Isolation reduces contextual but not auditory-cue fear conditioning: a role for endogenous opioids.

Isolation for several hours after fear conditioning reduces contextual but not auditory-cue fear conditioning (J. W. Rudy, 1996). This isolation effect is reversed by both, centrally and peripherally acting opioid receptor antagonists. As in isolation, systemically administered morphine given immediately after conditioning also reduces contextual fear conditioning. Morphine's effect is also reversed by both centrally and peripherally acting opioid receptor antagonists. Exposure to the conditioning context has been shown to eliminate the effect of isolation on contextual fear conditioning (J. W. Rudy, 1996). Context preexposure also eliminated the effect of morphine on contextual fear conditioning. These results imply that opioids released in the periphery play an important role in producing the isolation effect and that they do so by disrupting the postconditioning memory consolidation processes.