Galanin receptors are activated by the endogenous peptides galanin and galanin-like peptide. Human galanin is a 30 amino-acid non-amidated peptide ; in other species, it is 29 amino acids long and C-terminally amidated. Amino acids 1-14 of galanin are highly conserved in mammals, birds, reptiles, amphibia and fish. Shorter peptide species (e.g. human galanin-1-19  and porcine galanin-5-29 ) and N-terminally extended forms (e.g. N-terminally seven and nine residue elongated forms of porcine galanin [3,4]) have been reported. More recently, the newly-identified peptide, spexin (SPX), has been reported to activate human GAL2 and GAL3 (but not GAL1) receptors in heterologous expression systems; and to alter GAL2/3 receptor-related behaviours in animals .
Bersani M, Thim L, Rasmussen TN, et al. Galanin and galanin extended at the N-terminus with seven and nine amino acids are produced in and secreted from the porcine adrenal medulla in almost equal amounts. Endocrinology 1991;129:2693-8.
Kim DK, Yun S, Son GH, et al. Coevolution of the spexin/galanin/kisspeptin family: Spexin activates galanin receptor type II and III. Endocrinology 2014;155:1864-73.
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