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Suppression of I_h Contributes to Propofol-Induced Inhibition of Mouse Cortical Pyramidal Neurons

¹Departments of Pharmacology and ²Anesthesiology, University of Virginia, Charlottesville, Virginia

Submitted 15 April 2005; accepted in final form 4 August 2005

The contributions of the hyperpolarization-activated current, I_h, to generation of rhythmic activities are well described for various central neurons, particularly in thalamocortical circuits. In the present study, we investigated effects of a general anesthetic, propofol, on native I_h in neurons of thalamus and cortex and on the corresponding cloned HCN channel subunits. Whole cell voltage-clamp recordings from mouse brain slices identified neuronal I_h currents with fast activation kinetics in neocortical pyramidal neurons and with slower kinetics in thalamocortical relay cells. Propofol inhibited the fast-activating I_h in cortical neurons at a clinically relevant concentration (5 µM); inhibition of I_h involved a hyperpolarizing shift in half-activation voltage (V1/2 approximately –9 mV) and a decrease in maximal available current (36% inhibition, measured at –120 mV). With the slower form of I_h expressed in thalamocortical neurons, propofol had no effect on current activation or amplitude. In heterologous expression systems, 5 µM propofol caused a large shift in V1/2 and decrease in current amplitude in homomeric HCN1 and linked heteromeric HCN1–HCN2 channels, both of which activate with fast kinetics but did not affect V1/2 or current amplitude of slowly activating homomeric HCN2 channels. With GABA_A and glycine receptor channels blocked, propofol caused membrane hyperpolarization and suppressed action potential discharge in cortical neurons; these effects were occluded by the I_h blocker, ZD-7288. In summary, these data indicate that propofol selectively inhibits HCN channels containing HCN1 subunits, such as those that mediate I_h in cortical pyramidal neurons—and they suggest that anesthetic actions of propofol may involve inhibition of cortical neurons and perhaps other HCN1-expressing cells.

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