Influence of differential housing on emotional behaviour and neurotrophin levels in mice

Environmental enrichment condition (EC) induces profound behavioural, neurochemical and neuroanatomical changes. Increasing evidence has shown that the hippocampus, which is implicated in a range of cognitive functions, including learning and memory, is one of the most susceptible brain areas to the effects of enriched rearing. Recent work also suggests that the hippocampus is functionally segregated; lesion studies have shown that the dorsal hippocampus is important for spatial learning, whereas the ventral part is critical in emotional behaviour in rats. We investigated the effects of differential housing environments on anxiety-related behaviour and neurotrophin levels in dorsal and ventral hippocampus, and other brain regions. Ninety-six male and female C57BL/6 mice were reared in EC or standard housing condition (SC) for 4 months after weaning. Thereafter sixty-four animals were tested in the elevated plus-maze, open-field, novel-objects exploration and food neophobia. Thirty-two animals remained as untested. Subsequently, brain nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) were analysed in selected brain regions of the tested and non-tested animals. Differential housing influenced anxiety-related behaviour in the plus-maze and brain neurotrophins. Baseline levels of BDNF and NGF protein were differently distributed in dorsal and ventral parts of hippocampus in both male and female mice, with levels in the dorsal hippocampal being consistently higher than those in ventral hippocampus. Exposure to behavioural testing induced complex changes on neurotrophin levels in selected brain regions. This study demonstrates for the first time the differential distribution of normal levels of neurotrophin protein in dorsal and ventral hippocampus in mice, and these levels can be affected by environmental enrichment and have an impact on emotional behaviour.