DESCENDING NORADRENERGIC SYSTEMS PROTECT SPINAL CORD NEURONS FROM THE DELETERIOUS EFFECTS OF NOCICEPTIVE STIMULATION. E. Crown*, A. Ferguson, S. Dhruv, B. Patton, & J. Grau, Dept. of Psychology, Texas A&M University, College Station, TX 77843.

We have developed a paradigm to assess spinal cord function that relies on instrumental learning. Rats are tested by applying shock to one hindleg whenever that limb is extended. In spinally transected rats, exposure to response-contingent shock causes an increase in flexion duration that decreases net shock exposure. Rats given shock independent of leg position (noncontingent shock) do not exhibit an increase flexion duration. They also fail to learn when response-contingent shock is applied to the contralateral leg. Just 6 min of noncontingent shock to the leg or the tail undermines spinal cord plasticity for 48 hrs.

Rats given shock prior to spinalization do not exhibit a behavioral deficit. This suggests that descending supraspinal projections protect spinal cord neurons from the effects of noncontingent shock. The present experiments explore the fibers that mediate this protective effect. In Experiment 1, intact rats received dorsolateral funiculus (DLF) lesions or sham surgery prior to noncontingent shock. After shock exposure, these rats were spinalized and tested 24 hrs later. We found that rats developed the behavioral deficit if the dorsolateral funiculus was lesioned prior to shock, suggesting that these descending projections are responsible for the protective effect. Experiment 2 examined whether descending noradrenergic projections within the DLF mediate this protective effect. We found that intrathecal application of the a-1 adrenergic agonist clonidine prevents noncontingent shock from inducing a behavioral deficit. Funded by MH60157 to JWG & RCM.