INSTRUMENTAL LEARNING IN SPINALIZED RATS: CHARACTERIZING THE BEHAVIORAL DEFICIT OBSERVED AFTER NONCONTINGENT SHOCK. James W. Grau* & Eric D. Crown. Psychology Dept., Texas A&M Univ., College Station, TX 77843.
Neurons within the spinal cord can support a simple form of instrumental conditioning (Grau et al., 1998, Behav. Neurosci., 112, 1366). Rats receive a spinal transection at the second thoracic vertebrae (T2) and are trained 24 hrs later. One group (master) receives shock to one hindleg whenever the leg is extended (contingent shock). A second group (yoked) receives the same amount of shock independent of leg position (noncontingent shock). Subjects in the master group learn to maintain the shocked leg in a flexed position, effectively minimizing net shock exposure. Yoked rats fail to learn. Interestingly, yoked rats also fail to learn when they are subsequently given contingent shock to the contralateral hindleg.
Does exposure to a noncontingent nociceptive stimulus hurt spinal cord function in intact subjects? To address this issue, unoperated rats received 6 min of tailshock. Two hrs later, the spinal cord was transected. They were then tested with response-contingent shock 18 hrs later. Two control groups were transected prior to noncontingent shock; one group equated the interval between initial shock exposure and testing, while the other equated the interval between surgery and testing. In both of the control groups, noncontingent shock exposure prevented instrumental learning. Subjects exposed to noncontingent shock prior to surgery learned normally. It appears that, in intact organisms, brain-dependent processes protect spinal cord neurons from the deleterious effects of noncontingent nociceptive stimulation.
In spinally transected subjects, exposure to noncontingent shock could undermine behavioral potential by inducing an irreversible process that has a permanent effect. Alternatively, the effect could be temporary and passively decay with time. To evaluate these alternatives, subjects were tested with response contingent shock 9 days after they received a transection at T8. One, 2, 4, or 8 days before testing, subjects received 6 min of noncontingent tailshock. We found that the behavioral deficit decayed after 2 days. Subjects given 30 min of noncontingent shock exhibited a longer-lasting effect. Supported by MH60157.
Published in Society for Neuroscience Abstracts, 26, 2000, 2208.
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