Presence of cholinoceptors in mesencephalic raphe nuclei concerned in thermoregulation in rabbits

The thermal effects of cholinomimetics and cholinoceptor blocking agents microinjected into mesencephalic nucleus raphe medianus (NRM) were investigated in rabbits to determine the nature and role of these cholinoceptors in thermoregulation. Microinjection of cholinoceptor agonists, carbachol and pilocarpine, into NRM resulted in significant hyperthermia which could be blocked by local pretreatment with chlorisondamine (a nicotinic receptor blocker) as well as by ethybenztropine (a muscarinic receptor blocker). Intracerebroventricular pretreatment with LM 5008 (serotonin reuptake blocker) significantly inhibited the carbachol-induced hyperthermia. Both nicotinic and muscarinic cholinoceptors are present in mesencephalic NRM which may be involved in thermoregulation in rabbits. Activation of these cholinoceptors in NRM results in hyperthermia which seems to be due to an inhibition of a serotonin sensitive hypothalamic heat loss mechanism.