Cyproheptadine enhances the I_K 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 α-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-, D₂-, H₁- or H₂- 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 α-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 H₁ and serotonin receptors.