By screening a rat brain cDNA library using a cloned mu opioid receptor cDNA as probe, a clone was identified that is very similar to both the mu and delta opioid receptor sequences. Transient expression of this clone in COS-7 cells showed that it encodes a kappa opioid receptor, designated KOR-1, which is capable of high-affinity binding to kappa-selective ligands. Treatment of transfected cell membranes with bremazocine, a kappa-selective agonist, resulted in a 53% decrease in adenylate cyclase activity, indicating that this kappa opioid receptor displays inhibitory coupling to adenylate cyclase. Thus, one member from each of the three opioid receptor types, mu, kappa and delta, has been molecularly cloned. Analysis of sequence similarities among these opioid receptors, as well as between opioid receptors and other G-protein-coupled receptors, revealed regions of sequence conservation that may underlie the ligand-binding and functional specificities of opioid receptors.

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