Antagonism by thyrotropin-releasing hormone of brain temperature in response to pentobarbital in the rat: possible involvement of cholinergic mechanism.

The effects of thyrotropin-releasing hormone (TRH) on brain temperature in response to pentobarbital were examined in male rats. After intraperitoneal injection of pentobarbital sodium (55 mg/kg body wt), the rats were fixed stereotaxically and received intraventricular (ivt) injection of varying doses (0.03-30 nmol) of TRH and 17 nmol atropine. Following the injection of 3 nmol TRH, 100 nmol of carbocholine was administered in the same manner. A thermocouple microprobe was unilaterally placed in the midbrain reticular formation so that brain temperature was continuously monitored at room temperature. Brain temperature after pentobarbital injection progressively decreased. While ivt injection of saline did not affect this change in temperature, ivt administration of TRH produced a dose-dependent antagonism of the brain hypothermia induced by pentobarbital. Atropine injection also reversed the pentobarbital-induced decrease in brain temperature. Carbocholine injection led to a significant decrease in brain temperature in response to TRH administration. The present study indicates that brain TRH may play a pivotal role in brain thermoregulation and its mechanism may involve at least in part the central cholinergic pathway in the rat.