Skip Navigation LinksHome > December 2008 - Volume 109 - Issue 6 > Xenon Reduces N-Methyl-d-aspartate and α-Amino-3-hydroxy-5-m...
Anesthesiology:
doi: 10.1097/ALN.0b013e31818d6aee
Perioperative Medicine

Xenon Reduces N-Methyl-d-aspartate and α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid Receptor–mediated Synaptic Transmission in the Amygdala

Haseneder, Rainer M.D.*; Kratzer, Stephan M.S.†; Kochs, Eberhard M.D.‡; Eckle, Veit-Simon M.D.*; Zieglgänsberger, Walter M.D.§; Rammes, Gerhard Ph.D.∥

Free Access
Article Outline
Collapse Box

Author Information

Collapse Box

Abstract

Background: The neuronal and molecular targets of the inhalational general anesthetic xenon are a matter of debate. The current knowledge is largely based on studies using neurons in culture or heterologous expression systems. In the current study, the authors evaluated for the first time the effect of xenon on synaptic transmission in the basolateral amygdala in an in vitro brain slice preparation of the mouse.
Methods: A patch clamp technique was used to evaluate the effects of xenon on N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor–mediated excitatory postsynaptic currents (EPSCs), as well as on γ-aminobutyric acid type A receptor–mediated inhibitory postsynaptic currents. The currents were either evoked upon electrical stimulation (NMDA-eEPSCs, AMPA-eEPSCs) or upon focal, laser-guided photolysis of caged l-glutamate (p-NMDA-Cs, p-AMPA-Cs). In addition, the authors investigated the effects of xenon on miniature EPSCs.
Results: Xenon reversibly reduced basal synaptic transmission but had no effect on γ-aminobutyric acid type A receptor–mediated inhibitory synaptic transmission. Xenon concentration-dependently diminished NMDA-eEPSCs and p-NMDA-Cs to the same amount. Likewise, xenon-induced reduction of AMPA-eEPSCs and p-AMPA-Cs did not differ. Xenon did not affect the frequency of miniature EPSCs but reduced their amplitude.
Conclusions: In the current study, xenon considerably depressed NMDA and AMPA receptor–mediated synaptic transmission in the basolateral amygdala without affecting inhibitory synaptic transmission. The results provide evidence that the effects of xenon on NMDA- and AMPA-EPSCs are primarily mediated via postsynaptic mechanisms.
THE noble gas xenon is an inhalational anesthetic with analgesic and neuroprotective properties combined with beneficial pharmacokinetic and cardiovascular characteristics.1 During the past two decades, the safety and efficacy of xenon anesthesia in human patients has been demonstrated in numerous clinical trials.2–4 However, the mechanisms how xenon exerts its anesthetic and analgesic properties have not been clarified yet.
In a simplified manner, general anesthesia can be described as a state with reduced central nervous system excitability. This state can be reached by enhancement of inhibitory or reduction of excitatory neurotransmission, or the combination of both.5
Enhancement of inhibitory γ-aminobutyric acid–mediated (GABAergic) synaptic transmission is considered to be of fundamental importance for the anesthetic mechanism of, e.g., volatile and intravenous anesthetics.6,7 Two studies8,9 showed an enhancement of Cl currents through heterologously expressed recombinant γ-aminobutyric acid type A (GABAA) receptors by xenon. In contrast, other studies10,11 demonstrated that xenon exerts no effects on GABAA receptor–mediated inhibitory postsynaptic currents (GABAA-IPSCs) recorded from cultured hippocampal neurons. Therefore, there is general agreement that effects on GABA receptors most probably do not contribute to the xenon anesthetic state.12
Glutamate is the major excitatory neurotransmitter, and it is generally accepted that xenon reduces glutamatergic current responses. However, it is not yet clear which subtype of glutamate receptors (N-methyl-d-aspartate [NMDA], α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA], or kainic acid receptors) plays the most important role in mediating the xenon anesthetic state.12 The first investigations on neuronal mechanisms of xenon postulated an exclusive action on NMDA receptors.10,11 Meanwhile, several in vitro studies showed that xenon reduces currents not only through recombinant NMDA receptors,9 but also through recombinant non-NMDA receptors.13,14 Furthermore, an in vivo study using the model organism Caenorhabditis elegans showed diverse anesthesia-associated behavioral changes, which were induced by xenon via AMPA but not NMDA receptors.15
To our knowledge, there are no data yet available investigating the effect of xenon on synaptic transmission in acute brain slice preparations. Current insights of the neuronal and molecular targets of xenon are instead based on studies using heterologous expression systems or neurons in culture. The aim of the current study was to determine the effects of xenon on inhibitory and excitatory synaptic transmission mediated via native GABAA, NMDA, and AMPA receptors in an acute mice brain slice preparation, which provides a considerable degree of presynaptic and postsynaptic functionality and thus represents a much less artificial model for central nervous system neuronal signaling.
Back to Top | Article Outline

