Alpha-2-adrenergic and opioid receptor additivity in rat locuscoeruleus neurons

Functional interactions between G protein-coupled receptors (GPCRs) are widely reported whereas the mechanisms underlying these interactions remain unclear. A rapidly growing literature suggests that many GPCRs form heteromeric complexes and that these complexes may possess pharmacological and functional properties that differ from their respective parent receptors. In order to investigate the functional relationship between co-localized GPCRs in intact, native CNS neurons, we used intracellular recording techniques in locus coeruleus (LC) slices. The LC consists of a homogenous population of cells, all of which co-express μ-opioid and α_2A-adrenergic receptors (μOR and α_2AARs, respectively). Co-administration of μOR and α_2AAR agonists in vivo results in synergistic interactions in analgesic assays and heteromeric μOR-α_2AAR complexes have been detected in HEK-293 co-expressing the two receptors. The LC is therefore an excellent model system to explore these interactions. Dose-response and isobolographic analysis of agonist-induced hyperpolarization in individual LC neurons revealed that the interaction between these receptors was not synergistic but additive. This result suggests that co-localized μOR and α₂ARs are either not physically associated in these neurons or that heteromer formation may not fully explain the μOR-α_2AAR synergistic interactions reported following agonist administration in vivo.