Sensitivity Of 5-HT1A Autoreceptors Research Articles

Physical Interactions and Functional Relationships of Neuroligin 2 and Midbrain Serotonin Transporters.

The neurotransmitter serotonin [5-hydroxytryptamine (5-HT)] modulates many key brain functions including those subserving sensation, emotion, reward, and cognition. Efficient clearance of 5-HT after release is achieved by the antidepressant-sensitive 5-HT transporter (SERT, SLC6A4). To identify novel SERT regulators, we pursued a proteomic analysis of mouse midbrain SERT complexes, evaluating findings in the context of prior studies that established a SERT-linked transcriptome. Remarkably, both efforts converged on a relationship of SERT with the synaptic adhesion protein neuroligin 2 (NLGN2), a post-synaptic partner for presynaptic neurexins, and a protein well-known to organize inhibitory GABAergic synapses. Western blots of midbrain reciprocal immunoprecipitations confirmed SERT/NLGN2 associations, and also extended to other NLGN2 associated proteins [e.g., α-neurexin (NRXN), gephyrin]. Midbrain SERT/NLGN2 interactions were found to be Ca2+-independent, supporting cis vs. trans-synaptic interactions, and were absent in hippocampal preparations, consistent with interactions arising in somatodendritic compartments. Dual color in situ hybridization confirmed co-expression of Tph2 and Nlgn2 mRNA in the dorsal raphe, with immunocytochemical studies confirming SERT:NLGN2 co-localization in raphe cell bodies but not axons. Consistent with correlative mRNA expression studies, loss of NLGN2 expression in Nlgn2 null mice produced significant reductions in midbrain and hippocampal SERT expression and function. Additionally, dorsal raphe 5-HT neurons from Nlgn2 null mice exhibit reduced excitability, a loss of GABAA receptor-mediated IPSCs, and increased 5-HT1A autoreceptor sensitivity. Finally, Nlgn2 null mice display significant changes in behaviors known to be responsive to SERT and/or 5-HT receptor manipulations. We discuss our findings in relation to the possible coordination of intrinsic and extrinsic regulation afforded by somatodendritic SERT:NLGN2 complexes.

Reduced vesicular monoamine transport disrupts serotonin signaling but does not cause serotonergic degeneration

We previously demonstrated that mice with reduced expression of the vesicular monoamine transporter 2 (VMAT2 LO) undergo age-related degeneration of the catecholamine-producing neurons of the substantia nigra pars compacta and locus ceruleus and exhibit motor disturbances and depressive-like behavior. In this work, we investigated the effects of reduced vesicular transport on the function and viability of serotonin neurons in these mice. Adult (4–6months of age), VMAT2 LO mice exhibit dramatically reduced (90%) serotonin release capacity, as measured by fast scan cyclic voltammetry. We observed changes in serotonin receptor responsivity in in vivo pharmacological assays. Aged (months) VMAT2 LO mice exhibited abolished 5-HT1A autoreceptor sensitivity, as determined by 8-OH-DPAT (0.1mg/kg) induction of hypothermia. When challenged with the 5HT2 agonist, 2,5-dimethoxy-4-iodoamphetamine (1mg/kg), VMAT2 LO mice exhibited a marked increase (50%) in head twitch responses. We observed sparing of serotonergic terminals in aged mice (18–24months) throughout the forebrain by SERT immunohistochemistry and [3H]-paroxetine binding in striatal homogenates of aged VMAT2 LO mice. In contrast to their loss of catecholamine neurons of the substantia nigra and locus ceruleus, aged VMAT2 LO mice do not exhibit a change in the number of serotonergic (TPH2+) neurons within the dorsal raphe, as measured by unbiased stereology at 26–30months. Collectively, these data indicate that reduced vesicular monoamine transport significantly disrupts serotonergic signaling, but does not drive degeneration of serotonin neurons.