Materials and Methods

The experimental protocols were approved by the Ethical Committee on Animal Care and Use of the Government of Bavaria, Germany. Male C57Bl6 mice, 28–42 days old, were killed by cervical dislocation, and the brains were rapidly removed into ice-cold artificial cerebrospinal fluid (ACSF) containing 125 mm NaCl, 2.5 mm KCl, 25 mm NaHCO3, 2 mm CaCl2, 2 mm MgCl2, 25 mm d-glucose, and 1.25 mm NaH2PO4 (all from RBI/Sigma, Deisenhofen, Germany). Saturation with a mixture of 95% O2–5% CO2 (carbogen gas) led to a pH of 7.4. Coronal slices (350 μm thick) were prepared using a microtome (HM 650 V; Microm International, Walldorf, Germany). Slices were allowed to recover in a storage chamber (34°C) for at least 1 h before being transferred to the recording chamber.
A platinum ring with nylon filaments was used to fix the slice on the bottom of the recording chamber, which was continuously perfused (2 ml/min) with ACSF. We used infrared-phase contrast-enhanced videomicroscopy (Zeiss, Oberkochen, Germany; for details, see Dodt et al.16) to visualize the somata of neurons within the basolateral amygdala (BLA). Whole cell patch clamp recordings were performed from principal neurons, which were identified according to their accommodating firing pattern and long-lasting afterhyperpolarizations.17
The patch pipettes were pulled from thin-walled borosilicate glass tubes with inner filament (OD 1.5 mm, ID 1.17 mm, GC150TF-10; Clark Electromedical Instruments, Pangbourne Reading, United Kingdom) and heat polished using a two-step horizontal puller (DMZ-Universal Puller; Zeitz-Instruments, Munich, Germany). Pipettes had a series resistance of 4–6 MΩ when filled with a solution containing 130 mm K-d-gluconate, 5 mm KCl, 0.5 mm EGTA, 2 mm MgCl2, 10 mm HEPES, 5 mm d-glucose, and 20 mm Na2-phosphocreatine (all from RBI/Sigma). Currents were recorded with a switched voltage clamp amplifier (SEC 10 l; NPI Electronic, Tamm, Germany) with switching frequencies of 60–80 kHz (25% duty cycle). Series resistance was monitored continuously and compensated in bridge mode. All patch clamp experiments were performed at room temperature (22°–25°C) to ensure a sufficient oxygenation of the neurons.
Fig. 1
Fig. 1
Image Tools
Figure 1 depicts a scheme of the stimulation and recording conditions. Electrically evoked postsynaptic currents (ePSCs) were induced by square pulse stimuli (6–20 V, 0.05–0.2 ms, interstimulus interval 15 s) delivered via an ultrafine bipolar tungsten electrode (insulated to the tip, 5-μm tip diameter), which was placed to the lateral nucleus of the amygdala. Preceding each tissue stimulation, neuronal input resistance was determined by injecting a current pulse hyperpolarizing by 10 mV for 200 ms.
Photolytically evoked excitatory currents were induced upon focal photolysis of caged l-glutamate.16 Thereby, the beam of an ultraviolet laser (355-nm wavelength, frequency-tripled Nd:YVO4, 100-kHz pulse repetition rate; DPSS Lasers, San Jose, CA) was focused by the objective (60×, 0.9 numerical aperture; Olympus, Tokyo, Japan) on a small spot (5 μm in diameter) positioned on a dendrite approximately 10–20 μm from the soma. Laser stimulation was delivered alternating with electrical stimulation in intervals of 15 s. Once a stable whole cell recording had been obtained, “caged glutamate” (γ-α-carboxy-2-nitrobenzyl [γ-CNB]–caged glutamate) was added to the recirculating perfusate (0.25 mm). Caged glutamate had no discernible effect on neurons per se.16 Glutamate was released by Q-switching brief laser pulses (3–5 ms; intensity 1–2 mW), applied at regular intervals of 30 s throughout the experiment.
To isolate specific currents, we used d(−)-2-amino-5-phosphonopentanoic acid (AP5; 50 μm), 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (5 μm), 3-amino-propyl(diethoxymethyl)phosphinic acid (CGP35348; 200 μm), and bicuculline methiodide (20 μm). Of these four receptor antagonists, an appropriate cocktail was used, with 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo- benzo[f]quinoxaline-7-sulfonamide omitted for AMPA, AP5 omitted for NMDA, and bicuculline methiodide omitted for GABAA receptor–mediated responses. The metabotropic GABAB receptors were blocked, because their activation would interfere with either AMPA or NMDA receptor currents and would not allow a detailed analysis of receptor kinetics. For AMPA receptor–mediated currents, holding potential was set to −70 mV, and for NMDA and GABAA receptor–mediated currents, holding potential was set to −30 and −50 mV, respectively.
Miniature excitatory postsynaptic currents (mEPSCs) were continuously recorded (10 min) at −70 mV in the presence of 1 μm tetrodotoxin, 50 μm AP5, 200 μm CGP35348, and 20 μm bicuculline methiodide. mEPSCs were completely blocked in the presence of 5 μm 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (data not shown).
