The social environment may represent an important source of challenge in individuals belonging to species with complex social organization and likely had a major evolutionary role in shaping the stress response phenotype as it is. In many instances, social disputes over resources (territory, food and sexual partners) involve agonistic behaviors with different degrees of aggression, which may result in wounding, exhaustion and sometimes even death. In humans, social stress episodes do not necessarily imply overt aggressive acts; nevertheless, intraspecific interactions involve competitive/hostile behaviors, which may represent a severe challenge to physiological and psychological homeostasis. For all these reasons, social stress paradigms are thought to bear a high face validity, and they are being adopted more and more in animal research as preclinical models with the aim of clarifying the mechanisms underlying adaptive stress responsivity and stress-related psychological (e.g. depression) and psychosomatic (e.g. cardiovascular) disorders. A wide literature on rodents reveals that acute and chronic social challenge (mostly defeat or subordination stress) produces a variety of behavioral, physiological, and molecular changes that are sometimes long-lasting. These include decreased locomotor and exploratory activity, anxiety, reduced sexual behavior, impaired social memory and cross-sensitization to psychostimulants. Physiologically, defeated animals show increased hypothalamic-pituitary-adrenocortical activity, altered circadian rhythms of heart rate and body temperature, as well as impaired immunological function. Social defeat also produces a variety of changes in neurotransmitter systems, including altered dopamine turnover in different brain areas, and changes in GABA A, glutamate and serotonin (5-HT) receptor binding. Interestingly, animal research shows that although the social environment can have detrimental health implications, it is the quality of the social environment itself (i.e. via social support or prevention of social isolation) that can provide beneficial effects and promote resilience.
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