Multiple Opiate Receptors: Different Regional Distribution in the Brain and Differential Binding of Opiates and Opioid Peptides

In rat brain membrane preparations, the parenterally and orally active peptide, [D-Ala2, MePhe4, Met(O)5-ol]-enkephalin, binds to morphine receptor sites ([3H]naloxone or [3H]dihydromorphine binding sites) with an affinity higher than that for enkephalin receptor sites ([125I] [D-Ala2, D-Leu5]-enkephalin binding sites). [125I] [D-Ala2, MePhe4, Met(O)5-ol]-enkephalin binds to morphine receptor sites stereospecifically, in a saturable manner and with characteristics similar to that of [3H]dihydromorphine; this ligand can be used as an 125I-labeled probe to measure specific binding to morphine receptor sites. Na+ decreases and Mn2+ increases the binding capacity with a concomitant reduction of affinity for [125I] [D-Ala2, MePhe4, Met(O)5-ol]-enkephalin. This peptide does not bind to neuroblastoma cells with high affinity. The brain regional distribution of binding of [125I] [D-Ala2, MePhe4, Met(O)5-ol]-enkephalin or [3H]naloxone and [125I] [D-Ala2, D-Leu5]-enkephalin are different. The differential potency of binding of opiate agonists, antagonists, mixed agonist-antagonists, enkephalins and enkephalin analogues is studied by competition of binding of [3H]naloxone or [125I] [D-Ala2, MePhe4, Met(O)5-ol]-enkephalin (morphine receptor) and of [125I] [D-Ala2, D-Leu5]-enkephalin sites (enkephalin receptor). All of these results support the contention that there are multiple opiate receptors with differing characteristics. ACKNOWLEDGMENT We are grateful to Mark Collins for excellent technical assistance.