The whole cell responses were amplified, low-pass filtered (3 kHz), and then digitized (ITC-16 Computer Interface; Instrutech Corp., Port Washington, NY) with a sampling frequency of 9 kHz, and stored to a hard drive (Power Macintosh G3 computer, data acquisition software Pulse version 8.5; Heka Electronic GmbH, Lambrecht, Germany). The obtained data were analyzed with IGOR Pro software (WaveMetrics, Portland, OR). For detection of mEPSCs, the amplitude threshold was defined as the threefold amplitude of the baseline variance (noise level), and the events identified were subsequently verified visually. We then quantified the frequency and peak amplitudes of the detected events.
Under control conditions, the superfusing ACSF was exposed to a mixture of 65% N2–30% O2–5% CO2; for xenon application, the ASCF was gassed with a mixture of 65% xenon–30% O2–5% CO2. In some subsets of experiments, xenon was applied at three lower concentrations using gas mixtures of 30% xenon–35% N2, 18% xenon–47% N2, and 5% xenon–60% N2, respectively (each gas mixture supplemented with 30% O2 and 5% CO2). Replacement of nitrogen with xenon did not change the pH of the gassed ACSF. All gas mixtures were purchased from Linde AG (Unterschleissheim, Germany) and applied at a flow rate of 0.3–0.5 l/min to the ACSF reservoir. Oxygen and carbon dioxide concentrations were verified with a calibrated gas monitor (Datex Capnomac Ultima; Duisburg, Germany). Tubing was made of polytetrafluoroethylene (Teflon; VWR International, Darmstadt, Germany) to minimize loss of xenon. Concentration measurements of dissolved xenon were accomplished using Headspace-Gaschromatography (RCC Ltd., Itingen, Switzerland). When gassing the ACSF with the 65% (30%, 18%, or 5%) xenon gas mixture, the revealing mean ± SD (n = 4) concentration of xenon in ACSF was 1.9 ± 0.2 mm (1.1 ± 0.1, 0.6 ± 0.1, or 0.2 ± 0.1 mm). Saturation (> 95%) was achieved within 5 min after the start of gassing. For comparison, other studies describe xenon saturation of 3.9 mm or 2.0 mm when gassing with pure xenon,8,9 3.4 mm when gassing with 80 vol% xenon,11,13 or 3.54 mm when gassing with 84 vol%,14 which would result in a calculated mean of approximately 2.3 mm at an assumed 65 vol% xenon saturation. The difference from our obtained concentration of 1.9 mm might be explained by an increased evaporation due to our open-chamber system.
Tetrodotoxin, AP5, 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide, bicuculline methiodide, γ-CNB–caged glutamate (all from RBI/Sigma, Deisenhofen, Germany), GYKI52466 (a kind gift from David Leander, Ph.D., Neuroscience Division, Lilly Research Laboratories, Eli Lilly and Co., Indianapolis, IN), and CGP35348 (Novartis Laboratories, Basel, Switzerland) were bath applied at known concentrations via the superfusion system.
Back to Top | Article Outline
Statistical Analysis
For statistical evaluation of the xenon effect on the neuronal excitability, we first partitioned the 40-min recording time into equidistant subintervals of 5-min length and then determined the averaged relative amplitudes in each of them. A two factorial multivariate analysis of variance with repeated measures design was applied on the averaged relative amplitudes with interval as a within-subjects factor and kind of receptor as a between-subjects factor. By the analysis of variance, differences between the various intervals were tested by tests with contrasts, whereas differences between the various receptors were tested by post hoc tests. Averaged resting membrane potential, input resistance, and series resistance of neurons as well as averaged decay time constants and charge transfer of the current responses over the three phases—baseline (5–10 min), xenon (20–25 min), and washout (35–40 min)—were tested for significant differences also by a multivariate analysis of variance with repeated-measures design. Differences in the amplitudes and frequency of mEPSCs were tested about significances with one-factorial (one-way) analysis of variance. As a nominal level of significance, we accepted α = 0.05. It was corrected (according to the Bonferroni procedure) for all posteriori tests (tests with contrasts and post hoc tests).
The IC50 values and Hill coefficients were assessed on mean normalized data with explicit weighing by SEM according to the four parameter logistic equation %Control = 100/(1 + ([Antagonist]/IC50)Hill Coef). Numerical data are presented as mean ± SEM, with the number of experiments (= neurons) indicated, if not stated otherwise. In graphs where error bars are not shown, they are smaller than the size of the symbol.
Back to Top | Article Outline

