Vasoactive intestinal peptide and pituitary adenylyl cyclase-activating polypeptide inhibit tumor necrosis factor-alpha production in injured spinal cord and in activated microglia via a cAMP-dependent pathway

Tumor necrosis factor-alpha (TNF-alpha) production accompanies CNS insults of all kinds. Because the neuropeptide vasoactive intestinal peptide (VIP) and the structurally related peptide pituitary adenylyl cyclase-activating polypeptide (PACAP) have potent anti-inflammatory effects in the periphery, we investigated whether these effects extend to the CNS. TNF-alpha mRNA wa s induced within 2 hr after rat spinal cord transection, and its upregulati on was suppressed by a synthetic VIP receptor agonist. Cultured rat microgl ia were used to examine the mechanisms underlying this inhibition because m icroglia are the likely source of TNF-alpha in injured CNS. In culture, inc reases in TNF-alpha mRNA resulting from lipopolysaccharide (LPS) stimulatio n were reduced significantly by 10(-7) M VIP and completely eliminated by P ACAP at the same concentration. TNF-alpha protein levels were reduced 90% b y VIP or PACAP at 10(-7) M. An antagonist of VPAC(1) receptors blocked the action of VIP and PACAP, and a PAC(1) antagonist blocked the action of PACA P. A direct demonstration of VIP binding on microglia and the existence of mRNAs for VPAC(1) and PAC(1) (but not VPAC(2)) receptors argue for a recept or-mediated effect. The action of VIP is cAMP-mediated because (1) activati on of cAMP by forskolin mimics the action; (2) PKA inhibition by H89 revers es the neuropeptide-induced inhibition; and (3) the lipophilic neuropeptide mimic, stearyl-norleucine(17) VIP (SNV), which does not use a cAMP-mediate d pathway, fails to duplicate the inhibition. We conclude that VIP and PACA P inhibit the production of TNF-alpha from activated microglia by a cAMP-de pendent pathway.