Agonists of GABA(B) receptors exert a bi-directional effect on the activity of dopamine (DA) neurons of the ventral tegmental area, which can be explained by the fact that coupling between GABA(B) receptors and G protein-gated inwardly rectifying potassium (GIRK) channels is significantly weaker in DA neurons than in GABA neurons. Thus, low concentrations of agonists preferentially inhibit GABA neurons and thereby disinhibit DA neurons. This disinhibition might confer reinforcing properties on addictive GABA(B) receptor agonists such as gamma-hydroxybutyrate (GHB) and its derivatives. Here we show that, in DA neurons of mice, the low coupling efficiency reflects the selective expression of heteromeric GIRK2/3 channels and is dynamically modulated by a member of the regulator of G protein signaling (RGS) protein family. Moreover, repetitive exposure to GHB increases the GABA(B) receptor-GIRK channel coupling efficiency through downregulation of RGS2. Finally, oral self-administration of GHB at a concentration that is normally rewarding becomes aversive after chronic exposure. On the basis of these results, we propose a mechanism that might underlie tolerance to GHB.

Downregulation of G-protein-coupled receptors (GPCRs) provides an important mechanism for reducing neurotransmitter signaling during sustained stimulation. Chronic stimulation of M2muscarinic 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 GABABR2 receptors (GBR2s) rescues both surface expression and function of M2R, including M2R-induced activation of GIRKs and inhibition of cAMP production. GBR2 showed significant association with M2R 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 and M2R mediate specific binding between M2R and GBR2. In the brain, GBR2, but not GBR1, biochemically coprecipitates with M2R and overlaps with M2R 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.

Many inhibitory neurotransmitters in the brain

Mutations in Kir2