Different endogenous analgesia systems are activated by noxious stimulation of different body regions.

Noxious pinch of the neck and the base of the tail can produce equipotent analgesia as measured by the tail flick method. However, noxious stimulation of the neck can suppress pain responsiveness both at the site of stimulation and at sites remote from the stimulated area while noxious stimulation of the tail produces analgesia only at sites remote from the stimulated area. Thus, neck pinched animals are immobile and completely unresponsive to the noxious pinch whereas pinch to the base of the tail, which results in tail flick suppression, causes vocalization and well organized biting behavior directed at the pinched area. The analgesia elicited by noxious stimulation applied to both body regions is eliminated by spinalization, the administration of intermediate doses of barbiturates (30 and 45 mg/kg) and transection at the midcollicular, but not more rostral, brain level. Concurrent with the elimination of the analgesic effect of noxious pinch on tail flick is the emergence of responses to noxious neck pinch with vocalization and intense motor reactions now elicited by noxious stimulation of the nape of the neck. These results indicate that different analgesic systems are activated by noxious tail and neck pinch both requiring the integrity of mesencephalic structures for their normal function. Furthermore, these systems can be distinguished by their ability to produce recurrent, inhibitory, supraspinal effects on nociceptive information originating at different body regions.