We next asked whether the reduction in GABABR-activated GIRK currents could be explained by a decrease in surface expression of GABAB receptors or alternatively, a decrease in GIRK channel expression (Cruz et al., 2004; Padgett et al., 2012; Terunuma et al., 2010). To investigate a change in surface expression of GIRK channels, we examined the effect of intracellular GTPγS, a non-hydrolyzable form of GTP that bypasses GPCR activation and constitutively activates GIRK channels via a Gβγ-dependent mechanism (Logothetis et al., 1987). If the absence of SNX27 protein affected the function of only GABAB receptors, then GTPγS-stimulated currents would be expected to be similar in size to those of control neurons. In control neurons (WT and DAT-Cre+/−), inclusion of 100 µM GTPγS in the intracellular solution led to slow activation of a large outward potassium current, which was subsequently inhibited with extracellular Ba2+, indicating activation of GIRK channels (Figure 4A). In VTA DA neurons of SNX27DA KO mice, however, the GTPγS-induced Ba2+-sensitive currents were significantly reduced (Figure 4B). On average, the GTPγS-stimulated currents were ~75% smaller in SNX27DA KO mice (161.0
