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Influence of social housing conditions on mouse mammary tumor growth and behavior Grimm, Michele S.


Epidemiological evidence suggests that human breast cancer may be influenced by a number of psychosocial factors. In the present study we investigated psychosocial variables that may influence growth of the Shionogi mouse mammary carcinoma (SC115). Change in social housing condition, direction of change [individual to group (IG) or group to individual (GI)), group size, sibling relationship and dominance were the major variables examined. Male mice were housed from weaning in groups (G) or individually housed (I), and at age 2-4 months mice were injected with 3 x 106 tumor cells/mouse and placed into four treatment conditions. In the first two treatments, animals remained in their rearing conditions, group (GG) or individual (II); in the remaining treatments, animals were rehoused from group to individual (GI) or individual to group (IG). All animals were exposed to daily novelty stress in the post-injection period. The frequency of aggressive, defensive, social, nonsocial, sleep and fighting behaviors were measured three times per week in the home cage for about 10 days before and 17-18 days after tumor cell injection and rehousing. Behavioral frequencies were also measured immediately after each exposure to daily novelty stress. Rehousing from group to individual (GI) produced the highest tumor growth rates, and rehousing from individual to group (IG) produced the lowest tumor growth rates. Animals remaining in their rearing conditions (GG and II) showed tumor growth rates intermediate to the rehoused animals. Thus, changes in housing rather than individual or group housing per se, were found to be modulators of tumor growth rate. In the IG treatment, dominant mice showed faster tumor growth than subordinate mice, whereas in the GG treatment, dominant mice showed slower tumor growth than subordinate mice. Within the IG treatment, no sibling groups of three (IN3) showed the highest tumor growth rates. Pre-injection, individually housed animals showed more nonsocial behavior and less sleep than group housed animals, and groups of three showed more social behaviors than groups of five. In both the pre- and post-injection groups, dominant animals displayed a more active role than subordinate animals in agonistic, social and nonsocial behaviors. Post-injection, overall behavioral activity was highest on the day of injection and rehousing. GI mice showed more nonsocial behavior and less sleep than IG mice. For group housed animals, IG mice showed more defensive and fighting behaviors than GG mice. Active, passive and total social behaviors were highest in the IS5 group, and fights were highest in groups of five. Differences among groups in tumor growth rate and behavior were discussed in terms of arousal, social dominance, and stability of social status.

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