Loss of Interneurons Innervating Pyramidal Cell Dendrites and Axon Initial Segments in the CA1 Region of the Hippocampus following Pilocarpine- Induced Seizures CELINE DINOCOURT, 1,3 ZDRAVKO PETANJEK, 1,2 TAMAS F. FREUND, 3 YEZEKIEL BEN-ARI, 1 AND MONIQUE ESCLAPEZ 1 * 1 INMED, INSERM U29, Parc scientifique de Luminy, B.P. 13 13273 Marseille, France 2 Croatian Institute for Brain Research, School of Medicine, University of Zagreb, 10000 Zagreb, Croatia 3 Institute of Experimental Medicine, Hungarian Academy of Sciences, H-1450 Budapest, Hungary ABSTRACT In the pilocarpine model of chronic limbic seizures, vulnerability of GABAergic interneurons to excitotoxic damage has been reported in the hippocampal CA1 region. However, little is known about the specific types of interneurons that degenerate in this region. In order to characterize these interneurons, we performed quantitative analyses of the different populations of GABAer- gic neurons labeled for their peptide or calcium-binding protein content. Our data demonstrate that the decrease in the number of GAD mRNA-containing neurons in the stratum oriens of CA1 in pilocarpine-treated rats involved two subpopulations of GABAergic interneurons: interneu- rons labeled for somatostatin only (O-LM and bistratified cells) and interneurons labeled for parvalbumin only (basket and axo-axonic cells). Stratum oriens interneurons labeled for somatostatin/calbindin or somatostatin/parvalbumin were preserved. The decrease in number of somatostatin- and parvalbumin-containing neurons was observed as early as 72 hours after the sustained seizures induced by pilocarpine injection. Many degenerating cell bodies in the stratum oriens and degenerating axon terminals in the stratum lacunosum-moleculare were observed at 1 and 2 weeks after injection. In addition, the synaptic coverage of the axon initial segment of CA1 pyramidal cells was significantly decreased in pilocarpine-treated animals. These results indicate that the loss of somatostatin-containing neurons corresponds preferentially to the degeneration of interneurons with an axon projecting to stratum lacunosum-moleculare (O-LM cells) and suggest that the death of these neurons is mainly responsible for the deficit of dendritic inhibition reported in this region. We demonstrate that the loss of parvalbumin-containing neurons corresponds to the death of axo-axonic cells, suggesting that perisomatic inhibition and mechanisms controlling action potential generation are also impaired in this model. J. Comp. Neurol. 459:407– 425, 2003. © 2003 Wiley-Liss, Inc. Indexing terms: temporal lobe epilepsy; rat; GAD; somatostatin; parvalbumin; fluoro-jade B Vulnerability of subpopulations of GABAergic neurons to seizure-induced damage has been reported in two major regions of the hippocampal formation in experimental and human temporal lobe epilepsy (TLE). The loss of GABAergic interneurons is now well demonstrated in the hilus of the dentate gyrus in several models of TLE (Obenaus et al., 1993; Houser and Esclapez, 1996; Buck- master and Jongen-Relo, 1999) and the vulnerability of specific subpopulations of hilar interneurons including somatostatin- and parvalbumin-containing neurons is es- tablished in experimental (Sloviter, 1987; 1991; Ylinen et al., 1991; Sperk et al., 1992; Magloczky and Freund, 1995; Buckmaster and Dudek, 1997; Gorter et al., 2001) as in Grant sponsor: Institut National de la Sante ´ et de la Recherche Me ´dicale; Grant sponsor: the Howard Hughes Medical Institute (T.F.F); Grant sponsor: National Institute of Health; Grant number: NS 30549; Grant sponsor: Orsza ´- gos Tudoma ´ nyos Kutata ´ si Alapprogramok Hungary; Grant number: T 32251. *Correspondence to: Monique Esclapez, INMED, INSERM U29, Parc scientifique de Luminy, 163 Route de Luminy, B.P 13 13273 Marseille, Cedex 9, France. E-mail: esclapez@inmed.univ-mrs.fr Received 25 July 2002; Revised 13 December 2002; Accepted 20 Decem- ber 2002 DOI 10.1002/cne.10622 Published online the week of March 24, 2003 in Wiley InterScience (www.interscience.wiley.com). THE JOURNAL OF COMPARATIVE NEUROLOGY 459:407– 425 (2003) © 2003 WILEY-LISS, INC.