Electrophysiological evidence for rapid 5-HT1A autoreceptor inhibition by vilazodone, a 5-HT1A receptor partial agonist and 5-HT reuptake inhibitor

This study examined the effect of vilazodone, a combined serotonin (5-HT) reuptake inhibitor and 5-HT1A receptor partial agonist, paroxetine and fluoxetine on the sensitivity of 5-HT1A autoreceptors of serotonergic dorsal raphe nucleus neurons in rats. These effects were assessed by determining the intravenous dose of (±)-8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) required to suppress the basal firing rate of these neurons by 50% (ID50) in anesthetized rats using in vivo electrophysiology. 5-HT uptake inhibition was determined by the ability of the compounds to reverse (±)-p-chloroamphetamine (PCA)-induced rat hypothalamic 5-HT depletion ex vivo. Acute vilazodone administration (0.63 and 2.1µmol/kg, s.c.), compared with vehicle, significantly increased (2–3-fold) the ID50 of 8-OH-DPAT at 4h, but not 24h after administration. Subchronic administration (3 days) significantly increased the ID50 value at 4h (3–4-fold) and at 24h (~2-fold). In contrast, paroxetine and fluoxetine at doses that were supramaximal for 5-HT uptake inhibition did not significantly alter the ID50 value of 8-OH-DPAT after acute or subchronic administration. Vilazodone antagonized the action of PCA 3.5h and 5h after a single dose (ID50 1.49 and 0.46µmol/kg, s.c., respectively), but was inactive 18h post-administration, corroborating the electrophysiological results at 24h following acute administration. The results are consistent with the concept of rapid and, following repeated treatment, prolonged inhibition of 5-HT1A autoreceptors by vilazodone. This effect could occur by either direct interaction with, or desensitization of, these receptors, an effect which cannot be ascribed to vilazodone's 5-HT reuptake inhibiting properties.

Decline in serotonergic firing activity and desensitization of 5-HT1A autoreceptors after chronic unpredictable stress

Chronic stressful life events are risk factors for contracting depression, the pathophysiology of which is strongly associated with impairments in serotonergic (5-HT) neurotransmission. Indeed, in rodents, exposure to chronic unpredictable stress (CUS) produces depressive-like behaviours such as behavioural despair and anhedonia. To date, there have not been many studies that especially explore in vivo changes in 5-HT neurotransmission associated with CUS in the rat. Therefore, using in vivo electrophysiology, we evaluated whether CUS that induces anhedonia-like behaviours concurrently impairs midbrain raphe 5-HT neuronal activity. Unlike unstressed and acutely stressed rats, CUS produced progressive reductions in sucrose intake and preference (anhedonia-like). These were associated with a decrease in the spontaneous firing activity (35.4%) as well as in the number of spontaneously active 5-HT neurons, and a desensitization of somatodendritic 5-HT1A autoreceptors in the dorsal raphe. These results suggest that CUS dramatically decreases 5-HT neural activity and 5-HT1A autoreceptor sensitivity, and may represent endophenotypic features of depressive-like states.

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.

Thyroid hormones, serotonin and mood: of synergy and significance in the adult brain.

The use of thyroid hormones as an effective adjunct treatment for affective disorders has been studied over the past three decades and has been confirmed repeatedly. Interaction of the thyroid and monoamine neurotransmitter systems has been suggested as a potential underlying mechanism of action. While catecholamine and thyroid interrelationships have been reviewed in detail, the serotonin system has been relatively neglected. Thus, the goal of this article is to review the literature on the relationships between thyroid hormones and the brain serotonin (5-HT) system, limited to studies in adult humans and adult animals. In humans, neuroendocrine challenge studies in hypothyroid patients have shown a reduced 5-HT responsiveness that is reversible with thyroid replacement therapy. In adult animals with experimentally-induced hypothyroid states, increased 5-HT turnover in the brainstem is consistently reported while decreased cortical 5-HT concentrations and 5-HT2A receptor density are less frequently observed. In the majority of studies, the effects of thyroid hormone administration in animals with experimentally-induced hypothyroid states include an increase in cortical 5-HT concentrations and a desensitization of autoinhibitory 5-HT1A receptors in the raphe area, resulting in disinhibition of cortical and hippocampal 5-HT release. Furthermore, there is some indication that thyroid hormones may increase cortical 5-HT2 receptor sensitivity. In conclusion, there is robust evidence, particularly from animal studies, that the thyroid economy has a modulating impact on the brain serotonin system. Thus it is postulated that one mechanism, among others, through which exogenous thyroid hormones may exert their modulatory effects in affective illness is via an increase in serotonergic neurotransmission, specifically by reducing the sensitivity of 5-HT1A autoreceptors in the raphe area, and by increasing 5-HT2 receptor sensitivity.

