HCN1 Channels Contribute to the Effects of Amnesia and Hypnosis but not Immobility of Volatile Anesthetics

BACKGROUND: Hyperpolarization-activated, cyclic nucleotide-gated (HCN) subtype 1 (HCN1) channels have been identified as targets of ketamine to produce hypnosis. Volatile anesthetics also inhibit HCN1 channels. However, the effects of HCN1 channels on volatile anesthetics in vivo are still elusive. This study uses global and conditional HCN1 knockout mice to evaluate how HCN1 channels affect the actions of volatile anesthetics. METHODS: Minimum alveolar concentrations (MACs) of isoflurane and sevoflurane that induced immobility (MAC of immobility) and/or hypnosis (MAC of hypnosis) were determined in wild-type mice, global HCN1 knockout (HCN1(-/-)) mice, HCN1 channel gene with 2 lox-P sites flanking a region of the fourth exon of HCN1 (HCN1(f/f)) mice, and forebrain-selective HCN1 knockout (HCN1(f/f): cre) mice. Immobility of mice was defined as no purposeful reactions to tail-clamping stimulus, and hypnosis was defined as loss of righting reflex. The amnestic effects of isoflurane and sevoflurane were evaluated by fear-potentiated startle in these 4 strains of mice. RESULTS: All MAC values were expressed as mean +/- SEM. For MAC of immobility of isoflurane, no significant difference was found among wild-type, HCN1(-/-), HCN1(f/f), and HCN1(f/f): cre mice (all ~1.24%-1.29% isoflurane). For both HCN1(-/-) and HCN1(f/f): cre mice, the MAC of hypnosis for isoflurane (each ~1.05% isoflurane) was significantly increased over their nonknockout controls: HCN1(-/-) versus wild-type (0.86% +/- 0.03%, P < 0.001) and HCN1(f/f): cre versus HCN1(f/f) mice (0.84% +/- 0.03%, P < 0.001); no significant difference was found between HCN1(-/-) and HCN1(f/f): cre mice. For MAC of immobility of sevoflurane, no significant difference was found among wild-type, HCN1(-/-), HCN1(f/f), and HCN1(f/f): cre mice (all ~2.6%-2.7% sevoflurane). For both HCN1(-/-) and HCN1(f/f): cre mice, the MAC of hypnosis for sevoflurane (each ~1.90% sevoflurane) was significantly increased over their nonknockout controls: HCN1(-/-) versus wild-type (1.58% +/- 0.05%, P < 0.001) and HCN1(f/f): cre versus HCN1(f/f) mice (1.56% +/- 0.05%, P < 0.001). No significant difference was found between HCN1(-/-) and HCN1(f/f): cre mice. By fear-potentiated startle experiments, amnestic effects of isoflurane and sevoflurane were significantly attenuated in HCN1(-/-) and HCN1(f/f): cre mice (both P < 0.002 versus wild-type or HCN1(f/f) mice). No significant difference was found between HCN1(-/-) and HCN1(f/f): cre mice. CONCLUSIONS: Forebrain HCN1 channels contribute to hypnotic and amnestic effects of volatile anesthetics, but HCN1 channels are not involved in the immobilizing actions of volatile anesthetics.