Endogenous Opioids Interfere with Pavlovian Second-Order Fear Conditioning

Abstract

Second-order classical (Pavlovian) aversive conditioning was used to determine whether endogenous opiates play a role in the acquisition of associative learning. Second-order conditioning avoids confounding drug effects on associative learning with altered responsiveness to unconditioned stimuli (e.g., hyperalgesia). Administration of the prototypical opiate antagonist naloxone (2.0 mg/kg) prior to the second-order training session specifically facilitated associative learning as indicated by greater lick suppression over 3 days of testing. This result suggests that endogenous opiates retard the acquisition of associative learning, which complements previous studies demonstrating an opioid attenuation of memory consolidation.