A novel modulatory mechanism of sodium currents: frequency-dependence without state-dependent binding

We have previously found that the dopamine uptake inhibitor 1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (GBR 12909) inhibits neuronal sodium channels. The inhibition was profoundly dependent on the voltage protocol, suggesting that the effect is determined by the activity pattern of individual neurons. Our present study was aimed to understand more thoroughly the mechanism of this inhibition. The effect of GBR 12909 on sodium currents was investigated using whole-cell patch clamp recordings on cultured hippocampal neurons. Repetitive trains of depolarizations revealed two distinct components of inhibition: a frequency-dependent, transient and a frequency-independent, sustained component. Frequency-dependent inhibition can reflect dynamic equilibrium of binding or gating. In order to decide which is the dominant mechanism in the case of GBR 12909, we studied the rates of association and dissociation. We found an unexpectedly fast rate of association (tau=819.2 ms) to resting ion channels kept at hyperpolarized membrane potential (-150 mV), while the rate of dissociation was too slow to explain recovery between trains of stimulation (tau=248 s). These data suggest that frequency-dependent inhibition cannot be explained by binding and unbinding, but rather it is due to conformational transitions of the liganded channel, which can only be explained if ligand binding is assumed to enhance slow inactivation. We studied, therefore, the rate of slow inactivation in the presence of different concentrations of GBR 12909. We have found that GBR 12909 accelerates slow inactivation substantially (time constants more than hundredfold lower at concentrations above 10 microM), causing the time range of slow inactivation to overlap with the time range of fast inactivation. Slow inactivation can even be the dominant process, especially during subthreshold depolarizations in the presence of >10 microM of GBR 12909. This mechanism of inhibition could provide a selective inhibition of neurons not only with high frequency bursting activity but also with moderately depolarized membrane potential.