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Dopamine (DA) exerts a strong influence on inhibition in prefrontal cortex. The main cortical interneuron subtype targeted by DA are fast-spiking γ-aminobutyric acidergic (GABAergic) cells that express the calcium-binding protein parvalbumin. D1 stimulation depolarizes these interneurons and increases excitability evoked by current injection. The present study examined whether this direct DA-dependent modulation of fast-spiking interneurons involves DARPP-32. Whole-cell patch-clamp recordings were made from fast-spiking interneurons in brain slices from DARPP-32 knockout (KO) mice, wild-type mice, and rats. Low concentrations of DA (100 nM) increased interneuron excitability via D1 receptors, protein kinase A, and cyclic adenosine 3′,5′-monophosphate in slices from both normal and DARPP-32 KO mice. Immunohistochemical staining of slices from normal animals revealed a lack of colocalization of DARPP-32 with calcium-binding proteins selective for fast-spiking interneurons, indicating that these interneurons do not express DARPP-32. Therefore, although DARPP-32 impacts cortical inhibition through a previously demonstrated D2-dependent regulation of GABAergic currents in pyramidal cells, it is not involved in the direct D1-mediated regulation of fast-spiking interneurons.