Results

All control recordings were made from slices superfused with ACSF, which was saturated with a mixture of 65% N2–30% O2–5% CO2. Despite the reduced oxygen concentration, the recording of stable postsynaptic currents was feasible. In a recent work,18 we showed in detail that decreasing the oxygen concentration in the ACSF from 95% to 30% did not produce changes in the resting membrane potential or input resistance (Rin) of neurons in the BLA. Furthermore, neither rise time and decay time constant of electrically evoked NMDA-eEPSCs nor rate and amplitude of miniature AMPA-mEPSCs were changed when reducing oxygen concentration from 95% to 30%.18
Table 1
Table 1
Image Tools
For xenon application, the gas mixture of 65% N2–30% O2–5% CO2 was changed to a mixture containing 65% xenon–30% O2–5% CO2. Analysis of variance revealed that 1.9 mm xenon did not alter the resting membrane potential, input resistance, or series resistance of neurons in the BLA (table 1).
Fig. 2
Fig. 2
Image Tools
To evaluate whether xenon reduces overall neuronal excitability, the effect of 1.9 mm xenon on the basal synaptic transmission in the BLA was examined. Compound postsynaptic currents evoked by electrical stimulation (ePSCs) were recorded in the absence of any receptor blockers from neurons visualized by infrared videomicroscopy. Ten to 15 min after xenon application (1.9 mm), the ePSCs recorded from neurons of the BLA were reduced to 80.2 ± 4.2% of control responses (fig. 2A). The averaged relative amplitude in this interval was significantly lower than in the two baseline intervals (tests with contrasts in multivariate analysis of variance, P < 0.05). Upon termination of xenon application, the ePSCs recovered to control level. Application of xenon for longer than 15 min did not result in an additional reduction of ePSCs (data not shown).
Table 2
Table 2
Image Tools
To evaluate whether xenon exerts any effects on inhibitory synaptic transmission in the BLA, electrically evoked GABAA receptor–mediated eIPSCs were recorded in the presence of 50 μm AP5, 5 μm 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxo-benzo[f]quinoxaline-7-sulfonamide, and 200 μm CGP35348 at a holding potential of −50 mV. Xenon (1.9 mm) did not affect the amplitudes of GABAA-eIPSCs (fig. 2B). In agreement with other authors,19,20 the decay of GABAA-eIPSCs was fitted biexponentially, with two time constants, τdecay fast and τdecay slow. Under xenon, τdecay fast was significantly and reversibly prolonged (table 2). However, charge transfer of GABAA-eIPSCs, which was calculated by integrating the area of the current traces, was not changed (table 2).
Fig. 3
Fig. 3
Image Tools
Table 3
Table 3
Image Tools
N-Methyl-d-aspartate receptor–mediated EPSCs were evoked using either electrical stimulation (NMDA-eEPSCs) or photolytic uncaging of glutamate (p-NMDA-Cs) (fig. 3). Xenon (1.9 mm) reversibly diminished electrically evoked NMDA-eEPSCs recorded from neurons in the BLA to 64.6 ± 5.3% of control (n = 6; fig. 3A). Photolytically evoked p-NMDA-Cs were significantly reduced by 1.9 mm xenon to 72.9 ± 5.1% of control (n = 6; fig. 3B; tests with contrasts in multivariate analysis of variance, P < 0.05). When comparing the degree of reduction between NMDA-eEPSCs and p-NMDA-Cs with Bonferroni post hoc tests in multivariate analysis of variance, we did not find any significant difference. Current decay of NMDA-eEPSCs was fitted biexponentially. Both time constants of NMDA-eEPSCs, τdecay fast and τdecay slow, remained unchanged under 1.9 mm xenon (table 3).
Fig. 4
Fig. 4
Image Tools
Electrically (AMPA-eEPSCs) or photolytically (p-AMPA-Cs) evoked AMPA receptor–mediated currents were recorded in the BLA (fig. 4). Xenon (1.9 mm) reduced AMPA-eEPSCs to 56.2 ± 4.9% of control responses (n = 6; fig. 4A). p-AMPA-Cs were reduced in the presence of 1.9 mm xenon to 62.1 ± 4.8% of control (n = 6; fig. 4B). In both cases, the averaged relative amplitudes in the third 5-min interval after xenon application was significantly lower than in the first and second baseline intervals (tests with contrasts, P < 0.05). Reduction of the amplitudes of AMPA-eEPSCs and p-AMPA-Cs was not significantly different. Current decays of AMPA-eEPSCs were fitted monoexponentially and were not altered when 1.9 mm xenon was applied (table 3).
In some additional sets of experiments, we evaluated whether the effect of xenon on NMDA and AMPA receptor currents is concentration dependent. Therefore, we used xenon gas mixtures containing 30% xenon–35% N2–30% O2–5% CO2, 18% xenon–47% N2–30% O2–5% CO2, or 5% xenon–60% N2–30% O2–5% CO2. Saturating the ACSF with these gas mixtures revealed a mean ± SD concentration of xenon of 1.1 ± 0.1, 0.6 ± 0.1, or 0.2 ± 0.1 mm, respectively. Under 1.1 mm (0.6 mm) xenon, electrically evoked NMDA-eEPSCs were reversibly diminished to 74.8 ± 5.9% (81.2 ± 4.1%) of control (n = 4 for each concentration; fig. 3A), whereas photolytically evoked p-NMDA-Cs were reduced to 76.8 ± 1.9% (86.3 ± 4.9%) of control (n = 4 for each concentration; fig. 3B). There was no significant difference in the degree of reduction of NMDA-eEPSCs and p-NMDA-Cs under 1.1 or 0.6 mm xenon (fig. 3C). Xenon at 1.1 mm (0.6 mm) reduced electrically evoked AMPA-eEPSCs to 68.3 ± 7.2% (78.0 ± 6.1%) of control responses (n = 4 for each concentration; fig. 4A) and photolytically evoked p-AMPA-Cs to 71.9 ± 3.2% (86.0 ± 5.0%) of control responses (n = 4; fig. 4B). Likewise, there was no significant difference in the degree of reduction of AMPA-eEPSCs and p-AMPA-Cs under 1.1 or 0.6 mm xenon (fig. 4C). Application of 0.2 mm xenon did not result in a significant change of NMDA-eEPSC amplitudes (95.3 ± 4.3% of control, n = 5), p-NMDA-C amplitudes (96.0 ± 3.8% of control, n = 5), AMPA-eEPSC amplitudes (93.1 ± 4.7% of control, n = 6), or p-AMPA-C amplitudes (95.3 ± 2.9% of control, n = 6). Moreover, we did not find a significant difference in the degree of reduction when comparing NMDA-eEPSCs and AMPA-eEPSCs, or p-NMDA-Cs and p-AMPA-Cs under 1.9, 1.1, 0.6, or 0.2 mm xenon, indicating a similar sensitivity of the NMDA and AMPA receptors to xenon.
The calculated IC50 values for xenon against the NDMA and AMPA receptor–mediated eEPSCs were 3.5 mm (Hill = −0.9) and 2.3 mm (Hill = −1.0). The IC50 values for xenon against p-NMDA-Cs and p-AMPA-Cs were 5.6 mm (Hill = −0.8) and 2.7 mm (Hill = −1.2).
Fig. 5
Fig. 5
Image Tools
Miniature EPSCs (mEPSCs) were recorded in the presence of 1 μm tetrodotoxin from neurons that also yielded current responses upon electrical stimulation. mEPSCs recorded from neurons in the BLA occurred at a frequency of 8.9 ± 1.3 Hz and had a mean amplitude of 5.6 ± 0.9 pA. Application of xenon (1.9 mm) reduced the mEPSC amplitudes (fig. 5A). Figure 5B shows the effect of xenon on cumulative distributions of mEPSC amplitudes and interevent intervals. Xenon increased the proportion of mEPSCs with smaller amplitudes, but had no effect on the distribution of interevent intervals. Figure 5C shows pooled data from five experiments. Xenon reduced the mean mEPSC amplitude to 4.1 ± 0.3 pA, whereas the mean frequency remained unchanged.
Back to Top | Article Outline

