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

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

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

Role of the serotonergic system in alcohol dependence: from animal models to clinics

Alcohol dependence remains among the most common substance abuse problems worldwide, and compulsive alcohol consumption is a significant public health concern. Alcohol is an addictive drug that alters brain function through interactions with multiple neurotransmitter systems. These neurotransmitter systems mediate the reinforcing effects of alcohol. Specifically, the serotonergic system is important in mediating alcohol reward, preference, dependence, and craving. In this review chapter, we first discuss the serotonin system as it relates to alcoholism, and then outline interactions between this system and other neurotransmitter systems. We emphasize the serotonin transporter and its possible role in alcoholism, then present several serotonergic receptors and discuss their contribution to alcoholism, and finally assess the serotonin system as a target for pharmacotherapy, with an emphasis on current and potential treatments.

Stereoselective HPLC analysis of tertatolol in rat plasma using macrocyclic antibiotic chiral stationary phase

Enantiomeric resolution of teratolol was achieved on a vancomycin macrocyclic antibiotic chiral stationary phase known as Chirobiotic V with UV detection set at 220 nm. The polar ionic mobile phase (PIM) consisted of methanol-glacial acetic acid-triethylamine (100:0.01:0.015, v/v/v) has been used at a flow rate of 0.8 ml min(-1) . The calibration curves in plasma were linear over the range of 5-500 ng ml(-1) for each enantiomer with detection limit of 2 ng ml(-1) . The proposed method was validated in compliance with the international conference on harmonization (ICH) guidelines. The developed method applied for the trace analyses of tertatolol enantiomers in plasma and for the pharmacokinetic study of tertatolol enantiomers in rat plasma. The assay proved to be suitable for therapeutic drug monitoring and chiral quality control for tertatolol formulations by HPLC.

Organocatalytic Highly Enantioselective Tandem Michael-Knoevenagel Reaction for the Synthesis of Substituted Thiochromanes

Enantioenriched tetrasubstituted thiochromanes have been synthesized using a tandem Michael addition-Knoevenagel reaction between 2-mercaptobenzaldehydes and benzylidenemalonates with a 9-epi-aminoquinine thiourea derivative as the catalyst. Steric and electron effects were found to affect profoundly the enantioselectivity and diastereoselectivity of the reaction.

5-HT1A receptor knockout mouse as a genetic model of anxiety

Low levels of the serotonin(1A) (5-HT(1A)) receptor have been repeatedly found in mood and anxiety disorders. Stress often exacerbates psychiatric disease and can also reduce 5-HT(1A) receptor levels. When receptor deficiency was produced in mice by genetic knockout, an anxiety-like phenotype was observed. Anxiety in mice is defined as a high level of avoidance of novel and unfamiliar environment and increased fear reaction. Other aspects of anxiety such as autonomic activation, increased stress responsiveness, and neuroendocrine abnormalities have also been described in receptor knockout mice. These data indicate that 5-HT(1A) receptor knockout mice represent a genetic animal model of anxiety with both construct and face validities. Although the core phenotype of anxiety can be reproduced in knockout mice in various inbred and outbred backgrounds, abnormalities in 5-HT dynamics and resistance to the anxiolitic drug diazepam have been seen in one but not on other genetic backgrounds. This indicates that while the development of anxiety is an invariable consequence of receptor deficit, other features induced by receptor loss are strongly modulated by other gene(s). Strain-dependent variability within the core phenotype does not diminish the value of 5-HT(1A) receptor knockout mice as a model of anxiety. Indeed, it is consistent with the manifestation of anxiety in genetically heterogeneous human population.

Mechanism of action of flibanserin in the learned helplessness paradigm in rats

The mechanism of action of flibanserin, a 5-HT(1A) receptor agonist and a 5-HT(2A) receptor antagonist, was investigated in learned helplessness in rats. The effect of flibanserin (32 mg/kg, i.p. 30 min before testing) on learned helplessness was not antagonized by the (a) 5-HT synthesis inhibitor parachlorophenylalanine (pCPA; 150 mg/kg p.o.x3 times), which reduced brain 5-HT by 89%; (b) 5-HT(1A) receptor antagonists (+/-)-N-tert-butyl-3-4-(2-ethoxyphenyl)piperazin-1yl-2-phenyl propionamide [WAY100135; 10 mg/kg, i.p. 30 min before flibanserin, or 40 mg/kg, s.c. 15 min before flibanserin] and tertatolol (2.5 and 5 mg/kg, i.p. 30 min before flibanserin); and (c) 5-HT(2) receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI; 3 mg/kg, s.c. simultaneously with flibanserin). The effect of flibanserin on learned helplessness was antagonized by the dopamine D(1) receptor antagonist [R]-3-methyl-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH 23390; 0.1 mg/kg, i.p. 30 min before flibanserin) and by the opioid receptor antagonist naloxone (3 mg/kg, s.c. 15 min before flibanserin). Flibanserin (32 and 64 mg/kg) did not induce conditioned place preference. In conclusion, flibanserin improved rats' performance in the learned helplessness paradigm, by stimulating dopamine D1 and opioid receptors, probably indirectly, since flibanserin has a low affinity for these receptors.

Serotonin(1A) receptors in mood disorders: a combined genetic and genomic approach

The serotonin1A (5-HT1A) receptor has been under intense investigation, mostly due to its putative role in both the etiology and therapeutic treatments of depression and anxiety-related behaviors. However, the exact contribution of this receptor to normal brain physiology and disease processes remains poorly understood, due to a complex expression pattern and multiple functions. Recent development in genetic and genomic approaches allows not only for more refined functional dissection, but also for probing large gene databases for unknown gene product interactions. Here, we describe an experimental approach that is based on a combination of regional and temporal genetic manipulations of the 5-HT1A receptor with large-scale gene expression profiling to attempt to untangle the distinct roles for this receptor in particular brain regions, as well as to identify molecular partners that mediate its function. In turn, new leads for understanding mechanisms of anxiety, depression and their pharmacological treatments may be generated.

Differential effects of 5-HT1A receptor deletion upon basal and fluoxetine-evoked 5-HT concentrations as revealed by in vivo microdialysis

An involvement of serotonin (5-HT) 1A receptors in the etiology of psychiatric disorders has been suggested. Hypo-responsiveness of the 5-HT1A receptor is linked to anxiety and constitutive deletion of the 5-HT1A receptor produces anxiety-like behaviors in the mouse. Evidence that 5-HT1A receptor inactivation increases the therapeutic effects of antidepressants has also been presented. The present studies used in vivo microdialysis and homologous recombination techniques to examine the contribution of 5-HT1A autoreceptors to these effects. Basal and fluoxetine-evoked extracellular concentrations of 5-HT were quantified in the striatum, a projection area of dorsal raphe neurons (DRN), of wild-type (WT) and 5-HT1A receptor knock out (KO) mice. The density of 5-HT transporters was also determined. Basal 5-HT concentrations did not differ in WT and KO mice. Fluoxetine (10 mg/kg) increased 5-HT concentrations in both genotypes. This increase was, however, 2-fold greater in KO mice. In contrast, no differences in K(+)-evoked 5-HT concentrations were seen. Similarly, neither basal nor stimulation-evoked DA differed across genotype. Autoradiography revealed no differences between genotype in the density of 5-HT transporters or post-synaptic 5-HT2A receptors, an index of 5-HT neuronal activity. These experiments demonstrate that, under basal and KCl stimulated conditions, adaptive mechanisms in the 5-HT system compensate for the lack of 5-HT1A autoreceptor regulation of DRN. Furthermore, they suggest that the absence of release-regulating 5-HT1A autoreceptors in the DRN can not account for the anxiety phenotype of KO mice. The enhanced response to fluoxetine in KO mice is consistent with pharmacological studies and suggests that adaptive mechanisms that occur in response to 5-HT1A receptor deletion are insufficient to oppose increases in 5-HT concentrations produced by acute inhibition of the 5-HT transporter.

