Abstracts

Neonatal seizures have been associated with decreased seizure threshold and cognitive impairment in adulthood. One mechanism that may contribute to the long-term effects of repeated seizures is the upregulation of trophic factors induced by seizure activity. If seizures change trophic factor expression during infancy, this could disrupt the normal developmental regulation of neuronal growth and survival patterns in the immature brain.
In adult rats, we have previously demonstrated an increase in basic fibroblast growth factor (bFGF, FGF-2) in several limbic regions after repeated brief seizures evoked by exposure to low-intensity (minimal) electroconvulsive shock (ECS). The purpose of the present study is to determine whether repeated exposure to ECS-evoked seizures can increase bFGF expression in the immature rat brain.
Minimal ECS was applied to rat pups from P5 to P7 via corneal electrodes (200 msec, 35 mA) which induced minimal limbic motor seizures lasting 5-10 sec. A single daily session consisted of 3 ECS seizures, given at 0, 30, and 60 min. Daily sessions were given for either 1 d or 3d. Control (sham-treated) rats received the same handling but no current was passed through the electrodes. bFGF protein expression was measured in several forebrain regions by Western blotting and immunohistochemistry.
A single ECS session resulted in a significant increase in bFGF protein levels in the rhinal cortex of neonatal rats as determined by Western blotting. The levels of all three isoforms of bFGF were upregulated (24 kDa isoform by 36%, 22.5/22 kDa [ndash] by 27%, and the secreted, 18 kDa isoform [ndash] by 43.8%) by ECS. Three daily sessions of ECS increased only the 18 kDa isoform (by 29.1%). The other isoforms showed increases that did not reach significance. Upregulation of bFGF protein was detectable at 6 hr after the last ECS treatment and disappeared by 24hr. A comparable upregulation of FGF-2 protein was detected by immunohistochemistry in the hippocampal CA2 subfield.
Our data indicate that repeated brief seizures can affect the expression pattern of bFGF in the developing brain. This effect has a more rapid onset but a reduced magnitude compared to what was previously observed in adults. Moreover, unlike the effect in the adult brain, the repeated ECS treatment in the neonatal rat did not cause an augmented accumulation of bFGF protein. This may reflect a finer degree of coupling of bFGF biosynthesis and utilization in the immature brain. Measurement of FGF mRNA will allow us to assess the extent to which repeated seizures cause a sustained stimulation of bFGF biosynthesis in the immature brain.
[Supported by: NIH grants NS 20576, MH 02040, and a predoctoral fellowship by the Epilepsy Foundation]