Differential regulation of somatodendritic serotonin 5-HT1A receptors by 2-week treatments with the selective agonists alnespirone (S-20499) and 8-hydroxy-2-(Di-n-propylamino)tetralin: microdialysis and autoradiographic studies in rat brain

Vortioxetine Subchronically Activates Serotonergic Transmission via Desensitization of Serotonin 5-HT1A Receptor with 5-HT3 Receptor Inhibition in Rats

Vortioxetine is a novel, multimodal antidepressant with unique targets, including the inhibition of the serotonin transporter (SET), of serotonin 5-HT₃ (5-HT3R), and of 5-HT₇ (5-HT7R) receptors and partial agonism to serotonin 5-HT_1A (5-HT1AR) receptors in humans. Vortioxetine has a lower affinity to 5-HT1AR and 5-HT7R in rats compared with humans, but several behavior studies have demonstrated its powerful antidepressant-like actions. In spite of these efforts, detailed effects of the subchronic administration of vortioxetine on serotonergic transmission remain to be clarified. This study examined the mechanisms underlying the clinical effects of vortioxetine by measuring the releases of 5-HT and GABA in the medial prefrontal cortex (mPFC) of freely moving rats compared with the selective SET inhibitor, escitalopram. Inhibition of 5-HT3R in the mPFC enhanced regional 5-HT release via GABAergic disinhibition. Activation of somatodendritic 5-HT1AR in the dorsal raphe nucleus (DRN) and presynaptic 5-HT1AR in the mPFC inhibited 5-HT release in the mPFC. Escitalopram subchronically activated mesocortical serotonergic transmission via desensitization of 5-HT1AR in the mPFC and DRN and of 5-HT3R in the mPFC; however, vortioxetine also subchronically activated mesocortical serotonergic transmission via desensitization of 5-HT1AR in the mPFC and DRN but not of 5-HT3R in the mPFC. These demonstrations, the desensitization of 5-HT1AR with the inhibition of 5-HT3R (without 5-HT3R desensitization), at least partially, contribute to the multimodal antidepressant action of vortioxetine in rats.

Cannabidiol induces rapid-acting antidepressant-like effects and enhances cortical 5-HT/glutamate neurotransmission: role of 5-HT1A receptors

Cannabidiol (CBD), the main non-psychotomimetic component of marihuana, exhibits anxiolytic-like properties in many behavioural tests, although its potential for treating major depression has been poorly explored. Moreover, the mechanism of action of CBD remains unclear. Herein, we have evaluated the effects of CBD following acute and chronic administration in the olfactory bulbectomy mouse model of depression (OBX), and investigated the underlying mechanism. For this purpose, we conducted behavioural (open field and sucrose preference tests) and neurochemical (microdialysis and autoradiography of 5-HT1A receptor functionality) studies following treatment with CBD. We also assayed the pharmacological antagonism of the effects of CBD to dissect out the mechanism of action. Our results demonstrate that CBD exerts fast and maintained antidepressant-like effects as evidenced by the reversal of the OBX-induced hyperactivity and anhedonia. In vivo microdialysis revealed that the administration of CBD significantly enhanced serotonin and glutamate levels in vmPFCx in a different manner depending on the emotional state and the duration of the treatment. The potentiating effect upon neurotransmitters levels occurring immediately after the first injection of CBD might underlie the fast antidepressant-like actions in OBX mice. Both antidepressant-like effect and enhanced cortical 5-HT/glutamate neurotransmission induced by CBD were prevented by 5-HT1A receptor blockade. Moreover, adaptive changes in pre- and post-synaptic 5-HT1A receptor functionality were also found after chronic CBD. In conclusion, our findings indicate that CBD could represent a novel fast antidepressant drug, via enhancing both serotonergic and glutamate cortical signalling through a 5-HT1A receptor-dependent mechanism.

5-HT(1A) [corrected] receptors in mood and anxiety: recent insights into autoreceptor versus heteroreceptor function

RATIONALE

Serotonin (5-HT) neurotransmission is intimately linked to anxiety and depression and a diverse body of evidence supports the involvement of the main inhibitory serotonergic receptor, the serotonin-1A (5-HT(1A)) subtype, in both disorders.

OBJECTIVES

In this review, we examine the function of 5-HT(1A) receptor subpopulations and re-interpret our understanding of their role in mental illness in light of new data, separating both spatial (autoreceptor versus heteroreceptor) and the temporal (developmental versus adult) roles of the endogenous 5-HT(1A) receptors, emphasizing their distinct actions in mediating anxiety and depression-like behaviors.

RESULTS

It is difficult to unambiguously distinguish the effects of different populations of the 5-HT(1A) receptors with traditional genetic animal models and pharmacological approaches. However, with the advent of novel genetic systems and subpopulation-selective pharmacological agents, direct evidence for the distinct roles of these populations in governing emotion-related behavior is emerging.

