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The Journal of Neuroscience, February 1, 1998, 18(3):854-867

Sodium Current in Rat and Cat Thalamocortical Neurons: Role of a Non-Inactivating Component in Tonic and Burst Firing

H. Rheinallt Parri and Vincenzo Crunelli

Physiology Unit, School of Molecular and Medical Biosciences, University of Wales Cardiff, Cardiff CF1 3US, United Kingdom

The properties of the Na⁺ current present in thalamocortical neurons of the dorsal lateral geniculate nucleus were investigated in dissociated neonate rat and cat neurons and in neurons from slices of neonate and adult rats using patch and sharp electrode recordings.

The steady-state activation and inactivation of the transient Na⁺ current (I_Na) was well fitted with a Boltzmann curve (voltage of half-maximal activation and inactivation, V_1/2, 29.84 mV and 70.04 mV, respectively). Steady-state activation and inactivation curves showed a small region of overlap, indicating the occurrence of a I_Na window current. I_Na decay could be fitted with a single exponential function, consistent with the presence of only one channel type.

Voltage ramp and step protocols showed the presence of a noninactivating component of the Na⁺ current (I_NaP) that activated at potentials more negative (V_1/2 = 56.93 mV) than those of I_Na. The maximal amplitude of I_NaP was ~2.5% of I_Na, thus significantly greater than the calculated contribution (0.2%) of the I_Na window component. Comparison of results from dissociated neurons and neurons in slices suggested a dendritic as well as a somatic localization of I_NaP. Inclusion of papain in the patch electrode removed the fast inactivation of I_Na and induced a current with voltage-dependence (V_1/2 = 56.92) and activation parameters similar to those of I_NaP.

Current-clamp recordings with sharp electrodes showed that I_NaP contributed to depolarizations evoked from potentials of approximately 60 mV and unexpectedly to the amplitude and latency of low-threshold Ca²⁺ potentials, suggesting that this noninactivating component of the Na⁺ channel population plays an important role in the integrative properties of thalamocortical neurons during both tonic and burst-firing patterns.

Key words: thalamus; action potential; persistent Na⁺ current; inactivation; burst firing; dorsal lateral geniculate nucleus

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Copyright © 1998 Society for Neuroscience  0270-6474/98/183854-14$05.00/0

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