Changes in gonadotrophin-releasing hormone (GnRH-I) in the pre-optic area and median eminence of starlings (Sturnus vulgaris) during the recovery of photosensitivity and during photostimulation.

Changes in GnRH-I in the pre-optic (POA) and medio-basal (MBH) areas of the hypothalamus and in pituitary and plasma LH were measured in starlings (Sturnus vulgaris) during the recovery of photosensitivity under short days, and following photostimulation at various times during the recovery of photosensitivity. During exposure to short days there was a significant increase in GnRH-I in the POA, with the first detectable increase after only 10 days. There was no increase in GnRH-I in the MBH or in pituitary or plasma LH. In birds photostimulated after 10 short days, there was an increase in GnRH-I in the POA, but this was no greater than that in birds remaining under short days. There was no increase in GnRH-I in the MBH or in plasma LH. Photostimulation after 20 short days caused an immediate increase in GnRH-I in the POA, delayed increase in GnRH-I in the MBH, but no increase in plasma LH. Photostimulation after 30 short days caused an immediate increase in GNRH-I in the POA and the MBH and in plasma LH. The results show that the recovery of photosensitivity is gradual; the first measurable change occurs in the POA, consistent with photosensitivity being due to renewed GnRH-I synthesis. The effects of photostimulation increase, both in magnitude and in terms of how far 'downstream' of the POA changes are apparent, as photosensitivity is gradually restored. The results support the hypothesis that daylength has a dual role, controlling both synthesis and secretion of GnRH.