Abstract
The structural requirements for VIP interaction with receptors on synaptosomes from rat cerebral cortex was investigated by the ability of VIP and VIP fragments, secretin analogues and fragments, peptides of the VIP/secretin family and several other regulatory peptides to inhibit specific 125I-VIP binding. Only large VIP fragments interacted with the VIP receptors with potencies relative to VIP ranging from 0.9–0.006%. The rank order of inhibition was: VIP 7–27 > VIP 11–28 > VIP 1–22-NH 2 > VIP 16–28. Shorter fragments: VIP 18–28; VIP 18–28-NH 2; VIP 19–28; VIP 21–28; VIP 22–28; VIP 1–18; VIP 1–18-NH 2; VIP 1–10-NH 2; VIP 1–6; VIP 16–20 and VIP 16–19 had no effect. Secretin fragments and analogues inhibited 125I-VIP binding with potencies of 2.2–0.01% relative to VIP in the order: secretin > (Ala 4, Val 5) secretin > (D-Ala 4) secretin > (D-Phe 6) secretin > secretin 5–27 > secretin 14–27. Other peptides of the VIP/secretin family inhibited 125I-VIP binding with potencies of 200-1%; avian VIP > porcine VIP > PHI = secretin > human GRF, whereas glucagon and GIP showed no inhibition. Among twenty-five other regulatory peptides only avian PP and somatostatin were inhibitors with relative potencies of 0.02% and 0.03%, respectively. In conclusion it may be emphasized that the intact VIP molecule is essential for VIP interaction with its receptors in the rat brain cortex. The central amino acid sequence in the molecule (residues 11–22), which forms the α-helix, seems to be required for VIP receptor recognition, whereas the N- and C-terminal sequences (residues 1–10 and 23–28) seems to be important for the affinity of VIP receptor binding.
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