Prior research has shown that neurons within the spinal cord can support some simple forms of learning. Learning in the isolated spinal cord can be studied by cutting communication with the brain by means of a thoracic transection. Transected rats given shock to one hind leg whenever the leg is extended soon learn to maintain the leg in a flexed position that minimizes net shock exposure, a form of instrumental conditioning. Rats that receive shock independent of leg position (uncontrollable shock) do not learn and exhibit a learning deficit when later tested with controllable shock. This learning deficit can be prevented, and reversed, by training with controllable shock. Instrumental training also enables learning when subjects are tested with a more difficult response criterion. Our hypothesis is that instrumental training enables learning within the spinal cord, and has a protective effect, because it promotes the synthesis and release of the neurotrophin BDNF.
Aim 1 explores this hypothesis using pharmacological techniques. The necessity of BDNF is evaluated using drug manipulations that disrupt BDNF function. Sufficiency is examined by artificially applying BDNF to the spinal tissue. If BDNF plays a key role, disrupting BDNF should eliminate the beneficial effect of instrumental training and the application of BDNF should have a protective effect. Preliminary data suggest that training with controllable shock up-regulates BDNF mRNA expression while uncontrollable stimulation down-regulates expression.
Aim 2 uses assays for mRNA expression to examine the duration of these effects and their anatomical locus. Protein assays will evaluate how this expression affects BDNF levels and the signal pathways involved. Prior work has shown that uncontrollable, but not controllable, stimulation disrupts recovery after a spinal contusion injury. We outline a novel procedure to maximize the beneficial effect of controllable stimulation and seek evidence that instrumental training has a lasting effect in a contusion model. Additional work will evaluate whether training affects recovery because it promotes the release of BDNF. The long-term goal of this research is to characterize the mechanisms that underlie spinal plasticity at both a functional and neurobiological level. Instrumental training provides a model of a common behavioral technique (functional electrical stimulation [FES]) used to foster recovery after spinal injury in humans. By identifying key instrumental relations, and the neurochemical systems involved, we hope to develop more effective procedures to promote recovery. Further, procedures designed to promote neural growth across an injury require techniques to shape the appropriate pattern of neural innervation. Instrumental training could provide the procedure needed to select adaptive neural connections. ? ? ?
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