Melatonin receptors are for the birds: Molecular analysis of two receptor subtypes differentially expressed in chick brain

Two receptors (CKA and CKB) of the G protein-coupled melatonin receptor family were cloned from chick brain. CKA encodes a protein that is 80% identical at the amino acid level to the human Mel 1a melatonin receptor and is thus designated the chick Mel 1a melatonin receptor. CKB encodes a protein that is 80% identical to the Xenopus melatonin receptor and defines a new receptor subtype, the Mel 1c melatonin receptor, which is distinct from the Mel 1a and Mel 1b melatonin receptor subtypes. A melatonin receptor family consisting of three subtypes is supported by PCR cloning of distinct melatonin receptor fragments from Xenopus and zebrafish. Expression of CKA and CKB results in similar ligand binding and functional characteristics. The widespread distribution of CKA and CKB mRNA in brain provides a molecular substrate for the profound actions of melatonin in birds.