The Effect of Melatonin and Exercise on Social Isolation-Related Behavioral Changes in Aged Rats

Abstract

Social isolation (SI) is well established as an environmental factor that negatively influences different behavioral parameters, including cognitive function, anxiety, and social interaction, depending on the age of isolation. Aging is a physiological process that is associated with changes in cognitive function, locomotor activity, anxiety and emotional responses. Few studies have investigated the effect of SI in senescence, or possible interventions. In the current study, we investigated the possible complementary effects of melatonin (MLT) and exercise (Ex) in improving SI-related behavioral changes in aged rats. Forty aged Wistar rats (24 months old) were randomly divided into five groups (n = 8 per group): Control (group housing), SI (individual housing for 7 weeks), SI + MLT (SI rats treated with 0.4 mg MLT/ml in drinking water), SI + Ex (SI rats treated with 60 min of swimming), and SI + MLT + Ex (SI rats treated with both MLT and Ex). Different behavioral tasks were conducted in the following sequence: open field test, elevated plus maze test, sucrose preference test, Y maze test, and Morris water maze test. Locomotor activities measured by total distance moved and velocity revealed that SI + Ex (P = 0.0038; P = 0.0015) and SI + MLT + Ex (P = 0.0001; P = 0.0003) significantly improved the locomotor activity compared with SI rats but SI + MLT (P = 0.0599; P = 0.0627) rats showed no significant change. Anxiety index score was significantly improved in SI + MLT + Ex (P = 0.0256) compared with SI rats while SI + MLT (P > 0.9999) and SI + Ex (P = 0.2943) rats showed no significant change. Moreover, latency to reach the platform in Morris water maze was significantly reduced at day 5 in SI + MLT + Ex (P = 0.0457) compared with SI rats but no change was detected in SI + MLT (P = 0.7314) or SI + Ex (P = 0.1676) groups. In conclusion, this study supports the possible potential of MLT in combination with Ex in improving physical activity, anxiety, and cognitive functions in aging population.

Keywords: aging; anxiety; exercise; melatonin; social isolation (SI); spatial memory.

FIGURE 1

(A) Timeline of the experiment and overview of the behavioral tests. (B) MWM test. MLT, melatonin; Ex, exercise; SI, social isolation; EPM, elevated plus maze; SPT, sucrose preference test; MWM, Morris water maze. Created by BioRender.com.

FIGURE 2

(A) The effect of MLT and Ex on the percentage change in body weight among groups of aged rats. (B) The effect of MLT and Ex on food consumption among groups of aged rats. Data are presented as mean ± standard error of the mean (SEM). Two-way repeated measures ANOVA was used, followed by Tukey’s multiple comparisons test. * indicates a significant difference between the treated groups and the control group at p > 0.05; # indicates a significant difference between the treated groups and the SI group at p > 0.05; α indicates a significant difference between the treated groups and the combination group at p > 0.05; **P < 0.01; ##P < 0.01. The color of the symbol denotes comparisons between groups with the matching color and line.

FIGURE 3

The effect of MLT and Ex in the open field test on (A) total distance moved, (B) velocity, (C) rearing frequency, and (D) immobility frequency among groups. Data are presented as the mean ± SEM. One-way ANOVA was used, followed by Tukey’s multiple comparisons test. * indicates a significant difference between the treated groups and the control group at p > 0.05; # indicates a significant difference between the treated groups and the SI group at p > 0.05; ##P < 0.01; ###P < 0.001.

FIGURE 4

The effect of MLT and Ex on the central preference percentage among groups in the open field test. Data are presented as the mean ± SEM. One-way ANOVA was used, followed by Tukey’s multiple comparisons test. * indicates a significant difference between the treated groups and the control group at p > 0.05; # indicates a significant difference between the treated groups and the SI group at p > 0.05.

FIGURE 5

The effect of MLT and Ex on the anxiety index among groups in the EPM. Data are presented as mean ± SEM. One-way ANOVA was used, followed by Tukey’s multiple comparisons test. * indicates a significant difference between the treated groups and the control group at p > 0.05; # indicates a significant difference between the treated groups and the SI group at p > 0.05; α indicates a significant difference between the treated groups and the combination group at p > 0.05.

FIGURE 6

The effect of MLT and Ex on SPT performance (%) among groups. Data are presented as mean ± SEM. One-way ANOVA was used. ns, non-significant.

FIGURE 7

The effect of MLT and Ex on spontaneous alternation (%) (A), and frequency of arm entries (B) among groups in the Y maze. Data are presented as the mean ± SEM. One-way ANOVA was used. ns, non-significant.

FIGURE 8

The effect of MLT and Ex on the latency to reach the platform in the MWM among groups of aged rats over 5 days of training. Data are presented as mean ± SEM. Two-way repeated measures ANOVA was used, followed by Tukey’s multiple comparisons test. # indicates a significant difference between treated groups and SI group at p > 0.05; ##p < 0.01. The color of the asterisks denotes comparisons between groups with the matching color and line.