Dopamine Directly Modulates GABAA Receptors

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Dopamine is a critical neuromodulator that governs movement and motivation. Although dopamine is only thought to target GPCRs in vertebrates (D1-5), it can activate a Cys-loop pentameric ligand-gated ion channel in C. elegans (LGC-53). This channel shares homology with mammalian GABAA receptors, among other Cys-loop channels. The GABAA β3 subunit in particular is important for mediating tonic GABA signaling in dopamine-projecting striatal medium spiny neurons. We found 30% residue identity between ligand-binding domains of LGC-53 and β3, and hypothesized that β3-containing GABAA receptors may be directly modulated by DA. To address this, we used immunolabeling with specific anti-β3 antibodies and whole-cell patch clamp electrophysiology in cultured rat striatal neurons and transfected HEK 293 cells. We determined that 79 ±3% of cultured cells endogenously expressed β3, and 94% of neurons gave tonic-level GABA-evoked currents blocked by phasic concentrations of dopamine (0.1-10 mM). Inhibition was recapitulated in HEK 293 cells transfected with α1β3, α1β3δ or α1β2γ2 subunits, but we instead observed potentiation in α1β3γ2 and α5β3γ2. Surprisingly, dopamine (1 mM) evoked rapid currents in α1β2γ2, α1β3γ2 and α5β3γ2 in the absence of GABA. In α1β3(H267A)γ2 (α1Zβ3γ2) receptors insensitive to trace Zn2+ inhibition, we found that other biogenic amines evoked comparatively smaller currents than DA. When the ratio of α1 was increased or we mutated a critical Zβ3 tyrosine residue (Y62L), relative dopamine responses decreased. Furthermore, dopamine activity was retained but GABA activity was drastically reduced in Zβ3γ2 receptors, while Zβ3 or γ2 alone did not elicit currents. Finally, dopamine currents were picrotoxin- but not bicuculline- or gabazine-sensitive. Taken together, dopamine at phasic levels is a GABAA allosteric modulator that may inhibit tonic GABA in prototypical α-containing GABAA receptors. However, at β/γ-containing receptors that do not need α subunits, dopamine acts as a positive allosteric modulator.