Sexual attraction and copulation in male rats: effects of the dopamine agonist SND 919.

Behavioral differences towards receptive females, involving latency to the first contact, amicable behavior, genital exploration, and copulatory pattern, were seen in sexually active (A), sluggish (S), and inactive (I) male rats classified on the basis of 11 consecutive mating tests. The D2 dopamine agonist SND 919 (1 mg/kg) was intraperitoneally administered to the three groups 25 min before a 12th test; the drug stimulated the copulatory behavior of A and S but not of I rats in which it diminished genital exploration and amicable behavior. In a 13th test, conducted 1 week later, 31% of I initiated mating, 16% of them reaching ejaculation. The stimulant effect of SND 919 on copulation in A rats was confirmed in further experiments where it was injected at 0.1 mg/kg, a dose selective for the D2 autoreceptors.