Social Stress in Hamsters: Defeat Activates Specific Neurocircuits within the Brain

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Abstract
During an agonistic encounter, subordinate male hamsters display defensive and submissive postures and show increased secretion of glucocorticoids, whereas dominant males do not. To determine whether specific neuronal pathways are activated during the behavioral and neuroendocrine responses of subordinate males, expression of c-fosmRNA within the brains of subordinate males was compared with the pattern in dominant males after fighting. After 1 week of handling, pairs of hamsters were either swapped between cages (handled control males), or were allowed to interact for 30 min [dominant (DOM) males and subordinate (SUB) males]. A second group of control animals that received no handling or social stimulation (unhandled control males) were also included. After testing, all animals were killed by decapitation, their brains were removed for c-fos in situhybridization, and trunk blood was collected for analysis of plasma cortisol and corticosterone levels. Exposure of males to their partner’s cage for 30 min resulted in increased expression ofc-fosmRNA in multiple brain regions. In addition, fighting increased c-fosexpression in the medial amygdaloid nucleus of both DOM and SUB males as well as having more selective effects. In DOM males, c-fosexpression was elevated within the supraoptic nucleus of the hypothalamus. In SUB males, c-fosexpression increased within a multitude of brain areas, including cingulate cortex, lateral septum, bed nucleus of the stria terminalis, medial preoptic area, several hypothalamic nuclei, central amygdaloid nucleus, amygdalohippocampal area, dorsal periaqueductal gray, dorsal raphe, cuneiform nucleus, and locus coeruleus. These findings are discussed in relation to neurocircuits associated with behavioral arousal and stress.