Robert Dickinson

How does xenon produce anaesthesia

Since the discovery that the gas xenon can produce general anaesthesia without causing undesirable side effects, we have remained surprisingly ignorant of the molecular mechanisms underlying this clinical activity of an ‘inert’ gas. Although most general anaesthetics enhance the activity of inhibitory GABAA (γ-aminobutyric acid type-A) receptors,, we find that the effects of xenon on these receptors are negligible. Instead, xenon potently inhibits the excitatory NMDA (N-methyl-D-aspartate) receptor channels, which may account for many of xenons attractive pharmacological properties.

Contrasting Synaptic Actions of the Inhalational General Anesthetics Isoflurane and Xenon

Background: The mechanisms by which the inhalational general anesthetics isoflurane and xenon exert their effects are unknown. Moreover, there have been surprisingly few quantitative studies of the effects of these agents on central synapses, with virtually no information available regarding the actions of xenon. Methods: The actions of isoflurane and xenon on &ggr;-aminobutyric acid–mediated (GABAergic) and glutamatergic synapses were investigated using voltage-clamp techniques on autaptic cultures of rat hippocampal neurons, a preparation that avoids the confounding effects of complex neuronal networks. Results: Isoflurane exerts its greatest effects on GABAergic synapses, causing a marked increase in total charge transfer (by approximately 70% at minimum alveolar concentration) through the inhibitory postsynaptic current. This effect is entirely mediated by an increase in the slow component of the inhibitory postsynaptic current. At glutamatergic synapses, isoflurane has smaller effects, but it nonetheless significantly reduces the total charge transfer (by approximately 30% at minimum alveolar concentration) through the excitatory postsynaptic current, with the N-methyl-D-aspartate (NMDA) and &agr;-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor–mediated components being roughly equally sensitive. Xenon has no measurable effect on GABAergic inhibitory postsynaptic currents or on currents evoked by exogenous application of GABA, but it substantially inhibits total charge transfer (by approximately 60% at minimum alveolar concentration) through the excitatory postsynaptic current. Xenon selectively inhibits the NMDA receptor–mediated component of the current but has little effect on the AMPA/kainate receptor–mediated component. Conclusions: For both isoflurane and xenon, the most important targets appear to be postsynaptic. The authors’ results show that isoflurane and xenon have very different effects on GABAergic and glutamatergic synaptic transmission, and this may account for their differing pharmacologic profiles.

Neuroprotection against traumatic brain injury by xenon, but not argon, is mediated by inhibition at the N-methyl-D-aspartate receptor glycine site.

Background: Xenon, the inert anesthetic gas, is neuroprotective in models of brain injury. The authors investigate the neuroprotective mechanisms of the inert gases such as xenon, argon, krypton, neon, and helium in an in vitro model of traumatic brain injury. Methods: The authors use an in vitro model using mouse organotypic hippocampal brain slices, subjected to a focal mechanical trauma, with injury quantified by propidium iodide fluorescence. Patch clamp electrophysiology is used to investigate the effect of the inert gases on N-methyl-D-aspartate receptors and TREK-1 channels, two molecular targets likely to play a role in neuroprotection. Results: Xenon (50%) and, to a lesser extent, argon (50%) are neuroprotective against traumatic injury when applied after injury (xenon 43 ± 1% protection at 72 h after injury [N = 104]; argon 30 ± 6% protection [N = 44]; mean ± SEM). Helium, neon, and krypton are devoid of neuroprotective effect. Xenon (50%) prevents development of secondary injury up to 48 h after trauma. Argon (50%) attenuates secondary injury, but is less effective than xenon (xenon 50 ± 5% reduction in secondary injury at 72 h after injury [N = 104]; argon 34 ± 8% reduction [N = 44]; mean ± SEM). Glycine reverses the neuroprotective effect of xenon, but not argon, consistent with competitive inhibition at the N-methyl-D-aspartate receptor glycine site mediating xenon neuroprotection against traumatic brain injury. Xenon inhibits N-methyl-D-aspartate receptors and activates TREK-1 channels, whereas argon, krypton, neon, and helium have no effect on these ion channels. Conclusions: Xenon neuroprotection against traumatic brain injury can be reversed by increasing the glycine concentration, consistent with inhibition at the N-methyl-D-aspartate receptor glycine site playing a significant role in xenon neuroprotection. Argon and xenon do not act via the same mechanism.

