The central 5-HT1A receptors: pharmacological, biochemical, functional, and regulatory properties

Effect of 5-HT on binding of [(11)C] WAY 100635 to 5-HT(IA) receptors in rat brain, assessed using in vivo microdialysis nd PET after fenfluramine

By using a combination of positron emission tomography (PET) and postmortem tissue dissection, the effect of increased endogenous serotonin on specific binding of [(11)C]WAY 100635 to the 5-HT(1A) receptor was investigated in rat brain in vivo. The binding studies were complemented by in vivo microdialysis to monitor 5-HT levels in similarly treated isoflurane-anaesthetised rats, with the dialysis probe locations corresponding to two of the tissues sampled for specific binding of the radioligand. Fenfluramine treatment (10 mg/kg i.p.) resulted in a approximately 5-fold increase in extracellular 5-HT in medial prefrontal cortex and a approximately 15-fold increase in lateral hippocampus, maximal at approximately 40 min after injection. PET scan duration was either 60 or 90 min, beginning 30 min after fenfluramine injection. The specific binding of [(11)C]WAY 100635 was reduced by 10-20% in hippocampus, which showed highest binding in control animals. Specific binding, however, was unaffected in both prefrontal cortex and midbrain raphe, each additional high binding regions. The minimal effects are consistent with a low baseline occupancy of the 5-HT(1A) receptor by 5-HT in vivo, so that only a large change in endogenous agonist concentration will affect radioligand binding. This implies that utilisation of [(11)C]WAY 100635 in human PET to quantify 5-HT(1A) receptor expression can be extended to pathology where synaptic 5-HT levels are altered as a consequence of the disease state.

Further characterization of 5-HT1A receptors in the goldfish retina: role of cyclic AMP in the regulation of the in vitro outgrowth of retinal explants

The presence of serotonin 5-HT1A receptors and their physiological role were further characterized in the goldfish retina. The effects of the 5-HT6/7 receptor antagonists pimozide, fluphenazine and amoxapine, the 5-HT1A receptor antagonist WAY-100,135, and the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline, on the 5-HT1A receptor agonist [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding to retinal membranes, were evaluated. In addition, the effects of serotonin, 8-hydroxy-2-(di-n-propylamino)tetralin, WAY-100,135, the adenylate cyclase inhibitors SQ22536 and MDL12330A, and the cyclic AMP analog 8-bromoadenosine-3':5' cyclic monophosphate were also studied on neuritic outgrowth from retinal explants. WAY-100,135 but not 5-HT6/7 receptor antagonists inhibited [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding to retinal membranes N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline decreased [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding sites up to 70%, while receptor turnover was similar to that reported in other tissues. Serotonin and 8-hydroxy-2-(di-n-propylamino)tetralin stimulated cyclic AMP production, both ex vivo and in vitro, and these increases were related to inhibition of neuritic outgrowth. The inhibitory effect was reduced by SQ22536 and by WAY-100,135, and was mimicked by 8-bromoadenosine-3':5'cyclic monophosphate. This study supports previous findings about the role of serotonin as a regulator of axonal outgrowth during in vitro regeneration of the goldfish retina and demonstrates that this effect is mediated, at least in part, by 5-HT1A receptors through a mechanism which involves an increase of cyclic AMP levels.

5-HT(1A) receptor-mediated inhibition and 5-HT(2) as well as 5-HT(3) receptor-mediated excitation in different subdivisions of the rat amygdala

The techniques of extracellular single cell recording and microiontophoresis were used to study the effects of serotonin (5-HT) and of 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptor agonists on the spontaneous activity of amygdaloid neurons in rats anesthetized with urethane. The background discharge rate was modified by 5-HT as well as by 5-HT agonists in about two-thirds of neurons tested in different nuclei of the amygdaloid complex. Whereas the 5-HT(2) and 5-HT(3) agonists significantly increased the neuronal discharge rate in nearly all subdivisions of the amygdala, the 5-HT(1A) agonist significantly inhibited the firing rate. Co-administration of bicuculline and 5-HT receptor agonists prevented the 8-OH-DPAT-induced increases in the firing rate in most cases tested, as well as the inhibitory effects of DOI or 2-methyl-5HT. Therefore, GABAergic interneurons seem to be involved in the mediation of serotonergic effects. The action of 5-HT agonists on the neuronal discharge rate was blocked by different receptor-specific antagonists. The results support the hypothesis that 5-HT exerts control throughout the amygdala by acting at least on 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptors seemingly located both on projection and interneurons.

Altered serotonin and dopamine metabolism in the CNS of serotonin 5-HT(1A) or 5-HT(1B) receptor knockout mice

Measurements of serotonin (5-HT), dopamine (DA), and noradrenaline, and of 5-HT and DA metabolites, were obtained by HPLC from 16 brain regions and the spinal cord of 5-HT(1A) or 5-HT(1B) knockout and wild-type mice of the 129/Sv strain. In 5-HT(1A) knockouts, 5-HT concentrations were unchanged throughout, but levels of 5-HT metabolites were higher than those of the wild type in dorsal/medial raphe nuclei, olfactory bulb, substantia nigra, and locus coeruleus. This was taken as an indication of increased 5-HT turnover, reflecting an augmented basal activity of midbrain raphe neurons and consequent increase in their somatodendritic and axon terminal release of 5-HT. It provided a likely explanation for the increased anxious-like behavior observed in 5-HT(1A) knockout mice. Concomitant increases in DA content and/or DA turnover were interpreted as the result of a disinhibition of DA, whereas increases in noradrenaline concentration in some territories of projection of the locus coeruleus could reflect a diminished activity of its neurons. In 5-HT(1B) knockouts, 5-HT concentrations were lower than those of the wild type in nucleus accumbens, locus coeruleus, spinal cord, and probably also several other territories of 5-HT innervation. A decrease in DA, associated with increased DA turnover, was measured in nucleus accumbens. These changes in 5-HT and DA metabolism were consistent with the increased aggressiveness and the supersensitivity to cocaine reported in 5-HT(1B) knockout mice. Thus, markedly different alterations in CNS monoamine metabolism may contribute to the opposite behavioral phenotypes of these two knockouts.

Central 5-HT(1A) receptors: regional distribution and functional characteristics

Among the multiple receptors for serotonin identified to date, the 5-hydroxytryptamine (5-HT)(1A) type is among the best known because selective ligands have been available for more than 15 years. Radioactive derivatives allowed the demonstration of the presence of 5-HT(1A) binding sites mainly in the limbic areas and the raphe nuclei in the brain, where they correspond to postsynaptic receptors and "presynaptic" autoreceptors, respectively. This review article summarizes key data on the molecular, pharmacological, and differential functional properties of pre- versus postsynaptic 5-HT(1A) receptors.

Pindolol occupancy of 5-HT(1A) receptors measured in vivo using small animal positron emission tomography with carbon-11 labeled WAY 100635

Positron emission tomography (PET), following an intravenous injection of [carbonyl-(11)C]WAY 100635, was used to image central 5-HT(1A) receptors in rat following pretreatment with graded doses of (-)-pindolol (0.001-3 mg/kg, i.v.). The use of PET had advantages over ex vivo radioligand binding methods in that it produced parametric image volumes and reduced errors due to inter-rat variability. Time-radioactivity curves from regions of interest (ROI) acquired from individual rats enabled the estimation of specific binding of the radioligand using a compartmental model with reference tissue input. Binding potential (BP) of [(11)C]WAY 100635 was estimated for frontal cortex and hippocampus (postsynaptic), and midbrain raphe nuclei (presynaptic). In the latter ROI, pindolol dose-dependently decreased BP. The saturation curve could be fitted to a single-site model up to the lowest dose of pindolol used, giving an ED(50) (dose to cause 50% occupancy) value of 0.26 +/- 0. 05 mg/kg, and inclusion of control (nonpindolol-treated) rats did not affect the fit. In contrast, in cortex and hippocampus ROI, low doses of pindolol caused an increase in BP compared with controls. Pindolol doses greater than approximately 0.1 mg/kg, resulted in a dose-dependent decrease in BP, and ED(50) values in cortex and hippocampus were estimated as 0.44 +/- 0.13 and 0.48 +/- 0.12 mg/kg, respectively. The increase in [(11)C]WAY 100635 binding at low pindolol doses is feasibly related to a decrease in basal receptor occupancy following reduced release of endogenous 5-HT. Considering the apparently greater potency of pindolol at the midbrain raphe ROI, this effect could be mediated via agonist activity at the autoreceptor.

