μ-opioid and 5-HT1A receptors heterodimerize and show signalling crosstalk via G protein and MAP-kinase pathways

mu-opioid receptors have been shown to form heterodimers with several G protein coupled receptors involved in pain regulation such as alpha(2A)-adrenergic and neurokinin 1 receptors. Because the 5-HT1A receptor is also involved in pain control, we investigated whether it can interact with the mu-opioid receptor in cell lines. Using epitope-tagged mu-opioid and 5-HT1A receptors, we show that both receptors can co-immunoprecipate when expressed in the same cells. This physical interaction was corroborated by a Bioluminescence Resonance Energy Transfer signal between the mu-opioid receptor fused to Renilla luciferase and the 5-HT1A receptor fused to the Green Fluorescent Protein. Consistent with the presence of functional heterodimers, the mu-opioid receptor activated a G alpha(o) protein covalently fused to the 5-HT1A receptor in membrane preparations as well as a G alpha(15) protein fused to the 5-HT1A receptor in living cells. We demonstrate that both receptors can coexerce control of the ERK1/2 pathway: for example, mu-opioid receptor-induced ERK1/2 phosphorylation was selectively desensitized by 5-HT1A receptor activation. Although 5-HT1A and mu-opioid receptors were capable to internalize in response to their own activation, they were ineffective to induce the co-internalization of their partners. Thus, we show a functional heterodimerization of mu-opioid and 5-HT1A receptors in cell lines, a complex that might play a role in the control of pain in vivo. These results also support the potential therapeutic action of 5-HT1A agonists against nociceptive processes. (C) 2012 Elsevier Inc. All rights reserved.