No abstract

Neuropeptides are supposed to be implicated in the regulation of hormone as well as nonhormone dependent behavioral processes in birds. Previous immunohistochemical studies have suggested that in birds opioid pentapeptides, Met- and Leu-enkephalins, may be present in the brain including the regions that regulate sex hormone dependent behaviors, such as reproductive behaviors. To determine biochemically the presence of opioid peptides in the avian brain, a study was conducted to isolate these peptides from Japanese quails and zebra finches. Acetic acid extracts of the quail and finch brains were respectively forced through disposable C-18 reversed-phase cartridges, and then the retained material was subjected to the cation-exchange and reversed-phase high performance liquid chromatographic (HPLC) purifications. All of the purified substances showed a single peak on the reversed-phase HPLC and these substances enhanced spontaneous contractions of the avian rectum. The purified bioactive substances were further subjected to amino acid sequence analysis and were characterized as peptides with the following sequences: Tyr-Gly-Gly-Phe-Met, Tyr-Gly-Gly-Phe-Leu, and Tyr-Gly-Gly-Phe-Met-Arg-Phe. These three peptides were identical with opioid pentapeptides, Met- and Leu-enkephalins, and a heptapeptide, Met-enkephalin-Arg6-Phe7, which had been previously isolated from mammalian species. This is the first direct demonstration of the presence of opioid peptides in the avian brain and confirms previous immunohistochemical findings suggesting a functional role for the opioid peptide in neural mechanisms of avian reproductive behavior.

We have recently isolated three opioid peptides, i.e., Met- and Leu-enkephalins and Met-enkephalin-Arg6-Phe7, from the avian brain. In the present study, therefore, effects of these endogenous opioid peptides on the electrical activity of preoptic and hypothalamic neurons of the adult male Japanese quail were examined using a brain slice preparation. All of the three opioid peptides inhibited the spontaneous firing activities of subsets of neurons in the preoptic area and the paraventricular nucleus in the hypothalamus. Threshold concentrations for the inhibitory action were between 10(-7) and 10(-6) M in Met- and Leu-enkephalins and approximately 10(-6) M in Met-enkephalin-Arg6-Phe7, respectively. In a few cells in these brain areas, however, Leu-enkephalin rather potentiated the spontaneous activities, resulting in an increase of firing rates or a decrease of interburst intervals. The inhibitory effect of Met-enkephalin was completely blocked by naloxone, an opioid receptor antagonist, but not affected by bicuculline, a gamma-aminobutyric acid A (GABAA) receptor antagonist. These results suggest that there are functional opiate receptors in subsets of preoptic/hypothalamic neurons and that one of their main physiological functions in these areas is an inhibition of neuronal activities. Because these brain regions are considered to be involved with the regulation of a variety of male reproductive behaviors, opioid peptides may regulate some reproductive behavior through the mechanism that provokes such an inhibition.

Reproductive behaviors of the male bird,