MOLECULAR CHARACTERIZATION AND EXPRESSION OF CLONED HUMAN GALANIN RECEPTORS GALR2 AND GALR3

Galanin is a 29- or 30-amino acid peptide with wide-ranging effects on hormone release, feeding behavior, smooth muscle contractility, and s omatosensory neuronal function. Three distinct galanin receptor (GALR) subtypes, designated GALR1,2, and 3, have been cloned from the rat, W e report here the cloning of the human GALR2 and GALR3 genes, an initi al characterization of their pharmacology with respect to radioligand binding and signal transduction pathways, and a profile of their expre ssion in brain and peripheral tissues. Human GALR2 and GALR3 show, res pectively, 92 and 89% amino acid sequence identity with their rat homo logues. Radioligand binding studies with I-125-galanin show that recom binant human GALR2 binds with high affinity to human galanin (K-D = 0. 3 nM). Human GALR3 binds galanin with less affinity (IC50 of 12 nM for porcine galanin and 75 nM for human galanin). Human GALR2 was shown t o couple to phospholipase C and elevation of intracellular calcium lev els as assessed by aequorin luminescence in HEK-293 cells and by Xenop us melanophore pigment aggregation and dispersion assays, in contrast to human GALR1 and human GALR3, which signal predominantly through inh ibition of adenylate cyclase. GALR2 mRNA shows a wide distribution in the brain (mammillary nuclei, dentate gyrus, cingulate gyrus, and post erior hypothalamic, supraoptic, and arcuate nuclei), and restricted pe ripheral tissue distribution with highest mRNA levels detected in huma n small intestine. In comparison, whereas GALR3 mRNA was expressed in many areas of the rat brain, there was abundant expression in the prim ary olfactory cortex, olfactory tubercle, the islands of Calleja, the hippocampal CA regions of Ammon's horn, and the dentate gyrus, GALR3 m RNA was highly expressed in human testis and was detectable in adrenal gland and pancreas. The genes for human GALR2 and 3 were localized to chromosomes 17q25 and 22q12.2-13.1, respectively.