Inactive but awake behaviour as indicating a depression-like state in mice: Aetiological factors and association with adult hippocampal neurogenesis

In laboratory mice, ‘inactive but awake’ (IBA) home-cage behaviour
involves animals being spontaneously motionless with eyes open, not
interacting with their surroundings. Conventional (barren) housing
typically triggers IBA more than comparatively enriched environments.
Compellingly greater IBA is associated with some depression-like features
in mice and we further explored this through three aims. First, we aimed
to replicate previous results highlighting environmental and genetic
(using two strains of mice: DBA/2J and C57BL/6J) aetiological contributors
to IBA. Second, we explored whether the performance of IBA varied as the
level of enrichment was either increased or reduced. Third, we
opportunistically investigated whether elevated IBA predicted lower
density of immature neurons in the dorsal (dDG) or ventral dentate gyrus
(vDG) of the hippocampus. As expected, mice housed in conventional cages
displayed more IBA than those in comparatively enriched cages and even
more so in DBA/2J mice. As predicted enrichment loss generally increased
IBA while enrichment gain decreased IBA. Unsurprisingly, immature neuron
density was lower in conventional compared to enriched cages, although
only for vDG. Elevated IBA predicted reduced immature neuron density in
the dDG, and this effect tended to be stronger for C57BL/6Js.