Changes in task demands alter the pattern of zif268 expression in the dentate gyrus

Abstract

Granule cells of the dentate gyrus (DG) are thought to disambiguate similar experiences--a process termed pattern separation. Using zif268 as a marker of cellular activity, DG function was assessed in rats performing two tasks: a place task (go east) and a response task (turn right). As these tasks occurred within the same physical space (a plus maze) without any physical cue to indicate the correct strategy in a given trial, this scenario critically involves disambiguation of task demands and presumably pattern separation. Performance of the two tasks induced zif268 expression in distinct populations of granule cells within the suprapyramidal but not the infrapyramidal blade of the DG. Repeated performance of the same task (i.e., two response-task trials or two place-task trials), however, elicited zif268 expression within a single subset of the granule cell population. This differential transcription pattern shows that the retrieval of different behavioral strategies or mnemonic demands recruit distinct ensembles of granule cells, possibly to prevent interference between memories of events occurring within the same physical space to permit the selection of appropriate responses.

Figure 1.

Schematic outline of the experiment. a, b, Rats were trained in a plus maze (a) to navigate to a target arm for food reward using either a place (i.e., go east) or a response (i.e., turn right) strategy (b). Rats were trained to use one of four strategies: consistent spatial strategy (place-place), consistent response strategy (response-response), response strategy during the first session and a place strategy during the second (response-place), and place strategy during the first session and a response strategy during the second (place-response). Note that when the animal began from the south arm (green), both strategies resulted in an identical trajectory. c, During the final session, all groups were given two sessions of three trials on the maze beginning from the south arm, separated by a 25 min interval. d, Following these trials, the animals' brains were harvested and the compartmental expression of zif268 (red) was measured to provide a histological record of activity in granule cells (blue) Scale bar, 20 μm. Cells active during the first epoch on the maze expressed zif268 within the cytoplasm (left); those active during the second epoch express zif268 within the nucleus (right). Granule cells active during both epochs express zif268 within both cellular compartments (bottom).

Figure 2.

The pattern of DG zif268 expression is altered by task demands. a, b, Representative montages of confocal images of the DG demonstrate little zif268 expression in caged control animals (a), whereas navigation of the plus maze (b) induces a robust increase in zif268 localized in the suprapyramidal (DG_SP) but not the infrapyramidal (DG_IP) blade. c–e, Higher-magnification suprapyramidal DG micrographs illustrate the differences in zif268 transcription patterns in the three behavioral conditions. Compared to caged controls (c), the granule cells of animals that navigated the same maze using different strategies (d) expressed zif268 solely within the nucleus (short arrow) or cytoplasm (long arrows), and granule cells in animals that used the same strategy (e) expressed zif268 predominantly within both compartments (arrowheads). Granule cell nuclei are counterstained with DAPI (blue) and zif268 is labeled with Cy3 (red). Scale bars: a, b, 200 μm; c, d, e, 60 μm.

Figure 3.

Granule cells expressing zif268 across the DG. Within the suprapyramidal blade (DG_SP) of both the dorsal (a) and ventral (b) DG, rats that navigated the plus maze using different strategies (P/R and R/P) consistently had more granule cells expressing zif268 within either the nucleus (Foci) or the cytoplasm. These results indicate that unique populations of granule cells were recruited to express zif268 during each navigation session. In contrast, granule cells of animals that navigated the maze using the same strategy twice (P/P and R/R) expressed zif268 in both compartments (Foci + Cytoplasm), showing that the same cells were active during both navigation sessions. In the infrapyramidal blade (DG_IP), no differences were observed in either the dorsal (c) or ventral (d) DG (mean ± SEM; *p < 0.05, relative to caged).

Figure 4.

Overlap in repeated zif268 expression in the DG. When the same navigational strategy is used twice, ∼70% of granule cells in the suprapyramidal blade (DG_SP) that expressed zif268 during the first session also expressed it in the second session. This overlap is significantly greater than is seen in animals that navigated the plus maze using two different navigational strategies, showing that changing task demands recruited a distinct population of granule cells. In the infrapyramidal blade (DG_IP), no differential recruitment was apparent (mean ± SEM; *p < 0.05, same strategy vs different strategy within the same region).