Synaptic Excitation in Spinal Motoneurons Alternates with Synaptic Inhibition and Is Balanced by Outward Rectification during Rhythmic Motor Network Activity

Regular firing in spinal motoneurons of red-eared turtles (Trachemys scripta elegans, either sex) evoked by steady depolarization at rest is replaced by irregular firing during functional network activity. The transition caused by increased input conductance and synaptic fluctuations in membrane potential was suggested to originate from intense concurrent inhibition and excitation. We show that the conductance increase in motoneurons during functional network activity is mainly caused by intrinsic outward rectification near threshold for action potentials by activation of voltage and Ca²⁺ gated K channels. Intrinsic outward rectification facilitates spiking by focusing synaptic depolarization near threshold for action potentials. By direct recording of synaptic currents, we also show that motoneurons are activated by out-of-phase peaks in excitation and inhibition during network activity, whereas continuous low-level concurrent inhibition and excitation may contribute to irregular firing.

SIGNIFICANCE STATEMENT Neurons embedded in active neural networks can enter a high-conductance state. High-conductance states were observed in spinal motoneurons during rhythmic motor behavior. Assuming no change in intrinsic conductance, it was suggested that the high-conductance state in motoneurons originated from balanced inhibition and excitation. In this study, we demonstrate that intrinsic outward rectification significantly contributes to the high-conductance state. Outward rectification balances synaptic excitation and maintains membrane potential near spike threshold. In addition, direct synaptic current recordings show out-of-phase excitation and inhibition in motoneurons during rhythmic network activity.

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