Discussion

In the current study, for the first time, brain slices were used to study effects of xenon on glutamatergic and GABAergic synaptic transmission at the single neuron level. We found that xenon concentration-dependently reduces pharmacologically isolated NMDA and AMPA receptor–mediated synaptic transmission in slices of the BLA of adult mice, while not affecting GABAergic IPSCs. Furthermore, the analysis of receptor currents elicited by photolytic uncaging of glutamate and mEPSCs revealed that the xenon-induced effects are primarily mediated by postsynaptic mechanisms.
Published values of minimum alveolar concentrations of xenon for humans range from 63 to 71 vol%.21,22 Using a published23 solubility coefficient of 0.0887 for xenon at 37°C, calculated24 minimum alveolar concentration equivalents for dissolved xenon range from 2.2 to 2.5 mm. Therefore, with a measured xenon concentration of 1.9 ± 0.3 mm dissolved in ACSF at room temperature, the amount of xenon concentration applied in our experiments lies well within the clinically relevant range. As specific test experiments in the current study and others18 demonstrated, supplying brain slices with 30% O2 is obviously sufficient to maintain complete neuronal functionality and induced no change in the physiology of the neurons.
Enhancement of GABAA receptor–mediated inhibitory synaptic transmission is considered to be a crucial mechanism for numerous general anesthetics to reversible depress central nervous system function.6,7,25 Under our experimental conditions, we observed virtually no effect of xenon on GABAA receptor–mediated IPSCs: GABAA-eIPSCs amplitudes remained unchanged, and although τdecay fast of GABAA-eIPSCs was prolonged under xenon, total charge transfer, which determines the strength of inhibition,26,27 was not changed by xenon. Enhancement of GABAergic synaptic transmission seems to be critically involved in the anesthetic mechanisms of, e.g., volatile anesthetics.6,7 It has been shown that these agents prolong the decay time constant of GABAA IPSC to 260–500% and consecutively increase charge transfer.28,29 In comparison, the prolongation of τdecay fast of GABAA-eIPSCs that was observed under xenon in the current study is considerably small (138%) and not associated with an increased charge transfer. Therefore, a relevant enhancement of GABAergic synaptic transmission by xenon under our conditions seems very unlikely.
These results are in agreement with studies demonstrating that xenon exerts no effects on isolated GABAA-IPSCs evoked from hippocampal autaptic synapses.10,11 However, for xenon, two reports8,9 describe a xenon-induced increase of Cl currents through α1β2, α1β2γ2L, or α1β2γ2S GABAA receptors that were heterologously expressed in Xenopus oocytes9 or human embryonic kidney 293 cells.8 Although by far the largest population (representing almost half of all GABAA receptors in the brain) is that containing the α1β2γ2 subunit combination,30 neither in the current study nor in that by De Sousa et al.11 was the specific subunit composition determined. It has been shown that changes in the β subunit31 or the presence of additional Δ or θ subunits32,33 substantially affect GABAA receptor response to neuroactive drugs. In the amygdala, the subunit composition of α2βnγ1 predominates.30 As such, different subunit compositions might thus explain the different sensitivity of GABAA receptors to xenon. However, one should consider that the subunit distribution in neurons of brain slices rather resembles the in vivo distribution than in defined heterologous expression systems. Overall, our data support the generally accepted hypothesis12 that xenon-induced enhancement of GABAergic inhibition does not crucially account for the xenon anesthetic state in vivo.
Nevertheless, xenon reversibly diminished basal synaptic transmission in the BLA. This might account for the anesthetic properties of xenon, which is in accord with the overall view of the anesthetic state being described as a state with reduced central nervous system excitability. At a holding potential of –70 mV, basal synaptic transmission is governed primarily by NMDA and AMPA receptor activity. Using pharmacologically isolated NMDA- and AMPA-eEPSCs, we analyzed in detail how xenon affects excitatory synaptic transmission.
Xenon induced a reversible reduction of NMDA receptor–mediated eEPSCs, whereas current kinetics remained unchanged. NMDA receptor depression might be a pivotal mechanism for mediating anesthesia and analgesia,34,35 and an NMDA receptor–depressing effect has been described for, e.g., propofol, nitrous oxide, ketamine, and some volatile anesthetics.7 A reduced NMDA receptor function was accepted to be the major mechanism for the action of xenon.10–12 Doubtlessly, when regarding single synaptic transmission, the NMDA receptor activation plays only a minor role in governing the excitatory signaling. However, frequent stimulation activates NMDA receptors, which might result in synaptic plasticity, and excessive NMDA receptor activation causes excitotoxicity. Hence, antagonizing NMDA receptors might induce acute effects, which on the one hand induce amnesia and on the other hand prevent neurotoxicity.
Whether xenon also affects non-NMDA receptors was considered to be unlikely, mainly because xenon had little effect on AMPA/kainate receptors of dissociated hippocampal neurons forming autaptic synapses.10,11 Recently, however, it has been shown that xenon diminishes current responses from heterologously expressed AMPA13 and kainate receptors,14 as well as AMPA- and kainate-induced current responses from cultivated mice cortical neurons.14 Moreover, recent experiments15 obtained from C. elegans suggest that the xenon anesthetic state might not be mediated via NMDA but via AMPA receptor depression.
In the current study, we could demonstrate antagonistic properties of xenon against non-NMDA receptor–mediated EPSCs. The generic term non-NMDA receptor comprises AMPA and kainate receptors, with the fast glutamatergic synaptic transmission being primarily mediated via AMPA receptors,36,37 as tested also in the current study when non-NMDA-eEPSCs were entirely blocked by the specific AMPA receptor antagonist GYKI52466 (data not shown). Our findings fit well with data from studies13,14 demonstrating a sensitivity of agonist-induced AMPA receptor currents to xenon.
We found that xenon concentration-dependently reduces NMDA and AMPA receptor–mediated synaptic currents with similar potency. The dose–response relation for each receptor was calculated by the four-parameter logistic equation. Because of methodologic limitations, xenon concentrations beyond 1.9 mm cannot be tested. Therefore, the respective IC50 and Hill values can only be regarded as extrapolated. Furthermore, for calculating the respective IC50 and Hill values, it has to be assumed that xenon is a full antagonist. A full antagonism of xenon on excitatory ion channels under normobaric conditions has not been shown yet. However, one cannot exclude that under hyperbaric conditions, NMDA and AMPA receptor currents can be completely blocked by xenon. Although the IC50 values for xenon against NMDA receptor–mediated currents are higher than the IC50 values for the AMPA receptor–mediated currents, this difference is not statistically significant and might result from the aforementioned limitations. Therefore, we conclude that xenon equipotently antagonizes NMDA and AMPA receptors. Nevertheless, these extrapolated IC50 values for each receptor subtype are in the range of the EC50 values for xenon calculated from in vivo data.24
Although xenon impairs NMDA- and AMPA-eEPSCs with a similar potency as described for AMPA receptor currents induced in cultured neurons or expression systems, it is still unclear whether xenon acts via presynaptic or postsynaptic mechanisms. The similar influence of xenon on electrically and photolytically evoked currents combined with the analysis of mEPSCs is in favor of a postsynaptic mechanism of xenon on excitatory synaptic transmission. Xenon reduced mEPSC amplitudes with no effect on glutamatergic mEPSC frequency, providing further evidence that xenon-induced suppression of synaptic transmission might not result from a lower frequent synaptic glutamate release. A reduction of the mEPSC amplitude might be due to blocked postsynaptic receptors and/or a reduced size of quantal transmitter release. Although we cannot distinguish between these two mechanisms, a postsynaptic mechanism seems much more likely, because xenon depressed NMDA-eEPSCs and p-NMDA-Cs, and AMPA-eEPSCs and p-AMPA-Cs to the same extent, excluding additional presynaptic mechanisms.
In summary, we showed that clinical relevant concentrations of xenon depress NMDA and AMPA receptor–mediated synaptic transmission in the BLA. Our data provide evidence that this depression might be mainly due to postsynaptic mechanisms. The amygdala, especially the basolateral nucleus, seems to play an important role in anesthetic-induced amnesia,38,39 pain processing,40 and regulation of sympathetic tone.41 Xenon is an anesthetic agent that mediates profound analgesia and amnesia with only moderate cardiovascular side effects.1 Regarding these particular properties, the observed depression of excitatory synaptic transmission in the BLA might be crucial for the xenon anesthetic state.
The authors thank Kurt Biedermann, Ph.D. (Chemist, Department of Analytics, RCC Ltd., Itingen, Switzerland), for concentration measurements of dissolved xenon and Alexander Yassouridis, Ph.D. (Biostatistician, Department of Biostatistics, Max-Planck-Institute of Psychiatry, Munich, Germany), for statistical advice.
Back to Top | Article Outline

