Molecular and functional characterization of a partial cDNA encoding a novel chicken brain melatonin receptor

An approach based on homology probing was used to clone a partial cDNA encoding a novel melatonin (ML) receptor (MLR) from chicken (Gallus domesticus) brain. Based on available deduced amino-acid sequence, the chicken MLR (cMLR) displayed greater sequence homology to the frog (Xenopus) MLR than cloned human/mammalian receptors, with overall identities of 73% and 66%, respectively. In order to gain functional expression, a chimeric frog/chicken (flc)MLR was constructed in which the 5' end of the cMLR, including the N-terminus, TM1 and part of the first intracellular loop was substituted by fMLR sequence. [125I]Iodo-ML bound with high affinity (Kd of approximately 35 pM) to COS-7 cells transiently expressing the flcMLR in a saturable and guanine nucleotide-sensitive manner with the following rank order of potency: 2-iodo-ML > ML > 6-Cl-ML > S20750 > 6-OH-ML > S20642 > S20753 > N-acetyl-5HT >> 5-HT. Estimated Ki values for these compounds at the flcMLR correlated well to those obtained in native chicken brain membranes. In line with the observed structural similarity to the fMLR, the flcMLR exhibited affinities for ML, 6-Cl-ML and 6-OH-ML approximately 10-fold lower than mammalian receptors. Functionally, opposing interactions between ML and dopamine receptor signal transduction pathways were observed with ML potently inhibiting dopamine D1A-receptor-mediated cAMP accumulation in cells (HEK-293) transiently co-expressing these receptors. cMLR mRNAs were found expressed in chicken brain and kidney with trace levels observed in the lung. The availability of cloned vertebrate MLRs distinct at both the amino acid and pharmacological level from their mammalian counterparts may now allow for the identification of those amino-acid residues and structural motifs that regulate ML-binding specificity and affinity.