Hippocampal AMPA and NMDA mRNA levels correlate with aberrant fascia dentata mossy fiber sprouting in the pilocarpine model of spontaneous limbic epilepsy

Abstract

There is considerable controversy whether aberrant fascia dentata (FD) mossy fiber sprouting is an epiphenomena related to neuronal loss or a pathologic abnormality responsible for spontaneous limbic seizures. If mossy fiber sprouting contributes to seizures, then reorganized axon circuits should alter postsynaptic glutamate receptor properties. In the pilocarpine-status rat model, this study determined if changes in alpha amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) and n-methyl-D-aspartic acid (NMDA) receptor subunit mRNA levels correlated with mossy fiber sprouting. Sprague-Dawley rats were injected with pilocarpine (320 mg/kg; i.p.) and maintained in status epilepticus for 6 to 8 hours (pilocarpine-status). Rats were killed during the: (1) latent phase after neuronal loss but before spontaneous limbic seizures (day 11 poststatus; n = 7); (2) early seizure phase after their first seizures (day 25; n = 7); and (3) chronic seizure phase after many seizures (day 85; n = 9). Hippocampi were studied for neuron counts, inner molecular layer (IML) neo-Timm's staining, and GluR1-3 and NMDAR1-2b mRNA levels. Compared with controls, pilocarpine-status rats in the: (1) latent phase showed increased FD GluR3, NMDAR1, and NMDAR2b; greater CA4 and CA1 NMDAR1; and decreased subiculum GluR1 hybridization densities; (2) early seizure phase showed increased FD GluR3, increased CA1 NMDAR1, and decreased subiculum NMDAR2b densities; and (3) chronic seizure phase showed increased FD GluR2; increased FD and CA4 GluR3; decreased CA1 GluR2; and decreased subiculum GluR1, GluR2, NMDAR1, and NMDAR2b levels. In multivariate analyses, greater IML neo-Timm's staining: (1) positively correlated with FD GluR3 and NMDAR1 and (2) negatively correlated with CA1 and subiculum GluR1 and GluR2 mRNA levels. These results indicate that: (1) hippocampal AMPA and NMDA receptor subunit mRNA levels changed as rats progressed from the latent to chronic seizure phase and (2) certain subunit alterations correlated with mossy fiber sprouting. Our findings support the hypothesis that aberrant axon circuitry alters postsynaptic hippocampal glutamate receptor subunit stoichiometry; this may contribute to limbic epileptogenesis.

Reduced Inhibition of Dentate Granule Cells in a Model of Temporal Lobe Epilepsy

“…It seems unlikely that new GABAergic synapses at distal dendritic sites went undetected, because we detected IPSCs evoked with minimal stimulation of the outer molecular layer. We cannot exclude the possibility that functional evidence for additional GABAergic synapses would have eventually developed after longer periods of chronic epilepsy, but at the stage examined, glutamatergic axon sprouting and synaptogenesis by granule cells are well established (Mello et al, 1993;Mathern et al, 1998;Okazaki et al, 1999;Wuarin and Dudek, 2001), so one might expect that GABAergic synaptogenesis would have occurred. Perhaps surviving interneurons only appear to sprout axon collaterals, because seizure activity increases antigen expression (Feldblum et al, 1990;Wanscher et al, 1990;Shinoda et al, 1991;Schwarzer et al, 1995;Houser and Esclapez, 1996;Esclapez and Houser, 1999), making preexisting axon collaterals more visible by immunocytochemical methods.…”

“…Changes in glutamate receptors (Mody and Heinemann, 1987;Mathern et al, 1998) and excitatory circuitry might enhance excitatory synaptic input to granule cells and make them hyperexcitable. We found evidence of changes in excitatory circuitry that included abnormal morphological characteristics reported previously for granule cells in epileptic tissue (Fig.…”

Electrophysiological Evidence of Monosynaptic Excitatory Transmission Between Granule Cells After Seizure-Induced Mossy Fiber Sprouting

“…Furthermore, it will be necessary to understand the impact of other changes in the epileptic brain, including alterations in axonal structure (Pierce and Milner 2001) and neurotransmitters/neuromodulators (Elmer et al 1996;de Lanerolle et al 1998;Gall 1993;Mathern et al 1997Mathern et al , 1998.…”

Pathomechanismen der Epilepsie

“…" Key words EpilepsyNeuronal excitability -bursting -I m -I hpersistent sodium currentsynaptic plasticity -NMDA -GABA -sprouting " Zusammenfassung Die Epilepsieforschung ist kein neues wissenschaftliches Feld -klinische und experimentelle Untersuchungen werden seit Jahrzehnten be- [26,27]: Im Gyrus dentatus epilepsiechirurgischer Präparate fanden sich gegenüber Nagergewebe deutlich -neben molekularen Verschiebungen der Rezeptordichten des Hippocampus [39,40] -verlängerte NMDA-vermittelte synaptische Potentiale und Ströme, während AMPA/Kainat-vermittelte Reaktionen unverändert blieben (Abb. 8).…”