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Galanin Receptor

The galanin family of peptides is encoded by two separate genes: galanin/GMAP prepropeptide (GAL) and galanin-like peptide (GALP).  Galanin expression is modulated in a cell type-specific manner in humans, and tissue and cell type-specific hormonal regulators of the galanin gene include vasoactive intestinal peptide, activity-dependent neuroprotective protein, thyroid hormone, progesterone, GnRH, dexamethasone, nerve growth factor, brainderived nerve growth factor, and leukemia inhibitory factor. Some of the most potent inducers of galanin gene expression are protein kinase C (PKC) after activation with phorbol ester, protein kinase A activated by forskolin, and colchicine, which interferes with microtubules and alters intraneuronal transport  to produce a marked increase in GAL mRNA expression. 
The three galanin receptors share a number of characteristics as they are members of the 7-TM GPCRs, but their functional coupling and signal transduction pathways are substantially different, thus contributing to the diversity of galanin-mediated effects, depending on the cell type and its particular G protein repertoire.Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein–coupled receptors, GAL1, GAL2, and GAL3.  GAL1 activation results in an inhibitory action on adenylate cyclase (AC), leading to reduced cAMP concentrations, opening of G protein–regulated inwardly rectifying K+channels, and stimulation of mitogen-activated protein kinase (MAPK) activity. 
Activation of the MAPK/ERK pathway via GAL1 is not linked to the phosphatidylinositol 3-kinase pathway and leads to the induction of the cell-cycle control proteins p27Kip1 and p57Kip2 and suppression of cyclin D1 in GAL1-transfected human squamous cell carcinoma cells. GAL2 signals through multiple classes of G proteins to stimulate multiple intracellular pathways. Activation of GAL2 is capable of stimulating the MAPK/ERK pathway in a PTX-sensitive, PKC-dependent fashion, indicative of coupling to a G0 protein in GAL2-transfected cell lines. GAL1 and GAL2 are the most studied of the three galanin receptors, and the signaling properties of GAL3 are still poorly defined.