Antagonism of General Anesthesia by Naloxone in the Rat

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Abstract
The effect of naloxone, a narcotic antagonist, on the response of animals to painful stimuli during anesthesia was studied. Rats were anesthetized with cyclopropane, halothane or enflurane in groups of 12. Following induction, inspired anesthetic concentration was gradually reduced to a point at which 35-60% of animals responded to tail clamping. Thereafter the anesthetic concentration was held constant for 30 min. Rats in each group then received saline solution or naloxone, 10 mg/kg, given i.v. The response to tail clamping was retested 5 min later. In additional experiments EEG were recorded from rats anesthetized with 1 of these anesthetics. After a stable light plane of anesthesia was attained, each animal was given naloxone, 10 mg/kg, i.v. and the EEG recorded for an additional 5 min. In the tail-clamping experiments, naloxone approximately doubled the number of rats responding during cyclopropane, halothane or enflurane anesthesia. The EEG patterns of several animals anesthetized with either cyclopropane or halothane changed to patterns consistent with lighter planes of anesthesia after naloxone administration. That naloxone alters the depth of inhalational anesthesia suggests that anesthetics may release an endogenous morphine-like factor in the CNS.

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