Diurnal variation in 5-HT1B autoreceptor function in the anterior hypothalamus in vivo: effect of chronic antidepressant drug treatment

Brain Microdialysate Monoamines in Relation to Circadian Rhythms, Sleep, and Sleep Deprivation - a Systematic Review, Network Meta-analysis, and New Primary Data

Disruption of the monoaminergic system, e.g. by sleep deprivation (SD), seems to promote certain diseases. Assessment of monoamine levels over the circadian cycle, during different sleep stages and during SD is instrumental to understand the molecular dynamics during and after SD. To provide a complete overview of all available evidence, we performed a systematic review. A comprehensive search was performed for microdialysis and certain monoamines (dopamine, serotonin, noradrenaline, adrenaline), certain monoamine metabolites (3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindoleacetic acid (5-HIAA)) and a precursor (5-hydroxytryptophan (5-HTP)) in PubMed and EMBASE. After screening of the search results by two independent reviewers, 94 publications were included. All results were tabulated and described qualitatively. Network-meta analyses (NMAs) were performed to compare noradrenaline and serotonin concentrations between sleep stages. We further present experimental monoamine data from the medial prefrontal cortical (mPFC). Monoamine levels varied with brain region and circadian cycle. During sleep, monoamine levels generally decreased compared to wake. These qualitative observations were supported by the NMAs: noradrenaline and serotonin levels decreased from wakefulness to slow wave sleep and decreased further during Rapid Eye Movement sleep. In contrast, monoamine levels generally increased during SD, and sometimes remained high even during subsequent recovery. Decreases during or after SD were only reported for serotonin. In our experiment, SD did not affect any of the mPFC monoamine levels. Concluding, monoamine levels vary over the light-dark cycle and between sleep stages. SD modifies the patterns, with effects sometimes lasting beyond the SD period.

5-HT1B receptor agonist CGS12066 presynaptically inhibits glutamate release in rat hippocampus

CGS12066, a 5-hydroxytryptamine 1B (5-HT_1B) receptor agonist, has been reported to exhibit antidepressant activity. Considering that glutamatergic dysfunction is implicated in depression, the effect of CGS12066 on glutamate release in rat hippocampal nerve terminals and possible underlying mechanism were investigated. We observed that CGS12066 inhibited 4-aminopyridine (4-AP)-evoked glutamate release, and that a 5-HT_1B receptor antagonist blocked this inhibition. Western blot analysis and immunocytochemistry confirmed the presence of presynaptic 5-HT_1B receptor proteins. CGS12066-mediated inhibition of 4-AP-evoked glutamate release was completely abolished in the synaptosomes pretreated with inhibitors of Gi/Go-protein, adenylate cyclase (AC), and protein kinase A (PKA), namely pertussis toxin, MDL12330A, and H89, respectively. CGS12066 reduced the elevation of 4-AP-evoked intrasynaptosomal Ca²⁺ and cyclic AMP (cAMP) levels, but did not affect the synaptosomal membrane potential. Furthermore, in the presence of ω-conotoxin MVIIC, a N- and P/Q-type channel blocker, CGS12066-mediated inhibition of 4-AP-evoked glutamate release was markedly reduced; however, the intracellular Ca²⁺-release inhibitors dantrolene and CGP37157 did not affect the CGS12066 effect. Furthermore, CGS12066 reduced glutamatergic miniature excitatory postsynaptic current (mEPSC) frequency but did not affect mEPSC amplitude or glutamate-activated currents in hippocampal slices. Our data are the first to suggest that CGS12066 reduces AC/cAMP/PKA activation, through the activation of Gi/Go protein-coupled 5-HT_1B receptors present on hippocampal nerve terminals, subsequently reducing Ca²⁺ entry through voltage-dependent Ca²⁺ channels and reducing 4-AP-evoked glutamate release. This investigation into the role of 5-HT_1B receptors in glutamate release provides crucial information regarding the potential therapeutic role of 5-HT_1B receptors for treating depression.

In Vivo Ambient Serotonin Measurements at Carbon-Fiber Microelectrodes

The mechanisms that control extracellular serotonin levels in vivo are not well-defined. This shortcoming makes it very challenging to diagnose and treat the many psychiatric disorders in which serotonin is implicated. Fast-scan cyclic voltammetry (FSCV) can measure rapid serotonin release and reuptake events but cannot report critically important ambient serotonin levels. In this Article, we use fast-scan controlled adsorption voltammetry (FSCAV), to measure serotonin's steady-state, extracellular chemistry. We characterize the "Jackson" voltammetric waveform for FSCAV and show highly stable, selective, and sensitive ambient serotonin measurements in vitro. In vivo, we report basal serotonin levels in the CA2 region of the hippocampus as 64.9 ± 2.3 nM (n = 15 mice, weighted average ± standard error). We electrochemically and pharmacologically verify the selectivity of the serotonin signal. Finally, we develop a statistical model that incorporates the uncertainty in in vivo measurements, in addition to electrode variability, to more critically analyze the time course of pharmacological data. Our novel method is a uniquely powerful analysis tool that can provide deeper insights into the mechanisms that control serotonin's extracellular levels.

