Effect of acute and repeated administration of 5-HT1A receptor agonists on 5-HT release in rat brain in vivo

Differential Modulation of Dorsal Raphe Serotonergic Activity in Rat Brain by the Infralimbic and Prelimbic Cortices

The reciprocal connectivity between the medial prefrontal cortex (mPFC) and the dorsal raphe nucleus (DR) is involved in mood control and resilience to stress. The infralimbic subdivision (IL) of the mPFC is the rodent equivalent of the ventral anterior cingulate cortex, which is intimately related to the pathophysiology/treatment of major depressive disorder (MDD). Boosting excitatory neurotransmission in the IL-but not in the prelimbic cortex, PrL-evokes depressive-like or antidepressant-like behaviors in rodents, which are associated with changes in serotonergic (5-HT) neurotransmission. We therefore examined the control of 5-HT activity by both of the mPFC subdivisions in anesthetized rats. The electrical stimulation of IL and PrL at 0.9 Hz comparably inhibited 5-HT neurons (53% vs. 48%, respectively). However, stimulation at higher frequencies (10-20 Hz) revealed a greater proportion of 5-HT neurons sensitive to IL than to PrL stimulation (86% vs. 59%, at 20 Hz, respectively), together with a differential involvement of GABA_A (but not 5-HT_1A) receptors. Likewise, electrical and optogenetic stimulation of IL and PrL enhanced 5-HT release in DR in a frequency-dependent manner, with greater elevations after IL stimulation at 20 Hz. Hence, IL and PrL differentially control serotonergic activity, with an apparent superior role of IL, an observation that may help to clarify the brain circuits involved in MDD.

Pindolol Rescues Anxiety-Like Behavior and Neurogenic Maladaptations of Long-Term Binge Alcohol Intake in Mice

Long-term binge alcohol consumption alters the signaling of numerous neurotransmitters in the brain including noradrenaline (NE) and serotonin (5-HT). Alterations in the signaling of these neuronal pathways result in dysfunctional emotional states like anxiety and depression which are typically seen during alcohol withdrawal. Interestingly, studies have demonstrated that the development of alcohol-induced negative affective states is linked to disrupted neurogenesis in the dentate gyrus (DG) region of the hippocampus in alcohol-dependent animals. We have previously shown that modulation of NE and 5-HT activity by pharmacological targeting of β-adrenoreceptors (β-ARs) and 5-HT1A/1B receptors with pindolol reduces consumption in long-term alcohol-consuming mice. Since these receptors are also involved in emotional homeostasis and hippocampal neurogenesis, we investigated the effects of pindolol administration on emotional and neurogenic deficits in mice consuming long-term alcohol (18 weeks). We report that acute administration of pindolol (32 mg/kg) reduces anxiety-like behavior in mice at 24 h withdrawal in the marble-burying test (MBT) and the elevated plus-maze (EPM). We also show that chronic (2 weeks) pindolol treatment (32 mg/kg/day) attenuates alcohol-induced impairments in the density of immature neurons (DCX⁺) but not newborn cells (BrdU⁺) in the hippocampal DG. Pindolol treatment also restores the normal proportion of newborn proliferating cells (BrdU⁺/Ki67⁺/DCX⁻), newborn proliferating immature neurons (BrdU⁺/Ki67⁺/DCX⁺) and newborn non-proliferating immature neurons (BrdU⁺/Ki67⁻/DCX⁺) following long-term alcohol intake. These results suggest that pindolol, through its unique pharmacology may rescue some but not all deficits of long-term alcohol abuse on the brain, adding further value to its properties as a strong pharmaceutical option for alcohol use disorders (AUDs).

Intraperitoneal 8-OH-DPAT reduces competition from contextual but not discrete conditioning cues

The effects of the serotonergic (5-hydroxytryptamine, 5-HT) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.2 and 0.4 mg/kg i.p.) were examined in trace conditioning (Experiment 1) and overshadowing (Experiment 2) procedures. Both experiments used a fear conditioning procedure conducted off-the-baseline in water deprived male Wistar rats. 8-OH-DPAT was administered during conditioning and its effects were examined drug free as the suppression of an established licking response, both upon re-exposure to the cues provided by the conditioning chambers and upon presentation of experimental stimuli. There were no statistically significant effects of 8-OH-DPAT on conditioning to the discrete cue provided by a 5 s conditioned stimulus (CS), irrespective of the length of the trace interval used in Experiment 1, and irrespective of whether the CS took the form of a light alone, or a noise plus light compound in the Experiment 2 overshadowing procedure. The successful demonstration of overshadowing required the use of a second conditioning session which allowed further evaluation of the effects of 8-OH-DPAT in that neither a weak nor a strong overshadowing effect was modulated by either drug dose. Nonetheless conditioning to contextual cues was attenuated by treatment with 8-OH-DPAT at the 30 s trace interval. We therefore conclude that 8-OH-DPAT reduces competition from contextual but not discrete conditioning cues. This pattern of results lends further support to the view that contextual cue conditioning and discrete cue conditioning are modulated by different neuropharmacological mechanisms.

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8-OH-DPAT (0.2 and 0.4 mg/kg i.p.) was tested in two fear conditioning procedures.

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8-OH-DPAT reduced conditioning to contextual cues at a 30 s trace.

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However, overshadowing produced by presentation of a compound cue was unaffected by 8-OH-DPAT.

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8-OH-DPAT reduced competition from contextual but not discrete conditioning cues.

Effects of Estrogens on Central Nervous System Neurotransmission: Implications for Sex Differences in Mental Disorders

Nearly one of every five US individuals aged 12 years old or older lives with certain types of mental disorders. Men are more likely to use various types of substances, while women tend to be more susceptible to mood disorders, addiction, and eating disorders, all of which are risks associated with suicidal attempts. Fundamental sex differences exist in multiple aspects of the functions and activities of neurotransmitter-mediated neural circuits in the central nervous system (CNS). Dysregulation of these neural circuits leads to various types of mental disorders. The potential mechanisms of sex differences in the CNS neural circuitry regulating mood, reward, and motivation are only beginning to be understood, although they have been largely attributed to the effects of sex hormones on CNS neurotransmission pathways. Understanding this topic is important for developing prevention and treatment of mental disorders that should be tailored differently for men and women. Studies using animal models have provided important insights into pathogenesis, mechanisms, and new therapeutic approaches of human diseases, but some concerns remain to be addressed. The purpose of this chapter is to integrate human and animal studies involving the effects of the sex hormones, estrogens, on CNS neurotransmission, reward processing, and associated mental disorders. We provide an overview of existing evidence for the physiological, behavioral, cellular, and molecular actions of estrogens in the context of controlling neurotransmission in the CNS circuits regulating mood, reward, and motivation and discuss related pathology that leads to mental disorders.

Chronic treatment with tandospirone, a 5-HT(1A) receptor partial agonist, suppresses footshock stress-induced lactate production in the prefrontal cortex of rats

Serotonin 1A receptor (5-HT1A-R) agonists have been demonstrated to elicit antidepressant and anxiolytic effects. Lactate has been considered to play a major role in energy metabolism in the brain. Specifically, extracellular lactate concentrations (eLAC) have been suggested to reflect neural activity. Mild physical (e.g., handling) and non-physical (e.g., psychological) stressors have been shown to increase eLAC in several brain regions, including the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA). Using in vivo microdialysis technique, we measured eLAC in the mPFC and BLA of rats under electric footshock stress to clarify the effect of repeated injection procedure (saline, once daily for 14 days) as a stressor on brain energy metabolism. Then, we examined the effect of chronic treatment with tandospirone, a 5-HT1A-R partial agonist, on eLAC during footshock stress in the mPFC. Footshock stress led to an increase in eLAC both in the mPFC and BLA in rats without injections. Repeated saline injection increased basal eLAC in the BLA, while footshock-induced lactate increment was reduced. In the mPFC, repeated saline injection did not affect basal eLAC and footshock-induced eLAC increments. Chronic treatment with tandospirone, at 0.2 and 1.0 mg/kg/day, but not 2.0 mg/kg/day, attenuated footshock stress-induced eLAC elevation in the mPFC. These observations suggest that eLAC in the BLA is sensitive to repeated exposure to physical stress. Data also indicate chronic treatment with tandospirone diminishes acute energy demands during neural activation in the mPFC. The implications of the present findings in relation to clinical efficacy of 5-HT1A agonists are discussed.