Desensitization of 5-HT1A Autoreceptors by a Low Chronic Fluoxetine Dose Effect of the Concurrent Administration of WAY-100635

Using microdialysis, receptor autoradiography and in situ hybridization, we examined the effects of fluoxetine alone or with WAY-100635 on: (a) extracellular 5-HT in frontal cortex; and (b) density and sensitivity of 5-HT1A autoreceptors in rat brain. WAY-100635 (0.3 mg/kg, s.c.) doubled the increase in extracellular 5-HT produced by fluoxetine (3 mg/kg, i.p.) in frontal cortex. Two-week minipump treatments with these daily doses significantly raised extracellular 5-HT to 275 ± 33% (fluoxetine) and 245 ± 10% (fluoxetine plus WAY-100635) of controls. Fluoxetine 3 mg/kg·day desensitized dorsal raphe 5-HT1A autoreceptors, an effect prevented by the concurrent WAY-100635 administration. However, WAY-100635 (alone or with fluoxetine) did not change 5-HT1A autoreceptor sensitivity. The density of 5-HT1A receptors and its encoding mRNA, was unaffected by these treatments. These results suggest that prolonged blockade of 5-HT1A receptors in vivo prevents the autoreceptor desensitization induced by fluoxetine but does not result in receptor sensitization.

The effect of the selective 5-HT1A agonists alnespirone (S-20499) and 8-OH-DPAT on extracellular 5-hydroxytryptamine in different regions of rat brain.

1. We have examined the effects of the systemic administration of the selective 5-HT1A agonist alnespirone (S-20499) on in vivo 5-hydroxytryptamine (5-HT) release in the dorsal raphe nucleus, the median raphe nucleus and four forebrain areas innervated differentially by both (dorsal striatum, frontal cortex, ventral hippocampus and dorsal hippocampus). 2. Alnespirone (0.1-3 mg kg(-1), s.c.) dose-dependently reduced extracellular 5-HT in the six areas examined. In forebrain, the maximal reductions occurred in striatum and frontal cortex (maximal reduction to 23 and 29% of baseline, respectively). Those in dorsal and ventral hippocampus were more moderate (to ca 65% of baseline). In contrast, the decrease in 5-HT elicited in the median raphe nucleus was more marked than that in the dorsal raphe nucleus (to ca 30 and 60% of baseline, respectively). The selective 5-HT1A antagonist WAY-100635 (0.5 mg kg(-1), s.c.) prevented the decrease in 5-HT induced by alnespirone (0.3 mg kg(-1), s.c.) in frontal cortex. 3. 8-OH-DPAT (0.025, 0.1 and 0.3 mg kg(-1), s.c.) also reduced extracellular 5-HT in a regionally-selective manner (e.g., to 32% of baseline in striatum and to 69% in dorsal hippocampus at 0.1 mg kg(-1), s.c.). In midbrain, 8-OH-DPAT reduced the dialysate 5-HT slightly more in the median than in the dorsal raphe nucleus at all doses examined. 4. Doses of both compounds close to their respective ED50 values (0.3 mg kg(-1) alnespirone, 0.025 mg kg(-1) 8-OH-DPAT) reduced 5-HT to a comparable extent in all regions examined. However, the reductions attained at higher doses were more pronounced for 8-OH-DPAT. 5. These data show that the reduction of 5-HT release elicited by alnespirone and 8-OH-DPAT is more important in forebrain areas innervated by 5-hydroxytryptaminergic neurones of the dorsal raphe nucleus. This regional selectivity seems unlikely to be accounted for by differences in the sensitivity of 5-HT1A autoreceptors controlling 5-HT release, given the dissimilar effects of these two 5-HT1A agonists in regions rich in cell bodies and nerve terminals. This suggests the presence of complex mechanisms of control of 5-HT release by 5-HT1A receptors.

