The opiate antagonist naloxone does not consistently block inhibition of spinal nociceptive transmission produced by stimulation in lateral midbrain reticular formation of the cat.

Electrical stimulation in the lateral midbrain reticular formation (LRF) inhibited the responses of spinal dorsal horn neurons to noxious (50 degrees C) skin heating. Systemic administration of naloxone (0.1-2 mg/kg) had no effect on inhibition produced by LRF stimulation in 10 units, while this inhibition was partially blocked in 3 units. The results indicate that endogenous opiates are not primarily involved in mediating descending inhibition from LRF stimulation.