Abstracts

The syndrome of mesial temporal lobe epilepsy is generally associated with neuronal loss in the hippocampus and re-organization of neuronal circuits leading to chronic hyperexcitability. The pattern of neuronal loss involves subpopulations of GABAergic inhibitory interneurons. It has been hypothesised that this loss of inhibitory interneurons may play a role in the process of epileptogenesis. However this remains unclear and controversial, as some changes appear to be pro and others antiepileptogenic in classical epilepsy models. In the present study, using a neurotoxin SSP-saporin conjugate, we selectively destroyed these interneurons to investigate their role in the process of epileptogenesis. The SSP-saporin congugate was injected in adult Sprague-Dawley rats into the left ventricle (AP = -0.8, L = +1.5 and DV = -4.5 mm, ref. to bregma), in a concentration of 4 [mu]g /10 [mu]l in 0.9% sterile saline at a rate of 1 [mu]l/min. Control rats received an equivalent volume of saline. At the same time, two EEG bipolar electrodes were implanted into the ventral hippocampus (AP= - 3.8; ML= +2.0; DV= -3.0 mm ref. to bregma) and cortex (AP= -1.0; ML= +4.0; DV= -1.5 mm ref. to bregma). Video- EEG recordings were performed 30 min/day. Once the incidence and timing of the EEG and clinical epileptic events were defined, the rats were sacrificed at different time points. The nature, extent and timing of cellular loss were afterward assessed, using Cresyl violet staining and different imunohistological markers for GABAergic interneurons. The EEG recordings of the SSP-rats were characterised, 24-48 hrs after the SSP-injection, by periods (30-60 sec) of slow (2-3 Hz) rhythmic delta activity of high amplitude ([gt]150 mv) in the hippocampus and cortex. These rhythmic discharges were not associated with apparent clinical events, but with a loss of GABAergic interneurons, without any principal cell loss. At 48-72 hrs after SSP injection, the EEG showed periods (15-60 sec) of slow spikes-and-wave discharges (3-4 Hz) of moderate amplitude (20-50 mv) originating from the hippocampus. These were correlated clinically with wet dog shakes and myoclonic jerks. Freezing was observed in half of the SSP-rats, the EEG were then characterised by periods (10-30 sec) of rapid spikes-and-wave discharges (11-13 Hz) of moderate (20-50 mv) and high amplitude ([gt]150 mv).Six days after the SSP injection, a loss of principal cells was observed in the CA1 and CA3 regions of the hippocampus. Our results confirm that an initial loss of GABAergic interneurons leads to electrical discharges followed by clinical spontaneous recurrent seizures even before the loss of principal cells. This argues for a key role of GABAergic inhibitory interneurons in the process of epileptogenesis and for therapeutic interventions targeted to prevent their selective loss following an acute insult. (Supported by Savoy Fundation)