Fig. 2: Arc and c-Fos expression patterns diverge at all memory states in the DG.

A Left: Arc (green) and c-Fos (magenta) expression; Hoechst nuclear counterstain (blue). Right: inset. White arrow: example Arc⁺c-Fos⁺ cell. B–E Arc⁺ and c-Fos⁺ cell densities in the upper blade were greater than those in the lower blade during no shock, encoding, and 5-day retrieval. At 1-day retrieval, c-Fos⁺ cell density was greater in the upper than the lower blade, and there was a main effect of IEG (*p = 0.0236). F Mean Arc⁺c-Fos⁺ co-labeling normalized to chance (dotted line) was greater in the lower than upper blade during no shock, encoding, and 5-day retrieval. G Among complements of labeled DG cells, Arc⁺c-Fos⁻ cell density did not differ between blades but varied across memory states (main effect, *p = 0.0157). H, I Arc⁺c-Fos⁺ and Arc⁻c-Fos⁺ cell densities were greater in the upper than the lower blade during no shock, encoding, and 5-day retrieval. Arc⁺c-Fos⁺ cell density was greater in the upper than the lower blade at 1-day retrieval, and within the upper blade, greater during no shock than 1-day and 5-day retrieval. All labeled upper (J) and lower blade (K) cells for each group, summed across all mice, as a Venn diagram of Arc, c-Fos, and their overlap (blue) (upper: n = 1179, n = 3445, n = 3751, and n = 3871 cells; lower: n = 521, n = 1513, n = 1950, and n = 1953 cells). L The upper:lower density ratio was greater for c-Fos than Arc but not modulated by memory state. M This ratio for all labeled cells did not change across timepoints but was greater than 1 (dotted line) at each (one-sample t-tests, *p = 0.0375, ****p < 0.0001, ***p = 0.0002, and ***p = 0.0008). N Among complements, the Arc⁺c-Fos⁻ blade ratio was modulated by memory state, with that of encoding greater than that of all other groups, while both encoding and 1-day retrieval mean ratios were greater than 1 (one-sample t-tests, ***p = 0.0004, and **p = 0.0093). O, P Arc⁺c-Fos⁺ and Arc⁻c-Fos⁺ blade ratios were both greater than 1 at encoding and retrieval but not no shock timepoints (one-sample t-tests, ***p = 0.0005, **p = 0.0024, and ***p = 0.0009; **p = 0.0068, *p = 0.0129, and **p = 0.0024) but were not modulated by memory state. N = 4 (no shock) or 8 (encoding, 1-day retrieval, 5-day retrieval) mice. Error bars indicate ±SEM. *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001. DG dentate gyrus, ∪ union.