Aversive conditioning in the rat: effects of a benzodiazepine and of an opioid agonist and antagonist on conditioned hypoalgesia and fear.

Hypoalgesia and fear co-occurred in rats trained on a heated floor and tested for their latencies to paw lick on that floor and to step down onto a nonheated floor. These responses were extinguished, suggesting a mediation by aversive conditioning processes. A benzodiazepine impaired the acquisition of aversive conditioning, but it did not attenuate the expression of conditioned hypoalgesia. The opioid agonist morphine also impaired acquisition across a range of drug doses and variations in hypoalgesic tolerance, whereas the opioid antagonist naloxone facilitated acquisition. The results are discussed in terms of the perceptual-defensive-recuperative (Fanselow, 1986) and working memory (Grau, 1987) models of the mechanisms for the co-occurrence of conditioned hypoalgesia and fear.