate the contribution of serotonin and 5-HT1a receptors to the novelty-mediated enhancement of LTP. In freely moving animals, systemic administration of the selective 5-HT1a antagonist WAY-100635 (WAY) attenuated LTP in a dose-dependent manner when LTP was induced while animals explored novel cages. In contrast, LTP was completely unaffected by WAY when induced in familiar environments. LTP was also blocked in anesthetized animals by direct application of WAY to the dentate gyrus, but not to the median raphe nucleus (MRN), suggesting the effect of systemic WAY is mediated by a block of dentate 5-HT1a receptors. Paradoxically, systemic administration of the 5-HT1a agonist 8-OH-DPAT also attenuated LTP. This attenuation was mimicked in anesthetized animals following application of 8-OH-DPAT to the MRN, but not the dentate gyrus. In addition, application of a 5-HT1a agonist to the dentate gyrus reduced somatic GABAergic inhibition. Because serotonergic projections from the MRN terminate on dentate inhibitory interneurons, these data suggest 5-HT1a receptors contribute to LTP induction via inhibition of GABAergic interneurons. Moreover, activation of raphe 5-HT1a autoreceptors, which inhibits serotonin release, attenuated LTP induction even in familiar environments. This suggests that serotonin normally contributes to dentate LTP induction in a variety of behavioral states. Together, these data suggest that serotonin and dentate 5-HT1a receptors play a permissive role in dentate LTP induction, particularly in novel conditions, and presumably, during the encoding of novel, hippocampus-relevant information.

Galanin Receptor Antagonists

The pathophysiology of mood disorders involves several genetic and social predisposing factors, as well as a dysregulated response to chronic stress. Accumulated evidence during the last two decades has implicated disturbances in brain serotonin and/or noradrenaline (norepinephrine) neurotransmission in the aetiology of depression. In fact, current pharmacological treatment for mood disorders is based on the use of drugs that act mainly by enhancing brain serotonin and noradrenaline neurotransmission by blockade of the active reuptake mechanism for these neurotransmitters. However, current antidepressant drugs have a delayed onset of therapeutic action, and a substantial number of patients do not respond adequately to them. In addition, these drugs have a number of adverse effects that limit patient compliance. In view of this, there is an intense search to identify novel (receptor) targets for antidepressant therapy. Recent studies have indicated that several neuropeptides and their receptors are potential candidates for the development of novel antidepressant treatment. In this context, galanin is of particular interest, since it is co-localised with serotonin in the dorsal raphe nucleus and with noradrenaline in the locus coeruleus, nuclei known to play a major role in affective disorders and in the action of antidepressant drugs. The actions of galanin are mediated by three receptor subtypes (GAL1, GAL2 and GAL3), which are coupled to different intracellular effector systems. Studies in rats have shown that galanin administered intracerebroventricularly is a potent inhibitor of mesencephalic serotonergic neurotransmission, as indicated by a long-lasting reduction in the release of serotonin in the hippocampus. This inhibitory effect is related to activation of the galanin receptors located on the dorsal raphe neurons. Moreover, intracerebroventricular galanin alters the gene expression of serotonin 5-HT1A autoreceptors in the dorsal raphe and also changes their functional activity. In addition, galanin produces a functional blockade of postsynaptic 5-HT1A receptor-mediated responses. Both pharmacological and genetic studies suggest a role for galanin in depression-like behaviour in rodent models. Transgenic mice overexpressing galanin under the control of the platelet-derived growth factor-beta promoter display increased immobility in the forced swim test. Intracerebroventricular administration of galanin in the rat increases depression-like behaviour, and this is fully blocked by the nonselective peptide galanin receptor antagonist M35. Importantly, M35 alone administered intracerebroventricularly produces an antidepressant-like effect. Recently, newly developed receptor-specific nonpeptidergic galanin GAL3 receptor antagonists (SNAP-37889 and SNAP-398299), which cross the blood-brain barrier after systemic administration, have shown antidepressant-like activity in several animal models. On the other hand, stimulation of the GAL2 receptor at the raphe level by local application of the GAL2 receptor agonist galanin (2-11) has been shown to increase serotonin levels in the hippocampus and dorsal raphe. These results indicate an important (mainly inhibitory) role of galanin as a regulator of brain serotonin and 5-HT1A receptor-mediated transmission, which may be of potential importance for understanding mood disorders and for the development of antidepressant drugs. Taken together, the present evidence suggests that antidepressant efficacy may be associated with compounds acting as antagonists at the GAL3 and/or possibly GAL1 receptors, and/or agonists at the GAL2 receptor.

