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 [gamma]-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 [alpha]-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.     

Selective Synaptic Actions of Thiopental and Its Enantiomers

Background: There is conflicting evidence concerning the extent to which the intravenous general anesthetic thiopental acts by enhancing inhibitory [gamma]-aminobutyric acid-mediated (GABAergic) synaptic transmission or by inhibiting excitatory glutamatergic transmission. Yet there are remarkably few studies on the effects of thiopental on functional synapses. In addition, the degree of stereoselectivity of thiopental acting at synapses has yet to be tested.

Methods: The actions of thiopental and its enantiomers on GABAergic and glutamatergic synapses were investigated using voltage clamp techniques on microisland cultures of rat hippocampal neurons, a preparation that avoids the confounding effects of complex neuronal networks.

Results: Racemic thiopental markedly enhanced the charge transfer at GABAergic synapses without significantly affecting the peak of the postsynaptic current. At a surgically relevant concentration (25 [mu]m), charge transfer was increased by approximately 230%. However, even at twice this concentration there were no significant effects on glutamatergic postsynaptic currents. At GABAergic synapses, thiopental acted stereoselectively, with the S(-) enantiomer being approximately twice as effective as the R(+) enantiomer at enhancing charge transfer.     

Selective synaptic actions of thiopental and its enantiomers

BACKGROUND: There is conflicting evidence concerning the extent to which the intravenous general anesthetic thiopental acts by enhancing inhibitory gamma-aminobutyric acid-mediated (GABAergic) synaptic transmission or by inhibiting excitatory glutamatergic transmission. Yet there are remarkably few studies on the effects of thiopental on functional synapses. In addition, the degree of stereoselectivity of thiopental acting at synapses has yet to be tested. METHODS: The actions of thiopental and its enantiomers on GABAergic and glutamatergic synapses were investigated using voltage clamp techniques on microisland cultures of rat hippocampal neurons, a preparation that avoids the confounding effects of complex neuronal networks. RESULTS: Racemic thiopental markedly enhanced the charge transfer at GABAergic synapses without significantly affecting the peak of the postsynaptic current. At a surgically relevant concentration (25 microm), charge transfer was increased by approximately 230%. However, even at twice this concentration there were no significant effects on glutamatergic postsynaptic currents. At GABAergic synapses, thiopental acted stereoselectively, with the S(-) enantiomer being approximately twice as effective as the R(+) enantiomer at enhancing charge transfer. CONCLUSIONS: Thiopental stereoselectively enhances inhibitory GABAergic synaptic transmission in a way that reflects animal potencies, supporting the idea that this is a principal mode of action for this drug. The absence of any effect on glutamatergic synapses at surgically relevant concentrations suggests that the inhibition of these excitatory synapses is not an important factor in producing thiopental general anesthesia.     

Sevoflurane depresses glutamatergic neurotransmission to brainstem inspiratory premotor neurons but not postsynaptic receptor function in a decerebrate dog model

BACKGROUND: Inspiratory bulbospinal neurons in the caudal ventral medulla are premotor neurons that drive motoneurons, which innervate pump muscles such as the diaphragm and external intercostals. Excitatory drive to these neurons is mediated by N-methyl-d-aspartate (NMDA) receptors and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors and is modulated by an inhibitory gamma-aminobutyric acid type A (GABAA)ergic input. The authors investigated the effect of sevoflurane on these synaptic mechanisms in decerebrate dogs. METHODS: Studies were performed in decerebrate, vagotomized, paralyzed, and mechanically ventilated dogs during hypercapnic hyperoxia. The effect of 1 minimum alveolar concentration sevoflurane on extracellularly recorded activity of single neurons was measured during localized picoejection of the GABAA receptor blocker bicuculline and the glutamate agonists AMPA and NMDA. Complete blockade of the GABAAergic mechanism by bicuculline allowed differentiation between the effects of sevoflurane on overall GABAAergic inhibition and on overall glutamatergic excitation. The neuronal responses to exogenous AMPA and NMDA were used to estimate the anesthetic effect on postsynaptic glutamatergic neurotransmission. RESULTS: One minimum alveolar concentration sevoflurane depressed the spontaneous activity of 23 inspiratory premotor neurons by (mean +/- SD) 30.0 +/- 21.0% (P < 0.001). Overall glutamatergic excitation was depressed 19.2 +/- 18.5% (P < 0.001), whereas overall GABAAergic inhibition was enhanced by 11.9 +/- 25.1% (P < 0.05). The postsynaptic responses to exogenous AMPA and NMDA did not change. CONCLUSION: One minimum alveolar concentration depressed the activity of inspiratory premotor neurons by a reduction of glutamatergic excitation and an increase in overall inhibition. The postsynaptic AMPA and NMDA receptor response was unchanged. These findings contrast with studies in inspiratory premotor neurons where halothane did not change overall inhibition but significantly reduced the postsynaptic glutamate receptor response.     

