The effects of a vasoactive intestinal peptide (VIP) receptor antagonist (VIPhyb) on human glioblastoma cells were characterized. Pituitary adenylate cyclase activating polypeptide (125I-PACAP-27) bound with high affinity to U87, U118, and U373 cells. Specific 125I-PACAP-27 binding to U87 cells was inhibited, with high affinity, by PACAP but not VIP or VIPhyb (IC50 = 10, 1500, and 500 nM, respectively). By reverse transcriptase-polymerase chain reaction (RT-PCR), a major 305 bp band was observed indicative of PAC1 receptors. PACAP-27 caused cAMP elevation and the increase in cAMP caused by PACAP-27, was inhibited by the VIPhyb. Also, PACAP-27 caused cytosolic Ca2+ elevation in Fura-2AM loaded U87 cells and the VIPhyb inhibited this increase. Using the MTT growth assay, the VIPhyb was shown to inhibit glioblastoma growth in a concentration-dependent manner. Using a clonogenic assay in vitro, 10 microM VIPhyb significantly inhibited proliferation of U87, U118, and U373 cells. In vivo, 0.4 microg/kg VIPhyb inhibited U87 xenograft proliferation in nude mice. These results suggest that the VIPhyb antagonizes PAC1 receptors on glioblastoma cells and inhibits their proliferation.