CONCLUSIONS

There is strong and growing evidence for a functional dissociation between auto- and heteroreceptor populations in mediating anxiety and depressive-like behaviors, respectively. Furthermore, while it is well established that 5-HT(1A) receptors act developmentally to establish normal anxiety-like behaviors, the developmental role of 5-HT(1A) heteroreceptors is less clear, and the specific mechanisms underlying the developmental role of each subpopulation are likely to be key elements determining mood control in adult subjects.

Effects of early serotonin programming on behavior and central monoamine concentrations in an avian model

Serotonin (5-HT) acts as a neurogenic compound in the developing brain; however serotonin altering drugs such as SSRIs are often prescribed to pregnant and lactating mothers. Early agonism of 5-HT receptors could alter the development of serotonergic circuitry, altering neurotransmission and behaviors mediated by 5-HT signaling, including memory, fear and aggression. This study was designed to investigate the effects of early serotonin agonism on later behaviors. An extremely aggressive White leghorn strain (15I5) was used in the study. The chicks were injected with 5-MT (a serotonin agonist) at 2.5mg/kg (low dose), 10mg/kg (high dose) or saline (control) on the day of hatch and a second dose 24h later (n=9/sex/trt). Chicks' fear response and memory were tested at 2 weeks of age. In the fear test, chicks were subjected to a social isolation test for 20min, time to first vocalization and numbers of vocalizations were recorded. In the memory test, chicks were placed in a running wheel and presented with an imprinted object (white box with a red light) and a similar shaped novel object (blue box with a white light), respectively. The distance traveled in the wheel toward each object was measured. At 10 weeks of age birds were tested for aggression and concentrations of catecholamines were determined from the raphe nucleus and hypothalamus by HPLC (n=12). Expression of 5-HT1A and 5-HT1B receptor genes were measured by RT-PCR. Both high and low dose chicks tended to have shorter latency to first vocalization and a greater number of vocalizations compared with control chicks. Memory test showed that chicks from all groups traveled a similar distance toward a familiar object. However, control chicks walked the least toward a novel object, low dose chicks tended to walk further, and high dose chicks walked significantly further for a novel object. In aggression tests, both high and low dose males exhibited greater frequency of aggressive behaviors compared to controls, while no difference in aggression was evident in the females. Norepinephrine concentrations were also reduced in the low dose birds in the hypothalamus and in the raphe nucleus. Serotonin concentrations tended to be lower only in the both hypothalamus and raphe nucleus of the low dose birds. 5-HT1A expression was greatest in the hypothalamus and raphe nucleus of low dose birds. The agonism of the serotonin system during neural development of birds genetically predisposed to aggression alters both the dopaminergic and serotonergic systems further increasing their aggressiveness.

Rethinking 5-HT1A receptors: emerging modes of inhibitory feedback of relevance to emotion-related behavior

The complexities of the involvement of the serotonin transmitter system in numerous biological processes and psychiatric disorders is, to a substantial degree, attributable to the large number of serotonin receptor families and subtypes that have been identified and characterized for over four decades. Of these, the 5-HT(1A) receptor subtype, which was the first to be cloned and characterized, has received considerable attention based on its purported role in the etiology and treatment of mood and anxiety disorders. 5-HT(1A) receptors function both at presynaptic (autoreceptor) and postsynaptic (heteroreceptor) sites. Recent research has implicated distinct roles for these two populations of receptors in mediating emotion-related behavior. New concepts as to how 5-HT(1A) receptors function to control serotonergic tone throughout life were highlights of the proceedings of the 2012 Serotonin Club Meeting in Montpellier, France. Here, we review recent findings and current perspectives on functional aspects of 5-HT(1A) auto- and heteroreceptors with particular regard to their involvement in altered anxiety and mood states.

Transcriptional regulation of the 5-HT1A receptor: implications for mental illness

The serotonin-1A (5-HT(1A)) receptor is an abundant post-synaptic 5-HT receptor (heteroreceptor) implicated in regulation of mood, emotion and stress responses and is the major somatodendritic autoreceptor that negatively regulates 5-HT neuronal activity. Based on animal models, an integrated model for opposing roles of pre- and post-synaptic 5-HT(1A) receptors in anxiety and depression phenotypes and response to antidepressants is proposed. Understanding differential transcriptional regulation of pre- versus post-synaptic 5-HT(1A) receptors could provide better tools for their selective regulation. This review examines the transcription factors that regulate brain region-specific basal and stress-induced expression of the 5-HT(1A) receptor gene (Htr1a). A functional polymorphism, rs6295 in the Htr1a promoter region, blocks the function of specific repressors Hes1, Hes5 and Deaf1, resulting in increased 5-HT(1A) autoreceptor expression in animal models and humans. Its association with altered 5-HT(1A) expression, depression, anxiety and antidepressant response are related to genotype frequency in different populations, sample homogeneity, disease outcome measures and severity. Preliminary evidence from gene × environment studies suggests the potential for synergistic interaction of stress-mediated repression of 5-HT(1A) heteroreceptors, and rs6295-induced upregulation of 5-HT(1A) autoreceptors. Targeted therapeutics to inhibit 5-HT(1A) autoreceptor expression and induce 5-HT(1A) heteroreceptor expression may ameliorate treatment of anxiety and major depression.

