Persistent regional increases in brain-derived neurotrophic factor in the flurothyl model of epileptogenesis are dependent upon the kindling status of the animal.

Brain-derived neurotrophic factor (BDNF) appears to be both regulated by and a regulator of epileptogenesis. In the flurothyl (HFE) model of kindling mice exposed to successive flurothyl trials over 8 days express a rapid, long-lasting reduction in generalized seizure threshold and a more slowly evolving change in seizure phenotype in response to subsequent flurothyl exposure. The BDNF genotype of particular mouse strains appears to influence the epileptogenic progression in this model. Thus, we hypothesized that BDNF signaling pathways are altered by flurothyl-induced seizures. Following HFE kindling, fully kindled (eight seizures) adult male C57BI/6J mice had significantly elevated whole brain BDNF levels through at least 28 days after their final seizure. Mice that received only four HFE seizures (not kindled) had elevated BDNF levels, but only at 1 day post-seizure (DPSz), while BDNF levels were not significantly altered in mice receiving just one HFE seizure at any time point studied. Regional expression patterns of BDNF in the hippocampus, hypothalamus, and frontal cortex were also elevated by one DPSz and returned to control values by 14 DPSz in mice that received four HFE seizures. No changes were seen in the cerebellum, striatum, or piriform cortex. In contrast, fully kindled mice had significantly elevated BDNF levels within the hippocampus, hypothalamus, neocortex, and striatum that remained elevated through at least 14 DPSz, while levels were unchanged in the cerebellum and piriform cortex. Regional results were confirmed using anti-BDNF immunohistochemistry (IHC). Despite changes in BDNF levels following HFE kindling, we were unable to demonstrate alterations either in full-length tyrosine kinase receptor B (TrkB) expression (Western blot and IHC) or in truncated TrkB (IHC) expression levels. Together, these data suggest a model of a positive feedback loop involving seizure activity and seizure number and persistently modified BDNF signaling pathways that influences seizure phenotypes within the HFE kindling paradigm. Thus, long-term elevations in BDNF may be responsible in part for epileptogenic processes and the development of human refractory epilepsies.