OPERANT-CONDITIONING OF H-REFLEX IN FREELY MOVING RATS

1. Primates can increase or decrease the spinal stretch reflex and its electrical analogue, the H-reflex (HR), in response to an operant conditioning task. This conditioning changes the spinal cord itself and thereby provides an experimental model for defining the processes and substrates of a learned change in behavior. Because the phenomenon has been demonstrated only in primates, its generality and theoretical implications remain unclear, and its experimental use is restricted by the difficulties of primate research. In response to these issues, the present study explored operant conditioning of the H-reflex in the rat. 2. Seventeen Sprague-Dawley rats implanted with chronic electromyographic (EMG) recording electrodes in one soleus muscle and nerve cuff stimulating electrodes on the posterior tibial nerve were rewarded (either with medial forebrain bundle stimulation or food) for increasing (HRup conditioning mode) or decreasing (HRdown conditioning mode) soleus H-reflex amplitude without change in background EMG or M response (direct muscle response) amplitude. 3. H-reflex amplitude changed appropriately over 3-4 wk. Under the HRup mode, it rose to an average of 158 +/- 54% (mean +/- SD) of initial value, whereas under the HRdown mode it fell to an average of 67 +/- 11% of initial value. Background EMG and M response amplitude did not change. 4. Operant conditioning of the H-reflex in the rat appears similar in rate and final magnitude of change to that observed in the monkey. Additional studies are needed to determine whether it displays comparable muscular specificity, follows a similar two-phase course, and is also accompanied by spinal cord modifications. 5. The results indicate that adaptive plasticity of the behavior mediated by the spinal stretch reflex pathway can occur in a subprimate species. With further refinement of the experimental protocol and careful attention to the inherent variability of the rat H-reflex, operant conditioning of the H-reflex in the rat should provide a useful model for defining the plasticity underlying an adaptive change in vertebrate behavior and the learning processes responsible for it.