D₁ dopamine receptor activation reduces GABA(A) receptor currents in neostriatal neurons through a PKA/DARPP-32/PP1 signaling cascade

Dopamine is a critical determinant of neostriatal function, but its impact on intrastriatal GABAergic signaling is poorly understood. The role of D₁ dopamine receptors in the regulation of postsynaptic GABA(A) receptors was characterized using whole cell voltage-clamp recordings in acutely isolated, rat neostriatal medium spiny neurons. Exogenous application of GABA evoked a rapidly desensitizing current that was blocked by bicuculline. Application of the D₁ dopamine receptor agonist SKF 81297 reduced GABA- evoked currents in most medium spiny neurons. The D₁ dopamine receptor antagonist SCH 23390 blocked the effect of SKF 81297. Membrane-permeant cAMP analogues mimicked the effect of D₁ dopamine receptor stimulation, whereas an inhibitor of protein kinase A (PKA; Rp-8-chloroadenosine 3',5' cyclic monophosphothioate) attenuated the response to D₁ dopamine receptor stimulation or cAMP analogues. Inhibitors of protein phosphatase 1/2A potentiated the modulation by cAMP analogues. Single-cell RT-PCR profiling revealed consistent expression of mRNA for the β1 subunit of the GABA(A) receptor-a known substrate of PKA-in medium spiny neurons. Immunoprecipitation assays of radiolabeled proteins revealed that D₁ dopamine receptor stimulation increased phosphorylation of GABA(A) receptor β1/β3 subunits. The D₁ dopamine receptor-induced phosphorylation of B₁/β3 subunits was attenuated significantly in neostriata from DARPP-32 mutants. Voltage-clamp recordings corroborated these results, revealing that the efficacy of the D₁ dopamine receptor modulation of GABA(A) currents was reduced in DARPP-32-deficient medium spiny neurons. These results argue that D₁ dopamine receptor stimulation in neostriatal medium spiny neurons reduces postsynaptic GABA(A) receptor currents by activating a PKA/DARPP-32/protein phosphatase 1 signaling cascade targeting GABA(A) receptor β1 subunits.