Adaptive dynamics of the 5-HT systems following chronic administration of selective serotonin reuptake inhibitors: a meta-analysis

Selective serotonin reuptake inhibitors (SSRIs) are the most frequently prescribed antidepressants. However, a major concern is their delayed onset of action, which is hypothesized to be associated with the time required for serotonin (5-HT) autoreceptors to desensitize, which should be reflected by actual neurochemical changes. Numerous in vivo microdialysis studies have been published that report on 5-HT levels in different brain sites following SSRI administration. Here, we performed a meta-analysis on dynamic changes of 5-HT neurotransmission during the course of chronic SSRI treatment. We conducted a meta-analysis on research articles of 5-HT neurotransmission measured by in vivo microdialysis in rat brain after subchronic and chronic SSRI administrations. In total, data from 42 microdialysis studies (798 rats) were analyzed. Within the first week of SSRI treatment, extracellular 5-HT concentrations drop in frontal cortex. Over the next 2 weeks of treatment, a linear increase in extracellular 5-HT levels up to 350% of prior treatment baseline is evident (n = 269). However, in hippocampus, prefrontal cortex, nucleus accumbens, and ventral tegmental area we found increased 5-HT levels within the first 3 days of SSRI administration. The time course of 5-HT dynamics in frontal cortex is in line with the hypothesis that 5-HT autoreceptors desensitize over 2-3 weeks of SSRI treatment and thereby enhanced extracellular 5-HT levels ensue. Yet, in other regions we did not find evidence supporting the traditional autoreceptor-mediated feedback loops hypothesis and thus other neurobiological adaptation mechanisms may also play a role in the delayed onset of SSRI action.

5-HT(1B) autoreceptor regulation of serotonin transporter activity in synaptosomes

Serotonin-1B (5-HT(1B) ) autoreceptors are located in serotonin (5-HT) terminals, along with serotonin transporters (SERT), and play a critical role in autoregulation of serotonergic neurotransmission and are implicated in disorders of serotonergic function, particularly emotional regulation. SERT modulates serotonergic neurotransmission by high-affinity reuptake of 5-HT. Alterations in SERT activity are associated with increased risk for depression and anxiety. Several neurotransmitter receptors are known to regulate SERT K(m) and V(max) , and previous work suggests that 5-HT(1B) autoreceptors may regulate 5-HT reuptake, in addition to modulating 5-HT release and synthesis. We used rotating disk electrode voltammetry to investigate 5-HT(1B) autoreceptor regulation of SERT-mediated 5-HT uptake into synaptosomes. The selective 5-HT(1B) antagonist SB224289 decreased SERT activity in synaptosomes prepared from wild-type but not 5-HT(1B) knockout mice, whereas SERT uptake was enhanced after pretreatment with the selective 5-HT(1B) agonist CP94253. Furthermore, SERT activity varies as a function of 5-HT(1B) receptor expression-specifically, genetic deletion of 5-HT(1B) decreased SERT function, while viral-mediated overexpression of 5-HT(1B) autoreceptors in rat raphe neurons increased SERT activity in rat hippocampal synaptosomes. Considered collectively, these results provide evidence that 5-HT(1B) autoreceptors regulate SERT activity. Because SERT clearance rate varies as a function of 5-HT(1B) autoreceptor expression levels and is modulated by both activation and inhibition of 5-HT(1B) autoreceptors, this dynamic interaction may be an important mechanism of serotonin autoregulation with therapeutic implications.

Regulation of dorsal raphe nucleus function by serotonin autoreceptors: a behavioral perspective

Neurotransmission by serotonin (5-HT) is tightly regulated by several autoreceptors that fine-tune serotonergic neurotransmission through negative feedback inhibition at the cell bodies (predominantly 5-HT(1A)) or at the axon terminals (predominantly 5-HT(1B)); however, more subtle roles for 5-HT(1D) and 5-HT(2B) autoreceptors have also been detected. This review provides an overview of 5-HT autoreceptors, focusing on their contribution in animal behavioral models of stress and emotion. Experiments targeting 5-HT autoreceptors in awake, behaving animals have generally shown that increasing autoreceptor feedback is anxiolytic and rewarding, while enhanced 5-HT function is aversive and anxiogenic; however, the role of serotonergic activity in behavioral models of helplessness is more complex. The prevailing model suggests that 5-HT autoreceptors become desensitized in response to stress exposure and antidepressant administration, two seemingly opposite manipulations. Thus there are still unresolved questions regarding the role of these receptors-and serotonin in general-in normal and pathological states.

Fenfluramine-induced serotonin release decreases [11C]AZ10419369 binding to 5-HT1B-receptors in the primate brain

The need for positron emission tomography (PET)-radioligands that are sensitive to changes in endogenous serotonin (5-HT) levels in brain is recognized in experimental and clinical psychiatric research. We recently developed the novel PET radioligand [(11)C]AZ10419369 that is highly selective for the 5-HT(1B) receptor. In this PET-study in three cynomolgus monkeys, we examined the sensitivity of [(11)C]AZ10419369 to altered endogenous 5-HT levels. Fenfluramine-induced 5-HT release decreased radioligand binding in a dose-dependent fashion with a regional average of 27% after 1 mg/kg and 50% after 5 mg/kg. This preliminary study supports that [(11)C]AZ10419369 is sensitive to endogenous 5-HT levels in vivo and may serve as a tool to examine the pathophysiology and treatment of major psychiatric disorders.

