A nitric oxide synthesis inhibitor administered into the medial preoptic area increases seminal emissions in an ex copula reflex test.

Nitric oxide synthesis inhibitors, when administered systemically or into the ventricles of the brain, affect several indices of male sexual behavior. Some of the systemic effects are assumed to be due to local vasoconstriction at the penis. Others are suggested to be mediated within the brain. In these experiments, the nitric oxide synthesis inhibitor L-NMMA, and its less active enantiomer, D-NMMA, were microinjected into the medial preoptic area of male rats. In an ex copula test of genital reflexes, L-NMMA increased the number of seminal emissions, while D-NMMA had no effect. These results are consistent with the hypothesis that nitric oxide is a tonic inhibitor of sympathetic nervous system tone, possibly in part through an influence on dopamine synthesis or release.