Affinity of (+/-)-pindolol, (-)-penbutolol, and (-)-tertatolol for pre- and postsynaptic serotonin 5-HT(1A) receptors in human and rat brain

There is considerable interest in the use of drugs that selectively block presynaptic (somatodendritic) serotonin 5-HT(1A) receptors for the adjunctive treatment of major depressive disorder. The 5-HT(1A)/beta-adrenoceptor ligands (+/-)-pindolol, (-)-tertatolol, and (-)-penbutolol are currently under clinical investigation, and knowledge of their affinity at different populations of central 5-HT(1A) receptors is needed. Here we have determined the affinity of these drugs for presynaptic and postsynaptic 5-HT(1A) receptors in postmortem human and rat brain using receptor autoradiography and the selective 5-HT(1A) radioligand [(3)H]WAY-100635. The binding of [(3)H]WAY-100635 was specific and saturable and showed high affinity in the rat dorsal raphe nucleus and hippocampus (K(D) = 1.5-1.7 nM). In competition studies, the three compounds had nanomolar affinity and produced monophasic displacement of [(3)H]WAY-100635 binding in all regions of both species. (-)-Penbutolol and (-)-tertatolol had similar affinity for pre-and postsynaptic 5-HT(1A) receptors in both rat and human brain. However, in the human, but not the rat, the affinity of (+/-)-pindolol in dorsal raphe nucleus (K(i) = 8.9 +/- 1. 1 nM) was slightly but significantly higher than that in hippocampus (K(i) = 14.4 +/- 1.5 nM in CA1). In summary, our data show that (+/-)-pindolol, (-)-tertatolol, and (-)-penbutolol are all high-affinity ligands at native human and rat 5-HT(1A) receptors. (-)-Penbutolol and (-)-tertatolol do not discriminate between the pre- and postsynaptic 5-HT(1A) sites tested in either species, but (+/-)-pindolol showed a slightly higher affinity for the presynaptic site in human brain. Further work is needed to establish whether the latter difference is clinically relevant.

5-HT(1A) agonist potential of pindolol: electrophysiologic studies in the dorsal raphe nucleus and hippocampus

BACKGROUND

The ability of pindolol to block 5-HT(1A) autoreceptors on serotonin-containing neurons in the raphe nuclei is thought to underlie the clinical reports of enhanced efficacy and rate of improvement in depressed patients treated with pindolol/selective serotonin reuptake inhibitor (SSRI) combinations. Selectivity for somatodendritic 5-HT(1A) autoreceptors is a crucial requirement, as blockade of postsynaptic 5-HT(1A) sites may jeopardize the therapeutic response. Previous investigators have probed the effects of pindolol on serotonergic dorsal raphe cell firing in animal species; here we confirm their findings and extend them to include observations on postsynaptic 5-HT(1A) receptors in the hippocampus.

METHODS

Extracellular single-unit recordings were made in rats using standard electrophysiologic techniques. Firing rates of serotonin-containing neurons in the dorsal raphe nucleus and CA3 hippocampal pyramidal neurons were monitored and the effects of pindolol given alone or in combination with an SSRI (fluoxetine) or a 5-HT(1A) antagonist (WAY-100,635) were determined.

RESULTS

Pindolol inhibited the firing rates of serotonergic dorsal raphe neurons in a dose-dependent manner. Recovery to baseline firing rates was gradual, but this inhibition could be acutely reversed by WAY-100,635. A range of pindolol doses failed to block the inhibitory effects of fluoxetine on dorsal raphe cell firing. In the hippocampus, pindolol also inhibited cell firing as a function of dose, although these effects were insensitive to WAY-100,635 treatment.

CONCLUSIONS

The ability of pindolol to inhibit serotonergic dorsal raphe cell firing is indicative of its agonist potential and is consistent with previous studies. The lack of observable antagonism of the SSRI-induced slowing of raphe unit activity casts doubt on the suitability of this mechanism of action to account for the positive findings in clinical studies utilizing pindolol/SSRI combinations. The 5-HT(1A)-independent inhibition of hippocampal CA3 cell firing by pindolol suggests that this compound invokes multiple pharmacologic actions, all of which need to be assimilated into any proposed mechanism of action.

Genetic inactivation of the Serotonin(1A) receptor in mice results in downregulation of major GABA(A) receptor alpha subunits, reduction of GABA(A) receptor binding, and benzodiazepine-resistant anxiety

Anxiety is a common psychiatric illness often treated by benzodiazepines (BZs). BZs, such as Valium, bind to the alpha subunit of the pentameric GABA(A) receptor and increase inhibition in the CNS. There is considerable evidence for abnormal GABA(A) receptor function in anxiety, and a significant proportion of anxiety patients has a reduced sensitivity to BZs. Here, we show that serotonin(1A) (5-HT(1A)) receptor knock-out mice display BZ-resistant anxiety. Consistent with this finding, binding of both BZ and non-BZ GABA(A) receptor ligands were reduced and GABAergic inhibition was impaired in mutant mice. These changes were reflected by abnormal alpha subunit expression in the amygdala and hippocampus, two important limbic regions involved in fear and anxiety. These data suggest a pathological pathway, initiated by a 5-HT(1A) receptor deficit, leading to abnormalities in GABA(A) receptor composition and level, which in turn result in BZ-insensitivity and anxiety. This model mechanistically links together the 5-HT and GABA systems, which both have been implicated in anxiety. A related mechanism may underlie reduced BZ sensitivity in certain forms of anxiety.

Effect of flibanserin (BIMT 17), fluoxetine, 8-OH-DPAT and buspirone on serotonin synthesis in rat brain

In male rats, the effects of the administration of the novel serotonergic agent flibanserin on the synthesis of 5-HT were evaluated in the frontal cortex (FC), hippocampus (Hip) and brainstem (Br). The selective serotonergic uptake blocker, fluoxetine, and two serotonin1A (5-HT1A) agonists, 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) and buspirone, were used as reference compounds. The synthesis of 5-HT was assessed by measuring the accumulation of 5-hydroxytryptophan (5-HTP) after blockade of aromatic amino acid decarboxylase induced by m-hydroxybenzylhydrazine (NSD-1015), at 100 mg/kg i.p., 30 min before sacrifice. Flibanserin, 8-OH-DPAT and buspirone were given 15 min before NSD-1015, while fluoxetine 120 min before NSD-1015. In our experimental conditions, a different efficacy, expressed as percentage of maximal inhibition (Max) of 5-HTP accumulation, and a different potency, expressed in terms of minimal effective dose (MED), were observed in different brain areas with tested compounds. Flibanserin (1-32 mg/kg) decreased 5-HT synthesis with preferential activity in the FC, compared to the Hip and Br, both in terms of potency (MED=2 mg/kg in FC, 16 mg/kg in Hip and Br) and efficacy (Max=65% in FC, 44% in Hip and 29% in Br). Fluoxetine (1-30 mg/kg) decreased 5-HT synthesis with preferential activity in FC than in Hip and Br, only in terms of potency (MED=3 mg/kg in FC, 10 mg/kg in Hip and Br), this result being similar to that observed for flibanserin. In contrast, it showed greater efficacy both in FC and Hip (Max about 60%), than in Br (Max=49%). On the contrary, 8-OH-DPAT (0.3-3 mg/kg) decreased 5-HT synthesis with the same potency in all brain regions (MED=3 mg/kg) and showed the greatest efficacy in FC than in Hip and Br (Max=56% in FC, 49% in Hip and 40% in Br). Furthermore, buspirone (3-30 mg/kg), while inhibiting 5-HTP accumulation in all areas with the same efficacy (Max about 30%), seemed to have higher potency in Br than in FC and Hip (MED=3 mg/kg in Br, 10 mg/kg in FC and Hip). The results in terms of regional differences are discussed.

