Physiological Unmasking of New Glutamatergic Pathways in the Dentate Gyrus of Hippocampal Slices From Kainate-Induced Epileptic Rats

Abstract

In humans with temporal lobe epilepsy and kainate-treated rats, the mossy fibers of the dentate granule cells send collateral axons into the inner molecular layer. Prior investigations on kainate-treated rats demonstrated that abnormal hilar-evoked events can occasionally be observed in slices with mossy fiber sprouting when gamma-aminobutyric acid-A (GABAA)-mediated inhibition is blocked with bicuculline. However, these abnormalities were observed infrequently, and it was unknown whether these rats were epileptic. Wuarin and Dudek reported that in slices from kainate-induced epileptic rats (3-13 mo after treatment), hilar stimulation evoked abnormal events in most slices with mossy fiber sprouting exposed simultaneously to bicuculline and elevated extracellular potassium concentration [K+]o. Using the same rats, extracellular recordings were obtained from granule cells in hippocampal slices to determine whether 1) hilar stimulation could evoke abnormal events in slices with sprouting in normal artificial cerebrospinal fluid (ACSF), 2) adding only bicuculline could unmask hilar-evoked abnormalities and glutamate-receptor antagonists could block these events, and 3) increasing only [K+]o could unmask these abnormalities. In normal ACSF, hilar stimulation evoked abnormal field potentials in 27% of slices with sprouting versus controls without sprouting (i.e., saline-treated or only 2-4 days after kainate treatment). In bicuculline (10 microM) alone, hilar stimulation triggered prolonged field potentials in 84% of slices with sprouting, but not in slices from the two control groups. Addition of the N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonopentanoic acid (AP5), either blocked the bursts or reduced their probability of occurrence. The alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), always eliminated the epileptiform bursts. In kainate-treated rats with sprouting, but not in saline-treated controls, abnormal hilar-evoked responses were also revealed in 6-9 mM [K+]o. Additionally, 63% of slices with sprouting generated spontaneous bursts lasting 1-40 s in ACSF containing 9 mm [K+]o; similar bursts were not observed in controls. These results indicate that 1) mossy fiber sprouting is associated with new glutamatergic pathways, and although NMDA receptors are important for propagation through these circuits, AMPA receptor activation is crucial, 2) modest elevations of [K+]o, in a range that would have relatively little effect on granule cells, can unmask these new excitatory circuits and generate epileptiform bursts, and 3) this new circuitry underlies an increased electrographic seizure susceptibility when inhibition is depressed or membrane excitability is increased.

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).…”

Mossy fiber plasticity and enhanced hippocampal excitability, without hippocampal cell loss or altered neurogenesis, in an animal model of prolonged febrile seizures

“…However, seizure-induced, newly sprouted mossy fibers terminate predominantly on GC dendrites (Okazaki et al, 1995;Wenzel et al, 2000;Buckmaster et al, 2002), forming recurrent excitatory circuits. These new excitatory circuits, if not counterbalanced (e.g., by concurrent sprouting of GABAergic fibers; Davenport et al, 1990;Deller et al, 1995;André et al, 2001), can indeed facilitate GC excitation and thus render the hippocampal network hyperexcitable (Tauck and Nadler, 1985;Wuarin and Dudek, 1996;Patrylo and Dudek, 1998;Okazaki et al, 1999;but see Sloviter, 1992). Thus, a causal contribution of the sprouted fibers to hippocampal hyperexcitability in HT animals is theoretically possible.…”

Physiological and Morphological Characterization of Dentate Granule Cells in the p35 Knock-Out Mouse Hippocampus: Evidence for an Epileptic Circuit

“…Extracellular field recordings were used to construct input-output (I/O) curves, under normal bathing medium conditions and when the tissue was challenged by the application of either an elevated concentration of extracel- lular potassium (Rutecki et al, 1985;Hardison et al, 2000) or introduction of BMI, a GABA A antagonist (Wuarin and Dudek, 1996;Patrylo and Dudek, 1998;Patrylo et al, 1999). A bipolar stimulating electrode was placed on the surface of the slice over the outer molecular layer of the superior blade of the dentate near the apex.…”

“…The other approach to uncovering potential excitability, one used in several studies on other MTLE-like tissues (Patrylo and Dudek, 1998), was to partially block GABAergic inhibition with bicuculline. Under this disinhibition condition, perforant path stimulation had a differential effect on the short-latency response that was similar to the effect seen with high potassium (i.e., enhanced population spiking in wild-type dentate) but only a mild fEPSP increase in the p35Ϫ/Ϫ tissue.…”