Biphasic competition between opiates and enkephalins: Does it indicate the existence of a common high affinity ("MU-1") binding site?

Displacement from brain membranes of labeled opiates by low concentrations of enkephalins and of labeled enkephalins by low concentrations of opiates has been previously explained by the existence of a common high affinity site termed mu-1. An alternative interpretation of the same results is that the trough seen in the low concentration zone of the displacement curves represents cross binding of mu and delta opioid ligands to delta and mu receptors, respectively. In three sets of experiments with brain membranes, the size of the trough is shown to be dependent on the labeled ligand used: The ratio between the size of troughs seen with [³H]D-Ala, D-Leu enkephalin and with [³H]morphine varies with experimental conditions (storage of membranes at 40°C for 72h), with ratio of mu: delta receptors (e.g. in thalamus and cortex which are enriched in mu and delta sites, respectively) and with pretreatment of membranes with naloxonazine. These results can not be explained by a common high affinity site, but rather by binding of [³H]D-Ala, D-Leu enkephalin to mu and of [³H]morphine to delta opioid receptors.