Regulation of monoamine receptors in the brain: dynamic changes during stress

Monoamine receptors are membrane-bound receptors that are coupled to G-proteins. Upon stimulation by agonists, they initiate a cascade of intracellular events that guide biochemical reactions of the cell. In the central nervous system, they undergo diverse regulatory processes, among which are receptor desensitization, internalization into the cell, and downregulation. These processes vary among different types of monoamine receptors. alpha 2-Adrenoceptors are often downregulated by agonists, and beta-adrenoceptors are internalized rapidly. Others, such as serotonin1A-receptors, are controlled tightly by steroid hormones. Expression of these receptors is reduced by the "stress hormones" glucocorticoids, whereas gonadal hormones such as testosterone can counterbalance the glucocorticoid effects. Because of this, the pattern of monoamine receptors in certain brain regions undergoes dynamic changes when there are elevated concentrations of agonists or when the hormonal milieu changes. Stress is a physiological situation accompanied by the high activity of brain monoaminergic systems and dramatic changes in peripheral hormones. Resulting alterations in monoamine receptors are considered to be in part responsible for changes in the behavior of an individual.

The effects of the novel anxiolytic drug lesopitron, a full and selective 5-HT1A receptor agonist, on pupil diameter and oral temperature in man: comparison with buspirone

We investigated the effects of two 5-HT1A receptor agonists, buspirone and lesopitron, upon pupil size in human volunteers at an ambient luminance level of 32 Cd m(-2) and in darkness. Pupil diameter was monitored with a binocular infrared television pupillometer, before and after the administration of treatments for 4 h at 20-min intervals. Two experiments were conducted. In Experiment 1, 14 healthy male volunteers participated in seven weekly sessions, each associated with the ingestion of one capsule (buspirone 5, 10 and 20 mg, lesopitron 10, 20 and 40 mg and placebo), according to a double-blind balanced, cross-over design. Both buspirone and lesopitron tended to decrease pupil diameter. In darkness, only the highest dose of buspirone (20 mg) caused a miosis that was statistically significant. However, at the luminance level of 32 Cd m(-2) buspirone 10 and 20 mg evoked statistically significant miotic effects, as did the highest dose of lesopitron (40 mg). The miotic effect was significantly greater at 32 Cd m(-2) than in darkness after each dose of buspirone and the highest dose (40 mg) of lesopitron. In Experiment 2, pupil diameter and oral temperature were monitored with an electronic thermometer at 40-min intervals. Twenty healthy male volunteers participated in two weekly sessions, each associated with the sublingual application of 100 microl hydroalcoholic solution (lesopitron 20 mg, placebo), according to a double-blind balanced cross-over design. Lesopitron caused a significant miosis both in darkness and at the luminance level of 32 Cd m(-2); the miosis was greater at 32 Cd m(-2) than in darkness. Lesopitron tended to decrease oral temperature; this effect however, was not statistically significant. The greater effectiveness on the pupil of lesopitron administered sublingually in a solution indicates the importance of first-pass metabolism in reducing the effectiveness of the drug when administered by the mouth. The miosis observed in both experiments may be due to either a sympatholytic or a parasympathomimetic effect of the drugs, or both. The light-dependence of the miosis indicates that the 5-HT1A receptor agonists can modulate the light reflex, possibly via the noradrenergic control of central cholinergic neurones in the Edinger-Westphal nucleus.

Gender differences in acute and chronic stress in Wistar Kyoto (WKY) rats

While females are considered more susceptible to depressive behavior, this assertion is not strongly supported by the experimental literature. Since stress contributes to depressive behavior, male and female Wistar Kyoto (WKY) rats were exposed to either one session (acute stress) or 5 sessions (chronic stress) of restraint plus cold in order to study depressive behavior in male and female rats. After their respective treatment exposure, rats were tested in the open field test (OFT) and for retention of a passive-avoidance (P-A) task. One stress session resulted in significant immobility in the OFT for males, whereas 5 sessions were required to produce similar immobility in female rats. Acute stress interfered with the retention of the P-A response for males, while both acute and chronic stress produced poor P-A responses in female rats. Food consumption decreased progressively, as a function of stress sessions, in female rats, whereas feeding in males returned to control levels after five stress days. Both acute and chronic stress exacerbated the stress ulcer response in male rats, but not in female rats. Chronic, but not acute, stress resulted in an increase in serotonin transporter mRNA levels in the dorsal raphe nucleus of both male and female rats. The general consensus from these data suggested that female rats were more vulnerable to chronic stress and consequently supported the notion that females may be more susceptible to stress-induced behavioral depression. Key Words: WKY rats, acute and chronic stress, gender, passive avoidance, open field behavior, stress-ulcer, adrenal weight, serotonin, dorsal raphe nucleus

Behavioral, neuroendocrine and serotonergic consequences of single social defeat and repeated fluoxetine pretreatment in the Lewis rat strain

We have analysed some behavioral, neuroendocrine and serotonergic consequences of a single (30-min) social defeat followed by 14-18 h of sensory contact with the aggressor, in Lewis rats, an inbred strain highly sensitive to chronic social stressors [Berton O. et al. (1998) Neuroscience 82, 147-159]. In addition, we have investigated how the aforementioned consequences are affected by pretreatment with the selective serotonin reuptake inhibitor, fluoxetine (7.5 mg/kg/day for 21 days). A single social defeat triggered hypophagia and body weight loss, and increased anxiety in the elevated plus-maze. It did not affect baseline plasma adrenocorticotropic hormone levels and renin activity, but decreased plasma corticosterone levels. On the other hand, the responses of the latter variables to subsequent acute forced swim stress were blunted (corticosterone) or amplified (adrenocorticotropic hormone, renin activity) by prior defeat. The density of hippocampal serotonin transporters, but not that of hippocampal serotonin-1A and cortical serotonin-2A receptors, was decreased by a single social defeat; in addition, neither tryptophan availability and serotonin synthesis/metabolism, nor serotonin-1A autoreceptor-mediated functions (inhibition of serotonin synthesis, hyperphagia) were affected. Fluoxetine pretreatment diminished social defeat-induced hypophagia, body weight loss and anxiety without affecting these variables in control animals. This pretreatment increased plasma corticosterone levels in resting and acutely stressed rats, but abolished social defeat-elicited corticosterone hyporesponsiveness to acute forced swim stress. Except for a decrease in midbrain serotonin transporter density, fluoxetine did not affect the other serotonergic indices analysed herein, i.e. serotonin-1A and serotonin-2A receptor densities, serotonin synthesis/metabolism. A single social defeat in Lewis rats produces behavioral and endocrine alterations that may model some aspects of human anxiety disorders. In this paradigm, prior fluoxetine treatment is endowed with adaptive behavioral, and possibly neuroendocrine, effects without affecting the key elements of central serotonergic systems analysed herein.

Cocaine-induced alterations in the density of monoaminergic receptors in the embryonic guinea pig cerebral wall

Quantitative receptor autoradiography was used to examine the effect of chronic cocaine exposure on the density of alpha1-, alpha2- and beta-adrenergic, 5-HT1A- and 5-HT2-serotonergic, and D1- and D2-dopaminergic receptors in the fetal guinea pig cerebral wall which contained forming motor area of the cerebral cortex. The pregnant guinea pig received two daily subcutaneous injections of 20 mg/kg cocaine beginning on the 20th day of pregnancy (E20). The control animals received injections of equivalent volume of saline. The receptor densities were examined between days 5-30 of the treatment, which corresponds to E25-E50. By the fifth day of treatment (E25), cocaine produced downregulation of all receptors studied throughout the entire depth of the fetal cerebral wall. More extended treatment, however, resulted in recovery of receptor levels. Finally, from days 20-30 of treatment (E40-E50) there was a significant upregulation of noradrenergic and dopaminergic receptor sites. These findings demonstrate that exposure to cocaine in utero can influence adrenergic, serotonergic, and dopaminergic receptors in the embryonic cerebral wall, which may lead to alteration in corticogenesis. Furthermore, the present study reveals that, in the course of chronic treatment, cocaine may completely reverse its receptor regulatory activity in the fetal brain.

GR 127935 and (+)-WAY 100135 do not affect TFMPP-induced inhibition of 5-HT synthesis in the midbrain and hippocampus of Wistar-Kyoto rats

Wistar-Kyoto (WKY) rats display high emotivity (e.g. anxiety), compared to Wistar rats. The key role of serotonin (5-HT)1B/1D autoreceptors in 5-HT neurotransmission, and its consequences on emotivity, led us to measure the effects of the nonselective 5-HT1B/1D) receptor agonist m-trifluoromethyl-phenylpiperazine (TFMPP) on central tryptophan hydroxylase activity in male WKY and Wistar rats. In addition to strain-dependent differences in central 5-HT synthesis (WKY > Wistar), acute administration of TFMPP (1.5 and 3 mg/kg) decreased the amplitude of m-hydroxy-benzylhydrazine-elicited accumulation of hippocampal, striatal and cortical 5-hydroxytryptophan (5-HTP) in both strains. In midbrain, however, TFMPP decreased 5-HTP accumulation (but not tryptophan levels) in WKY rats only, whereas the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.2 mg/kg) decreased midbrain 5-HTP levels to a similar extent in both strains. Pretreatment of WKY rats with the selective 5-HT1B/1D receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1, 2,4-oxadiozol-3-yl)-biphenyl-4-carboxamide (GR 127935, 1.5 and 3 mg/kg) slightly increased midbrain tryptophan hydroxylase activity but did not affect the negative effect of TFMPP on 5-HTP formation. Pretreatment with the 5-HT1A receptor antagonist (+)-N-tert-butyl-3-(4-[2-methoxyphenyl]piperazin-1-yl)-2-phenylpro panamide ((+)-WAY 100135; 3 mg/kg), which decreased the inhibitory effect of 8-OH-DPAT on midbrain 5-HTP levels by 50%, did not alter that of TFMPP. Lastly, neither reserpine (5 mg/kg), ketanserin (1 mg/kg) mianserin (2 mg/kg) nor idazoxan (1 mg/kg) pretreatments affected TFMPP-induced inhibition of midbrain 5-HTP formation, ruling out a role for monoamine release, 5-HT2 receptors and alpha2-adrenoceptors. Our data show that TFMPP, an agonist often used to stimulate 5-HT1B/1D receptors, may inhibit central 5-HT synthesis through nonserotonergic mechanisms.

