Background
Numerous classes of anesthetic agents have been shown to enhance the effects mediated by the postsynaptic gamma-aminobutyric acid A (GABAA) receptor-coupled chloride channel in the mammalian central nervous system. However, presynaptic actions of anesthetics potentially relevant to clinical anesthesia remain to be clarified. Therefore, in this study, the effects of intravenous and volatile anesthetics on both the uptake and the depolarization-evoked release of GABA in the rat striatum were investigated.
Methods
Assay for specific GABA uptake was performed by measuring the radioactivity incorporated in purified striatal synaptosomes incubated with 3H-GABA (20 nM, 5 min, 37 degrees C) and increasing concentrations of anesthetics in either the presence or the absence of nipecotic acid (1 mM, a specific GABA uptake inhibitor). Assay for GABA release consisted of superfusing 3H-GABA preloaded synaptosomes with artificial cerebrospinal fluid (0.5 ml.min-1, 37 degrees C) and measuring the radioactivity obtained from 0.5 ml fractions over 18 min, first in the absence of any treatment (spontaneous release, 8 min), then in the presence of either KCl alone (9 mM, 15 mM) or with various concentrations of anesthetics (5 min), and finally, with no pharmacologic stimulation (5 min). The following anesthetic agents were tested: propofol, etomidate, thiopental, ketamine, halothane, enflurane, isoflurane, and clonidine.
Results
More than 95% of 3H-GABA uptake was blocked by a 10(-3)-M concentration of nipecotic acid. Propofol, etomidate, thiopental, and ketamine induced a dose-related, reversible, noncompetitive, inhibition of 3H-GABA uptake: IC50 = 4.6 +/- 0.3 x 10(-5) M, 5.8 +/- 0.3 x 10(-5) M, 2.1 +/- 0.4 x 10(-3) M, and 4.9 +/- 0.5 x 10(-4) M for propofol, etomidate, thiopental, and ketamine, respectively. Volatile agents and clonidine had no significant effect, even when used at concentrations greater than those used clinically. KCl application induced a significant, calcium-dependent, concentration-related, increase from basal 3H-GABA release, +34 +/- 10% (P < 0.01) and +61 +/- 13% (P < 0.001), respectively, for 9 mM and 15 mM KCl. The release of 3H-GABA elicited by KCl was not affected by any of the anesthetic agents tested.
Conclusions
These results indicate that most of the intravenous but not the volatile anesthetics inhibit the specific high-affinity 3H-GABA uptake process in vitro in striatal nerve terminals. However, this action was observed at clinically relevant concentrations only for propofol and etomidate. In contrast, the depolarization-evoked 3H-GABA release was not affected by anesthetics. Together, these data suggest that inhibition of GABA uptake, which results in synaptic GABA accumulation, might contribute to propofol and etomidate anesthesia.
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