Sevoflurane Depresses Glutamatergic Neurotransmission to Brainstem Inspiratory Premotor Neurons but Not Postsynaptic Receptor Function in a Decerebrate Dog Model

Background: Inspiratory bulbospinal neurons in the caudal ventral medulla are premotor neurons that drive motoneurons, which innervate pump muscles such as the diaphragm and external intercostals. Excitatory drive to these neurons is mediated by N-methyl-d-aspartate (NMDA) receptors and [alpha]-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors and is modulated by an inhibitory [gamma]-aminobutyric acid type A (GABAA)ergic input. The authors investigated the effect of sevoflurane on these synaptic mechanisms in decerebrate dogs.

Methods: Studies were performed in decerebrate, vagotomized, paralyzed, and mechanically ventilated dogs during hypercapnic hyperoxia. The effect of 1 minimum alveolar concentration sevoflurane on extracellularly recorded activity of single neurons was measured during localized picoejection of the GABAA receptor blocker bicuculline and the glutamate agonists AMPA and NMDA. Complete blockade of the GABAAergic mechanism by bicuculline allowed differentiation between the effects of sevoflurane on overall GABAAergic inhibition and on overall glutamatergic excitation. The neuronal responses to exogenous AMPA and NMDA were used to estimate the anesthetic effect on postsynaptic glutamatergic neurotransmission.

Results: One minimum alveolar concentration sevoflurane depressed the spontaneous activity of 23 inspiratory premotor neurons by (mean +/- SD) 30.0 +/- 21.0% (P < 0.001). Overall glutamatergic excitation was depressed 19.2 +/- 18.5% (P < 0.001), whereas overall GABAAergic inhibition was enhanced by 11.9 +/- 25.1% (P < 0.05). The postsynaptic responses to exogenous AMPA and NMDA did not change.     

Halothane depresses glutamatergic neurotransmission to brain stem inspiratory premotor neurons in a decerebrate dog model

BACKGROUND: Inspiratory bulbospinal neurons in the caudal ventral medulla are premotor neurons that drive phrenic motoneurons and ultimately the diaphragm. Excitatory drive to these neurons is mediated by N-methyl-d-aspartate (NMDA) receptors and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors and modulated by an inhibitory gamma-aminobutyric acid(A) (GABA(A))ergic input. The authors investigated the effect of halothane on these synaptic mechanisms in decerebrate dogs. METHODS: Studies were performed in decerebrate, vagotomized, paralyzed, and mechanically ventilated dogs during hypercapnic hyperoxia. The effect of 1 minimum alveolar concentration (MAC) halothane on extracellularly recorded neuronal activity was measured during localized picoejection of the GABA(A) receptor blocker bicuculline and the glutamate agonists AMPA and NMDA. Complete blockade of the GABA(A)ergic mechanism by bicuculline allowed differentiation between the effects of halothane on overall GABA(A)ergic inhibition and on overall glutamatergic excitation. The neuronal responses to exogenous AMPA and NMDA were used to estimate the anesthetic effect on postsynaptic glutamatergic neurotransmission. RESULTS: Halothane, 1 MAC, depressed the spontaneous activity of 21 inspiratory neurons by 20.6 +/- 18.0% (mean +/- SD; P = 0.012). Overall glutamatergic excitation was depressed 15.4 +/- 20.2% (P = 0.001), while overall GABA(A)ergic inhibition did not change. The postsynaptic responses to exogenous AMPA and NMDA were also depressed by 18.6 +/- 35.7% (P = 0.03) and 22.2 +/- 26.2% (P = 0.004), respectively. CONCLUSION: Halothane, 1 MAC, depressed the activity of inspiratory premotor neurons by a reduction of glutamatergic excitation. Overall inhibitory drive did not change. The postsynaptic AMPA and NMDA receptor response was significantly reduced. These findings contrast with studies in expiratory premotor neurons in which overall inhibition was significantly increased by halothane and there was no reduction in the postsynaptic glutamate receptor response.     