Isoindol-1-one analogues of 4-(2'-methoxyphenyl)-1-[2'-[N-(2"-pyridyl)-p-iodobenzamido]ethyl]pipera zine (p-MPPI) as 5-HT1A receptor ligands

In developing radioiodinated antagonists for in vivo imaging of 5-HT1A receptors with SPECT, a series of new arylpiperazine benzamido derivatives, including 4-(2'-methoxyphenyl)-1-[2'-[N-(2"-pyridyl)-p-iodobenzamido]ethyl]p iperazine (p-MPPI, 31) (Kd = 0.36 nM), as potential ligands for 5-HT1A receptors were reported previously. However, rapid in vivo metabolism may have caused the breakdown of the amide bond of [123I]-31 and rendered this agent obsolete as an in vivo imaging agent in humans. To improve the in vivo stability of 31, a series of cyclized amide analogues were designed and synthesized. In vitro binding, metabolic stability, and in vivo biodistribution of these new derivatives were investigated. Several five-membered-ring isoindol-1-ones displayed very high in vitro binding affinity, especially 2-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}-6-nitro-3-phenyl-2, 3-dihydroisoindol-1-one, 15, 3-hydroxy-6-iodo-2-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}- 3- phenyl-2,3-dihydroisoindol-1-one, 18, and 6-iodo-2-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}-3-phenyl-2, 3-dihydroisoindol-1-one, 21, which showed Ki values of 0.05, 0.65, and 0.07 nM, respectively. The affinities for 5-HT1A receptors of other cyclized amide derivatives, 5-(4-bromophenyl)-1-{2-[4-(2-methoxyphenyl)- piperazin-1-yl]ethyl}pyrrolidin-2-one, 25, 5-(4-iodophenyl)-1-{2-[4-(2-methoxyphenyl)piperazin- 1-yl]ethyl}pyrrolidin-2-one, 27, and 2-{2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl}-2,3-dihydro- isoindol-1-one, 29, were 1.09, 2.54, and 14.9 nM, respectively. Compared to [125I]-31, iodinated cyclized amide derivatives [125I]-21 and [125I]-27 displayed a slower metabolism in human liver microsomal and cytosolic preparations. Biodistribution of [125I]-21 and [125I]-27 in rats (after an i.v. injection) displayed moderate to low brain uptakes with little or no specific localization in hippocampal region, where 5-HT1A receptors are concentrated. These data indicate that the new iodinated ligands showed high binding affinities and better metabolic stability but displayed unexpectedly low selective binding to 5-HT1A receptors in vivo. Additional structural modifications may be needed to correct the unfavorable properties displayed for these iodinated cyclized amide derivatives for in vivo biodistribution in rats.

Effects of a chronic lithium treatment on cortical serotonin uptake sites and 5-HT1A receptors

The objectives of this study were to characterize the effects of a chronic lithium (Li+) treatment on serotonin (5-HT) uptake sites and on 5-HT1A receptors, and to determine the eventual reversibility of the treatment. The experiments were carried out with membranes from rat cerebral cortex using 8-hydroxy-2-(propylamino)tetralin, or [3H]8-OH-DPAT, and [3H]citalopram to label 5-HT1A receptors and 5-HT uptake sites, respectively. Endogenous levels of 5-HT and 5-hydroxyindole-3-acetic acid (5-HIAA) were measured by high-performance liquid chromatography in the cingulate cortex. The saturation curves with [3H]8-OH-DPAT were always best fitted a two-site model. After a treatment with Li+ for 28 days, no alterations in the binding parameters of [3H]8-OH-DPAT to the high- and low-affinity binding sites could be documented. However, competition curves with 5-HT to inhibit [3H]8-OH-DPAT binding revealed a decreased proportion of sites with high affinity for the agonist, together with an increased density of sites with low affinity for 5-HT, suggesting an alteration in the coupling efficacy between 5-HT1A receptors and their transduction systems. Saturation studies with [3H]citalopram showed an increase (> 40%) in the density of 5-HT uptake sites after chronic Li+, suggesting a more efficient 5-HT uptake process for the treated animals, in accord with clinical observations. Although 5-HT contents in cingulate cortex remained unchanged after the treatment, 5-H[AA levels decreased (> 30%), leading to a diminished (almost 50%) 5-HT turnover; and also reflecting a more efficient uptake in the treated rats, so that less 5-HT could be degraded by extracellular monoamine oxidase. All the effects revealed by [3H]8-OH-DPAT and [3H]citalopram were reversed following a recovery period of two days without Li+. Since symptoms of bipolar affective disorders may reappear if the chronic Li+ treatment is interrupted, the reversibility of the observed effects further supports the importance of central 5-HT synaptic transmission in the pathophysiology and treatment of human affective disorders.

Ultrastructural localization of 5-hydroxytryptamine1A receptors in the rat brain

5-Hydroxytryptamine1A (5-HT1A) receptors have been visualized at the electron microscopic level in selected areas (dorsal raphe nucleus, hippocampus, septum) of the rat brain using specific anti-peptide antibodies. 5-HT1A receptor immunoreactivity was found almost exclusively in the somatodendritic compartment of neurons and was very rarely observed within processes possibly belonging to glial cells. The immunoenzymatic reaction product was associated exclusively with dendritic spines in the dorsal hippocampus, whereas in the dorsal raphe nucleus and the septal complex, immunoreactivity was found in both dendritic processes and somata. Although some immunolabeling was observed within the cytoplasm of cell bodies, 5-HT1A receptor immunoreactivity was essentially confined to the plasma membrane where it was unevenly distributed. It was frequently associated with synapses (except in the dorsal raphe nucleus), but was also found extrasynaptically in both somata and dendrites. These data suggest that the action of serotonin via 5-HT1A receptor could occur through junctional as well as nonjunctional transmission.

5HT1A receptor agonist differentially increases cyclic AMP concentration in intact and lesioned goldfish retina. In vitro inhibition of outgrowth by forskolin

5HT1A receptors occur in the retina of various species and the administration of 5HT1A agonists results in the inhibition of outgrowth from postcrush goldfish retinal explants. The levels of cyclic AMP (cAMP) play a role in the modulation of the outgrowth of the nevous system. Moreover, the stimulation of central 5HT1A receptors with the agonist 8-hydroxy-2-(di-n-propylamino)tetralin has been reported to produce an increase or decrease in the activity of adenylate cyclase. In the present investigation we studied the effect of adenylate cyclase stimulation by forskolin, as well as the modulatory effects of 5HT1A receptor agonists and antagonists on the production of cAMP in the goldfish retina, and on the outgrowth of this tissue in vitro. 8-Hydroxy-2-(di-n-propylamino)tetralin produced a significant and dose-dependent increase in cAMP concentration. This effect was not additive to the stimulation produced by forskolin. By contrast, as previously described, the 5HT1A agonist decreased cAMP concentration in the hippocampus of the rat. Both effects were significantly impaired by the 5HT1A antagonist WAY-100,135. A significant effect of the antagonist alone was observed only in the goldfish retina. The increase in cAMP levels was greater in the intact than in the postcrush retina. In addition, forskolin decreased the outgrowth of postcrush retinal explants in a dose-dependent manner, suggesting the importance of critical levels of cAMP in this process. Taken together, 5HT1A receptors seem to be positively coupled to adenylate cyclase in the goldfish retina, where cAMP plays a role as a modulator of outgrowth and regeneration. The inhibitory effect of 5HT1A receptor agonists on retinal outgrowth might be mediated through the production of cAMP. The activation of other subtypes of 5HT receptors positively coupled to adenylate cyclase by the 5HT1A agonist, such as 5HT7, cannot be discarded.

