Allylprodine Analogues as Receptor Probes. Evidence That Phenolic and Nonphenolic Ligands Interact with Different Subsites on Identical Opioid Receptors

The m-hydroxy analogues of allylprodine and related structures have been synthesized and tested for narcotic agonist and antagonist activity on the electrically stimulated guinea pig ileum and by the hot-plate procedure in mice. It has been found that m-hydroxyallylprodine (α-2) is neither an agonist nor antagonist. Other phenolic congeners similarly have little or no activity. The fact that these results are in dramatic contrast with the structure-activity profile of morphine and closely related opiates has led to the proposal that the interaction of morphine and allylprodine (α-l) with the µ opioid receptor differs. This difference is postulated to arise from the recognition of the aromatic groups of morphine and α-1 by different aromatic-binding subsites of the receptor. These subsites are suggested to be identical with those which recognize the aromatic rings of the Tyr¹ and Phe⁴ of the enkephalins and endorphins. A receptor model consistent with these results is proposed.