, 2005; Sugar and James, 2003) using 15 electrophysiological and morphological properties (see Experimental Procedures). Using all CA1 and subicular pyramidal neurons (n = 110 cells), K-means PI3K Inhibitor Library manufacturer cluster analysis revealed two completely nonoverlapping groups of cells (Figure 2A), which aligned perfectly with the two step current-induced firing

patterns (Figure 2B). We did not find evidence of more than two clusters, subgroups within either the regular-spiking or bursting populations, or separation of CA1 and subicular neurons within either of the two clusters (Figures 2C and 2D). A bootstrap analysis (see Experimental Procedures and Sugar and James, 2003) demonstrated that the data are best represented by two clusters (see Figure S1 available online). Furthermore, a principal component analysis revealed that the first three principal components produced perfect separation of the regular-spiking and bursting cell types (Figure 2E). Finally, a plot of only two properties (one physiological and one morphological) reveals a large degree of separation between the two cell types (Figure 2F). Taken together, these results strongly indicate the presence of two (and not more) distinct classes of pyramidal neurons, with each cell type present in both CA1 and the subiculum. These regular-spiking and bursting cell types VE-821 molecular weight differ not

only in their firing patterns, but also in several additional properties (Figure 3 and Table 1). There are clear morphological differences between the two cell types: bursting cells have more extensive tuft dendrites (the distal third of the apical tree), whereas regular-spiking cells have more extensive basal dendrites (Figures 3A–3C). Regular-spiking cells also have a higher input 4-Aminobutyrate aminotransferase resistance (RN), a smaller postspike afterdepolarization (ADP), a more depolarized bursting threshold, as well as several other electrophysiological differences, relative to bursting

cells (Figure 3D and Table 1). The morphological differences in particular suggest that the regular-spiking and bursting populations do not reflect transient variance in excitability (Babadi, 2005; Beurrier et al., 1999), but rather that the two populations are discrete, stable cell types. As both of these cell types have characteristic pyramidal cell morphology and electrophysiological properties, and as we show that they are both immunopositive for a glutamatergic marker (the excitatory amino acid transporter 3, EAAT-3) and negative for a marker of GABAergic neurons (glutamic acid decarboxylase 2, GAD-2), these cell types are clearly both excitatory pyramidal neurons (Figure S2). To further investigate the properties of these two types of neurons, we evoked action potential firing using trains of brief excitatory postsynaptic current (EPSC)-like current injections (Moore et al., 2009).