Cyproheptadine Enhances the IK of Mouse Cortical Neurons through Sigma-1 Receptor-Mediated Intracellular Signal Pathway

Cyproheptadine (CPH) is a histamine- and serotonin-receptor antagonist, and its effects are observed recently in the modulation of multiple intracellular signals. In this study, we used cortical neurons and HEK-293 cells transfected with Kv2.1 alpha-subunit to address whether CPH modify neural voltage-gated K+ channels by a mechanism independent of its serotonergic and histaminergic properties. Our results demonstrate that intracellularly delivered CPH increased the I-K by reducing the activity of protein kinas A (PKA). Inhibition of G(i) eliminated the CPH-induced effect on both the I-K and PKA. Blocking of 5-HT-, M-, D-2-, H-1- or H-2-type GPCR receptors with relevant antagonists did not eliminate the CPH-induced effect on the I-K. Antagonists of the sigma-1 receptor, however, blocked the effect of CPH. Moreover, the inhibition of sigma-1 by siRNA knockdown significantly reduced the CPH-induced effect on the I-K. On the contrary, sigma-1 receptor agonist mimicked the effects of CPH on the induction of I-K. A ligand-receptor binding assay indicated that CPH bound to the sigma-1 receptor. Similar effect of CPH were obtained from HEK-293 cells transfected with the alpha-subunit of Kv2.1. In overall, we reveal for the first time that CPH enhances the I-K by modulating activity of PKA, and that the associated activation of the sigma-1 receptor/G(i)-protein pathway might be involved. Our findings illustrate an uncharacterized effect of CPH on neuron excitability through the I-K, which is independent of histamine H-1 and serotonin receptors.