Brain derived neurotrophic factor promoter 4 (BP4) and brain derived neurotrophic factor activity in primary neuronal cultures following potassium chloride (KCl) depolarization. Primary hippocampal (A, D) cortical (B, E), and amygdala (C, F) cultures were co-transfected with BP4 and renilla plasmid. Cells were either untreated (A–C) or treated (D–F) with 60 mmol/L KCl for 24 hours before harvesting for analysis by dual luciferase assay. Relative light units were calculated by normalizing luciferase signal to renilla signal. n ≥ 3, **p < .01.