Thyrotropin releasing hormone (TRH) in pineal and hypothalamus of the frog: effect of season and illumination.

The influence of photo-illumination and season on the pineal and hypothalamic content of TRH in the leopard frog (Rana pipiens) was studied. Animals (4-6 in each group) were exposed to constant light or darkness for 72 h and then sacrificed. The pineal and hypothalamus from each frog were extracted for TRH measurement by radioimmunoassay. The experiment was performed in spring, autumn, early winter and mid-winter. In early winter, mid-winter and spring the pineal content of TRH ranged from 0.14-0.70 ng. Significant differences between groups due to season and illumination were recorded. In autumn, mean levels of TRH were 12.95 ng (dark exposed) and 4.19 ng (light exposed)-values 10-20 times higher than at other times of the year (P less than 0.001). The hypothalamic content of TRH ranged from 11.4 ng in spring to 19.5 ng in autumn. Seasonal differences were present, but no effect of light or darkness was found. There was no definite relation between the TRH levels in hypothalamus and pineal. The alteration in pineal and hypothalamic TRH produced by season, and the effect of illumination on pineal TRH content, support the view that TRH has a neuronal function in vertebrates, possibly as a neurotransmitter.