BJA Advance Access originally published online on September 12, 2008
British Journal of Anaesthesia 2008 101(5):673-679; doi:10.1093/bja/aen266
Carbon dioxide negatively modulates N-methyl-D-aspartate receptors
Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, USA
* Corresponding author. E-mail: rjbrosnan{at}ucdavis.edu
Accepted for publication August 8, 2008.
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Abstract |
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Background: Carbon dioxide (CO2) dose-dependently decreases minimum alveolar concentration (MAC) of anaesthetics in rats. CO2 also dose-dependently decreases cerebrospinal fluid pH. N-methyl-D-aspartate (NMDA) channels exhibit pH sensitivity and are putative targets for inhaled anaesthetics. We hypothesized that CO2 dose-dependently decreases rat NMDA channel current via an acidifying effect at concentrations relevant to CO2 MAC.
Methods: To test this hypothesis, we studied rat NR1/NR2A glutamate receptors expressed in voltage-clamped Xenopus oocytes. To measure pH effects, we used perfusates adjusted between 7.3 and 5.3 with HCl. To measure CO2 effects, we used equimolar sodium perfusates containing either 0 or 24 mM NaHCO3 and CO2 between 0% and 87% atm. Solution compositions were measured using a blood gas analyser with values corrected using a calibrated pH meter and gas chromatograph with solutions at 37°C.
Results: We found that decreasing pH decreased NMDA current. Moreover, pH effects produced by adding CO2 to NaHCO3-containing perfusates were identical to those produced by adding HCl to normal perfusates. The pH inhibiting 50% of NMDA current was 6.52. The CO2 concentration inhibiting 50% of rat NMDA current was 63% for solutions with 24 mM NaHCO3. CO2 exhibited a linear dose-dependent NMDA response analogous to that observed for in vivo CO2 anaesthetic potency in rats.
Conclusions: CO2 and hydrogen ions act via the same mechanism to inhibit NMDA receptors. Moreover, CO2 inhibits rat NMDA receptors in a manner that is consistent with CO2 MAC-sparing effects in rats.
Keywords: anaesthesia; anaesthetic gases; carbon dioxide; hypercapnia; ion channels; N-methyl-D-aspartate; pH; receptor pharmacology
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Introduction |
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Since at least the time of the Egyptian pharaohs, carbon dioxide (CO2) has been used as an anaesthetic.1 CO2 is also a general anaesthetic, producing immobility to noxious stimuli at 1/3 atm in dogs2 and at 1/2 atm in rats.3 However, the mechanism for CO2 general anaesthesia, as true for the mechanism for all inhaled anaesthetics, remains unknown.4
The potency of conventional inhaled anaesthetics increases as a function of their lipid solubility.5 However, the anaesthetic potency of CO2 is much greater than would be predicted by its solubility in oil.6 Unlike conventional inhaled anaesthetics, CO2 decreases solution pH and decreasing cerebrospinal fluid (CSF) buffering capacity enhances CO2 anaesthetic potency.2 Thus, the narcotic effects of CO2 may not be due to the CO2 molecule itself. Rather, CSF acidification alone could be the mechanism for CO2 anaesthesia.
The N-methyl-D-aspartate (NMDA) receptor is a glutamatergic heterotetrameric cation channel with high calcium conductance composed of NR1 and NR2 subunits.7 NMDA receptors are inhibited by acidosis8 and NMDA-evoked neurone potentials are decreased by hypercapnia.9 NMDA antagonism is thought to contribute to the immobilizing action of volatile anaesthetics10 and the gaseous anaesthetics nitrous oxide and xenon.11 Likewise, NMDA antagonism could be important for CO2 anaesthetic actions.
We hypothesized that CO2 is an NMDA antagonist and that this antagonism, at least in part, explains CO2 narcosis. To be correct, several criteria must be fulfilled. CO2 should significantly and reversibly inhibit NMDA currents in vitro at concentrations that significantly decrease anaesthetic requirements for other inhaled agents in vivo. CO2 should also inhibit NMDA currents as a function of extracellular pH; therefore, more alkaline solutions should obtund CO2 antagonism. Finally, since PCO2 produces a linear dose-dependent decrease in inhaled anaesthetic requirement in rats, PCO2 should also produce a linear dose-dependent decrease in rat NMDA current.