Effects of sleep deprivation on serotonergic neuronal activity in the dorsal raphe nucleus of the freely moving cat

Total sleep deprivation (TSD) for one or more nights produces a rapid antidepressant response in humans. Since most pharmacological treatments for depression increase brain serotonin neurotransmission, the purpose of the present study was to determine whether TSD increases the activity of serotonergic neurons in the dorsal raphe nucleus (DRN) in cats. Cats were prevented from sleeping by the experimenter, who monitored the behavioral state of each animal on a polygraph. Firing rates during quiet waking (QW) and active waking (AW) were obtained throughout a 24-h sleep deprivation period and subsequent 6-h recovery period. During the experiments, unit activity was also recorded during exposure to loud white noise, which elicited strong behavioral arousal. The inhibitory response of serotonergic DRN neurons to systemic administration of the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was determined before and after TSD to assess possible changes in 5-HT1A autoreceptor sensitivity. TSD increased mean firing rates by as much as 18% during both AW and white noise exposure. Maximal effects were observed after 15 h of TSD for AW, and after 18 h for white noise. QW firing rates also tended to be elevated throughout TSD. Firing rates for all conditions during the recovery period were not significantly different from baseline. The neuronal inhibition produced by 8-OH-DPAT was significantly diminished after TSD. Overall, these results indicate that TSD increases the firing rate of serotonergic DRN neurons during AW and arousal. This effect may be attributable to a decrease in the sensitivity of 5-HT1A autoreceptors. These findings are consistent with the hypothesis that TSD exerts its antidepressant action, at least in part, through an activation of brain serotonergic neurons.

Effects of Food Deprivation on Midbrain 5-HT1A Autoreceptors in Lewis and SHR Rats

Food deprivation stimulates the activity of the hypothalamo-pituitary-adrenal axis and brain serotonin (5-hydroxytryptamine, 5-HT) synthesis. Because midbrain somato-dendritic 5-HT1A autoreceptors may obey homologous and heterologous (e.g. by glucocorticoids) down-regulation, we have analyzed whether 24 hr of fasting affects midbrain 5-HT1A receptor binding and sensitivity in Lewis and SHR rats (i.e. strains that differ in behavioral/neuroendocrine responses to stressors). Fasting affected neither [3H]8-hydroxy-2-(di-N-propylamino)tetralin ([3H]8-OH-DPAT) binding at 5-HT1A autoreceptors nor 8-OH-DPAT-induced inhibition of midbrain 5-HT synthesis (an index of 5-HT1A autoreceptor sensitivity). Because fasting increased 5-HT precursor (tryptophan) levels to similar extents in the midbrains of saline- and 8-OH-DPAT-treated rats, we conclude that food deprivation does not affect 5-HT1A autoreceptors. In turn, our results suggest that the differential effects of 5-HT1A receptor agonists on food intake, in fed and fasted rats may be independent from 5-HT1A autoreceptors. © 1997 Elsevier Science Ltd.

