Novel delta opioid receptor signaling pathways

The purpose of these studies was to provide a further understanding of δ-opioid receptor pharmacology by examining the δ-receptor signaling pathways. A cellular model was utilized, comprised of rat anterior pituitary cells (GH₃) transfected with either μ-, δ-, or both μ- and δ-opioid receptors (GH₃MOR, GH₃DOR, and GH₃MORDOR, respectively). The first study demonstrated that co-expressed μ- and δ-opioid receptors produced altered binding and functional characteristics. Simultaneous exposure of μ- and δ-receptors to selective agonists resulted in an interaction that translated into a functional synergy. Similar interactions, and the resulting signaling pathways, occurring in neurons/cells co-expressing μ- and δ-opioid receptors may explain the observed synergy in in vivo analgesic assays, and even multiple opioid receptor subtypes.; The second study investigated the novel agonist activity of the δ-antagonists TIPP (H-Tyr-Tic-Phe-Phe-OH) and its derivative, TIPP-Ψ (H-Tyr-Tic[CH ₂NH]-Phe-Phe-OH), to inhibit adenylyl cyclase activity. The agonist activity was characterized as selective for, and mediated by, the δ-opioid receptor. In addition, the activity was concentration-dependent, independent of δ-receptor density, sensitive to pertussis toxin (PTX), and occurred in cells containing endogenous or transfected δ-opioid receptors. These results demonstrated novel agonist activity of compounds previously characterized as δ-opioid receptor antagonists.; The third study further investigated the agonist activity of TIPP and identified unique agonist properties at points along the signal transduction pathway. Results examining the ligand/receptor interaction and G-protein activation indicated activity more consistent with an antagonist. However, TIPP produced inhibition of adenylyl cyclase activity in GH₃DORT membranes, and this inhibition was reversed by PTX pretreatment, implying agonist activity and, indirectly, G-protein activation. In addition, chronic treatment with TIPP produced receptor downregulation and desensitization, and most remarkably, antagonist-precipitated cAMP rebound independent of an upregulation of adenylyl cyclase activity. The unique agonist properties of TIPP are attributed to novel signaling pathways of the δ-opioid receptor.; The results of these studies demonstrate novel signaling pathways of the δ-opioid receptor resulting in synergistic interactions with co-expressed μ-opioid receptors and in the unique agonist activity of compounds classified as selective δ-antagonists. These findings are important for understanding the mechanism(s) underlying opioid receptor pharmacology and the future development of improved therapeutic opioid analgesics.