The internal solution contained (in mM) potassium gluconate 140, NaCl 4, MgCl2 2, EGTA 1.1, HEPES 5, Na2ATP 2, sodium creatine phosphate 5 and Na3GTP 0.6 (pH 7.3) with KOH. Whole-cell voltage-clamp recordings were used to measure GIRK currents. For agonist-induced currents, changes in holding currents (Vm = −35 mV with junction potential −15.7mV) in response to bath application of a saturating dose of baclofen (300 µM) or quinpirole (30 µM) were measured (at −50 mV every 10 sec). Currents were amplified (Molecular Devices Axopatch 200B), filtered at 1 kHz and digitized at 5 kHz (Molecular Devices Digidata 1320). Ih currents were monitored through a series of hyperpolarizing 200 ms voltage steps to −120 mV. Series resistance (Rs) was monitored throughout the experiment and recordings were excluded from analysis if the Rs varied by more than 20%. Clampex 9.0 software was used for data acquisition and analysis. GIRK currents were confirmed by inhibition with Ba2+ (1mM), a selective inhibitor of inward rectifiers. For the GTPγS experiment, GTPγS (0.1 µM) was added to the internal solution in place of Na3GTP. For
