A nitric oxide synthesis inhibitor in the medial preoptic area inhibits copulation and stimulus sensitization in male rats.

Dopamine in the medial preoptic area (MPOA) facilitates copulation in male rats, and nitric oxide (NO) regulates basal and female-stimulated MPOA dopamine release. Microinjection of L-nitro-arginine methyl ester (L-NAME, an NO synthesis inhibitor) into the MPOA blocked copulation in naive rats and impaired copulation in sexually experienced males. In other naive rats, L-NAME or saline was microinjected into the MPOA before each of 7 daily exposures to a receptive female placed over their cage. In a drug-free test on Day 8, copulation by L-NAME-treated rats was similar to that of unexposed controls and was impaired relative to saline-treated males. Therefore, NO in the MPOA is important for copulation and stimulus sensitization in male rats.