Effects of Halothane on Excitatory Neurotransmission to Medullary Expiratory Neurons in a Decerebrate Dog Model

Background: The activity of canine expiratory (E) neurons in the caudal ventral respiratory group is primarily dependent on N-methyl-d-aspartic acid (NMDA) receptor-mediated excitatory chemodrive inputs and modulated by an inhibitory mechanism mediated via [gamma]-aminobutyric acidA (GABAA) receptors. In an intact canine preparation, halothane depressed the activity of these neurons mainly by reduction in overall glutamatergic excitation. A new decerebrate preparation allows comparison of the effects of halothane on these synaptic mechanisms with an anesthetic-free baseline state.

Methods: Two separate studies were performed in decerebrate, vagotomized, paralyzed, mechanically ventilated dogs during hypercapnic hyperoxia. In study 1, the effect of 1 minimum alveolar concentration (MAC) halothane on extracellularly recorded E neuronal activity was studied before and during complete GABAA receptor blockade by localized pressure ejection of bicuculline. Complete blockade of the inhibitory mechanism allowed differentiation between the effects of halothane on overall GABAA-mediated inhibition and on overall NMDA receptor-mediated excitation. In study 2, the effect of 1 MAC halothane on the dose response of neurons to localized picoejection of the glutamate agonist NMDA was used to estimate halothane effect on postsynaptic glutamatergic excitatory neurotransmission.

Results: In study 1, the spontaneous activity of 14 E neurons was depressed 38.6 +/- 20.6% (mean +/- SD) by 1 MAC halothane. Overall excitation was depressed 31.5 +/- 15.5%. The GABAergic inhibition showed a 11.7 +/- 18.3% enhancement during halothane. In study 2, the spontaneous activity of 13 E neurons was again significantly depressed by 1 MAC halothane (27.9 +/- 10.6%), but the postsynaptic response of the neurons to exogenous NMDA was not significantly depressed by halothane (3.3 +/- 38.4%).     

Halothane Depresses Glutamatergic Neurotransmission to Brain Stem Inspiratory Premotor Neurons in a Decerebrate Dog Model

Background: Inspiratory bulbospinal neurons in the caudal ventral medulla are premotor neurons that drive phrenic motoneurons and ultimately the diaphragm. Excitatory drive to these neurons is mediated by N-methyl-d-aspartate (NMDA) receptors and [alpha]-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors and modulated by an inhibitory [gamma]-aminobutyric acidA (GABAA)ergic input. The authors investigated the effect of halothane on these synaptic mechanisms in decerebrate dogs.

Methods: Studies were performed in decerebrate, vagotomized, paralyzed, and mechanically ventilated dogs during hypercapnic hyperoxia. The effect of 1 minimum alveolar concentration (MAC) halothane on extracellularly recorded neuronal activity was measured during localized picoejection of the GABAA receptor blocker bicuculline and the glutamate agonists AMPA and NMDA. Complete blockade of the GABAAergic mechanism by bicuculline allowed differentiation between the effects of halothane on overall GABAAergic inhibition and on overall glutamatergic excitation. The neuronal responses to exogenous AMPA and NMDA were used to estimate the anesthetic effect on postsynaptic glutamatergic neurotransmission.

Results: Halothane, 1 MAC, depressed the spontaneous activity of 21 inspiratory neurons by 20.6 +/- 18.0% (mean +/- SD;P = 0.012). Overall glutamatergic excitation was depressed 15.4 +/- 20.2% (P = 0.001), while overall GABAAergic inhibition did not change. The postsynaptic responses to exogenous AMPA and NMDA were also depressed by 18.6 +/- 35.7% (P = 0.03) and 22.2 +/- 26.2% (P = 0.004), respectively.     

