Abstracts

RATIONALE: BDNF is known to foster the development and maintenance of neurons, and is highly expressed in the mossy fiber axons of dentate gyrus granule cells, where it is increased after seizures. BDNF also appears to increase seizure susceptibility in vivo and cause hyperexcitability in area CA3 in vitro (Scharfman, J. Neurophysiol., 1997; Croll et al., Neuroscience, 1999). Therefore, we hypothesized that BDNF infusion into the hippocampus might cause seizures and increased neurogenesis of granule cells. Of particular interest was the potential development of granule-like neurons in the hilar region following BDNF, which occurs after pilocarpine-induced status epilepticus (Scharfman et al., J. Neurosci., 2000) and may be fostered by BDNF production in adult granule cells after seizures.
METHODS: BDNF (0.5 [mu]l/hr, 3mg/ml), phosphate-buffered saline (PBS), or albumin (3 mg/ml) were infused for 2 weeks by osmotic pumps attached to a cannula lowered into the dorsal dentate gyrus of one hemisphere. Bromo-deoxyuridine (BrdU) was injected 2x/day from day 4 to day 14 (50 mg/kg i.p.). Animals were perfused with 4% paraformaldehyde at least 2 weeks after infusion stopped. Immunocytochemistry using monoclonal antibodies to NeuN (Chemicon, 1:5000), calbindin (Sigma, 1:100,000, monoclonal), or BrdU (Boehringer-Mannheim, 1:1000) were used to examine neurogenesis of granule cells. Double labeling was performed using diaminobenzidine for BrdU and NovaRed (Vector) for calbindin or NeuN.
RESULTS: BDNF-treated animals (n = 16/17) had increased numbers of BrdU-labeled cells in the dentate gyrus granule cell layer compared to PBS-infused (n=8) or albumin-infused (n=6) animals. Some of the new cells were located in the hilar region and were double labeled with NeuN, indicating that they were neurons, or with calbindin, a marker of granule cells. Behavioral (motor) seizures (stages 3-5) were observed in 5 of the BDNF-treated rats but not in PBS- or albumin-treated rats. Seizures may have occurred more often, because observation was not continuous.
CONCLUSIONS: The results suggest that BDNF infusion into the hippocampus can result in seizures, increased neurogenesis, and aberrant production of granule-like neurons in the hilar region. This was similar, but less dramatic, than the effects of pilocarpine-induced status epilepticus reported previously. These changes may occur as a result of direct effects of BDNF, secondary to BDNF-induced seizure activity resulting from BDNF treatment, or a combination, as well as BDNF induction of neuropeptide Y, which can influence seizure activity and proliferation.
Support: This work was supported by NS 37562 and NS 38285. We thank Amgen-Regeneron Partners for BDNF.
Disclosure: Grant - One of the authors, Scharfman, is principal investigator on an NIH grant (NS 37562) that relates to the subject of the abstract.