Effect of triiodothyronine on 5-HT1A and 5-HT1B receptor expression in rat forebrain and on latency to feed in the novelty suppressed feeding test

Thyroid hormones, particularly triiodothyronine (T3), have long been used for the treatment of depression, most frequently to enhance the therapeutic activity of other antidepressants. The purpose of this study was to evaluate possible underlying mechanisms for the antidepressant activity of T3. The effects of T3 20 microg/kg/d S.C. and fluoxetine 5mg/kg/d I.P. given alone or in combination for 7 days on the transcription rates of inhibitory serotonergic receptors (5-HT1A and 5-HT1B) were studied in different brain areas of male Sabra rats using real-time PCR. Significant effects of fluoxetine were found on the expression of 5-HT1B receptors in the frontal cortex and of T3 on the expression of 5-HT1A receptors in the amygdala and hippocampus and 5-HT1B receptors in the frontal and entorhinal cortices, the expression being reduced in all cases. An effect of the combination of T3 plus fluoxetine to reduce transcription was observed for 5-HT1A receptors, in the amygdala and dentate gyrus and for 5-HT1B receptors in the entorhinal cortex and anterior raphe nucleus. In the second experiment, the novelty suppressed feeding test (NFST) was used to examine the effects of fluoxetine 5mg/kg/d I.P. and T3 20 or 50 microg/kg/d, alone or in combination for 12 days, on latency to feed. Only the combinations of T3 (20 or 50 microg/kg/d) and fluoxetine (5mg/kg/d) yielded significant behavioral effects in this test. The results of our studies suggest that the mechanism underlying the antidepressant effect of T3 may involve a reduction in 5-HT1A and 5-HT1B receptor transcription rates.

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.

Fluoxetine disrupts food intake and estrous cyclicity in Fischer female rats

Adult, regularly cycling female Fischer rats were injected daily with 10 mg/kg fluoxetine for 12-23 days. In the first experiment, body weight and vaginal smears were monitored daily. Fluoxetine treatment reduced body weight within the first 24 h of treatment. Fluoxetine treatment also elongated the estrous cycle, reduced blood levels of progesterone, and eliminated lordosis behavior. In the second experiment, body weight and food intake were examined and a pair-fed group was included to determine if fluoxetine-induced anorexia contributed to the disturbance of the estrous cycle. In pair-fed rats, effects similar to fluoxetine treatment were present. These results lead to the suggestion that fluoxetine's anorectic properties could disrupt the female's normal endocrine cyclicity and that this disruption could be relevant to the reduction in sexual behavior and motivation. However, when the duration of fluoxetine treatment was extended beyond 16 to 17 days, fluoxetine-treated female rats reinitiated vaginal cyclicity and showed evidence of normal sexual receptivity. In contrast, the estrous cycles of their pair-fed counterparts remained disrupted. Thus, restricted food intake appears to contribute to the disruption of the estrous cycle and elimination of sexual receptivity during fluoxetine treatment. However, compensatory changes in the serotonergic system that are associated with chronic fluoxetine administration may contribute to the gradual recovery of estrous cyclicity and sexual receptivity of the fluoxetine-treated animals.

Effects of triiodothyronine and fluoxetine on 5-HT1A and 5-HT1B autoreceptor activity in rat brain: regional differences

The thyroid hormone triiodothyronine (T3) augments and accelerates the effects of antidepressant drugs. Although the majority of studies showing this have used tricyclics, a few studies have shown similar effects with the selective serotonin re-uptake inhibitor (SSRI) fluoxetine. In this study we investigated the effects of fluoxetine (5 mg/kg), T3 (20 microg/kg) and the combination of these drugs, each administered daily for 7 days, on serotonergic function in the rat brain, using in vivo microdialysis. Fluoxetine alone induced a trend towards desensitization of 5-HT1A autoreceptors as shown by a reduction in the effect of 8-OH-DPAT to lower 5-HT levels in frontal cortex, and desensitized 5-HT1B autoreceptors in frontal cortex. The combination of fluoxetine and T3 induced desensitization of 5-HT1B autoreceptors in hypothalamus. Since there is evidence linking hypothalamic function and depression, we suggest that this effect may partly account for the therapeutic efficacy of the combination of an SSRI and T3.

Effects of triiodothyronine on 5-HT(1A) and 5-HT(1B) autoreceptor activity, and postsynaptic 5-HT(1A) receptor activity, in rat hypothalamus: lack of interaction with imipramine

Triiodothyronine (T3) is effective in both augmenting and accelerating the therapeutic response to antidepressant drugs, especially tricyclics, and there is evidence from both human and animal studies that it acts on serotonergic neurotransmission. In this work we examined the effects of T3 alone and together with imipramine on 5-HT levels in the hypothalamus and on 5-HT(1A) and 5-HT(1B) autoreceptor sensitivity, using in vivo microdialysis in the rat. The effects of T3 on postsynaptic 5-HT(1A) receptor activity in the hypothalamus were also determined using a neuroendocrine challenge procedure. T3 administered daily at 20 microg/kg s.c. for 2 weeks reduced the sensitivity of 5-HT(1A) autoreceptors which control 5-HT release, as measured by the effect of 8-OH-DPAT to decrease 5-HT in the hypothalamus, and also the sensitivity of hypothalamic 5-HT(1B) receptors as measured by the effect of the 5-HT(1B) receptor agonist CP 93129 to decrease 5-HT release. Imipramine at 10 mg/kg daily for 4 weeks by osmotic minipump reduced 5-HT(1A) autoreceptor activity, as measured by the effect of 8-OH-DPAT in the hypothalamus, but the combination of T3 and imipramine given for 2 weeks did not affect either 5-HT(1A) or 5-HT(1B) autoreceptor activity. T3 at 20 microg/kg s.c. given daily for 1 week also reduced the sensitivity of postsynaptic 5-HT(1A) receptors in the hypothalamus, as measured by injection of 8-OH-DPAT and determination of the plasma ACTH and corticosterone responses. Animals which received T3 for 7 days showed a dose-dependent reduction in plasma free T4 levels but no change in total T3 levels. We conclude that while T3 alone affects both presynaptic and postsynaptic components of the serotonergic system, these effects may not be responsible for the therapeutic acceleration action seen with a combination of a tricyclic drug and T3.

