Different subunit requirements for volatile and nonvolatile anesthetics at gamma-aminobutyric acid type A receptors.

The ability of volatile (halothane and isoflurane) and nonvolatile (alphaxalone and pentobarbital) general anesthetics to modulate radioligand binding to gamma-aminobutyric acid (GABA)A receptors was examined in an immortalized cell line (WSS-1) expressing rat alpha 1 and gamma 2 subunits. Volatile anesthetics enhance [3H]flunitrazepam binding to WSS-1 cells in a concentration-dependent manner, with potencies and efficacies comparable to those found with native GABAA receptors. Transfection of these cells with cDNAs encoding rat beta 2 or beta 3 subunits had a significant influence on anesthetic efficacy but not potency in this assay. Thus, transfection with the beta 2 subunit reduced the efficacy of both isoflurane and halothane, whereas transfection with the beta 3 subunit increased the efficacy of isoflurane but not halothane, compared with values obtained in WSS-1 cells. In contrast, alpha-xalone (an anesthetic steroid) had no effect, whereas at high concentrations pentobarbital (an anesthetic barbiturate) produced a modest inhibition of [3H]flunitrazepam binding to GABAA receptors in WSS-1 cells. Transfection of WSS-1 cells with cDNAs encoding either beta 2 or beta 3 subunits resulted in a concentration-dependent enhancement of [3H]flunitrazepam binding by these nonvolatile anesthetics. Moreover, pentobarbital was significantly more potent in enhancing [3H]flunitrazepam binding to WSS-1 cells transfected with the beta 2 subunit, compared with the beta 3 subunit. The difference in subunit requirements between volatile and nonvolatile anesthetics for enhancement of [3H]flunitrazepam binding indicates that these classes of agents affect GABAA receptor function at distinct loci. These studies also provide evidence that the beta subunit is required for these nonvolatile anesthetics to positively modulate GABAA receptors.