5-HT-and norepinephrine-induced release of ACh from the thalamus and mesencephalon of the monkey during thermoregulation.

In the unanesthetized pigtailed monkey (Macaca nemestrina), trained to sit in a primate chair, the body temperature was monitored from the brain and colon. After an array of guide tubes had been implanted stereotaxically, isolated sits within the thalamus and mesencephalon were perfused by means of a push-pull cannula system. The perfusate was an artificial CSF containing 1.0 mug/ml of the anticholinesterase, neostigmine; each sample of effluent was assayed for the content of acetylcholine (ACh) on the eserinized guinea pig ileum. The micro-injection of 5-HT and norepinephrine into the monkey's hypothalamus in doses of 2.5-10.0 mug evoked a rise or fall in body temperature, respectively. In addition, these amines caused a concurrent change in the release of ACh at perfusion loci in both thalamus and mesencephalon. A composite anatomical mapping of the active releasing sites revealed that the output of ACh was elevated at 19 of 36 sites during the 5-HT-induced hyperthermia. The region of maximum sensitivity to 5-HT in terms of the percent change in ACh output at thalamic and mesencephalic sites was the rostral hypothalamus. These results provide further support for a neurochemical model of thermoregulation which postulates that 5-HT activates a cholinergic pathway originating in the hypothalamus which transmits the efferent signals for heat production.