Alterations of perisomatic GABA synapses on hippocampal CA1 inhibitory interneurons and pyramidal cells in the kainate model of epilepsy

In the kainate model of epilepsy, electrophysiological and anatomical modif ications occur in inhibitory circuits of the CAI region of the rat hippocam pus. Using postembedding GABA immunocytochemistry and electron microscopy, we characterized perisomatic GABA and non-GABA synaptic contacts in CA1 pyr amidal cells, and GABAergic interneurons of stratum oriens/alveus and strat um lacunosum-moleculare, and examined if changes occurred at these synapses at two weeks post-kainate treatment. We found that, in control rats, the n umber and total length of perisomatic GABA synapses were significantly smal ler (approximately 40-50%) in lacunosum-moleculare interneurons than in ori ens/alveus interneurons and pyramidal cells. Additionally, the number and t otal length of perisomatic non-GABA synapses were different among all cell types, with these parameters increasing significantly in the following orde r: pyramidal cells < lacunosum-moleculare interneurons <oriens/alveus inter neurons. Following kainate treatment, we found that the number and total le ngth of GABA synapses were significantly increased in lacunosum-moleculare interneurons (by 76% and 100%, respectively), but were unchanged in pyramid al cells and oriens/alveus interneurons. In addition, the mean length of in dividual GABA synapses was significantly increased (by 17%) in pyramidal ce lls after kainate treatment. In contrast, no changes were observed at non-G ABA synapses in any cell type examined after kainate treatment. These results indicate that, in control animals, the ultrastructural correl ates of perisomatic GABA inhibition are less pronounced in lacunosum-molecu lare than oriens/alveus interneurons or pyramidal cells, whereas those of p erisomatic excitation are more prominent in oriens/alveus than lacunosum-mo leculare interneurons, and much less present in pyramidal cells. In additio n, our results with kainate-treated animals suggest that cell-specific chan ges in perisomatic inhibition may occur in CA1 inhibitory interneurons in t he chronically hyperexcitable hippocampus. The ultrastructural correlates o f perisomatic inhibition were increased in lacunosum-moleculare interneuron s, which may thus suggest some disinhibition of pyramidal cells. However, t he ultrastructural correlates of perisomatic inhibition were increased in p yramidal cells, implying some enhancement of perisomatic inhibition of prin cipal cells in the hyperexcitable hippocampus. (C) 1999 IBRO. Published by Elsevier Science Ltd.