Autoanalgesia: acquisition, blockade and relationship to opiate binding

Acquisition of autoanalgesia (behaviorally activated antinociception) was assessed across 7 consecutive days by shocking rats 10 sec after the determination of their tail-flick latencies. Thus the effect of conditioned fear on antinociception was being investigated, since each shock preceded the subsequent tail-flick test by 24 h. Autoanalgesia was acquired by the second fear-conditioning trial. Although pretreatment with naltrexone or diazepam had no effect, spinal cord transection at the thoracic level effectively obviated autoanalgesia. Investigations of opiate and opioid binding indicated significantly less binding in the fear-conditioned rats as well as an inverse relationship between binding and antinociception. These changes in binding are suggestive of partial mediation of autoanalgesia by an endogenous opiate peptide that is released by the fear-conditioning procedure.