Nov. 14, 2011 Just as in humans, there are also the tough types or those with a more delicate personality among mice, as Eneritz Gómez, a psychologist at the University of the Basque Country (UPV/EHU), has been able to confirm. Some adopt an active strategy when faced with stress situations and somehow try to tackle the problem, whereas others display a passive attitude. Those in the second group are more vulnerable: some of the physiological characteristics resemble those attributed to human depression.
These results have been defended in her thesis entitled Diferencias individuales en ratones derrotados crónicamente: cambios conductuales, neuroendocrinos, inmunitarios y neurotróficos como marcadores de vulnerabilidad a los efectos del estrés (Individual differences in chronically defeated mice: behavioural, neuroendocrine, immune and, neurotrophic changes as indicators of vulnerability to effects of stress.)
Specifically, Gómez has used defeat-induced chronic social stress as the basis for her study. "Mice are very territorial. Five males and one female tend to live together. Only one male mates with the female, the same one that gains control of all the resources right from the start," she explains. These males fight amongst themselves, and the same mouse always wins; that is why it takes possession of everything, while the rest suffer defeat-induced chronic social stress. However, not all the losing mice end up equally affected; some get so low that they become ill, while others do not. Setting out to clarify why these differences arise, she focused on how the mice react to stress.
Active vs. Passive
"Stress is related to psychological disorder, but not all the subjects develop this disorder. This happens because they have different ways of acting when faced with stress," explains Gómez. This is her conclusion after filming the performance of all types of mice, analysing it and classifying it in terms of a passive or active strategy. And how do the two strategies differ? For example, if the dominant mouse were to attack, the dominated passive would not even move, whereas the active one would flee.
"The passive ones remain still nearly all the time, and as the stress goes on, they become even stiller. The active one is also aware that the stress is very hard and that it cannot get out of it, but it puts up greater resistance," says Gómez. There is also a great difference with respect to how they interact with the group leader. In fact, the one that makes use of the active strategy is interested in what is going on around it, it sniffs the dominant one, and tries to interact with it… Whereas the one adopting the passive strategy does not even look at it. The researcher can see similarities with human attitudes in all this.
Apart from behaviour, Gómez has studied the physiological signs (neuroendocrine, immune, and neurochemical alterations) of these dominated mice, and has seen that in this case, too, it is possible to make a classification on the basis of the strategy chosen. And also to draw comparisons with human beings. Specifically, she has confirmed that the clinical state of the passive mouse is not far removed from that of human depression: some of the physiological alterations displayed by these mice have been previously linked to disorders associated with stress, like depression.
With all this, the researcher has found the answer to the question that "they all get stressed but not all of them fall ill." And the fact is, the passive strategy reflects greater vulnerability, so these mice are more likely to fall ill.
The thesis is a theoretical contribution but could well be useful in the practical field. For example, in the design of therapies that can help to alter status perception in cases like those of cancer patients. At the same time, the physiological study done by Gómez could provide clues in the area of pharmacology for treating depression: "If we can see what physiological mechanisms are involved, the treatments could be more specialised."
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