Background

Volatile anesthetics moderately depress respiratory function at clinically relevant concentrations. Phox2b-expressing chemosensitive neurons in the retrotrapezoid nucleus, a respiratory control center, are activated by isoflurane, but the underlying mechanisms remain unclear. The hypothesis of this study was that the sodium leak channel contributes to the volatile anesthetics-induced modulation of retrotrapezoid nucleus neurons and to respiratory output.

Methods

The contribution of sodium leak channels to isoflurane-, sevoflurane-, and propofol-evoked activity of Phox2b-expressing retrotrapezoid nucleus neurons and respiratory output were evaluated in wild-type and genetically modified mice lacking sodium leak channels (both sexes). Patch-clamp recordings were performed in acute brain slices. Whole-body plethysmography was used to measure the respiratory activity.

Results

Isoflurane at 0.42 to 0.50 mM (~1.5 minimum alveolar concentration) increased the sodium leak channel–mediated holding currents and conductance from −75.0 ± 12.9 to −130.1 ± 34.9 pA (mean ± SD, P = 0.002, n = 6) and 1.8 ± 0.5 to 3.6 ± 1.0 nS (P = 0.001, n = 6), respectively. At these concentrations, isoflurane increased activity of Phox2b-expressing retrotrapezoid nucleus neurons from 1.1 ± 0.2 to 2.8 ± 0.2 Hz (P < 0.001, n = 5), which was eliminated by bath application of gadolinium or genetic silencing of sodium leak channel. Genetic silencing of sodium leak channel in the retrotrapezoid nucleus resulted in a diminished ventilatory response to carbon dioxide in mice under control conditions and during isoflurane anesthesia. Sevoflurane produced an effect comparable to that of isoflurane, whereas propofol did not activate sodium leak channel–mediated holding conductance.

Conclusions

Isoflurane and sevoflurane increase neuronal excitability of chemosensitive retrotrapezoid nucleus neurons partly by enhancing sodium leak channel conductance. Sodium leak channel expression in the retrotrapezoid nucleus is required for the ventilatory response to carbon dioxide during anesthesia by isoflurane and sevoflurane, thus identifying sodium leak channel as a requisite determinant of respiratory output during anesthesia of volatile anesthetics.

Editor’s Perspective
What We Already Know about This Topic
  • The voltage-independent sodium leak channel is expressed in chemosensitive retrotrapezoid nucleus neurons of the rostral medulla and is required for physiologic respiratory activity

  • Whether sodium leak channel in retrotrapezoid nucleus neurons contributes to the effects of volatile anesthetics on breathing under general anesthesia has not been previously investigated

What This Article Tells Us That Is New
  • Isoflurane increased neuronal activity of chemosensitive retrotrapezoid nucleus neurons in brain slices of neonatal mice

  • Genetic ablation of the voltage-independent sodium leak channel from retrotrapezoid nucleus neurons decreased respiratory rate and ventilatory response to carbon dioxide upon isoflurane and sevoflurane but not after propofol exposure

  • These observations suggest that the voltage-independent sodium leak channel in retrotrapezoid nucleus neurons of the medulla is a target of volatile anesthetics to activate respiratory activity

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