RESUMO
Opioid agonists produce analgesia in humans and other mammals by binding to three distinct types of G protein-coupled receptors; mu (MOR), delta (DOR), and kappa (KOR) opioid receptors. A fourth member of the opioid receptor family is the nociceptin or orphanin FQ receptor (ORL), however the role of the ORL receptor in analgesia is less clear. In the Northern grass frog, Rana pipiens, systemic and central administration of morphine and selective MOR, DOR, and KOR agonists produced dose-dependent antinociceptive effects blocked by the general opioid antagonist, naltrexone. The present study reports on the sequence, expression, and bioinformatics of four opioid receptor cDNAs cloned from Rana pipiens; rpMOR, rpDOR, rpKOR, and rpORL. These were the first opioid receptors cloned from a species of Class Amphibia, are selectively expressed in brain tissue, and show 70-84% identity to their homologous mammalian opioid receptors. Comparisons within species showed that MOR, DOR, and KOR proteins are significantly less divergent in earlier-evolved vertebrates compared to humans and other mammals. Among the four types of opioid receptors, MOR proteins show the least sequence variation among the six vertebrate species. Additionally, phylogenetic analysis supports the hypothesis that the family of opioid receptor proteins are coded by four genes that arose from two gene duplications of a single ancestral opioid receptor gene.