Transcriptional dysregulation of 5-HT1A autoreceptors in mental illness

The serotonin-1A (5-HT1A) receptor is among the most abundant and widely distributed 5-HT receptors in the brain, but is also expressed on serotonin neurons as an autoreceptor where it plays a critical role in regulating the activity of the entire serotonin system. Over-expression of the 5-HT1A autoreceptor has been implicated in reducing serotonergic neurotransmission, and is associated with major depression and suicide. Extensive characterization of the transcriptional regulation of the 5-HT1A gene (HTR1A) using cell culture systems has revealed a GC-rich "housekeeping" promoter that non-selectively drives its expression; this is flanked by a series of upstream repressor elements for REST, Freud-1/CC2D1A and Freud-2/CC2D1B factors that not only restrict its expression to neurons, but may also regulate the level of expression of 5-HT1A receptors in various subsets of neurons, including serotonergic neurons. A separate set of allele-specific factors, including Deaf1, Hes1 and Hes5 repress at the HTR1A C(-1019)G (rs6295) polymorphism in serotonergic neurons in culture, as well as in vivo. Pet1, an obligatory enhancer for serotonergic differentiation, has been identified as a potent activator of 5-HT1A autoreceptor expression. Taken together, these results highlight an integrated regulation of 5-HT1A autoreceptors that differs in several aspects from regulation of post-synaptic 5-HT1A receptors, and could be selectively targeted to enhance serotonergic neurotransmission.

F15063, a potential antipsychotic with dopamine D2/D3 receptor antagonist, 5-HT1A receptor agonist and dopamine D4 receptor partial agonist properties: influence on neuronal firing and neurotransmitter release

F15063 (N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)-ethyl]-(3-cyclopenten-1-yl-benzyl)-amine) is a potential antipsychotic with dopamine D2/D3 receptor antagonist, 5-HT1A receptor agonist and dopamine D4 receptor partial agonist properties. Herein, we compared its effects on rat ventral tegmental area dopamine and dorsal raphe serotonin electrical activity with those of the dopamine D2 receptor partial agonist/5-HT1A receptor agonist, SSR181507. Further, we investigated the modulation of extracellular dopamine and noradrenaline in the medial prefrontal cortex and serotonin in the hippocampus of freely moving rats by F15063 using in vivo microdialysis. In the ventral tegmental area, F15063 (200-700 microg/kg, i.v.) did not alter the electrical activity of dopamine neurons whereas SSR181507 (250-1000 microg/kg, i.v.) partially inhibited it, consistent with dopamine D2 receptor partial agonism. Both compounds reduced the inhibition of firing rate induced by the full agonist apomorphine. In the dorsal raphe, both ligands suppressed firing activity, consistent with agonism at 5-HT1A autoreceptors, although SSR181507 (25-75 microg/kg, i.v.) was more potent than F15063 (100-300 microg/kg, i.v.). F15063 (0.63-40 mg/kg, i.p.) dose-dependently increased dopamine levels in the prefrontal cortex and decreased hippocampal 5-HT. These effects were reversed by the selective 5-HT1A receptor antagonist WAY100635 (0.16 mg/kg, s.c.), indicating that they were mediated by 5-HT1A receptors (at post- and pre-synaptic levels, respectively). In the medial prefrontal cortex, noradrenaline levels were moderately but significantly increased by F15063 at 2.5 mg/kg. In conclusion, whereas SSR181507 exhibits (partial) agonism at dopamine D2 and 5-HT1A receptors, F15063 blocks dopamine D2-like receptors whilst activating 5-HT1A receptors. Such a profile distinguishes F15063 from SSR181507 and currently available antipsychotic drugs.

Investigating the motivational mechanism of altered saline consumption following 5-HT(1A) manipulation

The precise role played by serotonin (5-HT) in taste--an issue of great interest given the involvement of serotonin in human sensory and eating disorders--is a matter of considerable debate, perhaps because of the variety of methodologies that have been brought to bear by different researchers. Here, we use multiple methods to reveal the motivational mechanism whereby 5-HT(1A) receptor activation modulates drinking behavior. Subcutaneous injections of the selective 5-HT(1A) agonist 8-hydroxy-2-di-n-propylamino-tetralin (8-OH-DPAT), a drug that reduces 5-HT release by acting on presynaptic autoreceptors, dose-dependently increased consumption of 0.45 M NaCl in a one-bottle test. In a two-bottle test, however, 8-OH-DPAT-treated animals (30 microg/kg/ml) demonstrated decreased NaCl preference--although our detection of this effect was obscured by adaptation to the drug across days. Rats' performance in a brief access test confirmed that 8-OH-DPAT decreased preference for saline by both increasing water consumption and decreasing NaCl consumption. Finally, taste reactivity tests demonstrated that the latter result does not reflect decreased NaCl palatability. Overall, the results suggest that 8-OH-DPAT-induced 5-HT hypofunction increases thirst without substantially affecting the palatability of NaCl.