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Oocyte preparation and NMDA expression
Adult female South African clawed toed frogs (Xenopus laevis) were anesthetized in a 0.3% tricaine bath and unilaterally ovariectomized using a protocol approved by the Animal Use and Care Committee at the University of California, Davis. Eggs were defolliculated using 0.2% collagenase Type I (Worthington Biochemical, Lakewood, NJ, USA) and stored in modified Barth solution [88 mM NaCl, 1 mM KCl, 2.4 mM NaHCO3, 20 mM HEPES, 0.82 mM MgSO4, 0.33 mM Ca(NO3)2, 0.41 mM CaCl2, 5 mM sodium pyruvate, gentamycin, penicillin, streptomycin, filtered, pH 7.4]. All salts were of analytical grade and obtained from Fisher Scientific (Pittsburgh, PA, USA).
Glutamate receptors were expressed using rat NMDA NR1 clones in a pCDNA3 vector and rat NMDA NR2A clones in a Bluescript vector, both of which were generously provided by the RA Harris laboratory (University of Texas, Austin, TX, USA). Genes were sequenced and compared with sequences in the National Center for Biotechnology Information database (BLAST) to confirm identities. Plasmids were linearized with the restriction enzymes ApaI and XhoI, respectively, and RNA was synthesized using T7 RNA from a commercially available kit (mMessage mMachine; Ambion; Austin, TX, USA). Transcripts were mixed in a 1:1 ratio, and 1–10 ng of total RNA was injected into oocytes that were studied 1–2 days later. Oocytes injected with similar volumes of water served as controls.
Study solutions
It is not possible to study NMDA current responses to solutions ver a wide range of CO2 tensions at a fixed pH. The CO2–HCO3 chemical equilibrium is described by the equation: 
. Consequently, addition of NaOH to carbonated solutions to maintain constant pH reduces the PCO2, and subsequent addition of more CO2 generates more NaHCO3. Carried to completion, isohydric solutions at PCO2 tensions between 0% and 60% atm have different ionic activities, sodium concentrations, and may even exceed the solubility product for bicarbonate. Instead, a different study design was used. NMDA receptor responses to pH changes generated by direct addition of hydrogen ions were compared with pH changes generated by addition of CO2 to isonatremic solutions that contained either 0 or 24 mM sodium bicarbonate. New solutions were made immediately before each experiment using 
18.2 M
distilled water and analytical grade chemicals (Fischer Scientific, Hampton, NH, USA). All work was performed in Davis, CA, which is located 16 m above sea level. Electrophysiology measurements were made at ambient room temperature (23°C). Only glass or polytetrafluoroethylene (PTFE) containers and tubing were used to avoid solution contamination by phthalates.12
For PCO2 test solutions, volumes of 99.999% CO2 (Matheson Trigas, Newark, CA, USA) were added to sealed 100 ml gastight glass syringe pairs containing approximately 60 ml each of either barium frog Ringer’s solution (BaFR: 115 mM NaCl, 2.5 mM KCl, 1.8 mM BaCl2, 10 mM HEPES, 0.1 mM EGTA, filtered, pH 7.4) or BaFR plus agonist (BaFREG: barium frog Ringer’s solution plus 0.1 mM glutamate and 0.01 mM glycine; the single-letter amino acid codes for glutamic acid and glycine are E and G, respectively). Barium was substituted for calcium in perfusion solutions to avoid activating calcium-dependent chloride currents in Xenopus oocytes.13 The concentrations of glycine and glutamate in the BaFREG solutions produce maximal NMDA current responses. Solutions were vigorously shaken for 5 min to equilibrate gas partial pressures. After expulsion of the gas headspace, PCO2 in solutions were measured at 37°C using a commercially available calibrated blood gas analyser (ABL5, Radiometer America, Westlake, OH) from samples collected via gastight syringe stopcocks. This was done to simultaneously measure pH and PCO2, which was otherwise not possible using headspace gas chromatography and a standard benchtop pH meter. Oxygen was added as needed to the syringe headspace to ensure an O2 tension between 12 and 33 kPa (90 and 250 mm Hg). Further CO2 or air headspace additions and equilibrations were used to achieve solutions pairs having the following PCO2 (in % atm): 0, 10, 20, 30, 40, and 50. PCO2 of each solution was measured again at the end of each experiment.
