Physiologic and Morphologic Characteristics of Granule Cell Circuitry in Human Epileptic Hippocampus

Abstract

Morphological and electrophysiological techniques were used to examine granule cells and their mossy fiber axons in nine surgically resected hippocampal specimens from temporal lobe epilepsy (TLE) patients. Timm histochemistry showed mossy fiber sprouting into the inner molecular layer (IML) of the dentate in a subset of tissue samples. In slices from five tissue samples, stimulus-induced bursting activity could be induced with a low concentration (2.5 microM) of bicuculline; bursts were sensitive to the N-methyl-D-aspartate (NMDA) blocker, APV. There was a general correlation between such sprouting and experimentally induced hyperexcitability. Fourteen granule cells from five tissue samples were intracellularly stained [with lucifer yellow (LY) or neurobiotin]. Axons from a subset of these neurons showed axon collaterals reaching into the IML, but this axon projection pattern for single cells was not directly correlated with degree of mossy fiber sprouting shown grossly by Timm staining. Electron microscopic examination of intracellularly stained elements showed mossy fiber axon terminals making asymmetric synaptic contacts (including autapses on the granule cell dendrite) with dendritic shafts and spines in both apical and basal domains. These data are consistent with the hypothesis that mossy fiber sprouting provides a structural basis for recurrent excitation of granule cells, but does not provide direct support of the hypothesis that mossy fiber sprouting causes hyperexcitability. The data suggest that granule cell bursting activity is at least in part a function of compromised synaptic inhibition, since levels of gamma-aminobutyric acid (GABA) blockade that are generally subthreshold for burst induction were epileptogenic in some tissue samples from human epileptic hippocampus.

Structural and functional asymmetry in the normal and epileptic rat dentate gyrus

“…These data argue that infrapyramidal blade excitability may relate, or even depend, on the anatomic differences. Indeed, this possibility supports the hypothesis in the literature that mossy fiber sprouting increases dentate gyrus excitability, based on anatomic (Frotscher and Zimmer, 1983;Represa et al, 1993;Franck et al, 1995;Okazaki et al, 1995;Sutula et al, 1998;Wenzel et al, 2000) and physiological data (Tauck and Nadler, 1985;Sutula et al, 1992Sutula et al, , 1998Dudek et al, 1994;Pollard et al, 1996;Wuarin and Dudek, 1996;Patrylo and Dudek, 1998;Molnar and Nadler, 1999;Okazaki et al, 1999).…”

Ultrastructural localization of zinc transporter-3 (ZnT-3) to synaptic vesicle membranes within mossy fiber boutons in the hippocampus of mouse and monkey

“…For light microscopy detection in mouse and monkey brains, a Timm's-staining protocol described by Franck et al (18) was adapted. Fixed hippocampal slices (1 mm) were immersed in 0.4% Na 2 S for 30 min, then fixed ϳ16 hr in 1% paraformaldehyde and 1.25% glutaraldehyde.…”

Mossy Fiber Zinc and Temporal Lobe Epilepsy: Pathological Association with Altered “Epileptic”γ‐Aminobutyric Acid A Receptors in Dentate Granule Cells

“…Targets for these sprouted terminals include both DGCs and inhibitory interneurons (21,22). Extracellular, intracellular, and optical recording studies have demonstrated that these sprouted MFs are functional, i.e., activation of these collaterals elicits excitatory responses in the dentate gyrus (21,(23)(24)(25).…”

“…Targets for these sprouted terminals include both DGCs and inhibitory interneurons (21,22). Extracellular, intracellular, and optical recording studies have demonstrated that these sprouted MFs are functional, i.e., activation of these collaterals elicits excitatory responses in the dentate gyrus (21,(23)(24)(25). However, some degree of disinhibition appears necessary for pathological (epileptic) activity to be manifest in the dentate gyrus of slices prepared from sprouted, epileptic animals (24,26), suggesting that normal or augmented inhibitory responses may keep the function of these aberrant collaterals in check under normal conditions.…”