Inactivation of voltage-activated Na+ currents contributes to different adaptation properties of paired mechanosensory neurons

Voltage-activated sodium current (I-Na) is primarily responsible for the le ading edge of the action potential in many neurons. While I-Na generally ac tivates rapidly when a neuron is depolarized, its inactivation properties d iffer significantly between different neurons and even within one neuron, w here I-Na often has slowly and rapidly inactivating components. I-Na inacti vation has been suggested to regulate action potential firing frequency in some cells, but no clear picture of this relationship has emerged. We studi ed I-Na in both members of the paired mechanosensory neurons of a spider sl it-sense organ, where one neuron adapts rapidly (type A) and the other slow ly (type B) in response to a step depolarization. In both neuron types I-Na activated and inactivated with single time constants of 2-3 ms and 5-10 ms , respectively, varying with the stimulus intensity. However, there was a c lear difference in the steady-state inactivation properties of the two neur on types, with the half-maximal inactivation (V-50) being -40.1 mV in type A neurons and -58.1 mV in type B neurons. Therefore I-Na inactivated closer to the resting potential in the more slowly adapting neurons. I-Na also re covered from inactivation significantly faster in type B than type A neuron s, and the recovery was dependent on conditioning voltage. These results su ggest that while the rate of I-Na inactivation is not responsible for the d ifference in the adaptation behavior of these two neuron types, the rate of recovery from inactivation may play a major role. Inactivation at lower po tentials could therefore be crucial for more rapid recovery.