[Changes in TRH and cyclo (His-Pro) concentrations in the rat brain after cold exposure]

Intraventricular administration of TRH and cyclo (His-Pro) has been investigated to cause the respectively, increase and decrease, in body temperature. We scrutinized whether cold exposure (4 degrees C) may change TRH and cyclo (His-Pro) concentrations in the rat brain. While the blood TSH level rose after cold exposure, the rectal temperature was not significantly changed by the same exposure. Cyclo (His-Pro) concentrations in the hypothalamus increased significantly 15 and 30 min after cold exposure and returned to the basal level after 60 min. The extrahypothalamic concentrations of cyclo (His-Pro) decreased 60 min after cold exposure. No significant changes were observed in either the hypothalamic or extrahypothalamic concentrations of TRH under the cold status. It is postulated from the present data that the cold exposure-induced increase in cyclo (His-Pro) concentrations in the hypothalamus may be a potent hypothermic signal to elicit the counter regulatory mechanism(s) for the decrease in body temperature.