Abstract
In adult chickens, the housing system influences hippocampal morphology and neurochemistry. However, no work has been done investigating the effects of the early life environment on chicken brain development. In the present study, we reared 67 commercial laying hens (Gallus gallus domesticus) in two environments that differed in the degree of complexity (aviary or cage system). These two groups were further divided into two age groups. At 20 weeks of age, 18 aviary-reared birds and 15 cage-reared birds were humanely euthanized and their brains dissected. At 24 weeks of age, a further 16 brains from aviary-reared birds and 18 brains from cage-reared birds were collected. These brains were prepared for immunohistochemical detection of tyrosine hydroxylase (TH), the rate-limiting enzyme in the biosynthesis of dopamine, in the hippocampus and the caudolateral nidopallium (NCL). There were no differences between the treatment groups in TH staining intensity in the hippocampus or the NCL. In the medial hippocampus, the right hemisphere had higher TH staining intensity compared to the left hemisphere. The opposite was true for the NCL, with the left hemisphere being more strongly stained compared to the right hemisphere. The present study supports the notion that the hippocampus is functionally lateralized, and our findings add to the body of knowledge on adult neural plasticity of the avian brain.
Highlights
Environmental complexity and enrichment influence brain morphology and neurochemistry in mammals and birds [1], with enriched environments promoting brain development [2]
Due to technical difficulties in dissection and cryosectioning, some brains had to be excluded from the study resulting in a total sample size of 67 brains, 34 from aviary-reared birds (18 brains collected at 20 weeks of age from birds with the body weight: mean 1.62 kg ± 0.11 SD; and 16 brains collected at 24 weeks of age from birds with the body weight: mean 1.71 kg ± 0.12 SD) and 33 from cage-reared birds (15 brains collected at 20 weeks of age from birds with the body weight: mean 1.56 kg ± 0.08 SD; and 18 brains collected at 24 weeks of age from birds with the body weight: mean 1.67 ± 0.12 SD)
The results showed no effects of treatment or for any of the interactions between fixed factors on the staining intensity of tyrosine hydroxylase (TH) in the medial hippocampus (mHp)
Summary
Environmental complexity and enrichment influence brain morphology and neurochemistry in mammals and birds [1], with enriched environments promoting brain development [2]. Alterations in the brain as a response to environmental complexity have been described in the fruit fly (Drosophila melanogaster), in fish and in crayfish [reviewed in Ref. It was once believed that neural plasticity was limited to a critical period early in life [5]. Later research in the 70s and 80s showed that juvenile-young adult neural plasticity is not limited to a critical period but is triggered by experience and is independent of age [6, 7]. Hippocampal volume can be modified by experiences such as food storing [8] and migration [9]. On the other hand, continued deprivation of experience can result in attrition of hippocampal volume over time [7]
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