Propofol block of I_h contributes to the suppression of neuronal excitability and rhythmic burst firing in thalamocortical neurons

Abstract

Although the depressant effects of the general anesthetic propofol on thalamocortical relay neurons clearly involve gamma-aminobutyric acid (GABA)(A) receptors, other mechanisms may be involved. The hyperpolarization-activated cation current (I(h)) regulates excitability and rhythmic firing in thalamocortical relay neurons in the ventrobasal (VB) complex of the thalamus. Here we investigated the effects of propofol on I(h)-related function in vitro and in vivo. In whole-cell current-clamp recordings from VB neurons in mouse (P23-35) brain slices, propofol markedly reduced the voltage sag and low-threshold rebound excitation that are characteristic of the activation of I(h). In whole-cell voltage-clamp recordings, propofol suppressed the I(h) conductance and slowed the kinetics of activation. The block of I(h) by propofol was associated with decreased regularity and frequency of delta-oscillations in VB neurons. The principal source of the I(h) current in these neurons is the hyperpolarization-activated cyclic nucleotide-gated (HCN) type 2 channel. In human embryonic kidney (HEK)293 cells expressing recombinant mouse HCN2 channels, propofol decreased I(h) and slowed the rate of channel activation. We also investigated whether propofol might have persistent effects on thalamic excitability in the mouse. Three hours following an injection of propofol sufficient to produce loss-of-righting reflex in mice (P35), I(h) was decreased, and this was accompanied by a corresponding decrease in HCN2 and HCN4 immunoreactivity in thalamocortical neurons in vivo. These results suggest that suppression of I(h) may contribute to the inhibition of thalamocortical activity during propofol anesthesia. Longer-term effects represent a novel form of propofol-mediated regulation of I(h).

PIP2-Mediated HCN3 Channel Gating Is Crucial for Rhythmic Burst Firing in Thalamic Intergeniculate Leaflet Neurons

“…Rats and mice (P40 -P60) were used for this study; immunolabeling experiments and image analyses were performed much as previously described Ying et al, 2006 and2007b). Briefly, coronal sections (30 m) containing the IGL were cut using a microslicer as described above.…”

“…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).…”

“…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.…”

“…One of most striking features of I h is its activation during hyperpolarization and its ability to depolarize the membrane toward threshold for rebound burst firing (McCormick and Pape, 1990;Bal and McCormick, 1997;Ying et al, 2006); thus, we examined I h regulation of low-threshold rebound burst firing in IGL neurons. The membrane was transiently hyperpolarized to approximately Ϫ125 mV from Ϫ60 mV by intracellular injection of hyperpolarizing current pulses (see Materials and Methods) in the absence (control) and presence of ZD7288 (30 M, applied by superfusion).…”

NPY signaling through Y1 receptors modulates thalamic oscillations

“…Indispensable components for generation of these oscillations are GABA B and AMPA receptors as well as T-type calcium channels [4], however, many other factors modulate oscillatory activity, e.g. HCN channels, neuropeptides and electrical synapses [3,16,17,18,27,34]. Neuropeptide Y (NPY) is an inhibitor of absence seizures [23] and thalamic oscillations [25].…”