Effects of isolation, handling and novelty on the pituitary--adrenal response in the mouse

Male Swiss strain mice were individually- or group-housed for four weeks. Basal corticosterone levels did not differ with the type of housing, providing no support for the suggestion that the condition of the individually-housed mouse is stressful. Plasma corticosterone levels also were determined for mice which had been either left undisturbed or exposed to new cages which differed from their home cages by varying degrees. There were elevations in mean plasma corticosterone levels corresponding to the degree of difference between the home cage and the new cage. This finding supports the suggestion that changes in 11-OHCS levels are sensitive measures of environmental changes. Mice forced to remain in novel places exhibited higher plasma corticoid concentrations than animals which were given the opportunity to move freely between familiar and novel places. Corticoid values, as well as neurophysiological and behavioral responses, suggested that the stress induced by forced exploration might be due to the fact that animals are prevented from freely regulating their exposure to novel places rather than to novelty per se.