Anatomical locations of recordings in CA subfields (CA1 vs. CA3)

Before describing the basic properties of place cells recorded in the familiar environment across development, one important point concerns the brain region from which these cells were recorded. Most place cells in this study were recorded from the CA1 region of the hippocampus proper (P14-P21: n=276, 75% of all place cells recorded across these age bins).

However, a smaller subset between P14-P21 was recorded from CA3 (P14-P21: n=67, 18% of all place cells recorded across these age bins). No place cells were recorded from CA3 in animals older than P21. In one animal the recording site was situated at the border between CA1 and CA3 (see Figure III-44). This dataset was excluded from the comparison of place cell properties across CA subfields (this dataset accounts for the remaining 7% of the place cell

161 population). Before pooling data obtained from CA1 and CA3 for the subsequent analyses, it is important to demonstrate that place cells recorded from both CA subfields do not differ across the parameters measured throughout this thesis. These are quality (spatial information, spatial coherence and intra-trial correlation) and stability (inter-trial correlation and centre of mass shift between trials) of the spatial signal, as well as some basic properties like firing rates (mean and peak firing rates), average size of the main firing field and number of place fields per cell. The following figures (see Figure III-8, Figure III-9, Figure III-10 and Figure III-11) show a comparison of the means for the above mentioned parameters for units recorded from CA1 and CA3, while animals explored the familiar environment before any probe was conducted.

162 Figure III-8: Quality of the spatial signal of place cells recorded from CA1 (wine) and CA3 (dark cyan) between P14-15 and P20-21. Depicted are spatial information (A, bits/spike), spatial coherence (B, r) and intra-trial correlation (C, r). Values are means from recordings inside the familiar environment before any probe trial was run. All values are population means±SEM. *p<0.05, **p<0.01

A MANOVA for within-trial measures (spatial information, spatial coherence, intra-trial correlation, mean and peak firing rate, field size, number of firing fields) with a 2-by-4 design (brain region by age bin) reveals a main effect of age (F21,993=3.09, p<0.001), no effect of brain region (F7,329=0.9, p=0.5) and a significant interaction between age and brain region (F21,993=2.22, p=0.001). The main effect of age and the underlying between-subjects effects will be discussed in more detail for the analysis of place cell properties in recording trials inside the familiar environment in the following section (see section III.3).

163 The between-subjects effects for the interaction between age and brain region are non-significant for spatial information (F3,335=1.37, p=0.25) and intra-trial correlation (F3,335=1.26, p=0.29), while the interaction between the two factors is significant for spatial coherence (F3,335=5.61, p=0.001). The pairwise comparisons for the latter measure show significant differences between spatial coherence values at P18-19 (p=0.004) and P20-21 (p=0.02), respectively.

Figure III-9: Firing rates of place cells recorded from CA1 (wine) and CA3 (dark cyan) between P14-15 and P20-21. Depicted are mean firing rate (A, Hz) and peak firing rate (B, Hz). Values are means from recordings inside the familiar environment before any probe trial was run. All values are population means±SEM.

Although there seems to be a trend for generally higher firing rates in CA1 (see Figure III-9), these differences between CA1 and CA3 place cells do not reach statistical significance. The between-subjects effects for the interaction between age and brain region are both non-significant for peak (F3,335=1.39, p=0.25) as well as for mean firing rate (F3,335=1.39, p=0.25).

164 Figure III-10: Field properties of place cells recorded from CA1 (wine) and CA3 (dark cyan) between P14-15 and P20-21. Depicted are mean place field size (A, cm²) and number of firing fields per cell (B).

Field size corresponds to the area of the main firing field. Values are means from recordings inside the familiar environment before any probe trial was run. All values are population means±SEM. *p<0.05

The properties of place fields in CA1 and CA3 in terms of size of main firing field and average number of subfields per cell show a similar change across development in both regions (see Figure III-10). The size of the main firing field is almost identical for both CA subfields at all age bins (interaction age*brain region: F3,335=0.65, p=0.57). In both CA subfields field sizes decrease steadily across development (see Figure III-10A). The average number of subfields per cell decrease as well across development (see Figure III-10B). There is significant interaction for this parameter between age and brain region (F3,335=2.68, p=0.047) and the pairwise comparisons show that average numbers of firing fields differ at P20-21 between CA1 and CA3 (p=0.03). Note that this data point also exhibits a quite large standard error of the mean for CA3 (ca. 0.25), indicating a large variance for the data obtained at this age bin.

165 Figure III-11: Stability of the spatial signal of place cells recorded from CA1 (wine) and CA3 (dark cyan) between P14-15 and P20-21. Values are means across recording trials inside the familiar environment before any probe trial was run Depicted are inter-trial correlation (A, r) and centre of mass shift (B, cm).

All values are population means±SEM.

A MANOVA for the across-trial measures (inter-trial correlation and centre of mass shift) only reveals a main effect of age (F8,630=2.25, p=0.022), while both region and the interaction between age and region are non-significant, respectively (region: F2,314=2.23, p=0.11; age*brain region: F6,630=1.44, p=0.2). The stability of the spatial signal increases linearly across development in CA1 and CA3 (see Figure III-11A). The average shift of the centre of mass of place fields from trial to trial decreases almost continuously across development in both CA subfields (see Figure III-11B).

From this analysis it is clear that place cell properties do not show any systematic difference across development between CA1 and CA3. That is why these data were pooled for the subsequent analysis of place cell properties obtained from recordings inside the familiar environment.

166

III.3 Properties of place cells in a familiar