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.