Abstract

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.