Abstracts

Extracellular signal-regulated kinases (ERKs) appear to be particularly important in activity-dependent forms of plasticity. While previous studies have reported a significant increase in phosphorylated ERK (pERK) levels in the hippocampus following the induction of severe seizures, it is unknown whether ERK is activated during spontaneous seizures. The goals of this study were to determine if ERK phosphorylation increases after spontaneous seizures and describe the time course and distribution of pERK labeling., Adult C57BL/6 mice were injected with pilocarpine (320 mg/kg) to induce status epilepticus that was then stopped by injection of diazepam (5 mg/kg) 2 h after seizure onset. Beginning two weeks after status epilepticus, animals were observed for spontaneous behavioral seizures and perfused transcardially with fixative at intervals between 2 min and 24 h after the onset of spontaneous seizures. Tissue containing the hippocampal formation was processed for immunohistochemical localization of pERK., In control mice, moderate to strong pERK immunoreactivity was detected in cell bodies and dendrites of neurons that were scattered throughout the hippocampal formation. Labeled neurons were most prominent in the CA1 pyramidal cell layer, subiculum and entorhinal cortex. In mice sacrificed 24 h and longer after spontaneous seizures (n=22), pERK labeling was substantially decreased throughout the hippocampal formation. However, as early as 2 min after the onset of a spontaneous seizure (n=5), the cell bodies of many dentate granule cells were moderately to strongly labeled. Increased pERK immunoreactivity was also evident in the entorhinal cortex, subiculum and pyramidal cell layer of the hippocampus. At this time point, pERK labeling in the neocortex was also increased but to a lesser extent than in the hippocampal formation. By 5 min after the onset of spontaneous seizure (n=3), moderate to strong pERK labeling was evident in virtually all dentate granule cells, the majority of CA1 pyramidal cells and numerous neurons in the entorhinal cortex. At 15 min after spontaneous seizures (n=6), pERK labeling in these regions was lower than at earlier time points. By 30 min and longer after spontaneous seizures, pERK-immunoreactivity had returned to very low levels, similar to that in animals at 24 h or more after spontaneous seizures., ERK was activated in neurons throughout the hippocampal formation near the time of spontaneous seizures. This seizure-related phosphorylation of ERK could be detected early but for only a short period. The functional significance of such ERK activation is unknown but could be part of the seizure initiation process and potentially contribute to long-term plastic changes in the epileptic animals., (Supported by VA Medical Research Funds and NIH Grant NS046524 to CRH.)