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Persistent Sodium Currents in Mesencephalic V Neurons Participate in Burst Generation and Control of Membrane Excitability

¹Department of Physiological Science, University of California, Los Angeles, California; ²1st Department of Oral and Maxillofacial Surgery, Graduate School of Dentistry, Osaka University; ³Department of Oral and Maxillofacial Surgery, Matsumoto Dental University, Nagano, Japan; ⁴School of Mathematics, University of Minnesota, Minneapolis, Minnesota; and ⁵The Neurosciences Institute, San Diego, California

Submitted 23 October 2004; accepted in final form 22 December 2004

The functional and biophysical properties of a persistent sodium current (I_NaP) previously proposed to participate in the generation of subthreshold oscillations and burst discharge in mesencephalic trigeminal sensory neurons (Mes V) were investigated in brain stem slices (rats, p7–p12) using whole cell patch-clamp methods. I_NaP activated around –76 mV and peaked at –48 mV, with V_1/2 of –58.7 mV. Ramp voltage-clamp protocols showed that I_NaP undergoes time- as well as voltage-dependent inactivation and recovery from inactivation in the range of several seconds (_onset = 2.04 s, _recov = 2.21 s). Riluzole (5 µM) substantially reduced I_NaP, membrane resonance, postinhibitory rebound (PIR), and subthreshold oscillations, and completely blocked bursting, but produced modest effects on the fast transient Na⁺ current (I_NaT). Before complete cessation, burst cycle duration was increased substantially, while modest and inconsistent changes in burst duration were observed. The properties of the I_NaT were obtained and revealed that the amplitude and voltage dependence of the resulting "window current" were not consistent with those of the observed I_NaP recorded in the same neurons. This suggests an additional mechanism for the origin of I_NaP. A neuronal model was constructed using Hodgkin-Huxley parameters obtained experimentally for Na⁺ and K⁺ currents that simulated the experimentally observed membrane resonance, subthreshold oscillations, bursting, and PIR. Alterations in the model g_NaP parameters indicate that I_NaP is critical for control of subthreshold and suprathreshold Mes V neuron membrane excitability and burst generation.

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