The influence of inhalational anesthetics on in vivo and in vitro benzodiazepine receptor binding in the rat cerebral cortex

The effect of volatile anesthetics on benzodiazepine receptor binding was examined autoradiographically in the rat brain both in vivo and in vitro with the use of [³H]-Ro-15-1788, a benzodiazepine antagonist. For in vitro studies, slide-mounted brain sections were incubated at 37°C in Tris buffer (50 mM, pH 7.4) with [³H]-Ro-15-1788 (flumazenil, 0.5-12.0 nM) in the presence of air (control) or 1 MAC concentrations of halothane or isoflurane. Brain sections were exposed to x-ray film and their images digitized, and specific cortical [³H]-Ro-15-1788 binding was determined. A Scatchard plot of specific cortical binding was constructed, and the dissociation constant (K(D)) and maximum bound ligand per milligram tissue (B(max)) were determined for each experimental group. In the in vivo trials, rats were anesthetized with 1 MAC halothane or isoflurane; 0.5 μCi/g [³H]-Ro-15-1788 was given intravenously, and the animals were killed 15 min later. Seven standardized sagittal brain sections were examined from autoradiographs. Mean specific cortical binding was determined for each group and was compared with binding in unanesthetized control rats. A third experimental trial analyzed the timed arterial blood history of [³H]-Ro-15-1788 in animals prepared exactly as in the in vivo study. The [³H]-Ro-15-1788 blood clearance over 20 min and plasma [³H]-Ro-15-1788 levels at 15 min after injection of isotope were evaluated. In vitro Scatchard analysis showed no difference in experimental groups in K(D) or B(max) at 37°C. In vivo trials demonstrated enhanced cortical binding in animals treated with halothane or isoflurane (halothane = 125 ± 23 fmol/mg; isoflurane = 130 ± 25 fmol/mg) compared with control animals (80 ± 9 fmol/mg). However, plasma levels at 15 min were higher in anesthetic-treated groups (halothane = 8.13 ± 0.86 · 10^-7 mg/ml; isoflurane = 7.91 ± 1.0 · 10^-7 mg/ml). Furthermore, the clearance of [³H]-Ro-15-1788 was reduced in the presence of anesthetics (halothane = 6.46 ± 0.40 ml · kg^-1 · min^-1; isoflurane = 6.42 ± 0.61 ml · kg^-1 · min^-1). These findings indicate that neither halothane nor isoflurane acts directly through the benzodiazepine receptor in vitro. In vivo, volatile anesthetics apparently enhance cortical receptor binding, however, this difference can be explained in part by factors not directly related to receptor-anesthetic interactions, i.e., reduced blood ligand clearance during anesthesia.