References

1. Hecker K, Baumert JH, Horn N, Rossaint R: Xenon, a modern anaesthesia gas. Minerva Anestesiol 2004; 70:255–60

2. Lachmann B, Armbruster S, Schairer W, Landstra M, Trouwborst A, Van Daal GJ, Kusuma A, Erdmann W: Safety and efficacy of xenon in routine use as an inhalational anaesthetic. Lancet 1990; 335:1413–5

3. Rossaint R, Reyle-Hahn M, Schulte Am Esch J, Scholz J, Scherpereel P, Vallet B, Giunta F, Del Turco M, Erdmann W, Tenbrinck R, Hammerle AF, Nagele P, Xenon Study Group: Multicenter randomized comparison of the efficacy and safety of xenon and isoflurane in patients undergoing elective surgery. Anesthesiology 2003; 98:6–13

4. Wappler F, Rossaint R, Baumert J, Scholz J, Tonner PH, van Aken H, Berendes E, Klein J, Gommers D, Hammerle A, Franke A, Hofmann T, Schulte Am Esch J, Xenon Multicenter Study Research Group: Multicenter randomized comparison of xenon and isoflurane on left ventricular function in patients undergoing elective surgery. Anesthesiology 2007; 106:463–71

5. Tanelian DL, Kosek P, Mody I, MacIver MB: The role of the GABAA receptor/chloride channel complex in anesthesia. Anesthesiology 1993; 78:757–76

