Luzindole and ramelteon share a melatonin receptor-independent blocking effect on voltage-gated potassium K_V4.2...

Melatonin is a pineal hormone that regulates circadian rhythms primarily through melatonin receptors. We recently demonstrated that melatonin inhibits the voltage-gated potassium K_V4.2 channel (IC₅₀ = 479 μM) through a melatonin receptor-independent mechanism. However, it remains unclear whether this inhibitory effect is specific to melatonin or shared by structurally related compounds. In the present study, we examined the effects of melatonin-related compounds on K_V4.2 channels using human embryonic kidney 293 cells stably expressing K_V4.2 channels. Whole-cell patch-clamp recordings showed that luzindole, a melatonin receptor antagonist, inhibited K_V4.2 currents (IC₅₀ = 33 μM). Notably, ramelteon, a melatonin receptor agonist, also inhibited K_V4.2 currents (IC₅₀ = 177 μM). These results indicate that the inhibitory effects of luzindole and ramelteon are mediated by direct interactions with K_V4.2 channels rather than through melatonin receptors. In addition, luzindole inhibited K_V1.5 channels. In contrast, melatonin biosynthetic precursors failed to inhibit K_V4.2 currents, suggesting that the indole or indole-like structural motif alone is insufficient for K_V4.2 channel inhibition. Instead, specific structural features, including the methoxy group in melatonin and the furan ring in ramelteon, together with increased hydrophobicity, may be critical determinants of K_V4.2 channel blockade. Collectively, these findings reveal previously unrecognized pharmacological properties of melatonin-related compounds.