Differences between strychnine and penicillin epileptogenesis suggest a laminar organization of neocortical inhibition

Transient foci of epileptiform alteration in neuronal population activity were induced by microinjection of strychnine sulfate into different layers of cat striate cortex. Potentials evoked by visual field-specific photic stimulation were recorded from microelectrodes at the injection site and in adjacent laminae. Epileptogenesis, characterized by an enhancement of the normal primary response followed by the development of a large late potential, occurred only with strychnine injections into superficial pyramidal layers 2 and 3. By contrast, stellate layer 4 has been shown to be most susceptible to epileptogenic effects of penicillin and bicuculline. Since disinhibitory convulsants should be most effective where their actions antagonize the prevalent type of inhibition, these findings suggest that there may be a laminar segregation of neocortical inhibition, possibly glycine-mediated in layers 2–3 and probably gamma aminobutyric acid (GABA)-mediated in layer 4.