5-HT1A and 5-HT1B receptors control the firing of serotoninergic neurons in the dorsal raphe nucleus of the mouse: studies in 5-HT1B knock-out mice

The characteristics of the spontaneous firing of serotoninergic neurons in the dorsal raphe nucleus and its control by serotonin (5-hydroxytryptamine, 5-HT) receptors were investigated in wild-type and 5-HT1B knock-out (5-HT1B-/-) mice of the 129/Sv strain, anaesthetized with chloral hydrate. In both groups of mice, 5-HT neurons exhibited a regular activity with an identical firing rate of 0.5-4.5 spikes/s. Intravenous administration of the 5-HT reuptake inhibitor citalopram or the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) induced a dose-dependent inhibition of 5-HT neuronal firing which could be reversed by the selective 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xane carboxamide (WAY 100635). Both strains were equally sensitive to 8-OH-DPAT (ED50 approximately 6.3 microgram/kg i.v.), but the mutants were less sensitive than wild-type animals to citalopram (ED50 = 0.49 +/- 0.02 and 0.28 +/- 0.01 mg/kg i.v., respectively, P < 0.05). This difference could be reduced by pre-treatment of wild-type mice with the 5-HT1B/1D antagonist 2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carbox yli c acid [4-methoxy-3-(4-methyl-piperazine-1-yl)-phenyl]amide (GR 127935), and might be accounted for by the lack of 5-HT1B receptors and a higher density of 5-HT reuptake sites (specifically labelled by [3H]citalopram) in 5-HT1B-/- mice. In wild-type but not 5-HT1B-/- mice, the 5-HT1B agonists 3-(1,2,5, 6-tetrahydro-4-pyridyl)-5-propoxypyrrolo[3,2-b]pyridine (CP 94253, 3 mg/kg i.v.) and 5-methoxy-3-(1,2,3, 6-tetrahydropyridin-4-yl)-1H-indole (RU 24969, 0.6 mg/kg i.v.) increased the firing rate of 5-HT neurons (+22.4 +/- 2.8% and +13.7 +/- 6.0%, respectively, P < 0.05), and this effect could be prevented by the 5-HT1B antagonist GR 127935 (1 mg/kg i.v.). Altogether, these data indicate that in the mouse, the firing of 5-HT neurons in the dorsal raphe nucleus is under both an inhibitory control through 5-HT1A receptors and an excitatory influence through 5-HT1B receptors.

Production and characterization of an anti-serotonin 1A receptor antibody which detects functional 5-HT1A binding sites

We describe the production and characterization of a specific anti-5-HT1A receptor antibody made against a fusion protein consisting of glutathione-S-transferase (GST) coupled to a 75-amino acid sequence from the middle portion of the third intracellular loop (5-HT1A-m3i, serine253-arginine327) of the rat 5-HT1A receptor protein. This region was chosen to avoid putative phosphorylation and glycosylation sites and regions of known homology with other 5-HT receptors. Western blot analysis indicated that the polyclonal anti-5-HT1A-m3i antibody accurately recognized the fusion protein expressed in bacteria and labeled a prominent 67 kDa protein band in the hippocampus, cortex, brainstem, cerebellum and kidney with a density profile corresponding to the relative abundance of the 5-HT1A receptor in these tissues. No protein was detected in liver or muscle tissue preparations, and no protein bands were labeled in any of the above tissues following preabsorption of the antibody with the 5-HT1A-m3i fusion protein. Immunohistochemistry revealed prominent labeling in limbic structures including the hippocampus, amygdala, entorhinal cortex, and septum as well as in raphe nuclei. In the hippocampus, 5-HT1A-m3i labeling revealed a characteristic laminar pattern that coincided with that seen by autoradiographic binding of the 5-HT1A agonist [3H]-8-OH-DPAT in all strata of the hippocampal formation. In the dorsal and medial raphe nuclei, anti-5-HT1A-m3i antibodies labeled the somatodendritic membranes of 5-HT neurons, consistent with its role as an autoreceptor. The detailed matching of the anti-5-HT1A-m3i antibody with [3H]-8-OH-DPAT binding suggests that the antibody recognizes a functionally active form of the 5-HT1A receptor protein capable of binding 5-HT1A agonist ligands. These anti-5-HT1A antibodies may therefore be useful tools in localizing functional 5-HT1A receptors in specific regions of the brain as well as in studying the plasticity and ontogeny of the 5-HT1A receptor at the cellular and subcellular level.

Effects of (-)-tertatolol, (-)-penbutolol and (+/-)-pindolol in combination with paroxetine on presynaptic 5-HT function: an in vivo microdialysis and electrophysiological study

The antidepressant efficacy of selective serotonin reuptake inhibitors (SSRIs) might be enhanced by co-administration of 5-HT1A receptor antagonists. Thus, we have recently shown that the selective 5-HT1A receptor antagonist, WAY 100635, blocks the inhibitory effect of an SSRI on 5-HT cell firing, and enhances its ability to elevate extracellular 5-HT in the forebrain. Here we determined whether the beta-adrenoceptor/5-HT1A receptor ligands (+/-)-pindolol, (-)-tertatolol and (-)-penbutolol, interact with paroxetine in a similar manner. Both (-)-tertatolol (2.4 mg kg(-1) i.v.) and (-)-penbutolol (2.4 mg kg(-1) i.v.) enhanced the effect of paroxetine (0.8 mg kg(-1) i.v.) on extracellular 5-HT in the frontal cortex, whilst (+/-)-pindolol (4 mg kg(-1) i.v.) did not. (-)-Tertatolol (2.4 mg kg(-1) i.v.) alone caused a slight increase in 5-HT however, (-)-penbutolol (2.4 mg kg(-1) i.v.) alone had no effect. In electrophysiological studies (-)-tertatolol (2.4 mg kg(-1) i.v.) alone had no effect on 5-HT cell firing but blocked the inhibitory effect of paroxetine. In contrast, (-)-penbutolol (0.1-0.8 mg kg(-1) i.v.) itself inhibited 5-HT cell firing, and this effect was reversed by WAY 100635 (0.1 mg kg(-1) i.v.). We have recently shown that (+/-)-pindolol inhibits 5-HT cell firing via a WAY 100635-sensitive mechanism. Our data suggest that (-)-tertatolol enhances the effect of paroxetine on forebrain 5-HT via blockade of 5-HT1A autoreceptors which mediate paroxetine-induced inhibition of 5-HT cell firing. In comparison, the mechanisms by which (-)-penbutolol enhances the effect of paroxetine on extracellular 5-HT is unclear, since (-)-penbutolol itself appears to have agonist properties at the 5-HT1A autoreceptor. Indeed, the agonist action of (+/-)-pindolol at 5-HT1A autoreceptors probably explains its inability to enhance the effect of paroxetine on 5-HT in the frontal cortex. Overall, our data suggest that both (-)-tertatolol and (-)-penbutolol are superior to (+/-)-pindolol in terms of enhancing the effect of an SSRI on extracellular 5-HT. Both (-)-tertatolol and (-)-penbutolol are worthy of investigation for use as adjuncts to SSRIs in the treatment of major depression.

Role of somatodendritic 5-HT autoreceptors in modulating 5-HT neurotransmission

A very important element controlling serotonin (5-HT) release throughout the brain is the 5-HT1A autoreceptor present on the soma and dendrites of 5-HT neurons since it exerts a negative feedback influence on their firing activity. This 5-HT1A autoreceptor receives an increased activation by endogenous 5-HT at the beginning of a treatment with a selective 5-HT reuptake inhibitor (SSRI) and, consequently, a decreased 5-HT neuronal firing activity is obtained. As the SSRI treatment is prolonged, the 5-HT1A autoreceptor desensitizes and firing activity is restored in the presence of the SSRI. That this adaptive change underlies, at least in part, the delayed therapeutic effect of SSRI in major depression is supported by the acceleration of the antidepressant response by the concomitant administration of the 5-HT1A autoreceptor antagonist pindolol with SSRIs.

Increased anxiety of mice lacking the serotonin1A receptor

Brain serotonin (5-HT) has been implicated in a number of physiological processes and pathological conditions. These effects are mediated by at least 14 different 5-HT receptors. We have inactivated the gene encoding the 5-HT1A receptor in mice and found that receptor-deficient animals have an increased tendency to avoid a novel and fearful environment and to escape a stressful situation, behaviors consistent with an increased anxiety and stress response. Based on the role of the 5-HT1A receptor in the feedback regulation of the 5-HT system, we hypothesize that an increased serotonergic neurotransmission is responsible for the anxiety-like behavior of receptor-deficient animals. This view is consistent with earlier studies showing that pharmacological activation of the 5-HT system is anxiogenic in animal models and also in humans.

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.

