To associate subclusters with putative neuron types, we compared published GPe markers to selective markers we identified (Figure 6D). Only Pvalb and Penk/Foxp2 were selectively expressed in GPe subclusters (Kita, 1994; Voorn et al., 1999), suggesting 2–14 represents fast-spiking “prototypical” neurons, while 2–19 represents the slow-firing “arkypallidal” population (Abdi et al., 2015; Mallet et al., 2012). Interestingly, 2–13 is similar to 2–14, sharing markers like Grem1, but is distinguished by stronger expression of Scn4b, Kcnc3 and other “Neurofilament program” genes (Data S5). The fourth GPe subcluster (2–17) is enriched for Elfn1/Grik3 and has not been characterized. Several subclusters (2–15, 2–21, and 2–18) expressed markers in the VP and bordering GPe (Figure 6D,E), suggesting neuron types shared across regions with different behavioral functions and connectivity (Gittis et al., 2014; Kita, 2007; Smith et al., 2009). Border-spanning subclusters may explain a neuron type that is synaptically incorporated into the GPe, but exhibits VP-like axonal projections (Chen et al., 2015; Saunders et al., 2015).
