D2-like receptors mediate the expulsion phase of ejaculation elicited by 8-hydroxy-2-(di-N-propylamino) tetralin in rats

The mechanism of action by which 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT) facilitates ejaculation in conscious rats is not clearly established. The serotonin (5-HT) 1A agonist 8-OH-DPAT may actually act on cerebral dopaminergic receptors to exert its proejaculatory effect. The present work was undertaken to clarify this issue by testing various compounds i.c.v. delivered in an experimental model of the expulsion phase of ejaculation in anesthetized Wistar rats. Intrace-rebroventricular delivery of 8-OH-DPAT dose-dependently (ED50 = 17 mu g) induced rhythmic contractions of bulbospongiosus (BS) muscles, which are of paramount importance for the expulsion of semen, occurring in the form of cluster of bursts evidenced by the recording of BS muscle electrical activity. The 5-HT1A antagonist WAY100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide) (20 mu g) i.c.v. coadministered with 8-OH-DPAT (20 mu g) was unable to inhibit the effect of 8-OH-DPAT on BS muscle contractile activity. Conversely, raclopride ( 40 mu g) and spiperone (10 mu g), both dopamine D-2-like receptor antagonists, i.c.v. coinjected with 8-OH-DPAT (20 mu g), abolished BS muscle contractions. The involvement of D-2-like receptors was further supported by the fact that the D-2-like agonist quinelorane (20 mu g i. c. v.) also induced BS muscle rhythmic contractions. Our data demonstrate that D-2-like receptors mediate the induction by 8-OH-DPAT of rhythmic BS muscle contractions and suggest that i. c. v. delivery of D-2-like receptor agonists to anesthetized rats represents a relevant experimental model to study the expulsion phase of ejaculation.