Assessment of relative efficacies of 5-HT1A receptor ligands by means of in vivo animal models

We have evaluated the effects of ligands with varying efficacies at beta-adrenoceptors and 5-HT1A receptors in three in vivo models reflecting pre- and/or postsynaptic 5-HT1A receptor activation. Forepaw treading in rats is mediated by postsynaptic 5-HT1A receptors, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamin)tetralin)-induced discriminative stimulus is predominantly mediated by postsynaptic, but presynaptic 5-HT1A receptors might also be involved, and footshock-induced ultrasonic vocalization involves predominantly presynaptic 5-HT1A receptors. In vitro receptor binding studies demonstrated high beta-adrenoceptor and 5-HT1A receptor affinity of (-)-penbutolol, high beta-adrenoceptor and 60 times lower 5-HT1A receptor affinity of (+)-penbutolol, high beta-adrenoceptor affinity and about 100 times lower 5-HT1A receptor affinity of pindolol and (-)-tertatolol, only affinity for beta-adrenoceptors of metoprolol and ICI 118,551 (erythro-D,L-1-(7-methylindan-4-yloxy)-3-isopropylamine-b utan-2-ol, and only affinity for 5-HT1A receptors of WAY 100.635 ((N-[2-[4- (2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexane-carboxamide). (-)-Penbutolol, (-)-tertatolol, pindolol and WAY 100.635 antagonized 5-MeODMT-induced (5-methoxy-N, N-dimethyltryptamine) forepaw treading in rats, and (+)-penbutolol, ICI 118,551 and metoprolol were inactive. (-)-Penbutolol, WAY 100.635 and (-)-tertatolol antagonized 8-OH-DPAT-induced discriminative stimulus in rats, pindolol and metoprolol showed a mixed antagonistic and agonistic profile. Pindolol antagonized footshock-induced ultrasonic vocalization in rats, tertatolol inhibited maximum 36% and WAY 100.635, (-)-penbutolol, (+)-penbutolol, metroprolol and ICI 118,551 were inactive. (-)-Penbutolol and WAY 100.635 reversed 8-OH-DPAT-induced inhibition of ultrasonic vocalization completely, (-)-tertatolol reversed maximum 52% and (+)-penbutolol and pindolol were inactive. It is concluded, that efficacies at 5-HT1A receptors can be estimated by applying a battery of in vivo test models that involve post- and presynaptic receptors to a variable degree. The in vivo ranking order of efficacy at 5-HT1A receptors was: WAY 100.635 = (-)-penbutolol < (-)-tertatolol < pindolol.

Comparative study of pre- and postsynaptic 5-HT1A receptor modulation of anxiety in two ethological animal tests

The purpose of this study was to determine the roles of the presynaptic 5-hydroxtryptamine1A (5-HT1A) receptors in the median raphé nucleus (MRN) and of the postsynaptic 5-HT1A receptors in its projection area of the dorsal hippocampus in the social interaction and elevated plus-maze tests of anxiety. Direct administration of the 5-HT1A receptor agonist (+/-)-8-hydroxy-dipropylaminotetralin (8-OH-DPAT, 200 ng) into the MRN had significant anxiolytic effects in all three test situations examined (social interaction, plus-maze trails 1 and 2). These anxiolytic effects were antagonized by a silent dose (200 ng) of the 5-HT1A receptor antagonist WAY 100635, confirming that they were mediated by 5-HT1A receptors. In contrast, after bilateral administration to the dorsal hippocampus, 8-0H-DPAT (100 ng) had significant anxiogenic effects in the social interaction test and in plus-maze trial 2. These anxiogenic effects were antagonized by silent doses of 5-HT1A receptor antagonists [(+)WAY 100135, 10 mg/kg s.c., and intrahippocampal (+/-)tertatolol, 3 microg, respectively], confirming mediation by 5-HT1A receptors. In rats naive to the plus-maze, neither 8-OH-DPAT (50, 100, or 200 ng) nor the 5-HT1A receptor antagonist (+/-)tertatolol (3 microgram) had any significant effect when administered to the dorsal hippocampus. This demonstrates that previous experience of a rat in the plus-maze has a major effect on the sensitivity of dorsal hippocampal 5-HT1A receptors, as we have demonstrated previously for the 5-HT1A receptors in the dorsal raphé nucleus. Overall, our results provide evidence that stimulation of the presynaptic 5-HT1A receptors in the MRN results in an anxiolytic action, whereas stimulation of the post-synaptic 5-HT1A receptors in its projection area results in an anxiogenic effect. These results are consistent with an overall reduction in 5-HT neurotransmission in the dorsal hippocampus having an anxiolytic effect, and they explain the relatively weak anxiolytic profile detected when 5-HT1A receptor agonists are given systemically.

5-HT1A and benzodiazepine receptors in the basolateral amygdala modulate anxiety in the social interaction test, but not in the elevated plus-maze

In order to investigate the role of the 5-HT1A receptors of the amygdala in modulating anxiety, rats were implanted with bilateral cannulae aimed at the basolateral nucleus of the amygdala complex and infused with either artificial cerebrospinal fluid (aCSF) or the selective 5-HT1A receptor agonist 8-OH-DPAT (50-200 ng) and tested in two animal models of anxiety. In the elevated plus-maze test, no significant effects were detected in this dose range. In contrast, 8-OH-DPAT caused an overall reduction in levels of social investigation, thus indicating anxiogenic actions in the social interaction test. At 50 ng, 8-OH-DPAT had a selective action on anxiety, while at 200 ng there was a concomitant reduction in locomotor activity and, in some animals, signs of the 5-HT1A syndrome. Evidence that the anxiogenic effect of 8-OH-DPAT (50 ng) was due to activation of 5-HT1A receptors came from the finding that (-)-tertatolol, a 5-HT1A receptor antagonist, reversed this effect at a dose (1.5 micrograms) which was silent when given alone. The benzodiazepine receptor agonist, midazolam (1 and 2 micrograms) was bilaterally administered into the basolateral nucleus of the amygdala and evoked clear-cut anxiolytic effects in the social interaction test. These data indicate that the agonist activation of post-synaptic 5-HT1A receptors in the basolateral nucleus of the amygdala may produce anxiogenic effects, while agonist activation of BDZ receptors in the same areas evokes anxiolytic effects. Our results from the social interaction test are similar to those previously reported from tests of anxiety using punished paradigms, but contrast with those found in the elevated plus-maze. Thus, it is concluded that either the two tests have different sensitivities to midazolam and 8-OH-DPAT or more intriguingly, the tests are evoking fundamentally different states of anxiety, with that evoked by the plus-maze being mediated via brain areas or receptors different from those studied here.

The mechanism by which the selective 5-HT1A receptor antagonist S-(-) UH 301 produces head-twitches in mice

Electrophysiological studies indicate that certain 5-HT1A receptor antagonists increase the basal firing rate of some but not all raphe neurons by antagonizing the inhibitory endogenous serotonin tone operating on the somatodendritic pulse-modulating presynaptic 5-HT1A autoreceptors. This effect should enhance the synaptic concentration of 5-HT (5-hydroxytryptamine) in serotonergic terminal fields, which may then activate postsynaptic 5-HT receptors. However, in vivo microdialysis studies show that generally such 5-HT1A antagonists by themselves do not increase the basal 5-HT release but potentiate the ability of serotonin reuptake blockers to increase the neuronal serotonin terminal output in the rat brain via the above mechanism. The purpose of the present study was to determine whether antagonism of the proposed endogenous serotonin tone on the 5-HT1A autoreceptors can potentiate the activity of other postsynaptic serotonin receptors. To this end, we utilized the head-twitch response (HTR) in mice as an in vivo model of postsynaptic 5-HT2A receptor function. The selective and silent 5-HT1A receptor antagonist, S-(-)UH 301, by itself, in a dose-dependent manner, produced the HTR in normal but not in reserpinized animals. The 5-HT2A antagonist, SR 46349B, completely prevented S-(-)UH 301-induced HTR. Pretreatment with S-(-)UH 301 also potentiated 5-hydroxytryptophan (5-HTP)-induced HTR both in normal and in the reserpinized mice. At low doses (0.06-0.25 mg/kg), the 5-HT2A selective agonist, 8-OH DPAT, significantly but partially inhibited 5-HTP-induced HTR. However, further attenuation was not observed following the administration of larger doses of 8-OH DPAT. Depending upon the dose used, S-(-)UH 301 pretreatment not only antagonized but also broke through the inhibitory effect of 8-OH DPAT on 5-HTP-induced HTR. The selective (sertraline) and nonselective (cocaine) serotonin reuptake blockers potentiated the ability of 5-HTP to induce the head-twitch behavior in mice. Pretreatment with S-(-)UH 301 enhanced the potentiating effect of serotonin reuptake blockers on the 5-HTP induced HTR. These results suggest that an endogenous 5-HT tone via the discussed mechanism controls the terminal field synapticactivity of serotonergic neurons in mice. In addition, disinhibition of pulse-modulating 5-HT1A autoreceptors by S-(-)UH 301 can potentiate the synaptic effects of serotonin reuptake blockers as well as the serotonin precursor 5-HTP. However, a more firm general conclusion regarding antagonism of presynaptic 5-HT1A receptors leading to indirect functional enhancement of other postsynaptic serotonergic receptors can only be made when the above hypothesis is further tested with other selective 5-HT1A receptor antagonists (such as WAY 100 635), which we were unable to obtain. The present study is the first report to show that a selective 5-HT1A antagonist by itself can produce a serotonin-mediated function via indirect stimulation of another serotonin receptor subtype in mice.

