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

Five experiments with 240 male albino rats examined the influence of opiate antagonists (naltrexone; 1–24 mg/kg, ip) on both 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 hrs after inescapable shock exposure. Exp I showed that the long-term analgesic reaction could be reduced by administration of naltrexone prior to exposure to inescapable tail shock. Exp II showed that the reduction in the long-term analgesic reaction produced by naltrexone was dose-dependent. Exp III showed that the long-term analgesic reaction could also be reduced by administration of naltrexone prior to reexposure to shock. Exp IV showed that the long-term analgesic reaction could be reduced by administration of a large dose of naloxone prior to reexposure to shock. Exp V showed that the short-term analgesic reaction was reduced by naltrexone administered prior to inescapable shock. Implications for the biochemical substrates of both learned helplessness and stress-induced analgesia are discussed. (56 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)