Abstract

For a better understanding of clinical pain, several groups involved in the study of basic pain mechanisms have proposed the use of various experimental models close to clinical situations. They are based either on neurogenic or inflammatory processes. Data obtained with three of these models will be developed in the paper: rats rendered arthritic by Freund's adjuvant injection into the tail, rats with an intraplantar injection of carrageenin in one hind-paw, rats with a moderate ligature of one common sciatic nerve. The various pharmacological approaches revealed dramatic changes of the analgesic effects of morphine and other opioid substances, and a spectacular modification of the endogenous opioid reactivity. A further enhancement of the initial hyperalgesia was observed with high doses (1-3 mg/kg iv) of naloxone (known as an antagonist of morphine), contrasting with the paradoxical analgesia induced with the low dose (peaking up for 3 micrograms/kg iv). Electrophysiological studies emphasized dramatic changes of neuronal responsiveness in structures involved in the transmission of the nociceptive messages. In each of these models, electrophysiological data provide new insights on the physiopathological mechanisms of the related clinical pain.