Serotonin1A receptors are expressed by a subpopulation of cholinergic neurons in the rat medial septum and diagonal band of Broca--a double immunocytochemical study

The possible colocalization of 5-hydroxytryptamine1A receptors and choline acetyltransferase in the same neurons of the medial septum and diagonal band of Broca was investigated using double immunocytochemical techniques, either on the same section or on adjacent thin sections of the rat brain. The presence of both antigens in the same neurons was demonstrated at the light and electron microscopic levels. The proportion of cholinergic neurons that express 5-hydroxytryptamine1A receptors was similar in the different parts of the septal complex (around 25%). By contrast, the proportion of 5-hydroxytryptamine1A receptor-positive neurons also exhibiting choline acetyltransferase immunoreactivity was much higher (40-44%) in the dorsal and ventral groups of cholinergic cells, than in the intermediate group (18%). In line with the topographical distribution of cholinergic projections, this result points out the potential involvement of 5-hydroxytryptamine1A receptors in the control of the septohippocampal cholinergic projection by serotonin. This connection might be relevant to learning and memory, and in the appearance of age-dependent or neurodegenerative cognitive deficits, which have been shown to involve alterations in both the serotoninergic and the cholinergic systems.

Synthesis of serotonin from 5-hydroxytryptophan in the post-crush retina: inhibition of in vitro outgrowth by the intraocular administration of the precursor

Serotonin is present in the retina of many species, in which plays roles as a neurotransmitter, as a modulator of regeneration, and as the precursor of melatonin. The turnover of serotonin in the goldfish retina is modified by the lesion of the optic nerve and, in postcrush goldfish retinal explants, serotonin inhibits the outgrowth. In the present study, the modification of the serotonergic system of the retina induced by the process of regeneration was explored. The addition of the precursor of serotonin, 5-hydroxytryptophan, to retinal explants, increased the levels of serotonin in a concentration-dependent manner. The concentration of serotonin differentially increased in control and postcrush explants cultured in the presence of 5-hydroxytryptophan for various periods of time, indicating a greater accumulation of the indoleamine at early periods of time in the control than in the postcrush tissue culture. This observation, together with the fact that serotonin concentration in postcrush retina cultured in the absence of 5-hydroxytryptophan and exposed to the precursor for 60 min increased less than in the control, indicates a saturation of the serotonergic system produced by the lesion. The addition of imipramine or citalopram, serotonin uptake blockers, did not significantly change the concentration of serotonin in the cultures, thus, the elevation of serotonin accumulation, especially in the post-crush tissue, might not be due to the transport from the medium. The intraocular injection of 5-hydroxytryptophan after the crush of the optic nerve resulted in a decrease in the outgrowth of retinal explants, supporting the in vivo role of serotonin during the regenerating process in situ. The lesion of the optic nerve did not affect the specific cells, since the number of serotonin-immunoreactive neurons in the retina were not modified by the crush. Taken together, retinal serotonin system is regulated after producing a lesion of the optic nerve, a modulation which has been demonstrated in vivo and in vitro. Thus, there is a reciprocal interaction, since serotonin influences outgrowth in the postcrush retina and the serotonergic system is modulated by the crush, indicating a mechanism of feed-back regulation.

Effect of stress on the behavior and 5-HT system in Sprague-Dawley and Wistar Kyoto rat strains

The effects of chronic novel stressors, for 21 days, on the behavior and the serotoninergic (5-HT) system in Sprague-Dawley (SD) and Wistar Kyoto (WKY) rats were studied. Open-field and forced-swim tests revealed a significantly greater behavioral depression in the WKY strain. SD rats showed a decrease in 3H-DPAT binding to 5-HT1A receptors in the hippocampus, whereas WKY rats revealed an increase in 3H-DPAT binding in the hippocampus and hypothalamus. Stress did not appear to alter the binding of 3H-DPAT to 5-HT1A sites in the dorsal raphe or median raphe in either strains. SD rats revealed a modest increase in 5-HT transporter (5-HTT) sites in the cortex; WKY rats revealed a decrease in 5-HTT sites in the cortex and the hippocampus. Stress caused an increase in 3H-CNIMI binding to 5-HTT sites in the dorsal and median raphe nuclei in both strains. The results suggest that the greater susceptibility to behavioral depression in WKY rats may account for the differential effects on 5HT1A sites as well as 5-HTT sites in limbic regions and cell body area as compared to SD rats.

Effects of repeated doses of azapirones on rat brain 5-HT1A receptors and plasma corticosterone levels

Chronic buspirone or ipsapirone (3 mg/kg, twice daily) administration to rats for 10 days decreased the sensitivity of inhibition of single-unit activity of serotonergic dorsal raphe neurons to a challenge by each drug. The ED50 for buspirone was increased from 0.1 mg/kg to 1.8 mg/kg, and the ED50 for ipsapirone was increased from 0.7 mg/kg to 1.2 mg/kg. The binding properties (Kd and Bmax) of [3H]8-OH-DPAT to membranes of cerebral cortex and hippocampus were unaffected by chronic administration of either buspirone or ipsapirone. Chronic buspirone or ipsapirone administration increased the tolerance of the hypothalamic-pituitary-adrenal axis (HPAA) following a challenge by each drug. The ED50 for elevation of plasma corticosterone levels was increased from 4.0 mg/kg to 7.6 mg/kg for buspirone and 6.2 mg/kg to 8.0 mg/kg for ipsapirone. Chronic buspirone administration decreased the basal activity of the HPAA by 63%. Chronic buspirone administration did not alter the plasma corticosterone response of the HPAA to a 1-min episode of rotational stress. (Mg2+)-ATPase, (Na+ + K+)-ATPase, (Ca2+ + Mg2+)-ATPase and calmodulin-stimulated (Ca2+ + Mg2+)-ATPase activities of erythrocyte plasma membrane were unaffected by either chronic or acute buspirone treatment, or by the addition of the drug to the in vitro assay systems.

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).

Serotonin turnover rate, [3H]paroxetine binding sites, and 5-HT1A receptors in the hippocampus of rats subchronically treated with clonazepam

Selective central benzodiazepine agonists, such as clonazepam, are known to modify serotonin and 5-hydroxyindoleacetic content in the brain. In order to further study the effect of this benzodiazepine on serotonin turnover rate, rats received clonazepam, 10 mg/kg for 10 days, and the concentrations of serotonin and 5-hydroxyindoleacetic acid were determined in the hippocampus after inhibition of monoamineoxidase with pargyline. The results indicate a reduction in the turnover rate of the monoamine. In addition, the systemic administration of clonazepam produced a decrease in the Bmax of [3H]DPAT binding to 5-HT1A sites in the hippocampus. By contrast, this effect was not observed if clonazepam was delivered into the dorsal raphe nucleus by osmotic minipumps. The binding of [3H]paroxetine to 5-HT reuptake sites was increased by the treatment with clonazepam. The present observations indicate that clonazepam produces a reduction of serotonin turnover rate in the hippocampus of the rat concomitant with a down-regulation of 5-HT1A binding sites, probably by an effect at the forebrain projections. There is also an up-regulation of the serotonin transporter, which might contribute to a reduction in the synaptic availability of serotonin during clonazepam treatment.

5-HT1A receptor localization on the axon hillock of cervical spinal motoneurons in primates

Serotonin (5-HT) has direct and specific effects on the activity of spinal cord motoneurons. The 5-HT1A receptor has been shown to mediate motoneuron responses in spinal reflex pathways using the highly selective 5-HT1A receptor agonist 8-OH-DPAT. We have developed an antipeptide antibody that recognizes a specific region (the second external loop) of the 5-HT1A receptor. This 5-HT1A receptor antibody labels populations of neurons and glia in the primate cervical spinal cord. The highest receptor density is present in the superficial lamina of the dorsal horn, around the central canal, and on the axon hillock of large ventral horn motoneurons. The cellular labeling pattern on motoneurons shows a single, densely stained, tapering process emanating from the perikaryon. A more diffuse label is also present throughout the soma. Dendritic labeling was not apparent. These results suggest that post-synaptic 5-HT1A receptors may be involved in modulating spinal motoneuron activity at the key site of action potential initiation, the axon hillock.

Evidence for 5-HT autoreceptor-mediated, nerve impulse-independent, control of 5-HT synthesis in the rat brain

To gain further insight into the operation of 5-HT autoreceptor-mediated feedback control of 5-HT biosynthesis in serotonergic nerve terminal areas, the effect of the 5-HT1B and the 5-HT1A receptor agonists, TFMPP and 8-OH-DPAT, respectively, were investigated in the rat central nervous system (CNS) using in vivo and in vitro neurochemical approaches. TFMPP suppressed 5-HT synthesis (5-HTP accumulation after decarboxylase inhibition) both in vivo and in vitro. In vivo, the 5-HT synthesis-suppressing effect of the drug (3.0 mg/kg, s.c.) proved resistant to either acute hemitransection or reserpine (5 mg/kg, i.p.; 90 min before) pretreatment. In vitro, in cortical, hippocampal and striatal slice preparations, TFMPP (0.1-10 microM) decreased 5-HT synthesis under basal and stimulated (30 mM K+) conditions, an effect which was unaltered by prior in vivo reserpine-induced 5-HT depletion but was attenuated in the presence of 5-HT1B receptor antagonists such as methiothepin, cyanopindolol or propranolol. The 8-OH-DPAT (0.1 mg/kg, s.c.)-induced decrease of 5-HT synthesis in vivo was abolished by hemitransection but resistant to acute reserpine pretreatment; 8-OH-DPAT (10 microM) did not decrease 5-HT synthesis in vitro. In conclusion, the present study confirms the importance of 5-HT autoreceptors in the feedback control of nerve terminal 5-HT biosynthesis. Specifically, our data indicate: (1) that the reduction of rat brain 5-HT synthesis after TFMPP is mediated by 5-HT1B autoreceptors located on the serotonergic axon terminals, and (2) that the effect is directly mediated and occurs independently of 5-HT neuronal firing and intact monoamine stores.