Efficacy of buspirone for attenuating cocaine and methamphetamine reinstatement in rats

BACKGROUND

There are no approved pharmacotherapies for preventing psychomotor stimulant relapse. The operant reinstatement model has been suggested as a screen for identifying candidate medications. The present study examined if the anxiolytic buspirone could attenuate reinstatement of extinguished responding in Long-Evans rats that previously self-administered intravenous cocaine or methamphetamine.

METHODS

Rats were trained in 2-h daily sessions to self-administer 0.5mg/kg cocaine or 0.1mg/kg methamphetamine infusions followed by 12 days of instrumental extinction. Reinstatement was evoked by 17mg/kg i.p. cocaine primes or response-contingent cocaine-paired cues in cocaine-reinforced rats, and by 1mg/kg i.p. methamphetamine primes or response-contingent methamphetamine-paired cues in methamphetamine-reinforced rats.

RESULTS

Buspirone (1 and 3mg/kg) significantly (p<0.05) attenuated cocaine cue but not cocaine prime reinstatement. Buspirone (1 and 3mg/kg) also significantly attenuated methamphetamine cue reinstatement. Buspirone (3mg/kg) significantly attenuated methamphetamine prime reinstatement. During all reinstatement tests, 3mg/kg buspirone reduced levels of inactive lever pressing relative to those of vehicle, significantly so during the cocaine cue-induced reinstatement tests.

CONCLUSIONS

Given the complexity of buspirone's neuropharmacology consisting of serotonin 5-HT1A receptor partial agonist activity, and dopamine D2, D3 and D4 receptor antagonist effects, it is uncertain which of these activities or their combination is responsible for the present results. Overall, these results suggest that buspirone may reduce the likelihood of relapse to cocaine and methamphetamine use under some conditions, although this speculation must be interpreted with caution given buspirone's similar potency to attenuate inactive-lever responding.

Effects of citalopram on serotonin and CRF systems in the midbrain of primates with differences in stress sensitivity

This chapter reviews the neurobiological effects of stress sensitivity and s-citalpram (CIT) treatment observed in our nonhuman primate model of functional hypothalamic amenorrhea (FHA). This type of infertility, also known as stress-induced amenorrhea, is exhibited by cynomolgus macaques. In small populations, some individuals are stress-sensitive (SS) and others are highly stress-resilient (HSR). The SS macaques have suboptimal secretion of estrogen and progesterone during normal menstrual cycles. SS monkeys also have decreased serotonin gene expression and increased CRF expression compared to HSR monkeys. Recently, we found that CIT treatment improved ovarian steroid secretion in SS monkeys, but had no effect in HSR monkeys. Examination of the serotonin system revealed that SS monkeys had significantly lower Fev (fifth Ewing variant, rodent Pet1), TPH2 (tryptophan hydroxylase 2), 5HT1A autoreceptor and SERT (serotonin reuptake transporter) expression in the dorsal raphe than SR monkeys. However, CIT did not alter the expression of either Fev, TPH2, SERT or 5HT1A mRNAs. In contrast, SS monkeys tended to have a higher density of CRF fiber innervation of the dorsal raphe than HSR monkeys, and CIT significantly decreased the CRF fiber density in SS animals. In addition, CIT increased CRF-R2 gene expression in the dorsal raphe. We speculate that in a 15-week time frame, the therapeutic effect of S-citalopram may be achieved through a mechanism involving extracellular serotonin inhibition of CRF and stimulation of CRF-R2, rather than alteration of serotonin-related gene expression.

Repeated cortisol administration attenuates the EEG response to buspirone in healthy volunteers: evidence for desensitization of the 5-HT1A autoreceptor

It has previously been postulated that the therapeutic effect of antidepressants, particularly selective serotonin re-uptake inhibitors (SSRIs), is mediated by a down-regulation of somatodendritic (presynaptic) 5-HT(1A) autoreceptors with chronic treatment. Animal studies have revealed that repeated administration of corticosteroids similarly down-regulate this receptor. However, it has previously been difficult to explore if this receptor is similarly modulated in man in vivo. The objective of this study was to explore the effect of repeated administration of cortisol to healthy volunteers utilising a novel putative index of somatodendritic 5-HT(1A) autoreceptor function. This method involves the administration of the 5-HT(1A) agonist buspirone and observing the subsequent negative shift in the frequency spectrum of the electroencephalogram (EEG). Healthy male volunteers were treated with cortisol 20 mg, or placebo, orally twice daily for 7 days in a double-blind random-order crossover study. After each treatment period volunteers were administered buspirone 30 mg orally prior to EEG recordings. Following a week's treatment with placebo, buspirone led to a negative shift in the EEG frequency spectrum as previously reported. However, following treatment with cortisol, the effect of buspirone was significantly attenuated. This is consistent with corticosteroids having a similar effect on somatodendritic 5-HT(1A) autoreceptors in man as seen in rodents.

Acute flesinoxan treatment produces a different effect on rat brain serotonin synthesis than chronic treatment: an alpha-methyl-l-tryptophan autoradiographic study

5-HT(1A) receptor agonists display anxiolytic and anti-depressant properties in clinical studies. In this study, we used the alpha-[(14)C]methyl-l-tryptophan (alpha-MTrp) autoradiographic method to evaluate the effects of the 5-HT(1A) agonist, flesinoxan, on regional 5-HT synthesis in the rat brain, following acute or a 14-day continuous treatment. In the first series of experiments, flesinoxan (5mg/kg; i.p.) was administered 40min before the alpha-MTrp. It resulted in a significant increase of the arterial blood oxygen partial pressure (pO(2)) and a reduction of the regional rate of 5-HT synthesis throughout the brain, with the exception of a few regions (medial geniculate body and thalamus). In the second series of experiments, flesinoxan (5mg/kgday) was administered for 14 days, using an osmotic minipump implanted subcutaneously. When compared to rats treated with saline, there was an overall significant (p<0.05) reduction in the synthesis (one-sample two-tailed t-test). However, there was no significant influence on the 5-HT synthesis rate in the dorsal and median raphe nuclei and the majority of their projection areas. A significant (p<0.05) reduction was observed in the nucleus raphe magnus, medial caudate, ventral thalamus, amygdala, ventral tegmental area, medial forebrain bundle, nucleus accumbens, medial anterior olfactory nucleus and superior olive. The unaltered 5-HT synthesis rates in a large majority of regions following the 14-day treatment of flesinoxan may reflect the normalization (implies to not be different from salne treated control) of synthesis due to a desensitization of 5-HT(1A) autoreceptors on the cell body of 5-HT neurons as well as at postsynaptic sites, which is known to occur following long-term treatment with 5-HT(1A) agonists. It is of some importance to note that the normalization of the synthesis occurred in the majority of the brain limbic structures, the brain areas implicated in affective disorders and the corresponding successful treatments, as well as in the cortical regions, which are implicated in mood. However, there were some terminal regions (e.g., accumbens, anterior olfactory, lateral thalamus, raphe magnus and obscurus) in which the chronic flesinoxan treatment resulted in a significant reduction of synthesis, suggesting that there was not a full desensitization across the brain of the receptors controlling 5-HT synthesis.

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.

