AbstractBackgroundDopamine transmission is involved in the maintenance of the structural plasticity of direct‐pathway and indirect‐pathway striatal projection neurons (d‐SPNs and i‐SPNs, respectively). The lack of dopamine in Parkinson's disease produces synaptic remodeling in both types of SPNs, reducing the length of the dendritic arbor and spine density and increasing the intrinsic excitability. Meanwhile, the elevation of dopamine levels by levodopa recovers these alterations selectively in i‐SPNs. However, little is known about the specific role of the D1 receptor (D1R) in these alterations.MethodsTo explore the specific role of D1R in the synaptic remodeling of SPNs, we used knockout D1R mice (D1R−/−) and wild‐type mice crossed with drd2‐enhanced green fluorescent protein (eGFP) to identify d‐SPNs and i‐SPNs. Corticostriatal slices were used for reconstruction of the dendritic arbors after Lucifer yellow intracellular injection and for whole‐cell recordings in naïve and parkinsonian mice treated with saline or levodopa.ResultsThe genetic inactivation of D1R reduces the length of the dendritic tree and the spine density in all SPNs, although more so in d‐SPNs, which also increases their spiking. In parkinsonian D1R−/− mice, the spine density decreases in i‐SPNs, and this spine loss recovers after chronic levodopa.ConclusionsD1R is essential for the maintenance of spine plasticity in d‐SPNs but also affects i‐SPNs, indicating an important crosstalk between these 2 types of neurons. © 2020 International Parkinson and Movement Disorder Society