One of the most profound changes resulting from social subordination is decreased body weight. While DOM animals maintain their original body weight and control animals gain weight, SUB animals consistently show a rapid and sustained weight loss of up to 10-15% of their body weight that occurs over 2 weeks in the VBS [10
] (). Body composition of all animals was determined by NMR (EchoMRI, Waco, TX) prior to VBS colony formation (Day 0) and at the end of VBS housing (Day 13). These data are depicted in . There was no significant difference in body composition prior to VBS housing suggesting that body composition prior to VBS colony formation is not a reliable predictor of social status. Plasma testosterone is inversely related to adipose tissue mass and influences fat distribution [44
]. In addition, higher testosterone levels influence behavior and intermale aggression in rats [46
]. These observations then suggest that perhaps leaner rats that have higher testosterone levels would be predisposed to becoming DOM of the colony. However, our data are to the contrary and suggest that neither plasma testosterone nor body composition can determine dominance in this model of social stress.
Body weight during VBS. SUB in colonies lost a significant amount of body weight over 14-days in the VBS compared to DOM. * P < 0.05 vs. CON; ** P < 0.05 vs. CON and DOM.
Figure 2 Body composition prior to (Day 0) and on Day 13 of VBS housing measured by NMR. Following 14 days in the VBS, both DOM and SUB had significantly decreased % body fat. SUB did not gain lean body mass as CON and DOM did. DOM did not lose body weight and (more ...)
additionally shows that weight loss in SUB during social stress was attributable to loss of adipose tissue and maintenance of lean tissue, while DOM only lost adipose tissue. DOM did not lose weight but had decreased adiposity, suggesting that their body composition changed during social stress, shifting body weight from adipose to lean tissue.
We further analyzed adipose tissue distribution in VBS animals by separating subcutaneous fat (including the dorsosubcutaneous and inguinal fat pads) from visceral fat depots within the carcass (including retroperitoneal, perirenal, mesenteric and epididymal fat pads) using the method of Clegg, et al. [48
]. Although SUB lost adipose tissue during VBS stress, they retained a higher percentage of visceral fat than CON and DOM by the end of VBS housing ().
Adipose tissue distribution of VBS rats sacrificed following 14 days in VBS (n = 4 colonies). * P < 0.05 vs. CON.
As shown in , DOM and SUB both had lower levels of the adiposity hormone leptin, consistent with the decreased amount of adipose tissue [49
]. There was a significant correlation between these two parameters, providing a degree of cross-validity. Insulin was lower in SUB than in DOM and CON, consistent with previous reports [34
]. Our data also suggest that body weight loss results from social stress associated with dominance hierarchy formation, and not merely from being housed in an “enriched environment,” since colonies composed of 4 males without females do not form hierarchies and do not display the physiological or body weight changes as with DOM and SUB in mixed-gender colonies, yet appear to be comparably active although activity levels have not been directly measured [28
Table 2 Endocrine parameters following 14 days of social stress in the VBS. Adapted from .
There are several possible mechanisms that may account for decreased weight in SUB. Other investigators report that chronic stress in subordinate tree shrews results in weight loss that has been attributed to stress-induced enhancement of metabolic activity [19
] and to a lesser extent to reduced food intake [21
]. In studies using restraint stress, rats lose weight and remain hypophagic until a few days after the stressor has ended [23
]. Our next series of studies was aimed at determining whether the weight loss seen in SUB is the result of decreased food intake.
As described earlier, the VBSs in our laboratory are fitted with food intake monitors and microchip identification systems (AccuDiet ID system, AccuScan Instruments, Columbus, OH) that uses subcutaneous microchip identification technology to identify each animal while simultaneously recording the time and amount of food eaten. Using this system we determined that SUB lose body weight because they eat significantly less food compared to DOM beginning immediately upon entering the VBS and continuing throughout the 14 days of housing in the VBS. Thus, body weight loss in SUB can be attributed, at least in part, to decreased food intake in the VBS. Whether decreased food intake was the sole reason for body weight loss in SUB remains to be determined through implementation of an additional control group, animals that are pair-fed to match the food intake of the SUB group. Energy expenditure may also play a significant role in weight loss in SUB and this is currently being examined in the lab.