TDIQ (5,6,7,8-tetrahydro-1,3-dioxolo[4,5-g]isoquinoline) exhibits anxiolytic-like activity in a marble-burying assay in mice

Numerous studies have suggested that the central alpha2-adrenergic receptor system may exert an important role in some types of human anxiety. The anxiolytic-like activity and potential side effect-like activities of the novel and purported alpha2-adrenergic compound TDIQ (5,6,7,8-1,3-dioxolo[4,5-g]isoquinoline) were compared to those of the anxiolytic drugs diazepam and buspirone, and the nonselective alpha2-adrenergic agent clonidine. Anxiolytic-like behavior was assessed in an object (marble)-burying assay, a selective test for the evaluation of known anxiolytics and identification of putative antianxiety compounds, that used mice housed either alone or in groups (5/cage). The rodents' antianxiety-like effect was defined as dose-related increases in the number of marbles that remained uncovered in their bedding material without concomitant disruption of their motor activities. Rotarod and inclined screen procedures were employed as potential indicators of side effects. An additional test monitored the heart rate (HR) and blood pressure (BP) of mice after the intravenous (IV) administration of doses of TDIQ. The reference compounds inhibited marble-burying behavior in a dose-related manner and produced various degrees of impairment in the side effect tests. TDIQ also inhibited object burying and displayed a wide separation between doses that produced anxiolytic-like activity and doses that produced some, if any, disruption of coordinated movement and/or motor activity. Moreover, the IV administration of TDIQ, up to 10 mg/kg, produced negligible effects on the HR and BP of mice. TDIQ could be a lead candidate for a new type of structural compound in the treatment of certain forms of anxiety.

Role of 5-HT(1A) receptors in the modulation of stress-induced lactate metabolism in the medial prefrontal cortex and basolateral amygdala

RATIONALE

Lactate has been shown to play a significant role in energy metabolism and reflect neural activity in the brain.

OBJECTIVES

Using in vivo microdialysis technique, we measured extracellular lactate concentrations in the medial prefrontal cortex (mPFC) and the basolateral amygdaloid (BLA) nucleus of rats under electric foot shock stress. Moreover, to examine the role of serotonin (5-HT)(1A) receptors in brain energy metabolism in response to stressors, we attempted to determine whether the stress-induced changes of extracellular lactate levels in the mPFC and BLA are attenuated by tandospirone, a partial agonist at 5-HT(1A) receptors, or perospirone, a novel atypical antipsychotic with a 5-HT(1A) receptor partial agonist and 5-HT(2A)/dopamine-D(2) antagonist property.

RESULTS

Foot shock stress led to an increase in extracellular lactate concentrations both in the mPFC and BLA. Tandospirone (2 mg/kg) attenuated the foot shock stress-induced increase of extracellular lactate concentrations in both of the brain regions, which was blocked by pretreatment with WAY-100635, a selective 5-HT(1A) antagonist. On the other hand, perospirone (0.3 mg/kg) attenuated the increment of extracellular lactate concentrations in the mPFC and BLA, which was unaltered by pretreatment with WAY-100635.

CONCLUSIONS

These results indicate that the foot shock stress-induced increase in lactate metabolism is partly regulated by 5-HT(1A) receptors both in cortical and limbic regions.

Effects of 8-OH-DPAT on hippocampal NADH fluorescence in vivo in anaesthetized rats

Systemic administration of the 5-HT1A receptor agonist 8-OH-DPAT modifies 5-HT neuronal transmission via stimulation of presynaptic and postsynaptic receptors. Compared to the effects of presynaptic receptor stimulation, there are less data on the effects of postsynaptic 5-HT1A receptors and the net effects of a stimulation of pre- and postsynaptic 5-HT1A receptors available. We measured the neuronal activity in the rat hippocampus after systemic treatment with 8-OH-DPAT in doses (30-300 microg/kg) known to reduce 5-HT release and anxiety-like behavior in rodents. Neuronal activity was assessed by laser-induced fluorescence spectroscopy determining changes in nicotinamide adenine dinucleotide (NADH) fluorescence in the ventral hippocampus of anaesthetized rats in vivo. NADH, a co-substrate for energy transfer in the respiratory chain, mirrors mitochondrial activity. Increased NADH fluorescence signals lower consumption of NADH caused by neuronal inhibition. 8-OH-DPAT in a dose of 300 microg/kg, but not 100 microg/kg and 30 microg/kg, increased NADH fluorescence by maximal +27 +/- 3.5%, suggesting a decreased neuronal activity in the ventral hippocampus. The selective 5-HT1A antagonist WAY-100635 (3 mg/kg) prevented the increased NADH fluorescence after 8-OH-DPAT, but had no own effect. The results show that systemic administration of the 5-HT1A agonist 8-OH-DPAT dose-dependently affects neuronal activity in the ventral hippocampus. The dose of 300 microg/kg seemingly activates presynaptic and postsynaptic receptors with dominating inhibitory postsynaptic effects.

Effects of repetitive transcranial magnetic stimulation on behavioral and neurochemical changes in rats during an elevated plus-maze test

In transcranial magnetic stimulation (TMS) the regional electrical activity in the brain is influenced by a pulsed magnetic field. The rapidly changed magnetic field produces electrical currents that activate neurons. Repetitive TMS (rTMS) treatment can cause functional changes in the cortex. The present study clarified the effects of rTMS treatment on behavioral changes in rats, focusing on anxiety by using an elevated plus-maze (plus-maze) test. The effects of rTMS treatment on neurochemical changes during the plus-maze test were investigated by determining the extracellular levels of serotonin (5-HT) and dopamine (DA) in the prefrontal cortex by using in vivo microdialysis. Each rat received rTMS of the frontal brain for 3 days, during which 125 stimuli from five trains in a day were applied at 25 Hz for 1 s with 2-min intervals between trains. Three-day series of (chronic) rTMS treatment caused significant increases in the time spent in open arms and the number of entries into open arms of the plus-maze compared with non-treated and sham-treated rats, which were not observed in 1-day series of (acute) rTMS treatment. Chronic rTMS treatment suppressed the increases in 5-HT levels induced by the plus-maze test, but did not influence the elicited DA levels. These data suggest that chronic rTMS treatment of the frontal brain has anxiolytic effects in rats, which are related to the 5-HTergic neuronal system.

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.

EEG effects of buspirone and pindolol: a method of examining 5-HT1A receptor function in humans

RATIONALE

An involvement of 5-HT(1A) receptors is postulated in the pathophysiology of affective disorders and mechanism of action of antidepressants. Methods for studying their functional integrity in humans are, however, limited. Preliminary data suggests that activation of somatodendritic 5-HT(1A) receptors cause a negative shift in the EEG frequency spectrum. Animal research suggests that pindolol is an agonist at these receptors but an antagonist at postsynaptic 5-HT(1A) receptors.

OBJECTIVE

We postulated that while pindolol would antagonise known postsynaptic mediated neuroendocrine responses to the 5-HT(1A) agonist buspirone, both drugs would have a similar effect on the EEG frequency spectrum.

METHODS

Fourteen healthy men were administered placebo or pindolol (20 mg orally) 90 min before placebo or buspirone (30 mg orally) in a double blind cross-over study. Plasma prolactin and growth hormone were assayed and EEGs recorded before and after drug administration.

RESULTS

A significant negative shift in the EEG frequency spectrum was found for both buspirone and pindolol, with the combination producing a similar effect to each drug alone. In contrast, the neuroendocrine response to buspirone was significantly attenuated by pindolol.

CONCLUSIONS

The data obtained are consistent with the EEG effects of buspirone and pindolol being mediated by somatodendritic 5-HT(1A) receptors, in contrast to the neuroendocrine response, which is known to be mediated by postsynaptic receptors. The development of this novel method of assessing somatodendritic 5-HT(1A) receptors in humans is a potentially important advance which may allow the testing of hypotheses of its involvement in depression and response to antidepressants.

Is there a role for 5-HT1A agonists in the treatment of depression?

