Determination of diffusion and partition coefficients of propofol in rat brain tissue: implications for studies of drug action in vitro

Propofol (2,6-diisopropylphenol) is a widely used general anaesthetic that modulates γ-aminobutyric acid type A (GABAA) receptors, the major inhibitory neurotransmitter receptor in the brain. Previous studies have found that the concentration of propofol that is required to affect synaptic inhibition in brain slices is much higher than the free concentration that is achieved clinically and that modulates isolated receptors. We tested whether this is accounted for by slow equilibration in brain tissue, and determined the concentration that must be applied to achieve appropriate brain levels. Rat brain slices 300-μm thick were placed in a solution of 100 μM propofol in artificial cerebrospinal fluid for times ranging from 7.5 to 480 min. Concentrations in these slices were measured by HPLC to determine diffusion and partition coefficients. Electrophysiological measurements of the rate at which effects of 5 μM propofol developed were compared with the calculated rate of increase in tissue concentration. The diffusion coefficient was approximately 0.02×10−6 cm2 s−1, and the brain:artificial cerebrospinal fluid partition coefficient was 36. Diffusion times in brain slices agreed well with time course measurements of propofol-induced depression of synaptic responses, which continued to increase over 5 h. This depression was reversed by blocking GABA inhibition with picrotoxin (100 μM). Propofol does enhance inhibition in brain slices at a concentration of 0.63 μM in the superfusate, which produces brain concentrations corresponding with those achieved in vivo, but equilibration requires several hours. It is likely that slow diffusion to GABA receptors accounts for the high concentrations (>10 μM) that were needed to depress evoked responses in previous investigations.