Effects of halothane on excitatory neurotransmission to medullary expiratory neurons in a decerebrate dog model

BACKGROUND: The activity of canine expiratory (E) neurons in the caudal ventral respiratory group is primarily dependent on N-methyl-D-aspartic acid (NMDA) receptor-mediated excitatory chemodrive inputs and modulated by an inhibitory mechanism mediated via gamma-aminobutyric acidA (GABA(A)) receptors. In an intact canine preparation, halothane depressed the activity of these neurons mainly by reduction in overall glutamatergic excitation. A new decerebrate preparation allows comparison of the effects of halothane on these synaptic mechanisms with an anesthetic-free baseline state. METHODS: Two separate studies were performed in decerebrate, vagotomized, paralyzed, mechanically ventilated dogs during hypercapnic hyperoxia. In study 1, the effect of 1 minimum alveolar concentration (MAC) halothane on extracellularly recorded E neuronal activity was studied before and during complete GABA(A) receptor blockade by localized pressure ejection of bicuculline. Complete blockade of the inhibitory mechanism allowed differentiation between the effects of halothane on overall GABA(A)-mediated inhibition and on overall NMDA receptor-mediated excitation. In study 2, the effect of 1 MAC halothane on the dose response of neurons to localized picoejection of the glutamate agonist NMDA was used to estimate halothane effect on postsynaptic glutamatergic excitatory neurotransmission. RESULTS: In study 1, the spontaneous activity of 14 E neurons was depressed 38.6 +/- 20.6% (mean +/- SD) by 1 MAC halothane. Overall excitation was depressed 31.5 +/- 15.5%. The GABAergic inhibition showed a 11.7 +/- 18.3% enhancement during halothane. In study 2, the spontaneous activity of 13 E neurons was again significantly depressed by 1 MAC halothane (27.9 +/- 10.6%), but the postsynaptic response of the neurons to exogenous NMDA was not significantly depressed by halothane (3.3 +/- 38.4%). CONCLUSIONS: Together these results suggest that in our E neuron paradigm, halothane exerted its depressive effect mainly via reduction of glutamatergic presynaptic mechanisms.     

Xenon acts by inhibition of non-N-methyl-D-aspartate receptor-mediated glutamatergic neurotransmission in Caenorhabditis elegans

BACKGROUND: Electrophysiologic experiments in rodents have found that nitrous oxide and xenon inhibit N-methyl-D-aspartate (NMDA)-type glutamate receptors. These findings led to the hypothesis that xenon and nitrous oxide along with ketamine form a class of anesthetics with the identical mechanism, NMDA receptor antagonism. Here, the authors ask in Caenorhabditis elegans whether xenon, like nitrous oxide, acts by a NMDA receptor-mediated mechanism. METHODS: Xenon:oxygen mixtures were delivered into sealed chambers until the desired concentration was achieved. The effects of xenon on various behaviors were measured on wild-type and mutant C. elegans strains. RESULTS: With an EC50 of 15-20 vol% depending on behavioral endpoint, xenon altered C. elegans locomotion in a manner indistinguishable from that of mutants in glutamatergic transmission. Xenon reduced the frequency and duration of backward locomotion without altering its speed or other behaviors tested. Mutation of glr-1, encoding a non-NMDA glutamate receptor subunit, abolished the behavioral effects of xenon; however, mutation of nmr-1, which encodes the pore-forming subunit of an NMDA glutamate receptor previously shown to be required for nitrous oxide action, did not significantly alter xenon response. Transformation of the glr-1 mutant with the wild-type glr-1 gene partially restored xenon sensitivity, confirming that glr-1 was necessary for the full action of xenon. CONCLUSIONS: Xenon acts in C. elegans to alter locomotion through a mechanism requiring the non-NMDA glutamate receptor encoded by glr-1. Unlike for the action of nitrous oxide in C. elegans, the NMDA receptor encoded by nmr-1 is not essential for sensitivity to xenon.     

Xenon Acts by Inhibition of Non-N-methyl-d-aspartate Receptor-mediated Glutamatergic Neurotransmission in Caenorhabditis elegans

Background: Electrophysiologic experiments in rodents have found that nitrous oxide and xenon inhibit N-methyl-d-aspartate (NMDA)-type glutamate receptors. These findings led to the hypothesis that xenon and nitrous oxide along with ketamine form a class of anesthetics with the identical mechanism, NMDA receptor antagonism. Here, the authors ask in Caenorhabditis elegans whether xenon, like nitrous oxide, acts by a NMDA receptor-mediated mechanism.

Methods: Xenon:oxygen mixtures were delivered into sealed chambers until the desired concentration was achieved. The effects of xenon on various behaviors were measured on wild-type and mutant C. elegans strains.

