Direct interaction of GABA_B receptors with M₂ muscarinic receptors enhances muscarinic signaling

Downregulation of G-protein-coupled receptors (GPCRs) provides an important mechanism for reducing neurotransmitter signaling during sustained stimulation. Chronic stimulation of M₂ muscarinic receptors (M₂Rs) causes internalization of M₂R and G-protein-activated inwardly rectifying potassium (GIRK) channels in neuronal PC12 cells, resulting in loss of function. Here, we show that coexpression of GABA_B R₂ receptors (GBR2s) rescues both surface expression and function of M ₂R, including M₂R-induced activation of GIRKs and inhibition of cAMP production. GBR2 showed significant association with M ₂R at the plasma membrane but not other GPCRs (M₁R, μ-opioid receptor), as detected by fluorescence resonance energy transfer measured with total internal reflection fluorescence microscopy. Unique regions of the proximal C-terminal domains of GBR2 andM₂R mediate specific binding between M₂R and GBR2. In the brain, GBR2, but not GBR1, biochemically coprecipitates with M₂R and overlaps with M ₂R expression in cortical neurons. This novel heteromeric association between M₂R and GBR2 provides a possible mechanism for altering muscarinic signaling in the brain and represents a previously unrecognized role for GBR2.