Multiple neuroendocrine responses to chronic social stress: interaction between individual characteristics and situational factors.

After four weeks of individual housing, male Wistar rats (selected for high or low spontaneous aggressiveness by multiple round-robin encounters) were housed three per cage and submitted to four weeks of chronic social stress consisting of changing membership in the social groups by daily rotation of the animals among cages every day according to a random permutation procedure. In addition, half the males in each condition were housed with three females. Each environmental condition triggered different neuroendocrine changes. Cohabitation with females increased the hypothalamo-pituitary-adrenocortical axis activity, including enlargement of adrenals and increased circulating corticosterone levels. On the other hand, daily rotation of the rats between different social groups activated part of the sympathetic nervous system, such as increased phenylethanolamine N-methyl transferase (PNMT) activity in the adrenals. The level of aggressiveness, however, had no direct influence but interacted with environmental factors on such neuroendocrine measures as circulating testosterone or plasma renin activity. These results indicate that during chronic stress, there is no single, unique response by the animal, but a highly complex set of neuroendocrine changes, dependent on the interaction between individual characteristics (the level of aggressiveness is an example) and situational factors.