Interaction of co-expressed mu- and delta-opioid receptors in transfected rat pituitary GH(3) cells.

mu- and delta-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 mu- and delta-opioid receptors might be responsible for the observation of multiple opioid receptor subtypes. To examine potential interactions between mu- and delta-opioid receptors, we examined receptor binding and functional characteristics of mu-, delta-, or both mu- and delta-opioid receptors stably transfected in rat pituitary GH(3) cells (GH(3)MOR, GH(3)DOR, and GH(3)MORDOR, respectively). Saturation and competition binding experiments revealed that coexpression of mu- and delta-opioid receptors resulted in the appearance of multiple affinity states for mu- but not delta-opioid receptors. Additionally, coadministration of selective mu- and delta-opioid agonists in GH(3)MORDOR cells resulted in a synergistic competition with [(3)H][D-Pen(2,5)]enkephalin (DPDPE) for delta-opioid receptors. Finally, when equally effective concentrations of [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO) and two different delta-opioid agonists (DPDPE or 2-methyl-4a alpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12a alpha-octahydroquinolino-[2,3,3-g]-isoquinoline; TAN67) were coadministered in GH(3)MORDOR cells, a synergistic inhibition of adenylyl cyclase activity was observed. These results strongly suggest that cotransfection of mu- and delta-opioid receptors alters the binding and functional characteristics of the receptors. Therefore, we propose that the simultaneous exposure of GH(3)MORDOR cells to selective mu- and delta-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 mu- and delta-opioid receptors could explain observations of multiple opioid receptor subtypes in receptor binding studies and the synergistic interaction of mu- and delta-opioids in analgesic assays.