Preclinical profile of the mixed 5-HT1A/5-HT2A receptor antagonist S 21,357

This study evaluated the pharmacological and behavioral effects of S 21,357, a drug with high affinity for both 5-HT1A and 5-HT2A receptors. The drug behaved as antagonist at both 5-HT1A autoreceptors and postsynaptic 5-HT1A receptors, as it prevented the inhibitory effect of lesopitron on the electrical discharge of the dorsal raphé nucleus (DRN) 5-HT neurons and the activity of forskolin-stimulated adenylate cyclase in hippocampal homogenates. In addition, S 21,357 (4 and 128 mg/kg, PO) inhibited 5-HTP-induced head-twitch responses in mice, indicating that it possesses 5-HT2A antagonistic properties. In a test battery designed to assess defensive behaviors of Swiss-Webster mice to the presence of, or situations associated with, a natural threat stimulus (i.e., rat), S 21,357 (0.12-2 mg/kg, IP) reduced contextual defense reactions after the rat was removed, risk assessment activities when the subject was chased, and finally, defensive attack behavior. These behavioral changes are consistent with fear/anxiety reduction. Furthermore, the drug strongly reduced flight reactions in response to the approaching rat. This last finding, taken together with recent results with panic-modulating drugs, suggest that S 21,357 may have potential efficacy against panic attack. Finally, our results suggest that compounds sharing high affinities for both 5-HT1A and 5-HT2A receptors may directly or synergistically increase the range of defensive behaviors affected.

Anxiolytic effects in the plus-maze of 5-HT1A-receptor ligands in dorsal raphé and ventral hippocampus

The response of rats naive to, or experienced with, the elevated plus-maze test of anxiety was observed following direct administration of the 5-HT1A-receptor agonist (+/-)-8-hydroxy-dipropylaminotetralin (8-OH-DPAT) (50, 100, or 200 ng) or antagonist tertatolol (3 micrograms) into the dorsal raphé nucleus or bilaterally into the ventral hippocampus. In rats naive to the plus-maze, neither drug had a significant effect when microinjected into the dorsal raphé nucleus. However, in rats experienced with the plus-maze, 8-OH-DPAT (100 and 200 ng) had significant anxiolytic effects when administered to the dorsal raphé nucleus, which were antagonised by tertatolol (3 micrograms); this suggests they were mediated by 5-HT1A receptors. Hyperactivity (increased number of closed-arm entries) was found following bilateral injection of 8-OH-DPAT (100 ng) into the ventral hippocampus of rats naive to the plus-maze. This was not completely antagonised by tertatolol (3 micrograms). Interestingly, tertatolol (3 micrograms) itself had an anxiolytic effect which was not antagonised by 8-OH-DPAT (100 ng), suggesting the effect was not mediated by 5-HT1A receptors, and indeed other actions of tertatolol, such as those on 5-HT1B or beta-adrenergic receptors could have been involved. In rats experienced with the plus-maze, tertatolol (3 micrograms) again had a significant anxiolytic effect when administered bilaterally to the ventral hippocampus, and again, this was not antagonised by 8-OH-DPAT (100 ng). These results demonstrate that both the intracerebral location of the injection and test experience profoundly influence the effects of 5-HT1A ligands on behaviour of rats in the elevated plus-maze test of anxiety.

(-)-pindolol and (+/-)-tertatolol affect rat hippocampal 5-HT levels through mechanisms involving not only 5-HT1A, but also 5-HT1B receptors

The present work examined, using in vivo microdialysis, the effects of 0.16-10 mg/kg of the beta-adrenoceptor antagonists, (-)-pindolol and (+/-)-tertatolol, which have additional 5-HT1A receptor antagonist properties, on extracellular 5-HT levels in the ventral hippocampus of chloral hydrate-anaesthetized rats. These effects were compared with those observed when (-)-pindolol and (+/-)-tertatolol were given together with the 5-HT1A agonist 8-OH-DPAT (0.31 mg/kg i.p.). When given alone, (-)-pindolol and (+/-)-tertatolol increased 5-HT levels not only after systemic administration (at 2.5 and 10 mg/kg s.c.), but also when perfused locally through the dialysis probe (at a concentration of 10 microM). At doses equal to or lower than those that increased 5-HT when given alone, (-)-pindolol and (+/-)-tertatolol inhibited the decrease of extracellular 5-HT levels induced by 8-OH-DPAT. At higher doses, however, (-)-pindolol and (+/-)-tertatolol were less able to reverse these effects of 8-OD-DPAT. The selective beta 1-adrenoceptor antagonist, (+/-)-betaxolol, did not alter 5-HT levels, either when given alone or when given together with 8-OD-DPAT. Although the antagonism of the 8-OH-DPAT-induced decrease of 5-HT levels by (-)-pindolol and (+/-)-tertatolol is likely to be related to their 5-HT1A antagonist properties, their ability to increase extracellular 5-HT levels when given alone may involve interactions with 5-HT1B receptors at hippocampal 5-HT terminals.

Effects of 8-OHDPAT and 5-HT1A antagonists WAY100135 and WAY100635, on guinea-pig behaviour and dorsal raphe 5-HT neurone firing

1. The effects of 5-HT1A antagonists on guinea-pig behaviour and dorsal raphe neuronal activity were investigated. 2. WAY100135 (10 mg kg-1, s.c.) and WAY100635 (1 mg kg-1, s.c.) significantly reduced the behaviours induced by 8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) (1 mg kg-1, s.c.) indicative of post-synaptic 5-HT1A receptor antagonism. WAY100635 (10 mg kg-1, s.c.) alone induced ear twitches, which were antagonized by ketanserin (1 mg kg-1, s.c.), but no other overt behaviours. 3. WAY100635 (0.125 mg kg-1, i.v.) produced a right-ward shift in the dose-related inhibition of neuronal firing by 8-OHDPAT (5-100 micrograms kg-1, i.v.) but did not affect the maximum inhibition induced by 8-OHDPAT indicating competitive antagonism between 8-OHDPAT and WAY100635 at the 5-HT1A somato-dendritic autoreceptor in the dorsal raphe nucleus of the guinea-pig. WAY100635 also produced a dose-related increase in the basal firing of 5-HT neurones in the dorsal raphe nucleus and restored the firing of dorsal raphe neurones previously inhibited by 8-OHDPAT (10 micrograms kg-1, i.v.). 4. The results indicate that WAY100635 is a competitive 5-HT1A antagonist in the guinea-pig. Furthermore WAY100635 can increase 5-HT neuronal firing, suggesting that it blocks a 5-HT1A receptor-mediated inhibitory tone acting on guinea-pig 5-HT neurones resulting in increased 5-HT release and 5-HT2 receptor-mediated behaviours.

