Propofol and Sevoflurane Depress Spinal Neurons In Vitro via Different Molecular Targets

Background:The capacity of general anesthetics to produce immobility is primarily spinally mediated. Recently, compelling evidence has been provided that the spinal actions of propofol involve &ggr;-aminobutyric acid type A (GABAA) receptors, whereas the contribution of glycine receptors remains uncertain. The relevant molecular targets of the commonly used volatile anesthetic sevoflurane in the spinal cord are largely unknown, but indirect evidence suggests a mechanism of action distinct from propofol. Methods:The effects of sevoflurane and propofol on spontaneous action potential firing were investigated by extracellular voltage recordings from ventral horn interneurons in cultured spinal cord tissue slices obtained from embryonic rats (embryonic days 14–15). Results:Propofol and sevoflurane reduced spontaneous action potential firing of neurons. Concentrations causing half-maximal effects (0.11 &mgr;m propofol, 0.11 mm sevoflurane) were lower than the median effective concentration immobility (1–1.5 &mgr;m propofol, 0.35 mm sevoflurane). At higher concentrations, complete inhibition of action potential activity was observed with sevoflurane but not with propofol. Effects of sevoflurane were mediated predominantly by glycine receptors (45%) and GABAA receptors (38%), whereas propofol acted almost exclusively via GABAA receptors (96%). Conclusions:The authors’ results suggest that glycine and GABAA receptors are the most important molecular targets mediating depressant effects of sevoflurane in the spinal cord. They provide evidence that sevoflurane causes immobility by a mechanism distinct from the actions of the intravenous anesthetic propofol. The finding that propofol acts exclusively via GABAA receptors can explain its limited capacity to depress spinal neurons in the authors’ study.