Is the potent 5-HT1A receptor agonist, alnespirone (S-20499), affecting dopaminergic systems in the rat brain?

In 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-Treated Primates, the Selective 5-Hydroxytryptamine 1a Agonist (R)-(+)-8-OHDPAT Inhibits Levodopa-Induced Dyskinesia but Only with\ Increased Motor Disability

5-Hydroxytryptamine 1a (5-HT(1a)) receptor agonists, such as sarizotan and tandospirone, are reported to reduce levodopa-induced dyskinesia in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated macaques and in Parkinson's disease without worsening motor disability. However, these compounds are not specific for 5-HT(1a) receptors and also possess dopamine antagonist actions. We now report on the effects of (2R)-(+)-8-hydroxy-2-(di-n-propylamino)tetralin [(R)-(+)-8-OHDPAT], a selective 5-HT(1a) agonist lacking dopaminergic activity, on motor disability and dyskinesia (chorea and dystonia) in levodopa-primed MPTP-treated common marmosets. Administration of (R)-(+)-8-OHDPAT (0.2, 0.6, and 2.0 mg/kg s.c), in conjunction with levodopa/carbidopa (12.5 mg/kg each p.o.) to levodopa-primed animals, dose-dependently reduced levodopa-induced chorea but did not affect dystonic movements. However, (R)-(+)-8-OHDPAT treatment also reduced locomotor activity and the reversal of motor disability. Administration of (R)-(+)-8-OHDPAT alone had no effects of motor behaviors. The effects of (R)-(+)-8-OHDPAT on levodopa-induced motor behaviors were antagonized by the 5-HT(1a) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate (WAY-100635) (1.0 mg/kg s.c.). Administration of (R)-(+)-8-OHDPAT (0.6 mg/kg s.c.) also reduced chorea produced by the administration of the D(2)/D(3) dopamine receptor agonist pramipexole (0.06 mg/kg p.o.) to levodopa-primed MPTP-treated animals. However, again the increase in locomotor activity and reversal of motor disability produced by pramipexole were also inhibited. These data suggest that selective 5-HT(1a) agonists do not provide an effective means of suppressing levodopa-induced dyskinesia, except with worsening of parkinsonism.

Dorsal raphe vs. median raphe serotonergic antagonism. Anatomical, physiological, behavioral, neuroendocrinological, neuropharmacological and clinical evidences: relevance for neuropharmacological therapy

Monoaminergic neurons located in the central nervous system (CNS) are organized into complex circuits which include noradrenergic (NA), adrenergic (Ad), dopaminergic (DA), serotonergic (5-HT), histaminergic (H), GABA-ergic and glutamatergic systems. Most of these circuits are composed of more than one and often several types of the above neurons. Such physiologically flexible circuits respond appropriately to both external and internal stimuli which, if not modulated adequately, can trigger pathophysiologic responses. A great deal of research has been devoted to mapping the multiple functions of the CNS circuitry, thereby forming the basis for effective neuropharmacological therapeutic approaches. Such lineal strategies that seek to normalize complex and mixed physiological disorders, however, meet only partial therapeutic success and are often followed by undesirable side effects and/or total failure. In light of these, we have worked to develop possible models of CNS circuitry that are less affected by physiological interaction using the models to design more effective therapeutic approaches. In the present review, we cite and present evidence supporting the dorsal raphe versus median raphe serotonergic circuitry as one model of a reliable paradigm, necessary to the clear understanding and therapy of many psychiatric and even non-psychiatric disturbances.

A comparison of in vivo and in vitro neuroimaging of 5-HT1A receptor binding sites in the cat brain

To validate the cat as a suitable model for positron emission tomography imaging (PET) and to gain further knowledge on the anatomical distribution of the serotonin-1A receptor (5-HT 1A) in the feline brain, we used PET with [18F]MPPF and in vitro autoradiography with [3H]MPPF, [3H]8-OH-DPAT and [3H]paroxetine. PET radioactivity curves with [18F]MPPF were very reproducible in anaesthetized cats, with the highest radioactivity uptakes recorded in the hippocampus, cingulate cortex, septum, infralimbic cortex and raphe nucleus, whereas the lowest were found in the cerebellum. [3H]8-OH-DPAT binding displayed a comparable, albeit lower, regional distribution than with [3H]MPPF. Autoradiography also revealed the presence of 5-HT 1A receptor binding sites in the cortex and in the interpeduncular nucleus, due to its greater sensitivity and spatial resolution compared with PET imaging. The cat constitutes an interesting experimental model for PET imaging, as many physiological concepts have been well established with this animal. Our study also shows the advantages of combining complementary neuroimaging techniques such as in vivo PET imaging and in vitro autoradiography to visualize the distribution of the 5-HT 1A receptors.

