A COMPARISON OF THE RESPONSES TO GONADOTROPIN-RELEASING-HORMONE OF ADULT AND JUVENILE, AND PHOTOSENSITIVE AND PHOTOREFRACTORY EUROPEAN STARLINGS, STURNUS-VULGARIS

The reproductive system of juvenile European starlings appears to be s imilar to that of photorefractory adults, yet the increase in plasma l uteinizing hormone (LH) in juveniles in response to gonadotrophin-rele asing hormone (GnRH) is much less than that of photorefractory adults. To investigate this apparent anomaly, the effects of age, reproductiv e state, and sex on the increase in plasma LH concentration in respons e to im injections of GnRH were investigated. The results show that ph otorefractory juveniles needed 10 times the hose of GnRH to elicit the same increase in LH as photorefractory adults, and the response of ph otosensitive juveniles to 10 mu g GnRH was at least 15 times as great as that of photorefractory juveniles (e.g., 6.14 mu g/liter compared t o 0.35 mu g/liter 2 min after injection). However, the response of pho tosensitive adults was not greater than that of photorefractory adults . These differences were not due to differences in the amount of LH st ored in the pituitary: this was 298 +/- 34 and 306 +/- 51 ng/gland in photorefractory juveniles and adults, respectively, and 367 +/- 47 in photosensitive juveniles. Repeated weekly treatment with GnRH enhanced LH responses: LH levels 3 min after GnRH treatment increased in birds on short days from 7.7 mu g/liter after the first treatment to 24.6 m u g/liter after the sixth treatment and in birds on long days it incre ased from 0.54 to 1.8 mu g/liter. The greater response of photorefract ory adults compared to photorefractory juveniles may therefore be due to the self-priming effect of GnRH during a preceding period of photos ensitivity and/or photostimulation. The response to exogenous GnRH dep ends more on age and history than on prevailing physiological state. T here was also a marked sex difference: females showed a sevenfold grea ter response to GnRH. (C) 1995 Academic Press, Inc.