Xenon’s Sedative Effect Is Mediated by Interaction with the Cyclic Nucleotide-Binding Domain (CNBD) of HCN2 Channels Expressed by Thalamocortical Neurons of the Ventrobasal Nucleus in Mice

Abstract

Previous studies have shown that xenon reduces hyperpolarization-activated cyclic nucleotide-gated channels type-2 (HCN2) channel-mediated current (Ih) amplitude and shifts the half-maximal activation voltage (V1/2) in thalamocortical circuits of acute brain slices to more hyperpolarized potentials. HCN2 channels are dually gated by the membrane voltage and via cyclic nucleotides binding to the cyclic nucleotide-binding domain (CNBD) on the channel. In this study, we hypothesize that xenon interferes with the HCN2 CNBD to mediate its effect. Using the transgenic mice model HCN2EA, in which the binding of cAMP to HCN2 was abolished by two amino acid mutations (R591E, T592A), we performed ex-vivo patch-clamp recordings and in-vivo open-field test to prove this hypothesis. Our data showed that xenon (1.9 mM) application to brain slices shifts the V1/2 of Ih to more hyperpolarized potentials in wild-type thalamocortical neurons (TC) (V1/2: −97.09 [−99.56–−95.04] mV compared to control −85.67 [−94.47–−82.10] mV; p = 0.0005). These effects were abolished in HCN2EA neurons (TC), whereby the V1/2 reached only −92.56 [−93.16– −89.68] mV with xenon compared to −90.03 [−98.99–−84.59] mV in the control (p = 0.84). After application of a xenon mixture (70% xenon, 30% O2), wild-type mice activity in the open-field test decreased to 5 [2–10] while in HCN2EA mice it remained at 30 [15–42]%, (p = 0.0006). In conclusion, we show that xenon impairs HCN2 channel function by interfering with the HCN2 CNBD site and provide in-vivo evidence that this mechanism contributes to xenon-mediated hypnotic properties.

Xenon and nitrous oxide induced changes in resting EEG activity can be explained by systematic increases in the relaxation rates of stochastically driven alpha band oscillatory activity

“…For example neuroimaging evidence fairly unequivocally demonstrates that xenon and nitrous oxide induce changes in subcortical activity, particularly that of thalamus -a major anatomical source for broadband driving. Xenon has been reported to decrease thalamic regional cerebral blood flow [24,25] in clinical studies and to disrupt thalamocortical signal propagation, by inhibiting hyperpolarization-activated type 2 cyclic nucleotide-gated channels, in animal models [26,27]. By contrast nitrous oxide, in the few available studies, is fairly consistently reported to increase thalamic regional cerebral blood flow and glucose metabolism [28,29].…”

“…However, they each produce a different constellation of effects on the magnetoencephalogram (MEG) and electroencephalogram (EEG). For example, xenon while typically reported to induce increases in low-frequency delta (0 -4 Hz) and theta (4 -8 Hz) band activity, is associated with inconsistent changes in higher frequency alpha (8)(9)(10)(11)(12)(13), beta (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) and gamma activity (> 30 Hz) [1][2][3][4][5][6]. In contrast nitrous oxide is reported to have little or no effect on low frequency delta and theta activity, but is generally identified as suppressing alpha and increasing high frequency gamma [2,[7][8][9][10][11][12].…”