Surprisingly, about 4% of SPNs (Ppp1r1b+) were observed in a third, smaller cluster that also expressed Adora2a and Drd1 (cluster 13: n=2,744 cells; 4.5% of Ppp1r1b+ neurons; Figure 7A,B). These SPNs differed in expression from dSPNs+iSPNs by 110 genes (more than the 68 that distinguished dSPNs and iSPNs from each other) (Figure 7C), and expressed many genes that had little expression in the rest of the striatum (Figure 7D). Due to their transcriptional divergence from canonical SPNs, we call this population “eccentric” SPNs (eSPNs). eSPNs were intermixed with other SPNs in the striatum with no obvious spatial organization (Figure 7E,F). Our data account for all known striatal interneuron types (3.9% of total neurons)(Tepper and Bolam, 2004), suggesting by exclusion that eSPNs are not interneurons. We conclude that eSPNs are striatal principal neurons.
