Autoanalgesia: opiate and non-opiate mechanisms

Autoanalgesia (behaviorally-activated antinociception) was elicited by lesion-induced hyperemotionality or the classical conditioning of fear to the environmental stimuli associated with measuring antinociception. Both hyperemotionality and antinociception exhibited parallel decline in septal-lesioned rats with daily handling and in VMH-lesioned rats following treatment with diazepam. Autoanalgesia elicited by conditioned fear was blocked by spinal cord transection but not by diazepam. Although opiate binding experiments suggested the involvement of endorphins as mediators of autoanalgesia, hypophysectomy, morphine tolerance or very high doses of opiate antagonists failed to reduce the antinociception. Electrolytic lesions of the nucleus raphe magnus, a descending serotonergic system, did cause a significant reduction in autoanalgesia. Therefore, endorphin systems may be activated by the stress involved in autoanalgesic paradigms as a parallel system, whose functional integrity is not necessary for the expression of behaviorally-induced antinociception.