The reduction in glutamate release is predictive of cognitive and emotional alterations that are corrected by the positive modulator of AMPA receptors S 47445 in perinatal stressed rats.

S 47445 is a positive modulator of glutamate AMPA-type receptors, possessing neurotrophic and enhancing synaptic plasticity effects as well as pro-cognitive and anti-stress properties. Here, the drug was assessed in the perinatal stress (PRS) rat model, known to have a high predictive validity with monoaminergic antidepressants. The effects of a chronic treatment (i.p.) with S 47445 were investigated on risk-taking, motivational and cognitive behavior. S 47445 (1 and 10 mg/kg) increased the exploration of the elevated-plus maze and light/dark box as well as the time spent grooming in the splash test, and improved social memory in PRS rats. Also, the effects of S 47445 were examined on the synaptic neurotransmission. The reduced depolarization-evoked glutamate release induced by PRS was corrected with S 47445 (10 mg/kg). Remarkably, the reduction in glutamate release induced by PRS and corrected by S 47445 chronic treatment was correlated with all the behavioral changes. S 47445 at 10 mg/kg also normalized the lower levels of synaptic vesicle-associated proteins in ventral hippocampus in PRS rats. Finally, S 47445 reversed the decrease of mGlu5 receptors, GR and OXTR induced by PRS. Collectively, in an animal model of stress-related disorders, S 47445 corrected the imbalance between excitatory and inhibitory neurotransmission by regulating glutamate-evoked release that is predictive of PRS behavioral alterations, and also normalized the reduction of trafficking of synaptic vesicles induced by PRS. These results support the interest of glutamatergic-based therapeutic strategies to alleviate stress-related disorders.

Synergistic enhancing-memory effect of donepezil and S 47445, an AMPA positive allosteric modulator, in middle-aged and aged mice.

Positive allosteric modulators of AMPA receptors (AMPA-PAMs) are described to facilitate cognitive processes in different memory-based models. Among them, S 47445 is a novel potent and selective AMPA-PAM. In order to assess its efficacy after repeated administration, S 47445 effect was evaluated in two aging-induced memory dysfunction tasks in old mice, one short-term working memory model evaluated in a radial maze task and one assessing contextual memory performance. S 47445 was shown to improve cognition in both models sensitive to aging. In fact, administration of S 47445 at 0.3 mg/kg (s.c.) reversed the age-induced deficits of the working memory model whatever the retention interval. Moreover, in the contextual task, S 47445 also reversed the age-induced deficit at all tested doses (from 0.03 to 0.3 mg/kg, p.o.). Since donepezil, an acetylcholinesterase inhibitor, induces only moderate symptomatic effects on memory in Alzheimer's disease patients, an alternative strategy for treatment of cognitive symptoms could be to act simultaneously on both glutamatergic AMPA receptors and cholinergic pathways by combining pharmacological treatments. The present study further examined such effects by assessing combinations of S 47445 and donepezil given orally during 9 days in aged C57/Bl6J mice using contextual memory task (CSD) and the working memory model of serial alternation task (AT). Interestingly, a significant synergistic memory-enhancing effect was observed with the combination of donepezil at 0.1 mg/kg with S 47445 at 0.1 mg/kg p.o. in the CSD or with S 47445 at 0.1 and 0.3 mg/kg in AT in comparison to compounds given alone and without any pharmacokinetic interaction.

Behavioral and neurochemical characterization of TrkB-dependent mechanisms of agomelatine in glucocorticoid receptor-impaired mice.

