The effects of volatile anesthetics on NMDA receptor signaling were fully reversible and not dependent on holding potential (data not shown).
Volatile Anesthetics Inhibit Glutamate Signaling in a Noncompetitive Manner
We determined the concentration-response relationship for glutamate (in the presence of glycine at EC50) on NR1/NR2A and NR1/NR2B receptors alone and in the presence of each of the three anesthetics (administered at IC50). As shown in Figure 2A and B, and Table 1, each of the anesthetics decreased significantly (P < 0.001) the maximal glutamate effect (Emax). Thus, the inhibitory action of the anesthetics could not be overcome by large agonist concentrations. Even at millimolar concentrations of agonist, response sizes were reduced by >50%.
These results indicate that volatile anesthetics interact with glutamate binding in a noncompetitive manner.
Volatile Anesthetics Inhibit Glycine Signaling in a Noncompetitive Manner
Because glycine is an obligatory co-agonist at the NMDA receptor, its binding site could be a target for volatile anesthetic action. We therefore tested the effects of isoflurane, desflurane, and sevoflurane (at IC50) on glycine signaling (in the presence of glutamate at EC50). As indicated in Figure 2C and D, and Table 1, the anesthetics decreased Emax (P < 0.05) without affecting EC50. Therefore, volatile anesthetics are unlikely to act on the agonist binding pocket for glycine, but inhibit glycine signaling by an allosteric antagonism.
Mg2+ Potentiates the Inhibitory Effect of Volatile Anesthetics on NMDA Receptor Signaling
Next we assessed whether Mg2+ enhances the inhibitory effect of volatile anesthetics on NMDA receptor signaling. On both NR1/NR2A and NR1/NR2B receptors, we determined the IC50 for each volatile anesthetic alone, and in the presence of Mg2+ at IC50 (416 μM at NR1/NR2A and 629 μM at NR1/NR2B) (Fig. 3, Table 1). IC50 was shifted significantly to the left in the presence of Mg2+. On NR1/NR2A receptors, the reduction in IC50 was 28% (P = 0.02, t-test) for isoflurane, 16% (P = 0.038, t-test) for sevoflurane, and 24% (P = 0.035, t-test) for desflurane. On NR1/NR2B receptors, the shift was 30% (P < 0.001, t-test) for isoflurane, 12% (P = 0.028, t-test) for sevoflurane, and 14% (P = 0.024, t-test) for desflurane. There were no significant differences among the three anesthetics for NR1/NR2A receptors (P = 0.624), whereas for NR1/NR2B receptors, inhibition by isoflurane was significantly more (P < 0.05, analysis of variance, Student-Newman-Keuls) than by the other two anesthetics. Hill coefficients were similar in the presence and absence of Mg2+.
Hence, the presence of Mg2+ enhances the inhibitory action of volatile anesthetics on NMDA receptor functioning.
S(+)-Ketamine Potentiates Inhibition of NMDA Receptor Signaling by Volatile Anesthetics
To determine whether S(+)-ketamine also enhances volatile anesthetic inhibition of NMDA receptor functioning, we applied several concentrations of each anesthetic in the presence of S(+)-ketamine at IC50 (4.0 μM [NR1/NR2A] and 2.9 μM [NR1/NR2B]), and determined the anesthetic IC50 (Fig. 4, Table 1). As did Mg2+, S(+)-ketamine attenuated the anesthetic concentration required to achieve half-maximal inhibition. On NR1/NR2A receptors, calculated IC50 was reduced by 30% (P < 0.001, t-test) for isoflurane, 33% (P = 0.004, t-test) for sevoflurane, and 24% (P = 0.004, t-test) for desflurane. On NR1/NR2B receptors, IC50 was reduced by 38% (P = 0.001, t-test) for isoflurane, 33% (P < 0.001, t-test) for sevoflurane, and 24% (P = 0.006, t-test) for desflurane. In contrast to the findings with Mg2+, Hill coefficients were increased significantly in the presence of S(+)-ketamine. Although in the presence of multiple binding sites Hill coefficients are difficult to interpret (8), this difference between Mg2+ and ketamine suggests different modes of interaction among these drugs and volatile anesthetics.