5-HT1A and 5-HT1B receptor agonists and aggression: a pharmacological challenge of the serotonin deficiency hypothesis

More than any other brain neurotransmitter system, the indolamine serotonin (5-HT) has been linked to aggression in a wide and diverse range of species, including humans. The nature of this linkage, however, is not simple and it has proven difficult to unravel the precise role of this amine in the predisposition for and execution of aggressive behavior. The dogmatic view that 5-HT inhibits aggression has dominated both pharmacological research strategies to develop specific and effective novel drug treatments that reduce aggressive behavior and the pharmacological mechanistic interpretation of putative serenic drug effects. Our studies on brain serotonin and aggression in feral wild-type rats using the resident-intruder paradigm have challenged this so-called serotonin deficiency hypothesis of aggressive behavior. The well-known fact that certain 5-HT(1A/1B) receptor agonists potently and specifically reduce aggressive behavior without motor slowing and sedative effects is only consistent with this hypothesis under the assumption that the agonist mainly acts on the postsynaptic 5-HT(1A/1B) receptor sites. However, systemic injections of anti-aggressive doses of 5-HT(1A) and (1B) agonists robustly decrease brain 5-HT release due to their inhibitory actions at somatodendritic and terminal autoreceptors, respectively. The availability of the novel benzodioxopiperazine compound S-15535, which acts in vivo as a preferential agonist of the somatodendritic 5-HT(1A) auto-receptor and as an antagonist (weak partial agonist) at postsynaptic 5-HT(1A) receptors, allows for a pharmacological analysis of the exact site of action of this anti-aggressive effect. It was found that, similar to other prototypical full and partial 5-HT(1A) and/or 5-HT(1B) receptor agonists like repinotan, 8-OHDPAT, ipsapirone, buspirone, alnespirone, eltoprazine, CGS-12066B and CP-93129, also S-15535 very effectively reduced offensive aggressive behavior. Unlike the other ligands, however, a remarkable degree of behavioral specificity was observed after treatment with S-15535, in that the anti-aggressive effects were not accompanied by inhibiting (like other 5-HT(1A) receptor agonist with moderate to high efficacy at postsynaptic 5-HT(1A) receptors) or enhancing (like agonists with activity at 5-HT(1B) receptors and alnespirone) non-aggressive motor behaviors (e.g., social exploration, ambulation, rearing, and grooming) beyond the range of undrugged animals with corresponding levels of aggression. The involvement of 5-HT(1A) and/or 5-HT(1B) receptors in the anti-aggressive actions of these drugs was convincingly confirmed by showing that the selective 5-HT(1A) receptor antagonist WAY-100635 and/or the 5-HT(1B) receptor antagonist GR-127935, while inactive when given alone, effectively attenuated/prevented these actions. Furthermore, combined administration of S-15535 with either alnespirone or CGS-42066B elicited a clear additive effect, indicated by a left-ward shift in their dose-effect curves, providing further support for presynaptic sites of action (i.e., inhibitory somatodendritic 5-HT(1A) and terminal 5-HT(1B) autoreceptors). These findings strongly suggest that the specific anti-aggressive effects of 5-HT(1A) and 5-HT(1B) receptor agonists are predominantly based on reduction rather than enhancement of 5-HT neurotransmission during the combative social interaction. Apparently, normal display of offensive aggressive behavior is positively related to brief spikes in serotonergic activity, whereas an inverse relationship probably exists between tonic 5-HT activity and abnormal forms of aggression only.

The effect of a 5-HT1A receptor agonist on striatal dopamine release

5-HT1A receptor agonists consistently reduce neuroleptic induced catalepsy in rats. A serotonin-dopamine interaction has been proposed to underlie this effect. Specifically, 5-HT1A receptor agonists may reduce the activity of serotonergic projections that inhibit dopaminergic nigrostriatal neurones, therefore increasing dorsal striatal dopamine levels and partially overcoming the neuroleptic blockade of D2 receptors. We tested the hypothesis that 5-HT1A receptor agonists increase striatal dopamine release in man using PET scanning with the selective D2 receptor radioligand [11C]raclopride, which is sensitive to endogenous dopamine levels. Six healthy volunteers received two PET scans, one after placebo, the other after 1 mg flesinoxan, a selective 5-HT1A receptor agonist. Binding potential values for striatal subdivisions were determined using a simplified reference tissue model. We did not find any difference in striatal [11C]raclopride binding between conditions, even though flesinoxan lead to typical 5-HT1A receptor agonist side effects and produced elevation of growth hormone in five of the six subjects. Our results suggest that the anticataleptic effect of 5-HT1A receptor agonists is not mediated by striatal dopamine release, and indicates a need for further research with other suitable 5-HT1A receptor agonists.