Growing evidence indicates that impairment of the stress response, in particular the negative feedback regulation mechanism exerted by the hypothalamo-pituitary-adrenal (HPA) axis, might be responsible for the hippocampal atrophy observed in depressed patients. Antidepressants, possibly through the activation of BDNF signaling, may enhance neuroplasticity and restore normal hippocampal functions. In this context, glucocorticoid receptor-impaired (GR-i) mice-a transgenic mouse model of reduced GR-induced negative feedback regulation of the HPA axis-were used to investigate the role of BDNF/TrkB signaling in the behavioral and neurochemical effects of the new generation antidepressant drug, agomelatine. GR-i mice exhibited marked alterations in depressive-like and anxiety-like behaviors, together with a decreased cell proliferation and altered levels of neuroplastic and epigenetic markers in the hippocampus. GR-i mice and their wild-type littermates were treated for 21 days with vehicle, agomelatine (50mg/kg/day; i.p) or the TrkB inhibitor Ana-12 (0.5mg/kg/day, i.p) alone, or in combination with agomelatine. Chronic treatment with agomelatine resulted in antidepressant-like effects in GR-i mice and reversed the deficit in hippocampal cell proliferation and some of the alterations of mRNA plasticity markers in GR-i mice. Ana-12 blocked the effect of agomelatine on motor activity as well as its ability to restore a normal hippocampal cell proliferation and expression of neurotrophic factors. Altogether, our findings indicate that agomelatine requires TrkB signaling to reverse some of the molecular and behavioral alterations caused by HPA axis impairment.

Repeated restraint stress-induced atrophy of glutamatergic pyramidal neurons and decreases in glutamatergic efflux in the rat amygdala are prevented by the antidepressant agomelatine.

Major depressive illness is among the most prevalent neuropsychiatric disorders and is associated with neuroplasticity deficits in limbic structures such as the amygdala. Since exposure to stressful life events is proposed to contribute to depressive illness, our recent studies examined the effects of stress on amygdalar neuroplasticity. These studies determined that repeated stress elicits deficits in glutamatergic activity in the amygdala, neuroplasticity deficits that can be prevented by some but not all antidepressants. In view of these observations, the goal of the current study was to determine the effects of repeated restraint stress (RRS) on the dendritic architecture of pyramidal neurons in the rat basolateral nucleus of the amygdala (CBL), as well as glutamate efflux in the CBL and central nucleus of the amygdala (CMX) via in vivo microdialysis. We also examined the ability of the antidepressant agomelatine to prevent RRS-induced neuroplasticity deficits. Compared with control rats, rats subjected to RRS exhibited atrophy of CBL pyramidal neurons, including decreases in total dendritic length, branch points, and dendritic complexity index. In addition, glutamate efflux was significantly reduced in the CMX of rats subjected to RRS, thereby identifying a potential neurochemical consequence of stress-induced dendritic atrophy of CBL pyramidal neurons. Lastly, an acute stress challenge increased corticosterone (CORT) levels in the CBL, suggesting that stress-induced increases in CORT levels may contribute to the neuroanatomical and neurochemical effects of RRS in the CBL. Importantly, these RRS-induced changes were prevented by daily agomelatine administration. These results demonstrate that the neuroanatomical and neurochemical properties of glutamatergic neurons in the rat amygdala are adversely affected by repeated stress and suggest that the therapeutic effects of agomelatine may include protection of structural and neurochemical plasticity in limbic structures like the amygdala.

Hippocampal and behavioral dysfunctions in a mouse model of environmental stress: normalization by agomelatine.

Stress-induced alterations in neuronal plasticity and in hippocampal functions have been suggested to be involved in the development of mood disorders. In this context, we investigated in the hippocampus the activation of intracellular signaling cascades, the expression of epigenetic markers and plasticity-related genes in a mouse model of stress-induced hyperactivity and of mixed affective disorders. We also determined whether the antidepressant drug agomelatine, a MT1/MT2 melatonergic receptor agonist/5-HT2C receptor antagonist, could prevent some neurobiological and behavioral alterations produced by stress. C57BL/6J mice, exposed for 3 weeks to daily unpredictable socio-environmental stressors of mild intensity, were treated during the whole procedure with agomelatine (50 mg kg(-1) per day, intraperitoneal). Stressed mice displayed robust increases in emotional arousal, vigilance and motor activity, together with a reward deficit and a reduction in anxiety-like behavior. Neurobiological investigations showed an increased phosphorylation of intracellular signaling proteins, including Atf1, Creb and p38, in the hippocampus of stressed mice. Decreased hippocampal level of the repressive epigenetic marks HDAC2 and H3K9me2, as well as increased level of the permissive mark H3K9/14ac suggested that chronic mild stress was associated with increased gene transcription, and clear-cut evidence was further indicated by changes in neuroplasticity-related genes, including Arc, Bcl2, Bdnf, Gdnf, Igf1 and Neurod1. Together with other findings, the present data suggest that chronic ultra-mild stress can model the hyperactivity or psychomotor agitation, as well as the mixed affective behaviors often observed during the manic state of bipolar disorder patients. Interestingly, agomelatine could normalize both the behavioral and the molecular alterations induced by stress, providing further insights into the mechanism of action of this new generation antidepressant drug.

