Effects of adult housing conditions and dark brooder rearing on hippocampal plasticity in laying hens

Abstract

Hippocampal plasticity, the remodelling of immature neurons in the hippocampus, is downregulated in response to chronic stressors in rodents, especially in the ventral dentate gyrus (DG), while the dorsal DG has a role in spatial memory. A similar functional gradient has been hypothesised along the rostro-caudal axis of the avian hippocampal formation (HF), and previous studies in chickens have found that chronic stress suppresses hippocampal plasticity, especially in the caudal HF. I used hippocampal plasticity to measure the effects of housing systems and related health conditions on chronic stress in laying hens. I also investigated whether dark brooders, which simulate aspects of maternal care during rearing, confer stress resilience to adult hens. In my first study, I compared hippocampal plasticity between birds which were moved from the same rearing site to either a flat-deck or multi-tier laying system. Brains were stained to visualise doublecortin (DCX), a marker of neural plasticity. There was no difference in DCX+ cell density in the caudal or rostral HF, suggesting no detectable differences in chronic stress or spatial stimulation. In my second study, I measured DCX+ cell density in hens with footpad dermatitis (FPD) of varying severity. While hippocampal plasticity tended to decrease in response to FPD in conventionally reared birds, it tended to increase in response to FPD in dark brooder reared birds. Finally, I measured the effects of unpredictable chronic mild stress (UCMS) on behavioural responses, plasma corticosterone, and expression of genes related to hippocampal plasticity in hens reared with or without a dark brooder. In response to UCMS, DCX expression tended to decrease in the caudal HF of conventionally reared birds, but significantly increased in dark brooder reared birds. The results suggest that dark brooders confer stress resilience, and add to the call for their wider implementation on commercial rearing sites.