Unchanged density of 5-HT(1A) autoreceptors on the plasma membrane of nucleus raphe dorsalis neurons in rats chronically treated with fluoxetine

5-HT(1A) autoreceptors regulate the firing of 5-HT neurons and their release of 5-HT. In previous immuno-electron microscopic studies, we have demonstrated an internalization of 5-HT(1A) autoreceptors in the nucleus raphe dorsalis (NRD) of rats, after the acute administration of a single dose of the specific agonist 8-hydroxy-2-(di-n-propylamine)tetralin (8-OH-DPAT) or of the selective 5-HT reuptake inhibitor, fluoxetine. Twenty-four hours after either treatment, the receptors were back in normal density on the plasma membrane of NRD neurons. Here, we examined the subcellular localization of these receptors and the in vivo binding of the 5-HT(1A) radioligand 4,2-(methoxyphenyl)-1-[2-(N-2-pyridinyl)-p-fluorobenzamido]ethylpiperazine labeled with [(18)F]fluorine ([(18)F]MPPF) after chronic fluoxetine treatment (10 mg/kg daily for 3 weeks, by minipump). Unexpectedly, after such a treatment, there were no more differences between treated and control rats in either the density of plasma membrane labeling of NRD dendrites, or in the in vivo binding of [(18)F]MPPF, as measured with beta-microprobes. This was in keeping with earlier reports of an unchanged density of 5-HT(1A) receptor binding sites after chronic fluoxetine treatment, but quite unexpected from the strong electrophysiological and biochemical evidence for a desensitization of 5-HT(1A) autoreceptors under such conditions. Indeed, when the fluoxetine-treated rats were challenged with a single dose of 8-OH-DPAT, there was no internalization of the 5-HT(1A) autoreceptors, at variance with the controls. Interestingly, several laboratories have reported an uncoupling of 5-HT(1A) autoreceptors from their G protein in the NRD of rats chronically treated with fluoxetine. Therefore, the best explanation for our results is that, after repeated internalization and retargeting, functional 5-HT(1A) autoreceptors are replaced by receptors uncoupled from their G protein on the plasma membrane of NRD 5-HT neurons. Thus, the regulatory function of these autoreceptors may depend on a dynamic balance among their production, activation, internalization and recycling to the plasma membrane in inactivated (desensitized) form.

Acute and chronic tryptophan depletion differentially regulate central 5-HT1A and 5-HT 2A receptor binding in the rat

RATIONALE

Tryptophan depletion is used to reduce central serotonergic function and to investigate its role in psychiatric illness. Despite widespread clinical use, its effects on serotonin (5-HT) receptors have not been well characterized.

OBJECTIVE

The aim of this study was to examine the effect of acute (ATD) and chronic tryptophan depletion (CTD) on free-plasma tryptophan (TRP), central TRP and 5-HT and brain 5-HT(1A) and 5-HT(2A) receptor binding in the rat.

METHODS

TRP and 5-HT were measured by high-performance liquid chromatography and receptor levels determined by homogenate radioligand binding and in-vitro receptor autoradiography.

RESULTS

Free-plasma TRP, central TRP and central 5-HT levels were significantly and similarly reduced by ATD and 1- and 3-week CTD compared to controls. ATD significantly reduced 5-HT(1A) binding in the dorsal raphe (14%) but did not significantly alter postsynaptic 5-HT(1A) binding (frontal cortex, remaining cortex and hippocampus) or 5-HT(2A) binding (cortex and striatum). One-week CTD did not significantly alter cortical 5-HT(2A) binding or postsynaptic 5-HT(1A) binding. Furthermore, 3-week CTD did not significantly alter 5-HT(1A) binding but significantly increased cortical 5-HT(2A) binding without affecting striatal or hippocampal levels. In the CTD 1 and 3-week groups, rat body weight was significantly decreased as compared to controls. However, weight loss was not a confounding factor for decreased cortical 5-HT(2A)-receptor binding.

CONCLUSION

ATD-induced reduction in somatodendritic 5-HT(1A) autoreceptor binding may represent an intrinsic 'homeostatic response' reducing serotonergic feedback in dorsal raphe projection areas. In contrast, the increase in 5-HT(2A) receptor after CTD may be a compensatory response to a long-term reduction in 5-HT.

