Abstract
In the mammalian brain, adult hippocampal neurogenesis (AHN) is suppressed by chronic stress, primarily at the ventral pole of the hippocampus. Based upon anatomy, we hypothesise that the caudal pole of the avian Hippocampal Formation (HF) presents a homologous subregion. We thus investigated whether AHN is preferentially suppressed in the caudal chicken HF by unpredictable chronic mild stress (UCMS). Adult hens were kept in control conditions or exposed to UCMS for 8 weeks. Hens experiencing UCMS had significantly fewer doublecortin-positive multipolar neurons (p < 0.001) and beaded axons (p = 0.021) at the caudal pole of the HF than controls. UCMS birds also had smaller spleens and lower baseline plasma corticosterone levels compared to controls. There were no differences in AHN at the rostral pole, nor were there differences in expression of genetic mediators of the HPA stress response in the pituitary or adrenal glands. Duration of tonic immobility and heterophil/lymphocyte (H/L) ratios were also not responsive to our UCMS treatment. These results support the hypothesised homology of the caudal pole of the avian HF to the ventral pole of the rodent hippocampus. Furthermore, quantifying neurogenesis in the caudal HF post-mortem may provide an objective, integrative measure of welfare in poultry, which may be more sensitive than current welfare measures.
Highlights
The dentate gyrus is one of few discrete areas in the mammalian brain where adult neurogenesis, the production and integration of new neurons across the lifespan, occurs[12]
A Cox regression was conducted with stress treatment group and time-point as covariates, in order to determine whether the survival distribution of time until righting differed for birds allocated to the unpredictable chronic mild stress (UCMS) and control conditions pre-treatment and during week 6 of UCMS treatment (Fig. 2)
The caudal pole of the laying hen Hippocampal Formation (HF) is preferentially sensitive to the experience of unpredictable chronic stress, and the number of newly-generated neurons surviving, potentially to the point of functional integration, in this region is reduced in line with this negative welfare experience
Summary
The dentate gyrus is one of few discrete areas in the mammalian brain where adult neurogenesis, the production and integration of new neurons across the lifespan, occurs[12]. We hypothesise homology between these two areas, and between the caudal pole of the avian HF and the stress-responsive ventral hippocampal sub-region in rodents[37] Consistent with this notion, electrical stimulation of the pigeon HF has been demonstrated to suppress plasma corticosterone (CORT) titres more dramatically when performed at caudal sites than at more rostral points[35]. This suggests that the avian caudal pole may be the primary origin of hippocampal negative feedback inhibition to the HPA-axis, as characterises the ventral rodent hippocampus. Measurement of the neurogenic stress response in poultry has potential application to animal welfare in terms of facilitating comparison of the cumulative chronic stress experienced by commercial birds in different housing systems and husbandry conditions
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