Research reportEffect of one week of stress on emotional reactivity and learning and memory performances in Japanese quail
Research highlights
▶ A sub-chronic stress period induces a specific physiological and behavioural profile. ▶ In birds, a 8-day long exposure to stressors had no major impact on emotional reactivity. ▶ A 8-day long exposure to stressors did not affect corticosterone levels. ▶ One week of stress improved behavioural flexibility (i.e. reversal learning).
Introduction
Understanding the relationship between stress and cognitive functioning is an important area of research in pre-clinical studies. It is also relevant in animal research and specifically in farm animal research because stress may disrupt cognitive processes and lead to welfare and husbandry problems. Chronic stress was frequently reported being deleterious for memory performances [8], [22], [31], [34], [42], [47]. Nevertheless, recent studies reported the existence of individual differences in the vulnerability to develop stress-induced learning impairment [46]. In addition, either a lack of effect or even a facilitating effect of stress on spatial memory was previously reported [4], [14]. The most general view to make sense of the discrepancy in the literature is that the relationship between stress intensity–stress hormone levels - and learning and memory performances can take the form of an inverted-U shaped curve [1], [6], [21]. Under moderate or acute stressor, glucocorticoid hormones or other mediator associated with allostasis–a fundamental process that supports adaptation–contribute to maintain and even promote learning and memory capabilities. However, if there are many adverse life events, the secretion of hormone or mediators linked to allostasis can be dysregulated and have damaging effect on learning and memory [35].
Most of the experimental findings investigating learning and memory changes related to chronic stress used long periods of stress [46]. In rodents, a 3- to 5-week period of stress was frequently shown to impair spatial memory in a variety of spatial tasks such as the radial-arm maze [33], the Y-maze [7] and the Morris water maze [45]. Similarly in chicken, 10 weeks of unpredictable light: dark rhythm were shown to disrupt spatial memory [30], [31] and, in seabird chick, a chronically increased circulating level of corticosterone is associated with compromised spatial learning abilities later in life [28].
Studies that specifically investigated the effects of shorter duration of stress periods on cognitive functioning mainly focused on acute stress and only very limited research investigated consequences of intermediate stress duration. No differences in spatial memory are observed in control and stressed rodents following 7 days or 10 days of restrain stress but 13 days of stress were shown to enhance spatial memory [32], [37]. One week of restrain stress was also reported to enhance memory in female rats using the object placement task [3]. Nevertheless, the precise consequences of a sub-chronic period of stress on learning and memory still remain elusive. Moreover, although birds display comparable social and cognitive capacities to mammals [20], the impact of a short period of stress on spatial memory performances in birds, and more specifically in farm birds, still deserve consideration.
In the present study, two-week old quail were divided into control and treated groups. During 8 days, animals from the treated group were submitted to a series of aversive events at unpredictable times each day. To ascertain that the impact of the stress procedure could be investigated on different aspects of behaviour and physiology before sexual maturity, a developmental step that strongly orient the behaviour of the quail, the procedure of stress started when birds were 2 weeks old. A first cohort of animals was used for the measurement, after the treatment, of basal and stress activated corticosterone responses, a typical physiological marker of stress. The second cohort was tested for emotional reactivity and spatial memory performances. Fear reactivity was tested because previous studies reported that individuals frequently exposed to unpredictable environmental stressors tend to have greater response to anxiogenic or acute stressful events [48], [52]. Spatial training was tested to study the consequences of the stress procedure on learning and memory. The aim of this study was thus to provide, in quail, a sub-chronic stress related profile that included both physiological and behavioural consequences of stress.
Section snippets
Animals
Forty six female Japanese quail (Coturnix japonica) hatched and bred at the Pôle d’Expérimentation Avicole de Tours (UE PEAT, INRA, Nouzilly, France) were used in the present experiment. The present study used two independent cohorts of quail. The first cohort of 22 quail (Experiment 1) was used for measurement of tonic immobility and quantification of plasma corticosterone levels. The second cohort of 24 quail (Experiment 2) was used for behavioural investigations including measurement of
Body weight and tonic immobility
Treated animals had a lower body weight than controls [F(1, 20) = 5.53, p = 0.03] (Fig. 2). Note that pre-treatment body weights did not differ between control (81.20 ± 5.32 g) and treated birds (80.83 ± 6.39 g) [F < 1]. Tonic immobility duration was not different between quail from the control group (71.2 ± 16.2 s) and from the treated group (71.1 ± 12.1 s) [F < 1].
Plasma corticosterone levels
The 10 min-long period of contention produced a similar increase in corticosterone levels in both control and treated quail [effect basal vs.
Discussion
This study reveals that a one-week long period of exposure to various unpredictable stressors has no impact on emotional reactivity, basal and acute stress-induced plasma corticosterone levels but improved spatial reversal learning in Japanese quail.
The procedure of stress was applied when animals were 2 weeks old, a choice dictated by the behavioural characteristics of the species. First, some stressors could not be applied collectively but only individually (i.e. contention, capture). To
Conclusion
In conclusion, the present study reveals that one week of exposure to various unpredictable stressors had no major impact on emotional reactivity, basal and stress plasma corticosterone levels but improved spatial reversal learning in quail. Although it has been well established that long lasting chronic stress severely disrupt spatial learning and memory performances, our study reveals that a shorter period of stress can, on the contrary, facilitate spatial learning and memory performances
Acknowledgments
The work reported here was funded by the department PHASE of INRA. We are grateful to all the members of the experimental unit UEPEAT of INRA and particularly to F. Favreau, J. Delaveau, J.M. Hervouet for their technical assistance in the provision or maintenance of the birds. Authors also thank J.B. Sirerol, M. Traineau and M. Couty for their help during behavioural investigations or measurement of corticosterone levels.
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