g., Figures 1A versus 1B), or they could receive different amounts of input (e.g., Figures 1A versus 1C) or have different thresholds (e.g., Figures 1A versus 1D), with each such alternative having important implications for the origin of place and silent cells. With the extracellular recording methods used in nearly all previous place cell studies, one can attempt to infer the input into a place cell based on its spiking output (Mehta et al., 2000); find protocol however, this is problematic for studying silent cells because they rarely spike. More importantly, extracellular methods cannot

measure fundamental intracellular features such as the baseline Vm, AP threshold, or subthreshold Vm dynamics needed to reveal why spikes do or do not occur. But, recently, intracellular recording in freely moving animals has become possible (Lee et al., 2006, Lee et al., 2009 and Long et al., 2010), and hippocampal place cells have been recorded intracellularly in both freely moving (A.K. Lee et al., 2008, click here Soc. Neurosci., abstract [690.22]; Epsztein et al., 2010) and head-fixed (Harvey et al., 2009) rodents, providing an opportunity to directly measure inputs

and intrinsic properties during spatial exploration. Here, we used head-anchored whole-cell recordings in freely moving rats (Lee et al., 2006 and Lee et al., 2009) as they explored a novel maze in order to investigate what underlies the distinction between place and silent cells starting from the very beginning of map formation. We obtained whole-cell current-clamp recordings of dorsal hippocampal CA1 pyramidal neurons as rats moved around

a previously unexplored “O”-shaped arena (for 7.9 ± 2.3 min). Nine rats went around the maze a sufficient number of times in the same direction (clockwise, CW, or counterclockwise, Idoxuridine CCW) to allow determination of whether the recorded neuron was a place (PC, n = 4) or silent (SC, n = 5) cell in that environment based on its spiking (see Experimental Procedures). In three cases, both directions qualified. Since cells in one-dimensional mazes often have different place fields in each direction, including cases with a place field in one but not the other direction, this gave 12 directions (4.9 ± 0.9 laps each) to classify as having place fields (PD, n = 5) or being silent (SD, n = 7). These numbers agree with the extracellularly-determined fraction of place cells in a given environment (Thompson and Best, 1989, Wilson and McNaughton, 1993 and Karlsson and Frank, 2008), suggesting that extracellular methods can accurately sample silent cells. Figure 2 shows an intracellularly recorded place cell that fired in one corner of the maze (Figures 2A and 2B) and had place fields at that location in both directions (Figure 2C).