Naloxone and shock-elicited freezing in the rat

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In the 1st of 6 experiments with 325 Long-Evans rats it was found that the freezing behavior of the rat that occurs following painful electric shock increased when Ss were pretreated with the opiate antagonist naloxone. Freezing was a positive linear function of drug dose and shock intensity (Exp II). Naloxone pretreatment enhanced freezing only when Ss were given 2 or 3 shocks but did not affect freezing when Ss were given only 1 shock or were not shocked at all (Exps III, IV, and V). Naloxone had to be present during shock to increase freezing (Exp VI). Results suggest that when a rat is shocked, it releases endogenous analgesics (endorphins) that make a subsequent shock less aversive. Naloxone, by blocking the endorphin system, makes the shock more aversive than it would normally be. (23 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved)

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