In adult brain,
nerve growth factor (
NGF) gene expression is generally upregulated by neuronal activity. However, a single episode of hilus lesion (HL)-induced limbic
seizures stimulates a biphasic increase in
NGF mRNA expression with peaks at 4-6 and 24 hr after lesion and an intervening return to control levels
at 10-12 hr after lesion. In vitro studies suggest that
NGF transcription is regulated via an activating
protein 1 (AP-1) binding site in the first intron of the
NGF gene. To examine the relationship between seizure-induced
AP-1 binding and
NGF gene expression in this paradigm,
NGF mRNA levels and
AP-1 binding were examined after HL
seizures. Furthermore, to gain insight into the functional composition of the
AP-1 complex, supershift analysis was performed to characterize which Fos and Jun family members are included in the AP-1-binding complex at the different time points analyzed.
Solution hybridization analysis verified the biphasic increase in
NGF mRNA content of the dentate gyrus after HL
seizures. After an initial increase,
AP-1 binding slowly declined in a stepwise manner that encompassed, but did not correspond with, the two phases of
NGF mRNA expression. However, supershift analyses demonstrated that the relative contributions of JunD and JunB to the
AP-1 complex exhibited positive and negative correlations, respectively, with the phases of increased
NGF expression after HL. These results suggest that
AP-1 complexes containing JunD promote
NGF transactivation and that transient changes in the relative contributions of JunD and JunB to
AP-1 binding underlie the biphasic increase in
NGF gene expression induced by HL
seizures.