Block of persistent late Na+ currents by antidepressant sertraline and paroxetine

Antidepressants, such as traditional tricyclic antidepressants (TCAs), are the first-line treatment for various pain syndromes. Available evidence indicates that TCAs may target Na+ channels for their analgesic action. In this report, we examined the effects of contemporary antidepressants sertraline and paroxetine on (1) neuronal Na+ channels expressed in GH3 cells and (2) muscle rNav1.4 Na+ channels heterologously expressed in Hek293t cells. Our results showed that both antidepressants blocked Na+ channels in a highly state-dependent manner. The 50% inhibitory concentrations (IC50) for sertraline and paroxetine ranged approximately 18-28 microM: for resting block and approximately 2-8 microM: for inactivated block of neuronal and rNav1.4 Na+ channels. Surprisingly, the IC50 values for both drugs were about 0.6-0.7 microM: for the open channel block of persistent late Na+ currents generated through inactivation-deficient rNav1.4 mutant Na+ channels. For comparison, the open channel block in neuronal hNav1.7 counterparts yielded IC50 values around 0.3-0.4 microM: for both drugs. Receptor mapping using fast inactivation-deficient rNav1.4-F1579A/K mutants with reduced affinities toward local anesthetics (LAs) and TCAs indicated that the F1579 residue is not involved in the binding of sertraline and paroxetine. Thus, sertraline and paroxetine are potent open channel blockers that target persistent late Na+ currents preferentially, but their block is not mediated via the phenylalanine residue at the known LA/TCA receptor site.