Our behavioral studies indicate that DOM spent a majority of their time in the open surface area whereas SUB spent most time in the inner chambers and tunnels. It is important to note that SUB have adequate access to food and water even if they spend most of their time in the side chambers. Consistent with this, food intake data for individual colonies indicate that the DOM takes the majority of his meals in the open surface area while SUB eat their meals primarily in the inner chambers. Total food intake was higher for DOM compared to SUB, and the DOM did not take any meals in the inner chambers, suggesting that SUB did not decrease their intake as a result of the presence or threat of the DOM. Thus, these data suggest that although SUB have adequate access to food, they are nonetheless suppressing their food intake [50
]. There may be additional differences in meal patterns among SUB rats that also impact body weight and body composition and these studies are currently ongoing in our laboratory [51
In addition to determining how stress alters body weight and body composition in DOM and SUB during social housing in the VBS, it is also important to determine the long term consequences of stress on these parameters. After 14 days of social stress, SUB had elevated corticosterone and decreased testosterone compared to CON and DOM (), suggesting that SUB would be predisposed to regain body weight as adipose tissue rather than lean body mass [6
]. When SUB were removed from their VBS colonies and were allowed to recover in individual cages (“recovery” period), they regained body weight quickly and by the end of 3 weeks their body weight was only slightly lower than that of CON and DOM [24
](). SUB were immediately hyperphagic during the recovery period and re-gained body weight primarily as adipose tissue although they were on a standard chow diet that is low in fat content (Harlan Teklad 7012, approximately 5.67% fat). This change in body composition was maintained or increased following a second cycle of VBS social stress and recovery [53
]. Consistent with increased body fat, SUB were hyperinsulinemic and hyperleptinemic compared to DOM and CON after recovery [53
]. Together, these data suggest that the endocrine and metabolic status in SUB animals following chronic social stress (i.e. high corticosterone and low testosterone) may have increased SUB susceptibility to regain body weight primarily as fat and to preferentially deposit fat viscerally rather than subcutaneously [6
]. By separating subcutaneous fat (including the dorsosubcutaneous and inguinal fat pads) from visceral fat depots within the carcass (including retroperitoneal, perirenal, mesenteric and epididymal fat pads) we determined that indeed, SUB had a higher proportion of adipose tissue in the visceral fat depot compared to CON and DOM after recovery [53
]. Thus, following multiple cycles of social stress and recovery, SUB accumulate more adipose tissue primarily in the visceral depot suggesting that cumulative effects of chronic social stress may result in the development of symptoms related to the metabolic syndrome.
Figure 4 Percentage of original body weight over two cycles of chronic social stress in the VBS (14 days each) and recovery (21 days each) in individual cages. SUB rats lost a significant amount of body weight during both VBS 1 and VBS 2, while DOM lost little (more ...)
We further determined that changes in body weight and body composition are indeed attributable to social stress in the VBS. Male rats that are food restricted to match the body weight changes of SUB (“BW MATCH”) also lose a significant portion of adipose tissue and lean body mass such that their body composition is similar to that of SUB at this time point. However, BW MATCH rats did not have any significant differences in corticosterone or testosterone in response to food restriction. After 14 days, when allowed ad libitum
access to chow, BW MATCH rats were hyperphagic similar to SUB, but they regained body weight as adipose as well as lean tissue such that their ultimate body composition did not differ significantly from that of non-restricted controls. These data suggest that changes in body composition in SUB following repeated exposures to stress and recovery cannot be attributable to weight cycling alone [54
As mentioned earlier, we also included a second control group which consisted of VBS colonies composed of 4 males without females (“MALE ONLY”). In previous studies we determined that the males in all-male colonies do not form dominance hierarchies as mixed gender colonies do [28
], but are housed in the VBS which provides a larger, more complex living area compared to conventional shoebox cages in which rats are normally housed. There was no evidence of a dominance hierarchy among the rats in MALE ONLY colonies and although all rats maintained or lost a small amount of weight compared to individually housed controls, the difference was not significant. Body composition of rats in MALE ONLY colonies resembled that of DOM in mixed gender colonies. That is, they lost adipose tissue and maintained or increased lean body mass compared to singly housed control males. These findings suggest that body composition changes seen in DOM may be attributed in part to increased activity in the VBS. In contrast, this further suggests that body weight loss followed by increased adiposity after 2 cycles of social stress and recovery in SUB is attributable to stress derived from social subordination and not to exposure to an “enriched environment” [54
In humans, a recent study of young healthy men exposed to long-term stress revealed that long-term stress results in increased abdominal obesity and early signs of metabolic syndrome suggesting that stress plays an important role in the genesis of metabolic abnormalities [55
]. In that study, subjects lost both fat and lean body mass during the first stressful episode and subsequently regained body weight as fat resulting in an overall decrease in protein mass. This is similar to SUB body weight and body composition fluctuations through cycles of social stress in the VBS and subsequent recovery further suggesting that the VBS model may provide a means to study the development of stress-induced metabolic disorders.