Rapid desensitization of somatodendritic 5-HT1A receptors by chronic administration of the high-efficacy 5-HT1A agonist, F13714: a microdialysis study in the rat

BACKGROUND AND PURPOSE

Desensitization of somatodendritic 5-HT(1A) receptors is involved in the mechanism of action of several antidepressants, but the rapidity of this effect and the amount of agonist stimulation needed are unclear. We evaluated the capacity of the high-efficacy 5-HT(1A) agonist, F13714 (3-chloro-4-fluorophenyl-(4-fluoro-4-{[(5-methyl-6-methylamino-pyridin-2-ylmethyl)-amino]-methyl}-piperidin-1-yl-methanone) and of the partial agonist, flesinoxan, to desensitize somatodendritic 5-HT(1A) receptors involved in the control of 5-HT release.

EXPERIMENTAL APPROACH

Intracerebral microdialysis in the hippocampus of freely moving rats was used to examine the acute and chronic effects of the two compounds (administered by osmotic pumps for 3, 7 or 14 days) on extracellular 5-HT levels, measured by HPLC with electrochemical detection.

KEY RESULTS

When given acutely, F13714, flesinoxan and the low-efficacy 5-HT(1A) agonist, buspirone, dose-dependently decreased extracellular 5-HT concentrations (ED(50) values: 0.04, 0.77 and 5.6 mg kg(-1), respectively). The selective 5-HT(1A) antagonist WAY100635 inhibited the effects of the three compounds. F13714 (2.5 mg kg(-1) per day for 3, 7 or 14 days and 0.63 mg kg(-1) for 7 days) significantly attenuated the inhibition of 5-HT release induced by buspirone (10 mg kg(-1)). In contrast, flesinoxan (10 mg kg(-1) per day) failed to alter the response to buspirone at any of the treatment durations.

CONCLUSIONS AND IMPLICATIONS

Rat somatodendritic 5-HT(1A) receptors controlling hippocampal 5-HT release were rapidly desensitized by chronic activation with a high-efficacy 5-HT(1A) agonist, but not by chronic activation with a partial agonist. Thus, rapid 5-HT(1A) autoreceptor desensitization by high-efficacy agonists may accelerate the onset of the therapeutic effects of antidepressants.

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.

A serotonin 5-HT1A receptor agonist prevents behavioral sensitization to L-DOPA in a rodent model of Parkinson's disease

Marked fluctuation of dopamine concentration in the striatum following long-term L-DOPA administration contributes to the development of L-DOPA-induced motor complications including L-DOPA-induced dyskinesias and wearing-off in patients with Parkinson's disease. We have shown that pretreatment with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A (5-hydroxytryptamine) receptor agonist, alleviates fluctuation of dopamine levels in the dopamine-denervated striatum of 6-hydroxydopamine-lesioned (hemiparkinsonian) rats after L-DOPA treatment. To determine whether co-administration of 8-OH-DPAT with L-DOPA prevents L-DOPA-induced motor complications, we examined rotation behavior and levels of messenger RNAs coding for dynorphin and glutamic acid decarboxylase in the striatum of 6-hydroxydopamine-lesioned rats treated with L-DOPA alone or L-DOPA + 8-OH-DPAT, twice daily, for 2 weeks. Co-administration of 8-OH-DPAT inhibited an increase of rotation behavior to L-DOPA and L-DOPA-induced increases in levels of messenger RNAs coding for dynorphin and glutamic acid decarboxylase in the dopamine-denervated striatum, both of which are established indices of L-DOPA-induced motor complications. These results suggest that pharmaceutical products that stimulate 5-HT1A receptors could prove useful in prevention of the development of L-DOPA-induced motor complications in patients with Parkinson's disease.

Fatigue of cholestasis and the serotoninergic neurotransmitter system in the rat

Fatigue associated with cholestasis may impair health-related quality of life. The pathogenesis of this symptom is largely unknown, but it has been suggested that central serotoninergic neurotransmission may be implicated and that serotonin 1A receptor agonists may yield improvement. The aim of this study was to study the central serotoninergic system, specifically the serotonin (5-HT)(1A) receptor-mediated pathway of serotoninergic neurotransmission, in a bile duct resection rat model of cholestasis. Fatigue was assessed in the forced swim test in sham and bile duct-resected rats. The serotonin behavioral syndrome, which includes hyperlocomotion, was assessed in both groups of rats after escalating doses of the 5-HT(1A) receptor agonist 8-hydroxy(di-n-propylamine)tetralin (8-OH DPAT). 5-HT(1A) and 5-HT(2) receptor densities were explored in four brain regions using a receptor-binding assay. Extracellular 5-HT and 5-hydroxyindoleacetic acid were measured via in vivo brain dialysis. Bile duct-resected rats spent more time floating in the forced swim test, and 8-OH DPAT decreased floating time in cholestatic rats (P < .01). Dose-response curves created with 8-OH DPAT for the serotonin behavioral syndrome were similar in bile duct-resected and sham-operated rats. 5-HT(1A) and 5-HT(2) receptor densities in most brain regions and extracellular serotonin levels were similar in both groups of rats. In conclusion, 5-HT(1A) receptor agonist-induced amelioration of fatigue in cholestatic rats may be nonspecific and not linked to reversal of the pathophysiology of fatigue associated with cholestasis; however, these data do not exclude a potential role of the central serotoninergic system in the evolution of fatigue.

