The effects of intravenous fentanyl, morphine and naloxone on nociceptive responses of neurones in the rat caudal medulla

Fifty-three per cent of 106 spontaneously active single neurones, located in the caudal medullary reticular formation of the urethane or halothane anaesthetized rat, were found to respond to a noxious stimulation, such as immersion of the tail in water at 52–53°C for 30 sec. The evoked responses consisted of either an increase (82%), decrease (7%) or biphasic (11%) alteration in firing rate, and could be produced by other noxious stimuli, such as a pinch, but not by stroking, tapping or joint manipulation. Morphine (0.8–1.0 mg/kg) or fentanyl (5–10 μg/kg) administered intravenously blocked the acceleration in firing rate produced by noxious stimuli, and this effect was antagonized by naloxone administered intravenously, suggesting it to be an action on specific opiate receptors. Naloxone (0.5–10 mg/kg) blocked those decreases in firing rate produced by noxious stimuli, suggesting that an endogenous opioid may be released onto these neurones as a result of noxious stimuli applied to the periphery.