PACAP Receptor Signaling. In: Vaudry H, Arimura A, editors. Pituitary Adenylate Cyclase-Activating Polypeptide. Boston: Springer US; 2003. pp. 95-124. [DOI: 10.1007/978-1-4615-0243-2_5]

Pituitary adenylate cyclase-activating polypeptide (PACAP) decreases ischemic neuronal cell death in association with IL-6

Pituitary adenylate cyclase-activating polypeptide (PACAP) has been reported to decrease ischemic neuronal damage and increase IL-6 secretion in rats. However, the mechanisms underlying neuroprotection are still to be fully elucidated. The present study was designed to investigate the role played by PACAP and IL-6 in mediating neuroprotection after ischemia in a null mouse. Infarct volume, neurological deficits, and cytochrome c in cytoplasm were higher in PACAP(+/-) and PACAP(-/-) mice than in PACAP(+/+) animals after focal ischemia, although the severity of response was ameliorated by the injection of PACAP38. A decrease in mitochondrial bcl-2 was also accentuated in PACAP(+/-) and PACAP(-/-) mice, but the decrease could be prevented by PACAP38 injection. PACAP receptor 1 (PAC1R) immunoreactivity was colocalized with IL-6 immunoreactivity in neurons, although the intensity of IL-6 immunoreactivity in PACAP(+/-) mice was less than that in PACAP(+/+) animals. IL-6 levels increased in response to PACAP38 injection, an effect that was canceled by cotreatment with the PAC1R antagonist. However, unlike in wild-type controls, PACAP38 treatment did not reduce the infarction in IL-6 null mice. To clarify the signaling pathway associated with the activity of PACAP and IL-6, phosphorylated STAT (signal transducer and activator of transcription) 3, ERK (extracellular signal-regulated kinase), and AKT levels were examined in PACAP(+/-) and IL-6 null mice after ischemia. Lower levels of pSTAT3 and pERK were observed in the PACAP(+/-) mice, whereas a reduction in pSTAT3 was recorded in the IL-6 null mice. These results suggest that PACAP prevents neuronal cell death after ischemia via a signaling mechanism involving IL-6.

PACAP maintains cell cycling and inhibits apoptosis in chick neuroblasts

We previously reported that pituitary adenylate cyclase-activating polypeptide (PACAP) increased cAMP in neuroblast-enriched cultures from embryonic day 3.5 chick brain. Also, the neuroblasts expressed the mRNA, peptide, and receptor for PACAP. Here, we investigated downstream effects of increased cAMP by examining PACAP's role in regulating cell numbers during brain development. Using flow cytometry, we quantified proliferating cell nuclear antigen and DNA, and compared apoptotic cells and cells in cell cycle compartments under differing conditions. Untreated cultures showed high proliferative activity with little apoptosis. Addition of exogenous PACAP had no effect on this pattern. However, blocking endogenous PACAP with a receptor antagonist increased cell cycle exit, then increased apoptosis. We conclude that chick neuroblasts require production of PACAP to inhibit apoptosis and maintain full proliferative activity during early brain development.

[Prevention of delayed neuronal cell death by PACAP and its molecular mechanism]

Ischemic delayed neuronal cell death (apoptosis) in the hippocampus is prevented by PACAP. PACAP inhibits the increasing activity of the MAP kinase family, especially on JNK/SAPK and p38, thereby protecting against apoptotic cell death. After the ischemia-reperfusion, both pyramidal cells and astrocytes increased their expression of PACAP receptors (PAC1-Rs). The pyramidal cells degenerated but reactive astrocytes increased their expression of PAC1-R. PACAP does not only inhibit apoptotic cell death directly, but also affects astrocytes through PAC1-Rs. Interleukin-6 (IL-6), produced in astrocytes, has several effects on the prevention of brain ischemia and trauma and stimulating neuronal growth. IL-6 secretion into the CSF was markedly stimulated after PACAP infusion, suggesting that PACAP stimulates IL-6 secretion from astrocytes. We studied the effects of PACAP on the wild type and IL-6 KO mice after brain ischemia. In wild-type animals, PACAP significantly inhibited cell death, but in IL-6 KO animals, no cytoprotective effect of PACAP was seen. These results suggest that PACAP inhibits apoptotic cell death partly through IL-6. It is considered that PACAP itself and IL-6, stimulated secretion by PACAP, both synergistically inhibit the JNK/SAPK and p38 signaling pathway, thereby protecting against neuronal cell death.