6. Campagna JA, Miller KW, Forman SA: Mechanisms of actions of inhaled anesthetics. N Engl J Med 2003; 348:2110–24

7. Rudolph U, Antkowiak B: Molecular and neuronal substrates for general anaesthetics. Nat Rev Neurosci 2004; 5:709–20

8. Hapfelmeier G, Zieglgänsberger W, Haseneder R, Schneck H, Kochs E: Nitrous oxide and xenon increase the efficacy of GABA at recombinant mammalian GABAA receptors. Anesth Analg 2000; 91:1542–9

9. Yamakura T, Harris RA: Effects of gaseous anesthetics nitrous oxide and xenon on ligand-gated ion channels: Comparison with isoflurane and ethanol. Anesthesiology 2000; 93:1095–101

10. Franks NP, Dickinson R, de Sousa SL, Hall AC, Lieb WR: How does xenon produce anaesthesia? Nature 1998; 396:324

11. De Sousa S, Dickinson R, Lieb WR, Franks NP: Contrasting synaptic actions of the inhalational general anesthetics isoflurane and xenon. Anesthesiology 2000; 92:1055–66

12. Preckel B, Weber NC, Sanders RD, Maze M, Schlack W: Molecular mechanisms transducing the anesthetic, analgesic, and organ-protective actions of xenon. Anesthesiology 2006; 105:187–97

13. Plested AJ, Wildman SS, Lieb WR, Franks NP: Determinants of the sensitivity of AMPA receptors to xenon. Anesthesiology 2004; 100:347–58

14. Dinse A, Föhr KJ, Georgieff M, Beyer C, Bulling A, Weigt HU: Xenon reduces glutamate-, AMPA-, and kainate-induced membrane currents in cortical neurones. Br J Anaesth 2005; 94:479–85

15. Nagele P, Metz LB, Crowder CM: Xenon acts by inhibition of non-N-methyl-d-aspartate receptor-mediated glutamatergic neurotransmission in Caenorhabditis elegans. Anesthesiology 2005; 103:508–13

16. Dodt HU, Eder M, Schierloh A, Zieglgänsberger W: Infrared-guided laser stimulation of neurons in brain slices (protocol). Sci STKE 2002; 2002:L2

17. Washburn MS, Moises HC: Electrophysiological and morphological properties of rat basolateral amygdaloid neurons in vitro. J Neurosci 1992; 12:4066–79

18. Ranft A, Kurz J, Becker K, Dodt HU, Zieglgänsberger W, Rammes G, Kochs E, Eder M: Nitrous oxide (N2O) pre- and postsynaptically attenuates NMDA receptor-mediated neurotransmission in the amygdala. Neuropharmacology 2007; 52:716–23

19. Jones MV, Westbrook GL: Desensitized states prolong GABAA channel responses to brief agonist pulses. Neuron 1995; 15:181–91

20. Krampfl K, Lepier A, Jahn K, Franke C, Bufler J: Molecular modulation of recombinant rat alpha1beta2gamma2 GABAA receptor channels by diazepam. Neurosci Lett 1998; 256:143–6

21. Cullen SC, Eger EI II, Cullen BF, Gregory P: Observations on the anesthetic effect of the combination of xenon and halothane. Anesthesiology 1969; 31:305–9

22. Nakata Y, Goto T, Ishiguro Y, Terui K, Kawakami H, Santo M, Niimi Y, Morita S: Minimum alveolar concentration (MAC) of xenon with sevoflurane in humans. Anesthesiology 2001; 94:611–4

23. Weathersby PK, Homer LD: Solubility of inert gases in biological fluids and tissues: A review. Undersea Biomed Res 1980; 7:277–96

24. Franks NP, Lieb WR: Selective actions of volatile general anaesthetics at molecular and cellular levels. Br J Anaesth 1993; 71:65–76

25. Franks NP, Lieb WR: Molecular and cellular mechanisms of general anaesthesia. Nature 1994; 367:607–14

26. Mody I, De Koninck Y, Otis TS, Soltesz I: Bridging the cleft at GABA synapses in the brain. Trends Neurosci 1994; 17:517–25

27. Hentschke H, Schwarz C, Antkowiak B: Neocortex is the major target of sedative concentrations of volatile anaesthetics: Strong depression of firing rates and increase of GABAA receptor-mediated inhibition. Eur J Neurosci 2005; 21:93–102

28. Banks MI, Pearce RA: Dual actions of volatile anesthetics on GABAA IPSCs: Dissociation of blocking and prolonging effects. Anesthesiology 1999; 90:120–34

29. Nishikawa K, MacIver MB: Agent-selective effects of volatile anesthetics on GABAA receptor–mediated synaptic inhibition in hippocampal interneurons. Anesthesiology 2001; 94:340–7

30. McKernan RM, Whiting PJ: Which GABAA-receptor subtypes really occur in the brain? Trends Neurosci 1996; 19:139–43

31. Carlson BX, Engblom AC, Kristiansen U, Schousboe A, Olsen RW: A single glycine residue at the entrance to the first membrane-spanning domain of the gamma-aminobutyric acid type A receptor beta(2) subunit affects allosteric sensitivity to GABA and anesthetics. Mol Pharmacol 2000; 57:474–84

32. Mihalek RM, Banerjee PK, Korpi ER, Quinlan JJ, Firestone LL, Mi ZP, Lagenaur C, Tretter V, Sieghart W, Anagnostaras SG, Sage JR, Fanselow MS, Guidotti A, Spigelman I, Li Z, DeLorey TM, Olsen RW, Homanics GE: Attenuated sensitivity to neuroactive steroids in gamma-aminobutyrate type A receptor delta subunit knockout mice. Proc Natl Acad Sci U S A 1999; 96:12905–10