Increased expression of 5-HT1B receptor in dorsal raphe nucleus decreases fear-potentiated startle in a stress dependent manner

5-HT(1B) autoreceptors regulate serotonin release from terminals of dorsal raphe nucleus (DRN) projections. Due to postsynaptic 5-HT(1B) receptors in DRN terminal fields, it has not previously been possible to manipulate 5-HT(1B) autoreceptor activity without also changing 5-HT(1B) heteroreceptor activity. We have developed a viral gene transfer strategy to express epitope-tagged 5-HT(1B) and green fluorescent protein in vivo, allowing us to increase 5-HT(1B) expression in DRN neurons. We have shown that increased 5-HT(1B) autoreceptor expression reduced anxiety in unstressed animals but increased anxiety following inescapable stress. These findings suggest that effects of increased 5-HT(1B) autoreceptor expression are dependent on stress context. To better understand the mechanisms underlying these observations, we have used fear-potentiated startle (FPS). FPS is especially sensitive to the activity of the amygdala, which shares reciprocal connections with DRN. In the absence of an inescapable stressor, increased 5-HT(1B) autoreceptor expression attenuated FPS response compared with animals injected with a virus expressing only green fluorescent protein. Administration of the 5-HT(1B) antagonist SB224289 (5 mg/kg i.p.) before startle testing blocked the effects of increased 5-HT(1B) autoreceptor expression. Since SB224289 had no effect on FPS in the absence of viral gene transfer, these results suggest that the antagonist reversed the behavioral effects of increased 5-HT(1B) autoreceptor expression through blockade of transgenic receptors. When tested 24 h following water-restraint stress, animals with increased 5-HT(1B) autoreceptors demonstrated restoration of robust FPS response. These results extend our previous studies and suggest explanations for the complex relationship between 5-HT(1B) autoreceptor expression, stress, and anxiety behavior.

Chronic fluoxetine-induced desensitization of 5-HT1A and 5-HT1B autoreceptors: regional differences and effects of WAY-100635

Desensitization of 5-HT(1A) and 5-HT(1B) autoreceptors is thought to be the mechanism underlying the therapeutic effects of fluoxetine and other selective serotonin reuptake inhibitors when these are administered chronically. The blockade of 5-HT(1A) autoreceptors occurring on administration of a selective serotonin reuptake inhibitor together with a 5-HT(1A) autoreceptor antagonist is responsible for the acute increase in 5-hydroxytryptamine (serotonin, 5-HT) levels observed under these circumstances. The effects of repeated administration of selective serotonin reuptake inhibitors together with 5-HT(1A) receptor antagonists have not been widely studied. In this work, we found that the effects of fluoxetine (5 mg/kg, i.p., daily for 12 days) to desensitize 5-HT(1B) autoreceptors in the frontal cortex, as measured by the effect of the locally administered 5-HT(1B) receptor agonist, 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP 93129), and to desensitize 5-HT(1A) autoreceptors as measured by the action of the 5-HT(1A) receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT; 50 microg/kg, s.c.) to reduce 5-HT levels in cortex, were prevented by concomitant administration of the 5-HT(1A) receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide (WAY-100635; 0.3 mg/kg, s.c.). 5-HT(1B) receptor activity in the hypothalamus, as measured by the effects of locally administered CP 93129, and 5-HT(1A) autoreceptor activity, as determined by the effects of subcutaneous 8-OH-DPAT to reduce 5-HT levels in hypothalamus, were not altered either by fluoxetine alone or by fluoxetine in the presence of WAY-100635. The data suggest that the regulation of extracellular levels of 5-HT in the cortex and hypothalamus is subject to different autoregulatory mechanisms.

5-HT1B receptor mRNA levels in dorsal raphe nucleus: inverse association with anxiety behavior in the elevated plus maze

Serotonergic neurons in the dorsal raphe nucleus, the major source of forebrain serotonin projections, synthesize a terminal autoreceptor that inhibits serotonin release-the 5-HT(1B) autoreceptor. Overexpression of this autoreceptor is hypothesized to contribute to anxiety. Antidepressants decrease (while learned helplessness increases) 5-HT(1B) mRNA in dorsal raphe neurons, and viral-mediated overexpression of 5-HT(1B) here increases anxiety behavior after stress. However, 5-HT(1B) mRNA levels in dorsal raphe are substantially elevated in unstressed rats in two models of stress resistance. Thus, the role of dorsal raphe 5-HT(1B) autoreceptors in anxiety is complex. Therefore, we tested whether different stressors differentially affect dorsal raphe 5-HT(1B) mRNA [via in situ hybridization histochemistry] and anxiety behavior (using the elevated plus maze). Rats were assigned to a stressor (either forced swim, water restraint, dry restraint, or electric tail shock) or a control condition, then were tested and sacrificed 24 h later. Overall, controls exhibited less anxiety than stressed rats as indicated by a higher ratio of open arm to total arm entries (OTR). The stressors did not differentially affect the OTR, nor did any alter dorsal raphe 5-HT(1B) mRNA levels. There was, however, a significant positive correlation between the OTR and 5HT(1B) mRNA intensity in controls (r=.64; P=.006), but not in stressed rats (r=.16, P=.36), providing further evidence that elevated dorsal raphe 5-HT(1B) levels are associated with reduced anxiety in animals that have not been exposed to stress.

