EVALUATION OF TRUNCATED NEUROPEPTIDE-Y ANALOGS WITH MODIFICATIONS OF THE TYROSINE RESIDUE IN POSITION-1 ON Y1-RECEPTOR, Y2-RECEPTOR AND Y3-RECEPTOR SUBTYPES

Substitutions of the tyrosine residue in position 1 of truncated neuropeptide Y (N-terminal fragment 1-4 linked to C-terminal fragment 18-36 by the epsilon-aminocaproic acid) produced analogues that compete for specific [I-125]polypeptide YY (PYY) binding in the frontoparietal cortex (Y1-enriched) with a profile best fitted to a two site-model with K(D) values in the low and high nM range, respectively. In the hippocampal membrane preparations (Y2-enriched), halogen substitutions on the aromatic ring generated analogues with competition profiles best fitted to a one-site model, revealing differences between the two binding assays and the interaction of these analogues with the Y1 and Y2 receptor sub-types. In the rat vas deferens (Y2-enriched), all truncated analogues inhibited the twitch response with similar or slightly weaker potency than the native molecule. In contrast, these molecules were markedly less potent than neuropeptide Y (NPY) in the rabbit saphenous vein (Y1-enriched) and the rat distal colon (Y3-enriched). Some of the truncated analogues were inactive at up to muM concentrations in the rat distal colon, demonstrating the distinct structural requirement of the receptor sub-type present in this bioassay. These results revealed that amino acid residues between positions 5 and 17 are critical for the maintenance of optimal affinity for the NPY receptors present in the rabbit saphenous vein and the rat distal colon. They also further demonstrate the distinct structural requirements of the Y1, Y2 and Y3 receptor sub-types and the potential usefulness of truncated analogues to distinguish between Y1, Y2 and the newly characterized Y3 sub-type; some truncated analogues being inactive on the latter.