MOLECULAR-CLONING AND PHARMACOLOGICAL CHARACTERIZATION OF A NEW GALANIN RECEPTOR SUBTYPE

Galanin, a 29-30-amino acid neuropeptide, is widely distributed in cen tral and peripheral systems and mediates a variety of physiological fu nctions. Pharmacological studies have suggested the existence of multi ple receptor subtypes but only the type I (GalR1) galanin receptor has been cloned. Now we report the cloning by a combination of sib select ion and rapid amplification of cDNA ends of a cDNA encoding a new gala nin receptor (GalR2) from rat hypothalamus. The receptor is 372 amino acids in length and shares only 40% homology with the rat GalR1 recept or. It contains seven putative transmembrane domains with the amino an d carboxyl termini being least identical to GalR1. Northern blot analy ses revealed a 2-kilobase pair mRNA species distributed in several tis sues, suggesting a broader functional spectrum than GalR1. I-125-Label ed human galanin binding to rat GalR2 receptor expressed in COS-1 cell s was saturable (K-d = 0.59 nM) and could be displaced by galanin, sev eral galanin fragments, and chimeric peptides. The pharmacological pro files of GalR1 and GalR2 receptors were distinguishable by galanin fra gment(2-29), which bound the cloned GalR2 receptor with markedly highe r affinity than the GalR1 receptor. Activation of the cloned receptor by galanin led to inhibition of forskolin-stimulated intracellular cAM P production. The cloning of this new receptor subtype should provide further insights into the mechanisms by which galanin mediates its div erse physiological functions. The identification of galanin(2-29) as a receptor-specific ligand should enhance the understanding of specific ity of galanin-receptor interactions.