The roles of day length and the testes in the regulation of plasma LH levels in photosensitive and photorefractory willow ptarmigan ( Lagopus lagopus lagopus)

Abstract

The interaction between day length and testicular hormones in the regulation of seasonal breeding in willow ptarmigan was investigated by measuring the changes in plasma LH levels after castration of photosensitive and photorefractory birds subsequently exposed to long or short days. Plasma LH levels increased after castration in photorefractory and photosensitive birds exposed to long days but this increase was greater ( P < 0.001) in the photosensitive than in the photorefractory birds. Plasma LH levels increased immediately after castration in photosensitive but not in photorefractory birds exposed to short days. In the latter plasma LH levels increased steeply ( P < 0.001) after exposure to short days for more than 5 weeks. This increase may have occurred at the time when the birds regained photosensitivity. The increase in plasma LH after castration in photosensitive birds was greater ( P < 0.001) in those exposed to long days than in those kept on short days. We concluded that in the willow ptarmigan (1) there are seasonal changes in the sensitivity of the hypothalamo-hypophyseal unit to the inhibitory feedback action of steroids and (2) that the central nervous mechanism which controls seasonal breeding is stimulated more by long days than by short days in both photosensitive and photorefractory birds. These findings are consistent with the hypothesis that in the willow ptarmigan, seasonal breeding results from an interaction between a direct effect of day length on LH secretion and day length-induced changes in the sensitivity of the hypothalamus to the inhibitory feedback action of adrenal and testicular steroids.