For PCO2 plus bicarbonate test solutions, 24 mEq NaHCO3 replaced an equimolar quantity of NaCl to make bicarbonate-BaFR (BaFR: 91 mM NaCl, 24 mM NaHCO3, 2.5 mM KCl, 1.8 mM BaCl2, 10 mM HEPES, 0.1 mM EGTA, filtered, pH 7.4) and bicarbonate-BaFREG (bicarbonate-BaFR plus 0.1 mM glutamate and 0.01 mM glycine). CO2 was added to pairs of solutions and measured, as described above, to achieve the following PCO2 (in % atm): 0, 10, 30, 50, 70, and 85. Solutions in syringes containing 85% atm CO2 were first saturated with 100% O2 before CO2 additions, and the syringe gas cap was never exposed to room air. PCO2 of each solution was measured again at the end of each experiment.
For pH test solutions, 1 M HCl was added to 13 pairs of BaFR and BaFREG solutions to generate a range of pH values between 5.3 and 7.4. The pH was measured at room temperature using a calibrated benchtop pH analyser (Accumet XL20, Fischer Scientific).
Study design
Oocytes were studied in a linear-flow perfusion chamber that had a 250 µl channel volume. All solutions were administered by a syringe pump (Pump 33, Harvard Apparatus, Holliston, MA, USA) at 1.5 ml min–1 using glass syringes and PTFE tubing during which a –80 mV membrane potential was maintained using a standard two-electrode voltage clamp technique (GeneClamp 500B, Axon Instruments, Union City, CA, USA). After a 5 min baseline measurement during perfusion with BaFR, the perfusate was switched to BaFREG for 30 s followed by a 5 min washout with BaFR. This was repeated three to four times to verify constancy of the control agonist response (<10% change in peak current).
The perfusate was then switched to the test solution in BaFR for a 5 min washin, followed by a 30 s exposure to the counterpart test solution in BaFREG. Test solution washout with BaFR ensued for 5 min followed by another 30 s exposure to BaFREG. Data were only used for analysis if NMDA current responses after washout differed by 
10% from responses before washin of the test solution. Data were recorded by commercially available data acquisition software (Chart, version 5, AD Instruments, Colorado Springs, CO, USA).
Equipment calibration
The automated blood gas analyser was used to measure pH and PCO2 of test solutions, since placing solutions in open air vessels required for benchtop pH meter analysis and headspace gas chromatography would have resulted in CO2 loss and consequently an increase in measured pH. However, the blood gas analyser is not normally calibrated over the range of PCO2 and pH values used in this study; moreover, gas partial pressures are measured at 37°C which results in higher PCO2 and lower pH values compared with measurements at ambient temperature (23°C).
A CO2 standard curve was required to correct blood gas analyser values. Known CO2 standard concentrations spanning the range of experimental values were generated using Dalton’s Law and a digital barometer (Fisherbrand Traceable, Fischer Scientific) and used to calibrate a gas chromatograph (Clarus 500, Perkin Elmer, Waltham, MA, USA). Chromatography was performed by gas injections directly on to a 1.8 m CTR-I concentrically packed column (Grace, Deerfield, IL, USA) with a 0.25 ml sample loop, 20 ml min–1 carrier gas flow rate, 100°C oven temperature, and 120°C detector temperature. This produced a 1.4 min CO2 retention time and excellent separation from other air component peaks. Gas caps of different air and CO2 mixtures were added to gastight glass syringes containing 50 ml BaFR. After vigorous shaking, syringes were placed in a temperature-calibrated rotating hybridization incubator (Model 400, SciGene, Sunnyvale, CA, USA) at 37°C for 1 h. CO2 concentration in the syringe headspace was then immediately measured by gas chromatography, and the BaFR PCO2 was measured by the blood gas analyser. The resulting standard curve was linear only for 70% atm CO2. Consequently, the PCO2 of test solutions containing 87% CO2 was calculated by subtracting the PO2, which equalled 13.4 kPa (101 mm Hg) and fell within the instrument’s automated calibration range, from the total barometric pressure.
The pH values for all samples were corrected to 37°C. This was accomplished by constructing a standard curve for the benchtop pH meter using identical samples encompassing the entire study range measured at 25°C and 37°C. Because the pH of many test solutions were outside the automated calibration range, a standard curve was also constructed for the blood gas analyser and benchtop pH meter using samples measured at 37°C.