Bench-to-bedside review: Molecular pharmacology and clinical use of inert gases in anesthesia and neuroprotection

In the past decade there has been a resurgence of interest in the clinical use of inert gases. In the present paper we review the use of inert gases as anesthetics and neuroprotectants, with particular attention to the clinical use of xenon. We discuss recent advances in understanding the molecular pharmacology of xenon and we highlight specific pharmacological targets that may mediate its actions as an anesthetic and neuroprotectant. We summarize recent in vitro and in vivo studies on the actions of helium and the other inert gases, and discuss their potential to be used as neuroprotective agents.

Competitive Inhibition at the Glycine Site of the N -Methyl-d-Aspartate Receptor Mediates Xenon Neuroprotection against Hypoxia–Ischemia

Background:The general anesthetic gas xenon is neuroprotective and is undergoing clinical trials as a treatment for ischemic brain injury. A small number of molecular targets for xenon have been identified, the N-methyl-d-aspartate (NMDA) receptor, the two-pore-domain potassium channel TREK-1, and the adenosine triphosphate-sensitive potassium channel (KATP). However, which of these targets are relevant to acute xenon neuroprotection is not known. Xenon inhibits NMDA receptors by competing with glycine at the glycine-binding site. We test the hypothesis that inhibition of the NMDA receptor at the glycine site underlies xenon neuroprotection against hypoxia–ischemia. Methods:We use an in vitro model of hypoxia–ischemia to investigate the mechanism of xenon neuroprotection. Organotypic hippocampal brain slices from mice are subjected to oxygen-glucose deprivation, and injury is quantified by propidium iodide fluorescence. Results:We show that 50% atm xenon is neuroprotective against hypoxia–ischemia when applied immediately after injury or after a delay of 3 h after injury. To validate our method, we show that neuroprotection by gavestinel is abolished when glycine is added, confirming that NMDA receptor glycine site antagonism underlies gavestinel neuroprotection. We then show that adding glycine abolishes the neuroprotective effect of xenon, consistent with competitive inhibition at the NMDA receptor glycine site mediating xenon neuroprotection. Conclusions:We show that xenon neuroprotection against hypoxia– ischemia can be reversed by increasing the glycine concentration. This is consistent with competitive inhibition by xenon at the NMDA receptor glycine site, playing a significant role in xenon neuroprotection. This finding may have important implications for xenons clinical use as an anesthetic and neuroprotectant.

Xenon Improves Neurologic Outcome and Reduces Secondary Injury Following Trauma in an In Vivo Model of Traumatic Brain Injury

Objectives:To determine the neuroprotective efficacy of the inert gas xenon following traumatic brain injury and to determine whether application of xenon has a clinically relevant therapeutic time window. Design:Controlled animal study. Setting:University research laboratory. Subjects:Male C57BL/6N mice (n = 196). Interventions:Seventy-five percent xenon, 50% xenon, or 30% xenon, with 25% oxygen (balance nitrogen) treatment following mechanical brain lesion by controlled cortical impact. Measurements and Main Results:Outcome following trauma was measured using 1) functional neurologic outcome score, 2) histological measurement of contusion volume, and 3) analysis of locomotor function and gait. Our study shows that xenon treatment improves outcome following traumatic brain injury. Neurologic outcome scores were significantly (p < 0.05) better in xenon-treated groups in the early phase (24 hr) and up to 4 days after injury. Contusion volume was significantly (p < 0.05) reduced in the xenon-treated groups. Xenon treatment significantly (p < 0.05) reduced contusion volume when xenon was given 15 minutes after injury or when treatment was delayed 1 or 3 hours after injury. Neurologic outcome was significantly (p < 0.05) improved when xenon treatment was given 15 minutes or 1 hour after injury. Improvements in locomotor function (p < 0.05) were observed in the xenon-treated group, 1 month after trauma. Conclusions:These results show for the first time that xenon improves neurologic outcome and reduces contusion volume following traumatic brain injury in mice. In this model, xenon application has a therapeutic time window of up to at least 3 hours. These findings support the idea that xenon may be of benefit as a neuroprotective treatment in patients with brain trauma.

Identification of two mutations (F758W and F758Y) in the N-methyl-D-aspartate receptor glycine-binding site that selectively prevent competitive inhibition by xenon without affecting glycine binding.

