“…To date, however, detailed characterization of Y 4 -mediated responses has been hampered by the lack of selective ligands. The Y 1 -preferring agonist Pro 34 PYY has efficacy at the Y 4 receptor but only at high concentrations (Cox et al, 2001b). In addition GR231118, a homodimeric peptide based on the C-terminal sequence of NPY [(Ile, Glu,Pro,Dpr,Tyr,Arg,Leu,Arg,Try-NH 2 )-2-cyclic(2,4Ј),(2Ј,4)-diamide, also known as 1229U91] was originally identified as a competitive Y 1 receptor antagonist with low Y 2 affinity (Daniels et al, 1995;Hegde et al, 1995), but this dimeric nonapeptide also has Y 4 affinity and efficacy (Matthews et al, 1997;Parker et al, 1998;Schober et al, 1998Schober et al, , 2000.…”