Diffusion of penicillin in agar and cerebral cortex of the rat

The diffusion of penicillin was studied in agar gel and the cerebral cortex of the rat using pressure microinjection and ion-selective microelectrodes selective to penicillin. From the agar measurements a free diffusion coefficient for penicillin of 3.52 +/- 0.08 (mean +/- S.E.M.) X 10(-6) cm2.s-1 for 37 degrees C was determined. The tortuosity value in the cortex was 1.62 +/- 0.03 (mean +/- S.E.M.) at the same temperature implying an apparent diffusion coefficient of 1.34 +/- 0.07 (mean +/- S.E.M.) x 10(-6) cm2.s-1. This tortuosity value means that penicillin diffuses in the cortex in a similar manner to other extracellular substances. These diffusion values clarify previous estimates and permit accurate evaluation of epilepsy models based on the application of penicillin.