Differential Uptake of Volatile Agents into Brain Tissue In Vitro

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Abstract
Background: The rate of onset of drug actions in experiments with brain slices in vitro can vary widely. One factor that influences the rate is access to tissue sites of action. To study the effects of the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (F6, also termed 2N in the literature) on physiologic processes under defined tissue concentrations, the authors performed electrophysiologic measurements of the effects of F6 and halothane, measured the uptake of these agents into brain tissue, and performed computational modeling to determine concentration-depth profiles during drug application. Methods: Hippocampal brain slices 500 microm thick were prepared from adult rats. Evoked population responses in the CA1 region were obtained using extracellular recordings and electrical stimulation of the Schaffer collateral pathway. F6 (24 microm) and halothane (270 microm) were applied via superfusion for 40 min. Uptake of drug into tissue slices was measured using gas chromatography. Computational modeling was used to obtain estimates of drug diffusion coefficients in brain tissue and to calculate tissue concentration as a function of time and depth during drug application. Results: Halothane reduced the amplitude of the evoked population spike and reduced the population excitatory postsynaptic potential slope. F6 had no effect on either measure. Uptake experiments yielded a diffusion coefficient of 0.1 x 10-6 cm2/s for F6 and 0.8 x 10-6 cm2/s for halothane. After 40 min of drug application, the concentration reached at tissue depths from which physiologic signals were obtained, approximately the top 200 microm of the slice, was estimated to be 58% of the final equilibrium value for F6 and 93% for halothane. Conclusions: Diffusion into tissue is substantially slower for F6 than for halothane, and its impact is great enough that this must be considered when designing or interpreting in vitro experiments. However, impaired access does not account for the lack of effect of F6 on electrophysiologic responses in rat hippocampal slices.