PACAP-cytokine interactions govern adrenal neuropeptide biosynthesis after systemic administration of LPS

We have examined induction of neuropeptide expression in adrenal medulla after treatment of mice with lipopolysaccharide (LPS), a model for septic shock, which activates both immune and stress responses in vivo. Messenger RNAs encoding vasoactive intestinal polypeptide (VIP) and galanin, both modulators of steroidogenesis in neighboring adrenal cortex, are up-regulated at 24 h (eight-fold for VIP and two-fold for galanin) after LPS injection, and remain elevated for the following 24 h. Up-regulation of VIP and galanin by LPS is abrogated in pituitary adenylate cyclase-activating polypeptide (PACAP)-deficient mice, suggesting an interaction between LPS, or LPS-induced cytokines, and PACAP released in adrenal medulla from the splanchnic nerve. Treatment of cultured chromaffin cells with 100 nM PACAP and 10 nM tumor necrosis factor-alpha (TNF-alpha), a cytokine whose production is elevated by LPS, results in long-term synergistic up-regulation of VIP and galanin mRNA. PACAP blocks the earlier induction by TNF-alpha of mRNA encoding inhibitor of NF-kappaB alpha (I kappaB alpha), normally a negative autoregulator of TNF-alpha signaling through nuclear factor-kappaB (NF-kappaB), without affecting the induction of TNF-alpha-induced protein 3 (TNFAIP3), another NF-kappaB-dependent gene induced by TNF-alpha in chromaffin cells. By acting downstream of NF-kappaB to inhibit I kappaB alpha gene induction by TNF-alpha, PACAP may block I kappaB alpha-dependent negative autoregulation of TNF-alpha signaling through NF-kappaB, prolonging TNF-alpha-dependent signaling to neuropeptide-encoding genes in chromaffin cells. This mechanism may also underlie PACAP-dependent neuropeptide gene induction by LPS in vivo.