Preoptic and midbrain control of sexual motivation.

In a testing arena where a male rat can choose to spend time (and mate with) a sexually receptive female or choose a non-receptive female, a sexually motivated male will prefer the sexually receptive female and a decrease in preference for the receptive female can be said to reflect a decrease in sexual motivation. We have used a preference test to study the effects of castration and brain damage on sexual motivation. In neurologically intact males castration virtually eliminates copulation and decreases preference for a receptive female; copulation and preference are restored by replacement therapy with testosterone. Lesions of the medial preoptic area (MPOA) and lesions of the dorsolateral tegmentum (DLT) of the midbrain abolish copulation and decrease preference for a sexually receptive female. In lesioned males preference declines even further as testing is extended over a span of several months and is not affected by either castration or replacement therapy with testosterone. It seems likely that, at least in part, castration and brain damage decrease mating by decreasing sexual motivation. The MPOA and DLT are connected by axons running through the medial forebrain bundle, and we speculate that sex hormones may work on cells in the MPOA to increase sexual motivation and behavior, perhaps by altering the activity of axons projecting to the DLT which are principally involved in the mediation of sexual reward or "pleasure".