Differential use-dependent inactivation of Nav1.8 in the subpopulation of cultured dorsal root ganglion

Abstract

Sodium channel Nav1.8 is expressed preferentially in members of the peripheral nervous system such as nociceptive dorsal root ganglion (DRG) neurons. Using immunocytochemistry and eletrophysiological recording, we found that a subpopulation of small DRG neurons, grouped by isolectin B4 (IB4) immunoreactivity, showed a different use-dependent inactivation of Nav1.8 current, and this situation changed over time during in vitro culture. The IB 4 - immunoreactivity of DRG neurons was not changed during in vitro culture with the exception of this small population of IB4-negative small-diameter DRG neurons. The Nav1.8 channel in IB 4 + neurons underwent a level of use-dependent inactivation that was significantly stronger than that seen in IB 4 - neurons at 1 and 2 days-in-vitro (DIV). The use-dependent inactivation of the Nav1.8 channel in IB 4 + neurons at 1 DIV was significantly attenuated at 2 DIV. The values for voltage dependency of activation and steady-state inactivation of Nav1.8 were similar in all subpopulations of DRG neurons and did not change over time. The time constant for entry into slow inactivation of Nav1.8 in IB 4 + neurons was significantly faster than in IB 4 - neurons at 1 and 2 DIV, while the rate of recovery from slow inactivation of Nav1.8 in IB 4 + neurons was slower than that seen in IB 4 - neurons. Moreover, the time constant for entry into the slow inactivation of Nav1.8 in IB 4 + neurons after 1 DIV was significantly faster than at 2 DIV, and the rate of recovery from the slow inactivation of Nav1.8 in IB 4 + neurons at 1 DIV was slower than that at 2 DIV, which indicated that the strong use-dependent inactivation in IB 4 + neurons at 1 DIV was the result of a greater preference for the slow inactivation state than at 2 DIV. Our data suggest that the time-dependent change of the use-dependent inactivation of the Nav1.8 channel in DRG neurons cultured in vitro would contribute to the excitability of a subpopulation of DRG neurons and could play an important role in the development of inflammatory and neuropathic pain.