Effects of pain-attenuating brain stimulation and morphine on electrical activity in the raphe nuclei of the awake rat

Abstract

Evoked potential and multiple unit responses to noxious shock and pinch as well as to innocuous air puffs were recorded in the dorsal raphe, median raphe and raphe magnus nuclei of awake rats. Concurrent measurements of various behavioral responses to noxious stimuli were also made. Electrical stimulation of midbrain central gray and of medial thalamus, as well as systemic administration of morphine, greatly diminished all behavioral and electrophysiological responses to noxious stimuli without reliably affecting responses to air puffs. At the same time that brain stimulation and morphine attenuated nociceptive responses, a significant elevation was seen in the spontaneous multiple unit activity of these brain areas, particularly nucleus raphe magnus. In another group of animals, a comparison was made of the analgesic effectiveness of stimulation sites in the bulbar raphe (including raphe magnus) and sites dorsal or lateral to this region. More consistently potent effects were obtained from the raphe placements. These findings point to the importance of the bulbar raphe in mechanisms of analgesia. It is suggested that brain stem stimulation and morphine administration activate descending controls of raphe origin which selectively inhibit nociceptive elements in the spinal cord.