Opposing Actions of Sevoflurane on GABAergic and Glycinergic Synaptic Inhibition in the Spinal Ventral Horn

BACKGROUND: The ventral horn is a major substrate in mediating the immobilizing properties of the volatile anesthetic sevoflurane in the spinal cord. In this neuronal network, action potential firing is controlled by GABA(A) and glycine receptors. Both types of ion channels are sensitive to volatile anesthetics, but their role in mediating anesthetic-induced inhibition of spinal locomotor networks is not fully understood. METHODOLOGY/PRINCIPAL FINDINGS: To compare the effects of sevoflurane on GABAergic and glycinergic inhibitory postsynaptic currents (IPSCs) whole-cell voltage-clamp recordings from ventral horn interneurons were carried out in organotypic spinal cultures. At concentrations close to MAC (minimum alveolar concentration), decay times of both types of IPSCs were significantly prolonged. However, at 1.5 MAC equivalents, GABAergic IPSCs were decreased in amplitude and reduced in frequency. These effects counteracted the prolongation of the decay time, thereby decreasing the time-averaged GABAergic inhibition. In contrast, amplitudes and frequency of glycinergic IPSCs were not significantly altered by sevoflurane. Furthermore, selective GABA(A) and glycine receptor antagonists were tested for their potency to reverse sevoflurane-induced inhibition of spontaneous action potential firing in the ventral horn. These experiments confirmed a weak impact of GABA(A) receptors and a prominent role of glycine receptors at a high sevoflurane concentration. CONCLUSIONS: At high concentrations, sevoflurane mediates neuronal inhibition in the spinal ventral horn primarily via glycine receptors, and less via GABA(A) receptors. Our results support the hypothesis that the impact of GABA(A) receptors in mediating the immobilizing properties of volatile anesthetics is less essential in comparison to glycine receptors.