Differential use-dependent inactivation of Na_v1.8 in the subpopulation of cultured dorsal root ganglion

Backgrounds: Sodium channel Nav1.8 is expressed preferentially in members of the peripheral nervous system such as nociceptive dorsal root ganglion (DRG) neurons. Methods: 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 Na_v1.8 current, and this situation changed over time during in vitro culture. Results: The IB₄ ^-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 Na_v1.8 channel in IB₄ ⁺ neurons underwent a level of use-dependent inactivation that was significantly stronger than that seen in IB₄ ^- neurons at 1 and 2 days-in-vitro (DIV). The use-dependent inactivation of the Na_v1.8 channel in IB₄ ⁺ neurons at 1 DIV was significantly attenuated at 2 DIV. The values for voltage dependency of activation and steady-state inactivation of Na_v1.8 were similar in all subpopulations of DRG neurons and did not change over time. The time constant for entry into slow inactivation of Na_v1.8 in IB₄ ⁺ neurons was significantly faster than in IB₄ ^- neurons at 1 and 2 DIV, while the rate of recovery from slow inactivation of Na_v1.8 in IB₄ ⁺ neurons was slower than that seen in IB₄ ^- neurons. Moreover, the time constant for entry into the slow inactivation of Na_v1.8 in IB₄ ⁺ neurons after 1 DIV was significantly faster than at 2 DIV, and the rate of recovery from the slow inactivation of Na_v1.8 in IB₄ ⁺ neurons at 1 DIV was slower than that at 2 DIV, which indicated that the strong use-dependent inactivation in IB₄ ⁺ neurons at 1 DIV was the result of a greater preference for the slow inactivation state than at 2 DIV. Conclusion: 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.