Investigation of the possible cellular mechanisms of delta-opioid receptor subtype-pharmacology

Pharmacological evidence suggests that the δ opioid receptor exists as two distinct subtypes (δ1 and δ2). However, to date, only one δ opioid receptor gene has been cloned, and the source of this pharmacology has not been determined. Therefore, this study evaluated the cellular mechanisms of δ subtypes and investigated the consequences of δ-ligand pretreatment on the subsequent activity of agonists and antagonists.;In a cell line stably transfected with the human δ receptor, no evidence of δ subtypes in binding or functional assays was found. Additionally, there was a widespread lack of δ/μ selectivity. To study this lack of selectivity and because historical evidence demonstrated that δ-liganns influenced μ receptor activity, a cell line that stably expressing both δ and μ receptors was created. The effects of μ receptor expression on δ subtype pharmacology were then studied. However, the presence of the μ receptor did not change the binding or functional properties of any δ-ligand.;Previous research indicated that agonist and inverse agonist treatment changed the subsequent activity of antagonists and produced opposite effects on δ1- and δ2-selective antagonists. Therefore, the effect of ligand treatment on δ-subtype pharmacology was evaluated. Treatment with agonists did not produce changes in subtype pharmacology, but it did change antagonists into inverse agonists.;The environment of the receptor directly influences ligand activity. Thus, δ1- and δ2-ligands may bind differently in membranes and in intact cells. Additionally, the δ2-agonist, deltorphin II, is more cell permeable than the δ1-agonist, [D-Pen2, D-Pen5]enkephalin (DPDPE). Therefore, deltorphin II may be accessible to more receptors than DPDPE. Ligand binding experiments were performed with intact cells that stably expressed the human δ receptor. Expectedly, deltorphin II bound more receptor binding sites than DPDPE. However, this decrease in DPDPE binding relative to deltorphin II did not affect subtype pharmacology because δ1- and δ 2-selective antagonists had similar affinity at both [3H]DPDPE and [3H]deltorphin II binding sites. Our collected results suggest that the subtype pharmacology observed in vivo is not due to cellular or molecular entities. Instead, it is likely attributed to an undiscovered mechanism in vivo that is difficult to address in heterologous expression systems.