μ opioid and CB1 cannabinoid receptor interactions: Reciprocal inhibition of receptor signaling and neuritogenesis

1 Several studies have described functional interactions between opioid and cannabinoid receptors; the underlying mechanism(s) have not been well explored. One possible mechanism is direct receptor-receptor interactions, as has been demonstrated for a number of G-protein-coupled receptors. 2 In order to investigate interactions between opioid and cannabinoid receptors, we epitope tagged μ, δ and κ opioid receptors with Renilla luciferase and CB1 cannabinoid or CCR5 chemokine receptors with yellow fluorescent protein and examined the extent of substrate hydrolysis induced bioluminescence resonance energy transfer (BRET) signal. 3 We find that coexpression of opioid receptors with cannabinoid receptors, but not with chemokine receptors, leads to a significant increase in the level of BRET signal, suggesting that the opioid-cannabinoid interactions are receptor specific. 4 In order to examine the implications of these interactions to signaling, we used GTPγS binding and mitogen-activated protein kinase (MAPK) phosphorylation assays and examined the effect of receptor activation on signaling. 5 We find that the μ receptor-mediated signaling is attenuated by the CB1 receptor agonist; this effect is reciprocal and is seen in heterologous cells and endogenous tissue expressing both receptors. 6 In order to explore the physiological consequences of this interaction, we examined the effect of receptor activation on the extent of Src and STAT3 phosphorylation and neuritogenesis in Neuro-2A cells. 7 We find that the simultaneous activation of μ opioid and CB1 cannabinoid receptors leads to a significant attenuation of the response seen upon activation of individual receptors, implicating a role for receptor-receptor interactions in modulating neuritogenesis.