Serotonin1A autoreceptor activation by S 15535 enhances circadian activity rhythms in hamsters: Evaluation of potential interactions with serotonin2A and serotonin2C receptors

Mammalian circadian activity rhythms are generated by pacemaker cells in the suprachiasmatic nucleus (SCN). As revealed by the actions of diverse agonists, serotonergic input from raphe nuclei generally inhibits photic signaling in the suprachiasmatic nucleus. In contrast, the serotonin (5HT)1A partial agonist, 4-(benzodioxan-5-yl)1-(indan2-yl)piperazine (S 15535), was found to enhance the phase-shifting influence of light on hamster circadian rhythms [Gannon, Neuroscience 119 (2003) 567]. Herein, we extend this observation in showing that S 15535 (5.0mg/kg, i.p.) markedly (275%) enhanced the light-induced phase shift in circadian activity rhythms: further, this action was dose-dependently abolished by the highly-selective 5HT1A receptor antagonist, WAY 100,635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]N-2-pyridinyl-cyclohexane-carboxamide maleate) (0.1–0.5mg/kg, i.p.). WAY 100,635, which was inactive alone, shares the antagonist actions of S 15535 at postsynaptic 5HT1A sites, yet blocks its effects at their presynaptic counterparts. Thus, 5HT1A autoreceptor activation must be involved in this effect of S 15535 which contrasts with the opposite, inhibitory influence upon phase shifts of the “full” agonist, 8-OH-DPAT, which acts by stimulation of postsynaptic 5HT1A receptors [Rea et al., J Neurosci 14 (1994) 3635]. Despite the occurrence of 5HT2A and 5HT2C receptors in the (rat) suprachiasmatic nucleus, their influence on circadian rhythms is unknown since actions of selective ligands have never been evaluated. This issue was investigated with the most selective agents currently available. However, the 5HT2A agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.25 and 0.5mg/kg), and the 5HT2C agonist, αS-6-chloro-5-fluoro-a-methyl-1H-indole-1-ethanamine fumarate (Ro-60-0175) (1.0 and 5.0mg/kg), failed to affect light-induced phase shifts in hamsters. Moreover, even over broad dose-ranges, the 5HT2A antagonist, (+)-(2,3-dimethoxy-phenyl)-{1-[2-(4-fluoro-phenyl)-ethyl]-piperidin-4-yl}methanol (MDL 100,907) (0.1–1.0mg/kg), and the 5HT2C antagonist, 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy)pyridin-3-yl carbamoyl]indoline (SB 242,084) (1.0–10.0mg/kg), were likewise inactive. In view of evidence that 5HT2A and 5HT2C sites functionally interact with 5HT1A receptors, we also examined the influence of these agents upon the actions of S 15535, but no significant alteration was seen in its enhancement of rhythms. In conclusion, S 15535 elicits a striking enhancement of light-induced phase shifts in circadian rhythms by specifically recruiting 5HT1A autoreceptors, which leads to suppression of serotonergic input to the suprachiasmatic nucleus. Surprisingly, no evidence for a role of 5HT2A or 5HT2C sites was found, though comparable functional studies remain to be undertaken in rats. Indeed, the present work underlines the importance of comparative studies of circadian rhythms in various species, as well as the need for further study of potential interactions among 5HT receptor subtypes in their control.

Short‐term constant light potentiation of large‐magnitude circadian phase shifts induced by 8‐OH‐DPAT: effects on serotonin receptors and gene expression in the hamster suprachiasmatic nucleus

Nonphotic phase-shifting of mammalian circadian rhythms is thought to be mediated in part by serotonin (5-HT) acting in the suprachiasmatic nucleus (SCN) circadian clock. Previously we showed that brief (1-3 days) exposure to constant light (LL) greatly potentiates nonphotic phase-shifting induced by the 5-HT agonist, (+/-)2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT). Here we investigated potential mechanisms for this action of LL, including 5-HT receptor upregulation and SCN clock gene and neuropeptide gene expression. Autoradiographic analysis of ritanserin inhibition of [3H]8-OH-DPAT binding indicated that LL (approximately 2 days) did not affect 5-HT7 receptor binding in the SCN or dorsal raphe. Measurement of 5-HT1A autoreceptors in the median raphe and 5-HT1B receptors in the SCN also showed no effect of LL. In experiment 2, hamsters held under a 14-h light : 10-h dark photocycle (LD) or exposed to LL for approximately 2 days received an intraperitoneal injection of 8-OH-DPAT or vehicle at zeitgeber time (ZT) 6 or 0 and were killed after 2 h of dark exposure. 8-OH-DPAT suppressed SCN Per1 and Per2 mRNAs at both ZTs, as assessed by in situ hybridization. Per1 mRNA was also suppressed by LL alone. In addition, in situ hybridization of arginine vasopressin (AVP) mRNA and vasoactive intestinal polypeptide mRNA showed that LL significantly suppressed the former but not the latter. The LL-induced suppression of SCN Per1 mRNA and AVP mRNA may be involved in LL-induced potentiation of pacemaker resetting, especially as these data provide additional evidence that LL suppresses circadian pacemaker amplitude, thus rendering the clock more susceptible to phase-shifting stimuli.