Combined Application of Mg2+ and S(+)-Ketamine Profoundly Potentiates Inhibition of NMDA Receptor Functioning by Volatile Anesthetics
Results from Part I of this investigation suggest that combined administration of Mg2+ and S(+)-ketamine might further potentiate the inhibition of NMDA receptor signaling by volatile anesthetics. We therefore tested the inhibitory action of isoflurane, sevoflurane, and desflurane on glutamate/glycine-induced NMDA receptor currents in the presence and absence of Mg2+ and S(+)-ketamine (both at IC50, for Mg2+ 416 μM [NR1/NR2A] and 629 μM [NR1/NR2B], and for S(+)-ketamine 4.0 μM [NR1/NR2A] and 2.9 μM [NR1/NR2B]) (Fig. 5, Table 1). IC50 for each of the anesthetics was decreased by approximately 50% in the presence of Mg2+ and ketamine (P < 0.001, t-test). Furthermore, the steepness of the inhibition curve (Hill coefficient) for volatile anesthetics was increased significantly in the presence of Mg2+ and S(+)-ketamine. As a result, whereas in the absence of Mg2+ and ketamine, 1.22 to 1.33 MAC of each volatile anesthetic inhibited NMDA receptors by approximately 50%, in the presence of Mg2+ and ketamine, this anesthetic concentration reduced signaling to 5% ± 2.4% of control. These findings indicate that volatile anesthetics, ketamine, and Mg2+ interact profoundly at NMDA glutamate receptors, resulting in virtual elimination of receptor signaling at clinically relevant concentrations of all three compounds.
Our findings show that clinically relevant concentrations of isoflurane, sevoflurane, or desflurane inhibit functioning of NR1/NR2A and NR1/NR2B glutamate receptors expressed recombinantly in Xenopus oocytes. This inhibition is reversible, concentration-dependent, and voltage-insensitive, and results from noncompetitive antagonism of glutamate and glycine signaling, most likely by anesthetic interactions with the NR1 subunit of the receptor molecule. In addition, these effects can be potentiated significantly by co-application of either Mg2+, S(+)-ketamine, or—most profoundly—both. Therefore, the analgesic effects of ketamine and Mg2+ are likely to be enhanced in the presence of volatile anesthetics; the cerebral protective effects of the compounds may be potentiated in a similar manner.
Our results provide additional evidence for functional effects of clinical concentrations of volatile anesthetics on NMDA receptors. Such interactions are reported variably in the literature. Kirson et al. (9) found NMDA receptors to be relatively insensitive to halothane in concentrations <0.64 mM. IC50 for halothane on NMDA receptors in these studies was approximately 5.9 mM. In agreement with our data, these investigators showed a voltage-independent mechanism for volatile anesthetic inhibition of NMDA receptors. Black (10) suggests that enflurane increases NMDA receptor activity (consistent with the epileptiform discharges observed during anesthesia with this agent). A study by Pearce et al. (11) showed that volatile anesthetics do not block NMDA receptors in rat hippocampus, because long-term potentiation (which is inhibited by NMDA receptor antagonists) still occurred in the presence of 1.5–2.1 MAC of volatile anesthetic. Many of these studies have limitations (receptor subunit composition not defined, or no functional measurements obtained), and the discrepancies among these studies and those mentioned below are likely attributable to the model systems used.
