Figure 4.

Hypothetical diagram for dopaminergic regulation of Gα_olf protein levels in striatonigral and striatopallidal MSNs. The sizes of the circles, colored in red and blue, indicate the abundance of Gα_olf proteins in striatonigral MSNs expressing dopamine D₁ receptors (D₁Rs; D1-cells; red) and in striatopallidal MSNs expressing dopamine D₂ receptors (D₂Rs; D2-cells; blue), respectively. In the conditions of Parkinson’s disease (PD), D1-cells, but not D2-cells, might exhibit a DA D₁ hypersensitivity caused by a dramatic increase in their Gα_olf levels. In the conditions of PD with levodopa-induced dyskinesia (LID), D1-cells might show an increase in their Gα_olf levels, while D2-cells might show a decrease in their Gα_olf levels, which might result in an enhanced responsiveness to D₂R activation. ACh, acetylcholine; D1-cell, striatonigral medium spiny neuron expressing DA D₁ receptor; D2-cell, striatopallidal medium spiny neuron expressing DA D₂ receptor; PD, Parkinson’s disease; PD with LID, Parkinson’s disease with levodopa-induced dyskinesia.