Effects of acute and repeated administration of citalopram on extracellular levels of serotonin in rat brain

The effects of acute (2 days) and repeated (21 days) administration (50 mg/kg in the diet) of the selective serotonin (5-HT, 5-hydroxytryptamine) reuptake inhibitor, citalopram, on extracellular levels of 5-HT and their modulation by terminal autoreceptors in the hypothalamus of freely moving rats were compared in vivo by microdialysis. When studied without washout, extracellular levels of 5-HT were increased by both acute and repeated citalopram administration. In rats treated repeatedly, extracellular 5-HT levels were 43% (but not significantly) greater than in those treated acutely. Extracellular levels of 5-HT in control and citalopram-treated rats were similar when measured after 24 h washout. The enhancing effect of non-selective serotonergic autoreceptor antagonists, methiothepin (100 microM) or 1-(1-naphthyl)piperazine (NP) (10 microM), administered through the microdialysis probe, after 24 h washout, was similar in both control and chronically treated groups. These results suggest that repeated administration of citalopram followed by a washout of 24 h does not lead to desensitization of the terminal autoreceptor as measured in vivo in contrast to the effects we have shown previously in vitro. In rats treated chronically with citalopram without washout, methiothepin had a greater maximal effect on 5-HT outflow in comparison to rats receiving acute citalopram treatment. This finding suggests that a 5-HT autoreceptor antagonist or a combination of such a drug with a 5-HT uptake inhibitor would produce a greater increase of extracellular levels of 5-HT in hyposerotonergic states such as depression.

beta-Adrenoceptor regulation in rat heart, lung and skin after chronic treatment with (--)-tertatolol or (--)-propranolol

1. The effect of long-term treatment with the beta-adrenoceptor antagonists (--)-tertatolol and (--)-propranolol was studied. Sprague-Dawley rats were treated with either (--)-tertatolol (50 micrograms kg-1 hr-1), (--)-propranolol (250 micrograms kg-1 hr-1) or vehicle (1 mM HCl) for 14 days with osmotic minipumps implanted subcutaneously. 2. The mean daily systolic blood pressure and heart rate of rats treated with either (--)-tertatolol (108 +/- 1 mmHg/330 +/- 3 bpm) or (--)-propranolol (103 +/- 1 mmHg/330 +/- 2 bpm) were lower than in the control (126 +/- 1 mmHg/405 +/- 3 bpm, P < 0.001, n = 8-10) indicating the effectiveness of drug delivery. 3. Autoradiographic studies in areas of heart, lung and skin showed that beta-adrenoceptor populations were not significantly affected by the drug treatment (all regions P > 0.05). Nevertheless, the receptor population in the homogenates of (--)-tertatolol treated lung were halved (194 +/- 28 fmol mg protein-1 compared with a control value of 388 +/- 54 fmol mg protein-1, P < 0.01, n = 6). 4. In the presence of CGP 20712A, the left atrial inotropic and right atrial chronotropic responsiveness to (--)-isoprenaline were hypersensitive in both (--)-tertatolol and (--)-propranolol-treated groups (P < 0.005, ANCOVA). 5. (--)-Propranolol treated left ventricular free wall had lower basal [3H]-forskolin binding to adenylate cyclase (14.45 +/- 1.20 fmol mg protein-1 compared with a control value of 18.91 +/- 0.78 fmol mg protein-1, P = 0.01, n = 6). (--)-Tertatolol treatment had no effect on the basal binding. In the presence of the G-protein activators NaF and Gpp(NH)p, the enhancement of [3H]-forskolin binding did not differ between control and the drug treated groups. 6. Chronic (--)-tertatolol or (--)-propranolol treatment therefore did not produce an increase in receptors in heart, lung or skin but the beta-adrenoceptor-mediated responses were enhanced. In addition, [3H]-forskolin binding did not increase suggesting that the hypersensitivity was not due to changes in the number of receptors or adenylate cyclase. Hypersensitivity following beta-adrenoreceptor antagonist administration may therefore involve enhanced coupling of receptors to G-proteins.

BIMT 17, a 5-HT2A receptor antagonist and 5-HT1A receptor full agonist in rat cerebral cortex

In the search for antidepressant agents with a rapid onset of action, we have found that compound BIMT 17 (1-[2-[4-(3-trifluoromethylphenyl)piperazin-1- yl]ethyl]benzimidazol-[1H]-2-one) shows a good affinity for cerebral cortical 5-HT1A (pKi = 7.72) and 5-HT2A (pKi = 6.90) receptors, with no appreciable affinity for the other 5-HT receptor subtypes, including 5-HT2C. BIMT 17 reduced forskolin-stimulated cAMP accumulation in the cerebral cortex (pEC50 = 6.09) and in the hippocampus (pEC50 = 6.50), and antagonized 5-HT-induced phosphatidylinositol turnover (pKi = 6.96) in the cerebral cortex. The effect on cAMP accumulation was blocked by the 5-HT1A receptor antagonist tertatolol. Buspirone, 8-OH-DPAT and S 14671 (1-[2-(2-thenoylamino)ethyl]- 4[1-(7-methoxynaphtyl)]-piperazine), claimed to be 5-HT1A receptor agonists, did not reduce forskolin-stimulated cAMP formation in the cerebral cortex. On the basis of these data, it was concluded that BIMT 17 was the only compound that behaved as a full agonist with respect to the cAMP response in the cortex, while exerting concurrent agonism at 5-HT1A receptors and antagonism at 5-HT2A receptors. These characteristics might explain the peculiar behavior of BIMT 17 in mimicking the inhibitory action of 5-HT on the basal firing rate of the cortical neurons (see accompanying paper).

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)

Early desensitization of somato-dendritic 5-HT1A autoreceptors in rats treated with fluoxetine or paroxetine

Electrophysiological and autoradiographic approaches were used to assess possible changes in 5-hydroxytryptamine (serotonin) 5-HT1A receptors in the rat dorsal raphe nucleus after a subchronic treatment with fluoxetine or paroxetine, two specific serotonin reuptake inhibitors with antidepressant properties. Fluoxetine or paroxetine were injected daily (5 mg/kg, i.p.) for various time periods up to 21 days. Electrophysiological recordings performed 24 h after the last injection showed that the potency of the 5-HT1A receptor agonist, 8-OH-DPAT, to depress the firing of serotoninergic neurons in the dorsal raphe nucleus within brain stem slices was significantly reduced as early as after a 3-day treatment with either drug. The proportion of recorded neurons showing desensitization of somatodendritic 5-HT1A autoreceptors increased along the treatment from approximately 40% on the 3rd day to 60-80% on the 21st day. At no time during the treatment, was the specific binding of [3H]8-OH-DPAT (agonist radioligand) or [3H]WAY-100 635 (antagonist radioligand) to 5-HT1A receptors modified in the dorsal raphe nucleus or in other brain areas, suggesting that neither the density nor the coupling of these receptors to G-proteins were probably altered in rats injected with fluoxetine or paroxetine for up to 21 days. These results show that adaptive desensitization of somatodendritic 5-HT1A autoreceptors within the dorsal raphe nucleus can already be detected after a 3-day treatment with selective serotonin reuptake inhibitors. Rather than the desensitization per se, it may be the progressive increase in the number of serotoninergic neurons with desensitized 5-HT1A autoreceptors which plays a critical role in the (slowly developing) antidepressant action of these drugs.

Pharmacological characteristics and species-related variations of beta 3-adrenergic receptors

Beta-adrenergic receptors (beta-AR) belong to the large multigenic family of receptors coupled to GTP-binding proteins. Three subtypes have been identified: beta 1-, beta 2- and beta 3-AR. Much of the work delineating the precise pharmacological comparison of the three beta-ARs has come from investigations with stably transfected Chinese hamster ovary cells (CHO cells). This review discusses the structure and function of beta 3-AR in various species and presents new findings on a number of beta 3-AR ligands including carazolol, tertatolol and CL 316,243 which were found to be selective and potent beta 3-AR agonists and ZD 2079 and salmeterol which appear to display full but non-subtype selective agonistic activity. Species-related variations of the beta 3-AR pharmacology have been shown for propranolol and bupranolol. With the ongoing characterization of the beta 3-AR at the molecular and cellular level, and with the advent of computer-assisted molecular modelling to aid in the determination of the three-dimensional structure of the receptor, it is thought that novel beta 3-AR compounds will become available with improved selectivity and potency.

