Effects of testing moment in the circadian sleep-wake rhythm on learning, memory and behavioural flexibility.

The daily light-dark cycles allow the synchronization of behavioural and physiological processes to the external environment. Light is the most important environmental cue or zeitgeber that coordinates many aspects of physiology and behaviour such as body temperature, hormonal regulation, locomotor activity, maintenance behaviours, alertness or long-term potentiation (i.e. a process that plays a key role in memory consolidation). Mice are among the main animals used in behavioural neuroscience and preclinical research laboratories; although nocturnal, they are generally tested during day (i.e. during their resting phase, under the standard laboratory lightning condition). Convenient for the experimenter, manipulations during day can generate some stress to the animal. Moreover, several cognition studies show that mice performed better when tested during their active phase. Thereby, the testing moment might be a predominant variable affecting animal behaviour and therefore all the inferences we make about cognitive processes. Nevertheless, a lack of data related to the effect of testing moment on behavior was recently highlighted.
In this study, we focused on a memory test (the Object Recognition Test, ORT) and on a test learning and behavioral flexibility (the operant Attentional Set-Shifting Task, ASST). Due to their ease of use, both tasks are widely used but rarely with the testing moment as an independent variable. Moreover, particularly for the ORT, those studies have led to conflicting results. Theses discrepancies can be due to several methodological differences.
According to other published results in the literature and to some common sense, we supposed that mice should perform better when tested during their active phase. 24 female C57/BL6 mice divided in two groups were singly-housed under a standard light-dark cycle (lights on at 08.00 and off at 20.00, N=12) or under a reversed light-dark cycle (lights on at 20.00 and off at 08.00, N=12). Between-group ANOVAs and t-tests were applied to data.
Our findings highlight that mice tested during their active phase show a similar performance related to those tested during their inactive phase during the ASST. However, they discriminated better the new object during the ORT. As cognitive indexes are better for the active group or equivalent between groups, testing rodents during their active period should be privileged not only from an ethical perspective but also to improve the data quality. Moreover, further research should be conducted in this area in order to better understand the real effect of the testing moment on cognition.