Effects of chronic paroxetine treatment on dialysate serotonin in 5-HT1B receptor knockout mice

The role of serotonin (5-HT)1B receptors in the mechanism of action of selective serotonin re-uptake inhibitors (SSRI) was studied by using intracerebral in vivo microdialysis in conscious, freely moving wild-type and 5-HT1B receptor knockout (KO 5-HT1B) mice in order to compare the effects of chronic administration of paroxetine via osmotic minipumps (1 mg per kg per day for 14 days) on extracellular 5-HT levels ([5-HT]ext) in the medial prefrontal cortex and ventral hippocampus. Basal [5-HT]ext values in the medial prefrontal cortex and ventral hippocampus, approximately 20 h after removing the minipump, were not altered by chronic paroxetine treatment in both genotypes. On day 15, in the ventral hippocampus, an acute paroxetine challenge (1 mg/kg i.p.) induced a larger increase in [5-HT]ext in saline-pretreated mutant than in wild-type mice. This difference between the two genotypes in the effect of the paroxetine challenge persisted following chronic paroxetine treatment. Conversely, in the medial prefrontal cortex, the paroxetine challenge increased [5-HT]ext similarly in saline-pretreated mice of both genotypes. Such a challenge produced a further increase in cortical [5-HT]ext compared with that in saline-pretreated groups of both genotypes, but no differences were found between genotypes following chronic treatment. To avoid the interaction with raphe 5-HT1A autoreceptors, 1 micro m paroxetine was perfused locally through the dialysis probe implanted in the ventral hippocampus; similar increases in hippocampal [5-HT]ext were found in acutely or chronically treated wild-type mice. Systemic administration of the mixed 5-HT1B/1D receptor antagonist GR 127935 (4 mg/kg) in chronically treated wild-type mice potentiated the effect of a paroxetine challenge dose on [5-HT]ext in the ventral hippocampus, whereas systemic administration of the selective 5-HT1A receptor antagonist WAY 100635 did not. By using the zero net flux method of quantitative microdialysis in the medial prefrontal cortex and ventral hippocampus of wild-type and KO 5-HT1B mice, we found that basal [5-HT]ext and the extraction fraction of 5-HT were similar in the medial prefrontal cortex and ventral hippocampus of both genotypes, suggesting that no compensatory response to the constitutive deletion of the 5-HT1B receptor involving changes in 5-HT uptake capacity occurred in vivo. As steady-state brain concentrations of paroxetine at day 14 were similar in both genotypes, it is unlikely that differences in the effects of a paroxetine challenge on hippocampal [5-HT]ext are due to alterations of the drug's pharmacokinetic properties in mutants. These data suggest that there are differences between the ventral hippocampus and medial prefrontal cortex in activation of terminal 5-HT1B autoreceptors and their role in regulating dialysate 5-HT levels. These presynaptic receptors retain their capacity to limit 5-HT release mainly in the ventral hippocampus following chronic paroxetine treatment in mice.

Effects of triiodothyronine and imipramine on basal 5-HT levels and 5-HT(1) autoreceptor activity in rat cortex

Clinical studies have shown that triiodothyronine (T3) both augments and accelerates the therapeutic response to antidepressant drugs, particularly tricyclics. There is evidence that this effect is mediated by the serotonergic system. We show here that T3 administered daily for 7 days over the range 0.02-0.5 mg/kg increases basal serotonin (5-hydroxytryptamine, 5-HT) levels, as measured by in vivo microdialysis in rat cortex, in a dose-dependent fashion. All the doses of T3 examined reduced 5-HT(1A) autoreceptor activity, as measured by the effect of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.05 mg/kg s.c.) to decrease 5-HT levels in frontal cortex. T3 administered daily for 14 days at 0.02 mg/kg also reduced 5-HT(1B) autoreceptor activity, as measured by the effect of locally administered 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP 93129, 10 microM) to decrease 5-HT levels. In animals administered imipramine (10 mg/kg/day by osmotic minipump) concurrently with T3 injections, no further changes in either 5-HT(1A) or 5-HT(1B) autoreceptor activity were seen. We suggest that the effect of T3 to accelerate the therapeutic actions of antidepressant drugs may be due to a combination of the actions of T3 at autoreceptors and the actions of the drugs at postsynaptic 5-HT(1A) receptors.

Effects of chronic antidepressants and electroconvulsive shock on serotonergic neurotransmission in the rat hypothalamus

The hypothalamus may play a critical role in the pathophysiology and treatment of depression. There are two main lines of evidence for this: firstly, many of its functions correspond to those altered in depression; and secondly, many hypothalamic functions are regulated by the serotonergic system, which is a common target of antidepressant treatments. In keeping with observations from other laboratories, we have found that chronic antidepressants and electroconvulsive shock increase serotonergic neurotransmission in the rat hypothalamus by inducing desensitization of presynaptic autoreceptors. We have also found that chronic hypercorticosolemia, which constitutes a model of depression, has an opposite effect. We postulate that presynaptic autoregulation of serotonergic neurotransmission in the hypothalamus may play a critical role in the pathophysiology and treatment of depression.

