Opiate antagonists and long-term analgesic reaction induced by inescapable shock in rats.

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Abstract
Five experiments [exp] examined the influence of opiate antagonists on the short-term analgesic reaction resulting 30 min after exposure to inescapable shock and the long-term analgesic reaction resulting after reexposure to shock 24 h after inescapable shock exposure. Exp 1 showed that the long-term analgesic reaction could be reduced by administration of naltrexone prior to exposure to inescapable tail shock. Exp 2 showed that the reduction in the long-term analgesic reaction produced by naltrexone was dose-dependent. Exp 3 showed that the long-term analgesic reaction could also be reduced by administration of naltrexone prior to reexposure to shock. Exp 4 showed that the long-term analgesic reaction could be reduced by administration of a large dose of naloxone prior to reexposure to shock. Exp 5 showed that the short-term analgesic reaction was reduced by naltrexone administered prior to inescapable shock. Some implications of these results for the biochemical substrates of both learned helplessness and stress-induced analgesia are discussed.