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Contribution of Persistent Sodium Current to Locomotor Pattern Generation in Neonatal Rats

Laboratoire de Plasticité et Physio-Pathologie de la Motricité, Unité Mixte de Recherche 6196, Centre National de la Recherche Scientifique, Aix-Marseille Université, Marseille, France

Submitted 20 March 2007; accepted in final form 3 June 2007

The persistent sodium current (I_NaP) is known to play a role in rhythm generation in different systems. Here, we investigated its contribution to locomotor pattern generation in the neonatal rat spinal cord. The locomotor network is mainly located in the ventromedial gray matter of upper lumbar segments. By means of whole cell recordings in slices, we characterized membrane and I_NaP biophysical properties of interneurons located in this area. Compared with motoneurons, interneurons were more excitable, because of higher input resistance and membrane time constant, and displayed lower firing frequency arising from broader spikes and longer AHPs. Ramp voltage-clamp protocols revealed a riluzole- or TTX-sensitive inward current, presumably I_NaP, three times smaller in interneurons than in motoneurons. However, in contrast to motoneurons, I_NaP mediated a prolonged plateau potential in interneurons after reducing K⁺ and Ca²⁺ currents. We further used in vitro isolated spinal cord preparations to investigate the contribution of I_NaP to locomotor pattern. Application of riluzole (10 µM) to the whole spinal cord or to the upper lumbar segments disturbed fictive locomotion, whereas application of riluzole over the caudal lumbar segments had no effect. The effects of riluzole appeared to arise from a specific blockade of I_NaP because action potential waveform, dorsal root–evoked potentials, and miniature excitatory postsynaptic currents were not affected. This study provides new functional features of ventromedial interneurons, with the first description of I_NaP-mediated plateau potentials, and new insights into the operation of the locomotor network with a critical implication of I_NaP in stabilizing the locomotor pattern.

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