Overexpression of 5-HT1B receptor in dorsal raphe nucleus using Herpes Simplex Virus gene transfer increases anxiety behavior after inescapable stress

5-HT(1B) autoreceptors have been implicated in animal models of stress and are regulated selectively by serotonin-selective reuptake inhibitors such as fluoxetine. These terminal autoreceptors regulate serotonin release from dorsal raphe nucleus (DRN) projections throughout rat forebrain. However, it has not been previously possible to manipulate 5-HT(1B) autoreceptor activity selectively without also changing 5-HT(1B) activity in other neurons mediating different behavioral responses. Therefore, we have developed a viral-mediated gene transfer strategy to express hemagglutinin-tagged 5-HT(1B) and manipulate these autoreceptors in DRN. Green fluorescent protein (GFP) was coexpressed from a separate transcriptional unit on the same amplicon to assist in monitoring infection and expression. We confirmed the expression and biological activity of both transgenic proteins in vitro. When injected directly into DRN using stereotaxic procedure, HA-5-HT(1B) receptors were expressed in serotonergic neurons and translocated to the forebrain. The effect of DRN expression of HA-5-HT(1B) on stress-induced behaviors was compared with control rats that received GFP-only amplicons. There was no change in immobility in the forced swim test. However, HA-5-HT(1B) expression significantly reduced entrances into the central region of an open-field arena after water-restraint stress without altering overall locomotor activity, but not in the absence of stress exposure. HA-5-HT(1B) expression also reduced entries into the open arms of the elevated plus maze after water restraint. Because these tests are sensitive to increases in anxiety-like behavior, our results suggest that overactivity of 5-HT(1B) autoreceptors in DRN neurons may be an important mediator of pathological responses to stressful events.

Effects on serotonin in rat hypothalamus of D-fenfluramine, aminorex, phentermine and fluoxetine

Hypothalamic 5-HT (serotonin) regulates food intake, energy expenditure and bodyweight. Using in vivo microdialysis, we determined the effects of various anorectic drugs on hypothalamic extracellular 5-HT levels during the dark phase when rats predominantly feed. Phentermine and aminorex, which were originally considered to be catecholaminergic drugs, markedly increased 5-HT efflux in rat hypothalamus. Their actions were less profound than D-fenfluramine, but considerably greater than that of the selective 5-HT reuptake inhibitor, fluoxetine. This suggests that enhanced hypothalamic 5-HT function could be involved in their anorectic actions. Pharmacological characterization revealed that D-fenfluramine, aminorex and probably also phentermine potentiate synaptic 5-HT function predominantly by release, whereas fluoxetine acts exclusively by reuptake inhibition. The results also revealed that the combined actions of phentermine and D-fenfluramine on hypothalamic extracellular 5-HT levels were additive, but not synergistic. In contrast, there was a significant negative cooperative effect on extraneuronal 5-HT of combining phentermine with fluoxetine.

5-HT(1B) mrna regulation in two animal models of altered stress reactivity

BACKGROUND

Acute stress has profound effects on serotonergic activity, but it is not known whether alterations in the serotonin system can predispose individuals to exaggerated stress responses. We examined the regulation of 5-HT(1B) and 5-HT(1A) mRNA in two rodent models of differential sensitivity to stress: congenital learned helplessness (cLH) and handling and maternal separation (HMS).

METHODS

5-HT(1B) and 5-HT(1A) mRNAs in brain tissue sections were quantitated by in situ hybridization from control, stress-sensitive, and stress-resistant male rats in the HMS model and stress-sensitive and stress-resistant rats (both males and females) in the cLH model. Dorsal raphe nucleus, striatum, and hippocampus were examined.

RESULTS

The main result was that dorsal raphe 5-HT(1B) mRNA was substantially elevated (63-73%) in male rats in the stress-resistant group of both models compared with stress-sensitive animals. 5-HT(1B) mRNA in female rats did not differ between groups in the cLH model. There were no differences in 5-HT(1A) mRNA between HMS groups.

CONCLUSIONS

These findings suggest that 5-HT(1B) autoreceptor regulation is altered in animals with diminished stress reactivity. These results suggest that 5-HT(1B) autoreceptors in unstressed and acutely stressed animals differ, indicating the importance of state versus trait changes in serotonin function in animal models of anxiety and depression.

Effects of chronically administered venlafaxine on 5-HT receptor activity in rat hippocampus and hypothalamus

The effects of chronic administration of the mixed serotonin [5-hydroxytryptamine (5-HT)]/norepinephrine re-uptake inhibitor venlafaxine (5 mg/kg daily by osmotic minipump for 28 days) on the sensitivity of somatodendritic 5-HT(1A) autoreceptors on serotonergic neurons innervating the hypothalamus, and on 5-HT(1B) autoreceptors in both hypothalamus and hippocampus, were determined using in vivo microdialysis in freely moving rats. Venlafaxine induced a reduction in sensitivity of 5-HT(1B) autoreceptors in hypothalamus, but did not affect the sensitivity of 5-HT(1A) autoreceptors, or of 5-HT(1B) autoreceptors in hippocampus. The corticosterone and oxytocin responses to the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.05 or 0.2 mg/kg), a measure of postsynaptic 5-HT(1A) receptor activity in the hypothalamus, were reduced in animals administered 5 or 10 mg/kg venlafaxine daily by intraperitoneal injection for 21 days. This desensitization of post-synaptic 5- HT(1A) receptors in the hypothalamus may be a consequence of increased 5-HT levels induced by desensitization of the presynaptic 5-HT(1B) receptors. These results taken together with those of previous studies suggest that the hypothalamus might be an important site of drug action, and that venlafaxine has an overall mechanism similar to that of selective serotonin re-uptake inhibitors.

