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

Tumor necrosis factor-α (TNF-α) 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-α mRNA was induced within 2 hr after rat spinal cord transection, and its upregulation was suppressed by a synthetic VIP receptor agonist. Cultured rat microglia were used to examine the mechanisms underlying this inhibition because microglia are the likely source of TNF-α in injured CNS. In culture, increases in TNF-α mRNA resulting from lipopolysaccharide (LPS) stimulation were reduced significantly by 10^-7 M VIP and completely eliminated by PACAP at the same concentration. TNF-α protein levels were reduced 90% by VIP or PACAP at 10^-7 M. An antagonist of VPAC₁ receptors blocked the action of VIP and PACAP, and a PAC₁ antagonist blocked the action of PACAP. A direct demonstration of VIP binding on microglia and the existence of mRNAs for VPAC₁ and PAC₁ (but not VPAC₂) receptors argue for a receptor-mediated effect. The action of VIP is cAMP-mediated because (1) activation of cAMP by forskolin mimics the action; (2) PKA inhibition by H89 reverses the neuropeptide-induced inhibition; and (3) the lipophilic neuropeptide mimic, stearyl-norleucine¹⁷ VIP (SNV), which does not use a cAMP-mediated pathway, fails to duplicate the inhibition. We conclude that VIP and PACAP inhibit the production of TNF-α from activated microglia by a cAMP-dependent pathway.