Discharges were recorded from spinal dorsal horn neurons in response to noxious skin heating (e.g. 50°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.
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