The development of photorefractoriness in castrated willow ptarmigan (Lagopus lagopus lagopus).

Changes in plasma LH levels were observed in castrated willow ptarmigan exposed to different lighting patterns to determine whether they become photorefractory. LH levels did not increase in long-term castrated birds exposed to a natural (70 degrees N) increase in daylength and did not fall abruptly in June when intact birds become photorefractory. Plasma LH levels in naturally photostimulated, long-term castrates fell gradually during the summer and fall. Plasma LH levels increased steeply when these birds were transferred in winter from short days to continuous light. Thereafter the levels fell indicating that the birds became photorefractory. In a further experiment, photosensitive male willow ptarmigan were castrated 3 or 6 weeks after transfer from short days to continuous light. The postcastration increase in plasma LH levels was significantly greater in birds castrated after 3 weeks than in those castrated after 6 weeks of photostimulation. In both groups of castrates, LH levels began to fall when LH levels decreased in intact controls, indicating the development of photorefractoriness. These findings confirm that the development of photorefractoriness is progressive rather than abrupt and that it may be accelerated in castrated birds under natural increases in daylength but not after acute transfer from short days to continuous light. These observations support the view that the development of photorefractoriness in willow ptarmigan involves an interaction between the gonads and the neuroendocrine or circadian system. They are not entirely consistent with the simple concept that photorefractoriness is caused by a decrease in the sensitivity of the hypothalamus to inhibitory steroid feedback.