CLONED HUMAN AND RAT GALANIN GALR3 RECEPTORS - PHARMACOLOGY AND ACTIVATION OF G-PROTEIN INWARDLY RECTIFYING K+ CHANNELS

The neuropeptide galanin has been implicated in the regulation of proc esses such as nociception, cognition, feeding behavior, and hormone se cretion. Multiple galanin receptors are predicted to mediate its effec ts, but only two functionally coupled receptors have been reported. We now report the cloning of a third galanin receptor distinct from GALR 1 and GALR2. The receptor, termed GALR3, was isolated from a rat hypot halamus cDNA library by both expression and homology cloning approache s. The rat GALR3 receptor cDNA can encode a protein of 370 amino acids with 35% and 52% identity to GALR1 and GALR2, respectively. Localizat ion of mRNA, by solution hybridization/RNase protection demonstrates t hat the GALR3 transcript is widely distributed, but expressed at low a bundance, with the highest levels in the hypothalamus and pituitary. W e also isolated the gene encoding the human homologue of GALR3. The hu man GALR3 receptor is 90% identical to rat GALR3 and contains 368 amin o acids. Binding of porcine I-125-galanin to stably expressed rat and human GALR3 receptors is saturable (rat K-D = 0.98 nM and human K-D = 2.23 nM) and displaceable by galanin peptides and analogues in the fol lowing rank, order: rat galanin, porcine galanin similar or equal to M 32, M35 similar or equal to porcine galanin-(-7 to +29), galantide, hu man galanin > M40, galanin-(1-16) > [D-Trp(2)]galanin-(1-29), galanin- (3-29). This profile resembles that of the rat GALR1 and GALR2 recepto rs with the notable exception that human galanin, galanin-(1-16), and M40 show lower affinity at GALR3. In Xenopus oocytes, activation of ra t and human GALR3 receptors co-expressed with potassium channel subuni ts GIRK1 and GIRK4 resulted in inward K+ currents characteristic of G( i)/G(o)-coupled receptors, These data confirm the functional efficacy of GALR3 receptors and further suggest that GALR3 signaling pathways r esemble those of GALR1 in that both can activate potassium channels li nked to the regulation of neurotransmitter release.