Aggressive Behavior and Stress

An aggressive interaction elicits a strong stress response in both the winner and the loser of the interaction. Evidence shows that winning and losing (or offense and defense) elicits a differential stress response both in terms of the magnitude of responses and of the neuroendocrine stress systems involved. Because winning and losing social interactions form the basis of the social structure in groups of animals, this differential response might underlie the relationship between the position in the social structure and the vulnerability for stress pathology.

Winner. Relatively few studies consider the physiological changes during aggressive behavior in the victor of the social interaction. However, the available literature clearly shows that winning elicits a moderate activation of the HPA axis and of the sympathetic adrenomedulary system. Both human and animal studies also show an activation of the pituitary-gonadal axis in males. More chronically, dominant males in a social group may suffer from sympathetic nervous system-mediated forms of stress pathology. For example, dominant males have a higher incidence of cardiovascular abnormalities such as arteriosclerosis and hypertension, in particular in unstable social groups.

Loser. Most social stress studies concentrate on defense, that is, the loser of a social interaction, or the subordinate males in a social structure. A comparison of the stress response elicited by various stress procedures reveals that social defeat can be considered as one of the most severe stressors. Both the corticosterone response and the response of epinephrine and norepinephrine are by far the largest during social defeat stress.

Recent evidence reveals that stressors show not only quantitative differences but may also differ qualitatively in the pattern of activated physiological systems. For example, a comparison of the electrocardiovascular response to restraint stress and social defeat reveals that the two types of stressors differ strongly in the balance of activation of the two branches of the autonomic nervous system. Social defeat seems to activate exclusively the sympathetic branch, whereas during restraint stress the parasympathetic nervous system shows a considerable activation as well.

This differential autonomic balance may explain the high incidence of electrocardiovascular abnormalities observed during social defeat. Social defeat by a male conspecific induces not only strong acute increases in plasma catecholamines, corticosterone, heart rate, blood pressure, body temperature, prolactin, and testosterone but also increases in a variety of central nervous system neurotransmitters including serotonin, norepinephrine, dopamine, and endorphins.

There is a general tendency to extrapolate these acute effects observed during and shortly after a defeat to a lasting pathology when the stressor is present chronically, for example, in the subordinate males in a social group. Indeed chronic subordination stress has been reported to induce a downregulation of serotonergic and noradrenergic neurotransmission, lower plasma testosterone levels, and increased activation of the HPA axis. Most of these changes are consistent with the idea that in a variety of animals species subordinates develop depressionlike symptoms.

The subordinate position is also related to an increased sensitivity to addictive substances and immunosuppression, resulting in an enhanced vulnerability to infectious disease. Social defeat can be considered as a traumatic life event. Indeed, a single social defeat induces changes in physiology and behavior that can last from days to weeks and months. In fact, many of the behavioral and physiological changes observed after chronic subordination stress can also be observed weeks after a single social defeat.