Naloxone-reversible analgesia produced by microstimulation in the rat medulla.

Using microstimulation of the rostral medulla in the barbiturate-anesthetized rat, a map was constructed of loci for inhibition of the tail-flick response to noxious heat. Low threshold sites (less than or equal to 10 microA) were found in both the nucleus raphe magnus and the nucleus reticularis paragigantocellularis. Chronaxie determinations indicate that analgesia was not produced by activation of large myelinated axons of passage. Systemic naloxone only antagonized the inhibition generated from stimulation at low threshold sites. Inhibition from higher threshold sites, for example from the nucleus reticularis gigantocellularis, was not naloxone reversible. Depending on the area stimulated, either an opioid-or a non-opioid-mediated inhibition results from microstimulation within the rat medulla.