Compréhension du mécanisme d’action des antidépresseurs anciens ou nouveaux : apport des modèles de souris génétiquement modifiées en pharmacologie in vivo

The main hypothesis regarding the mechanism of action of antidepressant drugs is monoaminergic and mainly involves two neurotransmitters, serotonin and noradrenaline. Despite the well-recognized therapeutic efficacy of selective serotonin reuptake inhibitors (SSRIs), some disadvantages still occur. For example, they often require 4–6 weeks to achieve clinical benefits in depressed patients. In the past, some molecules that could shorten this long delay of action have been identified. The role of presynaptic autoreceptors – the activation of which leads to an inhibitory feedback control on neurotransmitter synthesis and release – has been extensively studied for antidepressant effects. In our laboratory, we studied the combined effects of an SSRI and a serotonin autoreceptor antagonist of the 5-HT1B subtype using intracerebral in vivo microdialysis in awake, freely moving mice. Important information on SSRIs has been obtained by applying this technique to genetically modified animals, such as constitutive knockout (KO) mice lacking 5-HT1B receptors (5-HT1B KO) generated by homologous recombination: we compared the effects of a combined treatment on extracellular/intrasynaptic levels of serotonin in various nerve terminals area in wild-type control and KO mice. Thus, we found that indirect activation of 5-HT1B autoreceptors limits the effects of SSRIs on dialysate 5-HT levels at serotonergic nerve terminals such as the ventral hippocampus. The study of substance P (neurokinin 1 receptor [R-NK1]) offers another example of the use of KO mice in the development of a new class of antidepressant drugs. NK1 receptor antagonists may display anxiolytic/antidepressant-like properties. The lack of selective compounds for each tachykinin receptor subtype (R-NK1, R-NK2 or R-NK3) and differences in their affinity between animal species have made R-NK1KO mice a very useful experimental tool. In collaborative work we found that genetic (R-NK1KO mice) or pharmacological (GR205171) blockade of R-NK1 is associated with several changes: the increase in cortical 5-HT outflow caused by systemic injection of paroxetine was 4- to 6-fold higher in freely moving R-NK1KO mice than in wild-type controls. The constitutive lack of NK1 receptors is associated with a functional desensitization of somatodendritic 5-HT1A autoreceptors, resembling that induced by chronic treatment with SSRI antidepressants. These results highlight the link between a neurotransmitter (serotonin) and a neuropeptide (substance P). This genetic strategy allowed us to point out that multiple targets participate to the effects of classical antidepressant drugs within the brain. We hope that, soon, some mice lines (constitutive or tissue specific, conditional rescue mice having alterations of sleep/wakefulness and/or food intake, altered central serotonin and/or noradrenaline neurotransmission, deficit in neurotrophic factors, but increases in intrasynaptic concentrations of substance P) could be a relevant model of the physiopathology of depressive disorders, and could help us understand the appearance of some symptoms. These recent findings suggest that instead of being rejected, the monoaminergic hypothesis of depression should be improved, corrected and completed by studying the role of other neurotransmitter, neuromodulatory compounds (substance P, BDNF [brain-derived neurotrophic factor]). By doing so, it thus could be possible to improve antidepressant drug treatment, i.e. shorten their long delay of action and/or to decrease treatment resistance or improve its tolerance.

Chronic administration of venlafaxine fails to attenuate 5-HT1A receptor function at the level of receptor-G protein interaction

In this study venlafaxine was administered to rats at a low, moderate or high dose; for comparison, the selective serotonin reuptake inhibitor (SSRI) sertraline and the tricyclic antidepressant (TCA) amitriptyline were also included. We evaluated, using quantitative autoradiography, the effect of these antidepressant treatments on [35S]GTPgammaS binding stimulated by the 5-HT1A receptor agonist 8-OH-DPAT, a measure of the capacity of 5-HT1A receptors to activate G proteins. Chronic administration of amitriptyline resulted in a marked increase in 5-HT1A receptor-stimulated [35S]GTPgammaS binding in the hippocampus which was accompanied by an increase in 5-HT1A receptor number. 5-HT1A receptor-stimulated [35S]GTPgammaS binding in the hippocampus was also increased by chronic treatment with the highest dose of venlafaxine; 5-HT1A receptor number, however, was not significantly altered. In serotonergic cell body areas (i.e. dorsal and median raphe nuclei), 5-HT1A receptor-stimulated [35S]GTPgammaS binding was not altered by chronic administration of amitriptyline, sertraline or venlafaxine. Chronic TCA treatment does not desensitize somatodendritic 5-HT1A autoreceptor function. However, the lack of effect of chronic sertraline treatment on 5-HT1A receptor-stimulated [35S]GTPgammaS binding is in contrast to what has been observed previously following chronic administration of the SSRI fluoxetine, and suggests that different SSRIs may regulate somatodendritic 5-HT1A autoreceptor function differently depending on their pharmacology. Our data also suggest that the desensitization of somatodendritic 5-HT1A autoreceptors observed in electrophysiological studies following chronic venlafaxine administration is not at the level of receptor-G protein interaction. The hypothermic response in vivo to acute injection of 8-OH-DPAT was significantly attenuated following chronic treatment with venlafaxine or sertraline, but not amitriptyline.

