Propofol-Block of SK Channels in Reticular Thalamic Neurons Enhances GABAergic Inhibition in Relay Neurons

Abstract

The GABAergic reticular thalamic nucleus (RTN) is a major source of inhibition for thalamocortical neurons in the ventrobasal complex (VB). Thalamic circuits are thought to be an important anatomic target for general anesthetics. We investigated presynaptic actions of the intravenous anesthetic propofol in RTN neurons, using RTN-retained and RTN-removed brain slices. In RTN-retained slices, focal and bath application of propofol increased intrinsic excitability, temporal summation, and spike firing rate in RTN neurons. Propofol-induced activation was associated with suppression of medium afterhyperpolarization potentials. This activation was mimicked and completely occluded by the small conductance calcium-activated potassium (SK) channel blocker apamin, indicating that propofol could enhance RTN excitability by blocking SK channels. Propofol increased GABAergic transmission at RTN-VB synapses, consistent with excitation of presynaptic RTN neurons. Stimulation of RTN resulted in synaptic inhibition in postsynaptic neurons in VB, and this inhibition was potentiated by propofol in a concentration-dependent manner. Removal of RTN resulted in a dramatic reduction of both spontaneous postsynaptic inhibitory current frequency and propofol-mediated inhibition of VB neurons. Thus the existence and activation of RTN input were essential for propofol to elicit thalamocortical suppression; such suppression resulted from shunting through the postsynaptic GABA(A) receptor-mediated chloride conductance. The results indicate that propofol enhancement of RTN-mediated inhibitory input via blockade of SK channels may play a critical role in "gating" spike firing in thalamocortical relay neurons.

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“…Brain slice preparation was previously described in detail (Ying and Goldstein, 2005), with the method slightly modified for this study. Briefly, mice or rats (P40 -P70) of either sex were anesthetized with isoflurane and decapitated.…”

“…Whole-cell voltage-and current-clamp recordings were performed as described previously (Ying and Goldstein, 2005;Ying et al, 2006Ying et al, , 2007b. Briefly, brain slices were visualized using a Nikon microscope equipped with a 4ϫ objective and a 40ϫ water-immersion objective.…”

“…Access resistance (R a ) and capacitance were compensated after a whole-cell configuration was established, and were monitored throughout recordings; data were discarded if either of the two parameters changed by Ͼ20% of the original values. Liquid junction potentials were calculated and corrected offline (Ying and Goldstein, 2005). For recordings of I h , neurons were voltage clamped at Ϫ50 mV, and voltage steps were applied from Ϫ50 to Ϫ130 mV (10 s, 10 mV/step).…”

“…The intracellular biocytin filling method was previously described (Ying and Goldstein, 2005) with a modification for this study. Briefly, IGL neurons were filled with 0.2% biocytin (in the pipette solution) during wholecell voltage-clamp recordings.…”

“…Data processing including the construction of activation curves, fit of time constants, and measurements of sag, burst and tonic spike firing was performed using MiniAnalysis (Synaptosoft) and Clampfit V.10, as previously described for neurons (Ying and Goldstein, 2005;Ying et al, 2006Ying et al, , 2007b. Data analyses for oocyte recordings were performed using PulseFit (HEKA Elektronik) and IgorPro (Wavemetrics, as previously reported (Santoro et al, 2000;Wainger et al, 2001;Fogle et al, 2007).…”