These results occurred when the intruder hamsters were placed in the foreign cage as few as four times, a total of 28 minutes, over the 33-day experiment, Bartness explained. Hamsters that were placed in the situation only once during the experiment did not eat more or gain weight compared to a control group. In addition, the intruder hamsters that were placed in the cage intermittently (at unpredictable times) showed comparable weight and fat gain compared to those placed in a foreign cage consecutively (at regular times).
However, while the intermittent group increased on all measures of fat gain, the consecutive group increased on only two of the fat measures. Still, this was an unexpected result.
"In humans, unpredictable [stress] events are more aversive than predictable events, causing greater alterations in homeostasis and thus increased stress," the authors wrote. "In addition, previous research suggests that unpredictable events cause greater activation in brain regions responsible for fear and anxiety in laboratory rats and reduction in immune function compared with events that are predictable."
Syrian hamsters provide a good model for obesity research, not only because they eat more and gain weight, but because, like humans, they add fat to their abdomens -- visceral fat. Visceral fat is particularly unhealthy because it affects the internal organs and is associated with diabetes, cancer and other serious illnesses, Bartness said.
Bartness' team began a second study to determine whether other stressors, such as a mild foot shock, produce the same effect as the social defeat model; and whether the dominant hamsters gain weight and fat as the result of the intrusion of the submissive hamsters.
Another line of inquiry would be to compare mice and rats to hamsters. Humans and hamsters, which eat more under stress, share the same predominant stress hormone, cortisol,
Source:American Physiological Society