Agomelatine: mechanism of action and pharmacological profile in relation to antidepressant properties.

Agomelatine behaves both as a potent agonist at melatonin MT1 and MT2 receptors and as a neutral antagonist at 5-HT2C receptors. Accumulating evidence in a broad range of experimental procedures supports the notion that the psychotropic effects of agomelatine are due to the synergy between its melatonergic and 5-hydroxytryptaminergic effects. The recent demonstration of the existence of heteromeric complexes of MT1 and MT2 with 5-HT2C receptors at the cellular level may explain how these two properties of agomelatine translate into a synergistic action that, for example, leads to increases in hippocampal proliferation, maturation and survival through modulation of multiple cellular pathways (increase in trophic factors, synaptic remodelling, glutamate signalling) and key targets (early genes, kinases). The present review focuses on the pharmacological properties of this novel antidepressant. Its mechanism of action, strikingly different from that of conventional classes of antidepressants, opens perspectives towards a better understanding of the physiopathological bases underlying depression.

Better sexual acceptability of agomelatine (25 and 50 mg) compared with paroxetine (20 mg) in healthy male volunteers. An 8-week, placebo-controlled study using the PRSEXDQ-SALSEX scale.

Sexual dysfunction (SD) is a common and underestimated effect of antidepressants. Healthy volunteers are the most adequate group to study this adverse event avoiding influence of depression itself. Sexual acceptability of agomelatine (a melatonergic agonist and 5HT(2C) antagonist) paroxetine and placebo by using the Psychotropic-Related Sexual Dysfunction Salamanca Sex Questionnaire (PRSEXDQ-SALSEX) was explored. A total of 92 healthy male volunteers were randomised to agomelatine (25 or 50 mg), paroxetine 20 mg or placebo for 8 weeks. SD, defined as at least one sexual impairment in one of the following PRSEXDQ-SALSEX items (decreased libido, delayed orgasm/ejaculation, anorgasmia/no ejaculation and erectile dysfunction), was evaluated at baseline and after 2, 4 and 8 weeks. At the last post-baseline assessment, SD was significantly lower in each agomelatine group (22.7% on 25 mg and 4.8% on 50 mg) than in the paroxetine group (85.7%; p < 0.0001). In the placebo group, 8.7% of volunteers reported a SD. The percentages of volunteers with moderate or severe SD were 4.5% for agomelatine 25 mg, 4.8% for agomelatine 50 mg, 61.9% for paroxetine 20 mg and 0% in the placebo group (p < or = 0.0001 agomelatine versus paroxetine). There is a much lower risk of having SD with agomelatine than paroxetine in healthy male volunteers, which confirms the better sexual acceptability profile of agomelatine compared with the SSRIs.

Chronic treatment with imipramine reverses immobility behaviour, hippocampal corticosteroid receptors and cortical 5-HT(1A) receptor mRNA in prenatally stressed rats.

Prenatal stress in the rat induces enhanced reactivity of the hypothalamus-pituitary-adrenal (HPA) axis, disturbances in a variety of circadian rhythms and increased anxiety-like behaviour. Such abnormalities parallel those found in human depressed patients. Prenatally stressed (PS) rats could represent, therefore, an interesting animal model for the evaluation of the efficacy of pharmacotherapeutic intervention in psychiatric disorders that has often been addressed using control animals. In the present study, PS and non-stressed rats were chronically treated with the tricyclic antidepressant imipramine (10 mg/kg i.p. for 21 days) and assessed in the forced swim test. Glucocorticoid receptor binding sites in the hippocampus were measured and 5-HT(1A) receptor mRNA levels in the frontal cortex were also assessed. PS rats were characterised by increased immobility in the forced swim test, reduced hippocampal corticosteroid receptor binding and increased levels of cortical 5-HT(1A) mRNA. All these parameters were significantly reversed by chronic imipramine treatment. Conversely, no significant effects were observed for non-stressed rats. All these effects are consistent with the expected pharmacotherapy of depression-like abnormalities in PS rats. These results further indicate that PS rats are a relevant animal model of depression.