33. Bonnert TP, McKernan RM, Farrar S, le Bourdelles B, Heavens RP, Smith DW, Hewson L, Rigby MR, Sirinathsinghji DJ, Brown N, Wafford KA, Whiting PJ: theta, a novel gamma-aminobutyric acid type A receptor subunit. Proc Natl Acad Sci U S A 1999; 96:9891–6

34. Hudspith MJ: Glutamate: A role in normal brain function, anaesthesia, analgesia and CNS injury. Br J Anaesth 1997; 78:731–47

35. Arhem P, Klement G, Nilsson J: Mechanisms of anesthesia: Towards integrating network, cellular, and molecular level modeling. Neuropsychopharmacology 2003; 28 (suppl 1):S40–7

36. Vignes M, Collingridge GL: The synaptic activation of kainate receptors. Nature 1997; 388:179–82

37. Cossart R, Epsztein J, Tyzio R, Becq H, Hirsch J, Ben-Ari Y, Crepel V: Quantal release of glutamate generates pure kainate and mixed AMPA/kainate EPSCs in hippocampal neurons. Neuron 2002; 35:147–59

38. Dutton RC, Maurer AJ, Sonner JM, Fanselow MS, Laster MJ, Eger EI: The concentration of isoflurane required to suppress learning depends on the type of learning. Anesthesiology 2001; 94:514–9

39. Alkire MT, Nathan SV: Does the amygdala mediate anesthetic-induced amnesia? Basolateral amygdala lesions block sevoflurane-induced amnesia. Anesthesiology 2005; 102:754–60

40. Neugebauer V, Li W, Bird GC, Bhave G, Gereau RW: Synaptic plasticity in the amygdala in a model of arthritic pain: Differential roles of metabotropic glutamate receptors 1 and 5. J Neurosci 2003; 23:52–63

41. Sajdyk TJ, Shekhar A: Excitatory amino acid receptor antagonists block the cardiovascular and anxiety responses elicited by GABAA receptor blockade in the basolateral amygdala of rats. J Pharmacol Exp Ther 1997; 283:969–77

Cited By:

This article has been cited 9 time(s).

Mini-Reviews in Medicinal Chemistry
Anesthesia and Postoperative Cognitive Dysfunction (POCD)
Jungwirth, B; Zieglgansberger, W; Kochs, E; Rammes, G
Mini-Reviews in Medicinal Chemistry, 9(): 1568-1579.

British Journal of Anaesthesia
Sodium channels and the synaptic mechanisms of inhaled anaesthetics
Hemmings, HC
British Journal of Anaesthesia, 103(1): 61-69.
10.1093/bja/aep144
CrossRef
Acta Neurobiologiae Experimentalis
Xenon blocks AMPA and NMDA receptor channels by different mechanisms
Weigt, HU; Fohr, KJ; Georgieff, M; Georgieff, EM; Senftleben, U; Adolph, O
Acta Neurobiologiae Experimentalis, 69(4): 429-440.

Suppressing the Mind: Anesthetic Modulation of Memory and Consciousness
Molecular Targets of General Anesthetics in the Nervous System
Hemmings, HC
Suppressing the Mind: Anesthetic Modulation of Memory and Consciousness, (): 11-31.
10.1007/978-1-60761-462-3_2
CrossRef
Molecular Pain
Xenon inhibits excitatory but not inhibitory transmission in rat spinal cord dorsal horn neurons
Georgiev, SK; Furue, H; Baba, H; Kohno, T
Molecular Pain, 6(): -.
ARTN 25
CrossRef
European Journal of Pharmacology
The xenon-mediated antagonism against the NMDA receptor is non-selective for receptors containing either NR2A or NR2B subunits in the mouse amygdala
Haseneder, R; Kratzer, S; Kochs, E; Hofelmann, D; Auberson, Y; Eder, M; Rammes, G
European Journal of Pharmacology, 619(): 33-37.
10.1016/j.ejphar.2009.08.011
CrossRef
Anesthesiology
Competitive Inhibition at the Glycine Site of the N-Methyl-d-Aspartate Receptor Mediates Xenon Neuroprotection against Hypoxia–Ischemia
Banks, P; Franks, NP; Dickinson, R
Anesthesiology, 112(3): 614-622.
10.1097/ALN.0b013e3181cea398
PDF (971) | CrossRef
Anesthesiology
Xenon Attenuates Excitatory Synaptic Transmission in the Rodent Prefrontal Cortex and Spinal Cord Dorsal Horn
Haseneder, R; Kratzer, S; Kochs, E; Mattusch, C; Eder, M; Rammes, G
Anesthesiology, 111(6): 1297-1307.
10.1097/ALN.0b013e3181c14c05
PDF (1278) | CrossRef
Anesthesiology
Xenon and the Pharmacology of Fear
Hemmings, HC; Mantz, J
Anesthesiology, 109(6): 954-955.
10.1097/ALN.0b013e31818d4964
PDF (84) | CrossRef
Back to Top | Article Outline

© 2008 American Society of Anesthesiologists, Inc.

Publication of an advertisement in Anesthesiology Online does not constitute endorsement by the American Society of Anesthesiologists, Inc. or Lippincott Williams & Wilkins, Inc. of the product or service being advertised.
Login

Article Tools

Images

Share