5-HT2C receptor activation is a common mechanism on proerectile effects of apomorphine, oxytocin and melanotan-II in rats

5-Hydroxytryptamine (5-HT), dopamine, oxytocin and melanocortin pathways are known to be involved in the induction of penile erections in rats. Although a dopamine–oxytocin–5-HT link in the central nervous system has been suggested to be important to the control of penile erections, the 5-HT receptor subtype that mediates dopamine–oxytocin–5-HT action and the relationship between the dopamine–oxytocin–5-HT pathway and melanocortin pathway have not been fully elucidated. In this study, in order to clarify these matters, we examined the effects of a selective 5-HT2B/5-HT2C receptors antagonist, 1-(1-methylindol-5-yl)-3-(3-pyridyl)urea (SB200646) and a selective 5-HT2C receptor antagonist, 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy) pyridin-3-yl carbamoyl] indoline (SB242084) on penile erections induced by a dopamine receptor agonist, 10, 11-dihydroxyaporphine (apomorphine), oxytocin, or a melanocortin receptor agonist, melanotan-II (MT-II) in rats. SB200646 at 10 mg/kg and SB242084 at 3 mg/kg, these doses which completely antagonize penile erections induced by 5-HT2C receptor agonists, m-chlorophenylpiperazine (mCPP) and (S)-2-(7-ethyl-1H-furo[2,3-g]indazol-1-yl)-1-methylethylamine (YM348), significantly inhibited penile erections elicited by apomorphine, oxytocin or MT-II. In addition, in order to clarify further the suggestion that the 5-HT pathway projecting from medulla oblongata to lumbosacral spinal site and lumbosacral 5-HT2C receptor are involved in the induction of penile erection, we also examined the proerectile effect of YM348 in spinal and a 5-HT depletor, p-chlorophenyl alanine (pCPA)-treated rats. YM348 induced intracavernous pressure increase in spinal and pCPA-treated rats as well as normal rats. These results suggest that 5-HT2C receptor in lumbosacral spinal sites mediates not only dopamine–oxytocin–5-HT action but melanocortin action on penile erections, and that the 5-HT pathway is located downstream from melanocortin pathway as well as the dopamine–oxytocin pathway.