The site and the mode of analgesic actions exerted by clonidine in monkeys

We used intracerebral administration of clonidine in monkeys to map effective sites for analgesia. The jaw opening reflex elicited by tooth pulp stimulation was used for analgesia testing. We found that the most consistently effective sites for analgesia in monkeys are in at least three brain regions: the diencephalic periventricular gray, the dorsal raphe nuclei, and the periaqueductal gray. In addition, the analgesia induced by intracerebral administration of clonidine was effectively antagonized by pretreatment of animals with either naloxone (a narcotic antagonist) or yohimbine (an alpha-adrenergic antagonist). These results suggest the existence of an opiate and an adrenergic antinociceptive mechanism in the diencephalic periventricular gray, the dorsal raphe nuclei, and the periaqueductal gray activated by clonidine in primates.