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.