A MODEL OF TRANSIENT HYPERALGESIA IN THE BEHAVING MONKEY INDUCED BY TOPICAL APPLICATION OF CAPSAICIN

The aim of this study was to develop a model of transient hyperalgesia in the awake monkey performing operant tasks. An adult male rhesus mo nkey was trained to press a lever to receive food reward for detecting a light or to escape mechanical or thermal stimuli applied in the max illary region of the face. A small contact thermode was positioned on one side of the face and a mechanical stimulator was placed on the oth er side. Noxious and innocuous thermal (43, 47 and 51 degrees C) or me chanical (245, 490, 736 and 1472 mN) stimuli of 4.5-s duration were pr esented in a pseudo-random order. The animal was tested before, 1 h an d 24 h after topical capsaicin application (0.3 ml; 0.004 M). At the s ite of capsaicin application, the monkey escaped more thermal and mech anical stimuli 1 h after than before capsaicin, suggestive of thermal and mechanical hyperalgesia. At 24 h post-capsaicin, mechanical escape behavior had returned to baseline, but thermal escapes were still sli ghtly elevated. Capsaicin had no significant effect on either mechanic al or thermal escape behavior for stimuli presented to the contralater al site. Seven human subjects tested with these procedures reported hi gher pain intensity for similar stimuli after capsaicin application, i n accordance with the monkey escape behavior. It is concluded that top ical application of capsaicin on the maxillary face of the awake behav ing monkey produces a transient thermal and mechanical hyperalgesia. T he procedure is repeatable and produces no overt signs of distress. Th us it could provide an important tool for studying neural mechanisms o f hyperalgesia and for testing analgesic treatments in primates. (C) 1 997 International Association for the Study of Pain. Published by Else vier Science B.V.