Low Rheobase cells are highly excitable neurons in the superficial retrosplenial granular cortex
We recorded from 167 cells in the superficial layers (L2/3) of the mouse retrosplenial granular cortex (RSG). Consistent with other cortical regions, fast-spiking (FS) interneurons were present in these RSG layers (Figure 1A). FS cells were identified by their unique spiking properties (Sempere-Ferràndez et al., 2018), including their narrow spike width and rapid, sharp AHPs. Regular-spiking (RS) pyramidal neurons were occasionally found, but far less often than in typical neocortex (Figure 1B). A third population of cells was identified with distinct and unique physiological and intrinsic properties. The action potentials of these neurons were narrower than typical RS cells and often displayed prominent afterdepolarizations (Figure 1C). Detailed analyses of physiological and intrinsic parameters revealed several distinct and unique properties of these neurons. Spike widths were between those of FS and RS cells (FS = 0.22 ± 0.05 ms, RS = 0.86 ± 0.05 ms, LR = 0.55 ± 0.02ms; p<0.01 for each comparison; Figure 1D; Table 1). Additionally, these cells had uniquely high input resistance (402.69 ± 16.75 MΩ; p<0.01), low input capacitance (38.42 ± 1.32 pF; p<0.01), and low rheobase (91.79 ± 12.89 pA; p<0.01), suggesting they are a class of highly excitable neurons compared to both FS and RS neurons (Figure 1E,F,G; Table 1). They also exhibited minimal spike frequency adaptation (ratio of 1.26 ± 0.06), far lower than the substantial spike frequency adaptation shown by RS cells (ratio of 3.42 ± 0.58; p<0.01), highlighting their potential ability to fire trains of action potentials at high frequencies with minimal adaptation (Figure 1J; Table 1). For reasons investigated and explained in detail below, we refer to these unique neurons as Low Rheobase (LR) cells in the rest of this manuscript.