OPERANTLY CONDITIONED MOTONEURON PLASTICITY - POSSIBLE ROLE OF SODIUM-CHANNELS

1. Learning is traditionally thought to depend on synaptic plas plasticity. However, recent work shows that operantly conditioned decrease in the primate H reflex is associated with an increase in the depolarization needed to fire the spinal motoneuron (V-DEP) and a decrease in its conduction velocity (CV). Furthermore, the increase in V-DEP appears to be largely responsible for the H-reflex decrease. The conjunction of these changes in V-DEP and CV suggests that an alteration in Na+ channel properties throughout the soma and axon could be responsible. 2. A mathematical model of the mammalian myelinated axon was used to test whether a positive shift in the voltage dependence of Na+ channel activation, a decrease in Na+ channel peak permeability, ability, or changes in other fiber properties could have accounted for the experimental findings. 3. A positive shift of 2.2 mV in Na+ channel activation reproduced the experimentally observed changes in V-DEP, and CV. whereas a reduction in Na- channel permeability or changes in other fiber properties did not. 4. These results are consistent with the hypothesis that operantly conditioned decrease in the primate H reflex is largely due to a positive shift in the voltage dependence of Na+- channel activation. Recent studies suggest that change in activation of protein kinase C may mediate this effect.