Synthesis of (+)-(R)- and (-)-(S)-trans-8-hydroxy-2-[N-n-propyl-N-(3'-iodo-2'-propenyl)] aminotetralin: new 5-HT1A receptor ligands

(R,S)-trans-8-Hydroxy-2-[N-n-propyl-N-(3'-iodo-2'- propenyl)amino]tetralin 7, a new radioiodinated ligand based on 8-OH-DPAT, was reported as a potential ligand for 5-HT1A receptors. The optically active (+)-(R)- and (-)-(S)-7 were prepared to investigate the stereoselectivity of (R,S)-7. Racemic intermediate 8-methoxy-2-N-n-propyltetralin was reacted with the acyl chloride of (-)-(R)-O-methylmandelic acid to form a mixture of (S,R)- and (R,R)-diastereoisomers, which were separated by flash column chromatography. After removing the N-acyl group from the diastereoisomers, the desired (+)-(R)- or (-)-(S)-7 was obtained by adding an N-iodopropenyl group. In vitro homogenate binding studies showed the stereoselectivity of this new compound for 5-HT1A receptors. (+)-(R)-7 isomer displayed 100-fold higher affinity than the (-)-(S)-7 isomer. Biochemical study indicated that (+)-(R)-7 potently inhibited forskolin-stimulated adenylyl cyclase activity in hippocampal membranes (Emax and EC50 were 24.5% and 5.4 nM, respectively), while (-)-(S)-7 showed no effect at 1 microM. The radioiodinated (+)-(R)- and (-)-(S)-[125I]7 were confirmed by coelution with the resolved unlabeled compound on HPLC (reverse phase column PRP-1, acetonitrile/pH 7.0 buffer, 80/20). The active isomer, (+)-(R)-[125I]7, displayed high binding affinity to 5-HT1A receptors (Kd = 0.09 +/- 0.02 nM). In contrast, the (-)-(S)-7 isomer displayed a significantly lower affinity to the 5-HT1A receptor (Kd > 10 nM).(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in exploratory activity following stimulation of hippocampal 5-HT1A and 5-HT1B receptors in the rat

The object exploration task allows the measure of changes in locomotor and exploratory activities, habituation, and reaction to a spatial change and to novelty. The effects of intrahippocampal (dorsal CA1 field) microinjections of serotonin 1 receptor (5-HT1) agonists on these behavioral components were evaluated in the rat. 8-Hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT, 5 micrograms/microliters) was used as a 5-HT1A agonist, 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP 93,129,16 micrograms/microliters) as a 5-HT1B agonist, and scopolamine (10 micrograms/microliters) as a muscarinic cholinergic antagonist. Scopolamine induced a long-lasting increase in locomotor activity and a lack of reaction to spatial change; both these results are in agreement with the known crucial influence of the septo-hippocampal cholinergic system in hippocampal functioning. Stimulation of 5-HT1A and 5-HT1B receptors induced a decrease in object exploration and habituation without affecting the retrieval of spatial information. But stimulation of hippocampal 5-HT1B receptors induced a selective change in the animal's emotional state, i.e., an initial decrease in locomotor activity and a neophobic reaction in response to a new object; such effects did not occur following stimulation of 5HT1A receptors. These results have to be considered in the light of the anxiogenic property of 5-HT1B agonists. On the whole, they support the hypothesis of the involvement of the serotonergic system, via 5HT1A and 5-HT1B receptors, in the modulation of hippocampal functions.

Evaluation of [O-methyl-3H]WAY-100635 as an in vivo radioligand for 5-HT1A receptors in rat brain

N-(2-(4-(2-Methoxyphenyl)-1-piperazinyl)ethyl)-N-(2- pyridyl)cyclohexanecarboxamide trihydrochloride (WAY-100635) is a new, potent and selective 5-HT1A receptor antagonist. We have evaluated radiolabelled WAY-100635 as a prospective radioligand for positron emission tomography (PET) by studying biodistribution in rat ex vivo. After intravenous injection, [O-methyl-3H]WAY-100635 cleared rapidly from plasma but was retained in brain. Specific binding was quantified from kinetic studies, using a reference-tissue compartment model, fitting for binding potential (k3/k4). The regional variation in binding potential correlated with the known distribution of 5-HT1A receptors. Saturation studies gave Bmax values in vivo that were consistent with those reported in vitro. At 60 min after injection, the ratio of radioactivity in 5-HT1A receptor-rich regions (e.g. septum, entorhinal cortex and hippocampus) to that in cerebellum reached approximately 16. Pre-dosing the rats with WAY-100635 (2 mg/kg) reduced this ratio to one, whereas similar pre-dosing with citalopram (5-HT uptake site inhibitor), prazosin (alpha 1A-adrenoceptor antagonist) or idazoxan (alpha 2-adrenoceptor antagonist) caused little or no reduction. Substantial (77%) blockade of [3H]WAY-100635 binding was achieved with the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), and the partial agonists, ipsapirone and buspirone. Thus, the properties of WAY-100635 are such that, when labelled with carbon-11, it could provide a radioligand suitable for clinical and pharmacological investigations of central 5-HT1A receptors in man using PET.

5-Hydroxytryptamine receptor subtypes in vertebrates and invertebrates

In the last few years, molecular biology has led to the cloning and characterization of several 5-HT receptors (serotonin receptors) in vertebrates and in invertebrates. These studies have allowed identification not only of 5-HT receptors already described but also of novel subtypes. The molecular cloning of 13 different mammalian receptor subtypes revealed an unexpected heterogeneity among 5-HT receptors. Except for the 5-HT3 receptors which are ligand-gated ion channel receptors, all the other 5-HT receptors belong to the large family of receptors interacting with G proteins. Based on their amino acid sequence homology and coupling to second messengers these receptors can be divided into distinct families: the 5-HT1 family contains receptors that are negatively coupled to adenylate cyclase: the 5-HT2 family includes receptors that stimulate phospholipase C; the adenylyl cyclase stimulatory receptors are a heterogeneous group including the 5-HT4 receptor which has not yet been cloned, the Drosophila 5-HTdro1 receptor and two mammalian receptors tentatively named 5-HT6 and 5-HT7 receptors. The 5-HT5A and 5-HT5B receptors might constitute a new family of 5-HT receptors whose effectors are unknown. This review focusses on the molecular characteristics of the cloned 5-HT receptors such as their structure, their effector systems and their distribution within the central nervous system. The existence of a large number of receptors with distinct signalling properties and expression patterns might enable a single substance like 5-HT to generate simultaneously a large panel of effects in many brain structures. The availability of the genes encoding these receptors has already allowed a partial characterization of their structure-function relationship and will probably allow in the future a dissection of the contribution of each of these receptor subtypes to physiology and behaviour.

Production and characterization of polyclonal antibodies recognizing the intracytoplasmic third loop of the 5-hydroxytryptamine1A receptor

The portion of the complementary DNA encoding the third intracellular loop of the rat 5-hydroxytryptamine1A (serotonin) receptor was subcloned into the vector pGEX-KG and expressed in Escherichia coli as a fusion protein coupled with the glutathione S-transferase of Schistosoma japonicum. The fusion protein was purified on a glutathione-agarose affinity column and used to immunize rabbits for the production of polyclonal anti-5-hydroxytryptamine1A receptor antibodies. Enzyme-linked immunosorbent assay revealed that antibodies were produced as early as one month after the first injection of the fusion protein, and immune response plateaued at a maximum after the third (monthly) booster injection. These antibodies only marginally affected the specific binding of [3H]8-hydroxy-2-(di-n-propyl-amino) tetralin to solubilized and membrane bound 5-hydroxytryptamine1A receptors, and did not interfere with serotonin-induced inhibition of forskolin-stimulated adenylate cyclase negatively coupled to 5-hydroxytryptamine1A receptors in rat hippocampal membranes. However, antibodies were able to immunoprecipitate 5-hydroxytryptamine1A receptor binding sites solubilized from rat hippocampal membranes. The distribution of immunoautoradiographic labelling and immunohistochemical staining of rat brain sections exposed to the antibodies raised against the fusion protein superimposed to that of 5-hydroxytryptamine1A receptor binding sites labelled by specific radioligands, with marked enrichment in the limbic areas (dentate gyrus and CA1 area in the hippocampus, lateral septum, entorhinal cortex) and the anterior raphe nuclei. The differential cellular location of immunoreactivity within the hippocampus (where dendritic fields but not pyramidal cell somas were immunostained) and the median raphe nucleus (where the plasmic membrane of somas was strongly immunoreactive) suggests that the addressing of 5-hydroxytryptamine1A receptors might differ from one neuronal cell type to another.

