Sodium current in rat and cat thalamocortical neurons: role of a non-inactivating component in tonic and burst firing

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 (INa) 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 / _Na window current.  / _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 (/ _NaP) that activated at potentials more negative (V_1/2 = -56.93 mV) than those of I_Na. The maximal amplitude of _{/ NaP} was approximately 2.5% of INa, 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 / _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.