Elsevier

Neuroscience

Volume 93, Issue 2, July 1999, Pages 457-467
Neuroscience

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

https://doi.org/10.1016/S0306-4522(99)00199-2Get rights and content

Abstract

In the kainate model of epilepsy, electrophysiological and anatomical modifications occur in inhibitory circuits of the CA1 region of the rat hippocampus. Using postembedding GABA immunocytochemistry and electron microscopy, we characterized perisomatic GABA and non-GABA synaptic contacts in CA1 pyramidal cells, and GABAergic interneurons of stratum oriens/alveus and stratum lacunosum-moleculare, and examined if changes occurred at these synapses at two weeks post-kainate treatment. We found that, in control rats, the number and total length of perisomatic GABA synapses were significantly smaller (approximately 40–50%) in lacunosum-moleculare interneurons than in oriens/alveus interneurons and pyramidal cells. Additionally, the number and total length of perisomatic non-GABA synapses were different among all cell types, with these parameters increasing significantly in the following order: pyramidal cells<lacunosum-moleculare interneurons<oriens/alveus interneurons. Following kainate treatment, we found that the number and total length of GABA synapses were significantly increased in lacunosum-moleculare interneurons (by 76% and 100%, respectively), but were unchanged in pyramidal cells and oriens/alveus interneurons. In addition, the mean length of individual GABA synapses was significantly increased (by 17%) in pyramidal cells after kainate treatment. In contrast, no changes were observed at non-GABA synapses in any cell type examined after kainate treatment.

These results indicate that, in control animals, the ultrastructural correlates of perisomatic GABA inhibition are less pronounced in lacunosum-moleculare than oriens/alveus interneurons or pyramidal cells, whereas those of perisomatic excitation are more prominent in oriens/alveus than lacunosum-moleculare interneurons, and much less present in pyramidal cells. In addition, our results with kainate-treated animals suggest that cell-specific changes in perisomatic inhibition may occur in CA1 inhibitory interneurons in the chronically hyperexcitable hippocampus. The ultrastructural correlates of perisomatic inhibition were increased in lacunosum-moleculare interneurons, which may thus suggest some disinhibition of pyramidal cells. However, the ultrastructural correlates of perisomatic inhibition were increased in pyramidal cells, implying some enhancement of perisomatic inhibition of principal cells in the hyperexcitable hippocampus.

Section snippets

Experimental procedures

For kainate lesions, male Sprague–Dawley rats (Charles River, Montre ́al, Que ́bec, Canada; n=4) were injected intracerebroventricularly with kainate.38., 41. Briefly, adult rats (150–160 g) were anesthetized with ketamine (165 mg/kg, i.m.), xylazine (10 mg/kg, i.m.) and atropine (0.27 mg/kg, i.p.). Rats were positioned in a stereotaxic apparatus and bilateral intracerebroventricular injections of kainate (0.55 μg/μl, 0.9% saline, pH 7.3–7.4) were made using a 10-μl Hamilton syringe. Stereotaxic

GABA and non-GABA synaptic contacts on interneurons and pyramidal cells

Postembedding GABA immunogold labeling was used to identify pyramidal cells and interneurons in the hippocampal CA1 region. GABA-immunonegative cells with a pyramidal-shaped soma located in the stratum pyramidale were identified as pyramidal cells, whereas GABA-immunopositive cells with a soma located either in the O/A or LM were selected as two groups of interneurons. In general, cells found in the O/A and LM were GABA immunopositive. GABA-immunonegative neurons in the O/A or LM were not

Discussion

Electron microscopy and postembedding immunogold staining for GABA were used (i) to characterize perisomatic GABA and non-GABA synaptic contacts on CA1 pyramidal cells and inhibitory interneurons located in the O/A and LM of normal rats, and (ii) to examine if these synaptic contacts were altered in the chronic hyperexcitable hippocampus following kainate treatment. In normal animals, our main findings were that the number and total length of perisomatic GABA synapses were significantly smaller

Conclusions

Using postembedding immunogold labeling for GABA and electron microscopy, we found that the number and total length of perisomatic GABA and non-GABA synaptic contacts were significantly different between O/A, LM and pyramidal cells in normal animals. Our results suggest that LM interneurons receive less perisomatic GABAergic innervation than O/A interneurons and pyramidal cells. In addition, O/A interneurons were preferentially innervated by perisomatic non-GABA inputs compared to LM

Acknowledgements

This work was supported by the Fonds de la Recherche en Santé du Québec (FRSQ; J.-C.L.), the Medical Research Council of Canada (MRCC; J.-C.L. and C.B.), the Savoy Foundation (F.M.), a Research Center grant from the Fonds pour la Formation de Chercheurs et l'Aide à la Recherche (FCAR) to the Groupe de Recherche sur le Système Nerveux Central and an Équipe de Recherche grant from the FCAR (J.-C.L.).

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