Enlarged housing space and increased spatial complexity enhance hippocampal neurogenesis but do not increase physical activity in mice

Abstract

IntroductionEnvironmental enrichment (EE) improves various health outcomes, such as hippocampal neurogenesis, in rodents, which is thought to be caused, in part, by increased physical activity. However, the specific effect of each enrichment component, such as enlarged housing spaces and increased spatial complexity with a variety of objects, on physical activity remains unclear because of methodological limitations in measuring physical activity. We aimed to examine whether enlarged housing spaces and increased spatial complexity increase physical activity in mice using a body-implantable actimeter.MethodsAdult male C57BL/6J mice were assigned to either standard housing or EE groups. The housing environment in the EE mice was gradually enriched by enlarging the housing space and the placement of a variety of objects. Physical activity was measured using a body-implanted actimeter. Hippocampal neurogenesis was immunohistochemically examined.ResultsEnlarged housing spaces and the placement of a variety of objects did not increase physical activity in mice. In contrast, hippocampal neurogenesis was enhanced in the EE mice, suggesting that environmental interventions successfully provided enriched housing conditions for these mice.ConclusionsThese results indicate that enlarged housing spaces and increased spatial complexity do not increase physical activity in mice. Furthermore, we found that EE enhanced hippocampal neurogenesis without increasing activity volume. Besides the current understanding that increasing the amount of physical activity is key to improving hippocampal function, our result suggests that the environment in which physical activity takes place is also a crucial contextual factor in determining the impact of physical activity on hippocampal function.

Environmental enrichment: a systematic review on the effect of a changing spatial complexity on hippocampal neurogenesis and plasticity in rodents, with considerations for translation to urban and built environments for humans

“…Thirdly, Funabashi et al (2023) investigated the effects of gradually enriching the housing environment on hippocampal neurogenesis in mice. The enrichment process involved progressively increasing the size of the housing space and introducing objects.…”

“…Based on Tables 3 , 4 , this systematic review proposes a regression model involving the duration of spatial complexity intervention, physical activity, frequency of changing in-space spatial complexity, and age. While the effect of housing space volume has been explicitly investigated only in one study that gradually increased the cage size but did not effectively explore the impact of this intervention before adding element complexity ( Funabashi et al, 2023 ), we consider housing space volume as another prospective independent variable for more precision, knowing that the rest of studies used larger cages for any EE intervention compared to any standard housing condition. However, we confidently exclude the social variable because some studies support that neurogenesis is present in both social isolation + EE (spatial complexity) and group housing + EE (spatial complexity), resulting in neurogenesis compared to both groups without EE ( Madronal et al, 2010 ; Monteiro et al, 2014 ).…”