Attenuation of Native Hyperpolarization-Activated, Cyclic Nucleotide-Gated Channel Function by the Volatile Anesthetic Sevoflurane in Mouse Thalamocortical Relay Neurons

Stefan Schwerin,Rainer Haseneder,Stephan Kratzer,Claudia Kopp,Matthias Kreuzer,Gerhard Schneider,Elisabeth Pircher

doi:10.3389/fncel.2020.606687

Stefan Schwerin, Rainer Haseneder + Show 5 more

Open Access

https://doi.org/10.3389/fncel.2020.606687

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Abstract

As thalamocortical relay neurons are ascribed a crucial role in signal propagation and information processing, they have attracted considerable attention as potential targets for anesthetic modulation. In this study, we analyzed the effects of different concentrations of sevoflurane on the excitability of thalamocortical relay neurons and hyperpolarization-activated, cyclic-nucleotide gated (HCN) channels, which play a decisive role in regulating membrane properties and rhythmic oscillatory activity. The effects of sevoflurane on single-cell excitability and native HCN channels were investigated in acutely prepared brain slices from adult wild-type mice with the whole-cell patch-clamp technique, using voltage-clamp and current-clamp protocols. Sevoflurane dose-dependently depressed membrane biophysics and HCN-mediated parameters of neuronal excitability. Respective half-maximal inhibitory and effective concentrations ranged between 0.30 (95% CI, 0.18–0.50) mM and 0.88 (95% CI, 0.40–2.20) mM. We witnessed a pronounced reduction of HCN dependent Ih current amplitude starting at a concentration of 0.45 mM [relative change at −133 mV; 0.45 mM sevoflurane: 0.85 (interquartile range, 0.79–0.92), n = 12, p = 0.011; 1.47 mM sevoflurane: 0.37 (interquartile range, 0.34–0.62), n = 5, p < 0.001] with a half-maximal inhibitory concentration of 0.88 (95% CI, 0.40–2.20) mM. In contrast, effects on voltage-dependent channel gating were modest with significant changes only occurring at 1.47 mM [absolute change of half-maximal activation potential; 1.47 mM: −7.2 (interquartile range, −10.3 to −5.8) mV, n = 5, p = 0.020]. In this study, we demonstrate that sevoflurane inhibits the excitability of thalamocortical relay neurons in a concentration-dependent manner within a clinically relevant range. Especially concerning its effects on native HCN channel function, our findings indicate substance-specific differences in comparison to other anesthetic agents. Considering the importance of HCN channels, the observed effects might mechanistically contribute to the hypnotic properties of sevoflurane.

Highlights

Since its introduction into clinical practice, the volatile anesthetic sevoflurane has been enjoying popularity worldwide with several studies demonstrating its safe and efficient use (Brioni et al, 2017)
As we previously showed in a set of experiments using voltage-sensitive dye imaging techniques, the application of sevoflurane led to a dose-dependent, global reduction of cortical depolarization as well as to deferred thalamocortical signal propagation in response to stimulation of the VB thalamus (Kratzer et al, 2017)
We hypothesized that these effects of sevoflurane might be attributable to a reduction of intrinsic neuronal excitability of thalamocortical relay neurons mediated by an impairment of HCN channel function, as tonic Ih is ascribed an important role in regulating membrane biophysics, like the resting membrane potential and the input resistance, as well as firing properties of neurons (He et al, 2014)

Summary

Introduction

Since its introduction into clinical practice, the volatile anesthetic sevoflurane has been enjoying popularity worldwide with several studies demonstrating its safe and efficient use (Brioni et al, 2017). -called ‘‘top-down’’ theories emphasize disruptive effects of general anesthetics on functional connectivity in cortical networks, whereas ‘‘bottomup’’ approaches focus on hierarchical processing of sensory information (Mashour, 2014). In this context, the thalamus as part of the intricately interconnected thalamocortical network (McCormick and Bal, 1997) as well as ascending subcortical pathways (Guillery and Sherman, 2011) has attracted considerable attention. More recent studies, which suggest that the anesthetic-induced loss of consciousness cannot sufficiently be explained by the interruption of the thalamocortical feed-forward relay of peripheral input to sensory cortices alone, have evolved our understanding of thalamic function. Preserved thalamocortical connectivity between higher-order nuclei and frontalparietal networks seems a likely prerequisite for maintaining consciousness (Liu et al, 2013; Mashour, 2014; Ranft et al, 2016: Sherman, 2016)

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