Effect of pertussis toxin on the response of rat medial prefrontal cortex cells to the iontophoresis of serotonin receptor agonists

In this study, we examined the response of spontaneously active as well as quiescent cells (L-glutamate-activated) in the rat medial prefrontal cortex (mPFc) to the iontophoresis of 2-methylserotonin (2-Me-5-HT, 5-HT3 receptor agonist), (+/-)-2,5-dimethoxy-(4-iodo-phenyl)-2-aminopropane (DOI, 5-HT2A,2C receptor agonist), 8-hydroxy-N,N-di-propylamino tetralin (8-OH-DPAT, 5-HT1A receptor agonist) and gamma-aminobutyric acid (GABA, a non-selective GABA receptor agonist) after the intracerebral administration of pertussis toxin, an inactivator of the Gi/o protein. This was accomplished using the techniques of extracellular single cell recording and iontophoresis. The administration of pertussis toxin (0.5 microgram, 24 hours before the experiment) into the mPFc did not alter the response of mPFc cells to the iontophoresis of DOI, 2-Me-5HT or GABA compared to saline treated controls. However, the response of mPFc cells to the iontophoresis of 8-OH-DPAT was significantly attenuated in the animals pretreated with pertussis toxin compared to controls. These results suggest that the 5-HT1A but not 5-HT2A,2C or 5-HT3 receptor is coupled to the Gi/o protein.

Postnatal development and localization of 5-HT1A receptor mRNA in rat forebrain and cerebellum

The localization of the rat brain 5-HT1A receptor mRNA was analyzed by RNAse mapping and in situ hybridization during postnatal development, particularly in the cerebellum. The regional distribution of 5-HT1A mRNA during the first 2 postnatal weeks was different from that found in adults. In some areas of the immature brain (hippocampus, cerebral cortex), 5-HT1A mRNA was found in lower density than in the adult brain. In contrast, high concentrations of the transcript were present in other brain structures only during the first days after birth. Thus, in the cerebellum, the density of 5-HT1A mRNA decreased markedly from day 2 to day 9 after birth and could hardly be detected in the adult animal. The localization of the mRNA in the molecular/Purkinje cell layer of the immature cerebellum agreed with that of the 5-HT1A receptor protein visualized by immunocytochemistry and was consistent with the hypothesis that Purkinje cells express this receptor.

Heterogeneity of cortical and hippocampal 5-HT1A receptors: a reappraisal of homogenate binding with 8-[3H]hydroxydipropylaminotetralin

The selective serotonin (5-HT) agonist 8-hydroxydipropylaminotetralin (8-OH-DPAT) has been extensively used to characterize the physiological, biochemical, and behavioral features of the 5-HT1A receptor. A further characterization of this receptor subtype was conducted with membrane preparations from rat cerebral cortex and hippocampus. The saturation binding isotherms of [3H]8-OH-DPAT (free ligand from 200 pM to 160 nM) revealed high-affinity 5-HT1A receptors (KH = 0.7-0.8 nM) and low-affinity (KL = 22-36 nM) binding sites. The kinetics of [3H]8-OH-DPAT binding were examined at two ligand concentrations, i.e., 1 and 10 nM, and in each case revealed two dissociation rate constants supporting the existence of high- and low-affinity binding sites. When the high-affinity sites were labeled with a 1 nM concentration of [3H]8-OH-DPAT, the competition curves of agonist and antagonist drugs were best fit to a two-site model, indicating the presence of two different 5-HT1A binding sites or, alternatively, two affinity states, tentatively designated as 5-HT1AHIGH and 5-HT1ALOW. However, the low correlation between the affinities of various drugs for these sites indicates the existence of different and independent binding sites. To determine whether 5-HT1A sites are modulated by 5'-guanylylimidodiphosphate, inhibition experiments with 5-HT were performed in the presence or in the absence of 100 microM 5'-guanylylimidodiphosphate. The binding of 1 nM [3H]8-OH-DPAT to the 5-HT1AHIGH site was dramatically (80%) reduced by 5'-guanylylimidodiphosphate; in contrast, the low-affinity site, or 5-HT1ALOW, was seemingly insensitive to the guanine nucleotide. The findings suggest that the high-affinity 5-HT1AHIGH site corresponds to the classic 5-HT1A receptor, whereas the novel 5-HT1ALOW binding site, labeled by 1 nM [3H]8-OH-DPAT and having a micromolar affinity for 5-HT, may not belong to the G protein family of receptors. To further investigate the relationship of 5-HT1A sites and the 5-HT innervation, rats were treated with p-chlorophenylalanine or with the neurotoxin p-chloroamphetamine. The inhibition of 5-HT synthesis by p-chlorophenylalanine did not alter either of the two 5-HT1A sites, but deafferentation by p-chloroamphetamine caused a loss of the low-affinity [3H]8-OH-DPAT binding sites, indicating that these novel binding sites may be located presynaptically on 5-HT fibers and/or nerve terminals.

Interactions of lesopitron (E-4424) with central 5-HT1A receptors: in vitro and in vivo studies in the rat

Previous studies have shown that the 5-HT1A receptor ligand, lesopitron (E-4424, 2-[4-[4-(4-chloro-1-pyrazolyl)butyl]-1-piperazinyl]pyrimidine), exerts potent anxiolytic-like effects in rodents and monkeys (Costall et al., 1992, J. Pharmacol. Exp. Ther. 262, 90). In an attempt to determine whether these effects are really mediated through the interaction of lesopitron with central 5-HT1A receptors, we investigated the agonistic and/or antagonistic nature of this interaction under in vitro and in vivo conditions in the rat. In vitro binding and autoradiographic studies with [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) and [3H]lesopitron as radioligands confirmed that lesopitron binds to 5-HT1A receptors in the rat brain with a relatively high affinity (pKi = 7.35). As expected of a full agonist at postsynaptic 5-HT1A receptors, lesopitron (IC50 = 125 nM) inhibited forskolin-stimulated adenylate cyclase activity in rat hippocampal membranes to the same extent as 5-HT, and this effect was preventable by potent 5-HT1A receptor antagonists such as (-)-tertatolol, (-)-propranolol and N-tert-butyl-3,4-(2-methoxyphenyl)piperazin-1-yl-2-phenyl- propanamide ((+)-WAY 100135). As previously shown for agonists acting at the somato-dendritic 5-HT1A autoreceptors in the dorsal raphe nucleus, lesopitron inhibited the firing of serotoninergic neurons both in vitro (in brainstem slices, IC50 = 120 nM) and in vivo (in chloral hydrate-anaesthetized rats, ID50 = 35 micrograms/kg i.v.), and this effect was preventable by (-)-tertatolol. Interestingly, the inhibition of the discharge due to lesopitron lasted for only a few minutes both in vitro and in vivo whereas the anxiolytic-like properties of this drug lasted for hours after a single injection in mice (Costall et al., 1992). In addition, the doses required for the stimulation of pre- and postsynaptic 5-HT1A receptors were markedly higher than those producing significant anxiolytic-like effects in rodents (Costall et al., 1992). It is therefore unlikely that the anxiolytic-like properties of lesopitron involve its stimulatory action at central 5-HT1A receptors.

Differential effects of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on various 5-HT receptor binding sites in the rat brain

The effects of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), an alkylating agent producing irreversible blockade of various membrane bound receptors in brain, were investigated on four different types of serotonin receptors, 5-HT1A, 5-HT1B, 5-HT2A and 5-HT3, in various brain regions in the rat. In addition, the fate of central benzodiazepine- and "R"-zacopride-specific binding sites was also examined in rats treated with EEDQ. Membrane binding assays and/or quantitative autoradiography with appropriate radioligands indicated that EEDQ inactivated 5-HT1A, 5-HT1B and 5-HT2A sites, but was poorly active on 5-HT3, benzodiazepine and "R" sites. Among the receptors affected by EEDQ, hippocampal 5-HT1A sites were the most sensitive to the alkylating agent (ID50 approximately 1 mg/kg i.p.), followed by the cortical 5-HT2A (ID50 approximately 3 mg/kg i.p.) and the striatal 5-HT1B (ID50 approximately 6 mg/kg i.p.) sites. Pretreatment by selective ligands partially protected hippocampal 5-HT1A sites from irreversible inactivation by EEDQ (10 mg/kg i.p.) with the following order of efficacy: WAY 100635 > spiperone > BMY 7378 > ipsapirone. Similarly, pretreatment by spiperone (5 mg/kg i.p.) also reduced the ability of EEDQ to inactivated cortical 5-HT2A receptors. Analyses of the time-course recovery of respective binding sites after EEDQ administration showed that the turnover rate of 5-HT1A sites did not significantly differ in the dorsal raphe nucleus and in various forebrain areas (hippocampus, septum, cerebral cortex; half-life: approximately 4 days), but was lower than that of cortical 5-HT2A sites (half-life: 2.9 days).