Results: With an EC50 of 15-20 vol% depending on behavioral endpoint, xenon altered C. elegans locomotion in a manner indistinguishable from that of mutants in glutamatergic transmission. Xenon reduced the frequency and duration of backward locomotion without altering its speed or other behaviors tested. Mutation of glr-1, encoding a non-NMDA glutamate receptor subunit, abolished the behavioral effects of xenon; however, mutation of nmr-1, which encodes the pore-forming subunit of an NMDA glutamate receptor previously shown to be required for nitrous oxide action, did not significantly alter xenon response. Transformation of the glr-1 mutant with the wild-type glr-1 gene partially restored xenon sensitivity, confirming that glr-1 was necessary for the full action of xenon.     

Action of Isoflurane on the Substantia Gelatinosa Neurons of the Adult Rat Spinal Cord

Background: Although isoflurane, a volatile anesthetic, can block the motor response to noxious stimulation (immobility and analgesia) and suppress autonomic responsiveness, how it exerts these effects at the neuronal level in the spinal cord is not fully understood.

Methods: The effects of a clinically relevant concentration (1 rat minimum alveolar concentration [MAC]) of isoflurane on electrically evoked and spontaneous excitatory/inhibitory transmission and on the response to exogenous administration of the [gamma]-aminobutyric acid type A receptor agonist muscimol were examined in lamina II neurons of adult rat spinal cord slices using the whole cell patch clamp technique. The effect of isoflurane on the action potential-generating membrane property was also examined.

Results: Bath-applied isoflurane (1.5%, 1 rat MAC) diminished dorsal root-evoked polysynaptic but not monosynaptic excitatory postsynaptic currents. Glutamatergic miniature excitatory postsynaptic currents were also unaffected by isoflurane. In contrast, isoflurane prolonged the decay phase of evoked and miniature [gamma]-aminobutyric acid type A receptor-mediated inhibitory postsynaptic currents and increased the amplitude of the muscimol-induced current. Isoflurane had little effect on action potential discharge activity.     

Competitive inhibition at the glycine site of the N-methyl-D-aspartate receptor by the anesthetics xenon and isoflurane: evidence from molecular modeling and electrophysiology

BACKGROUND: Inhibition of N-methyl-D-aspartate (NMDA) receptors by anesthetic gases and vapors may play an important role in anesthesia and neuroprotection. However, the site of action of these agents on the NMDA receptor is unknown. The authors show that xenon and isoflurane compete for the binding of the coagonist glycine on the NMDA receptor NR1 subunit. METHODS: Using a novel application of grand canonical Monte Carlo simulations, the authors predict the binding site of xenon on NMDA receptors. They test this prediction using electrophysiology on recombinant NMDA receptors. RESULTS: The authors' modeling predicts that xenon binds at the glycine site of the NMDA receptor. The authors show that inhibition of NMDA receptors by xenon and isoflurane increases as glycine concentration is decreased, consistent with the prediction of competitive inhibition at the glycine site. Lineweaver-Burk analysis shows that isoflurane inhibition seems purely competitive with glycine, but for xenon, there is an additional component of noncompetitive inhibition. The loss of inhibitory effect of xenon and isoflurane in mutant NR1(F639A)/NR2A receptors is explained by increased glycine affinity of the mutant receptors, and inhibition is restored at low glycine concentrations. CONCLUSIONS: Xenon and isoflurane inhibit NMDA receptors by binding at the same site as the coagonist glycine. This finding may have important implications for general anesthesia and neuroprotection. Neuroprotectants that act at the glycine site of the NMDA receptor antagonists are well tolerated in patients, being devoid of psychotomimetic side effects, and the mechanism of inhibition may play a role in their clinical profile.     

