Effects of hypothalamic and preoptic lesions on reproductive behavior in male rats.

Adult male rats, which were selected on the basis of showing complete patterns of male copulatory behavior on two prior tests, were castrated six weeks prior to brain surgery. Animals were divided into three groups and given small bilateral lesions in the dorsomedial preoptic area (POA), ventromedial hypothalamus (VMH), or sham operations. Starting 10 days postsurgery, all animals were injected with estrogen alone and estrogen plus progesterone, and tested twice for lordosis behavior. Ten days following the female behavior tests, animals were injected daily with testosterone propionate for 13 days and tested for masculine sexual behavior on injection 5, 9 and 13. Low levels of lordotic behavior were observed for POA and VMH animals on both tests for female sex behavior and were comparable to sham operated animals. However, in terms of all dependent measures of male copulatory behavior, animals with VMH lesions showed significantly higher levels of male sex behavior with shorter latencies than sham animals across all three behavior tests. In contrast, POA lesioned rats showed little or no male sex behavior on any test and were significantly inferior to sham operated animals. Thus, the POA and VMH appear to exert excitatory and inhibitory control, respectively, over male copulatory behavior in male rats.