Contrasting Contribution of 5-Hydroxytryptamine 1A Receptor Activation to Neurochemical Profile of Novel Antipsychotics: Frontocortical Dopamine and Hippocampal Serotonin Release in Rat Brain

Several novel antipsychotics, such as aripiprazole, bifeprunox, SSR181507 [(3-exo)-8-benzoyl-N-(((2S)7-chloro-2,3-dihydro-1,4-benzodioxin-1-yl)methyl)-8-azabicyclo(3.2.1)octane-3-methanamine], and SLV313 [1-(2,3-dihydro-benzo[1,4]dioxin-5-yl)-4-[5-(4-fluorophenyl)-pyridin-3-ylmethyl]-piperazine], activate serotonin 5-hydroxytryptamine (5-HT)1A receptors. Such activity is associated with enhanced treatment of negative symptoms and cognitive deficits, which may be mediated by modulation of cerebral dopamine and serotonin levels. We employed microdialysis coupled to high pressure liquid chromatography with electrochemical detection to examine 5-HT1A receptor activation in the modulation of extracellular dopamine in medial prefrontal cortex and serotonin in hippocampus of freely moving rats. The above compounds were compared with drugs that have less interaction with 5-HT1A receptors (clozapine, nemonapride, ziprasidone, olanzapine, risperidone, and haloperidol). Hippocampal 5-HT was decreased by bifeprunox, SSR181507, SLV313, sarizotan, and nemonapride, effects similar to those seen with the 5-HT1A agonist, (+)-8-hydroxy-2-(di-n-propylamino)tetralin [(+)8-OH-DPAT], consistent with activation of 5-HT1A autoreceptors. These decreases were reversed by the selective 5-HT1A antagonist, WAY100635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide]. In contrast, haloperidol, risperidone, clozapine, olanzapine, ziprasidone, and aripiprazole did not significantly modify hippocampal serotonin levels. In medial prefrontal cortex, dopamine levels were increased by SSR181507, SLV313, sarizotan, and (+)8-OH-DPAT. These effects were reversed by WAY100635, indicating mediation by 5-HT1A receptors. In contrast, the increases in dopamine levels induced by clozapine, risperidone, olanzapine, and ziprasidone were not blocked by WAY100635, consistent with predominant influence of other mechanisms in the actions of these drugs. Haloperidol, nemonapride, and the D2 partial agonists, aripiprazole and bifeprunox, did not significantly alter dopamine release. Taken together, these data demonstrate the diverse contribution of 5-HT1A receptor activation to the profile of antipsychotics and suggest that novel drugs selectively targeting D2 and 5-HT1A receptors may present distinctive therapeutic properties.

Effects of serotonin 5‐HT1A agonist in advanced Parkinson's disease

Intermittent stimulation of striatal dopaminergic receptors seems to contribute to motor dysfunction in advanced Parkinson's disease (PD). With severe dopaminergic denervation, exogenous levodopa is largely decarboxylated to dopamine in serotonergic terminals. If 5-HT1A autoreceptors regulate dopamine as well as serotonin release, in parkinsonian patients inhibition of striatal serotonergic neuron firing might help maintain more physiological intrasynaptic dopamine concentrations and thus ameliorate motor fluctuations and dyskinesias. To evaluate this hypothesis, effects of a selective 5-HT1A agonist, sarizotan, given orally at 2 and 5 mg twice daily to 18 relatively advanced parkinsonian patients, were compared with baseline placebo function during a 3-week, double-blind, placebo-controlled, proof-of-concept study. Sarizotan alone or with intravenous levodopa had no effect on parkinsonian severity. But at safe and tolerable doses, sarizotan coadministration reduced levodopa-induced dyskinesias and prolonged its antiparkinsonian response (P < or = 0.05). Under the conditions of this study, our findings suggest that 5-HT1A receptor stimulation in levodopa-treated parkinsonian patients can modulate striatal dopaminergic function and that 5-HT1A agonists may be useful as levodopa adjuvants in the treatment of PD.