Functional effects of corticosterone on 5-HT(1A) and 5-HT(1B) receptor activity in rat brain: in vivo microdialysis studies

Glucocorticoid hormones are known to be elevated in depression, and to interact with serotonin 5-HT(1A) receptors at both the presynaptic and postsynaptic levels. Since one of the presumed mechanisms of action of antidepressant drugs is induction of changes in sensitivity of 5-HT(1A) and also 5-HT(1B) receptors, the effects of repeated administration of corticosterone (50 mg/kg s.c. b.i.d. for 10 days) on activities of these receptors were determined using in vivo microdialysis in freely moving rats. Presynaptic 5-HT(1A) receptor activity, as measured by the effect of a challenge dose (0.2 mg/kg s.c.) of the 5-HT(1A) agonist 8-hydroxy-2 (di-n-propylamino) tetralin (8-OH-DPAT) to reduce 5-HT levels in the hypothalamus, was not affected by corticosterone administration. Presynaptic 5-HT(1B) receptor activity, as measured by the effect of the 5-HT(1B) receptor antagonist (N-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazole-3-yl)[1,1'-biphenyl]-carboxamide (GR 127935) (5 mg/kg s.c.) to increase 5-HT levels, was increased in hypothalamus but not hippocampus of corticosterone-treated rats. Postsynaptic 5-HT(1A) receptor activity, as measured by the effect of 8-OH-DPAT to increase cyclic AMP levels in the hippocampus, was not affected by corticosterone administration. The decrease in presynaptic 5-HT(1B) receptor activity after chronic administration of antidepressant drugs complements the increases in 5-HT(1B) receptor number observed in animal models of depression.

Circadian variation in the activity of the 5-HT(1B) autoreceptor in the region of the suprachiasmatic nucleus, measured by microdialysis in the conscious freely-moving rat

Intracerebral microdialysis was used to examine the function of the terminal 5-hydroxytryptamine(1B) (5-HT(1B)) autoreceptor in the region of the suprachiasmatic nuclei (SCN) of freely moving conscious rats at six time points or zeitgeber times (ZTs) across the light:dark cycle. Infusion of the 5-HT(1A/1B) agonist 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridyl)-1H-indole (RU24969) (1 microM) via the microdialysis probe produced a decrease in 5-HT output when applied at ZTs 3, 6, 15 and 21 (69.8+/-11.9, 59+/-11.7, 43.9+/-17.2 and 45.7+/-17.0% respectively). At ZTs 9 and 18 RU24969 (1 microm) failed to affect the 5-HT output significantly (28.0+/-11 and 32.8+/-24.6% decrease respectively). The profile of inhibition of 5-HT output following infusion of RU24969 (1 microM) at ZT 6 was unaffected by concurrent infusion of the specific 5-HT(1A) antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xanecarboxamide trihydrochloride (WAY100635) (1 microM) (52.48+/-17.5% decrease). The data demonstrate a circadian rhythm in the activity of the 5-HT(1B) autoreceptor in the region of the SCN.

Effects of acute and chronic tianeptine administration on serotonin outflow in rats: comparison with paroxetine by using in vivo microdialysis

Using in vivo microdialysis, we compared the effects of tianeptine (an antidepressant drug which, in marked contrast with other antidepressants, is thought to increase the uptake of serotonin (5-hydroxytryptamine, 5-HT) on extracellular 5-HT concentrations ([5-HT](ext)) in the frontal cortex and raphe nuclei of freely moving rats with those of paroxetine, a potent selective serotonin reuptake inhibitor. A single paroxetine dose (1 mg/kg, i.p.) increased [5-HT](ext) over baseline in the frontal cortex and raphe nuclei, respectively. A single administration of tianeptine (10 mg/kg, i.p.) did not change [5-HT(ext)] in the two brain regions studied. Repeated exposure to paroxetine (0.5 mg/kg) b.i.d. for 14 days induced a sixfold significant increase in basal [5-HT](ext) in the raphe nuclei. Administration of tianeptine (5 mg/kg) b.i.d. for 14 days did not affect 5-HT baseline concentrations. In rats chronically treated with either paroxetine or tianeptine, drug challenge did not alter area under the curve values. Thus, our in vivo data indicate that tianeptine and paroxetine do not exert a similar in vivo effect on the serotonergic system in rat brain.

Chronic clomipramine alters presynaptic 5-HT(1B) and postsynaptic 5-HT(1A) receptor sensitivity in rat hypothalamus and hippocampus, respectively

Clomipramine is a tricyclic antidepressant drug with a high affinity for the serotonin (5-HT) uptake site or transporter. Electrophysiological experiments have provided evidence that repeated administration of clomipramine induces an increase in the sensitivity of postsynaptic 5-HT(1A) receptors in the hippocampus. We have studied the effects of clomipramine, administered to rats at a dose of 10mg/kg/day for 28 days by osmotic minipumps, on presynaptic 5-HT(1A) and 5-HT(1B) autoreceptors in the hypothalamus, and on postsynaptic 5-HT(1A) receptors in the hippocampus, by using in vivo microdialysis to measure 5-HT and cyclic adenosine monophosphate (cAMP) levels. Postsynaptic 5-HT(1A) receptor sensitivity in the hypothalamus was determined by means of a neuroendocrine challenge procedure. Although the sensitivity of presynaptic 5-HT(1A) autoreceptors, as measured by the effect of a subcutaneous (s.c.) injection of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.2mg/kg or 50 microg/kg) to reduce 5-HT levels, did not change, there was a reduction in sensitivity of presynaptic 5-HT(1B) receptors, as measured by the effect of an injection of the 5-HT(1B/1D) antagonist GR 127935 (5mg/kg, s.c.) to increase 5-HT levels. This effect probably accounted for the increase in basal 5-HT levels observed in the hypothalamus after chronic clomipramine administration. Postsynaptic 5-HT(1A) receptor sensitivity in the hippocampus, measured by the effect of 8-OH-DPAT to increase cAMP levels in the dialysate, was increased after chronic clomipramine. Animals that had received daily intraperitoneal injections of 10mg/kg clomipramine for 28 days did not show a change in postsynaptic 5-HT(1A) receptor sensitivity in the hypothalamus as measured by the ability of 8-OH-DPAT (50 microg/kg, s.c.) to stimulate secretion of corticosterone. Taken together with the results of previous experiments involving the cerebral cortex, these in vivo results show that chronic clomipramine exerts effects on both pre- and postsynaptic serotonin receptors, but that these effects are highly region-specific.

