Abstract
Stress is a condition of human experience and an important factor in the onset of various diseases. There are numerous studies showing how stress can accelerate cell aging, immune senescence and some age-related diseases such as neurodegenerative disorders and osteoporosis. However, the effects of stress have different consequences depending on the type, duration or severity and predictability of the stressor applied. Although stress can be beneficial in its acute phase, repeated and severe stressful stimuli produce adverse effects. There are different models of stress depending on the exposure time; acute (when the stressor is applied for a short time, e.g. hours or days, and intensely) or chronic (when the stressor is applied for a long time, e.g. weeks or months, and less intensely. In these cases, the stressor can be repeated each time or different stressors can be used). The latter model is most frequently used to achieve similar conditions to those found in human diseases related to stress. Also, there are several different paradigms depending on the purpose of the study [development of drug therapies or modeling depressive behaviors; for the different paradigms see Dagnino-Subiabre, (2012)]. Here, we describe a 9-day variable-stressor paradigm with repeated and prolonged stimulation and a random daily stressor over days or weeks to minimize its predictability. This protocol has been adapted from other models of variable stress with significant modifications. The absence of predictability of the stressor applied is an important characteristic of this model compared to other models in which repeated stress is used. We avoid the use of a strong stressor, such as foot shock or tail pinch, and describe an easily reproducible new chronic mild stress model. Some models of chronic mild stress have been reported to lead to a wide range of behavioral disturbances and have been proposed as models of depression in animal studies (Cryan et al., 2005).
Keywords: Stress, Rat, Protocol, Chronic
Materials and Reagents
Equipment
Procedure
The stressors schedule used in this protocol is listed in Table 1. Application of stress starts at a different time every day to minimize its predictability (from 8:00 a.m. to 8:00 p.m.). Table 1. Schedule of stressors used during chronic variant stress treatment ---------------------------------------------- Day Stressor Time ----- ----------------------- ------------ 1 Forced swimming 10 min 2 Restraint 3 h 3 Water deprivation 24 h 4 Restrain at 4 °C 90 min 5 Isolation 24 h 6 Food deprivation 24 h 7 Water deprivation 24 h 8 Restrain at 4 °C 2 h 9 Food deprivation 24 h ----------------------------------------------- Day 1: Forced swimming
Representative data
Validation of the stress model: Changes in body and adrenal glands weight, in the blood levels of corticosterone, and in dopamine (DA) and 3,4-Dihydroxyphenylacetic acid (DOPAC) levels in the prefrontal cortex are typical effect of stress and are used as methods to assess stress models. With this chronic variant stress protocol, the body weight of the animals decreases, whereas adrenal weight and the blood levels of corticosterone increase (Figure 4). It has also been found that increased levels of dopamine and DOPAC in the prefrontal cortex are observed (Table 2). Notes about reproducibility and variability: Stress perception is very subjective and each animal, just as each person, is stressed to a different degree. Therefore, it is normal that the results present some variability. To avoid this, glucose preference test is recommended (Hu et al., 2010); then, only animals that have been actually stressed must be included in the study. Table 2. DA and DOPAC amount in brain cortex in control and stressed rats Animals were killed at different time points after treatment (0, 2, 4, 8, and 10 days respectively), and the prefrontal cortex was harvested and processed for DA and DOPAC quantification by HPLC as described in de Pablos et al. (2006). Numbers are expressed as nanograms per gram of wet tissue and are Mean ± SD of five independent experiments. *p<0.05, **p<0.01, statistical significance (Student’s t test) compared with control animals. Figure 4. A. Body weight gain (g); B. Adrenal glands weight (mg, bars) and ratio between adrenal glands weight and body weight gain (scatter plot and line). C. Serum corticosterone (percentage of control animals). Statistical signification: Student’s t test comparing before C and after 10 days of variate stress (S10d); *, p< 0.05; **, p< 0.01; #, p< 0.01 (for the ratio adrenal glands weight/body weight gain). One-way ANOVA followed by the LSD post hoc test for multiple range comparisons, p< 0.01; *, compared with the control; a, compared with the previous time point (S1d to S10d indicate the days subjected to variate stress).
Acknowledgments
Chronic-variable stress was adapted from other models of variable stress (Gamaro et al., 2003; Konarska et al., 1990; Murua and Molina, 1992; Muscat et al., 1992; Papp et al., 1991; Willner et al., 1987) with significant modifications. This work was supported by grant SAF-2012-39029 from the Spanish Ministry of Economy and Competitiveness and P10-CTS-6494 (Proyecto de Excelencia of Junta de Andalucia).
References
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