Role of the dopaminergic, serotonergic and cholinergic link in the expression of penile erection in rats

The neural mechanisms underlying the penile erection induced by serotonergic, cholinergic and dopaminergic stimulants were comparatively investigated. Fenfluramine (0.1-10 mg/kg, i.p.), pilocarpine (0.032-3.2 mg/kg) and apomorphine (0.01-1 mg/kg) induced penile erection in rats with bell-shaped dose-response curves. The penile erection induced by fenfluramine (1 mg/kg) was dose-dependently antagonized by pindolol (0.1-3.2 mg/kg), a 5-HT1 antagonist, or scopolamine (0.032-1 mg/kg), a muscarinic antagonist, but not by sulpiride (1-32 mg/kg), a dopaminergic antagonist. The penile erection induced by pilocarpine (0.32 mg/kg) was countered by pindolol or scopolamine but not by sulpiride, while that induced by apomorphine (0.032 mg/kg) was countered by all three antagonists. Septo-hippocampal cholinergic deafferentations by medial septum lesioning or fimbria-fornix transection also significantly attenuated the penile erection induced by fenfluramine or apomorphine, but scarcely affected that induced by pilocarpine. Raphe lesion by injections of 5,7-dihydroxytryptamine, a serotonergic neurotoxin, into the median- and dorsal-raphe nuclei significantly attenuated the penile erections induced by fenfluramine and apomorphine but not that by pilocarpine. These results suggest that a neuronal link between the dopaminergic, serotonergic and cholinergic systems plays a crucial role in the expression of penile erection; dopaminergic stimulation causes an activation of the raphe serotonergic neurons which in turn enhances the septo-hippocampal cholinergic pathway and results in expression of penile erection.