In contrast to these reports, our findings are in agreement with many studies supporting anesthetic interference with NMDA glutamate signaling. Isoflurane blocks NMDA-stimulated currents in cultured hippocampal neurons (12). Volatile anesthetics (IC50 for isoflurane 0.6–0.9 mM) depress glutamate-dependent intraneuronal translocation of Ca2+ (13), and halothane (0.8 mM) and enflurane (1 mM) (14) block NMDA receptor function (15). Glutamate (100 μM)-stimulated [3H]MK-801 binding to the NMDA receptor was suppressed by halothane and enflurane (16,17). Eighty percent xenon reduced NMDA-activated currents by approximately 60% in a noncompetitive manner, suggesting that the NMDA receptor is instrumental in the anesthetic and analgesic effects of this compound (18). Nitrous oxide was suggested to be a mixed competitive/noncompetitive NMDA antagonist (19).
It is of interest that we found volatile anesthetics to be mainly equipotent in their effects on NMDA glutamate signaling despite differences in the corresponding millimolar concentrations. In addition, the measured IC50 values were very close to 1 MAC for each of the three compounds. This relationship between anesthetic potency and ability to inhibit NMDA receptors suggests that inhibition of NMDA glutamate signaling in the brain and spinal cord may contribute considerably to the anesthetic state. This hypothesis has been formulated previously (20), and our findings support it completely. If so, our results suggest that the administration of ketamine and Mg2+ should have a noticeable effect on volatile anesthetic requirements: in the presence of concentrations of ketamine and Mg2+ as used in this study, the IC50 of volatile anesthetics on NMDA receptors is shifted to approximately 0.5 MAC (Fig. 5). It would be of interest to test this hypothesis in a clinical study.
In addition, similar effects on NMDA signaling might help to explain why anesthetics with very different effects on cerebral metabolic rates have similar neuroprotective properties (21).
Both S(+)-ketamine and Mg2+ enhanced the inhibitory potency of volatile anesthetics on NMDA receptor signaling. These findings can explain results from animal studies showing a dose-dependent volatile anesthetic sparing effect of NMDA receptor antagonists. In rabbits, MK-801 (dizoclipine, plasma level 103 ± 28 ng/mL), a noncompetitive NMDA receptor antagonist, reduced MAC requirements for halothane and isoflurane by 46% and 67%, respectively (4). Whereas 0.3 μM MK-801 decreased isoflurane MAC by 67%, it required an approximately 10-fold larger concentration of S(+)-ketamine (4 μM) to increase isoflurane sensitivity by 30% in our model. This difference is probably attributable to an at least 10-fold more potency of MK-801 as compared with ketamine. Similar results have been reported for other noncompetitive NMDA receptor blockers, like phencyclidine (5), as well as for competitive NMDA receptor antagonists like D-CPP-ene and CGS 19755 (Selfotel) (22). The ability of Mg2+ to decrease MAC has similarly been documented (23).
The combined application of ketamine and Mg2+ has not been studied. In our study, the combination profoundly enhanced inhibition of NMDA receptor functioning by volatile anesthetics, to the point in which clinically relevant concentrations of the compounds virtually eliminated NMDA receptor signaling. This has implications for neuroprotective and analgesic effects of these compounds. Selective noncompetitive and competitive NMDA receptor antagonists protect against focal cerebral ischemia (24). In agreement, ketamine is neuroprotective, with the S(+)-isomer being significantly more effective than the R(−) form (3). Mg2+ appears to have only a modest protective effect. Its benefit may be limited by the depolarization that takes place in damaged neurons, which in turn limits the (voltage-dependent) ability of Mg2+ to block NMDA receptors. The neuroprotective actions of volatile anesthetics are also thought to be mediated in part by inhibition of glutamate signaling (25). Volatile anesthetics with very different effects on cerebral metabolic rate show relatively similar neuroprotective potencies; this is in agreement with our findings of similar potency at the NMDA receptor. Our results suggest that combinations of these compounds might be significantly more effective than either compound used alone.
We thank Prof. Dr. med. Eike Martin (Ruprecht-Karls-Universität Heidelberg, Germany) and Prof. Dr. med. Hugo Van Aken (Westfälische-Wilhelms-Universität Muenster, Germany) for their support. Sincere thanks to Dagmar Westerling, MD, PhD, for a preliminary analysis of the results.
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© 2001 International Anesthesia Research Society
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