title: Emergence of functional delta-opioid receptors induced by long-term treatment with morphine.
abstract: Opioid analgesics remain the choice for the treatment of moderate to severe pain. Recent research has established that the mu-opioid receptor is predominantly responsible for mediating many opioid actions, including analgesia and opioid tolerance. However, the function of delta-opioid receptors is rather puzzling at present, with inconsistent reports of system effects by agonists of delta-opioid receptors. The functional interaction between mu-opioid receptors and delta-opioid receptors is also poorly understood. In this study, we demonstrated that in a brainstem site critically involved in opioid analgesia, agonists of delta-opioid receptors, ineffective in opioid naive rats, significantly inhibit presynaptic GABA release in the brainstem neurons from morphine-tolerant rats. In membrane preparation from control brainstem tissues, Western blot detected no proteins of delta-opioid receptors, but consistent delta-opioid receptor proteins were expressed in membrane preparation from morphine-tolerant rats. Immunohistochemical studies revealed that long-term morphine treatment significantly increases the number of delta-opioid receptor-immunoreactive varicosities that appose the postsynaptic membrane of these neurons. The colocalization of delta-opioid receptor-immunoreactive varicosities with the labeling of the GABA-synthesizing enzyme glutamic acid decarboxylase is also significantly increased. From a behavioral perspective, activation of delta-opioid receptors in the brainstem nucleus, lacking an effect in opioid naive rats, became analgesic in morphine-tolerant rats and significantly reduced morphine tolerance. These findings indicate that long-term morphine treatment induces the emergence of functional delta-opioid receptors and delta-opioid receptor-mediated analgesia, probably through receptor translocation to surface membrane in GABAergic terminals. They also suggest that opioid drugs with preference for delta-opioid receptors may have better therapeutic effect in a mu-opioid-tolerant state.
authors.affiliation: Department of Anesthesiology and Pain Medicine, Unit 110, University of Texas-MD Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030, USA.
authors.forename: Junyi
authors.initials: J
authors.lastname: Ma
authors.forename: Yong
authors.initials: Y
authors.lastname: Zhang
authors.forename: Alex E
authors.initials: AE
authors.lastname: Kalyuzhny
authors.forename: Zhizhong Z
authors.initials: ZZ
authors.lastname: Pan
mesh_terms: D000818:Animals; D001483:Base Sequence; D017931:DNA Primers; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D008845:Microinjections; D009020:Morphine / Q000494:pharmacology*; D051381:Rats; D017465:Receptors, Opioid, delta / Q000187:drug effects*; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D017931:DNA Primers; D017465:Receptors, Opioid, delta; D009020:Morphine