New and distinct hippocampal place codes are generated in a new environment during septal inactivation

Abstract

The hippocampus generates distinct neural codes to disambiguate similar experiences, a process thought to underlie episodic memory function. Entorhinal grid cells provide a prominent spatial signal to hippocampus, and changes in their firing pattern could thus generate a distinct spatial code in each context. We examined whether we would preclude the emergence of new spatial representations in a novel environment during muscimol inactivation of the medial septal area, a manipulation known to disrupt theta oscillations and grid cell firing. We found that new, highly distinct configurations of place fields emerged immediately and remained stable during the septal inactivation. The new place code persisted when theta oscillations had recovered. Theta rhythmicity and feedforward input from grid cell networks were thus not required to generate new spatial representations in the hippocampus.

Figure 1

Near-complete reduction of theta power in the familiar and novel room during septal inactivation. Coronal sections show (A) infusion cannula (white arrow) in the medial septal area (MSA, dashed line) and (B) tetrode tracks that terminated in hippocampal region CA1 (red lines). Scale bars, 0.5 mm. (C) LFP traces recorded simultaneously from the hippocampus (top row) and medial entorhinal cortex (MEC, bottom row). Theta oscillations are reduced during septal inactivation. All traces are from periods with running speeds > 20 cm/s. (D) (Top) Experimental timeline of familiar (F) and novel (N) room recordings during baseline (F1, F2), septal inactivation (F3, N1, N2, N3, F4), and recovery from inactivation (F5, F6). Bottom: Mean percentage of baseline theta (5–10 Hz) power after muscimol infusions into the septal area. The magnitude and timecourse of the reduction in theta power during hippocampal place cell recordings (mean ± 95% confidence intervals in black) was compared to the reduction in theta power in simultaneous LFP recordings from the hippocampus (in gray) and MEC (in purple). Recordings at 6 hours and at 24 hours after muscimol infusion (shown in intervals from 0 to 10 minutes) show recovery of theta oscillations. The timecourse of the reduction in theta power corresponded between the hippocampus and MEC. The theta reduction during place cell recordings closely matched the previously reported theta reduction during recordings in which grid cells were disrupted (Figure S2).

Figure 2

Place fields emerge in a novel room during septal inactivation. (A) Firing rate maps of 10 representative hippocampal neurons from 2 representative rats (separated by horizontal line) in the novel room after intraseptal muscimol infusion. Each row is a cell. (B) Same as in (A), but from different cells in a novel room after intraseptal aCSF infusion. (C) Heat map of vertically stacked autocorrelograms from cells recorded in the novel room after muscimol (top) and aCSF (bottom) infusions. The average population autocorrelogram is overlaid in white. (D) Theta rhythmicity, mean firing rate, peak firing rate, place field size, and spatial information for cells recorded in F3, N1, N2, and N3 after intraseptal muscimol (purple) or aCSF (black) infusions. (E–F) Cumulative distribution functions (CDFs) were generated to compare distributions of rate map correlation scores between conditions. The median and inter-quartile ranges are shown as stippled line in the CDFs and are displayed to the right of each panel. (E) Comparisons are between pairs of novel room sessions after muscimol infusion (shades of blue) and between pairs of familiar room sessions (shades of gray; familiar room sessions include recordings before and after the muscimol infusion). All novel room comparisons differ from the familiar room comparisons. (F) Distributions in the novel room during muscimol are re-plotted from (E) and compared to rate map correlations in a novel room after control infusions.

Figure 3

Global remapping in the novel room during septal inactivation. (A–B) Scatterplots compare the mean firing rate of each cell between repeated sessions in the novel room (N1 and N2) and between the familiar and novel room (F3 and N1). Firing rate maps of the cells with the top five overlap scores (circled in blue) are displayed to the right of each scatterplot. Cells with high overlap scores between F3 and N1 show dissimilar spatial firing patterns. (C) CDFs of the rate overlap scores and (D) CDFs of rate map correlations. Comparisons are between repeated recordings in the familiar room (gray), between repeated recordings in the novel room (blue), and between familiar and novel room recordings (purple). The median and inter-quartile ranges are shown as stippled lines in the CDF and are displayed to the right of each panel. Red lines display shuffled distributions. Novel to familiar comparisons were not different from the shuffled distributions.

Figure 4

Newly formed place fields are retained after recovery from septal inactivation. Novel room recordings were performed in either a circle or square environment in different experiments. (A) Firing rate maps show that the place fields that were formed in the novel room during septal inactivation (N1, N2, N3) are retained during retesting in the same room 24 hours after the infusion (N4, N5). In the familiar room, spatial firing patterns also match the patterns from the previous day. (B) Examples of place cells that were active in only the familiar room, but not in the novel room during any of the sessions. (C) Example of the only neuron (of twenty) that was silent during septal inactivation but generated a place field after recovery from septal inactivation. (D) Mean (± SEM) of cells’ rate map correlations between the inactivation and the recovery day. For the novel room, the averaged rate map during septal inactivation (N1, N2, N3) was compared to the average during recovery (N4, N5) (brown bar). For the familiar room, the average rate map of F1, F2, and F3 was compared to the average of the 24 hours maps (F6, F7). After 24 hours, fields that had emerged in the novel room during the inactivation on the previous day were as stable as those in a familiar room, and 24-hour stability within each of the rooms was substantially higher than for shuffled place field locations.