Circadian rhythm entrainment with melatonin, melatonin receptor antagonist S22153 or their combination in mice exposed to constant light.

The ability of daily melatonin and the melatonin receptor antagonist, S22153, to entrain circadian system function was investigated in mice with atypical melatonin rhythm. B6D2F(1) mice were first synchronized to a LD 12:12 for approximately 2 wk, then exposed to continuous light (LL) until study completion. After 10-18 days of LL exposure, mice received daily subcutaneous (s.c.) melatonin at a dose of 0.1, 1 or 10 mg/kg/day (exp. 1) or daily intraperitoneal (i.p.) S22153 (20 mg/kg/day) with or without melatonin (1 mg/kg/day, exp. 2) at subjective zeitgeber time (ZT) 10 for 19 days. Then all the mice were exposed to LL for another 10 days. Spectral analysis showed that initial LL lengthened the period of both rhythms by approximately 1.5 hr as compared with LD 12:12. No entrainment of either rhythm was found in controls. Conversely, daily melatonin-only, S22153-only or their combination set the temperature and activity periods to approximately 24 hr and produced a significant increase of the circadian amplitude of both rhythms as compared with controls. However, after treatment withdrawal, the dominant period lengthened to approximately 25.5 hr in mice receiving either melatonin or S22153. On the contrary, the period remained close to 24 hr for the 10 days following withdrawal of combined S22153 and melatonin. Such sustained pharmacological resetting of circadian function could display therapeutic potential against external resynchronization resulting from defective photoperiodic entrainment.

Melatonin or a melatonin agonist corrects age-related changes in circadian response to environmental stimulus.

The effects of a melatonin agonist, S-20098, included in the diet were tested on a specific effect of aging in hamsters: the marked decline in the phase shifting effects of a 6-h pulse of darkness on a background of constant light. In contrast to young hamsters, old hamsters fed with the control diet showed little or no phase shifts in response to a dark pulse presented in the middle of their inactive or active period. Old hamsters fed with S-20098 showed phase shifts that were ~70% of the ones in young animals and significantly greater than those in old controls. The phase advancing response to a dark pulse presented during the inactive period was dose dependent and reversed after S-20098 discontinuation. Melatonin included in the diet showed comparable restorative effects on the phase shifting response to a dark pulse in old hamsters. Replacement therapy with melatonin or melatonin-related compounds could prove useful in treating, preventing, or delaying disturbances of circadian rhythmicity and/or sleep in older people.

Population pharmacokinetic-pharmacodynamic modelling of S 15535, a 5-HT(1A) receptor agonist, using a behavioural model in rats.

The pharmacokinetic-pharmacodynamic relationship of S 15535 (1-(benzodioxan-5-yl) 4-(indan-2-yl)piperazine) and its active 5-hydroxy metabolite S 32784 (1-(benzodioxan-5-yl) 4-(5-hydroxyindan-2-yl)piperazine), and buspirone as a reference, were studied in male Wistar rats using a behavioural model of anxiety by determining the reduction in the number of fear-induced ultrasonic vocalisations. S 15535 and buspirone were administered p.o. and i.v. S 32784, present in man but not in rat, was administered i.v. The pharmacokinetics and pharmacokinetic-pharmacodynamic relationships were described using non-linear mixed effects modelling. The no-drug effect was constant and all compounds were active in the model, reducing ultrasonic vocalisations immediately after administration. The sigmoid E(max) model was used to describe the pharmacokinetic-pharmacodynamic relationships, with E(max) values of a 90% decrease in baseline ultrasonic vocalisations. Corrected for plasma protein binding, all compounds showed similar potency. The study shows that ultrasonic vocalisations can be considered a suitable endpoint for the anxiolytic effect when used in conjunction with non-linear mixed effects modelling to overcome the limited sampling and effect measurements.