Action of isoflurane on the substantia gelatinosa neurons of the adult rat spinal cord

BACKGROUND: Although isoflurane, a volatile anesthetic, can block the motor response to noxious stimulation (immobility and analgesia) and suppress autonomic responsiveness, how it exerts these effects at the neuronal level in the spinal cord is not fully understood. METHODS: The effects of a clinically relevant concentration (1 rat minimum alveolar concentration [MAC]) of isoflurane on electrically evoked and spontaneous excitatory/inhibitory transmission and on the response to exogenous administration of the gamma-aminobutyric acid type A receptor agonist muscimol were examined in lamina II neurons of adult rat spinal cord slices using the whole cell patch clamp technique. The effect of isoflurane on the action potential-generating membrane property was also examined. RESULTS: Bath-applied isoflurane (1.5%, 1 rat MAC) diminished dorsal root-evoked polysynaptic but not monosynaptic excitatory postsynaptic currents. Glutamatergic miniature excitatory postsynaptic currents were also unaffected by isoflurane. In contrast, isoflurane prolonged the decay phase of evoked and miniature gamma-aminobutyric acid type A receptor-mediated inhibitory postsynaptic currents and increased the amplitude of the muscimol-induced current. Isoflurane had little effect on action potential discharge activity. CONCLUSIONS: Isoflurane augments gamma-aminobutyric acid-mediated inhibitory transmission, leading to a decrease in the excitability of spinal dorsal horn neurons. This may be a possible mechanism for the antinociceptive effect of isoflurane in the spinal cord.     

Determinants of the Sensitivity of AMPA Receptors to Xenon

Background: There is substantial and growing literature on the actions of general anesthetics on a variety of neurotransmitter-gated ion channels, with the greatest attention being focused on inhibitory [gamma]-amino butyric acid type A receptors. In contrast, glutamate receptors, the most important class of fast excitatory neurotransmitter-gated receptor channels, have received much less attention, and their role in the production of the anesthetic state remains controversial.

Methods: [alpha]-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors formed from a variety of different subunits were expressed in Xenopus oocytes and HEK-293 cells, and their sensitivities to the inhalational general anesthetics xenon, isoflurane, and halothane were determined using two-electrode voltage clamp and patch clamp techniques. The effects of desensitization on anesthetic sensitivity were investigated using cyclothiazide and site-directed mutagenesis. An ultrarapid application system was also used to mimic rapid high-concentration glutamate release at synapses.

Results: The authors show that xenon can potently inhibit AMPA receptors when assayed using bath application of kainate. However, when the natural neurotransmitter l-glutamate is used under conditions in which the receptor desensitization is blocked and the peak of the glutamate-activated response can be accurately measured, the pattern of inhibition changes markedly. When desensitization is abolished by a single-point mutation (L497Y in GluR1 and the equivalent mutation L505Y in GluR4), the xenon inhibition is eliminated. When AMPA receptors are activated by glutamate using an ultrarapid application system that mimics synaptic conditions, sensitivity to xenon, halothane, and isoflurane is negligible.     

Thermoregulatory thresholds for vasoconstriction in patients anesthetized with various 1-minimum alveolar concentration combinations of xenon, nitrous oxide, and isoflurane.

BACKGROUND: Nitrous oxide limits intraoperative hypothermia because the vasoconstriction threshold with nitrous oxide is higher than with equi-minimum alveolar concentrations of sevoflurane or isoflurane, presumably because of its stimulating actions on the sympathetic nervous system. Xenon, in contrast, does not cause sympathetic activation. Therefore, the authors tested the hypothesis that the vasoconstriction threshold during xenon-isoflurane anesthesia is less than during nitrous oxide-isoflurane anesthesia or isoflurane alone. METHODS: Fifteen patients each were randomly assigned to one of three 1-minimum alveolar concentration anesthetic regimens: (1) xenon, 43% (0.6 minimum alveolar concentration) and isoflurane, 0.5% (0.4 minimum alveolar concentration); (2) nitrous oxide, 63% (0.6 minimum alveolar concentration) and isoflurane 0.5%; or (3) isoflurane, 1.2%. Ambient temperature was maintained near 23 degrees C and the patients were not actively warmed. Thermoregulatory vasoconstriction was evaluated using forearm-minus-fingertip skin temperature gradients. A gradient exceeding 0 degrees C indicated significant vasoconstriction. The core-temperature threshold that would have been observed if skin had been maintained at 33 degrees C was calculated from mean skin and distal esophageal temperatures at the time of vasoconstriction. RESULTS: The patients' demographic variables, preinduction core temperatures, ambient operating room temperatures, and fluid balance were comparable among the three groups. Heart rates were significantly less during xenon anesthesia than with nitrous oxide. The calculated vasoconstriction threshold was lowest with xenon (34.6+/-0.8 degrees C, mean +/- SD), intermediate with isoflurane alone (35.1+/-0.6 degrees C), and highest with nitrous oxide (35.7+/-0.6 degrees C). Each of the thresholds differed significantly. CONCLUSIONS: Xenon inhibits thermoregulatory control more than isoflurane, whereas nitrous oxide is the least effective in this respect.     

