Brain localization of cholinergic influence on male sex behavior in rats: antagonists.

The muscarinic receptor antagonist scopolamine was microinjected into either the preoptic area or the lateral ventricle, preceding sexual behavior tests. In Experiment 1 unilateral ventricular injections of scopolamine delayed the initiation of copulation, while unilateral preoptic injections had no effect. In Experiment 2 bilateral injections into the preoptic area produced dose-related decreases in the percentages of animals intromitting and ejaculating. In Experiment 3 scopolamine, injected alone into the preoptic area, again decreased the percentages of animals mounting, intromitting, and ejaculating. The muscarinic agonist oxotremorine, injected alone into the preoptic area, decreased ejaculatory threshold (i.e., decreased the number of intromissions preceding ejaculation) as previously reported. Concurrent oxotremorine and scopolamine injections into the preoptic area were not different from vehicle; thus, scopolamine blocked oxotremorine's effect. These data suggest that some cholinergic activation of the preoptic area is critical for normal copulation, since bilateral blockade of muscarinic receptors there dramatically decreased the number of animals copulating. However, increased cholinergic activity there only reduced ejaculation threshold.