Agomelatine (S20098) displayed pK i values of 6.4 and 6.2 at native (porcine) and cloned, human (h)5-hydroxytryptamine (5-HT) 2C receptors, respectively. It also interacted with h5-HT 2B receptors (6.6), whereas it showed low affinity at native (rat)/cloned, human 5-HT 2A (Ͻ5.0/5.3) and 5-HT 1A (Ͻ5.0/5.2) receptors, and negligible (Ͻ5.0) affinity for other 5-HT receptors. In antibody capture/ scintillation proximity assays, agomelatine concentration dependently and competitively abolished h5-HT 2C receptor-mediated activation of Gq/11 and Gi 3 (pA 2 values of 6.0 and 6.1). As measured by [ 3 H]phosphatidylinositol depletion, agomelatine abolished activation of phospholipase C by h5-HT 2C (pK B value of 6.1) and h5-HT 2B (pK B value of 6.6) receptors. In vivo, it dose dependently blocked induction of penile erections by the 5-HT 2C agonists (S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine (Ro60,0175) and 1-methyl-2-(5,8,8-trimethyl-8H-3-aza-cyclopenta[a]inden-3-yl) ethylamine (Ro60,0332). Furthermore, agomelatine dose dependently enhanced dialysis levels of dopamine in frontal cortex of freely moving rats, whereas they were unaffected in nucleus accumbens and striatum. Although the electrical activity of ventrotegmental dopaminergic neurons was unaffected by agomelatine, it abolished their inhibition by Ro60,0175. Extracellular levels of noradrenaline in frontal cortex were also dose dependently enhanced by agomelatine in parallel with an acceleration in the firing rate of adrenergic cell bodies in the locus coeruleus. These increases in noradrenaline and dopamine levels were unaffected by the selective melatonin antagonist N- [2-(5-ethyl-benzo[b]thien-3-yl)ethyl] acetamide (S22153) and likely reflect blockade of 5-HT 2C receptors inhibitory to frontocortical dopaminergic and adrenergic pathways. Correspondingly, in distinction to agomelatine, melatonin showed negligible activity at 5-HT 2C receptors and failed to modify the activity of adrenergic and dopaminergic pathways. In conclusion, in contrast to melatonin, agomelatine behaves as an antagonist at 5-HT 2B and 5-HT 2C receptors: blockade of the latter reinforces frontocortical adrenergic and dopaminergic transmission.

The present study evaluated, via a combined electrophysiological and dialysis approach, the potential influence of serotonin (5-HT)(2C) as compared to 5-HT(2A) and 5-HT(2B) receptors on dopaminergic, adrenergic, and serotonergic transmission in frontal cortex (FCX). Whereas the selective 5-HT(2A) antagonist MDL100,907 failed to modify extracellular levels of dopamine (DA), noradrenaline (NA) or 5-HT simultaneously quantified in single dialysate samples of freely-moving rats, the 5-HT(2B)/5-HT(2C) antagonist SB206,553 dose-dependently increased levels of DA and NA without affecting those of 5-HT. This action was attributable to 5-HT(2C) receptor blockade inasmuch as the selective 5-HT(2C) antagonist SB242,084 likewise increased FCX levels of DA and NA, whereas the selective 5-HT(2B) antagonist SB204,741 was ineffective. Further, the preferential 5-HT(2C) receptor agonist Ro60-0175 dose-dependently depressed FCX levels of DA. The suppressive influence of 5-HT(2C) receptors on DA release was also expressed on mesolimbic and nigrostriatal dopaminergic pathways, in that levels of DA in nucleus accumbens and striatum were likewise reduced by Ro60-0175 and elevated, though less markedly, by SB206,553. In line with the above findings, Ro60-0175 dose-dependently decreased the firing rate of ventrotegmental dopaminergic and locus coeruleus (LC) adrenergic perikarya, whereas their activity was dose-dependently enhanced by SB206,553. Furthermore, SB206,553 transformed the firing pattern of ventrotegmental dopaminergic neurons into a burst mode. In contrast to SB206,553, MDL100,907 had little affect on the firing rate of dopaminergic or adrenergic neurons. In conclusion, as compared to 5-HT(2A) and 5-HT(2B) receptors, 5-HT(2C) receptors exert a tonic, suppressive influence on the activity of mesocortical - as well as mesolimbic and nigrostriatal - dopaminergic pathways, likely via indirect actions expressed at the level of their cell bodies. Frontocortical adrenergic, but not serotonergic, transmission is also tonically suppressed by 5-HT(2C) receptors.

Cognitive deficits in schizophrenia severely compromise quality of life and are poorly controlled by current antipsychotics. While 5-HT6 receptor blockade holds special promise, molecular substrates underlying their control of cognition remain unclear. Using a proteomic strategy, we show that 5-HT6 receptors physically interact with several proteins of the mammalian target of rapamycin (mTOR) pathway, including mTOR. Further, 5-HT6 receptor activation increased mTOR signalling in rodent prefrontal cortex (PFC). Linking this signalling event to cognitive impairment, the mTOR inhibitor rapamycin prevented deficits in social cognition and novel object discrimination induced by 5-HT6 agonists. In two developmental models of schizophrenia, specifically neonatal phencyclidine treatment and post-weaning isolation rearing, the activity of mTOR was enhanced in the PFC, and rapamycin, like 5-HT6 antagonists, reversed these cognitive deficits. These observations suggest that recruitment of mTOR by prefrontal 5-HT6 receptors contributes to the perturbed cognition in schizophrenia, offering new vistas for its therapeutic control.

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