The role of serotonin (5-hydroxytryptamine; 5-HT) in the treatment of depressive and anxiety disorders is underscored by the therapeutic action of selective 5-HT reuptake inhibitors acting to enhance the degree of activation of various 5-HT receptor subtypes. The 5-HT1A receptors are particularly relevant to the antidepressant and anxiolytic responses in human beings. They are located presynaptically in the raphe nuclei, where they act as cell body autoreceptors to inhibit the firing rate of 5-HT neurons, and are located postsynaptically in limbic and cortical regions, where they also attenuate firing activity. The azapirones are full agonists at 5-HT1A autoreceptors and are generally, but not exclusively, partial agonists at postsynaptic 5-HT1A receptors. Some of these drugs, including gepirone and other 5-HT1A agonists such as buspirone, have been reported to exert anxiolytic and antidepressive activity in double-blind, placebo-controlled, and comparative trials. Their delayed therapeutic activity is believed to result from increased activation of postsynaptic 5-HT1A receptors occurring only after 5-HT neurons regain their normal firing activity. The recovery of this parameter, which is attributable to 5-HT1A autoreceptor desensitization, also restores 5-HT release. At this point, the summed effects of a normalized level of synaptic 5-HT and the exogenous 5-HT1A agonist can be exerted on postsynaptic 5-HT1A receptors. The widespread recognition of the clinical efficacy of such agents has largely been hampered by their undesirable pharmacokinetic properties. Most 5-HT1A agonists are indeed readily absorbed but are also rapidly eliminated, thereby often producing either suboptimal therapeutic responses at low doses, or cumbersome adverse effects at higher doses. Extended-release formulations allow once-daily dosing regimens, thus avoiding sharp peak plasma concentrations. This improves compliance and permits the use of higher dosages, which may be associated with enhanced efficacy and better tolerability relative to the immediate-release formulations. In sum, 5-HT1A receptor agonism represents a valuable and efficacious therapeutic approach to major depression.

Diverse actions of ovarian steroids in the serotonin neural system

All of the serotonin-producing neurons of the mammalian brain are located in 10 nuclei in the mid- and hindbrain regions. The cells of the rostal nuclei project to almost every area of the forebrain and regulate diverse neural processes from higher order functions in the prefrontal cortex such as integrative cognition and memory, to limbic system control of arousal and mood, to diencephalic functions such as pituitary hormone secretion, satiety, and sexual behavior. The more caudal serotonin neurons project to the spinal cord and interact with numerous autonomic and sensory systems. All of these neural functions are sensitive to the presence or absence of the ovarian hormones, estrogen and progesterone. We have shown that serotonin neurons in nonhuman primates contain estrogen receptor beta and progestin receptors. Thus, they are targets for ovarian steroids which in turn modify gene expression. Any change in serotoninergic neural function could be manifested by a change in any of the projection target systems and in this manner, serotonin neurons integrate steroid hormone information and partially transduce their action in the CNS. This article reviews the work conducted in this laboratory on the actions of estrogens and progestins in the serotonin neural system of nonhuman primates. Comparisons to results obtained in other laboratory animal models are made when available and limited clinical data are referenced. The ability of estrogens and progestins to alter the function of the serotonin neural system at various levels provides a cellular mechanism whereby ovarian hormones can impact cognition, mood or arousal, hormone secretion, pain, and other neural circuits.

Pindolol augmentation of antidepressant treatment: recent contributions from brain imaging studies

Preclinical studies suggest that augmentation of selective serotonin (5-HT) reuptake inhibitors by the 5-HT(1A) receptor agent pindolol might reduce the delay between initiation of treatment and antidepressant response, an effect largely mediated by blockade of 5-HT(1A) autoreceptors in the dorsal raphe nuclei. Although some controlled clinical trials suggest that pindolol might reduce latency to selective serotonin reuptake inhibitor response in acute depressive episodes, the effect is moderate and highly variable. Recent positron emission tomography studies investigating the occupancy of 5-HT(1A) receptors in humans by pindolol have shown that at the dose used most often in clinical trials the occupancy is low and variable, which might explain the inconsistent clinical results. Positron emission tomography studies also suggest that pindolol might be more potent at blocking 5-HT(1A) autoreceptors than at blocking postsynaptic receptors, a property that may be useful in this pharmacologic strategy. Thus, the positron emission tomography data support the potential of pindolol to augment the antidepressant response of selective serotonin reuptake inhibitors, but also imply that this potential has not been fully evaluated. Here we review the clinical trials, the positron emission tomography studies, and the possible mechanisms of pindolol augmentation. It is also suggested that positron emission tomography may be used to define therapeutic dosing early on in the process of clinical evaluation of new treatment strategies.

Study of mechanisms of calcitonin analgesia in mice. Involvement of 5-HT3 receptors

The analgesic effect of calcitonin when serotonin (5-HT) concentration is increased and the involvement of some 5-HT receptors were studied using the writhing test in mice. 5-hydroxytryptophan (5-HTP) administration increased both 5-HT levels in the central nervous system (CNS) and calcitonin analgesia. The 5-HT(1A) agonist (+/-)-8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT) diminished calcitonin analgesia, this effect being antagonised by the 5-HT(1A) antagonist (WAY 100, 135). As the stimulation of 5-HT(1A) autoreceptors reduces the turnover of 5-HT, the effect of 8-OH-DPAT on calcitonin analgesia may be attributed to this decrease. The 5-HT(2A-2C) agonist (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI) diminished calcitonin analgesia. A sub-analgesic dose of the 5-HT(2A) antagonist ketanserin failed to prevent this effect. The 5-HT(3) agonist (+/-)-2-methyl-5-hydroxytryptamine maleate (2-methyl-5-HT) potentiated calcitonin analgesia, whereas it was significantly reduced by the 5-HT(3) antagonist tropisetron. The effect of 2-methyl-5-HT on calcitonin analgesia was also reversed by tropisetron, This result suggests that the 5-HT(3) receptor may play an important role in the relationship between calcitonin and the serotonergic system. Tropisetron also reversed the analgesia induced by calcitonin plus 5-HTP corroborating importance of the 5-HT(3) receptors.

Displacement of the binding of 5-HT(1A) receptor ligands to pre- and postsynaptic receptors by (-)pindolol. A comparative study in rodent, primate and human brain

Using receptor autoradiography we examined the displacement of the binding of [(3)H]8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and [(3)H][N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridyl)cy clohexanecarboxamide. 3HCl] (WAY 100635) to 5-HT(1A) receptors by (-)pindolol in the brain of four different species, rat, guinea pig, monkey and human. (-)Pindolol completely displaced the binding of both tritiated ligands at 10(-6) M in all species and regions examined. The affinity of (-)pindolol for presynaptic 5-HT(1A) receptors in the dorsal raphe nucleus was similar to that observed in postsynaptic locations, such as hippocampus (areas CA1, CA3 and dentate gyrus) or entorhinal cortex. Affinity values (K(i)) were in the range 3.8 - 15.9 nM for [(3)H]8-OH-DPAT and 5.8 - 22.3 nM for [(3)H]WAY 100635. In human brain, the K(i) values using [(3)H]8-OH-DPAT as ligand were 10.8 nM in the dorsal raphe nucleus and 6.5 - 13.5 in postsynaptic sites. The present data do not support the hypothesis that (-)pindolol may displace 5-HT(1A) ligands preferentially from presynaptic 5-HT(1A) receptors in the dorsal raphe nucleus, as suggested by electrophysiological evidence. The affinity of (-)pindolol for human 5-HT(1A) receptors is below the mean plasma concentration attained in depressed patients treated with a combination of fluoxetine and pindolol, which indirectly supports an action of pindolol at 5-HT(1A) receptors in these patients.

Effects of acute and chronic administration of the serotonin1A agonist buspirone on serotonin synthesis in the rat brain

The effects of acute and chronic administration of buspirone, a serotonin 5-HT1A agonist, on the 5-HT synthesis rates in various rat brain structures were investigated using alpha-[14C]methyl-L-tryptophan (alpha-[14C]MTrp) and an autoradiographic method. In the acute treatment study, buspirone (10 mg/kg) was injected subcutaneously 30 min before alpha-[14C]MTrp administration (30 microCi over 2 min) into a femoral vein. In the chronic treatment study, buspirone was given in a sustained fashion (10 mg/kg/day) for 14 days using an osmotic minipump implanted subcutaneously. Rats were killed 60 and 150 min after alpha-[14C]MTrp administration (two-time point method). A single dose of buspirone induced a significant decrease of 5-HT synthesis throughout the brain with the exception of the pineal body. However, the chronic treatment with buspirone did not induce significant differences in 5-HT synthesis in the brain. There was no significant difference in plasma free tryptophan concentration between any of the groups. The unaltered 5-HT synthesis rates in the chronic treatment study likely reflect a normalization of this parameter due to a desensitization of 5-HT1A autoreceptors on the cell body of 5-HT neurons, which has been previously shown to occur following long-term treatment with 5-HT1A agonists.