Statistical analysis
Data were described by mean (SEM). The NMDA current–pH response curve was fit to the Hill equation from which the median effective pH (pH50) and Hill coefficient (nH) were estimated using a pharmacology modelling programme (WinNonlin, Pharsight, Cary, NC, USA). Differences between the pH response curve and those for CO2 or CO2+HCO3 were analysed using t-tests with Dunn-Sidak corrections for multiple comparisons. Buffering capacity of BaFR and BaFR+HCO3 solutions was analysed by linear regression (SPSS v.11) of pH and log10(CO2) followed by a t-test of the slopes. Linear portions of the NMDA current–CO2 response curves were analysed using linear regression, and the x-intercept was used to predict CO2 concentration producing maximal NMDA channel inhibition. A P-value of <0.05 was considered significant.
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Baseline NMDA currents in the presence of maximal agonist (BaFREG) ranged from 0.8–8.5 µA, indicating good receptor expression in oocytes. In contrast, sham-injected oocytes did not exhibit any current change when exposed to BaFREG solution. A sample NMDA current tracing is shown in Figure 1. CO2 and H+ both caused a concentration-dependent decrease in baseline NMDA current. After washout with BaFR solution, NMDA current responses to agonist were indistinguishable from baseline responses, indicating that effects of CO2 and H+ on NMDA receptors are reversible (Fig. 1).
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As expected, pH decreased as a function of increasing CO2. The log-log regressions of H+ and CO2 had an R2>98% for solutions with and without bicarbonate (Fig. 2); the slope of each line equals the buffering capacity of the corresponding solution. Differences between the buffering capacities for solutions with and without bicarbonate were modest, but statistically significant. The
pH/log(%CO2) was –1.39 (0.10) and –0.87 (0.02) for BaFR with and without bicarbonate, respectively.
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The sigmoid pH–current response curve (Fig. 3) was described by a Hill equation having an absolute maximum NMDA current change (
Imax) equal to –98 (1)%, a current change without pH inhibition (I0) equal to 9 (3)%, a pH50 equal to 6.52 (0.02), and an nH equal to 23 (1). Since Imax–I0=107 and (pH50)nH=6.31x1018, this relationship mathematically reduces to:
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I is the per cent change in NMDA current. Although control measurements were performed at pH=7.4 at 23°C, the pH after temperature correction to 37°C was 7.24. If the Hill relationship is extrapolated back to pH=7.4, I increases approximately 4%.
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When plotted as a function of pH, the effects of CO2+HCO3 on NMDA current were indistinguishable from the effects of pH alone (Fig. 3). However, the addition of CO2 alone to BaFR caused a left shift of the dose–response curve. Negative NMDA current modulation for the three highest values of the CO2 curve—corresponding to the three lowest CO2 concentrations in BaFR—was significantly less than for the comparable pH changes produced by HCl.
Equimolar NaCl replacement by NaHCO3 reduced CO2 potency as an NMDA antagonist (Fig. 4). Addition of 10% CO2 caused a much more precipitous decrease in NMDA current for BaFR solutions without NaHCO3, corresponding to a much greater decrease in solution pH. However, for larger partial pressures, absolute changes in NMDA inhibition for a given change in CO2 were identical. Least squares slope estimates were –0.94 (0.06) and –0.96 (0.09) for the linear portions of the CO2 and CO2+HCO3 responses, respectively. Relationships were described by lines with R2>0.97. Maximal NMDA inhibition was predicted for 56 (2)% atm CO2 in BaFR. It is unknown whether the CO2–HCO3 slope would remain constant at hyperbaric CO2 concentrations; such an assumption suggests a lower bound for maximal NMDA inhibition at 114 (6)% atm CO2, equal to the x-intercept (Fig. 4). The median inhibitory CO2 concentration with the bicarbonate-containing solution was 63 (2)% atm, but was <10% atm without bicarbonate addition.
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CO2 causes reversible and concentration-dependent inhibition of NMDA receptor current. Moreover, the magnitude, pH dependence, and rectilinear NMDA current responses all support the relevancy of this receptor to the mechanism of action for CO2 anaesthesia.
