Abstracts

Rationale: Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are neurotrophins that have been implicated in many aspects of epilepsy, but their roles are still not clear. One reason may be that effects are mediated not only by the mature peptides (NGF, BDNF) but also by their precursors, proneurotrophins (proNGF, proBDNF). To address this hypothesis, we compared the effects of the convulsant kainic acid (KA) in knock in mice that overexpress either proNGF or proBDNF, due to a mutation in the furin site where proneurotrophins are normally cleaved.Methods: Four genotypes were studied: 1) mice with one cleavage-resistant allele of NGF, tagged with hemagglutinin (HA; proNGF-HA/+), 2) their wild type littermates (+/+), 3) mice with one cleavage-resistant allele of BDNF and HA tag (proBDNF-HA/+), and 4) their wild type littermates (+/+). KA doses were 22mg/kg (proNGF mice; Sv129 background) and 25mg/kg (proBDNF mice; C57bl/6J background). Adult male mice (7-8 weeks old) were implanted with electrodes for video-EEG (1 subdural screws over left frontal cortex, 1 over right occipital cortex, 1 twisted bipolar depth electrode in each dorsal hippocampus) and recordings were made 2 weeks later (Pinnacle Tech.). A screw over the left olfactory bulb served as ground and a screw over the cerebellum was reference. Seizures were quantified as follows: latency to the first epileptiform activity; latency to the first nonconvulsive or convulsive seizure (stages 3-5, Racine scale); number of nonconvulsive or convulsive seizures. Eight and 20 weeks after KA administration, 2 weeks of 24 hr/7day/week video-EEG was used to determine if there were differences in spontaneous seizure frequency, using automated seizure detection (Sirenia, Pinnacle Tech.). Results: Immediately after KA administration, there were fewer convulsive seizures in proNGF-HA/+ mice (mean sem, 3.0 1.3, n=5) compared to +/+ mice (7.4 1.0, n=8; p=0.018). Other measurements were not significantly different. None of the proNGF-HA/+ mice developed epilepsy, defined by at least one spontaneous seizure during 2 weeks of continuous monitoring. In proBDNF mice, a similar number of mice in each group had a severe prolonged seizure in the first hours after KA injection and died (n=2/7, 3/8 respectively), which could be due to the strain, where more seizures elicited by KA cause mortality than other strains. In the remaining mice, proBDNF-HA/+ mice had more convulsive seizures (5.2 1.1, n=4) than +/+ mice (1.5 0.3, n=5; p=0.020) and other measurements were not significantly different. Conclusions: The results suggest that impaired cleavage of proneurotrophins influences convulsive seizures elicited by KA. However, in light of previous reports that mature NGF facilitates seizures, proNGF-HA/+ mice could be protected because of haploinsufficiency of mature NGF rather than accumulation of proNGF. Based on published studies showing that that infusion of mature BDNF into hippocampus induces convulsive seizures, it would seem unlikely that proBDNF-HA/+ mice are vulnerable because of haploinsufficiency of mature BDNF. Instead, proBDNF may be proconvulsant.