The opiate antagonist, naloxone, does not affect descending inhibition from midbrain of nociceptive spinal neuronal discharges in the cat.

Discharges were recorded from spinal dorsal horn neurons in response to noxious skin heating (e.g. 50 degrees C). Repetitive electrical stimulation of the mesencephalic periaqueductal gray (PAG) inhibited these nociceptive discharges. In each of 9 experiments systemic administration of naloxone (1-3 mg/kg) did not affect this descending inhibition, arguing against involvement of endogenous opiates.