SODIUM CURRENT IN RAT AND CAT THALAMOCORTICAL NEURONS - ROLE OF A NONINACTIVATING COMPONENT IN TONIC AND BURST FIRING

The properties of the Na+ current present in thalamocortical neurons o f the dorsal lateral geniculate nucleus were investigated in dissociat ed neonate rat and cat neurons and in neurons from slices of neonate a nd adult rats using patch and sharp electrode recordings. The steady-s tate activation and inactivation of the transient Na+ current (I-Na) w as well fitted with a Boltzmann curve (voltage of half-maximal activat ion and inactivation, V-1/2, -29.84 mV and -70.04 mV, respectively). S teady-state activation and inactivation curves showed a small region o f overlap, indicating the occurrence of a I-Na window current; I-Na de cay could be fitted with a single exponential function, consistent wit h the presence of only one channel type. Voltage ramp and step protoco ls showed the presence of a noninactivating component of the Na+ curre nt (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 similar to 2.5% of I-Na, thus significantly greater than the calculated contribut ion (0.2%) of the I-Na window component. Comparison of results from di ssociated neurons and neurons in slices suggested a dendritic as well as a somatic localization of I-NaP. Inclusion of papain in the patch e lectrode removed the fast inactivation of I-Na and induced a current w ith voltage-dependence (V-1/2 = -56.92) and activation parameters simi lar to those of I-NaP. Current-clamp recordings with sharp electrodes showed that I-NaP contributed to depolarizations evoked from potential s of approximately -60 mV and unexpectedly to the amplitude and latenc y of low-threshold Ca2+ potentials, suggesting that this noninactivati ng component of the Na+ channel population plays an important role in the integrative properties of thalamocortical neurons during both toni c and burst-firing patterns.