Agent-selective effects of volatile anesthetics on GABAA receptor-mediated synaptic inhibition in hippocampal interneurons

BACKGROUND: A relatively small number of inhibitory interneurons can control the excitability and synchronization of large numbers of pyramidal cells in hippocampus and other cortical regions. Thus, anesthetic modulation of interneurons could play an important role for the maintenance of anesthesia. The aim of this study was to compare effects produced by volatile anesthetics on inhibitory postsynaptic currents (IPSCs) of rat hippocampal interneurons. METHODS: Pharmacologically isolated gamma-aminobutyric acid type A (GABAA) receptor-mediated IPSCs were recorded with whole cell patch-clamp techniques in visually identified interneurons of rat hippocampal slices. Neurons located in the stratum radiatum-lacunosum moleculare of the CA1 region were studied. The effects of clinically relevant concentrations (1.0 rat minimum alveolar concentration) of halothane, enflurane, isoflurane, and sevoflurane were compared on kinetics of both stimulus-evoked and spontaneous GABAA receptor-mediated IPSCs in interneurons. RESULTS: Halothane (1.2 vol% approximately 0.35 mm), enflurane (2.2 vol% approximately 0.60 mm), isoflurane (1.4 vol% approximately 0.50 mm), and sevoflurane (2.7 vol% approximately 0.40 mm) preferentially depressed evoked IPSC amplitudes to 79.8 +/- 9.3% of control (n = 5), 38.2 +/- 8.6% (n = 6), 52.4 +/- 8.4% (n = 5), and 46.1 +/- 16.0% (n = 8), respectively. In addition, all anesthetics differentially prolonged the decay time constant of evoked IPSCs to 290.1 +/- 33.2% of control, 423.6 +/- 47.1, 277.0 +/- 32.2, and 529 +/- 48.5%, respectively. The frequencies of spontaneous IPSCs were increased by all anesthetics (twofold to threefold). Thus, the total negative charge transfer mediated by GABAA receptors between synaptically connected interneurons was enhanced by all anesthetics. CONCLUSIONS: Volatile anesthetics differentially enhanced GABAA receptor-mediated synaptic inhibition in rat hippocampal interneurons, suggesting that hippocampal interneuron circuits are depressed by these anesthetics in an agent-specific manner.     

Positron emission tomography study of regional cerebral metabolism during general anesthesia with xenon in humans

BACKGROUND: The precise mechanism by which the gaseous anesthetic xenon exerts its effects in the human brain remains unknown. Xenon has only negligible effects on inhibitory gamma-aminobutyric acid receptors, one of the putative molecular targets for most general anesthetics. Instead, xenon has been suggested to induce anesthesia by inhibiting excitatory glutamatergic signaling. Therefore, the authors hypothesized that xenon, similar to ketamine and nitrous oxide, increases global and regional cerebral metabolism in humans. METHODS: The regional cerebral metabolic rate of glucose (rcMRGlu) was sequentially assessed in two groups of six volunteers each, using F-fluorodeoxyglucose as tracer. In the xenon group, rcMRGlu was determined at baseline and during general anesthesia induced with propofol and maintained with 1 minimum alveolar concentration xenon. In the control group, rcMRGlu was measured using the identical study protocol but without administration of xenon. rcMRGlu was assessed after the plasma concentration of propofol had decreased to subanesthetic levels (< 1.0 microg/ml). rcMRGlu was quantified in 10 cerebral volumes of interest. In addition, voxel-wise changes in rcMRGlu were analyzed using statistical parametric mapping. RESULTS: Xenon reduced whole-brain metabolic rate of glucose by 26 +/- 7% (from 43 +/- 5 micromol x 100 g x min to 31 +/- 3 micromol x 100 g x min; P < 0.005) and significantly decreased rcMRGlu in all volumes of interest compared with the control group receiving propofol only. Voxel-based analysis revealed metabolic depression within the orbitofrontal, frontomesial, temporomesial, occipital, dorsolateral frontal, and lateral temporal cortices and thalami. No increases in rcMRGlu were detected during xenon anesthesia. CONCLUSIONS: Xenon induces metabolic depression in the human brain, suggesting that the inhibition of the glutamatergic system is likely to be of minor significance for the anesthetic action of xenon in vivo.     

