N. raphe magnus lesions disrupt stimulation-produced analgesia from ventral but not dorsal midbrain areas in the rat

We previously found that the opiate antagonist, naloxone, partially blocks stimulation-produced analgesia (SPA) elicited from ventral but not dorsal regions of the medial midbrain in rats. The present study compares the effects of n. raphe magnus (NRM) lesions on SPA from these same two midbrain areas. SPA thresholds were measured with the tail-flick method and compared before and for up to two weeks after NRM lesions. A high positive correlation was found between percent NRM destruction and percent increase in SPA threshold for rats with ventral but not dorsal electrode placements. Damage to brain areas other than NRM seemed not to contribute to these effects. We conclude that n. raphe magnus is a critical relay in the pain-suppressive path from that area of the rat midbrain mediating an opioid form of stimulation-produced analgesia.