Dopamine D2-like receptor agonists induce penile erection in male rats: differential role of D2, D3 and D4 receptors in the paraventricular nucleus of the hypothalamus.

Pramipexole, a dopamine D3/D2 receptor agonist, induces penile erection when administered subcutaneously (s.c.) or into the paraventricular nucleus of the hypothalamus of male rats, like apomorphine, a mixed D1/D2 receptor agonist, and PD 168,077, a D4 receptor agonist. A U-inverted dose-response curve was found with pramipexole and apomorphine, but not with PD 168,077 (0.025-0.5mg/kg s.c.). Pramipexole effect was abolished by L-741,626, a D2 receptor antagonist (2.5 and 5mg/kg s.c.) and raclopride, a D2/D3 receptor antagonist (0.025 and 0.1mg/kg s.c.), but not by SB277011A (2.5 and 10mg/kg s.c.) or FAUC 365 (1 and 2mg/kg s.c.), two D3 receptor antagonists, or L-745,870 (1 and 5mg/kg i.p.), a D4 receptor antagonist. Similar results were found with apomorphine (0.08mg/kg s.c.), although its effect was also partially reduced by L-745,870. In contrast, PD 168,077 effect was abolished by L-745,870, but not L-741,626, SB277011A, FAUC 365 or raclopride. Similar results were found when dopamine agonists (5-200ng/rat) and antagonists (1-5μg/rat) were injected into the paraventricular nucleus. However, no U-inverted dose-response curve was found with any of the three dopamine agonists injected into this nucleus. As pramipexole- and apomorphine-induced penile erection was reduced mainly by D2, but not D3 or D4 antagonists, D2 receptors are those that mediate the pro-erectile effect of these dopamine agonists. Although the selective stimulation of paraventricular D4 receptors induces penile erection, D4 receptors seem to play only a modest role in the pro-erectile effect of the above dopamine agonists.