In vitro characterisation of the muscarinic receptor partial agonist, sabcomeline, in rat cortical and heart membranes

We have investigated the pharmacology of the functionally selective muscarinic M1 receptor partial agonist, sabcomeline [SB-202026 (R-(Z)-(+)-alpha-(methoxyamino)-1-azabicyclo[2.2.2] octane-3-acetonitrile)], in rat cortex and heart using radioligand binding and functional studies. The Quinuclidinyl benzilate/Oxotremorine-M acetate ratio from radioligand binding studies suggested that sabcomeline and xanomeline [3(3-hexyloxy-1,25-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-met hylpyridine] are muscarinic receptor partial agonists in cortical and heart membranes. In [35S]GTPgammaS binding studies in rat cortex, carbachol stimulated binding via muscarinic M2/M4 receptors which could be blocked by sabcomeline with a pA2 of 7.2. In rat heart membranes, carbachol also stimulated [35S]GTPgammaS binding studies through muscarinic M2 receptors. Sabcomeline caused a small stimulation of basal [35S]GTPgammaS binding in both rat and heart tissues. Sabcomeline did not stimulate phosphoinositide hydrolysis in rat cortical slices, but did block the muscarinic M1 receptor-mediated response caused by carbachol with apparent pKb of 6.9. Xanomeline and milameline also had no effect on phosphoinositide hydrolysis up to 100 microM. In adenylyl cyclase studies in rat atria, sabcomeline inhibited forskolin-stimulated adenylyl cyclase activity to a similar extent to that of carbachol, xanomeline and milameline. The present study, using the techniques of radioligand binding, supports previous publications which have claimed that sabcomeline is a muscarinic receptor partial agonist. As expected, this study shows that the functional actions of this compound at muscarinic receptor subtypes and in different tissues will depend on receptor reserve.

Ovarian steroid regulation of serotonin-1A autoreceptor messenger RNA expression in the dorsal raphe of rhesus macaques

It is widely hypothesized that ovarian steroids act on serotonin neurons to modulate mood and alter neuroendocrine function in women. However, information is needed on the molecular consequences of estrogen and progesterone action in serotonin neurons. This study examined the effect of estrogen, with and without progesterone, on the expression of messenger RNA for the serotonin-1A autoreceptor in monkeys using in situ hybridization and a 432-bp serotonin-1A probe generated with polymerase chain reaction. Monkeys were spayed/ovariectomized (control; n=4), estrogen treated (28 days, n=4) and estrogen+progesterone treated (14 days estrogen+14 days estrogen+progesterone, n=4). Perfusion-fixed midbrain sections containing the dorsal raphe (10 microm) were hybridized at 60 degrees C with 35S antisense complementary RNA. After a final wash in 0.1 x standard saline citrate at 70 degrees C, sections were apposed to betamax film for four days and then emulsion fixed. Adjacent sections were immunostained for serotonin to confirm the location of the dorsal raphe. Densitometric analysis of autoradiographs with gray level thresholding was performed at five levels of the dorsal raphe. The number of pixels exceeding background in defined areas was obtained (pixel number), as well as the mean optical density. In the estrogen- and the estrogen+progesterone-treated groups compared to the control group, there was a 38% and 43% decrease in serotonin-1A messenger RNA signal, respectively, represented by pixel number (P<0.05). Mean optical density for serotonin-1A was significantly decreased by estrogen treatment (21%; P<0.05) and then further decreased with the addition of progesterone treatment (45%; P<0.01). Also, the number of positive cells and the grains/cell were counted. There were significantly fewer serotonin-1A messenger RNA-positive cells in the serotonergic neurons of the dorsal raphe in estrogen- and estrogen+progesterone-treated groups (P<0.001) than controls. There were significantly lower single-cell levels of serotonin-1A messenger RNA in serotonergic neurons of the dorsal raphe only in the estrogen+progesterone-treated group (P<0.05). These results suggest that estrogen reduces serotonin-1A gene expression and that the addition of progesterone further reduces serotonin-1A gene expression in non-human primates. If the changes in gene expression are manifested by alterations in protein expression, then, together, these actions of estrogen and progesterone could increase serotonin neurotransmission, thereby elevating mood and/or altering neuroendocrine functions.

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

Single treatment with the serotonin (5-hydroxytryptamine) 5-HT1A receptor agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and alnespirone (S-20499) reduces the extracellular 5-HT concentration (5-HText) in the rat midbrain and forebrain. Given the therapeutic potential of selective 5-HT1A agonists in the treatment of affective disorders, we have examined the changes in 5-HT1A receptors induced by 2-week minipump administration of alnespirone (0.3 and 3 mg/kg/day) and 8-OH-DPAT (0.1 and 0.3 mg/kg/day). The treatment with alnespirone did not modify baseline 5-HText but significantly attenuated the ability of 0.3 mg/kg s.c. alnespirone to reduce 5-HText in the dorsal raphe nucleus (DRN) and frontal cortex. In contrast, the ability of 8-OH-DPAT (0.025 and 0.1 mg/kg s.c.) to reduce 5-HText in both areas was unchanged by 8-OH-DPAT pretreatment. Autoradiographic analysis revealed a significant reduction of [3H]8-OH-DPAT and [3H]WAY-100635 [3H-labeled N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridyl)cyclohexa necarboxamide x 3HCl] binding to somatodendritic 5-HT1A receptors (but not to postsynaptic 5-HT1A receptors) of rats pretreated with alnespirone but not with 8-OH-DPAT. In situ hybridization analysis revealed no change of the density of the mRNA encoding the 5-HT1A receptors in the DRN after either treatment. These data indicate that continuous treatment for 2 weeks with alnespirone, but not with 8-OH-DPAT, causes a functional desensitization of somatodendritic 5-HT1A receptors controlling 5-HT release in the DRN and frontal cortex.

Ovarian steroids and serotonin neural function

The serotonin neural system originates from ten nuclei in the mid- and hindbrain regions. The cells of the rostral nuclei project to almost every area of the forebrain, including the hypothalamus, limbic regions, basal ganglia, thalamic nuclei, and cortex. The caudal nuclei project to the spinal cord and interact with numerous autonomic and sensory systems. This article reviews much of the available literature from basic research and relevant clinical research that indicates that ovarian steroid hormones, estrogens and progestins, affect the function of the serotonin neural system. Experimental results in nonhuman primates from this laboratory are contrasted with studies in rodents and humans. The sites of action of ovarian hormones on the serotonin neural system include effects within serotonin neurons as well as effects on serotonin afferent neurons and serotonin target neurons. Therefore, information on estrogen and progestin receptor-containing neurons was synthesized with information on serotonin afferent and efferent circuits. The ability of estrogens and progestins to alter the function of the serotonin neural system at various levels provides a cellular mechanism whereby ovarian hormones can impact mood, cognition, pain, and numerous other autonomic functions.

A single pretreatment with 8-OH-DPAT reduces behavioral indices of serotonin 1A receptor activation in ovariectomized rats

The effects of prior treatment with 8-OH-DPAT on 8-OH-DPAT-induced eating behavior, hypothermia, flat body posture and forepaw treading were examined in ovariectomized rats. Twenty-four hours after a single pretreatment with 0.25 mg/kg of the drug, the eating behavior and flat body posture induced by 8-OH-DPAT were reduced relative to that seen following the first treatment with the drug. In contrast, within 24 hr of the prior drug treatment, the drug's effects on forepaw treading and rectal temperature were unchanged. However, 7 days after prior treatment with 8-OH-DPAT, eating behavior, flat body posture and hypothermia induced by the drug were suppressed. Only forepaw treading remained unchanged. These findings are discussed in reference to similar studies of 5-HT1A receptor agonist-induced modulation of behaviors in male rats.