In vivo comparison of two 5-HT1A receptors agonists alnespirone (S-20499) and buspirone on locus coeruleus neuronal activity

The aim of the present study was to compare, in chloral-hydrate anaesthetized rats, the alpha(2)-adrenergic properties of the selective 5-HT(1A) receptor agonist, alnespirone (S-20499), with those of buspirone, a 5-HT(1A) receptor agonist exhibiting potent alpha(2)-adrenoceptor antagonist properties via its principal metabolite, 1-(2-pyrimidinyl)-piperazine. Both locus coeruleus spontaneous firing activity and noradrenaline release in the medial prefrontal cortex were potently inhibited by the alpha(2)-adrenoceptor agonist clonidine, at a dose of 40 microg/kg (i.p.). Such an inhibition was neither prevented nor reversed by alnespirone (10 mg/kg, i.p.), while buspirone, at the same dose, potently antagonized the locus coeruleus inhibitory effects of clonidine. These data demonstrate that, in contrast with some aryl-piperazine compounds (such as buspirone), alnespirone, either on its own or via a possible metabolite such as buspirone, is devoid in vivo of significant alpha(2)-adrenoceptor antagonist properties.

The 5-HT1A receptor in schizophrenia: a promising target for novel atypical neuroleptics?

Increasing attention is being directed towards the role of the serotonergic system in the neurochemistry of schizophrenia and antipsychotic drug treatment. This review considers the 5-HT1A receptor in this context. In patients with schizophrenia, the majority of post-mortem studies have reported increases in 5-HT1A receptor density in the prefrontal cortex in the approximate range 15-80%. Although the pathophysiological significance of this finding is unclear, given the location of a major proportion of these receptors on pyramidal cells, it may reflect an abnormal glutamatergic network. In terms of drug treatment, 5-HT1A agonists clearly display anticataleptic activity in rats. In addition, 5-HT1A agonists consistently increase dopamine release in the prefrontal cortex in rodents, which is an effect that might be predicted to improve negative symptoms. 5-HT1A agonists augment classical neuroleptics in some rat models of antipsychotic action and may be capable of modulating the glutamatergic network therapeutically. Despite the encouraging preclinical data, there is a paucity of clinical studies of 5-HT1A agonist augmentation of neuroleptics in the treatment of schizophrenia. However, the clinical relevance may be clarified by the atypical antipsychotic drugs clozapine, quetiapine and ziprasidone which combine D2 receptor antagonism and 5-HT1A agonism. In conclusion, given the increased prefrontal 5-HT1A receptor density in the illness, and the anticataleptic activity of 5-HT1A agonists combined with their ability to evoke prefrontal dopamine release, there is now a sufficient rationale to examine thoroughly the role of the 5-HT1A receptor in schizophrenia and antipsychotic drug treatment.

Alnespirone (S 20499), an agonist of 5-HT1A receptors, and imipramine have similar activity in a chronic mild stress model of depression

A chronic mild stress (CMS) model of depression was used to study an antidepressant-like activity of alnespirone (S 20499), a selective agonist of 5-HT1A receptors. In this model, a substantial decrease in consumption of a palatable sucrose solution over time is observed in rats subjected to a variety of mild stressors. This effect can be reversed by chronic administration of various classes of antidepressant drugs. Chronic (5 weeks) treatment with alnespirone, in a dose range between 1-5 mg/kg/day, gradually and dose dependently reversed the CMS-induced reductions in sucrose consumption without any significant effects in the non-stressed control animals. The onset of action of the most active doses (2.5 and 5 mg/kg/day) and the overall efficacy of alnespirone in the CMS model were comparable to those observed following similar administration of imipramine (10 mg/kg/ day). At the lower (0.5 mg/kg/day) and higher (10 and 20 mg/kg/day) doses, alnespirone was ineffective against the CMS-induced deficit in sucrose consumption. These data provide further support for previous suggestions, based on both the clinical observations and animal data, that agonism at 5-HT1A receptors may result in antidepressant action.

