Instrumental learning within the spinal cord: III. Prior exposure to noncontingent shock induces a behavioral deficit that is blocked by an opioid antagonist.

Spinally transected rats given legshock whenever one hindleg is extended learn to maintain a flexion response that decreases net shock exposure. Prior exposure to response-independent (noncontingent) shock prevents learning. This behavioral deficit was eliminated by systemic administration of the nonselective opioid antagonist naltrexone (Experiment 1). The deficit was also blocked by intrathecal (i.t.) naltrexone at a dose of 7 microg/microl (Experiment 2). Noncontingent shock undermined behavioral potential for 24 h (Experiment 3). The expression of the deficit was blocked by naltrexone (7 microg/microl, i.t.) given prior to testing. The same dose prior to initial shock exposure had no effect. Administration of an antagonist that acts on the kappa opioid receptor (nor-BNI) restored learning (Experiment 4). Equal molar concentrations of antagonists that act on the micro (CTOP) or delta (naltrindole) receptor had no effect.