Serotonin and the neuroendocrine regulation of the hypothalamic--pituitary-adrenal axis in health and disease

Serotonin (5-hydroxytryptamine, 5-HT)-containing neurons in the midbrain directly innervate corticotropin-releasing hormone (CRH)-containing cells located in paraventricular nucleus of the hypothalamus. Serotonergic inputs into the paraventricular nucleus mediate the release of CRH, leading to the release of adrenocorticotropin, which triggers glucocorticoid secretion from the adrenal cortex. 5-HT1A and 5-HT2A receptors are the main receptors mediating the serotonergic stimulation of the hypothalamic-pituitary-adrenal axis. In turn, both CRH and glucocorticoids have multiple and complex effects on the serotonergic neurons. Therefore, these two systems are interwoven and communicate closely. The intimate relationship between serotonin and the hypothalamic-pituitary-adrenal axis is of great importance in normal physiology such as circadian rhythm and stress, as well as pathophysiological disorders such as depression, anxiety, eating disorders, and chronic fatigue.

Activation of serotonergic neurotransmission during the performance of aggressive behavior in rats

High aggression is often linked to lowered serotonin (5-HT) neurotransmission. Although this may hold for high aggression as a trait characteristic of an individual, serotonergic activity is probably increased during performance of aggressive behavior. To test this hypothesis, first, the 5-HT1A agonist alnespirone and gamma aminobutyric acid-A agonist muscimol were administered into the dorsal raphe nucleus. These treatments, which inhibit 5-HT neuronal activity, were shown to decrease performance of aggressive behavior. Second, after a resident-intruder test, the activation of 5-HT neurons (measured by c-fos expression) was increased in high-aggressive rats, compared with low-aggressive rats or control rats that were not subjected to a social confrontation. Results show that performance of aggressive behavior increases 5-HT neuronal activity and that preventing this activation inhibits expression of aggressive behavior.

5-HT1A agonists: alcohol drinking in rats and squirrel monkeys

RATIONALE

Increased alcohol intake after administration of low doses of 5-HT(1A )agonists is thought to be due to a reduction in 5-HT impulse flow due to activation of 5-HT(1A) somatodendritic receptors, whereas decreased alcohol drinking found after administration of higher doses of 5-HT(1A) agonists may be mediated by action at postsynaptic 5-HT(1A) receptors.

OBJECTIVE

This study compares Long-Evans rats and squirrel monkeys to examine the hypothesis that low doses of the 5-HT(1A) selective agonists, 8-OH-DPAT and alnespirone, will preferentially increase, and at higher doses decrease alcohol drinking, and whether these effects can be antagonized by WAY 100635.

METHODS

Male Long-Evans rats were induced to drink from two bottles, one containing a solution of 10% ethanol and 1% sucrose (w/v), the other containing an equally preferred concentration of sucrose. Squirrel monkeys also drank from two bottles, one containing a solution of 2% ethanol and 15% sucrose (w/v), the other, water.

RESULTS

In rats, low doses of both 8-OH-DPAT (0.018-0.03 mg/kg) and alnespirone (0.3-3.0 mg/kg) increased alcohol drinking by ca. 100% without altering sucrose intake. The highest dose of 8-OH-DPAT (0.1 mg/kg) suppressed intake of both solutions without significant motor impairment. Pretreatment with WAY 100635 (0.1 mg/kg), shifted the entire dose-effect curve of 8-OH-DPAT to the right, and antagonized the effects of the 0.56 mg/kg dose of alnespirone. In the monkeys, administration of both agonists dose-dependently decreased alcohol intake and were behaviorally sedative.

CONCLUSIONS

These results support the hypothesis that in rats, 5-HT(1A) receptor stimulation activates somatodendritic receptors at lower doses and postsynaptic receptors at higher doses, each with opposite effects on alcohol intake. The absence of such biphasic dose-effect curves in monkeys suggests a different function of 5-HT(1A) somatodendritic receptors in rats and monkeys, at least with regard to alcohol drinking.