Subchronic fluoxetine administration to rats: effects on 5-HT autoreceptor activity as measured by in vivo microdialysis

Subchronic administration of fluoxetine to rats has been shown to induce subsensitivity of presynaptic 5-HT(1A) and 5-HT(1B) autoreceptors, and also postsynaptic 5-HT(1A) receptors in the hypothalamus. We investigated the effects of administration of fluoxetine (10 mg/kg i.p.) to rats for 6 days on presynaptic 5-HT(1A) receptor activity in the hypothalamus, postsynaptic 5-HT(1A) receptor activity in the hippocampus, and presynaptic 5-HT(1B) autoreceptor activity in both areas, using in vivo microdialysis. The effect of the 5-HT(1B/1D) antagonist (N-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-2'-methyl-4'-(5- methyl-1,2,4-oxadiazole-3-yl)[1,1'-biphenyl]-carboxamide (GR 127935) (5 mg/kg s.c.) to elevate 5-hydroxytryptamine (5-HT) levels was reduced in hippocampus but not hypothalamus of fluoxetine-treated rats. Fluoxetine did not alter either presynaptic 5-HT(1A) autoreceptor activity, as measured by the effect of injection of 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) (0.2 mg/kg or 50 microg/kg s.c.) on 5-HT levels in the hypothalamus, or postsynaptic 5-HT(1A) receptor activity, as measured by the effect of 8-OH-DPAT (0.2 mg/kg s.c.) on cyclic AMP accumulation, in the hippocampus.

Antidepressant-induced regulation of 5-HT(1b) mRNA in rat dorsal raphe nucleus reverses rapidly after drug discontinuation

Serotonin release from dorsal raphe projections in the forebrain is regulated by terminal 5-HT(1B) autoreceptors; dysregulation of these receptors may be involved in the pathophysiology of clinical depression. Using in situ hybridization, we have previously reported that fluoxetine reduces 5-HT(1B) mRNA in rat dorsal raphe nucleus (DRN) in a time-dependent and reversible manner. In this study we examined longer term treatment (8 weeks) with several different serotonin-selective reuptake inhibitors (SSRIs) or a tricyclic antidepressant on 5-HT(1B) mRNA regulation in DRN and hippocampus, and evaluated the stability of these drugs' effects after drug discontinuation. Fluoxetine (5 mg/kg/d), paroxetine (5 mg/kg/d), sertraline (10 mg/kg/d) or nortriptyline (10 mg/kg/d) was administered to rats via subcutaneous osmotic minipumps. Paroxetine and fluoxetine reduced DRN 5-HT(1B) mRNA by 36% and 27%, respectively whereas sertraline had a no significant effect. After 3-14 days of drug washout, DRN 5-HT(1B) mRNA levels in SSRI treated rats were no longer different from control. 5-HT(1B) mRNA levels in hippocampus were not affected by SSRI drugs at any timepoint. Nortriptyline had no significant effect on 5-HT(1B) mRNA in either DRN or hippocampus. These results confirm that SSRI antidepressants reduce presynaptic 5-HT(1B) mRNA selectively, and that this effect is maintained for at least 8 weeks of antidepressant treatment but reverses rapidly after discontinuation. Furthermore, it is possible that washout after chronic antidepressant treatment, that is routinely used in functional assays of autoreceptor action in animal models, may lead to more rapid reversal of biological effects than has previously been thought.

Effect of chronic paroxetine treatment on 5-HT1B and 5-HT1D autoreceptors in rat dorsal raphe nucleus

This study reports the effect of chronic paroxetine (10 mg/kg p.o., 21 days) on 5-HT1B and 5-HT1D autoreceptors controlling stimulated 5-HT efflux in slices of rat dorsal raphe nucleus. Electrically evoked 5-HT (10 pulses, 200 Hz, 0.1 ms, 10 mA) was measured using fast cyclic voltammetry. 5-HT efflux was inhibited by CP 93129 (10 nM-10 microM) and by sumatriptan (1 nM-1 microM) agonists at 5-HT1B and 5-HT1D receptors, respectively. Chronic paroxetine did not, initially, appear to alter the sensitivity of the 5-HT1B autoreceptors to CP 93129. However, when constructed in the presence of WAY 100635 (10 nM) the selective and silent 5-HT1A antagonist, there was a significant (P < 0.001) rightward shift of the CP 93129 concentration-response curve in the paroxetine-treated rats but not in the controls, implying a desensitisation of the 5-HT1B autoreceptor by paroxetine. Chronic paroxetine did not affect the sumatriptan concentration-response curve, even with WAY 100635 present, implying that there was no (de)sensitisation of the 5-HT1D autoreceptor. These data suggest that chronic paroxetine treatment may desensitise 5-HT1B autoreceptors in the dorsal raphe nucleus but that this effect is unmasked only when the dominant 5-HT1A autoreceptor control is antagonised.