The development of the reproductive system was studied in juvenile starlings during the acquisition of photosensitivity, the attainment of sexual maturation after photostimulation and the subsequent onset of photorefractoriness, using immunohistochemistry for LHRH and radioimmunoassay measurements of hypothalamic, pituitary and plasma hormone concentrations. The first stage of sexual development induced by exposure of photorefractory immature starlings to short days (8 h light:16 h darkness; 8L:16D) was characterized by a decrease in pituitary prolactin content within 1 week and an increase in hypothalamic LHRH content, in the size of the LHRH perikarya and in the intensity of immunostaining in the median eminence in 4-6 weeks. Sexual maturation occurring after exposure to long days (18L:6D) was associated with further increases in LHRH content and cell size, and increases in LH and prolactin concentrations. During testicular regression, LHRH perikarya were reduced in size and staining intensity but LHRH immunostaining in the median eminence and content in the hypothalamus remained high until gonadal regression was almost complete. Prolactin levels were maximal during testicular regression. These results suggest that gonadal regression is initiated by a reduction in LHRH synthesis and possibly, in addition, an external inhibitory influence on LHRH release. Hypothalamic LHRH content eventually declined and LHRH immunostaining in the median eminence was much reduced in fully photorefractory starlings maintained under long days.

Molt is induced in quail when they are transferred from photostimulatory long daylengths of 18L, with a fully active reproductive system, to short daylengths of 8L, when the reproductive system regresses. Gonadal steroids, but not gonadotrophins, inhibit molt in male and female Japanese quail. Molt can be induced in long-term photostimulated (18L) quail by withdrawing gonadal steroids 1) by transferring quail to 8L, 2 ) by castrating males and maintaining them on a photoperiodic regime of 18L, or 3) by treating females or males with the antiestrogen, tamoxifen. In contrast, treatment of male quail with the antiandrogen, flutamide, does not induce molt even though androgen secretion is inhibited and androgen-dependent tissues regress. Neither the exact mode of action nor the metabolite of gonadal steroids which inhibits molt is known, although estradiol is capable of inhibiting molt in females and to a lesser extent in males. Plasma levels of thyroxine are consistently elevated during molt. o 1993 WiIey-Liss, Inc.

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