Abstract: Novel compounds which are capable of binding with enhanced specificity to the mu opioid receptor are disclosed. The compounds are analogs of somatostain and have the formula: ##STR1## wherein X is CONH.sub.2 or CH.sub.2 OH;Y and Z are independently sulfur or CH.sub.2 ;R.sup.1 and R.sup.2, which may be the same or different, are hydrogen, methyl, ethyl, cyclopentamethylene, or a lower alkyl group having five or less carbon atoms;R.sup.3 and R.sup.4, which may be the same or different, are hydrogen, methyl, ethyl, cyclopentamethylene, or a lower alkyl group having five or less carbon atoms, provided, however, that R.sup.1, R.sup.2, R.sup.3, and R.sup.4 may not all be hydrogen;AA.sub.1 is Phe, D-Phe, phenyl-Gly, D-phenyl-Gly, Tyr, D-Tyr, L-1-Naphthylalanine, D-1-Naphthylalanine, or D-Phe(4-Me);AA.sub.2 is Tyr, Phe, Tyr(OMe), Phe(4-Me),Tyr(OEt), or Phe(4-Et); andAA.sub.3 is Lys, Arg, Orn or homo-Arg.

Abstract: Novel compounds which are analogs of .alpha.-melanotropin and which exhibit reactivity for the central nervous system receptors without exhibiting substantial reactivity for the peripheral receptors are disclosed. The compounds of the present invention are polypeptides of the formula: ##STR1## wherein R.sup.1 is a substituted or unsubstituted aromatic radical;R.sup.2 is hydrogen or a methyl group;R.sup.3 is carboxylate, carboxamide, hydroxymethyl, or aldehyde group or a peptide residue;R.sup.4 is a simple amino acid residue;R.sup.5 is a simple amino acid residue;R.sup.6 and R.sup.7, which may be the same or different, are hydrogen, methyl or lower alkyl having one to five carbon atoms;R.sup.8 and R.sup.9, which may be the same or different, are hydrogen, methyl or lower alkyl having one to five carbon atoms;X and Y are sulfur, methylene, SO, or SO.sub.2 ;Z is --NH.sub.2, ##STR2## and n is an integer greater than or equal to 2.