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Relative Location of Inhibitory Synapses and Persistent Inward Currents Determines the Magnitude and Mode of Synaptic Amplification in Motoneurons

Canadian Institutes of Health Research Group in Sensory-Motor Systems, Department of Physiology, Centre for Neuroscience Studies, Queen's University, Kingston, Canada

Submitted 27 June 2007; accepted in final form 27 November 2007

In some motoneurons, L-type Ca²⁺ channels that partly mediate persistent inward currents (PICs) have been estimated to be arranged in 50- to 200-µm-long discrete regions in the dendrites, centered 100 to 400 µm from the soma. As a consequence of this nonuniform distribution, the interaction between synaptic inputs to motoneurons and these channels may vary according to the distribution of the synapses. For instance, >93% of synapses from Renshaw cells have been observed to be located 65 to 470 µm away from the cell body of motoneurons. Our goal was to assess whether Renshaw cell synapses are distributed in a position to more effectively control the activation of the L-type Ca²⁺ channels. Using compartmental models of motoneurons with L-type Ca²⁺ channels distributed in 100-µm-long hot spots centered 100 to 400 µm away from the soma, we compared the inhibition generated by four distributions of inhibitory synapses: proximal, distal, uniform, and one based on the location of Renshaw cell synapses on motoneurons. Regardless of whether the synapses were activated tonically or transiently, in the presence of L-type Ca²⁺ channels, inhibitory synapses distributed according to the Renshaw cell synapse distribution generate the largest inhibitory currents. The effectiveness of a particular distribution of inhibitory synapses in the presence of PICs depends on their ability to deactivate the channels underlying PICs, which is influenced not only by the superposition between synapses and channels, but also by the distance away from the somatic voltage clamp.