Ancestral vertebrate complexity of the opioid system

The evolution of the opioid peptides and nociceptin/orphanin as well as their receptors has been difficult to resolve due to variable evolutionary rates. By combining sequence comparisons with information on the chromosomal locations of the genes, we have deduced the following evolutionary scenario: The vertebrate predecessor had one opioid precursor gene and one receptor gene. The two genome doublings before the vertebrate radiation resulted in three peptide precursor genes whereupon a fourth copy arose by a local gene duplication. These four precursors diverged to become the prepropeptides for endorphin (POMC), enkephalins, dynorphins, and nociceptin, respectively. The ancestral receptor gene was quadrupled in the genome doublings leading to delta, kappa, and mu and the nociceptin/orphanin receptor. This scenario is corroborated by new data presented here for coelacanth and spotted gar, representing two basal branches in the vertebrate tree. A third genome doubling in the ancestor of teleost fishes generated additional gene copies. These results show that the opioid system was quite complex already in the first vertebrates and that it has more components in teleost fishes than in mammals. From an evolutionary point of view, nociceptin and its receptor can be considered full-fledged members of the opioid system.