Ejaculatory response induced by a 5-HT2 receptor agonist m-CPP in rats: differential roles of 5-HT2 receptor subtypes

It has been reported that systemic administration of m-CPP (1-3-chlorophenyl] piperazine hydrochloride), a 5-HT(2) receptor agonist, produces a 5-HT(2C) receptor-mediated penile erections and self-grooming in rats. In the present study, we examined the ability of m-CPP to induce ejaculation in rats and determined which 5-HT(2) receptor subtypes may be involved in the m-CPP-induced ejaculation. The ejaculatory response was assessed by weighing the seminal materials accumulated over 30 min. In Experiment 1, systemic administration of m-CPP (0.1-3.0 mg/kg, i.p.) produced a dose-dependent increase in both the incidence of ejaculation and the weight of the seminal materials. The inverted U-shaped dose-response effects of m-CPP on penile erection and genital grooming were also observed, with maximum effects at 0.6 mg/kg. Pretreatment with SB242084 (0.1 and 0.3 mg/kg, i.p.), a selective 5-HT(2C) receptor antagonist, dose-dependently attenuated the ejaculatory response induced by m-CPP (3.0 mg/kg). The proejaculatory effect of m-CPP was also attenuated by ketanserin (0.3 and 1.0 mg/kg, i.p.), a 5-HT(2A) receptor antagonist, whereas SB204741 (0.1 and 0.3 mg/kg, i.p.), a selective 5-HT(2B) receptor antagonist, significantly potentiated the m-CPP-induced ejaculatory response. Penile erection and genital grooming induced by m-CPP (0.3 mg/kg, i.p.) was only blocked by SB242084. In Experiment 2 (termed as corset test), in rats fitted with a corset at the thoracic level to prevent the loss of seminal materials by genital grooming, the proejaculatory effect of m-CPP was more efficiently detected than in the non-fitted animals: the ED(50) value for inducing ejaculation was reduced to less than 50% of the ED(50) in non-fitted animals. In this test, the proejaculatory effect of m-CPP (0.6 mg/kg, i.p.) was completely blocked by SB242084 (0.3 mg/kg, i.p.), whereas ketanserin (0.3 mg/kg, i.p.) or SB204741 (0.3 mg/kg, i.p.) did not affect the m-CPP -induced ejaculation. From these observations, it is suggested that the 5-HT(2) receptor agonist m-CPP at low doses (0.3-1.0 mg/kg) possesses the proejaculatory as well as proerectile effects in rats that are primarily associated with the activation of 5-HT(2C) receptors, and that the activation of 5-HT2B receptors may produce an inhibitory effect on ejaculation induced by a high dose (3.0 mg/kg) of m-CPP. Furthermore, the results of the present study also indicate that the corset test employed in this study may be useful for detecting the proejaculatory effect of the compounds.