A comparison of the responses to gonadotrophin-releasing hormone of adult and juvenile, and photosensitive and photorefractory European starlings, Sturnus vulgaris.

The reproductive system of juvenile European starlings appears to be similar to that of photorefractory adults, yet the increase in plasma luteinizing hormone (LH) in juveniles in response to gonadotrophin-releasing hormone (GnRH) is much less than that of photorefractory adults. To investigate this apparent anomaly, the effects of age, reproductive state, and sex on the increase in plasma LH concentration in response to im injections of GnRH were investigated. The results show that photorefractory juveniles needed 10 times the dose of GnRH to elicit the same increase in LH as photorefractory adults, and the response of photosensitive juveniles to 10 micrograms GnRH was at least 15 times as great as that of photorefractory juveniles (e.g., 6.14 micrograms/liter compared to 0.35 microgram/liter 2 min after injection). However, the response of photosensitive adults was not greater than that of photorefractory adults. These differences were not due to differences in the amount of LH stored 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 micrograms/liter after the first treatment to 24.6 micrograms/liter after the sixth treatment and in birds on long days it increased from 0.54 to 1.8 micrograms/liter. The greater response of photorefractory adults compared to photorefractory juveniles may therefore be due to the self-priming effect of GnRH during a preceding period of photosensitivity and/or photostimulation. The response to exogenous GnRH depends more on age and history than on prevailing physiological state. There was also a marked sex difference: females showed a sevenfold greater response to GnRH.