Penicillin epilepsy in the rat: the responses of different layers of the cortex cerebri

Na penicillin G has been electrophoretically applied from a fluid-filled micropipette at different rates and depths within the somatosensory area of the cerebral cortex of rats anaesthetized with urethane. With low fluxes of penicillin, the chemical has to reach a critical concentration in the deep part of layer III and produces focal interictal epileptiform discharges (FIEDs) there, irrespective of the depth of application. Larger penicillin fluxes result in FIEDs maximal at the site of the tip of the penicillin electrode. Calculations showed that diffusion together with the need for the penicillin concentration to reach a threshold throughout a critical mass of tissue accounts for the observation that application of penicillin into a relatively insensitive layer of cortex can initiate spiking confined either to that lamina or to a distant more sensitive lamina, the site of the FIED depending on the rate of penicillin application. With a large penicillin flux, FIEDs persisted for many hours so that subsequent trials had to be undertaken in the presence of background spiking. Despite this, it proved possible to determine the rank order of sensitivity of the different cortical laminae. This was, in decreasing order of sensitivity, III > IV >> II > Va >> Vb. Evidence is presented to suggest that the difference in sensitivity of different cortical depths is not due to a difference in packing density of the sensitive neural elements; there seem to be genuine laminar gradients in the sensitivity of the neuronal structures in which synchronized electrical activity is induced.