Lumbar intrathecal naloxone blocks analgesia produced by microstimulation of the ventromedial medulla in the rat.

In lightly barbiturate-anesthetized rats. low threshold (less than 10 micro A) electrical stimulation within the rostral ventromedial medulla inhibited the tail-flick response to noxious heat. Naloxone applied intrathecally at the lumbar level reversed this inhibition, but the same dose of naloxone applied to the cervical intrathecal space had no effect. Doses of naloxone 1- to 4-fold greater than the intrathecal dose did not antagonize tail-flick suppression when given systemically. Because neither systemic nor intrathecal naloxone had any effect on base-line tail-flick latencies, we conclude that the inhibition of the tail-flick response resulting from microstimulation in the ventromedial medulla is mediated by a spinal opioid synapse.