Opiate withdrawal-like behavior induced by naloxone following exposure to a contextual fear stimulus

1. Considerable experimental attention has been directed at understanding the role of opioid peptides in mediating stress effects. Fewer studies have utilized non-physical or 'psychological' methods to investigate the role of the opioid system in stress. 2. Recent studies have shown that conditioned fear, a psychological stressor, can augment morphine induced analgesia and acute dependence. Two experiments were conducted to assess withdrawal-like behavioral changes induced by the general opioid antagonist naloxone, in the absence of morphine, following exposure to a context conditioned fear stimulus. 3. Experiment 1 demonstrated that a high dose of naloxone (10 mg/kg) produced a specific increase in one behavioral index of negative affect, forepaw tremor behavior, in rats exposed to a context fear stimulus. 4. Experiment 2 assessed the relative effects of several naloxone (0, 1, 5, 10 mg/kg) doses in inducing withdrawal-like behavioral changes in animals exposed to a conditioned fear context. This experiment revealed that low doses of naloxone produced an overall increase in all behaviors. High naloxone doses tended to increase only forepaw tremor behavior. 5. The results are discussed ih terms of opioid systems and stress.