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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 (P<0·01) higher than in birds kept on long days. Pituitary FSH content increased (P<0·01) but plasma FSH levels remained low and testes remained small in most birds. Pituitary and plasma prolactin levels decreased rapidly. In thyroidectomized starlings kept on long days, hypothalamic GnRH content increased to the same values as in birds on short days, pituitary FSH content increased markedly and testes grew although plasma FSH remained low. Pituitary and plasma prolactin levels decreased slowly.

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