Review of the role of inhibitory neurons in chronic epileptic foci induced by intracerebral tetanus toxin.

Blocking inhibition provides one of the most common experimental means of triggering epileptic activity in hippocampus and neocortex. However, it has proved much more difficult to show that chronic models of epilepsies are due to disinhibition. One problem is knowing how much inhibition needs to be blocked to provide a sufficient mechanism for epileptic activity. We have found that inhibitory (GABAA) transmission, estimated from evoked monosynaptic IPSCs, must be reduced to 17% of their control amplitude (by 4-7 microM bicuculline) before hippocampal slices generate all-or-none epileptic discharges. Similar estimates of inhibition in chronic epileptic foci induced by intrahippocampal injection of tetanus toxin showed that monosynaptic IPSCs dropped to 10% of control in the injected hippocampus during the first 2 weeks after injection. At all other stages of the active epileptic foci in the two hippocampi the reduction in IPSCs was not alone sufficient for epileptic activity; at 4-6 weeks IPSCs were normal despite continued epileptic activity. One likely mechanism for the late epileptic activity is a reduction of either the intrinsic excitability, or the synaptic excitation, of inhibitory interneurons so they fail to be recruited normally. Alternative mechanisms include the formation of new excitatory connections, as found at modest levels in the dentate gyrus. Several mechanisms may play a part in chronic foci such as those induced by tetanus toxin, either acting together, or sequentially during the progression of the epileptic focus.