Chronic alnespirone-induced desensitization of somatodendritic 5-HT1A autoreceptors in the rat dorsal raphe nucleus

The effects of long-term (7, 14 or 21 days) administration of the 5-HT1A receptor agonist alnespirone [5 mg/(kg day), i.p.] on the binding characteristics of 5-HT1A, 5-HT2A and 5-HT3 receptors, and the functional status of 5-HT1A autoreceptors were assessed using biochemical and electrophysiological approaches in rats. Whatever the treatment duration, the specific binding of [3H]8 hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT), [3H]trans,4-[(3Z)3-(2-dimethylaminoethyl) oxyimino-3(2-fluorophenyl) propen-1-yl] phenol hemifumarate ([3H]SR 46349B), and [3H]S-zacopride to 5-HT1A, 5-HT2A and 5-HT3 receptors, respectively, were unaltered in all the brain areas examined. In contrast, in vitro electrophysiological recordings performed 24 h after the last injection of alnespirone 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, was significantly reduced after a 21-day treatment with alnespirone. However, no changes were noted after a 7-day or 14-day treatment. These data indicate that desensitization of somatodendritic 5-HT1A autoreceptors is a selective but slowly developing adaptive phenomenon in response to their chronic stimulation in rats.

Differential regulation of somatodendritic serotonin 5-HT1A receptors by 2-week treatments with the selective agonists alnespirone (S-20499) and 8-hydroxy-2-(Di-n-propylamino)tetralin: microdialysis and autoradiographic studies in rat brain

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

Similar pharmacological properties of 8-OH-DPAT and alnespirone (S 20499) at dopamine receptors: comparison with buspirone

Alnespirone (S 20499) has previously been described as a potential anxiolytic drug that acts by stimulation of 5-HT1A receptors. Some data suggest that alnespirone might also be a weak dopamine D2 receptor agonist: it displays moderate affinity for dopamine D2 receptors in vitro and it inhibits prolactin release and induces yawning in rats. In order to test for possible interactions of alnespirone with dopamine receptors in vivo, we studied the changes of in vivo striatal [3H]SCH 23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benza zepine) and [3H]raclopride binding following the injection of a tracer dose of either tritiated ligand (4 microCi) in mice treated with increasing doses of alnespirone (5, 10, 20 and 40 mg/kg, i.p.) and, in the same animals, the changes in the levels of dopamine, 5-hydroxytryptamine (5-HT) and their metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA). These changes were compared with those produced by increasing doses of the reference 5-HT1A receptor agonist 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin, 0.25, 1 and 4 mg/kg, i.p.) or buspirone (5 and 20 mg/kg, i.p.). Decreased in vivo striatal [3H]SCH 23390 specific binding was observed in mice treated with 5, 10 and 40 mg/kg alnespirone. In contrast, increased in vivo striatal [3H]raclopride specific binding was observed in mice treated with 5 and 20 mg/kg alnespirone. In these animals, the striatal 5-HIAA/5-HT ratio was decreased by 5 to 40 mg/kg alnespirone, whereas the striatal HVA/DA ratio was unaffected at all tested doses of alnespirone. Similarly, 8-OH-DPAT decreased specific in vivo striatal [3H]SCH 23390 binding at 0.25, 1 and 4 mg/kg, and increased in vivo specific striatal [3H]raclopride binding at 1 and 4 mg/kg. In the same animals, all tested doses of 8-OH-DPAT decreased the striatal 5-HIAA/5-HT ratio but did not modify the striatal HVA/dopamine ratio. Buspirone (5 and 20 mg/kg) completely inhibited in vivo specific striatal [3H]raclopride binding and increased the striatal HVA/DA ratio but did not modify the striatal 5-HIAA/5-HT ratio, whereas apomorphine (3 mg/kg) decreased both in vivo specific striatal [3H]SCH 23390 and [3H]raclopride binding as well as the striatal HVA/DA and 5-HIAA/5-HT ratios. Finally, increasing doses of alnespirone or 8-OH-DPAT weakly increased sniffing induced by apomorphine (0.75 mg/kg, s.c.) in mice and decreased grooming induced by the dopamine D1 receptor agonist SK&F 39393 ((+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol, 1.87 mg/kg, s.c.), whereas buspirone decreased both apomorphine-induced sniffing and SK&F 39393-induced grooming. These results indicate that alnespirone and 8-OH-DPAT have a similar profile and do not seem to interact directly with dopamine receptors. The results also suggest that the stimulation of 5-HT1A receptors by either alnespirone or 8-OH-DPAT modulates the availability of striatal [3H]SCH 23390 and [3H]raclopride binding sites and possibly the functioning of striatal dopamine D1 and D2 receptors in opposite directions.