Abstract

μ- and δ-Opioid agonists interact in a synergistic manner to produce analgesia in several animal models. Additionally, receptor binding studies using membranes derived from brain tissue indicate that interactions between μ- and δ-opioid receptors might be responsible for the observation of multiple opioid receptor subtypes. To examine potential interactions between μ- and δ-opioid receptors, we examined receptor binding and functional characteristics of μ-, δ-, or both μ- and δ-opioid receptors stably transfected in rat pituitary GH₃ cells (GH₃MOR, GH₃DOR, and GH₃MORDOR, respectively). Saturation and competition binding experiments revealed that coexpression of μ- and δ-opioid receptors resulted in the appearance of multiple affinity states for μ- but not δ-opioid receptors. Additionally, coadministration of selective μ- and δ-opioid agonists in GH₃MORDOR cells resulted in a synergistic competition with [³H][d-Pen^2,5]enkephalin (DPDPE) for δ-opioid receptors. Finally, when equally effective concentrations of [d-Ala²,N-MePhe⁴,Gly-ol⁵]enkephalin (DAMGO) and two different δ-opioid agonists (DPDPE or 2-methyl-4aα-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12aα-octahydroquinolino-[2,3,3-g]-isoquinoline; TAN67) were coadministered in GH₃MORDOR cells, a synergistic inhibition of adenylyl cyclase activity was observed. These results strongly suggest that cotransfection of μ- and δ-opioid receptors alters the binding and functional characteristics of the receptors. Therefore, we propose that the simultaneous exposure of GH₃MORDOR cells to selective μ- and δ-opioid agonists produces an interaction between receptors resulting in enhanced receptor binding. This effect is translated into an augmented ability of these agonists to inhibit adenylyl cyclase activity. Similar interactions occurring in neurons that express both μ- and δ-opioid receptors could explain observations of multiple opioid receptor subtypes in receptor binding studies and the synergistic interaction of μ- and δ-opioids in analgesic assays.