S20098 affects the free-running rhythms of body temperature and activity and decreases light-induced phase delays of circadian rhythms of the rat.

Mammalian endogenous circadian rhythms are entrained to the environmental day-night cycle by light exposure. Melatonin is involved in this entrainment by signaling the day-night information to the endogenous circadian pacemaker. Furthermore, melatonin is known to affect the circadian rhythm of body temperature directly. A striking property of the endogenous melatonin signal is its synthesis pattern, characterized by long-term elevated melatonin levels throughout the night. In the present study, the influence of prolonged treatment with the melatonin agonist S20098 during the activity phase of free-running rats was examined. This was achieved by giving S20098 in the food. The free-running body temperature and activity rhythms were studied. The present study shows that enhancement of the melatonin signal, using S20098, affected the free-running rhythm by gradual phase advances of the start of the activity phase, consequently causing an increase in length of the activity phase. A well-known feature of circadian rhythms is its time-dependent sensitivity for light. Light pulse exposure of an animal housed under continuous dark conditions can cause a phase shift of the circadian pacemaker. Therefore, in a second experiment, the influence of melatonin receptor stimulation on the sensitivity of the pacemaker to light was examined by giving the melatonin agonist S20098 in the food during 1 day prior to exposure to a 60-min light pulse of 0, 1.5, 15, or 150 lux given at circadian time (CT) 14. S20098 pretreatment caused a diminished light pulse-induced phase shift when a light pulse of low light intensity (1.5 lux) was given. S20098 treatment via the food was sufficient to exert chronobiotic activity, and S20098 treatment resulting in prolonged overstimulation of melatonin receptors is able to attenuate the effect of light on the circadian timing system.

A melatonin agonist facilitates circadian resynchronization in old hamsters after abrupt shifts in the light-dark cycle.

Age-related changes in the mammalian circadian system may be associated with a decline in circulating melatonin levels. Using 'jet lag' paradigms involving abrupt shifts in the light-dark cycle, we showed that a melatonin agonist, S-20098, accelerated by approximately 25% resynchronization of the circadian activity rhythm in old hamsters to the new light-dark cycle. It suggests the usefulness of melatonin-related compounds to treat circadian disorders associated with aging.

Drug action at the 5-HT(1A) receptor in vivo: autoreceptor and postsynaptic receptor occupancy examined with PET and [carbonyl-(11)C]WAY-100635.

Serotonin(1A) (5-HT(1A)) receptors have been implicated in the pathophysiology and treatment of anxiety and depression and are a target for novel drug development. In this qualitative study, positron emission tomography (PET) and [carbonyl-(11)C]WAY-100635 were used to assess 5-HT(1A) autoreceptor and postsynaptic receptor occupancy in man in vivo by five different compounds with nanomolar affinity for this site. Occupancy by pindolol, penbutolol, buspirone, EMD 68843, and S 15535 was compared to test-retest data from 10 healthy volunteers. All drugs, apart from buspirone, displayed occupancy at the 5-HT(1A) receptor site. Pindolol demonstrated a preferential occupancy at the autoreceptor compared to the postsynaptic receptor over a plasma range of about 10-20 ng/mL. Differential occupancy may be an important component of novel drug action. The level of autoreceptor or postsynaptic occupancy needed to achieve significant physiological effects is not known, although it is of note that none of the drugs in this study achieved occupancies beyond 60%. Overall this study demonstrates the utility of PET in aiding novel drug development.

Somatodendritic 5-HT(1A) autoreceptors mediate the anti-aggressive actions of 5-HT(1A) receptor agonists in rats: an ethopharmacological study with S-15535, alnespirone, and WAY-100635.