Effects of gaseous anesthetics nitrous oxide and xenon on ligand-gated ion channels. Comparison with isoflurane and ethanol

BACKGROUND: Ligand-gated ion channels are considered to be potential general anesthetic targets. Although most general anesthetics potentiate the function of gamma-aminobutyric acid receptor type A (GABAA), the gaseous anesthetics nitrous oxide and xenon are reported to have little effect on GABAA receptors but inhibit N-methyl-d-aspartate (NMDA) receptors. To define the spectrum of effects of nitrous oxide and xenon on receptors thought to be important in anesthesia, the authors tested these anesthetics on a variety of recombinant brain receptors. METHODS: The glycine, GABAA, GABA receptor type C (GABAC), NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainate, 5-hydroxytryptamine3 (5-HT3), and nicotinic acetylcholine (nACh) receptors were expressed in Xenopus oocytes and effects of nitrous oxide and xenon, and as equipotent concentrations of isoflurane and ethanol, were studied using the two-electrode voltage clamp. RESULTS: Nitrous oxide (0.58 atmosphere [atm]) and xenon (0.46 atm) exhibited similar effects on various receptors. Glycine and GABAA receptors were potentiated by gaseous anesthetics much less than by isoflurane, whereas nitrous oxide inhibited GABAC receptors. Glutamate receptors were inhibited by gaseous anesthetics more markedly than by isoflurane, but less than by ethanol. NMDA receptors were the most sensitive among glutamate receptors and were inhibited by nitrous oxide by 31%. 5-HT3 receptors were slightly inhibited by nitrous oxide. The nACh receptors were inhibited by gaseous and volatile anesthetics, but ethanol potentiated them. The sensitivity was different between alpha4beta2 and alpha4beta4 nACh receptors; alpha4beta2 receptors were inhibited by nitrous oxide by 39%, whereas alpha4beta4 receptors were inhibited by 7%. The inhibition of NMDA and nACh receptors by nitrous oxide was noncompetitive and was slightly different depending on membrane potentials for NMDA receptors, but not for nACh receptors. CONCLUSIONS: Nitrous oxide and xenon displayed a similar spectrum of receptor actions, but this spectrum is distinct from that of isoflurane or ethanol. These results suggest that NMDA receptors and nACh receptors composed of beta2 subunits are likely targets for nitrous oxide and xenon.     

Determinants of the sensitivity of AMPA receptors to xenon

BACKGROUND: There is substantial and growing literature on the actions of general anesthetics on a variety of neurotransmitter-gated ion channels, with the greatest attention being focused on inhibitory gamma-amino butyric acid type A receptors. In contrast, glutamate receptors, the most important class of fast excitatory neurotransmitter-gated receptor channels, have received much less attention, and their role in the production of the anesthetic state remains controversial. METHODS: alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors formed from a variety of different subunits were expressed in Xenopus oocytes and HEK-293 cells, and their sensitivities to the inhalational general anesthetics xenon, isoflurane, and halothane were determined using two-electrode voltage clamp and patch clamp techniques. The effects of desensitization on anesthetic sensitivity were investigated using cyclothiazide and site-directed mutagenesis. An ultrarapid application system was also used to mimic rapid high-concentration glutamate release at synapses. RESULTS: The authors show that xenon can potently inhibit AMPA receptors when assayed using bath application of kainate. However, when the natural neurotransmitter l-glutamate is used under conditions in which the receptor desensitization is blocked and the peak of the glutamate-activated response can be accurately measured, the pattern of inhibition changes markedly. When desensitization is abolished by a single-point mutation (L497Y in GluR1 and the equivalent mutation L505Y in GluR4), the xenon inhibition is eliminated. When AMPA receptors are activated by glutamate using an ultrarapid application system that mimics synaptic conditions, sensitivity to xenon, halothane, and isoflurane is negligible. CONCLUSIONS: AMPA receptors, when assayed in heterologous expression systems, showed a sensitivity to inhalational anesthetics that was minimal when glutamate was applied rapidly at high concentrations. Because these are the conditions that are most relevant to synaptic transmission, the authors conclude that AMPA receptors are unlikely to play a major role in the production of the anesthetic state by inhalational agents.