Neuropeptide K (NPK) suppresses copulatory behavior in male rats

Recent studies show that neuropeptide K (NPK), a member of the tachykinin family of neuropeptides, is found in various hypothalamic sites implicated in the control of gonadotropin secretion, food intake and sexual behavior. Since our previous studies showed that NPK inhibited feeding and gonadotropin secretion in rats, we have now assessed the effects of NPK on sexual behavior in male rats. Copulatory behavior was recorded subsequent to injection of different doses of NPK either into the third ventricle of the brain or intraperitoneally in sexually experienced male rats. We observed that intraventricular (IVT) administration of 0.125 nmol NPK produced only a slight effect on behavior as indicated by a significant increase in intromission latency. However, a four-fold higher dose of 0.5 nmol of NPK completely eliminated copulatory behavior. These rats displayed no overt locomotor deficit but ignored the receptive females, rarely approaching them during the test period. Only two mounts and no intromissions were observed in 6 rats during the test period. In contrast, neither the same dose of 0.5 nmol nor a higher dose of 3.14 nmol injected intraperitoneally produced any discernible effect on sexual performance. These results show that NPK acts centrally to inhibit sexual behavior in male rats and that hypothalamic NPK may be an important part of the neural circuit that regulates reproduction and related behaviors.