Opiate microinjections into midbrain do not affect the aversiveness of caudal trigeminal stimulation but produce somatotopically organized peripheral hypoalgesia.

High-dose microinjections of morphine sulfate (15 micrograms) and (D-Ala2)-Met-enkephalin (30 micrograms) were made into the ventral periaqueductal gray of rats. Consistent with previous reports using lower doses, both opiates produced hypoalgesia for noxious thermal stimuli applied to the upper and lower body. More hypoalgesia was observed on the face than on the hind legs or tail. Current thresholds of aversive reaction to stimulation in the trigeminal subnucleus caudalis were unaffected by microinjection of either opiate. Systemic injections of 6 mg/kg morphine sulfate profoundly inhibited defense responses to peripheral noxious stimuli and significantly elevated aversive reaction thresholds for stimulation in the trigeminal subnucleus caudalis. Aversive reactions to stimulation in the dorsal periaqueductal gray remained unaffected by either microinjected or systemically administered opiates.