Direct interaction of GABAB 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 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, mu-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.