To elucidate the relative contribution of somatodendritic 5-HT(1A) autoreceptors and postsynaptic 5-HT(1A) receptors in the specific anti-aggressive properties of 5-HT(1A) receptor agonists, the influence of the novel benzodioxopiperazine compound S-15535, which behaves in vivo as a competitive antagonist at postsynaptic 5-HT(1A) receptors and as an agonist at 5-HT(1A) autoreceptors, upon offensive and defensive aggression was investigated in wild-type rats using a resident-intruder paradigm. S-15535 exerted a potent dose-dependent decrease in offensive, but not defensive, aggressive behavior (inhibitory dose (ID)(50) = 1.11 mg/kg). This anti-aggressive profile was roughly similar to that of the potent pre- and postsynaptic 5-HT(1A) full agonist alnespirone (ID(50) = 1. 24). The drug's profound anti-aggressive actions were not accompanied by sedative side effects or signs of the "5-HT(1A) receptor-mediated behavioral syndrome," which are characteristically induced by prototypical 5-HT(1A) receptor agonists like 8-OH-DPAT and buspirone. The selective pre- and postsynaptic 5-HT(1A) antagonist WAY-100635, which was inactive given alone, abolished the anti-aggressive effects of S-15535 and alnespirone, thereby confirming the involvement of 5-HT(1A) receptors. Furthermore, combined administration of S-15535 and alnespirone elicited an additive anti-aggressive effect, providing further support for somatodendritic 5-HT(1A) receptor involvement. Finally, the postsynaptic 5-HT(1A) antagonistic properties of S-15535 were confirmed by showing blockade of the alnespirone-induced hypothermia, a postsynaptic 5-HT(1A) mediated response in the rat. These data provide extensive evidence that the anti-aggressive effects of 5-HT(1A) receptor agonists are expressed via their action on somatodendritic 5-HT(1A) autoreceptors, thereby most likely attenuating intruder-activated serotonergic neurotransmission.

Roles of 5-HT(1A) receptors in the dorsal raphe and dorsal hippocampus in anxiety assessed by the behavioral effects of 8-OH-DPAT and S 15535 in a modified Geller-Seifter conflict model.

8-OH-DPAT [8-hydroxy-2-(di-N-propylamino)tetralin], a 5-HT(1A) receptor agonist, and S 15535 (4-benzodioxan-5-yl)1-(indan-2-yl)piperazine, a partial agonist at 5-HT(1A) receptors, were administered into the dorsal raphe nucleus and dorsal hippocampus and their behavioral effects were assessed in a modified Geller-Seifter conflict model. Injected into the dorsal raphe nucleus 8-OH-DPAT, 1 microg but not 0.04 or 0.2 microg 0.5 microl(-1), and S 15535, 2.5 microg but not 0.1 or 0.5 microg 0.5 microl(-1), significantly increased punished responding with no effect on rates of unpunished or time-out responding. WAY 100635, a selective 5-HT(1A) receptor antagonist, injected subcutaneously at 0. 3 mg kg(-1) 30 min before 1 microg 8-OH-DPAT or 2.5 microg S 15535 in the dorsal raphe, completely antagonized their effects on punished responding. At doses ranging from 1 to 10 microg microl(-1) injected into the CA1 region of the dorsal hippocampus neither 8-OH-DPAT nor S 15535 modified punished responding or the rates of time-out. At the highest doses, 8-OH-DPAT significantly reduced unpunished responding whereas S 15535 had the opposite effect. The results suggest that stimulation of 5-HT(1A) receptors in the dorsal raphe nucleus has anxiolytic-like effects whereas stimulation of postsynaptic receptors in the dorsal hippocampus has no anxiolytic or anxiogenic effects, at least judging from changes in rates of punished responding. These results are compatible with the hypothesis that 5-HT(1A) receptor agonists and partial agonists attenuate anxiety by reducing serotonergic transmission in brain areas innervated by the dorsal raphe nucleus.

Organization of rat circadian rhythms during daily infusion of melatonin or S20098, a melatonin agonist.

Daily administration of melatonin or S20098, a melatonin agonist, is known to entrain the free-running circadian rhythms of rats. The effects of the duration of administration on entrainment were studied. The animals demonstrated free-running circadian rhythms (running-wheel activity, body temperature, general activity) in constant darkness. Daily infusions of melatonin or S20098 for 1, 8, or 16 h entrained the circadian rhythms to 24 h. Two daily infusions of 1 h (separated by 8 h) entrained the activity peak within the shorter time interval. The entraining properties of melatonin and S20098 were similar and were affected neither by pinealectomy nor by infusion of 1- or 8-h duration. However, with 16-h infusion, less than half of the animals became entrained. Once entrained, the phase angle between the onset of infusion and the rhythms (onset of activity or acrophase of body temperature) increased with the duration of infusion. Before entrainment, the free-running period increased with the duration of infusion, an effect that was not predictable from the phase response curve.

