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Comparative Study
. 2008 Jan 30;28(5):1030-3.
doi: 10.1523/JNEUROSCI.4235-07.2008.

Immediate-early gene expression at rest recapitulates recent experience

Diano F Marrone  1 , Michael J Schaner, Bruce L McNaughton, Paul F Worley, Carol A Barnes
Affiliations
Comparative Study

Immediate-early gene expression at rest recapitulates recent experience

Diano F Marrone et al. J Neurosci. .

Abstract

Immediate-early genes (IEGs) are tightly coupled to cellular activity and play a critical role in regulating synaptic plasticity. While encoding spatial experience, hippocampal principal cells express IEGs in a behaviorally dependent and cell-specific manner. This expression can be detected through the use of cellular compartment analysis of temporal activity by fluorescence in situ hybridization to generate estimates of cellular activity that match direct neuronal recording under comparable conditions. During rest, IEG expression continues to occur in a small number of cells, and the role of this basal expression is unknown. Imaging IEGs expressed during exploration and adjacent rest periods reveals that "constitutive" IEG expression during rest is not random. Rather, consistent with proposed memory consolidation mechanisms, it recapitulates a subset of the pattern generated by recent experience.

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Figures

Figure 1.
Figure 1.
A timeline shows gene expression patterns indicative of cellular activity (top) and each group's behavior (bottom) during each episode. Cells were labeled with digoxigenin-conjugated Homer 1a (H1a, red), fluorescein-conjugated Arc (green), and counterstained with TO-PRO-3 iodide (blue). Cells active 60 min before the animals were killed exhibit cytoplasmic H1a (left), when E-R-E animals explored a novel environment and R-E-E animals rested. Thirty minutes before being killed, R-E-E animals explored and E-R-E animals rested. Cells active during this time exhibit cytoplasmic Arc and intranuclear H1a (middle). Five minutes before the animals were killed, active cells exhibit intranuclear Arc. Both R-E-E and E-R-E animals explored at this time (right). A third group (R-R-R) remained undisturbed in their home cages.
Figure 2.
Figure 2.
A, The proportion of CA1 principal cells transcribing IEGs across episodes. During exploration (hatched bars), IEG transcription is significantly higher in both E-R-E (white) and R-E-E (light gray) animals than controls (R-R-R; dark gray), whereas transcription during rest is comparable with controls. Most cells active during the first exploration are active in the second (behavioral overlap). B, The proportion of cells active in rest and both explorations (solid), and the overlap expected by random chance (cross-hatched). Significantly more cells were active during all episodes in E-R-E than R-R-R animals, and in R-R-R and E-R-E animals this overlap is significantly greater than expected by random chance, suggesting transcription recapitulates previous experience both in the exploration and home cage environments (mean ± SEM; **p < 0.001 vs R-R-R; ††p < 0.001 between behavioral groups; ‡p < 0.05 vs chance overlap).
Figure 3.
Figure 3.
A schematic diagram illustrates the main observations made in the experiment. At left, schematic illustrations show cells (blue) displaying typical patterns of Homer 1a (H1a; red) and Arc (green) expression in the three groups examined in this experiment. At right, Venn diagrams depict the degree of overlap in the active cell populations during episodes 1 (red), 2 (yellow), and 3 (green). A, B, In rest-rest-rest animals (A), a small number of neurons (∼10%) actively transcribe IEGs during any episode, and the degree of overlap in the activities during each episode is far greater than expected by random chance (B). C, In R-E-E animals, exploration induces a robust increase in cells expressing intranuclear H1a (indicating activity 30 min before the animal was killed), and both cytoplasmic and nuclear Arc (indicating activity at 30 and 5 min before the animal was killed, respectively), corresponding to the time of the two explorations. Relatively little cytoplasmic H1a, corresponding to activity during the initial rest period, can be observed. D, In addition, ∼40% of CA1 principle cells are active during the first exploration (yellow), and ∼90% of cells active during the first exploration are again active after returning to the same environment (green). Moreover, the degree of overlap between the population of cells that are active during rest (red) and those that are active during exploration is no greater than would be expected by random chance. E, F, Exploration induces a similar increase in IEG transcription in E-R-E animals (E), appearing as a large number of cells expressing cytoplasmic H1a (indicating activity 60 min before the animal was killed), and intranuclear Arc (indicating activity 5 min before the animal was killed) with only ∼10% of cells transcribing IEGs during the intervening rest period. In these animals (F), ∼40% of CA1 principle cells are active during the first exploration (red), and ∼90% of these cells are also active during the second exploration (green), similar to R-E-E animals. However, the majority of cells that are active during rest in E-R-E animals are the same cells activated by exploration (yellow). This degree of overlap is significantly greater than would be expected by random chance, and is a significant shift relative to the degree of overlap observed in R-E-E animals (dashed circle).
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