Figure 1.
Brain activation experimental design. A.
After 3 weeks in either standard condition (SC) or enriched condition (EC), memory performances of mice were tested in an object-recognition task performed in a Y-maze with an inter-session interval (ISI) of 24 h (“Test” groups, n = 12 for each housing condition). B. Ninety minutes after the presentation or after the test session, animals were euthanized and the brains were collected for c-Fos immunohistochemistry (“Presentation” groups and “Test” groups, n = 10 per housing condition). Additional mice were exposed to the same behavioral procedures but without any object (“Control of presentation” groups and “Control of test” groups, n = 8 and n = 10 per housing condition respectively).
Figure 2.
Effect of increasing ISI on object recognition memory performances in mice housed in SC.
Data are expressed as the mean exploration time (± SEM) of novel object for the test session with increasing ISI: 2 h (n = 10), 4 h (n = 11), 6 h (n = 11) and 24 h (n = 12). Exploration time of the novel object was significantly higher than the chance level (10 seconds) with 2 h and 4 h-ISI, suggesting that SC mice discriminated the novel object at both delays (univariate t-test: § denotes P<0.05).
Figure 3.
Effect of EC on object recognition memory tested with 2 h-ISI. A.
Data are the mean exploration time (± SEM) of the novel object during the test session in SC (n = 11) and EC (n = 9) groups. Exploration time of the novel object was significantly higher than the chance level (10 seconds) in the two groups (univariate t-test: § denotes P<0.05 for SC mice and EC mice), suggesting that all mice discriminated the novel object. Neither the percentage of entries, nor the percentage of time spent in the arm containing the novel object were different from the chance level (50%) in both groups. B. The time spent to reach the criterion (20 seconds of total exploration of the objects) did not differ between groups during the presentation session. By contrast, during the test session, the time spent to reach the criterion was significantly higher in EC mice than in SC ones (ANOVA with repeated measurements followed by a SNK multiple range test: ** denotes P<0.01, significantly different from SC group). The time spent to reach the criterion was significantly different between two sessions in EC group (ANOVA: # denotes P<0.05, significantly different from the presentation session). C. The total number of entries in the three arms did not differ between groups.
Figure 4.
Effect of EC on object recognition memory tested with 24 h-ISI. A.
Data are the mean exploration time (± SEM) of the novel object during the test session in SC (n = 10) and EC (n = 10) groups. Only EC mice discriminated the novel object compared to the chance level (10 seconds) (univariate t-test: §§§ denotes P<0.001). Moreover, the time spent to explore the novel object was significantly higher in EC mice compared to SC ones (ANOVA: *** denotes P<0.001). The percentage of entries and the percentage of time spent in the arm containing the novel object were significantly different from the chance level (50%) in EC group only (univariate t-test: § denotes P<0.05). A significant difference of the percentage of time spent in the arm containing the novel object was observed between groups (ANOVA: * denotes P<0.05). B. The time spent to reach the criterion did not differ between groups during the presentation session but was significantly higher in EC mice during the test session (ANOVA with repeated measurements followed by a SNK multiple range test: ** denotes P<0.01, significantly different from SC group). EC mice spent significantly more time reaching the criterion during the test session (ANOVA: ## denotes P<0.01, significantly different from the presentation session). C. The total number of entries in the three arms did not differ between groups.
Figure 5.
Effect of EC on neuronal activity in selected brain regions.
Results are expressed as mean percentage changes of relative number of c-Fos-positive cells compared to control animals (which performed the task without any objects: n = 8 “control of presentation” animals per housing condition for the presentation session and n = 10 “control of test” animals per housing condition for the test session). A. A significant activation of the hippocampus and the perirhinal cortex was observed in SC mice during the presentation session while a significant reduction of activation of the baso-lateral amygdala occurred in EC mice (ANOVA with repeated measurements followed by a SNK multiple range test: * denotes P<0.05, significantly different from SC group; univariate t-test: § denotes P<0.05 and §§ denotes P<0.01, significantly different from respective “control of presentation” animals (100%)). B. The novel object discrimination during the test session (24 h-ISI) was associated with a significant activation of the infralimbic cortex and the hippocampus in EC mice (univariate t-test: § denotes P<0.05, significantly different from respective “control of test” animals (100%)). Abbreviations: Fr, frontal cortex; PrL, prelimbic cortex; IL, infralimbic cortex; CgA, anterior cingulate cortex; Hpp, hippocampus; PRh, perirhinal cortex; BLA, amygdala, baso-lateral nucleus.
Figure 6.
Microphotographs of brain c-Fos immunostaining related to the presentation and the test sessions.
During the presentation session, the number of c-Fos-positive cells counted in the hippocampus (represented here by the granular cell layer of the dentate gyrus), the perirhinal cortex and the baso-lateral amygdala was lower in EC mice compared to SC ones. By contrast, the number of c-Fos-positive cells counted in the hippocampus and the infralimbic cortex during the test session was higher in EC mice. The dark arrows indicate examples of nuclei showing c-Fos immunoreactivity. Unannotated scale bars: 50 µm.
Table 1.
Interregional correlation matrix of the relative neuronal activity found in the hippocampus.