To evaluate whether correlation structure relates to electrophysiological properties, we identified neuron populations known to be fast-firing (Figure S3D). One set of co-expressed genes encoded proteins that enable fast and persistent action potentials, including the Kcnc1–3 (Kv3.1), Kcna1 (Kv1.1) and Scn8a (Nav1.6) channels (Chen et al., 2008; Goldberg et al., 2008; Rudy and McBain, 2001) (Figure 4D). Fast-firing cell types expressed high levels of Kcnc2/Kcnc1 and Scn1a/Scn8a, variable levels of Kcnc4, and low levels of Scn2a1, whose expression was inversely correlated with Kcnc1–3 and Scn1a (Figure 4D). These relationships nominate hypotheses about channel contributions to physiological properties: for example, Kcnc4 could be a Kv3 family member that tailors membrane properties orthogonal to firing frequency, and Scn2a1 might undermine fast-firing.