High PCO2 produces general anaesthesia. The minimum alveolar concentration (MAC), equal to the end-tidal anaesthetic concentration that prevents movement in response to a maximal noxious stimulus in 50% of individuals,14 is 32.6 kPa (245 mm Hg) in dogs2 and 53.6 kPa (403 mm Hg) in rats.3 Assuming an in vivo NMDA response similar to that for bicarbonate-buffered solutions, CO2 at MAC should inhibit 20% of NMDA current in dogs and 40% in rats. Contemporary volatile anaesthetics at MAC inhibit NMDA current by 22–34%,15 and spinal NMDA receptors contribute to the immobilizing action of these agents.10 NMDA receptor inhibition likely also contributes to the mechanism of action of CO2 anaesthesia. Since selective NMDA antagonists cannot by themselves cause complete anaesthesia in rats,10 CO2 must act on other anaesthetic- or pH-sensitive targets also.
Carbon dioxide exerts its effects on NMDA receptors principally through the acidification of extracellular fluid rather than by a direct interaction between receptors and the CO2 molecule itself. Increased H+ concentration has previously been shown to act on the NR1 subunit surface loop encoded by exon 5 to inhibit NMDA receptor function.16 When plotted as a function of pH, NMDA antagonism caused by HCl and CO2 in bicarbonate-buffered solutions is identical. This suggests that H+ ions generated by carbonation likewise inhibit NMDA via an identical extracellular receptor effect on the exon 5 surface loop. In dogs, CO2 potency is enhanced when CSF bicarbonate—and hence buffering capacity—is decreased. Moreover, halothane MAC-sparing effects are more closely correlated with CSF pH than PCO2, with CSF pH approximately equal to 6.7 at 1 MAC of CO2 in dogs.2 The CSF pH in rats at 1 MAC of CO2 is probably even more acidic, since the ED50 of CO2 is higher in rats than in dogs, resulting in an even greater degree of NMDA receptor negative modulation. Relevance of NMDA to CO2 narcosis in vivo is thus supported by the pH-mediated NMDA antagonism of CO2 in vitro.
Rat NMDA receptors exhibit rectilinear inhibition by CO2 in bicarbonate-buffered solutions; that is to say, there is no threshold response observed. Similarly, CO2 decreases volatile anaesthetic MAC rectilinearly in rats,3 except that the slope of the NMDA current response is only 40% of the slope of the MAC response. In dogs, however, there is a CO2 threshold response to decreasing halothane MAC.2 Yet, the CSF pH threshold for MAC reduction in hypercapnic dogs still approximates the pH at which significant rat NMDA inhibition was observed. Therefore, different CO2 MAC responses between dogs and rats may not arise from differences in NMDA or other receptor pH sensitivity, but rather from differences in CSF bicarbonate or CSF buffering capacity in the animals studied. Formulation of BaFR perfusates with a higher either bicarbonate concentration or buffering capacity than used in this study would require more CO2 to reach a pH that inhibits NMDA current, thereby resulting in an apparent threshold response to CO2 antagonism.
There remains a small, but significant, CO2 effect for which extracellular pH (pHe) does not account, as evidenced by differential inhibition from CO2- and HCl-containing solutions at equivalent pH. Xenopus oocytes lack an endogenous bicarbonate current;17 thus, the 24 mM bicarbonate vs chloride ion composition difference between perfusates is alone insufficient to explain this observation.
For a given pHe, CO2 tension is higher in BaFR solutions that contain bicarbonate than those that do not. Yet unlike hydrogen ions, CO2 can rapidly diffuse across cell membranes18 and decrease intracellular pH (pHi). Hence, under extracellular isohydric conditions, pHi is lower in BaFR solutions that contain bicarbonate than those that do not. Deviation of the CO2 dose–response curve from that of pH alone suggests that either CO2 or the attending acidosis can interact with a second site that is not easily accessible to H+—perhaps a transmembrane or intracellular site—that could positively modulate NMDA receptor function19 and thus oppose channel inhibition by extracellular protons. In contrast, the CO2+HCO3 response does not deviate from that of pH alone. Since PCO2 is higher in BaFR+HCO3 solutions than in isohydric BaFR counterparts, pHi must also be lower in BaFR+HCO3 solutions than in BaFR solutions for a given pHe. Thus, marked intracellular acidosis in BaFR+HCO3 solutions with high PCO2 may alter the structural conformation of transmembrane or intracellular portions of NMDA subunits, consequently nullifying the binding or efficacy of CO2 at this second site of action.
