Repeated interactions with females elevate metabolic capacity in the limbic system of male rats.

The effect of heterosexual social experience on brain metabolic capacity was investigated by measuring the activity of cytochrome oxidase, a rate-limiting enzyme in oxidative metabolism. Male Sprague-Dawley rats were kept naïve or allowed to copulate with receptive females three (3 F males) or 16 times (16 F males). Throughout the vomeronasal system and other limbic areas, 16 F males had elevated metabolic capacity relative to naïve and 3 F males, whereas no significant differences in brain metabolism were found between 3 F and naïve males. Behavioral differences were also found between 3 F and 16 F males. In a second experiment, we assessed differences in brain metabolism between sexually active and inactive males given only one opportunity to copulate and found no significant difference in neural metabolism between these males. This suggests that the differences found in the first experiment were primarily driven by differences in repeated experience rather than by sexual performance between 16 F and 3 F males. We speculate that these changes in brain metabolic capacity could be related to immediate early gene expression during copulation and could underlie the long-term behavioral changes accompanying heterosexual social experience.