Direct Interaction of GABA B Receptors with M 2 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 M 2 muscarinic receptors (M 2 Rs) causes internalization of M 2 R 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 2 R, including M 2 R-induced activation of GIRKs and inhibition of cAMP production. GBR2 showed significant association with M 2 R at the plasma membrane but not other GPCRs (M 1 R, 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 and M 2 R mediate specific binding between M 2 R and GBR2. In the brain, GBR2, but not GBR1, biochemically coprecipitates with M 2 R and overlaps with M 2 R expression in cortical neurons. This novel heteromeric association between M 2 R and GBR2 provides a possible mechanism for altering muscarinic signaling in the brain and represents a previously unrecognized role for GBR2.