Plasma thyroxine and triiodothyronine levels during the development of photorefractoriness in Willow Ptarmigan ( Lagopus lagopus lagopus) exposed to different photoperiods

Abstract

Changes in the concentrations of plasma thyroxine (T4) triiodothyronine (T3), and luteinizing hormone (LH) are measured in photosensitive Willow Ptarmigan ( Lagopus lagopus lagopus) transferred from a 6-hr daylength (6L:18D) to 14-hr (14L:10D), 18-hr (18L:6D), or continuous (LL) daylengths. The increase in plasma LH, and the moult from white to pigmented plumage, observed after transfer to the three stimulatory photoperiods, are not associated with any immediate changes in the concentrations of plasma T4 or T3 except for an increase ( P < 0.01) in T4 levels in birds transferred to 18L:6D. As indicated by a fall ( P < 0.01) in concentrations of plasma LH, the birds exposed to LL and 18L:6D are becoming photorefractory after 8 to 9 weeks of photostimulation. At this time, the concentration of plasma T3 but not of T4 increases significantly ( P < 0.001) and remains high for 5 and 9 weeks in birds exposed to LL and 18L:6D, respectively. Thereafter, levels of plasma T3 begin to fall concomitant with an increase in the levels of plasma T4. In birds exposed to 14L:10D, LH levels remain elevated during the 20-week study and minor increases are observed in the levels of plasma T3 and T4. These observations do not suggest a major role for thyroid hormones in the initiation of photorefractoriness in the Willow Ptarmigan.