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Changes in concentrations of hypothalamic gonadotrophin-releasing hormone (GnRH), pituitary and plasma LH, testicular mass, in-vitro release of testosterone, body mass and migratory activity were measured in male garden warblers (Sylvia borin) kept from November to June under a constant photoperiod of 12·8 h. Under such conditions garden warblers gradually change from the photorefractory to the photosensitive state and gonadal recrudescence then occurs. Hypothalamic GnRH content was low from December to March, but increased in April to reach the highest levels in June. The spontaneous increase in GnRH was paralleled by increases in pituitary LH content, testicular mass and in-vitro testosterone release. Body mass decreased 1 month and nocturnal activity 2 months before the spontaneous increase in GnRH. Ovine LH increased in-vitro testosterone release over basal release at all times. The results suggest that in garden warblers (1) changes in hypothalamic GnRH content can occur under constant photoperiodic condition, (2) the gradual change from the photorefractory to the photosensitive state is not characterized by a gradual increase in hypothalamic content of GnRH (cf. starlings), and (3) Leydig cells are capable of testosterone release even during the photorefractory state. Journal of Endocrinology (1991) 128, 339–345

When starlings (Sturnus vulgaris) are transferred from short to long days, hypothalamic content of gonadotrophin-releasing hormone (GnRH) and pituitary gonadotrophin content eventually decrease, as birds become photorefractory, to values lower than they were on short days. This implies that both should increase as photorefractoriness is terminated some time after transfer from long to short days. Further, since thyroidectomy causes termination of photorefractoriness in birds held on long days, this should also result in an increase in hypothalamic GnRH and pituitary gonadotrophin contents. This study tests these hypotheses. Male starlings were transferred from short to long days for 10 weeks, by which time they should have become photorefractory. One group of birds was then killed; blood was collected, hypothalami and pituitaries were excised, and the stage of moult and testicular weights recorded. The remaining birds were then kept on long days, transferred to short days or thyroidectomized and kept on long days. Groups of birds in each treatment group were killed and sampled 2, 8 and 14 weeks later. Hypothalamic content of GnRH, and pituitary and plasma FSH and prolactin concentrations, were measured by radioimmunoassay. In birds sampled after 10 weeks of long days, hypothalamic content of GnRH was quite low, pituitary and plasma levels of FSH were very low, testes were small and pituitary and plasma prolactin levels were high. In intact birds kept on long days, hypothalamic GnRH content decreased further and remained low. Pituitary and plasma FSH levels remained low, testes remained small and pituitary and plasma prolactin levels decreased slowly. In intact birds transferred to short days, hypothalamic GnRH content increased to values significantly (PP These results demonstrate that short days activate the hypothalamus to a state of 'readiness to function' and confirm that the thyroid glands are involved in this response. J. Endocr. (1986) 110, 73–79

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. Journal of Endocrinology (1989) 122, 255–268

Changes in concentrations of hypothalamic gonadotrophin-releasing hormone (GnRH) and pituitary and plasma FSH and prolactin were measured in intact and thyroidectomized female starlings (Sturnus vulgaris) after transfer from short to long photoperiods. In intact birds, hypothalamic GnRH did not increase significantly during the first 6 weeks of photostimulation, but by 12 weeks, as birds became photorefractory, it had decreased to levels significantly lower than those before photostimulation. In thyroidectomized birds, which did not become photorefractory, hypothalamic GnRH remained high after 12 weeks of photostimulation. Pituitary FSH increased in both intact and thyroidectomized birds; it then decreased to low levels in intact photorefractory birds, but remained high in thyroidectomized birds. Plasma FSH increased to a peak after 2 weeks, but by 6 weeks it had decreased to low levels in both groups. In intact birds there was a 70-fold increase in pituitary prolactin during the first 6 weeks, and levels were still high after 12 weeks of photostimulation. In thyroidectomized birds, pituitary prolactin remained low. The results suggest that while the initial effect of long daylengths is to cause gonadal maturation, the ultimate effect is to switch off the reproductive system. J. Endocr. (1985) 105, 71–77