Regulatory Mechanism of the Avian Reproductive Behavior

Abstract

Reproductive behaviors of the male bird, such as aggressive and sexual behaviors, are considered to be the typical instinct behavior and have been studied anatomically, ethologically, and endocrinologically. Since the electrical stimulation of the hypothalamus and preoptic area can elicit stereotyped reproductive behaviors in several avian species, these brain regions are considered to be involved with the control of male reproductive behaviors. The occurrence of a variety of reproductive behaviors shows a seasonal variation in most species of wild male birds inhabiting the temperate zone. Behavioral studies combined with endocrinological methods showed that sex steroids released from the testis, particularly testosterone, are essential to produce male reproductive behaviors in several temperate birds including the Japanese quail. One of the targets of testosterone is the preoptic and hypothalamic neurons, because testosterone or its metabolites concentrating cells was shown in the medial preoptic area, nucleus paraventricularis, regio lateralis hypothalami, nucleus inferior hypothalami, and nucleus infundibuli in the Japanese quail as well as other avian species. In addition, androgen actions in the avian brain are often mediated by the enzymatic activity that catalyzes the conversion of androgen to estrogen. Both aromatase and estrogen receptors are present in several brain regions including the hypothalamus and preoptic area. Thus, the reproductive behaviors of male birds are rather well understood in ethological and endocrinological terms.
In contrast, little is known regarding the cellular and molecular regulations on the central nervous system of these behaviors. Neuropeptides are supposed to be implicated in the regulation of hormone dependent behavioral processes in birds. Recently, we have isolated three opioid peptides, i. e., Met-enkephalin, Leu-enkephalin, and Met-enkephalin-Arg-Phe, from adult males of the Japanese quail and zebra finch. On the basis of immunohistochemical studies, these enkephalins were located in the preoptic and hypothalamic areas as well as other brain regions. A series of our experiments indicated that their main physiological functions in these areas is an inhibition of neuronal activities. Thus, opioid peptides may regulate some reproductive behavior through the mechanism that provokes such an inhibition. On the other hand, new findings that neurosteroids accumulate in the brain through mechanisms at least partly independent of peripheral steroidogenic glands have been available in an avian species. We have recently demonstrated that the avian brain produces pregnenolone, the main precursor of steroid hormones, on the basis of biochemical studies. Furthermore, the immunohistochemical and Western immunoblotting analyses showed that a specific enzyme of the pregnenolone synthesis, cytochrome P450scc, is located in the preoptic and hypothalamic areas in addition to other brain regions. The distribution of this enzyme suggests that pregnenolone acts as an important neurosteroid to regulate the avian reproductive behavior.
This paper summarizes the advances made in our understanding of regulatory mechanisms of the avian reproductive behavior.