MOLECULAR CHARACTERIZATION OF A 2ND MELATONIN RECEPTOR EXPRESSED IN HUMAN RETINA AND BRAIN - THE MEL(1B) MELATONIN RECEPTOR

A G protein-coupled receptor for the pineal hormone melatonin was rece ntly cloned from mammals and designated the Mel(1a) melatonin receptor . We now report the cloning of a second G protein-coupled melatonin re ceptor from humans and designate it the Mel(1b) melatonin receptor. Th e Mel(1b) receptor cDNA encodes a protein of 362 amino acids that is 6 0% identical at the amino acid level to the human Mel(1a) receptor. Tr ansient expression of the Mel(1b) receptor in COS-1 cells results in h igh-affinity 2-[I-125]iodomelatonin binding (K-d = 160 +/- 30 pM) In a ddition, the rank order of inhibition of specific 2-[125I]iodomelatoni n binding by eight ligands is similar to that exhibited by the Mel(1a) melatonin receptor. Functional studies of NIH 3T3 cells stably expres sing the Mel(1b) melatonin receptor indicate that it is coupled to inh ibition of adenylyl cyclase. Comparative reverse transcription PCR sho ws that the Mel(1b) melatonin receptor is expressed in retina and, to a lesser extent, brain, PCR analysis of human-rodent somatic cell hybr ids maps the Mel(1b) receptor gene (MTNR1B) to human chromosome 11q21- 22. The Mel(1b) melatonin receptor may mediate the reported actions of melatonin in retina and participate in some of the neurobiological ef fects of melatonin in mammals.