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 leading edge of the action potential in many neurons. While I(Na) generally activates rapidly when a neuron is depolarized, its inactivation properties differ significantly between different neurons and even within one neuron, where I(Na) often has slowly and rapidly inactivating components. I(Na) inactivation has been suggested to regulate action potential firing frequency in some cells, but no clear picture of this relationship has emerged. We studied I(Na) in both members of the paired mechanosensory neurons of a spider slit-sense organ, where one neuron adapts rapidly (type A) and the other slowly (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 clear difference in the steady-state inactivation properties of the two neuron 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 recovered from inactivation significantly faster in type B than type A neurons, and the recovery was dependent on conditioning voltage. These results suggest that while the rate of I(Na) inactivation is not responsible for the difference in the adaptation behavior of these two neuron types, the rate of recovery from inactivation may play a major role. Inactivation at lower potentials could therefore be crucial for more rapid recovery.