The selective 5-HT1A antagonist radioligand [3H]WAY 100635 labels both G-protein-coupled and free 5-HT1A receptors in rat brain membranes

The tritiated derivative of the novel silent 5-HT1A receptor antagonist WAY 100635 [N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide] was tested as a potential radioligand of 5-HT1A receptors in the rat brain. Binding assays with membranes from various brain regions showed that [3H]WAY 100635 specifically bound to a homogeneous population of sites, with a Kd of 0.10 nM. The regional distribution of [3H]WAY 100635 specific binding sites, as assessed in membrane binding assays and by autoradiography of labelled brain sections, superimposed exactly over that of 5-HT1A receptors specifically labelled by [3H]8-hydroxy-2-(di-n-propylamino) tetralin ([3H]8-OH-DPAT). Furthermore, the positive correlation (r = 0.96) between the respective pKi values of a large series of ligands as inhibitors of the specific binding of [3H]WAY 100635 and [3H]8-OH-DPAT in hippocampal membranes indicated that their pharmacological properties were similar. Nevertheless, marked differences also existed between [3H]8-OH-DPAT and [3H]WAY 100635 specific binding, as the former was inhibited by 1-100 microM GTP and GppNHp, whereas the latter was enhanced by these guanine nucleotides. In contrast, Mn2+ (1-10 mM) increased the specific binding of [3H]8-OH-DPAT, but inhibited that of [3H]WAY 100635. Treatment of membranes with N-ethylmaleimide (1-5 mM) markedly reduced their capacity to specifically bind [3H]8-OH-DPAT, but slightly increased (at 1 mM) or did not affect (at 5 mM) their [3H]WAY 100635 specific binding capacity. Finally, the Bmax of [3H]WAY 100635 specific binding sites was regularly 50-60% higher than that of [3H]8-OH-DPAT in the same membrane preparations from various brain regions (hippocampus, septum, cerebral cortex). These data are compatible with the idea that whereas [3H]8-OH-DPAT only binds to G-protein-coupled 5-HT1A receptors, [3H]WAY 100635 is a high affinity ligand of both G-protein-coupled and free 5-HT1A receptor binding subunits in brain membranes.

Selective in vivo labelling of brain 5-HT1A receptors by [3H]WAY 100635 in the mouse

The novel selective 5-HT1A receptor antagonist radioligand [3H]WAY 100635 ([O-methyl-3H]N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2- pyridyl)cyclohexane-carboxamide) was injected i.v. to mice in an attempt to label in vivo central 5-HT1A receptors. Although 5 min after the i.v. injection of [3H]WAY 100635 (4-7.6 muCi per mouse) the amount of tritium found in the whole brain only accounted for 1.5-1.8% of the injected radioactivity, regional differences in 3H accumulation already corresponded to those of 5-HT1A receptor density. Optimal data were obtained 1 h after [3H]WAY 100635 injection as the distribution of 3H in brain was exactly that of 5-HT1A receptor binding sites in mouse brain sections labelled in vitro with [3H]WAY 100635. In particular, high level of labelling was found in the lateral septum, gyrus dentatus and CA1 area of Ammon's horn in the hippocampus, dorsal raphe nucleus and entorhinal cortex. No labelling was found in he substantia nigra, and 3H accumulated in the cerebellum represented only 12-14% of that found in the hippocampus. Pretreatment with various drugs indicated that only 5-HT1A receptor ligands were able to decrease the accumulation of 3H in all the brain areas examined except in the cerebellum. Assuming that only non-specific binding took place in the latter structure, it was possible to calculate the ID50 values of 5-HT1A receptor agonists (8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), S 14506 (1-[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxynaphthyl+ ++)piperazine) and S 20499 ((+)-4-[N-(5-methoxy-chroman-3-yl)-N-propylamino]butyl-8- azaspiro-(4,5)-decane-7,9-dione)) and antagonists (spiperone, (-)-tertatolol, (+)-WAY 100135 (N-tert-butyl-3,4-(2-methoxyphenyl)piperazin-1-yl-2-phenyl- propanamide)) as inhibitors of 3H accumulation in the hippocampus of [3H]WAY 100635-injected mice. Comparison of these values with the in vitro affinity of the same ligands for hippocampal 5-HT1A receptors revealed marked variations in the capacity of 5-HT1A receptor agonists and antagonists to reach the brain when injected via the subcutaneous route in mice.

Reduction of in vivo striatal 5-hydroxytryptamine release by 8-OH-DPAT after inactivation of Gi/G(o) proteins in dorsal raphe nucleus

5-HT1A receptor agonists reduce firing-dependent terminal 5-HT synthesis and release by activating somatodendritic 5-HT1A receptors. We have examined the effects of 8-hydroxy-2-(di-n- propylamino)tetralin (8-OH-DPAT, 0.1 mg/kg s.c.) on in vivo striatal 5-HT release in conscious rats with somatodendritic 5-HT1A receptors inactivated by the application of pertussis toxin in the dorsal raphe nucleus. The uncoupling of 5-HT1A receptors from hyperpolarizing potassium channels was demonstrated by the inability of the intra-raphe application of citalopram to reduce striatal release (control animals had a 47% reduction, an effect prevented by previous treatment with the 5-HT1A antagonist (-)-tertatolol). Yet 8-OH-DPAT (0.1 mg/kg s.c.) decreased striatal 5-HT release by 66% (peak effect) in pertussis toxin-treated rats, a value comparable to that found in naive animals (74%). This raises the possibility that other 8-OH-DPAT-sensitive serotonergic receptors different from 5-HT1A autoreceptors may be involved in the control of terminal 5-HT release.

5-HT1A receptors in the median raphe nucleus and dorsal hippocampus may mediate anxiolytic and anxiogenic behaviours respectively

The behavioural response of rats in the high light unfamiliar condition of the social interaction test of anxiety was observed following direct administration of the 5-HT1A receptor agonist, (+/-)-8-hydroxy-dipropylaminotetralin (8-OH-DPAT, 50, 100 or 200 ng) or antagonist tertatolol (3 micrograms) into the median raphe nucleus or dorsal hippocampus. In the median raphe nucleus, 8-OH-DPAT (200 ng) significantly increased social interaction without changing locomotor activity; lower doses were inactive. In the dorsal hippocampus, bilateral injection of 8-OH-DPAT (100 ng) significantly decreased social interaction, without effect on locomotor activity; both 50 and 100 ng significantly changed grooming. Tertatolol had no effect on social interaction following administration to the median raphe nucleus, but significantly increased locomotor activity. Bilateral injection of tertatolol into the dorsal hippocampus decreased social interaction and changed grooming. These effects are similar to those of 8-OH-DPAT suggesting tertatolol may have 5-HT1A receptor agonist properties. In conclusion, the findings of this study demonstrate that 5-HT1A somatodendritic autoreceptors and post-synaptic receptors mediate anxiolytic and anxiogenic effects, respectively, in the social interaction test.

Neurotrophic effects of ipsapirone and other 5-HT1A receptor agonists on septal cholinergic neurons in culture

Repeated treatment of primary cultures of fetal rat septal neurons with 5-HT1A receptor agonists (8-OH-DPAT, ipsapirone, gepirone and buspirone) increased choline acetyltransferase activity after 6-7 days in culture. This effect was optimal with ipsapirone (+ 50-80% at 1 microM of the agonist), and could be prevented by potent 5-HT1A receptor antagonists such as (-)-tertatolol and (+)-WAY 100135. Under conditions where they completely suppressed the stimulatory effect of NGF on choline acetyltransferase in these cultures, specific anti-NGF antibodies did not alter the stimulatory effect of ipsapirone, suggesting that a possible release of NGF from some septal cells did not account for the effect of 5-HT1A receptor stimulation. Autoradiographic investigations with [3H]8-OH-DPAT as radioligand and immunocytochemistry with specific anti-choline acetyltransferase antibodies and anti-rat 5-HT1A receptor antibodies showed that 5-HT1A receptors were expressed on septal neurons in culture, notably on the cholinergic neurons identified by their positive staining with anti-choline acetyltransferase antibodies. Detailed morphometrical analysis by computer-assisted imaging revealed that repeated exposure to ipsapirone (1 microM for 7 days) did not influence the survival of cholinergic as well as non-cholinergic neurons, but specifically altered the neuritic tree (i.e. the total length of neurites and the number of branching points) of cholinergic neurons only. These data suggest that under in vitro conditions ipsapirone and other 5-HT1A receptor agonists may exert a direct trophic action on septal cholinergic neurons.

The 5-HT1A receptor antagonist (S)-UH-301 blocks the qR)-8-OH-DPAT-induced inhibition of serotonergic dorsal raphe cell firing in the rat

(S)-UH-301 [(S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin, 0.5-4.0 mg/kg i.v.] did not significantly alter the firing rate of 5-hydroxytryptamine (5-HT) containing neurons in the dorsal raphe nucleus (DRN) as a group, although some individual cells were activated whereas others were depressed. However, (S)-UH-301 (2.0 mg/kg i.v.) consistently reversed the inhibition of DRN-5-HT cells produced by the selective 5-HT1A receptor agonist (R)-8-OH-DPAT (0.5 microgram/kg i.v.) and the dose-response curve for this effect of (R)-8-OH-DPAT was markedly shifted to the right by pretreatment with (S)-UH-301 (1.0 mg/kg i.v.). These results support the notion that (S)-UH-301 acts as an antagonist at central 5-HT1A receptors.

