Several chimeric peptides were synthesized and found to be high-affinity ligands for both galanin and substance P receptors in membranes from the rat hypothalamus. The peptide galantide, composed of the N-terminal part of galanin and C-terminal part of substance P (SP), galanin-(1-12)-Pro-SP-(5-11) amide, which is the first galanin antagonist to be reported, recognizes two classes of galanin binding sites (KD(1) less than 0.1 nM and KD(2) approximately 6 nM) in the rat hypothalamus, while it appears to bind to a single population of SP receptors (KD approximately 40 nM). The chimeric peptide has higher affinity towards galanin receptors than the endogenous peptide galanin-(1-29) (KD approximately 1 nM) or its N-terminal fragment galanin-(1-13) (KD approximately 1 microM), which constitutes the N-terminus of the chimeric peptide. Galantide has also higher affinity for the SP receptors than the C-terminal SP fragment-(4-11) amide (KD = 0.4 microM), which constitutes its C-terminal portion. Substitution of amino acid residues, which is of importance for recognition of galanin by galanin receptors, such as [Trp2], in the galanin portion of the chimeric peptide or substitution of ([Phe7] or [Met11]-amide) in the SP portion of chimeric peptide both cause significant loss in affinity of the analogs of galantide for both the galanin- and the SP-receptors. These results suggest that the high affinity of the chimeric peptide, galantide, may in part be accounted for by simultaneous recognition/binding to both receptors.(ABSTRACT TRUNCATED AT 250 WORDS)