The actions of volatile anesthetics: a new perspective

Abstract

This article reviews recent work in applying neutron and X-ray scattering towards the elucidation of the molecular mechanisms of volatile anesthetics. Experimental results on domain mixing in ternary lipid mixtures, and the influence of volatile anesthetics and hydrostatic pressure are placed in the contexts of ion-channel function and receptor trafficking at the postsynaptic density.

Keywords: anesthesia; lipid rafts; membranes; neutron scattering; synaptic plasticity.

Figure 1

(a) Neutron diffraction. First-order diffraction peaks for a multilayered sample of deuterated d62-DPPC/DOPC (1:1) with 20% cholesterol. The first peak is from the L_o phase and the second is from the L_d phase. The black trace is for helium at atmospheric pressure, the red trace is for xenon at two times atmospheric pressure (3.2 MAC) and the blue trace is for xenon at four times atmospheric pressure (6.4 MAC). Experiments were performed at 28°C and 98% relative humidity. Traces are Gaussian fits to data. q = 4πsinθ/λ is the neutron momentum transfer. Bars indicate standard errors (1 atm = 101 kPa). (b) Neutron diffraction. First-order diffraction peaks for a multilayered sample of deuterated d31-palmitoyl sphingomyelin/DOPC (1:1) with 20% cholesterol. The first peak is from the L_o phase and the second is from the L_d phase. The black trace is for air and the red trace is for xenon at three times atmospheric pressure (4.6 MAC). Experiments were performed at 27°C and 98% relative humidity. Traces are Gaussian fits to data. Bars indicate standard errors (1 atm = 101 kPa). Reprinted with permission from Weinrich & Worcester (2013 ▸). Copyright 2013 American Chemical Society.

Figure 2

Change in the ratio of first-order peak areas versus concentration as MAC. Solid squares are for an SPM/DOPC/cholesterol mixture with xenon, solid triangles are for an SPM/DOPC/cholesterol mixture with nitrous oxide, open squares are for a DPPC/DOPC/cholesterol mixture with xenon and open triangles are for a DPPC/DOPC/cholesterol mixture with nitrous oxide. The MAC used for xenon is 0.63 bar and that for nitrous oxide is 1.04 bar. The dashed line represents the least-squares linear fit; the adjusted r ² is 0.89. Bars represent standard errors. Reprinted with permission from Weinrich & Worcester (2013 ▸). Copyright 2013 American Chemical Society.

Figure 3

Time course of first-order X-ray diffraction for a multilayered sample of 1:1 DPPC/DOPC with 20% cholesterol hydrated at 98% relative humidity at 27°C. Data are plotted as the ratio of first-order peak heights for the two domains: L_o/L_d. At 40 min, n-hexane was introduced into the sample chamber as a solution in n-hexadecane (1/10 by volume) to maintain an n-hexane partial pressure of 0.042 bar, which is the partial pressure required for narcosis in mice (White et al., 1981 ▸). The decrease in L_o/L_d results from mixing of L_o lipids into the L_d phase. At about 100 min equilibrium is reached and the sample chamber was kept undisturbed for 12 h to test stability. Subsequent data (blue) have 720 min subtracted from the time in order to include the results upon removal of n-hexane by replacing the hexane solution with n-hexadecane on the same plot. Anesthetic doses for n-hexane (MAC) in vivo are uncertain because one metabolic product is a neurotoxin. Error bars indicate standard error.

Figure 4

Neutron diffraction of 1:1 d62-DPPC/DLPC oriented multilayers on glass with q (1.4–1.5 Å⁻¹) directed parallel to the plane of the membrane. Neutron diffraction in this plane is generated by the d62-lipid chains and is greatly reduced by mixing of d62-lipid chains with h62-lipid chains so that d62-lipid chains are no longer adjacent. The midpoint of the phase-mixing transition for the native lipid mixture is about 29°C (squares) and corresponds well to the phase diagrams established by calorimetry (van Dijck et al., 1977 ▸). The addition of 1.5 mol% halothane (measured at 27°C) decreases the transition temperature to 25°C (circles), but 7.5 mol% F6 only decreases the transition to 27°C (triangles). Inset: neutron counts collected in θ/2θ scans across the chain diffraction peak for the 1:1 d62 DPPC/DLPC oriented multilayers at three temperatures: at the beginning, midpoint and end of the temperature scan (top, middle and bottom traces, respectively) in the absence of halothane. Peaks were integrated in the range q = 1.46–1.52 Å⁻¹ and background counts outside this region were subtracted to obtain the plotted chain diffraction intensity. Error bars represent one standard deviation and are from counting statistics. Reprinted with permission from Weinrich et al. (2012 ▸). Copyright 2012 American Chemical Society.

Figure 5

(a) Small-angle neutron scattering intensity versus q for d62-DPPC/DOPC/cholesterol (2:2:1 molar ratio) unilamellar vesicles in D₂O/H₂O (45% D₂O) at 25°C matched to the scattering length for vesicles at atmospheric pressure at 33°C: pink inverted triangles, 0 MPa; green triangles, 10 MPa; cyan circles, 21 MPa; purple squares, 31 MPa. The detector is at 5 m and the neutron wavelength is 6 Å. (b) Scattering invariant Q () for the SANS data as a function of pressure and temperature. The dotted line is drawn to guide the eye through the points at 0 MPa. The arrow indicates ΔT, while ΔP is the MPa value at 25°C and the ratio of these values gives dT/dP, assuming linearity. The dT/dP values thus obtained are 0.26, 0.24 and 0.20°C MPa⁻¹ from top to bottom. Error bars indicate standard error. Reprinted with permission from Worcester & Weinrich (2015 ▸). Copyright 2015 American Chemical Society.

Figure 6

(a) Gramicidin channels in 5:2 DOPC/cholesterol (CHL) at 30°C, 1 M KCl, 100 mV. The horizontal bar is 2 s in length. The vertical bar is 3 pA. (b) Logarithmically binned histogram of gramicidin lifetimes in 5:2 DOPC/CHL at 30°C and single-exponential fit to log probability. (c) Gramicidin channels in 1:1:1 DOPC/SPM/CHL at 32°C (7°C above the miscibility transition for this mixture), 1 M KCl, 100 mV. The horizontal bar is 2 s in length. The vertical bar is 3 pA. (d) Logarithmically binned histogram of gramicidin lifetimes in 1:1:1 DOPC/SPM/CHL at 32°C with single-exponential (green trace) and double-exponential fits (fuchsia trace). Reprinted with permission from Weinrich et al. (2017 ▸). Copyright 2017 Royal Society of Chemistry.