Narcotics do not alter the heat response of unmyelinated primary afferents in monkeys

Recent reports of opiate receptors in the peripheral nervous system have led to the hypothesis that the analgesic action of opiates might, in part, result from a reduction in response of peripheral nerve fibers thought to be concerned with signaling pain (nociceptive afferents). The authors examined the effects of the narcotics, fentanyl (up to 30 μg/kg, iv) and morphine (1 mg/kg, iv), on the response of single unmyelinated afferents (C-fiber nociceptors and warm fibers), recorded in monkeys, to heat stimuli applied to their receptive fields. Neither narcotic affected the response of the afferents. In addition, naloxone did not affect their response. Thus, an alteration of cutaneous nociceptor response is unlikely to contribute to the analgesic action of narcotics.