Regional differences in the effect of the combined treatment of WAY 100635 and fluoxetine: an in vivo microdialysis study

We studied the changes in extracellular serotonin (5-HT) levels in the frontal cortex (FC) and ventral hippocampus (vHi) in conscious rats, induced by the combined administration of a highly selective 5-HT1A receptor antagonist, WAY 100635 (0.1 mg/kg, i.v.), and fluoxetine (1 mg/kg, i.p.), a selective 5-HT reuptake inhibitor (SSRI). In the two brain areas studied, no change in extracellular 5-HT concentrations was observed following fluoxetine administration over the 210 min post-injection period. However, in animals co-administered with [WAY 100635 + fluoxetine], the maximal increase in 5-HT levels in the FC was to 215% of the respective basal value (100%), while no significant change in 5-HT was observed in dialysates from the vHi. Furthermore, the [norfluoxetine]-to-[fluoxetine] ratio in the FC was significantly higher than in the hippocampus as measured in homogenates of animals treated with either fluoxetine alone or a prior administration of WAY 100635. Thus, WAY 100635 made the fluoxetine short-lasting effect apparent in the FC, but not by interfering with pharmacokinetic parameters of fluoxetine. Taken together, our data suggest the possibility, that either 5-HT1A autoreceptor sensitivity or uptake carrier density or higher [metabolite]-to-[parent drug] ratios in the FC than in the hippocampus may be involved in regional specific responses to SSRIs.

Functional Correlates Of 5-HT1A Receptor Activation After Exposure To Repeated Cocaine Administration In Rats

It is well known that acute cocaine treatment has profound effects on serotonin (5-HT) neurons, but the consequences of chronic cocaine administration on 5-HT neurotransmission are not well studied. We used the 5-HT agonist 80H-DPAT as a probe to assess 5-HT1A receptor function in vivo in rats previously exposed to cocaine. Rats were treated with cocaine (15 mg/kg, ip) twice daily for one week. After 42 hr and 8 days of withdrawal, rats were challenged with 80H-DPAT (10-300 μg/kg, sc) or vehicle, and various endpoints were evaluated including: 5-HT synthesis in discrete brain regions, feeding, plasma prolactin, and the 5-HT behavioral syndrome. Prior cocaine exposure did not alter the 80H-DPAT-induced inhibition of 5-HT synthesis in any brain region examined. Although cocaine-treated rats exhibited higher basal rates of feeding, the hyperphagia produced by 80H-DPAT was comparable in cocaine- and saline-treated groups. The plasma prolactin response to 80H-DPAT was significantly blunted in cocaine-treated animals at 42 hr of withdrawal, and this effect was maintained at 8 days. Cocaine exposure did not affect the 5-HT behavioral syndrome elicited by 80H-DPAT. These data indicate that prior cocaine administration does not change 5-HT1A autoreceptor sensitivity (as measured by inhibition of 5-HT synthesis and hyperphagia). However, the attenuated prolactin response to 80H-DPAT suggests that cocaine renders postsynaptic 5-HT1A receptor mechanisms subsensitive in some brain regions. Deficits in 5-HT1A receptor function may underlie feelings of anxiety experienced by cocaine addicts during periods of drug withdrawal.

5-HT1A Receptor Sensitivity in Major Depression

The hypothermic, growth hormone and corticotrophin (ACTH) responses to the 5-HT1A receptor agonist buspirone (30 mg orally) were measured in 20 unmedicated patients with major depression and 20 healthy controls. Compared with the controls, the hypothermic responses of the depressed patients to buspirone were significantly attenuated, particularly in patients with melancholic depression. In contrast, the responses of growth hormone and ACTH to buspirone were unchanged. The data suggest that major depression may be associated with impaired sensitivity of 5-HT1A autoreceptors but that the function of the post-synaptic 5-HT1A receptors that mediate growth hormone and ACTH release is unaltered. Within the limitations that attend the use of buspirone as a 5-HT1A probe, our data suggest that the decrement in serotonin neurotransmission at post-synaptic 5-HT1A receptors in depression is due to decreased serotonin release rather than impaired responsivity of post-synaptic 5-HT1A receptors.