Abstract
New findings: What is the central question of this study? It is generally recognized that social isolation is associated with physical inactivity, but is social isolation a direct determinant of decreased physical activity? What is the main finding and its importance? We conducted a within-subjects experiment with the aid of a body-implantable actimeter. Our results clearly demonstrated that social isolation decreased home-cage activity in mice. This might have resulted from increased immobility and decreased vigorous activity, suggesting that avoidance of social isolation is important to prevention of physical inactivity. An inactive lifestyle can have a negative impact on physiological and mental health. Social isolation is associated with physical inactivity; however, it remains uncertain whether social isolation is a direct determinant of decreased physical activity. Hence, we assessed whether social isolation decreases home-cage activity using a within-subjects design and examined the effects of social isolation on hippocampal neurogenesis in mice. This study used a body-implantable actimeter called nanotag, which enabled us to measure home-cage activity despite housing the mice in groups. Initially, we examined the influence of the intraperitoneal implantation of nanotag on home-cage activity. Although nanotag implantation decreased home-cage activity temporarily, at 7 days postimplantation the activity recovered to the same level as that of control (non-implanted) mice, suggesting that implantation of nanotag does not have a negative influence on home-cage activity if mice undergo a 1 week recovery period after implantation. In the main experiment, after the 1 week baseline measurement performed with mice in group housing, the mice were placed in a group or in isolation. Home-cage activity was measured for an additional 4 weeks. Home-cage activity in isolated mice during the dark period decreased by 26% from pre-intervention to the last week of intervention. Furthermore, the reduction in the number of 5 min epochs during which the activity count exceeded 301 (an index of vigorous activity) was significantly larger for isolated mice. Contrary to expectations, social isolation did not impair hippocampal neurogenesis. Our results demonstrate that social isolation is a direct determinant of decreased physical activity, possibly because of reduced vigorous physical activity.