Naloxone does not consistently affect inhibition of spinal nociceptive transmission produced by medial diencephalic stimulation in the cat

In cats anesthetized with sodium pentobarbital and artificially ventilated with 70% N₂O, the response of single lumbar dorsal horn units to noxious radiant heat stimuli (50°C, 10 sec) applied to glabrous footpad skin were recorded with microelectrodes. Unit heat-evoked responses were markedly suppressed by electrical stimulation (100 msec pulse trains at 100 Hz, 3/sec, 50-300 μA) at sites in the medial basal diencephalic periventricular gray, lateral hypothalamic area, preoptic area, and posterior basal telencephalon. Inhibition of dorsal horn unit responses produced by brain stimulation was not consistently affected following systemic administration of the opiate antagonist naloxone (0.5-1 mg/kg i.v.), indicating that the endogenous opioid peptides are not primarily involved in the mediation of this descending inhibition under the experimental conditions.