Selective antiaggressive effects of alnespirone in resident-intruder test are mediated via 5-hydroxytryptamine1A receptors: A comparative pharmacological study with 8-hydroxy-2-dipropylaminotetralin, ipsapirone, buspirone, eltoprazine, and WAY-100635.

The present study characterized the effects of the novel, selective, and potent 5-hydroxytryptamine1A (serotonin) (5-HT1A) receptor agonist, alnespirone [S-20499, (S)-N-4-[5-methoxychroman-3-yl)propylamino)butyl- 8-azaspiro-(4,5)-diacetamide, hydrochloride] on offensive and defensive resident-intruder aggression in wild-type rats and compared its actions with those of the prototypical full 5-HT1A agonist 8-hydroxy-2- dipropylaminotetralin (8-OH-DPAT), the partial 5-HT1A agonists ipsapirone and buspirone, and the mixed 5-HT1A/1B agonist eltoprazine. All five agonists exerted effective dose-dependent decreases of offensive aggressive behavior in resident rats; 8-OH-DPAT was the most potent (ID50 = 0.074 mg/kg), followed by eltoprazine (0.24), buspirone (0.72), ipsapirone (1.08), and alnespirone (1.24). However, in terms of selectivity of the antiaggressive effects as determined by the absence of decrements in social interest and general motor activity, alnespirone appeared to be superior. In the defensive aggression test, neither alnespirone nor any of the other four agonists changed defensive behaviors in the intruder rats. The involvement of 5-HT1A receptors in the antiaggressive actions of these drugs was confirmed by showing that the selective 5-HT1A receptor antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2- pyridinyl)cyclohexanecarboxamide trihydrochloride), which was inactive alone, fully prevented the antiaggressive effects of alnespirone, 8-OH-DPAT, and buspirone and partly reversed those of ipsapirone and eltoprazine. The data clearly indicate that alnespirone effectively suppresses offensive aggression with an advantageous profile of action compared with other full or partial 5-HT1A agonists. These selective antiaggressive actions of alnespirone are mediated by stimulating 5-HT1A receptors, presumably the somatodendritic autoreceptors at the raphe nuclei. Furthermore, the data provide evidence for a major involvement of these 5-HT1A receptors in the modulation of aggressive behavior by 8-OH-DPAT, ipsapirone, buspirone, and eltoprazine.

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.

Effects of two melatonin analogues, S-20098 and S-20928, on melatonin receptors in the pars tuberalis of the rat.

By using quantitative autoradiography, we studied the effects of two drugs related to melatonin on the 2-(125)I-melatonin binding in the pars tuberalis (PT) of rats. The drugs tested were two naphthalenic analogues of melatonin, S-20098 (N-[2-(7-methoxy-1-naphthyl) ethyl] acetamide), an agonist, and S-20928 (N-[2-(1-naphthyl) ethyl] cyclobutyl carboxamide), a putative antagonist. Melatonin (s.c. and i.p.), S-20098 (s.c.), and S-20928 (i.p.) were injected 4 hr before sacrifice. Acute administration of both melatonin and S-20098 decreased melatonin receptor density. In contrast, the putative antagonist S-20928, at a low dose (1 mg/kg), was ineffective on melatonin receptors. It neither affected the 2-(125)I-melatonin specific binding observed in the control group nor did it prevent the decrease in binding induced by melatonin when injected 5 min before the hormone. At a high dose (10 mg/kg), S-20928 totally blocked the effect of melatonin on melatonin receptor density and induced a decrease in binding capacity as melatonin did when injected alone. These results indicate that in the rat pars tuberalis, the melatonin agonist, S-20098, is able to down-regulate melatonin receptors, whereas S-20928 seems to behave as a partial agonist.