Naloxone lowers brain-stimulation escape thresholds.

Rats were trained to escape from aversive electrical brain stimulation to the mesencephalic reticular formation (MRF). The threshold for this escape behavior was determined by a modification of the psychophysical method of limits. The administration of naloxone (4-16 mg/kg, IP) produced a decrease in escape threshold from MRF stimulation. These findings implicate the involvement of the MRF in the modulation of pain and suggest that threshold changes to stimulation at this level of the nociceptive neuro-axis may represent a change in a motivational-attentional dimension of pain.