Food restriction reduces neurogenesis in the avian hippocampal formation

Abstract

The mammalian hippocampus is particularly vulnerable to chronic stress. Adult neurogenesis in the dentate gyrus is suppressed by chronic stress and by administration of glucocorticoid hormones. Post-natal and adult neurogenesis are present in the avian hippocampal formation as well, but much less is known about its sensitivity to chronic stressors. In this study, we investigate this question in a commercial bird model: the broiler breeder chicken. Commercial broiler breeders are food restricted during development to manipulate their growth curve and to avoid negative health outcomes, including obesity and poor reproductive performance. Beyond knowing that these chickens are healthier than fully-fed birds and that they have a high motivation to eat, little is known about how food restriction impacts the animals' physiology. Chickens were kept on a commercial food-restricted diet during the first 12 weeks of life, or released from this restriction by feeding them ad libitum from weeks 7–12 of life. To test the hypothesis that chronic food restriction decreases the production of new neurons (neurogenesis) in the hippocampal formation, the cell proliferation marker bromodeoxyuridine was injected one week prior to tissue collection. Corticosterone levels in blood plasma were elevated during food restriction, even though molecular markers of hypothalamic-pituitary-adrenal axis activation did not differ between the treatments. The density of new hippocampal neurons was significantly reduced in the food-restricted condition, as compared to chickens fed ad libitum, similar to findings in rats at a similar developmental stage. Food restriction did not affect hippocampal volume or the total number of neurons. These findings indicate that in birds, like in mammals, reduction in hippocampal neurogenesis is associated with chronically elevated corticosterone levels, and therefore potentially with chronic stress in general. This finding is consistent with the hypothesis that the response to stressors in the avian hippocampal formation is homologous to that of the mammalian hippocampus.

Keel bone fractures induce a depressive-like state in laying hens

“…Hens with severe KBFs 3-4 weeks before tissue sampling had lower densities of DCX + multipolar and bipolar neurons in the HF when compared to birds with minimal KBFs, while the magnitude of keel bone injury an individual had sustained by this time also linearly predicted their number of surviving new-born cells. Such downregulated AHN occurs as part of a depression-like physiological state, arising from chronic exposure to stress across numerous paradigms in both mammalian 29,31,76 and avian 43,45,48,50 species. As previous work provided behavioural evidence of pain through a place preference for the location of analgesic administration in hens with KBFs 21 , altered HF morphology in terms of reduced AHN in the present study further suggests that the accumulation of fractures is sufficient to lead to long-term stress in affected birds.…”

“…Data were collected at 11 time points during the production cycle. For one pen (20 focal birds), this occurred when the birds were 21,24,27,31,35,39,44,48,52,57, and 61 weeks of age. For practical reasons, data from the remaining two pens (40 focal birds) were collected the subsequent week, i.e., 22,25,28,32,36,40,45,49,53,58, and 62 weeks of age.…”

Spatial Cognition and Range Use in Free-Range Laying Hens

“…In contrast, neuroanatomical studies on a separate group of birds from the same adult environments showed no rearing-treatment impacts on the immunohistochemical detection of tyrosine hydroxylase in the hippocampus at 20 or 24 weeks of age [ 12 ]. Differences may have been detected in the current study if other measures were made such as dendritic length [ 34 ], neurogenesis [ 35 ], lateralisation, or immunohistochemical staining [ 12 ]. Alternatively, the enriched rearing treatment may not have been sufficient to have neurological impact.…”

Smaller brains in laying hens: New insights into the influence of pure breeding and housing conditions on brain size and brain composition

“…The hippocampus is known to be especially sensitive to environmental enrichment and shows adaptive alterations in different domesticated species ( Cnotka et al., 2008 , Rehkämper et al., 2008 ). In addition, the hippocampus shows morphological and functional changes and suppressed neurogenesis in response to chronic stress ( McEwen, 1999 , Mirescu and Gould, 2006 , Robertson et al., 2019 , Smulders, 2017 ).…”