Preserving and restoring behavioral potential within the spinal cord using an instrumental training paradigm

We have shown that spinal cord neurons can support a simple form of instrumental learning. In a typical experiment, rats are spinalized at the second thoracic vertebra (T(2)) and given shock to one hindleg. One group (master) receives shock whenever the leg is extended. This response-contingent shock causes an increase in response duration that decreases net shock exposure. This instrumental learning is not observed in yoked controls that receive the same amount of shock independent of leg position (noncontingent shock). Interestingly, rats that have received noncontingent shock also fail to learn when they are subsequently exposed to response-contingent shock on either the ipsilateral or contralateral leg. Just 6 min of noncontingent nociceptive stimulation, applied to the leg or tail, undermines behavioral potential for up to 48 h. The present experiments explore whether a behavioral therapy can prevent and/or reverse this deficit. In experiment 1, spinalized rats received 30 min of training with contingent shock, noncontingent shock, or nothing prior to noncontingent tailshock. They were then tested with contingent shock to the contralateral hindleg. Rats that had received noncontingent shock alone failed to learn. Prior exposure to contingent shock had an immunizing effect that prevented the deficit. Experiment 2 examined whether training with contingent shock after noncontingent shock exposure would restore behavioral potential. To facilitate performance during contingent shock training, subjects were given an intrathecal injection of the opioid antagonist naltrexone, a drug treatment that temporarily blocks the expression of the behavioral deficit. Twenty-four hours later subjects were tested with contingent shock on either the ipsilateral or contralateral leg. We found that naltrexone combined with contingent shock therapy restored spinal cord function. Naltrexone alone had no effect. The results suggest that noncontingent nociceptive stimulation can undermine behavioral potential after spinal cord injury and that instrumental training can help preserve, and protect, spinal cord function.