Volatile Aromatic Anesthetics Variably Impact Human γ-Aminobutyric Acid Type A Receptor Function

In Brief BACKGROUND: The γ-aminobutyric acid type A (GABAA) and N-methyl-d-aspartate (NMDA) receptors are important inhibitory and excitatory neurotransmitter receptors, respectively, in the central nervous system. At the concentrations required to produce immobility in the face of a noxious stimulus, volatile aromatic anesthetics inhibit NMDA receptors to varying degrees, strongly suggesting that they also act at other targets to produce immobilization. In this study, we sought to assess the potential role that GABAA receptors play in mediating the behavioral actions of volatile aromatic anesthetics. METHODS: Electrophysiological techniques were used to quantify the effects of eight volatile aromatic anesthetics and three clinical anesthetics on currents mediated by α1β2γ2L GABAA receptors expressed in Xenopus oocytes. RESULTS: At equivalent minimal alveolar anesthetic concentration multiples, volatile aromatic anesthetics vary widely in the degrees to which they enhance GABAA receptor-mediated currents elicited by low concentrations of GABA. In general, anesthetics that inhibit NMDA receptors most, enhanced GABAA receptors least. This reciprocal relationship between anesthetic potency on GABAA versus NMDA receptors was also observed for the clinical anesthetics isoflurane, halothane, and cyclopropane. Studies using a range of GABA concentrations indicated that volatile aromatic anesthetics enhance GABAA receptor activity by shifting the open-close (gating) equilibrium towards the open channel state. CONCLUSIONS: These findings suggest that GABAA receptors contribute variably to the behavioral actions of volatile anesthetics and imply that the molecular determinants of anesthetic action on NMDA and GABAA receptors are distinctly different. IMPLICATIONS: Inhaled anesthetics enhanced the actions of γ-aminobutyric acid (GABA) on GABAA receptors. However, at equivalent minimal alveolar anesthetic concentration multiples, the magnitudes of these actions vary dramatically among anesthetics. In general, inhaled anesthetics (aromatic and clinical) that inhibit N-methyl-d-aspartate receptor currents most, enhance GABAA receptor currents least (and vice versa).