Direct evidence that RGS proteins can specifically influence receptor-GIRK coupling comes from several studies [28][29][2]. First, co-transfection of RGS3s reduces the coupling efficiency (i.e. increases the EC50) between muscarinic M2 or 5HT1A receptors and GIRK1/2 heteromeric channels in vitro [28,30]. Interestingly, the increase in EC50 was greater for RGS4 even though both RGS3s and RGS4 strongly accelerate the deactivation kinetics. Second, in acute slices of the VTA, selective expression of RGS2 in DA neurons modulates the coupling between GABAB receptors and GIRK channels [2]. Pharmacological inhibition of RGS proteins or genetic ablation of RGS2 in DA neurons decreases the EC50, from ~15 µM in control neurons to ~7 µM. Thus, the presence of RGS2 opposes G-protein activation of GIRK channels, leading to a higher EC50. The lower Gβγ affinity for GIRK2/GIRK3 heteromeric channels also contributes to the shift in EC50 for baclofen activation [1][29] and DA neurons uniquely express GIRK2/GIRK3 channels. We observed a similar decrease in EC50 in GIRK3 knockout mice as well as in RGS2/GIRK3 double-knockout mice [2]. These findings suggest that RGS2 specifically modulates the coupling to
