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Resident Intruder Paradigm-Induced Aggression Relieves Depressive-Like Behaviors in Male Rats Subjected to Chronic Mild Stress

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Resident Intruder Paradigm-Induced Aggression Relieves Depressive-Like Behaviors in Male Rats Subjected to Chronic Mild Stress

Sheng Wei et al. Med Sci Monit.

Abstract

Background: Accumulating epidemiological evidence shows that life event stressors are major vulnerability factors for psychiatric diseases such as major depression. It is also well known that the resident intruder paradigm (RIP) results in aggressive behavior in male rats. However, it is not known how resident intruder paradigm-induced aggression affects depressive-like behavior in isolated male rats subjected to chronic mild stress (CMS), which is an animal model of depression.

Material and methods: Male Wistar rats were divided into 3 groups: non-stressed controls, isolated rats subjected to the CMS protocol, and resident intruder paradigm-exposed rats subjected to the CMS protocol.

Results: In the sucrose intake test, ingestion of a 1% sucrose solution by rats in the CMS group was significantly lower than in control and CMS+RIP rats after 3 weeks of stress. In the open-field test, CMS rats had significantly lower open-field scores compared to control rats. Furthermore, the total scores given the CMS group were significantly lower than in the CMS+RIP rats. In the forced swimming test (FST), the immobility times of CMS rats were significantly longer than those of the control or CMS+RIP rats. However, no differences were observed between controls and CMS+RIP rats.

Conclusions: Our data show that aggressive behavior evoked by the resident intruder paradigm could relieve broad-spectrum depressive-like behaviors in isolated adult male rats subjected to CMS.

Figures

Figure 1
Figure 1
Experimental protocol. CMS procedures were performed on CMS and CMS+RIP animals for 3 weeks. The sucrose preference test (SPT) was performed at 12, 16, 23, and 30 days. The open-field test (OFT) was performed at 12, 16, 23, and 30 days. The forced swimming test (FST) was performed at day 35. The body weights were measured at 12, 16, 23, and 30 days.
Figure 2
Figure 2
Open-field test. The open-field test (OFT) was performed at 12 (baseline), 16 (week 1), 23 (week 2), and 30 (week 3) days after the start of CMS as shown in Figure 1. CMS rats had significantly lower open-field scores compared to control rats at 28 or 35 days. The total scores assigned the CMS group were significantly less than in the CMS+RIP rats at 35 days. There were no differences between controls and CMS+RIP rats. Values represent the mean ±SEM (n=12 for controls, n=11 for the CMS group, n=11 for the CMS+RIP group). * p<0.05, ** p<0.01, *** p<0.001.
Figure 3
Figure 3
Sucrose preference test. A sucrose intake test was performed at 12 (baseline), 16 (week 1), 23 (week 2), and 30 (week 3) days after the start of CMS as shown in Figure 1. The intake of 1% sucrose solution in CMS rats was significantly lower than in controls (at 28 and 35 days) or CMS+RIP rats (at 35 days) after the start of CMS. Values represent the mean ±SEM (n=12 for controls, n=11 for the CMS group, n=11 for the CMS+RIP group). * p<0.05, ** p<0.01, *** p<0.001.
Figure 4
Figure 4
Forced swimming test. CMS rats spent a significantly longer time immobile than did the control and CMS+RIP rats. No differences were observed between controls and the CMS+RIP rats. Values represent the mean ±SEM (n=6 for controls, n=6 for the CMS group, n=6 for the CMS+RIP group). * p<0.05, ** p<0.01, *** p<0.001, by one-way ANOVA.
Figure 5
Figure 5
Body weight gain. Body weight gain was measured at 12 (baseline), 16 (week 1), 23 (week 2), and 30 (week 3) days after the start of CMS as shown in Figure 1. There was a significant reduction in body weight gain in CMS animals after 2 and 3 weeks compared with control and CMS+RIP groups. There was no statistical difference between the controls and CMS+RIP rats during the entire experimental period. Values represent the mean ±SEM (n=12 for controls, n=11 for the CMS group, n=11 for the CMS+RIP group). * p<0.05, ** p<0.01, *** p<0.001, by two-way ANOVA, split-plot for time.

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