Synapse-specific changes in Arc and BDNF in rat hippocampus following chronic temporal lobe epilepsy.

Expression of immediate early genes (IEGs) in the brain is important for synaptic plasticity, and probably also in neurodegenerative conditions. To understand the cellular mechanisms of the underlying neuropathophysiological processes in epilepsy, we need to pinpoint changes in concentration of synaptic plasticity-related proteins at subsynaptic levels. In this study, we examined changes in synaptic expression of Activity-regulated cytoskeleton-associated (Arc) and Brai Derived Neurotrophic Factor (BDNF) in a rat model of kainate-induced temporal lobe epilepsy (TLE). Western blotting showed reduced concentrations of Arc and increased concentrations of BDNF in hippocampal synaptosomes in chronic TLE rats. Then, using quantitative electron microscopy, we found corresponding changes in subsynaptic regions in the hippocampus. Specifically, we detected significant reductions in the concentrations of Arc in the presynaptic terminal of Schaffer collateral glutamatergic synapses in the stratum radiatum of the CA1 area in TLE, as well as in their adjacent postsynaptic spines. In CA3, there was a significant reduction of Arc only in the presynaptic terminal cytoplasm. Conversely, in CA3, there was a significant increase in the expression of BDNF in the presynaptic terminal, but not in the postsynaptic spine. Significant increase in BDNF concentration in the CA1 postsynaptic density was also obtained. We hypothesize that the observed changes in Arc and BDNF may contribute to both cognitive impairment and increased excitotoxic vulnerability in chronic epilepsy.

Changes in synaptic AMPA receptor concentration and composition in chronic temporal lobe epilepsy.

Excitotoxicity caused by excessive stimulation of glutamate receptors, resulting in pathologically increased Ca2+-concentrations, is a decisive factor in neurodegenerative diseases. We investigated long-term changes in synaptic contents of AMPA receptor subunits that play important roles in calcium regulation in chronic epilepsy. Such plastic changes may be either adaptive or detrimental. We used a kainic acid (KA)-based rat model of chronic temporal lobe epilepsy (TLE). Using hippocampal synaptosomes, we found significant reductions in the concentration of the AMPA receptor subunits GluA1 and GluA2, and the NMDA receptor subunit NR2B. The relative size of GluA1 and GluA2 reductions were almost identical, at 28% and 27%, respectively. In order to determine whether the synaptic reduction of the AMPA receptor subunits actually reflected the pool of receptors present along the postsynaptic density (PSD), as opposed to cytoplasmic or extrasynaptic pools, we performed postembedding immunogold electron microscopy (EM) of GluA1 and GluA2 in Schaffer collateral synapses in the hippocampal CA1 area. We found significant reductions, at 32% and 52% of GluA1 and GluA2 subunits, respectively, along the PSD, indicating that these synapses undergo lasting changes in glutamatergic neurotransmission during chronic TLE. When compared to the overall concentration and composition of AMPA receptors expressed in the brain, there was a relative increase in GluA2-lacking AMPA receptor subunits following chronic epilepsy. These changes in synaptic AMPA receptor subunits may possibly contribute to further aggravate the excitotoxic vulnerability of the neurons as well as have significant implications for hippocampal cognitive functions.