Ontogeny of seizure-induced increases in BDNF immunoreactivity and TrkB receptor activation in rat hippocampus

The present work tested the hypothesis that the anatomic and developmental patterns of status epilepticus-induced increases of brain-derived neurotrophic factor (BDNF) protein coincided with status epilepticus-induced increases of phospho-Trk immunoreactivity, a measure of TrkB receptor activation, in rat hippocampus. In P22 rats, robust increases of phospho-Trk immunoreactivity were detected in the mossy fiber pathway of the hippocampus one day following kainate-induced status epilepticus. Conversely, no change in phospho-Trk immunoreactivity was detected in P8 or P14 rats. In P17 rats, intermediate levels of increased phospho-Trk immunoreactivity were detected, again in the mossy fiber pathway. Like phospho-Trk immunoreactivity, marked increases of BDNF immunoreactivity were detected in the mossy fiber pathway of P22 but not P14 rats. Dissociations were found in P17 rats following status epilepticus in that striking increases of BDNF, but not phospho-Trk immunoreactivity were detected. Immunoprecipitation and Western blot analyses of hippocampal extracts after status epilepticus showed increased phospho-TrkB, but not TrkB immunoreactivity in P22 rats, thereby confirming and extending the immunohistochemical findings. While most of the findings support the hypothesis, important dissociations among individual animals at P17 were identified. Together the findings are consistent with the proposal that status epilepticus-induced increase of BDNF content in the mossy fibers is necessary, but not sufficient, to effect activation of TrkB, as revealed by phospho-Trk immunoreactivity. Furthermore, these results provide the first characterization of seizure-induced increases in BDNF protein and TrkB receptor activation in developing animals.