The present study shows that footshock-stress caused a glucose intolerance accompanied by an increase in corticosterone level and IL-6 epididymal protein content. The fish oil consumption blunted these effects of stress, demonstrating the preventive effect of fish oil consumption in development of glucose intolerance promoted by footshock-stress.
The final body weight and carcasses lipid and protein content were not modified by 3 days of footshock-stress and/or fish oil consumption for 8 weeks. Previously, Papakonstantinou et al. , using a repeated restraint stress for 3 days, 3 hours per day, showed that consumption of hyperlipidic diet rich with fish oil (2 weeks before stress and 2 after) did not ameliorate stress-induced anorexia and weight loss. Similar results was observed in rats treated with normolipidic diet containing fish oil for 45 days submitted for 96 hours of sleep deprivation . Taken these results all together it could be suggested that the duration of fish oil ingestion and type of stressor could influence the effect of stress in body weight.
However, in the present study the footshock-stress in fish oil treated rats caused a decrease in RET weight. Fish oil reduces fatty acid accumulation [26, 27] and prevents diet induced obesity , mainly through the regulation of lipid metabolism by inhibiting lipogenesis, promoting lipolysis and fatty acid oxidation, and suppressing preadipocyte differentiation .
As previously reported [10, 11] footshock-stress caused a glucose intolerance and increase corticosterone serum levels, in addition in the present study we demonstrated that the ingestion for 8 weeks of normolipidic diet rich in fish oil prevented these effects.
Insulin resistance promoted by glucocorticoid has long been known and is associated with increased hepatic glucose production and decreased peripheral glucose transport and utilization . In the present study, CS exhibited a marked increase in corticosterone concentrations in relation to all other groups, including FS, which reflects OGTT response. This result suggests that glucose intolerance promoted by footshock occurred in response to activation of HPA axis, and that consumption of fish oil blunted this effect, protecting against glucose intolerance. This result underscores the anti-stress effect of fish oil consumption.
Based on these find, we seek to establish whether the effect of fish oil was not only related to the normal corticosterone level observed in FS group, but also if it could be associated to fish oil effects on adipokines related to glucose intolerance.
The increase in adipoR2 in fish oil fed rats might be involved in this response, but further investigations will be required to elucidate the mechanisms involved.
The insulin-sensitizing adipokine adiponectin is another important factor involved in glucose intolerance [30, 31]. Adiponectin is specifically expressed in adipose tissue and has antiatherogenic and antidiabetic properties [31–33] and its effects are mediated by adiponectin receptors, adipoR1 and adipoR2 . AdipoR1 is abundantly expressed in skeletal muscle, whereas adipoR2 is predominantly found in liver [31, 32] and both are also expressed in adipose tissue .
The effects of stress and glucocorticoids on adiponectin regulation are controversial. In human adipocytes, dexamethasone inhibits adiponectin release . In human volunteers treated with dexamethasone plasma adiponectin levels were unchanged , while in another study a small rise was found  but adiponectin was decreased in subjects treated with hydrocortisone . Catecholamine also modulates adiponectin, β-adrenergic stimulation downregulate adiponectin gene expression [30, 38] but upregulates AdipoR2 expression . These data suggest that fish oil consumption, although not alter serum levels of adiponectin, may have improved the sensitivity to this hormone contributing to the protective effect of fish oil on glucose intolerance induced by stress.
Increase in pro-inflammatory adipokines, such as TNF-α and IL-6, also leads to glucose intolerance and insulin resistance .
The fish oil and stress have been reported to exert pro and anti-inflammatory effects [29, 39]. In these sense, we decided to measured the retroperitoneal and epididymal white adipose tissue and liver TNF-α, IL-6 and IL-10 protein content.
Neither footshock-stress, fish oil consumption nor the interaction between fish oil and footshock-stress caused modification on TNF-α and IL-10 protein content in the studied tissues. However, the footshock-stress increased IL-6 protein concentration in EPI and fish oil consumption impaired this effect.
Glucocorticoids cause a decrease in adipocyte gene expression and secretion of adiponectin and IL- 6 [5, 6]. On the other hand, in primary cultures of murine adipocytes, norepinephrine, isoprenaline, and a β3-selective agonist have been shown to stimulate IL-6 gene expression and protein secretion . Conversely, fish oil rich diet decreased plasma levels of norepinephrine in healthy subjects . In the present study, the decrease in EPI IL-6 protein content in FS group is not related to an increase in glucocorticoid, since stress did not modified this hormone concentration in fish oil treated rats. However, taken our results with the literature ones, the decrease in IL-6 could be related to a decrease in norepinephrine caused by fish oil diet in stressed rats (FS). The catecholamines were not evaluated, which is a limiting factor in this study, since these hormones, like corticosterone, reflect the stress response and affects many metabolic effects. However the measurement of serum corticosterone demonstrated its important role in the footshock-stress and fish oil consumption response.
Adipose tissue IL-6 production may account for about 25% of the circulating IL-6 . Then, the decrease in IL-6 in EPI of FS comparing to CS reflect, probably, in the serum IL-6 concentration and in the normal glucose tolerance test observed in FS rats in relation to CS rats.
In summary, footshock-stress promoted glucose intolerance associated to corticosterone serum level and epididymal white adipose tissue IL-6 concentration increase. The fish oil consumption by stressed rats normalized the stress responses. These results suggested that fish oil intake could be useful to minimize or prevent the development of diseases associated to the stress.