The difference in the female rats' responses may stem from the demands of motherhood, researchers speculate in the study "Social isolation and the inflammatory response: sex differences in the enduring effects of a prior stressor" by Gretchen L. Hermes, Anthony Montag and Martha K. McClintock of the University of Chicago, and Louis Rosenthal of the University of Wisconsin, Madison. The study appears in the February issue of the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, published by the American Physiological Society.
The study reinforces a growing body of evidence on health disparities between men and women and may shed light on why socially isolated men are more vulnerable to disease and death than isolated women, Hermes said.
Previous studies have established a link between stress and immune function, Hermes said. But this study looked at the long-lasting effect that three months of isolation (the equivalent of chronic social stress) and one 30-minute episode of acute physical stress had on the inflammatory response, the body's innate immune response to bacteria, viruses, and parasites. The authors found that a full two to three weeks after being subjected to isolation and the acute physical stress, male rats showed a markedly slower healing response when injected with a foreign body compared to female rats, Hermes said.
This finding potentially has great clinical significance because the inflammatory response is an important immune response involved in many diseases, including heart disease, cancer, and infectious disease, said senior author Martha McClintock, director of the Institute for Mind and Biology at the University of Chicago.
Results show 'real life' complexity; evolutionary benefits of motherhood? The experiment involved Norway rats, a particularly social species of rats that lives in large colonies of closely spaced burrows with cooperative grooming, feeding and rearing of offspring. "They are not just any old animal, but are gregarious, particularly the females," Hermes noted. "Removing them from their social context produces a profound effect."
The study covered three months, a significant portion of the rat's life span, and showed the lasting effects of an acute stressor superimposed on the chronic social condition of isolation, Hermes said. Other studies have examined the cumulative effect of repeated stressors, with researchers measuring the immune response immediately after the stressful event. But in this study, the individual experiences a stressful event and sometime much later experiences an immune challenge. "This study is more like real life," she said.
This research dovetails with studies which have found that in times of stress, rats are more likely to give birth to female offspring than male offspring, McClintock said, suggesting that male offspring are less likely to survive under stressful circumstances. The study also fits with human studies, which found that socially isolated men are more likely to become ill and die sooner than similarly isolated women.
It is not clear why females heal more quickly than males under stress, but the authors said it may be a protection that evolved in the context of females protecting their offspring.
"While lactating, maternal rats become highly aggressive toward intruders and predators and are at high risk for wounding, particularly from neck bites that puncture the skin where the foreign substance was induced," the paper noted. "Stress-induced facilitation of the inflammatory response in threatened maternal rats would promote their healing and survival, with obvious benefit to her dependent offspring."
Females may have a different physiological response to stress that evolved differently from males because of motherhood. "For example, oxytocin, a hormone secreted by females in social contexts, may function as part of an alternative stress regulatory system that facilitates wound healing," the authors wrote.
Another possibility is that male and females experience stress differently, the study said. If females perceive the restraint as more traumatic, they may react more strongly to the stress associated with the induction of the foreign substance and have a stronger immune reaction.
Stressor simulates collapsed burrow The 120 rats in the study, 60 males and 60 females, were weaned 28 days after birth, and assigned to one of two groups: those that lived in their own cages (isolated) and those that lived in same-sex groups of five (non-isolated). The isolated and non-isolated groups had an equal number of males and females.
When the rats were 100 days old, the researchers subdivided the two groups. They placed half the isolated rats into the stressed group and half into the non-stressed group. They divided the non-isolated rats in the same way. The four resultant groups -- isolated and stressed, isolated and non-stressed, non-isolated and stressed, and non-isolated and non-stressed -- each had 15 females and 15 males.
The researchers then injected the two non-stressed groups, (isolated and non-isolated), with carrageenin (seaweed), a substance that doesn't harm the rats but does challenge their immune systems. The immune system responds by walling off the carrageenin with scar tissue, then neutralizing and reabsorbing it, Hermes explained.
The rats in the stressed groups, both isolated and non-isolated, however, weren't injected with the carrageenin at the 100-day mark. Instead, each was placed for 30 minutes in a restraint tube (the acute stressor). The restraint tube inhibits the rat from moving and simulates a collapsed burrow. The stressed animals were injected with carrageenin 14 days after they experienced the stress of the restraint tube.
When the researchers compared the two groups that were not stressed (isolated, non-stressed versus non-isolated, non-stressed), they found that the isolated female and male rats healed more slowly than the non-isolated female and male rats, Hermes said. In addition, the researchers found no significant differences in healing rates between the male and female rats within the isolated, non-stressed group. That is, healing was a function of whether the rat was isolated, not its gender.
But when rats experienced chronic social isolation and an acute stressor, the responses were the opposite for males and females: the stressor actually stimulated the healing response in females and delayed it in males, with stressed males generating a much weaker immune response to the carrageenin than the females, Hermes explained.
In fact, the females that experienced an acute restraint stress healed more rapidly than the female rats both isolated and non-isolated, that had not been stressed. "For whatever reason, a history of an acute stressor was immuno-enhancing for females of this species," Hermes said.
"This is striking, because it shows that, in effect, 'stress' is not 'stress,'" McClintock added. "For females, the long-term social stressor and the brief physical stressor had opposite effects on immune function."
Source and funding
"Social isolation and the inflammatory response: Sex differences in the enduring effects of a prior stressor," by Gretchen L. Hermes, of the Institute for Mind and Biology and the Department of Comparative Human Development, University of Chicago; Louis Rosenthal, Morris Institute, University of Wisconsin; Anthony Montag, Department of Pathology, University of Chicago; and Martha K. McClintock, Institute for Mind and Biology, Department of Comparative Human Development, and Department of Psychology, University of Chicago appears in the February issue of the American Journal of Physiology-Regulatory, Integrative, and Comparative Physiology published by the American Physiological Society.
Research was supported by a National Institutes of Health MERIT Award, a grant from the Mind-Body Network of the John D. and Catherine T. MacArthur Foundation (McClintock), National Institute of Aging Program Project grant and a National Institute of Environmental Health Science Center grant.
Editor's note: The media may obtain a copy of Hermes et al. by contacting Christine Guilfoy, American Physiological Society, (301) 634-7253, (978) 290-2400 (cell), or cguilfoy@the-aps.org.
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