Neuroimmune Interactions and Psychologycal Stress Induced by Cohabitation with a Sick Partner: A Review

This study reviews the neuroimmunological consequences elicited in mice from long-term cohabitation with tumor-bearing conspecifics. Two types of experiments were performed; one used Swiss female mice and Ehrlich tumor cells, and the other used C57Bl/6 female mice and B16F10 melanoma cells. The female Swiss mice and the C57Bl/6 mice were divided into two groups, i.e., control and experimental. One mouse in each control pair was treated with control solutions (1.0 mL/kg); the other was kept undisturbed and called the ‘companion of health partner’ (CHP). One mouse in each experimental pair was inoculated with 5 x 106 Ehrlich tumor cells or with 106 murine B16-F10 melanoma cells; the other mouse, which was the subject of the performed studies, was left undisturbed and called the ‘companion of sick partner’ (CSP). Although we used two different strains of mice and two different tumor types, the CSP mice presented, in relation to the CHP mice, an increased locomotion in the open field and plus maze apparatuses and no changes in the corticosterone serum levels before and after the immobilization-stress challenge. The Swiss CSP mice showed a reduced level and an increased turnover rate of hypothalamic noradrenaline (NE), as well as increased plasmatic levels of adrenaline and NE. Changes in the immune cell phenotype and activity were also observed in the Swiss and C57Bl/6 CSP mice. The study found that odor cues left by the Ehrlich tumor-injected Swiss mice are aversive and may therefore be responsible for the neuroimmune changes reported in the CSP mice. It is proposed that the final neural link between the neuroimmunological changes observed in the CSP mice involves psychogenic stress imposed by the housing condition and the activation of the brain catecholaminergic pathways and the sympathetic nervous systems.