Serotonin1A receptor agonists induce Fos protein expression in the locus coeruleus of the conscious rat

The azapirones, which are partial agonists of the serotonin (5-HT)1A receptor, possess anxiolytic activity. These agents may act at the pre- or postsynaptic 5-HT1A receptors, and involve the noradrenergic system. To determine whether these drugs activate noradrenergic neurons via 5-HT1A receptors, we have evaluated the expression of the immediate early gene c-fos in the locus coeruleus. Tandospirone and ipsapirone each induced expression of Fos protein in the noradrenergic neurons of the locus coeruleus of conscious rats. This effect was reversed by pretreatment with (+)-WAY100135, a specific 5-HT1A antagonist. These results clearly demonstrate that azapirones activate noradrenergic neurons via 5-HT1A receptors.

The role of 5-HT(1B/1D) receptors in the modulation of 5-hydroxytryptamine levels in the frontal cortex of the conscious guinea pig

The role of 5-HT(1B/1D) receptors in modulating extracellular 5-hydroxytryptamine (5-HT) levels in the guinea pig was investigated with the non-selective 5-HT(1B/1D) receptor inverse agonist, methiothepin, and the selective 5-HT(1B/1D) receptor partial agonists, GR 127935 (n-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-2'-methyl-4'-(5-me thyl-1,2,4-oxadiazole-3-yl)[1,1'-biphenyl]-4-carboxamide) and GR 125743 (n-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-3-methyl-4-(4-pyri dinyl)benzamide). Extracellular 5-HT levels were measured using the technique of brain microdialysis, in the frontal cortex of the freely moving guinea-pig. Extracellular 5-HT was tetrodotoxin sensitive and calcium dependent, and increased when perfused with a high concentration of K+. In addition, extracellular 5-HT levels were lowered by the 5-HT(1B/1D) receptor agonist, sumatriptan, and the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin, while perfusion of the selective serotonin re-uptake inhibitor, paroxetine, increased 5-HT in a concentration-dependent manner. Perfusion of methiothepin, GR 127935 and GR 125743 into the frontal cortex caused significant but transient increases of extracellular 5-HT. However, systemic administration of methiothepin, GR 127935 and GR 125743, at 0.3 mg/kg i.p., produced significant decreases in extracellular 5-HT, to minima of 27 +/- 3%, 31 +/- 12% and 27 +/- 13% of basal, respectively. The increase of extracellular 5-HT, following 5-HT(1B/1D) receptor inverse and partial agonist perfusion into the frontal cortex, was probably a consequence of attenuation of an endogenous 5-HT tone at terminal 5-HT autoreceptors. The unexpected decrease in 5-HT levels following systemic administration may be a result of additional attenuation of endogenous 5-HT tone at cell body autoreceptors in the raphe. Such an increase in local 5-HT levels could then stimulate 5-HT1A receptors to inhibit cell firing and hence decrease 5-HT levels in the terminal regions. This was confirmed when co-administration of the 5-HT1A receptor antagonist, WAY 100635, significantly attenuated the GR 127935 decrease in 5-HT.

Effect of repeated ipsapirone treatment on hippocampal excitatory synaptic transmission in the freely behaving rat: role of 5-HT1A receptors and relationship to anxiolytic effect

The effects of acute and repeated treatment with the 5-HT1A receptor ligand ipsapirone on hippocampal excitatory synaptic transmission and in an ultrasonic vocalization anxiety test were investigated in the rat. Synaptic responses in the CA1 region of the dorsal hippocampus of alert, freely behaving male Wistar rats were reduced after acute injection of ipsapirone (1 or 2 mg/kg, i.p.). This effect was prevented by pretreatment with the 5-HT1A receptor antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide trihydrochloride, 0.25 or 0.5 mg/kg, i.p.) but not by the 5-HT-depleting agent para-chlorophenylalanine (300 mg/kg per day for 3 days, i.p.). WAY-100635 (0.1-0.3 mg/kg, i.p.) also blocked the acute anti-aversive effects of ipsapirone (3 mg/kg, i.p.) in the anxiety test. Repeated administration of ipsapirone (1 or 2 mg/kg per day for 7-8 days, i.p.) produced a gradual reduction in baseline synaptic transmission which was transiently reversed by WAY-100635 (0.25 mg/kg, i.p.). Ipsapirone (1 mg/kg per day for 7 days) produced a gradual and sustained reduction in the duration of vocalizations in the anxiety test which paralleled the reduction in baseline synaptic responses in the same animals. The data indicate that with repeated administration of ipsapirone, a prolongation and enhancement of the 5-HT1A receptor-mediated reduction in hippocampal excitatory synaptic transmission occurs. This delayed effect may contribute to the sustained anxiolytic and/or antidepressant effect of ipsapirone.

Ex vivo inhibitory effect of the 5-HT uptake blocker citalopram on 5-HT synthesis

Serotonin (5-hydroxytryptamine, 5-HT) synthesis was determined in vivo by measuring the accumulation of 5-hydroxytryptophan (5-HTP) in rat frontal cortex after inhibition of aromatic amino acid decarboxylase by administrative of m-hydroxybenzylhydrazine (NSD 1015) (100 mg/kg, i.p.). The selective 5-HT reuptake inhibitor, citalopram, the 5-HT1A agonists, (+/-) 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), ipsapirone, gepirone and the 5-HT1A/B agonist, 7-trifluoromethyl-4(4-methyl-1-piperazinyl-pyrolo[1,2-a]-quinox ali ne (CGS 12066B), the 5-HT1A/B ligands and beta-adrenoceptor antagonists, (+/-) pindolol and (+/-) alprenolol, and the non-selective 5-HT ligands, m-chlorophenylpiperazine (mCPP) and metergoline, all inhibited the synthesis of 5-HT. The 5-HT1A/5-HT2 antagonist, spiperone, alone, had no effect on basal 5-HT synthesis, however it attenuated the effect of 8-OH-DPAT by 56% and CGS 12066B by 39% but only barely that of citalopram by 17%. The selective 5-HT1A antagonist, WAY 100635, which did not modify by itself 5-HT synthesis, had no effect on citalopram-induced reduction of 5-HT synthesis. Neither the 5-HT2 agonist, (+/-)1-(2,5-dimethoxy-4-indophenyl)-2-aminopropane (DOI) nor the 5-HT2 antagonist, ritanserin, had any effect on the synthesis of 5-HT. In addition, ritanserin did not modify the inhibitory effect of citalopram. Methiothepin was the only compound to increase 5-HT synthesis. These results suggest that the effect of citalopram on the synthesis of 5-HT is not mediated by 5-HT1A or 5-HT2 receptors and that other receptors may be involved.

Depression as a spreading neuronal adjustment disorder

The outlines of a theory of the pathophysiology of depression are presented. The classic monoamine theory of depression as well as its more recent elaborations suggests that a deficit in monoamine neurotransmitters in the synaptic cleft is the primary cause of depression. We suggest that the primary defect emerges in the regulation of firing rates in brainstem monoaminergic neurons, which brings about a decrease in the tonic release of neurotransmitters in their projection areas, an increase in postsynaptic sensitivity and, concomitantly, exaggerated responses to acute increases in presynaptic firing rate and transmitter release. We propose that the initial defect involves, in particular, the noradrenergic innervation from the locus coeruleus, which in turn leads to dysregulation of 5-HT-ergic and dopaminergic neurotransmission.

