Depolarization promotes PARP1-dependent H1 clearance in IEG promoters. (A) IEG expression is elevated in cultured PMNs after neuronal stimulation. mRNA levels of Bdnf, TrkB, and c-fos were measured by quantitative real-time PCR in resting and depolarized neurons treated with either NaCl or KCl for 30 min. (B) KCl-induced IEG up-regulation is blocked by PARP inhibitors. mRNA levels of c-Fos, c-Jun, and Egr1 were measured by quantitative real-time PCR in resting (NaCl) and depolarized (KCl) PMNS and in PMNs treated with KCl and DHIQ (KCl+DHIQ). *, P < 0.05, two-tailed U test. (C and D) PARP1 replaces H1 on IEG promoters. ChIP assays were performed for H1 and PARP1 at c-Fos, c-Jun, and Egr1 promoters in PMNs after depolarization and after treatment with PARP1 inhibitors. After neuronal excitation, H1 was released (C) and PARP1 was enriched (D) at IEG promoters. PARP1 inhibition restored both H1 and PARP1 levels back to their levels in resting neurons. No changes were found in the tubulin promoter (used as a control). *, P < 0.05, two-tailed U test. All ChIP experiments were performed at least twice. Error bars represent SEM.