Cold water stress analgesia in rats: differential effects of naltrexone.
Cold water swim (CWS, 2 degrees C, 3.5 min) decreases the responsiveness to nociceptive stimuli in rats. The influence of various parameters of the CWS condition on stress-induced analgesia were evaluated by means of naltrexone effects. Naltrexone dose-dependently (but significantly only at high doses--21 mg/kg) partially antagonized 3.5 minute continuous CWS analgesia. Its effect was proportional to the duration of CWS. Naltrexone (14 mg/kg) significantly antagonized intermittent CWS-analgesia (18 10-sec exposures, 3/min) and enhanced the analgesia induced by 60 consecutive exposures (1 sec each, 12/min). These results demonstrate that naltrexone differentially affects CWS-analgesia, depending on specific parametric conditions of the stressor. In addition to activation of a non-specific naltrexone-insensitive analgesia-inducing system (not reduced by the drug in all the conditions studied) there appear to be three naltrexone sensitive systems: (1) a non-opioid analgesia-inducing system which mediates continuous CWS-analgesia; (2) an opioid analgesia-inducing system, involved in intermittent CWS-analgesia; and (3) a naltrexone-sensitive system which opposes the analgesic effect of 60 consecutive exposures. Thus, a highly specific relationship exists between certain parameters of the cold water stressor and the nature of the mechanisms which subserve the induced analgesia.
D000818:Animals; D001143:Arousal / Q000187:drug effects*; D001833:Body Temperature Regulation / Q000187:drug effects; D003080:Cold Temperature; D008297:Male; D009270:Naloxone / Q000031:analogs & derivatives*; D009271:Naltrexone / Q000494:pharmacology*; D009619:Nociceptors / Q000187:drug effects*; D051381:Rats; D011919:Rats, Inbred Strains; D011930:Reaction Time / Q000187:drug effects; D011957:Receptors, Opioid / Q000187:drug effects; D013312:Stress, Physiological / Q000150:complications*
D011957:Receptors, Opioid; D009270:Naloxone; D009271:Naltrexone