Modulation of serotonergic function in rat brain by VN2222, a serotonin reuptake inhibitor and 5-HT1A receptor agonist

VN2222 (1-(benzo[b]thiophen-3-yl)-3-[4-(2-methoxiphenyl piperazin-1-yl]propan-1-ol) is a potential antidepressant with high affinity for the serotonin transporter and 5-HT(1A) receptors. Locally applied, VN2222 enhanced the extracellular 5-hydroxytryptamine (5-HT) concentration (5-HT(ext)) in rat striatum to 780% of baseline whereas its systemic administration (1-10 mg/kg s.c.) reduced 5-HT(ext). In the presence of citalopram, 8-OH-DPAT or VN2222 applied in medial prefrontal cortex reduced 5-HT(ext). Fluoxetine, VN2222, and 8-OH-DPAT suppressed the firing rate of dorsal raphe 5-HT neurons (ED(50): 790, 14.9, and 0.8 microg/kg i.v., respectively). These effects were antagonized by WAY 100635. Administration of VN2222 for 2 weeks desensitized 5-HT(1A) receptors as assessed by microdialysis and single-unit recordings (ED(50) values for 8-OH-DPAT were 0.45 and 2.34 microg/kg i.v. for controls and rats treated with 6 mg/kg day VN2222). These results show that VN2222 is a mixed 5-HT reuptake inhibitor/5-HT(1A) agonist that markedly desensitizes 5-HT(1A) autoreceptors. These properties suggest that it may be a clinically effective dual action antidepressant drug.

In vivo comparison of two 5-HT1A receptors agonists alnespirone (S-20499) and buspirone on locus coeruleus neuronal activity

The aim of the present study was to compare, in chloral-hydrate anaesthetized rats, the alpha(2)-adrenergic properties of the selective 5-HT(1A) receptor agonist, alnespirone (S-20499), with those of buspirone, a 5-HT(1A) receptor agonist exhibiting potent alpha(2)-adrenoceptor antagonist properties via its principal metabolite, 1-(2-pyrimidinyl)-piperazine. Both locus coeruleus spontaneous firing activity and noradrenaline release in the medial prefrontal cortex were potently inhibited by the alpha(2)-adrenoceptor agonist clonidine, at a dose of 40 microg/kg (i.p.). Such an inhibition was neither prevented nor reversed by alnespirone (10 mg/kg, i.p.), while buspirone, at the same dose, potently antagonized the locus coeruleus inhibitory effects of clonidine. These data demonstrate that, in contrast with some aryl-piperazine compounds (such as buspirone), alnespirone, either on its own or via a possible metabolite such as buspirone, is devoid in vivo of significant alpha(2)-adrenoceptor antagonist properties.

Origin and functional role of the extracellular serotonin in the midbrain raphe nuclei

There is considerable interest in the regulation of the extracellular compartment of the transmitter serotonin (5-hydroxytryptamine, 5-HT) in the midbrain raphe nuclei because it can control the activity of ascending serotonergic systems and the release of 5-HT in terminal areas of the forebrain. Several intrinsic and extrinsic factors of 5-HT neurons that regulate 5-HT release in the dorsal (DR) and median (MnR) raphe nucleus are reviewed in this article. Despite its high concentration in the extracellular space of the raphe nuclei, the origin of this pool of the transmitter remains to be determined. Regardless of its origin, is has been shown that the release of 5-HT in the rostral raphe nuclei is partly dependent on impulse flow and Ca(2+) ions. The release in the DR and MnR is critically dependent on the activation of 5-HT autoreceptors in these nuclei. Yet, it appears that 5-HT autoreceptors do not tonically inhibit 5-HT release in the raphe nuclei but rather play a role as sensors that respond to an excess of the endogenous transmitter. Both DR and MnR are equally responsive to the reduction of 5-HT release elicited by the local perfusion of 5-HT(1A) receptor agonists. In contrast, the effects of selective 5-HT(1B) receptor agonists are more pronounced in the MnR than in the DR. However, the cellular localization of 5-HT(1B) receptors in the raphe nuclei remains to be established. Furthermore, endogenous noradrenaline and GABA tonically regulate the extracellular concentration of 5-HT although the degree of tonicity appears to depend upon the sleep/wake cycle and the behavioral state of the animal. Glutamate exerts a phasic facilitatory control over the release of 5-HT in the raphe nuclei through ionotropic glutamate receptors. Overall, it appears that the extracellular concentration of 5-HT in the DR and the MnR is tightly controlled by intrinsic serotonergic mechanisms as well as afferent connections.

