Naloxone and shock-elicited freezing in the rat.

Abstract

The freezing behavior of the rat that occurs following painful electric shock increased when the animal was pretreated with the opiate antagonist naloxone. Freezing was a positive linear function of drug dose and shock intensity. Naloxone pretreatment enhanced freezing only when the animal was given 2 or 3 shocks but did not affect freezing when the animal was given only 1 shock or not shocked at all. Naloxone must be present during shock, not just during the observation period, to increase freezing. When an animal is shocked, it apparently 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.