Blockade of transmission at NMDA receptors facilitates the electrical and synthetic activity of ascending serotoninergic neurones

This study examined the influence of N-methyl-D-aspartate (NMDA) receptors upon the activity of serotoninergic neurones projecting from the rat dorsal raphe nucleus (DRN) to the striatum of rats. The channel blocker (+)-MK 801 (0.04-0.63 mg/kg, s.c.) augmented striatal accumulation of the serotonin (5-HT) precursor, 5-hydroxytryptophan (5-HTP), in rats treated with the inhibitor of decarboxylase, NSD 1015: the maximal effect of (+)-MK 801 was 164% relative to vehicle values (= 100%). In analogy, (+)-MK 801 (0.01-0.5 mg/kg, i.v.) increased the firing rate of DRN neurones with a maximal effect of 204%. This action was stereospecific in that (-)-MK 801, which shows lower affinity at NMDA receptors, enhanced firing only at higher doses. The selective, competitive antagonist at the NMDA recognition site, CPP (0.5-8.0 mg/kg, i.v.), also facilitated the firing rate of DRN neurones, though with a maximal effect (137%) less than that of (+)-MK 801. Further, CPP (40.0 mg/kg, s.c.) did not significantly modify striatal 5-HT synthesis. While NMDA did not significantly modify DRN firing alone, it abolished the facilitatory action of CPP, consistent with a competitive interaction at the NMDA recognition site. In conclusion, blockade of NMDA receptors specifically facilitates the activity of ascending serotoninergic neurones.

Clozapine inhibits serotoninergic transmission by an action at alpha 1-adrenoceptors not at 5-HT1A receptors

This study examined the mechanism underlying the influence of clozapine upon serotoninergic transmission in the rat. In vitro, clozapine manifested weak affinity at 5-HT1A receptors (pKi = 6.5) as compared to the agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (9.0), but high affinity at alpha 1-adrenoceptors (8.2) as compared to the alpha 1-adrenoceptor antagonist, prazosin (9.7). Ex vivo, clozapine (inhibitory dose (ID)50 = 0.7 mg/kg s.c.) mimicked prazosin (0.5) in potently occupying central alpha 1-adrenoceptors whereas, as compared to 8-OH-DPAT (0.2), it failed to occupy 5-HT1A receptors (> 10.0). The firing of serotoninergic neurones in the dorsal raphe nucleus was abolished by 8-OH-DPAT, clozapine and prazosin with ID50 values of 0.006, 0.09 and 0.07 mg/kg i.v., respectively. At comparable doses, they reduced striatal turnover of 5-HT. While the 5-HT1A receptors antagonists, (-)-tertatolol (2.0 mg/kg i.v.) and spiperone (0.63 mg/kg i.v.), blocked the action of 8-OH-DPAT upon dorsal raphe nucleus firing, they failed to modify the effect of clozapine and prazosin. In contrast, the alpha 1-adrenoceptor agonist, cirazoline (0.005 mg/kg i.v.) prevented the actions of clozapine and prazosin, but not that of 8-OH-DPAT. It is concluded that clozapine only weakly interacts with 5-HT1A receptors and that its potent alpha 1-adrenoceptor antagonist properties underlie inhibition of serotoninergic transmission.

Effects of 5-HT1A receptor ligands on a safety signal withdrawal procedure of conflict in the rat

The present study evaluated in the rat the ability of various 5-HT1A receptor agonists to exert an "anxiolytic-like" release of the suppression of lever pressing for food induced by the withdrawal of a conditioned signal for safety without presentation of a conditioned signal for punishment. During the period associated with the safety signal withdrawal (Saf.CS-/Pun.CS-), control rats exhibited a typical pattern of responding with an initial strong blockade of responding that lessened over the period as presses were rewarded and shocks omitted. The 5-HT1A receptor partial agonists buspirone (0.125-0.5 mg/kg) and 8-(2-[2,3-dihydro-1,4-benzodioxin-2-yl- methylamino]ethyl)-8-azaspiro[4,5]decane-7,9-dione methyl sulfonate (MDL 73005EF; 0.5-2 mg/kg) and the full agonist (+)-4-[N-(5-methoxy-chroman-3-yl)-N-propylamino]-butyl-8- azaspiro[4,5]decane-7,9-dione (S 20499; 0.125-1 mg/kg) produced a robust and dose-related release of pressing during the Saf.CS-/Pun.CS- period. This effect was less marked with ipsapirone (0.125-1 mg/kg). Conversely, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.06-0.25 mg/kg), a full agonist, was completely inactive and did not prevent MDL 73005EF (1-2 mg/kg) or diazepam (0.125 mg/kg) from releasing the suppressed behavior. The specific 5-HT1A antagonist (+)-N-tert-butyl-3-4-(2-methoxyphenyl)piperazin-1-yl-2-phenylpr opa namide [(+)-WAY 100135; 0.25-8 mg/kg] and the beta-adrenoceptor/5-HT1A antagonist (-)-tertatolol (2-8 mg/kg) did not modify the behavioral blockade, nor did (+)-WAY 100135 (2-4 mg/kg) reduce the ability of buspirone (0.25 mg/kg) to enhance responding during the Saf.CS-/Pun.CS- period.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Contrasting behavioural effects of 8-OH DPAT in the dorsal raphé nucleus and ventral hippocampus

Administration of the 5-HT1A receptor agonist (+-)-8-Hydroxy-dipropylaminotetralin (8-OH DPAT, 50 ng) into the dorsal raphé nucleus (DRN) increased social interaction but did not change the motor activity of rats tested in high light, thus indicating an anxiolytic response. This effect was blocked by coadministration of the 5-HT1A antagonist, tertatolol (3 micrograms). In contrast, 8-OH DPAT (50 and 100 ng) was without effect on social interaction when administered to the DRN projection area in the ventral hippocampus, but did change locomotor activity. The effects depended on the light level and dose: thus when the rats were tested in low light, 50 ng increased locomotor activity, but in high light a decrease was found with 100 ng and also an induction of wet dog-shakes. Thus, our results support the importance of the somatodendritic autoreceptors in the DRN in alleviating anxiety, whereas the post-synaptic receptors in the ventral hippocampus play no role. They do, however, mediate changes in activity and the 5-HT syndrome.

In vivo characterization of the putative 5-HT1A receptor antagonist SDZ 216,525 using two models of somatodendritic 5-HT1A receptor function

SDZ 216,525 has been proposed to be a silent 5-HT1A receptor antagonist. The present study examined the potential intrinsic agonist action of SDZ 216,525 using two in vivo models of somatodendritic 5-HT1A autoreceptor function: 5-HT release using microdialysis and feeding behaviour of satiated animals. SDZ 216,525 (1 mg/kg s.c.) and the alpha 1-adrenoceptor antagonist prazosin (1 mg/kg s.c.) significantly decreased hippocampal 5-HT release. In addition, SDZ 216,525 (3 and 10 mg/kg s.c.) and prazosin (3 and 10 mg/kg s.c.) significantly increased food intake in satiated rats. The selective 5-HT1A receptor antagonist (RS)-WAY100135 (10 mg/kg s.c.) which has been demonstrated to block the effects of 8-OH-DPAT on 5-HT release and food intake had no significant effect on the response induced by SDZ 216,525. In contrast, the non-selective 5-HT1A receptor antagonist (-)-pindolol (8 mg/kg s.c.) attenuated both SDZ 216,525 responses. The decrease in hippocampal 5-HT release and increase in food intake induced by SDZ 216,525 suggest that the compound may be a 5-HT1A receptor partial agonist. However, the failure of the 5-HT1A receptor antagonist (RS)-WAY100135 to block the SDZ 216,525 responses suggests that SDZ 216,525 decreases 5-HT release and increases food intake by a mechanism other than 5-HT1A receptor agonism. The high affinity of SDZ 216,525 for the alpha 1-adrenoceptor, and the ability of prazosin to decrease 5-HT release and increase food intake, suggest that the effects of SDZ 216,525 may be mediated via an alpha 1-adrenoceptor antagonist action.

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.