Activation of 5-HT(1A) but not 5-HT(1B) receptors attenuates an increase in extracellular dopamine derived from exogenously administered L-DOPA in the striatum with nigrostriatal denervation

In order to determine whether L-DOPA-derived extracellular dopamine (DA) in the striatum with dopaminergic denervation is affected by activation of serotonin autoreceptors (5-HT(1A) and 5-HT(1B) receptors), we applied in vivo brain microdialysis technique to 6-hydroxydopamine-lesioned rats and examined the effects of the selective 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the selective 5-HT(1B) receptor agonist CGS-12066 A on L-DOPA-derived extracellular DA levels. Single L-DOPA injection (50 mg/kg i.p.) caused a rapid increase and a following decrease of extracellular DA, with a peak value at 100 min after L-DOPA injection. Pretreatment with both 0.3 mg/kg and 1 mg/kg 8-OH-DPAT (i.p.) significantly attenuated an increase in L-DOPA-derived extracellular DA and the times of peak DA levels were prolonged to 150 min and 225 min after L-DOPA injection, respectively. These 8-OH-DPAT-induced changes in L-DOPA-derived extracellular DA were antagonized by further pretreatment with WAY-100635, a selective 5-HT(1A) antagonist. In contrast, intrastriatal perfusion with the 5-HT(1B) agonist CGS-12066 A (10 nM and 100 nM) did not induce any changes in L-DOPA-derived extracellular DA. Thus, stimulation of 5-HT(1A) but not 5-HT(1B) receptors attenuated an increase in extracellular DA derived from exogenous L-DOPA. These results support the hypothesis that serotonergic neurons are primarily responsible for the storage and release of DA derived from exogenous L-DOPA in the absence of dopaminergic neurons.

Repeated treatment with 8-OH-DPAT induces tolerance to its ability to produce the 5-HT1A behavioural syndrome, but not to its ability to attenuate haloperidol-induced catalepsy

When administered acutely, 5-hydroxytryptamine1A (5-HT1A) agonists attenuate the cataleptic side effects of antipsychotics. We investigated whether tolerance occurs to these effects after repeated administration of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). For comparison, we also assessed the ability of 8-OH-DPAT to produce elements of the 5-HT1A behavioural syndrome (i.e. forepaw treading, flat body posture and lower lip retraction), some of which readily demonstrate tolerance. Catalepsy was measured in rats using both the cross-legged position test and the bar test. Repeated treatment with 8-OH-DPAT (0.63-2.5 mg/kg subcutaneously), once daily for 4 days, did not significantly alter the ability of acute treatment with 8-OH-DPAT (0.01-2.5 mg/kg) to inhibit catalepsy induced by haloperidol (2.5 mg/kg) in either test. In contrast, the ability of 8-OH-DPAT to produce the 5-HT1A behavioural syndrome was significantly attenuated by the repeated treatment. The present data, showing an absence of tolerance to the anti-cataleptic effects of a 5-HT1A agonist, indicate that mixed dopamine antagonist/5-HT1A agonist compounds may continue to have a low propensity to induce extrapyramidal side effects during chronic treatment.

Increased binding at 5-HT(1A), 5-HT(1B), and 5-HT(2A) receptors and 5-HT transporters in diet-induced obese rats

5-Hydroxytryptamine (5-HT, serotonin), synthesized in midbrain raphe nuclei and released in various hypothalamic sites, decreases food intake but the specific 5-HT receptor subtypes involved are controversial. Here, we have studied changes in the regional density of binding to 5-HT receptors and transporters and the levels of tryptophan hydroxylase, in rats with obesity induced by feeding a palatable high-energy diet for 7 weeks. We mapped binding at 5-HT receptor subtypes and transporters using quantitative autoradiography and determined tryptophan hydroxylase protein levels by Western blotting. In diet-induced obese (DiO) rats, specific binding to 5-HT(1A) receptors ([3H]8-OH-DPAT) was significantly increased in the dorsal and median raphe by 90% (P<0.01) and 132% (P<0.05), respectively, compared with chow-fed controls. 5-HT(1B) receptor binding sites ([125I]cyanopindolol) were significantly increased in the hypothalamic arcuate nucleus (ARC) of DiO rats (58%; P<0.05), as were 5-HT(2A) receptor binding sites ([3H]ketanserin) in both the ARC (44%; P<0.05) and lateral hypothalamic area (LHA) (121%; P<0.05). However, binding to 5-HT(2C) receptors ([3H]mesulgergine) in DiO rats was not significantly different from that in controls in any hypothalamic region. Binding to 5-HT transporters ([3H]paroxetine) was significantly increased (P<0.05) in both dorsal and median raphe, paraventricular nuclei (PVN), ventromedial nuclei (VMH), anterior hypothalamic area (AHA) and LHA of DiO rats, by 47%-165%. Tryptophan hydroxylase protein levels in the raphe nuclei were not significantly different between controls and DiO rats. In conclusion, we have demonstrated regionally specific changes in binding to certain 5-HT receptor subtypes in obesity induced by voluntary overeating of a palatable diet. Overall, these changes are consistent with reduced 5-HT release and decreased activity of the 5-HT neurons. Reduction in the hypophagic action of 5-HT, possibly acting at 5-HT(1A), 5-HT(1B) and 5-HT(2A) receptors, may contribute to increased appetite in rats presented with highly palatable diet.