5-HT1A receptor-mediated inhibition of N-type calcium current in acutely isolated ventromedial hypothalamic neuronal cells

The effects of serotonin (5-hydroxytryptamine: 5-HT) and 5-HT1A agonist on voltage-gated calcium channels (VGCCs) in acutely isolated ventromedial hypothalamic (VMH) neuronal cells were studied using whole-cell configuration of the patch-clamp technique. 5-HT at 10 microM inhibited inward calcium current reversibly in 80% of cells. This inhibition was specific to N-type current. Because pindolol blocked the effect of 5-HT and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) mimicked the effect of 5-HT, the inhibitory effect of 5-HT appeared to be mediated via the 5-HT1A receptor. In the fura-2 fluorometry method, 8-OH-DPAT attenuated the [Ca2+]i increase induced by the depolarization stimulus of 50 mM K+. These results indicate that 5-HT suppresses Ca2+ entry through N-type channels in the VMH neurons via the 5-HT1A receptor and that the stimulating effect of 8-OH-DPAT on feeding behavior may be mediated by the blocking of Ca2+ entry through N-type channels.

Comparison of the effects of IVth ventricular administration of some tryptamine analogues with those of 8-OH-DPAT on autonomic outflow in the anaesthetized cat

1 The present study compares the effects on representative autonomic outflows of IVth ventricular application of tryptamine analogues which act at 5-HT1 receptors with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). 2 Cumulative doses of 8-OH-DPAT, N,N-di-n-propyl-5-carboxamidotryptamine (DP-5-CT) and 5-carboxamidotryptamine (5-CT, 2.5-40 nmol kg-1), sumatriptan (10-160 nmol kg-1), indorenate (100-800 nmol kg-1), 5-hydroxytryptamine (5-HT, 20-640 nmol kg-1) both alone and in the presence of cinanserin (0.1 mg kg-1) were given into the IVth ventricle of cats which were anaesthetized with a mixture of alpha-chloralose and pentobarbitone sodium, neuromuscularly blocked and artificially ventilated. Recordings were made of arterial blood pressure, heart rate, renal, cardiac, splanchnic and phrenic nerve activities, femoral arterial flow, tracheal and intragastric pressures. 3 Central application of each of the agonists evoked significant falls in arterial blood pressure. In addition 8-OH-DPAT, DP-5-CT, 5-CT and 5-HT all evoked a differential inhibition of sympathetic nerve activities, with renal nerve activity being the most sensitive and cardiac nerve activity the least sensitive. In the dose-ranges used, administration of sumatriptan evoked reductions only in renal and splanchnic nerve activities whilst indorenate reduced activity in all three sympathetic nerves to a similar extent. 4. The effect of the agonists on heart rate was more inconsistent than the effects on sympathetic outflow.IVth ventricular application of 5-CT and sumatriptan were without effect on heart rate whilst 8-OH-DPAT, DP-5-CT, indorenate and 5-HT alone and in the presence of cinanserin all evoked significant bradycardias. However, whilst atropine partially reversed the bradycardias evoked by 8-OHDPAT and only slightly reversed those caused by indorenate, atropine was without effect on those evoked by DP-5-CT or 5-HT.5. None of the analogues tested had significant effects on gut motility, phrenic nerve discharge or tracheal pressure. 8-OH-DPAT, DP-5-CT, indorenate and 5-HT were without effect on femoral arterial conductance. However, following pretreatment with cinanserin, 5-HT evoked a significant reduction in femoral arterial conductance. At its highest dose, sumatriptan evoked a significant increase in femoral arterial conductance as did 5-CT at the 20 nmol kg-1 dose.6. It is concluded that the present data support the view that 5-HT1A receptors at the level of the brainstem are involved in the central sympathoinhibitory effects caused by intravenous administration of 5-HT1A agonists. Further, brainstem 5-HT1A receptors play an important role in the control of renal sympathetic outflow while brainstem 5-HT2 receptors are involved in the control of skeletal muscle and/or skin blood flow. Selective tryptamine agonists for 5-HT1A receptors differ from non-tryptamine agonists in that they do not cause an increase in central cardiac vagal tone.

Further assessment of the antagonist properties of the novel and selective 5-HT1A receptor ligands (+)-WAY 100 135 and SDZ 216-525

In vitro biochemical and electrophysiological methods were used to assess the potential antagonist properties of the novel compounds (+)-WAY 100 135 [N-tert-butyl-3,4-(2-methoxyphenyl)piperazin-1-yl-2- phenylpropanamide dihydrochloride] and SDZ 216-525 [methyl 4-(4-(4-(1,1,3-trioxo-2H-1,2-benziosothiazol-2-yl)butyl)- 1-piperazinyl)1H-indole-2-carboxylate] at pre- (i.e. somatodendritic autoreceptors) and postsynaptic 5-HT1A receptors in the rat brain. Both (+)-WAY 100 135 and SDZ 216-525 were pure antagonists at postsynaptic 5-HT1A receptors negatively coupled to adenylate cyclase in rat hippocampal membranes. Competitive prevention of the inhibition by the 5-HT1A receptor agonists 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin], 5-HT (5-hydroxytryptamine), S-20499 [(+)-4-(N-(5-methoxychroman-3-yl)-N-propylamino)butyl-8-azaspir o(4,5)decane- 7,9-dione] and lesopitron occurred with a pA2 of 8.7 for (+)-WAY 100 135 and 9.9 for SDZ 216-525. The higher potency of the latter compound was also noted at the level of presynaptic 5-HT1A receptors where both (+)-WAY 100 135 and SDZ 216-525 prevented the negative influence of 5-HT1A receptor agonists (8-OH-DPAT, flesinoxan or lesopitron) on the nerve impulse flow within dorsal raphe nucleus 5-HT neurones in brain stem slices. At high concentrations, both (+)-WAY 100 135 (> 1 microM) and SDZ 216-525 (> or = 0.1 microM) inhibited the spontaneous cell discharge through different mechanisms. The blockade of alpha 1-adrenoceptors by (+)-WAY 100 135 apparently accounted for its inhibitory influence on the firing of 5-HT neurones, whereas 5-HT1A receptor agonist properties were responsible for the effect of SDZ 216-525. Although approximately 10 times less potent than SDZ 216-525, (+)-WAY 100 135 is therefore a pure antagonist at both pre- and postsynaptic 5-HT1A receptors in the rat brain.

Localization of 5-HT1A receptors to astroglial cells in adult rats: implications for neuronal-glial interactions and psychoactive drug mechanism of action

Although tissue culture studies have shown a variety of neurotransmitter receptors on astroglial cells, verifying these observations in adult animals has been difficult and rarely accomplished. In the current study we have used double immunocytochemistry to localize 5-HT1a receptors to astroglial cells in fixed sections of adult rat brain. The astroglial cells were identified using an antibody raised against the astroglial-specific protein glial fibrillary acidic protein (GFAP). To label the 5-HT1a receptor, we used an antibody we recently raised against a unique peptide sequence occurring in the second extracellular loop of the receptor. Our results show that the 5-HT1a receptor occurs in relatively high abundance on astroglial cells. There is regional specificity, the receptor being much more commonly found in septum and hippocampus than striatum. There are also intraregional differences in that even within a single brain region one astrocyte may have very high levels of the receptor while an adjacent cell has none. We propose that the cellular localization of this receptor could have significance in understanding the mechanism of action of 5-HT1a receptor active drugs in alleviating anxiety and depression. The mechanism may be through the release of a neurotrophic agent, S-100 beta, from astrocytes. This factor may then cause regeneration or sprouting of neuronal terminals which have been lost due to a disease process.

