HCN channels are essential for the escape response of Paramecium

When mechanical stimulation was applied to free swimming Paramecium, forward swimming velocity transiently increased due to activation of the posterior mechanosensory channels. The behavior response, known as "escape response," requires membrane hyperpolarization and the activation of K-channel type adenylate cyclases. Our hypothesis is that this escape response also involves activation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. HCN channels are activated by hyperpolarization and are modulated by cyclic nucleotides such as cAMP and cGMP. They play a critical role in many excitable cells in higher animals. If HCN channels act in Paramecium, this should help to enhance and prolong hyperpolarization, thereby increasing the swimming speed of Paramecium. This study used RNAi to examine the role of the HCN channel 1 in the escape responses by generating hcn1-gene knockdown cells (hcn1-KD). These cells showed reduced mechanically-stimulated escape responses and a lack of cGMP-dependent increases in swimming speed. Electrophysiological experiments demonstrated reduced hyperpolarization upon injection of large negative currents in hcn1-KD cells. This is consistent with a decrease in HCN1 channel activity and changes in the escape response. These findings suggest that HCN1 channels are K⁺ channels that regulate the escape response of Paramecium by amplifying the hyperpolarizations elicited by posterior mechanical stimulation.