Functional interaction between serotonin and other neuronal systems: focus on in vivo microdialysis studies

In this review, the functional interactions between serotonin (5-HT) and other neuronal systems are discussed with the focus on microdialysis studies in the mammalian brain (mainly rats). 5-HT release is negatively regulated not only by somatodendritic 5-HT1A and terminal 5-HT1B (5-HT1D) autoreceptors but also by alpha 2-adrenergic and mu-opioid heteroreceptors that are located on serotonergic nerve terminals. 5-HT by itself is involved in the inhibitory effects of noradrenaline release and the facilitatory regulation of dopamine release via multiple 5-HT receptors. Acetylcholine release appears to be regulated by inhibitory 5-HT1B heteroreceptors located on cholinergic nerve terminals. Long-term treatment with 5-HT-uptake inhibitors and noradrenaline-uptake inhibitor produces desensitization of 5-HT1A autoreceptors and alpha 2-heteroreceptors, respectively, which may be related therapeutically to the delayed onset of the effects of antidepressants. Some microdialysis studies have predicted that the combination of a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist might produce much greater availability of 5-HT in the synaptic cleft in terms of much faster induction of subsensitivity of 5-HT1A autoreceptors. Clinical trials based on this hypothesis have revealed that combination therapy with a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist ameliorated the therapeutic efficacy in depressive patients. Taken together, neurochemical approaches using microdialysis can contribute not only to clarification of the physiological role of the serotonergic neuronal systems but also might be a powerful pharmacological approach for the development of therapeutic strategies.

An autoradiographic study of serotonergic receptors in a murine genetic model of anxiety-related behaviors

Modifications in serotonin (5-HT) neurotransmission have been associated with the physiopathology of anxiety and depression. Among the numerous 5-HT receptor subtypes, several (5-HT1A, 5-HT1B, 5-HT2 and 5-HT3) could be involved in these etiologies. By using a murine genetic model, we attempted to correlate variations in the density of receptor subtypes with modifications of anxiety-related behaviors. From a classic inbred strain (C57BL/6ByJ) and a linkage-testing inbred strain (ABP/Le), segregated F(2) populations for 3 loci located in the 4th, 7th and 9th chromosomes have been selected for their different responses in anxiety-related behavioral tests. The regional density of 5-HT1A, 5-HT1B, 5-HT2A and 5-HT2B receptors has been measured in the brains of parental strains, F(1) and F(2) populations by quantitative autoradiography. The results suggest that chromosomal fragments containing the brown, pink-eyed dilution and the short-ear loci, previously shown to be involved in anxiogenic processes, are mainly associated with a variation in the density of the 5-HT1B receptors.

Role of 5-HT1A autoreceptors in the mechanism of action of serotoninergic antidepressant drugs: recent findings from in vivo microdialysis studies

Although a new generation of selective serotonin reuptake inhibitors (SSRIs) has been introduced in therapeutics as antidepressant drugs, a two to four week lag period still occurs between starting treatment with SSRIs and the onset of therapeutic effects in man. In vivo cerebral microdialysis can be used to measure extracellular concentrations of serotonin (5-hydroxytryptamine, 5-HT), which reflect intrasynaptic events. With the coupling of this new experimental method to very sensitive analytical assays such as liquid chromatography with electrochemical detection, it has recently been possible to obtain two major arguments supporting the hypothesis that somatodendritic 5-HT1A autoreceptors situated in the raphe nuclei play an important role in the mechanism of action of SSRIs. First, in the rat, single administration of SSRIs at low doses comparable to those used therapeutically increases extracellular 5-HT concentrations in the vicinity of the cell body and the dendrites of serotoninergic neurones of the raphe nuclei. This effect is more marked than that observed in regions rich in nerve endings (frontal cortex). The magnitude of the activation of the serotoninergic neurotransmission depends on the brain area studied and the dose of the SSRIs administered to rats. This could be explained by simultaneous activation of somatodendritic 5-HT1A autoreceptors by endogenous 5-HT in the raphe nuclei, thereby limiting the corticofrontal effects of the antidepressant. Second, SSRIs cause a larger increase in extracellular 5-HT concentrations in the nerve endings when administered chronically: 5-HT autoreceptors may have gradually desensitized during the 2-4 weeks of treatment with SSRIs. Preliminary studies of patients with depression appear to confirm these experimental results, as co-administration of a 5-HT1A autoreceptor antagonist and a SSRI accelerated the onset of the antidepressant effect (< 1 week).

5-HT1A receptor agonists: recent developments and controversial issues

During the last decade, serotonin (5-HT)1A receptors have been a major target for neurobiological research and drug development. 5-HT1A receptors have been cloned and a variety of selective agonists, such as the aminotetraline 8-OH-DPAT and the pyrimidinylpiperazine ipsapirone, have become available. Demonstrations of apparent intrinsic activity of these ligands at 5-HT1A receptors, however, depend highly on the particular assay system. This may be due to the possible existence of receptor subtypes and to assay (or brain region)-dependent differences in receptor reserve and the nature of receptor-effector coupling. Nevertheless, the apparent intrinsic activity of 8-OH-DPAT seems to be higher (although possibly not yet maximal) than that of the pyrimidinylpiperazines. In the brain, 5-HT1A receptors are located presynaptically as somatodendritic receptors on 5-HT neurons and postsynaptically in particular limbic and cortical regions. Although it is generally accepted that presynaptic 5-HT1A receptors control 5-HT neuronal activity, recent evidence suggests an additional role of postsynaptic 5-HT1A receptors in cortex as part of a negative feedback loop. Anxiolytic and antidepressive properties of selective 5-HT1A receptor agonists have now been confirmed by clinical studies. Although it is well established that the latter properties depend on the agonistic activity of these compounds, the optimal level of intrinsic activity is still a matter of debate and may be dependent on the clinical indication. Such compounds may also have antiaggressive effects, and possibly anticraving effects (manifested by their alcohol intake-reducing effects in dependent animals), but the specificity of these so-called anti-impulsivity effects is still controversial and not yet tested clinically. Anticataleptic, antiemetic and neuroprotective properties have been demonstrated in different species. Behavioral studies on the mechanisms underlying the anxiolytic and antidepressive effects have examined the relative contribution of pre- and postsynaptic 5-HT1A receptors by means of local cerebral application and lesion techniques. Most evidence points towards a critical involvement of presynaptic receptors in the anxiolytic effects of 5-HT1A receptor agonists (although a possible contribution of postsynaptic receptors cannot be excluded). With regard to the antidepressive properties, a case can be made for the reverse; i.e., a strong involvement of postsynaptic receptors and a questionable contribution of presynaptic receptors. However, as the therapeutic effects of those 5-HT1A receptor (partial) agonists which have been tested clinically require repeated administration, attention has been directed increasingly towards chronic studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Increase of noradrenaline release in the hypothalamus of freely moving rat by postsynaptic 5-hydroxytryptamine1A receptor activation

1. 5-Hydroxytryptamine (5-HT) plays a role in the regulation of noradrenergic neurones in the brain, but the precise mechanism of regulation of noradrenaline (NA) release by 5-HT1A receptors has not been defined. The present study describes the effect of a highly potent and selective 5-HT1A receptor agonist, 5-(3-[[(2S)-1,4-benzodioxan-2-ylmethyl)]amino]propoxy)-1,3-b enzodioxole HC1 (MKC-242), on NA release in the hypothalamus using microdialysis in the freely moving rat. 2. Subcutaneous injection of MKC-242 (0.5 mg kg-1) increased extracellular levels of NA and its metabolite, 3-methoxy-4-hydroxyphenylglycol, in the hypothalamus and hippocampus. 3. The 5-HT1A receptor agonists, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (0.2 mg kg-1) and buspirone (3 mg kg-1) mimicked the effect of MKC-242 in increasing NA release in the hypothalamus. 4. The effects of MKC-242 and 8-OH-DPAT in the hypothalamus were antagonized by pretreatment with WAY100135 (10 mg kg-1), a silent 5-HT1A receptor antagonist. 5. Local administration of 8-OH-DPAT (10-100 microM), citalopram (1 microM), a 5-HT reuptake inhibitor, and MDL72222 (10 microM), a 5-HT3 receptor antagonist, into the hypothalamus, had no effect on NA release. 6. Intracerebroventricular injection with 5,7-dihydroxytryptamine caused a marked reduction in brain 5-HT content, but the treatment affected neither basal NA levels nor the MKC-242-induced increase in NA release. 7. The effect of MKC-242 in increasing NA release was not attenuated by repeated treatment with the drug (0.5 mg kg-1, once a day for 2 weeks). 8. The present results suggest that activation of postsynaptic 5-HT1A receptors increases NA release in the hypothalamus.

