Dopamine D₃ receptors regulate GABA_A receptor function through a phospho-dependent endocytosis mechanism in nucleus accumbens

The dopamine D₃ receptor, which is highly enriched in nucleus accumbens (NAc), has been suggested to play an important role in reinforcement and reward. To understand the potential cellular mechanism underlying D ₃ receptor functions, we examined the effect of D₃ receptor activation on GABA_A receptor (GABA_AR)-mediated current and inhibitory synaptic transmission in medium spiny neurons of NAc. Application of PD128907 [(4aR,10bR)-3,4a,4,10b-tetrahydro-4-propyl-2H,5H-[1] benzopyrano-[4,3-b]-1,4-oxazin-9-ol hydrochloride], a specific D₃ receptor agonist, caused a significant reduction of GABA_AR current in acutely dissociated NAc neurons and miniature IPSC amplitude in NAc slices. This effect was blocked by dialysis with a dynamin inhibitory peptide, which prevents the clathrin/activator protein 2 (AP2)-mediated GABA_A receptor endocytosis. In addition, the D₃ effect on GABA_AR current was prevented by agents that manipulate protein kinase A (PKA) activity. Infusion of a peptide derived from GABA_AR β subunits, which contains an atypical binding motif for the clathrin AP2 adaptor complex and the major PKA phosphorylation sites and binds with high affinity to AP2 only when dephosphorylated, diminished the D₃ regulation of IPSC amplitude. The phosphorylated equivalent of the peptide was without effect. Moreover, PD128907 increased GABA_AR internalization and reduced the surface expression of GABA_A receptor β subunits in NAc slices, which was prevented by dynamin inhibitory peptide or cAMP treatment. Together, our results suggest that D₃ receptor activation suppresses the efficacy of inhibitory synaptic transmission in NAc by increasing the phospho-dependent endocytosis of GABA_A receptors.