Non-immobilizers put to the test: F6 and the GABAA receptor

In the ongoing search for the mechanisms of anesthetic action, a promising paradigm introduced approximately a decade ago was the use of non-immobilizers [D.D.,Koblin, et al., Polyhalogenated and perfluorinated compounds that disobey the Meyer-Overton hypothesis, Anesth. Analg. 79 (6) (1994) 1043-1048] [1]. These drugs have physicochemical properties similar to those of anesthetics but, except for amnesia, do not induce an anesthetic state. Since the GABAA receptor figures prominently in the list of general anesthetic targets, we investigated the effects of F6 (1,2-dichlorohexafluorocyclobutane, a.k.a. 2N) in vitro on expressed and native GABAA receptors and in vivo on field potentials in the hippocampus. The experiments in vitro were aimed at analyzing the interaction of F6 with the GABAA receptor, and in vivo on testing the hypothesis that nonconvulsive seizures cause F6-induced amnesia. We used three different types of preparations: transfected HEK-293 cells combined with a rapid drug application system for studies of expressed receptors; acute hippocampal slices from juvenile rats for investigations of synaptic and extrasynaptic receptors; and multisite electrodes chronically implanted into adult rats for experiments on hippocampal field potentials. All animal experiments were conducted in accordance with APS guidelines and with the approval of the IACUC. We found that with expressed receptors, F6 blocked currents mediated by α1β2 receptors with an EC50 of 8 μM (approx. 0.5 times predicted MAC) but did not alter current kinetics. By contrast, α1β2γ2s-containing receptors were insensitive to F6. Native GABAA receptors at subsynaptic and extrasynaptic locations on hippocampal pyramidal cells were also insensitive to F6. In vivo recordings showed no evidence of F6-induced nonconvulsive seizure activity at amnestic concentrations. We conclude that F6 causes amnesia neither by altering inhibition of hippocampal pyramidal neurons nor by inducing nonconvulsive hippocampal seizures. Instead, amnesia may result from effects on other types of hippocampal neurons or on receptors other than the GABAA receptor.