Pharmacological and functional characterization of galanin-like peptide fragments as potent galanin receptor agonists

The hypothalamic galanin-like peptide (GALP) was isolated by its ability to activate Galanin receptors. The mature porcine GALP is a 60-amino acid neuropeptide proteolytically processed from a 120-amino acid precursor protein. It contains a region identical to the N-terminal 13-amino acids of the neuropeptide Galanin. Within the sequence of human GALP (1-60) a potential proteolytic cleavage site between two basic Amino acids is present at position 33, which might lead to a shorter C-terminally amidated peptide. In addition, the first two Amino acids could be potentially removed via the action of dipeptidase IV. Ligand binding assays using the human neuroblastoma cell line SH-SY5Y transfected with the respective Galanin receptors revealed that human GALP (1-60) displayed the highest affinity for the Galanin receptor subtype GalR3 (IC50 = 10 nM) followed by GalR2 (IC50 = 28 nM) and GalR1 (IC50 = 77 nM). Ligand binding assays and functional studies showed that the human GALP (3-32) fragment was at least as potent as full length GALP (1-60). Other studies have shown that shorter fragments like human GALP (1-21) and GALP (22-60) were not effective on feeding responses in mice as compared to the full length peptide. Taken together these data suggest that the putative fragment GALP (3-32) might represent the strongest mediator of biological GALP activity. Furthermore it might be a useful tool to study the affinity of GALP to Galanin receptors and to search for specific GALP receptors.