Septohippocampal cholinergic pathway and penile erections induced by dopaminergic and cholinergic stimulants

Penile erection was studied in rats injected with pilocarpine, amantadine and apomorphine. Pilocarpine and apomorphine are direct stimulants of muscarinic and dopaminergic receptors, respectively, while amantadine is reported to be an indirect stimulant of dopaminergic nerves. Single intraperitoneal injection of pilocarpine (0.1-3.2 mg/kg), amantadine (1-32 mg/kg) and apomorphine (0.01-1 mg/kg) increased the number of penile erections with bell-shaped dose-response curves. The penile erections, irrespective of the stimulant used, were antagonized by doses (0.032-1 mg/kg) of scopolamine but not by those of methylscopolamine. Sulpiride treatment (1-32 mg/kg) had little effect on pilocarpine-induced penile erection while it attenuated dose-dependently the apomorphine- and amantadine-induced ones. Medial septum and fimbria-fornix lesions abolished penile erections by amantadine and apomorphine but not by pilocarpine. The similar results with amantadine and apomorphine suggest that the drugs activate postsynaptic dopamine receptors, which indirectly activate the septohippocampal cholinergic pathway to induce penile erections. Possible linkage between dopaminergic and cholinergic nerves is discussed.