(-)Tertatolol is a potent antagonist at pre- and postsynaptic serotonin 5-HT1A receptors in the rat brain

The potential 5-HT1A antagonist properties of the beta-antagonist tertatolol were assessed using biochemical and electrophysiological assays in the rat. (+/-) Tertatolol bound with high affinity (Ki = 38 nM) to 5-HT1A sites labelled by [3H]8-OH-DPAT in hippocampal membranes. The (-)stereoisomer (Ki = 18 nM) was about 50-fold more potent than the (+)stereoisomer (Ki = 864 nM) to inhibit the specific binding of [3H]-8-OH-DPAT. As expected of a 5-HT1A antagonist, (-)tertatolol prevented in a concentration-dependent manner (Ki = 24 nM) the inhibitory effect of 8-OH-DPAT on forskolin-stimulated adenylate cyclase activity in rat hippocampal homogenates. Furthermore in vivo pretreatment with (-)tertatolol (5 mg/kg s.c.) significantly reduced the inhibitory influence of 8-OH-DPAT (0.3 mg/kg s.c.) on the accumulation of 5-hydroxytryptophan in various brain areas after the blockade of aromatic L-amino acid decarboxylase by NSD-1015 (100 mg/kg i.p.). In vitro (in brainstem slices; Ki approximately 50 nM) and in vivo (in chloral hydrate anaesthetized rats; ID50 approximately 0.40 mg/kg i.v.), (-)tertatolol prevented the inhibitory effects of the 5-HT1A receptor agonists 8-OH-DPAT, ipsapirone and lesopitron on the firing rate of serotoninergic neurones within the dorsal raphe nucleus. In about 25% of these neurones, the basal firing rate was significantly increased by (-)tertatolol (up to +47% in vitro, and +30% in vivo). These data indicate that (-)tertatolol is a potent competitive antagonist at both pre (in the dorsal raphe nucleus)-and post (in the hippocampus)-synaptic 5-HT1A receptors in the rat brain.

Perfusate serotonin increases extracellular dopamine in the nucleus accumbens as measured by in vivo microdialysis

The effects of local application of serotonin (5-HT) on extracellular levels of dopamine (DA) in the nucleus accumbens (N. ACC) were assessed using in vivo microdialysis. At a perfusate flow rate of 0.3 microliter/min the baseline dialysate concentration of DA was 2.1 +/- 0.7 nM (mean +/- S.E.M.; n = 5) and significantly increased to 142 +/- 18%, 220 +/- 47% and 332 +/- 35% of baseline when 0.1 microM, 0.2 microM and 0.4 microM concentrations of 5-HT were included in the perfusate. Perfusate 5-HT concentrations below 0.1 microM had no effect on dialysate DA. The in vivo dialysis efficiency for 5-HT was found to be 39 +/- 12%, and thus the concentrations of 5-HT reaching the extracellular space at the surface of the dialysis membrane were estimated to be 40, 80 and 160 nM for the 0.1, 0.2 and 0.4 microM 5-HT perfusates, respectively. The serotonin-induced increase in dialysate DA was attenuated by co-perfusion of 0.4 microM 5-HT with 4 microM concentrations of pindolol (a relatively non-specific 5-HT1 antagonist; 151 +/- 7% vs. 332 +/- 35% baseline dialysate DA for 5-HT/antagonist and 5-HT-only perfusates, respectively), LY 53,857 (a specific 5-HT2 antagonist; 130 +/- 17% vs. 332 +/- 35%) and MDL 7222 (a specific 5-HT3 antagonist; 143 +/- 19% vs. 332 +/- 35%).(ABSTRACT TRUNCATED AT 250 WORDS)

5-HT-1A receptor-mediated effects of antidepressants

1. Antidepressant (AD) drugs in general induce subsensitivity of behavioural functions associated with activation of 5-HT-1a receptors in animals. 2. Electrophysiological studies in animals in general indicate increased serotonergic transmission after AD administration, mediated partly by increased functioning of post-synaptic 5-HT-1a receptors in the hippocampus. 3. Binding studies have in general shown no change in 5-HT-1a receptor number either pre-or post-synaptically, while results of second messenger studies (inhibition of adenylate cyclase) indicate subsensitivity after AD administration. 4. Human studies also indicate subsensitivity of 5-HT-1a receptors after ADs.

Physiopharmacological interactions between stress hormones and central serotonergic systems

The present review tries to delineate some mechanisms through which the sympathetic nervous system (SNS) and the hypothalamo-pituitary-adrenal (HPA) interact with central serotonergic systems. The recent progress in 5-hydroxytryptamine (5-HT) receptor pharmacology has helped to define the means by which central serotonergic activity may alter the respective activities of the SNS (sympathetic nerves and adrenomedulla) and of the HPA axis. These pharmacological findings have also helped to characterize the differential effects of central 5-HT upon different branches of the SNS and the numerous sites at which 5-HT exerts stimulatory influences upon the HPA axis. Although relevant to stress-related neuroendocrinology, the extent to which these interactions are involved in the antidepressant/anxiolytic properties of some serotonergic agents still remains to be clarified. Beside these findings, there is also abundant evidence for a tight control of central serotonergic systems by stress hormones. Activation of the SNS increases, by numerous means, central availability of tryptophan, whereas glucocorticoids exert differential actions upon the intra- and the extraneuronal regulation of 5-HT function. Actually, a significant number of these mechanisms is involved in the maintenance of homeostasis during stressful events, thereby conferring to these mechanisms a key role in adaptation processes.

(-)-Penbutolol as a blocker of central 5-HT1A receptor-mediated responses

Brain 5-HT1A and 5-HT1B receptors are important targets for drug-induced modulation of 5-HT function in vivo. However, very few compounds are available that are effective antagonists at 5-HT1 receptors, thus hampering the progress of fundamental as well as clinical research in this area. The present study assessed the usefulness of the beta-adrenolytic agent (-)-penbutolol (and its (+)-counterpart) as a 5-HT1A receptor-blocking agent. The compound was found to counteract, in a stereospecific fashion, not only the behavioural and hypothermic but also the in vivo 5-HT synthesis/turnover-reducing effects of the specific 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). These findings indicate that (-)-penbutolol is an antagonist at both postsynaptic receptors and somatodendritic autoreceptors of the 5-HT1A subtype. Thus, (-)-penbutolol represents a useful addition to the array of pharmacological tools available for the study of central 5-HT1 receptor-mediated functions.

Quantification of 5-hydroxytryptamine1A receptors in the cerebellum of normal and X-irradiated rats during postnatal development

5-Hydroxytryptamine1A receptors were studied in rats during the first postnatal month in the normal cerebellum and in the granule cell-deprived cerebellum produced by X-irradiation at postnatal day 5. Quantitative autoradiographic studies on sagittal sections of cerebellar vermis, using [1251]BH-8-MeO-N-PAT as radioligand or specific anti-receptor antibodies, revealed that 5-hydroxytryptamine1A receptors existed in the molecular/Purkinje cell layer but at variable density from one lobule to another. Thus, in both normal and X-irradiated rats, the posterior lobules were more heavily labelled than the anterior ones, and the density of 5-hydroxytryptamine1A sites decreased progressively in all the cerebellar folia down to hardly detectable levels at postnatal day 21. However, the intensity of labelling remained higher at postnatal day 8 and postnatal day 12 in X-irradiated rats than in age-paired controls. Measurements of [3H]8-OH-DPAT specific binding to membranes from whole cerebellum confirmed that the density of 5-hydroxytryptamine1A sites per mg membrane protein (Bmax) was higher in X-irradiated animals than in age-paired controls. However, on a "per cerebellum" basis, no significant difference could be detected between the total number of 5-hydroxytryptamine1A sites, which progressively increased in both control and X-irradiated animals during the first postnatal month. These results therefore show that 5-hydroxytryptamine1A receptors are not located on developing granule cells. The progressive decrease in 5-hydroxytryptamine1A receptor density during the first postnatal month did not reflect a transient expression of 5-hydroxytryptamine1A receptors in the cerebellum of newborn rats, but resulted from the progressive "dilution" of these sites in this growing structure. The higher density of 5-hydroxytryptamine1A sites in X-irradiated rats simply reflected a lower "dilution" due to the delayed growth of the cerebellum in these animals.

Differential coupling of 5-HT1A receptors occupied by 5-HT or 8-OH-DPAT to adenylyl cyclase

Human serotonin (5-hydroxytryptamine, 5-HT)-1A receptors have been transfected in NIH-3T3 cells, and their coupling to adenylyl cyclase was analysed depending on 1) the number of receptor expressed, 2) the experimental conditions used, 3) the nature of the agonists. Two monoclonal cell lines were used, expressing low (45 fmol/mg) and high (500 fmol/mg) levels of 5-HT1A receptor. Two methods were tested to study the negative coupling of the transfected 5-HT1A receptors to adenylyl cyclase: 1) measurement of cAMP production in intact cells, 2) measurement of adenylyl cyclase activity in vitro on membrane preparations. Studies on intact cells revealed that an increase in the receptor concentration was followed by 1) an increase in the efficacies of 5-HT, 5-CT (5-carboxamidotryptamine) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), 2) a 2 to 3-fold increase in the potency of 5-CT and 8-OH-DPAT, but no change in the potency of 5-HT. In membrane preparations, 8-OH-DPAT dose-response curve was shifted leftwards when the receptor concentration became higher whereas the corresponding shift was smaller for 5-HT and absent for 5-CT. Surprisingly, on membrane preparations, 8-OH-DPAT was a partial agonist relative to 5-HT. The relative efficacy of 8-OH-DPAT was lower in the clone expressing the lowest level of receptor. This partial agonist behavior of 8-OH-DPAT could be modulated by the ionic conditions under which the adenylyl cyclase activity was measured.(ABSTRACT TRUNCATED AT 250 WORDS)