Drug-induced defaecation in rats: role of central 5-HT1A receptors

1. We investigated the acute effects of 5-hydroxytryptamine (5-HT), and of the 5-HT1A receptor agonists, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), buspirone and SR 57746A, on rat faecal pellet output and water content. 2. 5-HT, 8-OH-DPAT, buspirone and SR 57746A, a new selective 5-HT1A receptor agonist, displaced [3H]-8-OH-DPAT from specific binding sites in rat hippocampus membranes (Ki, nM; 1.8, 1.2, 15, 3.1 respectively) and stimulated rat defaecation dose-dependently. SR 57746A and buspirone induced 1 g dry weight of faeces at 1.3 and 6.1 mg kg-1, p.o. (AD1) respectively. 8-OH-DPAT and 5-HT stimulated defaecation after s.c. injection (AD1, 0.07 and 7.5 mg kg-1, respectively). All these agents increased faecal water content. 3. The putative 5-HT1A receptor antagonist, pindolol, injected s.c. or i.c.v., significantly reduced the defaecation induced by systemically administered 8-OH-DPAT, buspirone or SR 57746A, but not 5-HT. 4. Pretreatment with p-chlorophenylalanine (i.p.) or 5,7-dihydroxytryptamine (i.c.v.), according to protocols designed to cause either generalized or CNS-limited 5-HT depletion respectively, also reduced the defaecation induced by buspirone or SR 57746A. 5. No specific 5-HT1A binding sites could be labelled by incubating rat colon membranes with [3H]-8-OH-DPAT, and in vitro preparations of rat colon segments showed no response to 8-OH-DPAT or SR 57746A up to 5 microM. 6. After eight days' repeated daily treatment, complete tolerance developed to the stimulant effects of SR 57746A and buspirone on faecal water content, but not on faecal pellet output. This suggests that faecal mass excretion and water exchange through the gut wall are affected by independent mechanisms.7. The present findings support the involvement of central 5-HTIA receptors in intestinal propulsion and regulation of luminal fluid content, presumably accounting for the drug-induced defaecation in rats.

Differential effects of WAY-100135 on the decrease in 5-hydroxytryptamine release induced by buspirone and NAN-190

1-(2-Methoxyphenyl)-4-[(phthalimido)butyl] piperazine (NAN-190) and 8-[4-[4-(2-pyrimidinyl)-1-piperazinyl]butyl]-8-azaspiro[4.5] decane-7,9-dione (buspirone) are 5-HT1A receptor partial agonists which decrease 5-hydroxytryptamine (5-HT) release in vivo. In order to assess whether these ligands decrease 5-HT release by stimulating 5-HT1A receptors we examined the ability of the selective 5-HT1A receptor antagonist N-tert-butyl 3-4-(2-methoxyphenyl) piperazin-1-yl-2-phenylpropanamide dihydrochloride (WAY-100135) to block their inhibitory effects on 5-HT. NAN-190 (0.1 mg/kg s.c.) and buspirone (1.0 mg/kg s.c.) significantly decreased extracellular levels of 5-HT in hippocampal dialysates. WAY-100135 (10.0 mg/kg s.c.) attenuated the effect of buspirone but had no significant effect on the NAN-190-induced decreased in 5-HT release. These data demonstrate that buspirone is an agonist at the somatodendritic 5-HT1A receptor but that the inhibitory effects of NAN-190 on 5-HT release may be mediated via a mechanism other than, or in addition to, 5-HT1A receptor agonism.

Adaptive changes in 5-HT1A receptor-mediated hippocampal inhibition in the alert rat produced by repeated 8-OH-DPAT treatment

1. The effect of acute and repeated treatment with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor ligand, on excitatory amino acid-mediated synaptic transmission was examined in the stratum radiatum CA1 region of the dorsal hippocampus of alert, gently restrained, rats. 2. Acute administration of 8-OH-DPAT transiently reduced the amplitude of the field excitatory postsynaptic potential (e.p.s.p.) in a dose-dependent (25-75 micrograms kg-1, i.p.) manner. This effect was blocked by the postsynaptic 5-HT1A receptor antagonist, MDL 73005EF (2 and 4 mg kg-1, i.p.). 3. 8-OH-DPAT (25 micrograms kg-1, i.p.) administered daily for 7 days produced a gradual reduction in the 24 h pre-injection baseline field e.p.s.p. amplitude. The reduction reached its lowest level after 7-8 days and was transiently reversed by acute injection of MDL 73005EF (2 mg kg-1, i.p.) on day 8. The field e.p.s.p. baseline amplitude recovered fully 5-8 days after cessation of drug treatment. 4. 8-OH-DPAT (25 micrograms kg-1, i.p.) administered daily for 7 days produced a marked reduction in acute response to 8-OH-DPAT (25 and 50 micrograms kg-1, i.p.) which did not recover until between day 36 and day 80 of the study. 5. It was concluded that repeated treatment with 8-OH-DPAT produced adaptive changes which resulted in a reduction in the dynamic range of 5-HT1A receptor-mediated transmission in the hippocampus.

Increase in extracellular serotonin produced by uptake inhibitors is enhanced after chronic treatment with fluoxetine

The effect of prolonged uptake inhibition with fluoxetine (10 mg/kg/day i.p. x 14 days) on extracellular serotonin (5-HT) in the rat diencephalon was monitored using in vivo microdialysis. The increase in extracellular 5-HT after repeated administration of fluoxetine was significantly greater than the increase produced by a single injection of this uptake blocker. This difference may have been due to a decrease in somatodendritic autoreceptor sensitivity, since the response to a low dose of the 5-HT1A agonist 8-OH-DPAT (25 micrograms/kg i.v.) was abolished in the chronic rats, while the response to a high dose (100 micrograms/kg i.v.) was attenuated as compared to animals injected once with fluoxetine.

Lack of 5-HT1A autoreceptor desensitization following chronic citalopram treatment, as determined by in vivo microdialysis

Electrophysiological studies suggest that 5-HT autoreceptor desensitization may be responsible for the delayed clinical efficacy of some antidepressant drugs, such as selective 5-HT reuptake inhibitors (SSRI) and certain MAO inhibitors (MAOI). In the present study we have used in vivo microdialysis to test this hypothesis. Rats were treated for 2 weeks with the antidepressant SSRI citalopram (5 mg/kg, s.c., b.i.d.). After 24 hr withdrawal, dialysis probes were implanted in the dorsal hippocampus (DH) and the frontal cortex (FCx). The rats then received as acute challenge, a 5-HT1A autoreceptor-active dose of the reference 5-HT1A agonist 8-OH-DPAT (0.025 mg/kg s.c.). The 8-OH-DPAT-induced changes in dialysate 5-HT from the DH and the FCx were monitored and taken as an index of autoreceptor sensitivity. Chronic citalopram and control animals responded similarly to 8-OH-DPAT with a drop of 5-HT of about 50-65%; no significant difference between the chronic citalopram and control groups were obtained, either in the DH or in the FCx. These data suggest that cell body 5-HT1A autoreceptors do not desensitize in response to repeated administration with antidepressant SSRI drugs such as citalopram.

Sustained 5-hydroxytryptamine release-inhibitory and anxiolytic-like action of the partial 5-HT1A receptor agonist, buspirone, after prolonged chronic administration

The effect of prolonged administration of high doses of buspirone on its 5-hydroxytryptamine (5-HT) release-inhibitory and anxiolytic-like properties was investigated. The 5-HT release-inhibitory effect of a challenge dose of buspirone (0.5 mg/kg, s.c.) was identical in rats chronically treated with vehicle or buspirone (10 mg/kg, b.i.d. for 10 weeks), as estimated by in vivo microdialysis in the ventral hippocampus. In the same set of animals there was a significant anxiolytic-like effect in the elevated plus-maze after 5 weeks of treatment with buspirone. The results indicate that the functional capacity of 5-HT release-controlling 5-HT1A autoreceptors is retained upon chronic administration of buspirone, and that this effect may well be associated with the anxiolytic-like action of the compound.