Agomelatine, an antidepressant with melatonin agonist and 5-HT(2C) antagonist properties, as well as two of its main metabolites, S 21517 (N-[2-(7-hydroxy-1-naphtyl)ethyl]acetamide) and S21540 (N-[2-(3-hydroxy-7-methoxynaphtalen-1-yl)ethyl]acetamide), have been assessed in vitro on pig choroid plexus preparations to determine their affinities for 5-HT(2C) receptors and their effects on inositol phosphate production. These compounds were also tested for their ability to inhibit the penile erections induced by the 5-HT(2C) receptor agonists, m-(chlorophenyl)piperazine (mCPP, 0.75 mg/kg, SC) and Ro 60-0175 (2.5 mg/kg, SC) in Wistar rats. These in vivo effects were compared to those of melatonin and the 5-HT antagonists pizotifen and SB 206,553. Agomelatine and S 21517 had moderate affinity for 5-HT(2C) receptors and behaved in vitro as weak antagonists at this receptor subtype. S 21540 had a 10-fold lower affinity. Pizotifen and SB 206,553 antagonized mCPP- and Ro 60-0175-induced penile erections, suggesting that penile erections induced by mCPP or Ro 60-0175 resulted from the stimulation of 5-HT(2C) receptors. Whereas increasing doses (from 1.25 to 40 mg/kg, IP) of melatonin were unable to modify the penile erections induced by mCPP and Ro 60-0175, agomelatine (from 1.25 to 40 mg/kg, IP) dose-dependently decreased mCPP- as well Ro 60-0175-induced penile erections. Furthermore, increasing doses (from 1.25 to 40 mg/kg, IP) of S 21517 and S 21540, the two main metabolites of agomelatine, did not affect the penile erections induced by mCPP and Ro 60-0175. Considering the similar activity of melatonin and agomelatine at melatonin receptors, these data suggested that the reported effects were not due to the stimulation of melatonin receptors and that, contrary to melatonin, agomelatine exerted 5-HT(2C) receptor antagonist properties in addition to its agonist activity at melatonin receptors. Finally, neither S 21517 nor S 21540 seemed to participate to the observed inhibition of penile erections by agomelatine.
