The D1 agonist-induced suppression of Na⁺ currents involves cyclic-AMP-dependent protein kinase (PKA). A, The suppression of Na⁺ current by D1 agonists was mimicked by the catalytic subunit of PKA (50–100 U/ml; n = 8), which was dialyzed into the neurons through the patch pipette. Na⁺ current reduction was observed 4 min after membrane seal rupture. B, The PKA inhibitorPKI [5–24] (10 μm) prevented the suppression of Na⁺ current by SKF 38393 after 4 min of dialysis into the neuron (n = 9).