Time-of-day influence on exploratory behaviour of rats exposed to an unfamiliar environment

in Behaviour
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It is well known that rats exhibit elevated levels of activity during the dark phase and reduced levels during the light phase of the photoperiod cycle. However, the information about the influence of the time-of-day on the strategies used to explore the environment is still not understood. Here we tested the hypothesis that time-of-day influences the fine-scale exploratory behaviour of rats, measured in the open field (OF) test, and emotionality of rats, measured in the elevated plus maze (EPM) test. Adult male Wistar rats were subjected to the OF and EPM tests during Morning, Afternoon, or Evening sessions. In the OF, a principal component analysis (PCA) revealed that the Evening group exhibited longer duration of locomotion and rearing, and also higher distance travelled, trip length, inter-stop distance, number of stops and stops per trip compared to other groups. PCA also revealed that the Evening group exhibited shorter time spent at the home base, duration of locomotion along the perimeter and distance travelled along the perimeter compared to other groups. In the EPM test, there was no difference between the groups in any of the parameters evaluated. Our results indicate that the time-of-day may influence the spatio-temporal organization of exploration of rats subjected to unfamiliar environments. These alterations appear to be unrelated to differences in the emotional state of the animals.

Time-of-day influence on exploratory behaviour of rats exposed to an unfamiliar environment

in Behaviour

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Figures

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    Locomotor activity. The end-point results of (a) distance travelled, (b) duration of locomotion, (c) number of stops and (d) inter-stop distance. The data are expressed as the means ± SEM. N=1012 animals per group. p<0.05; ∗∗p<0.01; ∗∗∗p<0.001. All data were analysed using one-way ANOVA followed by Tukey post hoc analysis.

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    Exploratory activity. The end-point results of accumulated (a) duration of rearing and (b) duration of grooming. The data are expressed as the means ± SEM. N=1012 animals per group. p<0.05. All data were analysed using one-way ANOVA followed by Tukey post hoc analysis.

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    Time-course analyses of the behavioural profile within each group. (a) The distance travelled, (b) duration of rearing, and (c) duration of grooming are represented as the percentage of each behaviour during each 5-min period (for example, the percentage of the distance travelled was calculated as the distance travelled in each 5-min period divided by the accumulated distance travelled during the entire test multiplied by 100). The data are expressed as the means ± SEM. N=1012 animals per group. p<0.05; ∗∗p<0.01; ∗∗∗p<0.001. Data were analysed using two-way ANOVA with repeated measures (within-subject factor: time period of the test; between-subject factor: time-of-day) followed by the Bonferroni post hoc test.

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    Comparison between the groups with respect to time spent at the home base zone during the final 10 min of testing. The data are expressed as the means ± SEM. N=1012 animals per group. p<0.05. Data were analysed using one-way ANOVA followed by Tukey post hoc analysis.

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    Temporal organization of exploration. (a) The number of trips, (b) the trip length and (c) the average number of stops during each trip. Data are expressed as the medians and the interquartile ranges. N=1012 animals per group. p<0.05. All data were analysed using the Kruskal–Wallis test followed by Dunn’s multiple comparisons test.

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    Principal component analysis. (a–c) The matrix correlation between the OF task variables and each individual PC (PC1, PC2 and PC3). (d–f) Comparison between the groups with respect to the PC1, PC2 and PC3 values. Dashed lines in panels a–c (values 0.3 and −0.3 on the y-axis) indicate the cut-off points (i.e., the variables which presented correlation values > −0.3 and <0.3 are not represented in the graphs). Data in panels d–f are expressed as the medians and the interquartile ranges. p<0.05; ∗∗p<0.01. Data were analysed using one-way ANOVA followed by Tukey post hoc analysis or using the Kruskal–Wallis test followed by Dunn’s multiple comparisons test. This figure is published in colour in the online edition of this journal, which can be accessed via http://booksandjournals.brillonline.com/content/journals/1568539x.

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    Pearson correlation between (a) the distance travelled and the duration of rearing, (b) the distance travelled and the duration of grooming, (c) the distance travelled and the duration of locomotion, (d) the duration of locomotion and the duration of rearing, (e) the duration of locomotion and the duration of grooming, and (f) the duration of rearing and the duration of grooming. The blue, red and green lines represent the correlation between the variables corresponding to the Morning, Afternoon and Evening groups, respectively. This figure is published in colour in the online edition of this journal, which can be accessed via http://booksandjournals.brillonline.com/content/journals/1568539x.

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    Microstructure of the behaviour. Schematic ethograms (a and c) and occupancy plots of each group (b and d) representing the behaviour of the animals during the first 5 min (perimeter patrolling phase) and the final 10 min (home base phase) of observation. In the home base phase an organized strategy is performed, in which the rats established a safe zone and performed exploratory tours through the arena starting from the home base and returning to it. A higher incidence of yellowish-red points during the final 10 min compared to the first 5 min indicates that little or no home base was established during the first 5 min of the test. In c, the length of the arrows represents the distance travelled during each trip, while the width of the arrows represents the frequency in which the trips occurred along each trail. This figure is published in colour in the online edition of this journal, which can be accessed via http://booksandjournals.brillonline.com/content/journals/1568539x.

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    Time-course analyses of the behaviour profile of each group with respect to the distance travelled (a and d), the duration of rearing (b and e), and the duration of grooming (c and f). The animals in the Evening group travelled a longer distance during the first and second 5-min periods of the test compared to the Morning and Afternoon groups. The Evening group exhibited a longer duration of rearing during the first and second 5-min periods than the Morning group. We did not detect any difference between the groups in the duration of grooming. The minute-by-minute habituation profile of the distance travelled, the duration of rearing, and the duration of grooming is illustrated in d–f, respectively. The data are expressed as the means ± SEM. N=1012 animals per group. p<0.05; ∗∗p<0.01; ∗∗∗p<0.001. Data were analysed using two-way ANOVA with repeated measures (within-subject factor: time period of the test; between-subject factor: time-of-day) followed by the Bonferroni post hoc test.

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