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Kindling epileptogenesis was induced by periodic electrical stimulation of the Schaffer collateral/commissural pathway in the CA1 area of the rat hippocampus. The progressive nature of hippocampal kindling is demonstrated by a detailed description of the behavioral signs and the progressive increase of the after-discharge duration in the course of kindling acquisition. Furthermore, the evolution of the alterations in the paired-pulse local evoked field potentials and the modifications of the GABAA receptor binding and of the expression of mRNAs encoding for the subunits of the GABAA and glutamate receptors are considered. Evidence is presented that during kindling opposite changes occur in the CA1 and the fascia dentata in terms of the balance between excitation and inhibition due to contrasting changes in GABA-mediated inhibitory function.

The kindling model of epilepsy, induced by tetanic stimulation of Schaffer collateral/commissural fibers, was studied in the rat hippocampus. Gamma-aminobutyric acid immunoreactivity was used to quantify the number of GABA-immunoreactive somata per mm2 in CA1 region, 28 days after the last generalized seizure. Comparison of the numbers obtained from kindled animals with those from controls, showed a significant decrease (18%) on the ipsilateral stimulated side but none on the contralateral side. In control rats injection of the GABA-transaminase inhibitor, amino oxyacetic acid (AOAA), led to a 46% increase in the number of cell somata immunoreactive for GABA. This probably results from an accumulation of GABA, reflecting GABA synthesis by glutamate decarboxylase (GAD) activity, in somata of interneurons that had initially a GABA content below the immunocytochemical detection threshold. In kindled rats, 31 days after the last seizure, the number of GABA-immunoreactive cells that could be observed after AOAA-treatment was significantly lower (35% ipsilateral and 25% contralateral) when compared to AOAA-treated controls. This suggests that in kindled animals a GAD dependent increase in GABA content did not take place in a subpopulation of interneurons. The observations for kindled rats are interpreted as a long-term decrease in GABA content and as an alteration in GABA turnover in a subpopulation of interneuron somata, the latter possibly due to a decrease in GAD activity. The long-term enhanced seizure sensitivity, characteristic for kindled animals, may be due to a decreased GABAergic inhibitory control of the neuronal circuitry in the CA1 region of the hippocampus.

Summary: Daily repeated tetanic electrical stimulation (kindling) of hippocampus or other brain structures leads to progressive increase in epileptiform activity. Since kindling may involve changes in the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), the distribution of this amino acid was studied. A significant decrease in number of GABA immunoreactive positive cell bodies in stimulated CA1 region of the rat hippocampus compared to the contralateral side was found.

We studied the binding of [35S]t-butylbicyclophosphorothionate to the GABAA receptor-mediated chloride channel in the CA1 area and fascia dentata of control and Schaffer collateral kindled rats, by means of semi-quantitative autoradiography. The [35S]t-butylbicyclophosphorothionate binding was determined at three stages during kindling aquisition: (i) after six afterdischarges, (ii) after 14 afterdischarges and (iii) after the induction of fully kindled seizures. Furthermore, the binding was studied at the long-term stage, 28 days after the last generalized tonic-clonic seizure [Racine R. J. (1972) Electroenceph. clin. Neurophysiol.32, 281–294]. The binding was investigated at three [35S]t-butylbicyclophosphorothionate concentrations, 4, 47.5 ( KD value) and 180 nM ( Bmax value).

The binding of the GABAA/benzodiazepine receptor agonist [3H]flunitrazepam was studied in the hippocampus of rats kindled by daily stimulation of the Schaffer collaterals, using semi-quantitative autoradiography. Two kindled stages were investigated: (i) 24 h after the last generalized tonic-clonic seizure (fully kindled) and (ii) 28 days after the last generalized seizure (long-term). The binding of [3H]flunitrazepam was determined at two concentrations, 3 and 16 nM.

To investigate the changes underlying kindling epileptogenesis in the rat hippocampus, the levels of the messenger RNAs encoding for the subunits of theN-methyl-d-aspartate-receptor (1,2A-D) and the kainate-receptor (1, -2, GluR-5, -6, -7) were determined in hippocaampal principal neurons usingin situ hybridization techniques and semi-quantitative analysis of the autoradiograms. Schaffer collateral-commissural pathway kindled rats were investigated at three different stages of kindling acquisition, always 24 h after the last stimulation. Furthermore, fully kindled rats were studied at long-term (28 days) after termination of kindling stimulations. NR1 messenger RNA levels were slightly decreased in CA1 area of fully kindled animals. In the fascia dentata region, a minor increase of NR2A and NR2B transcripts was found at all stages of kindling acquisition. KA-2 messenger RNA was enhanced in all hippocampal subfields during kindling development. However, none of these changes persisted at long-term after the last seizure and only the low-abundant GluR-7 expression was slightly depressed in the fascia dentata.