PHOTOREFRACTORINESS IN EUROPEAN STARLINGS (STURNUS-VULGARIS) IS NOT DEPENDENT UPON THE LONG-DAY-INDUCED RISE IN PLASMA THYROXINE

Transfer of intact, photosensitive starlings from short to long days c auses an increase in plasma thyroxine and gonadal maturation and later induces photorefractoriness. Thyroidectomy of starlings prevents the induction of photorefractoriness. This study investigated whether the long-day-induced increase in plasma thyroxine is necessary for the ind uction of photorefractoriness. Photosensitive starlings were thyroidec tomised, given thyroxine in their drinking water at concentrations tha t result in plasma thyroxine at short-day physiological concentrations or lower, and transferred to long days. Plasma thyroxine and prolacti n, gonadal size, and moult were monitored. The group with short-day co ncentrations of plasma throxine became photorefractory at the same tim e as intact controls transferred to long days. The other groups, with lower plasma throxine, also became photorefractory, but the onset of p hotorefractoriness was delayed. The increase in plasma prolactin follo wing photostimulation was proportional to plasma thyroxine concentrati ons. The onset of moult was also related to plasma thyroxine. We concl ude that the long-day concentrations of plasma thyroxine observed in t he plasma of intact starlings are not necessary for the induction of p hotorefractoriness. This suggests that thyroxine acts as a permissive factor rather than actively driving the photorefractory process. (C) 1 997 Academic Press.