Xenon inhibits excitatory but not inhibitory transmission in rat spinal cord dorsal horn neurons

Abstract

Background: The molecular targets for the promising gaseous anaesthetic xenon are still under investigation. Most studies identify N-methyl-D-aspartate (NMDA) receptors as the primary molecular target for xenon, but the role of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionic acid (AMPA) receptors is less clear. In this study we evaluated the effect of xenon on excitatory and inhibitory synaptic transmission in the superficial dorsal horn of the spinal cord using in vitro patch-clamp recordings from rat spinal cord slices. We further evaluated the effects of xenon on innocuous and noxious stimuli using in vivo patch-clamp method.

Results: In vitro, xenon decreased the amplitude and area under the curve of currents induced by exogenous NMDA and AMPA and inhibited dorsal root stimulation-evoked excitatory postsynaptic currents. Xenon decreased the amplitude, but not the frequency, of miniature excitatory postsynaptic currents. There was no discernible effect on miniature or evoked inhibitory postsynaptic currents or on the current induced by inhibitory neurotransmitters. In vivo, xenon inhibited responses to tactile and painful stimuli even in the presence of NMDA receptor antagonist.

Conclusions: Xenon inhibits glutamatergic excitatory transmission in the superficial dorsal horn via a postsynaptic mechanism. There is no substantial effect on inhibitory synaptic transmission at the concentration we used. The blunting of excitation in the dorsal horn lamina II neurons could underlie the analgesic effect of xenon.

Figure 1

Effect of xenon on eEPSCs. A - Excitatory post synaptic currents evoked at holding potential (HP) = -70 mV by primary afferents electrical stimulation recruiting Aδ- (left) and C-fibres (right) before (black), during (dark grey) and after (light grey) xenon application. Five consecutive traces are shown for each modality. Note the decrease in amplitude during xenon application. B - Xenon reversibly decreased the mean amplitude of monosynaptic currents evoked by Aδ- (left, n = 10) and C- (right, n = 9) afferent fibre stimulation.

Figure 2

Exogenous agonist-induced currents inhibited by xenon. A - Representative traces showing NMDA (50 μM, 30 s)-induced outward currents at HP = +40 mV, which were reversibly reduced by xenon (50%). Drug application is marked with bars. B - Xenon decreased the amplitude and integrated area of NMDA-induced currents in lamina II neurons (n = 8). *P < 0.05 compared with control. C - AMPA (10 μM, 30 s)-induced inward currents at HP = -70 mV, which were reversibly reduced by xenon (50%). D - Xenon decreased the amplitude and integrated area of NMDA-induced currents in lamina II neurons (n = 7). *P < 0.05 compared with control.

Figure 3

Xenon decreased mEPSC amplitude, but not mEPSC frequency. A - Two-scale traces of mEPSC recorded at HP = -70 mV in the presence of tetrodotoxin (1 μM) before, during and after xenon application. B - The effect of xenon on miniature excitatory postsynaptic current (mEPSC) amplitude (n = 16) and frequency (n = 16). *P < 0.05 compared with control. C - Cumulative histograms of mEPSC amplitude (left) and inter-event intervals (right) from a representative neuron showing the shift to the left of mEPSC amplitude under xenon exposure. Control trace was calculated from 470 events and xenon trace was calculated from 490 events.

Figure 4

Xenon inhibited the responses to touch and pinch in vivo. A - Representative traces showing the decrease in the response to light touch of the hind limb during xenon application (HP = -70 mV) recorded using in vivo patch-clamp method. B - Diagram showing the reversible decrease in mean amplitude and integrated area of the touch response during xenon application (n = 4). *P < 0.05 compared with control. C - Response to skin fold pinch during xenon application. D - The mean amplitude and integrated area of pinch-evoked EPSCs decreased in the presence of xenon (n = 6). n is the number of neurons.

Figure 5

Xenon did not affect inhibitory synaptic transmission in lamina II neurons. Representative traces and diagrams showing no change in amplitude and integrated area of the currents induced by exogenous GABA (A) or glycine (B) in the presence of xenon. C - Focal stimulation induced inhibitory postsynaptic currents before (grey) and during (black) xenon application. Neither amplitude nor integrated area changed in the presence of xenon. D - Trace and diagram showing the lack of an effect of xenon on the mean amplitude or frequency of miniature inhibitory postsynaptic currents.