Opioid Receptors (ORs) are involved in the pathophysiology of several neuropsychiatric conditions yet remain an untapped therapeutic resource. Although only mu-, delta-, and kappa-OR types have been cloned, additional subtypes result from complexes generated by direct receptor-receptor interactions. Mu- and delta-ORs form a heteromeric receptor complex with unique pharmacological and signalling properties distinct from those of mu- and delta-OR homomers. In these studies, we sought to characterize the ligand binding pocket and agonist-induced internalization profile of the mu-delta heteromer, to investigate mu-delta heteromer-specific signalling in brain, and to interrogate the contribution of this receptor complex to opioid-mediated behavioural effects.
In competition radioligand binding studies, delta-agonists displaced high affinity mu-agonist binding from the mu-delta heteromer but not the muOR homomer, suggestive of delta-agonists occupying or allosterically modulating the muOR ligand binding pocket within the heteromer. Delta-agonists induced internalization of the mu-delta heteromer in a dose-dependent, pertussis toxin resistant, and muOR- and deltaOR-dependent manner from the cell surface via the clathrin and dynamin endocytic machinery. Agonist-induced internalization of the mu-delta heteromer persisted following chronic morphine treatment conditions which desensitized the muOR homomer.
Using Galpha-specific GTPgammaS binding assays, we demonstrated that mu-delta heteromer signalling previously characterized in cell lines was present in the striatum and hippocampus, and did not desensitize following prolonged morphine treatment conditions which desensitized muOR homomer-mediated signalling.
Since delta-agonists which also target the mu-delta heteromer possess antidepressant-like and anxiolytic-like properties, we investigated the role of this receptor complex in mood regulation. We devised a strategy to selectively analyze the effects of the mu-delta heteromer by dissociating it using a specific interfering peptide aimed at a sequence implicated in mu-delta heteromerization. The interfering peptide abolished the unique pharmacological and trafficking properties of delta-agonists at the mu-delta heteromer and dissociated this receptor complex in vitro. Intra-accumbens administration of the interfering peptide disrupted the mu-delta interaction in vivo and allowed for isolation of the mu-delta heteromer contribution to the mood-regulatory effects of a delta-agonist with activity at the heteromer. Activation of the mu-delta heteromer in the nucleus accumbens produced antidepressant-like and anxiolytic-like actions in animal models of depression and anxiety.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:OTU.1807/65509 |
| Date | 20 June 2014 |
| Creators | Kabli, Noufissa |
| Contributors | George, Susan R., O'Dowd, Brian F. |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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