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Physiol Behav. 1984 Apr;32(4):547-55.

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.

[Indexed for MEDLINE]

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