The D1 agonist-induced suppression of Na⁺ current involves G_s protein activation of cAMP. A, This combination of time plot and current traces (inset) shows that the suppression produced by SKF 38393 was mimicked by the membrane-permeable cAMP analog 8-Br-cAMP (50 μm;n = 6; 37.3 ± 10.6% suppression), which also occluded the effect of SKF 38393. B, These time–response curves compare the suppression of Na⁺currents by SKF 38393 in a control neuron and a neuron that had been dialyzed with the nonhydrolyzable GDP analogGDP-β-S, which prevents activation of G_s proteins. Note the marked reduction in the efficacy of SKF 38393 (2.8 ± 1.4% suppression; n = 7).