The NMDA current pH50 in this study was 6.52. However, in rat cerebellar granule cells, NMDA currents are 50% of maximum at pH=7.3.8 This discrepancy could result from differences between the NMDA subunits expressed or between the cellular expression models. Additionally, neuronal NMDA measurements were reported for room temperature. If corrected to 37°C using the BaFR standard curve from this study, the NMDA pH50 in cerebellum neurones is approximately 7.1. Correcting pH and gas partial pressures to normal body temperature allows comparison of in vivo and in vitro physiologic responses for a comparable per cent of ionized imidazole moieties on the receptor protein.20 Moreover, if identical systems at different temperatures are compared at the same PCO2, then the cooler system will have a lower CO2 activity and a higher solubility coefficient and aqueous CO2 content. Hence, CO2 partial pressure would appear a more potent NMDA inhibitor at lower temperatures because there is more dissolved CO2. This is analogous to ED50–temperature relationships observed with alcohols and inhaled anaesthetics, for which potency is also a function of aqueous—and not gas phase—concentration.21 22 Nonetheless, the open probability and conductance through open NMDA receptors—and also agonist and antagonist association time constants with activated receptors—decrease as temperature decreases,23 24 and hence the temperature used for measurement may still cloud to some degree in vivo and in vitro comparisons.
NMDA inhibition by CO2 implies that hypoventilation might exhibit the pharmacologic effects common to other NMDA antagonists, such as amnesia and analgesia. In evidence, the long-term potentiation, an NMDA-mediated enhancement of synaptic connections important for learning and memory, is inhibited in rat hippocampus slices by 20% CO2.25 Remarkably, humans breathing only 6.5% CO2 perform markedly slower on reasoning tests and have a tendency, albeit not statistically significant, to forget new facts.26 Furthermore, rats that inhale 70/30% CO2/O2 for 30 s exhibit antinociception to hot plate, tail flick, and tail pinch tests for at least 1 h after exposure.27 If these phenomena are due in part to NMDA antagonism, then the present study would suggest that even small current decreases caused by CO2 could have important and long-lasting effects, perhaps most importantly during and after anaesthesia and surgery. Thus, the central nervous system effects of hypoventilation may be as widespread as the NMDA receptor itself.
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This work was funded by the School of Veterinary Medicine at the University of California, Davis.
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1 Plinius Secundus G. The Historie of the World. London: Adam Islip. 573.
2 Eisele JH, Eger EI 2nd, Muallem M. Narcotic properties of carbon dioxide in the dog. Anesthesiology (1967) 28:856–65.[CrossRef][Web of Science][Medline]
3 Brosnan RJ, Eger EI 2nd, Laster MJ, Sonner JM. Anaesthetic properties of carbon dioxide in the rat. Anesth Analg (2007) 105:103–6.
4 Sonner JM, Antognini JF, Dutton RC, et al. Inhaled anaesthetics and immobility: mechanisms, mysteries, and minimum alveolar anaesthetic concentration. Anesth Analg (2003) 97:718–40.
5 Overton CE. Studies of Narcosis (1991) London: Chapman and Hall.
6 Simon SA, Gutknecht J. Solubility of carbon dioxide in lipid bilayer membranes and organic solvents. Biochim Biophys Acta (1980) 596:352–8.[Medline]
7 Monyer H, Sprengel R, Schoepfer R, et al. Heteromeric NMDA receptors: molecular and functional distinction of subtypes. Science (1992) 256:1217–21.
8 Traynelis SF, Cull-Candy SG. Proton inhibition of N-methyl-D-aspartate receptors in cerebellar neurons. Nature (1990) 345:347–50.[CrossRef][Web of Science][Medline]
9 Urenjak J, Zilkha E, Gotoh M, Obrenovitch TP. Effect of acidotic challenges on local depolarizations evoked by N-methyl-D-aspartate in the rat striatum. Life Sci (1997) 61:523–35.[CrossRef][Web of Science][Medline]
10 Stabernack C, Sonner JM, Laster M, et al. Spinal N-methyl-D-aspartate receptors may contribute to the immobilizing action of isoflurane. Anesth Analg (2003) 96:102–7.
