Inactivation of Voltage-Activated Na+ Currents Contributes to Different Adaptation Properties of Paired Mechanosensory Neurons

Abstract

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 Nainactivation 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.