Background: Xenon is a general anesthetic with neuroprotective properties. Xenon inhibition at the glycine-binding site of the N-Methyl-D-aspartate (NMDA) receptor mediates xenon neuroprotection against ischemic injury in vitro. Here we identify specific amino acids important for xenon binding to the NMDA receptor, with the aim of finding silent mutations that eliminate xenon binding but leave normal receptor function intact. Methods: Site-directed mutagenesis was used to mutate specific amino-acids in the GluN1 subunit of rat NMDA receptors. Mutant GluN1/GluN2A receptors were expressed in HEK 293 cells and were assessed functionally using patch-clamp electrophysiology. The responses of the mutant receptors to glycine and anesthetics were determined. Results: Mutation of phenylalanine 758 to an aromatic tryptophan or tyrosine left glycine affinity unchanged, but eliminated xenon binding without affecting the binding of sevoflurane or isoflurane. Conclusions: These findings confirm xenon binds to the glycine site of the GluN1 subunit of the NMDA receptor and indicate that interactions between xenon and the aromatic ring of the phenylalanine 758 residue are important for xenon binding. Our most important finding is that we have identified two mutations, F758W and F758Y, that eliminate xenon binding to the NMDA receptor glycine site without changing the glycine affinity of the receptor or the binding of volatile anesthetics. The identification of these selective mutations will allow knock-in animals to be used to dissect the mechanism(s) of xenons neuroprotective and anesthetic properties in vivo.

Molecular modeling of a tandem two pore domain potassium channel reveals a putative binding Site for general anesthetics

Anesthetics are thought to mediate a portion of their activity via binding to and modulation of potassium channels. In particular, tandem pore potassium channels (K2P) are transmembrane ion channels whose current is modulated by the presence of general anesthetics and whose genetic absence has been shown to confer a level of anesthetic resistance. While the exact molecular structure of all K2P forms remains unknown, significant progress has been made toward understanding their structure and interactions with anesthetics via the methods of molecular modeling, coupled with the recently released higher resolution structures of homologous potassium channels to act as templates. Such models reveal the convergence of amino acid regions that are known to modulate anesthetic activity onto a common three- dimensional cavity that forms a putative anesthetic binding site. The model successfully predicts additional important residues that are also involved in the putative binding site as validated by the results of suggested experimental mutations. Such a model can now be used to further predict other amino acid residues that may be intimately involved in the target-based structure–activity relationships that are necessary for anesthetic binding.

Modelling Blast Brain Injury

The consequences of blast traumatic brain injury (blast-TBI) in humans are largely determined by the characteristics of the trauma insult and, within certain limits, the individual responses to the lesions inflicted [1]. In blast-TBI the mechanisms of brain vulnerability to the detonation of an explosive device are not entirely understood. They most likely result from a combination of the different physical aspects of the blast phenomenon, specifically extreme pressure oscillations (blast-overpressure wave – primary blast), projectile penetrating fragments (secondary blast) and acceleration-deceleration forces (tertiary blast), creating a spectrum of brain injury that ranges from mild to severe blast-TBI [2]. The pathophysiology of penetrating and inertially-driven blast-TBI has been extensively investigated for many years. However, the brain damage caused by blast-overpressure (primary blast) is much less understood and is unique to this type of TBI [3]. Indeed, there continues to be debate about how the pressure wave is transmitted and reflected through the brain and how it causes cellular damage [4]. No single model can mimic the clinical and mechanical complexity resulting from a real life blast-TBI [3]. The different models, non-biological (in silico or surrogate physical) and biological (ex vivo, in vitro or in vivo), tend to complement each other.

Argon: A Noble Foe for Subarachnoid Hemorrhage

Anaesthetics, Pain Medicine and Intensive Care Section, Department of Surgery and Cancer, Imperial College London, United Kingdom. Department of Life Sciences, Imperial College London, United Kingdom. # Correspondence should be addressed to: Dr Robert Dickinson, Department of Surgery & Cancer, Imperial College London, Sir Ernst Chain Building, South Kensington, London SW7 2AZ, UK. Tel: + 44 (0) 20 7594 7633 Fax: +44 (0) 20 7594 7628. E-mail: r.dickinson@imperial.ac.uk