11 Yamakura T, Harris RA. Effects of gaseous anaesthetics nitrous oxide and xenon on ligand-gated ion channels. Comparison with isoflurane and ethanol. Anesthesiology (2000) 93:1095–101.[CrossRef][Web of Science][Medline]
12 Yang L, Milutinovic PS, Brosnan RJ, Eger EI 2nd, Sonner JM. The plasticizer di(2-ethylhexyl) phthalate modulates gamma-aminobutyric acid type A and glycine receptor function. Anesth Analg (2007) 105:393–6.
13 Barish ME. A transient calcium-dependent chloride current in the immature Xenopus oocyte. J Physiol (1983) 342:309–25.
14 Eger EI 2nd, Saidman LJ, Brandstater B. Minimum alveolar anaesthetic concentration: a standard of anaesthetic potency. Anesthesiology (1965) 26:756–63.[Web of Science][Medline]
15 Solt K, Eger EI 2nd, Raines DE. Differential modulation of human N-methyl-D-aspartate receptors by structurally diverse general anaesthetics. Anesth Analg (2006) 102:1407–11.
16 Traynelis SF, Hartley M, Heinemann SF. Control of proton sensitivity of the NMDA receptor by RNA splicing and polyamines. Science (1995) 268:873–6.
17 Burckhardt BC, Thelen P, Burckhardt G. Expression of rat renal Na+HCO3– cotransporter in Xenopus laevis oocytes. Pflugers Arch (1994) 429:294–6.[CrossRef][Web of Science][Medline]
18 Caldwell PC. Studies on the internal pH of large muscle and nerve fibres. J Physiol (1958) 142:22–62.
19 Chen YH, Wu ML, Fu WM. Regulation of presynaptic NMDA responses by external and intracellular pH changes at developing neuromuscular synapses. J Neurosci (1998) 18:2982–90.
20 Reeves RB. An imidazole alphastat hypothesis for vertebrate acid–base regulation: tissue carbon dioxide content and body temperature in bullfrogs. Respir Physiol (1972) 14:219–36.[CrossRef][Web of Science][Medline]
21 Franks NP, Lieb WR. Temperature dependence of the potency of volatile general anaesthetics: implications for in vitro experiments. Anesthesiology (1996) 84:716–20.[CrossRef][Web of Science][Medline]
22 Won A, Oh I, Brosnan R, Eger EI 2nd, Sonner JM. Hypothermia decreases ethanol MAC in rats. J Anesth (2006) 20:247–50.[CrossRef][Web of Science][Medline]
23 Chung SH, Kuyucak S. Changes in the kinetics and conductance of N-methyl-D-aspartate (NMDA)-receptor activated single channels with temperature. Neurosci Lett (1995) 187:181–4.[CrossRef][Web of Science][Medline]
24 Lamdani-Itkin H, Kloog Y, Sokolovsky M. Modulation of glutamate-induced uncompetitive blocker binding to the NMDA receptor by temperature and by glycine. Biochemistry (1990) 29:3987–93.[CrossRef][Web of Science][Medline]
25 Velisek L. Extracellular acidosis and high levels of carbon dioxide suppress synaptic transmission and prevent the induction of long-term potentiation in the CA1 region of rat hippocampal slices. Hippocampus (1998) 8:24–32.[CrossRef][Web of Science][Medline]
26 Sayers JA, Smith RE, Holland RL, Keatinge WR. Effects of carbon dioxide on mental performance. J Appl Physiol (1987) 63:25–30.
27 Mischler SA, Alexander M, Battles AH, Raucci JA Jr, Nalwalk JW, Hough LB. Prolonged antinociception following carbon dioxide anesthesia in the laboratory rat. Brain Res (1994) 640:322–7.[CrossRef][Web of Science][Medline]
28 Pontén U. Consecutive acid–base changes in blood, brain tissue and cerebrospinal fluid during respiratory acidosis and baseosis. Acta Neurol Scand (1966) 42:455–71.[Web of Science][Medline]
29 Messeter K, Siesjo BK. Regulation of the CSF pH in acute and sustained respiratory acidosis. Acta Physiol Scand (1971) 83:21–30.[Web of Science][Medline]
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