Three subtypes of medium spiny neurons (MSNs, the GABAergic projection neurons of the striatum) were identified: D1-type MSNs, D2-type MSNs, and a third subtype marked by both DRD1 and DRD2 expression (D1/D2 neurons). This cluster showed strong expression of RXFP1, a known marker of D1/D2 hybrid neurons33. Interestingly, D1/D2 cells had a similar DRD1 expression level (average normalized expression = 83.8) to the D1 MSNs (86.2), but DRD2 expression was far lower in D1/D2 cells (41.9) compared to D2 MSNs (85.6). Similarly, the D1/D2 cluster had a higher total number of ATAC-seq reads surrounding the DRD1 transcription start site (TSS) (3079 within a 5 kb region) than the DRD2 TSS (959).
