Inhibitory role of opioid peptides in the regulation of aggressive and sexual behaviors in male Japanese quails

We have recently isolated three opioid peptides, i.e., Met- and Leu-enkephalins and Met-enkephalin-Arg⁶-Phe⁷, from the avian brain. Furthermore, electrophysiological studies have shown that the dominant effect of these enkephalins on preoptic and hypothalamic neurons is an inhibition of neuronal activities in the male Japanese quail. The hypothalamus and preoptic area are known to be involved in the control of male reproductive behaviors, such as aggressive and sexual behaviors. To determine the functional role of opioid peptides in these reproductive behaviors, therefore, the present study was undertaken using adult males of the Japanese quail. We examined behavioral changes following an injection of naloxone (0.2, 2.0, and 20.0 nmol), a nonselective opioid receptor antagonist, or D-Ala²-Met⁵-enkephalinamide (DALA; 0.2, 2.0, and 20.0 nmol), a selective δ opioid receptor agonist, into the preoptic and anterior hypothalamic regions. Naloxone treatment showed a significant increase in the frequency of several aggressive actions and the effect was dose dependent. In contrast, DALA treatment significantly decreased the frequency of aggressive actions in a dose-dependent manner. Similar significant effects of these two drugs were observed in the sexual behavior. These findings provide the first evidence for the role of opioid peptides in the reproductive behaviors in the bird and suggest an inhibitory action of opioid to evoke the behaviors.