Abstract
Simple SummaryThe hippocampus region of birds is a pivotal area for learning and memory. Early-life conditions can have a long-lasting impact on animals. However, the influence of early-life and later-life environments on animals’ health and welfare has not been well-studied. This study addresses the impact of early-life enrichment and the later environment on the learning ability and hippocampal responses in chicks. We found that the early-life environmental complexity did not prepare better for learning ability in response to life challenges in the future. This study indicates that perches and litter materials, which enriched early-life conditions, were conducive to improved stress responses later in life in terms of neural- and immune-related gene expression and functional pathways. This can be attributed to the “silver spoon” effect. However, the enrichment through litter materials alone in early life does not improve the hippocampal plasticity in later stressed environments. In addition, early-life barren conditions that match later-life conditions have beneficial impacts on neural development, supporting the match and mismatch theory. This study helps us to understand the relationship between early- and later-life environments from the perspective of animal neural- and immune-related development. It also has the potential to guide the treatment of mental problems and personal health in humans based on the interactions between the early-life and later-life environments. In this study, we hypothesized that complex early-life environments enhance the learning ability and the hippocampal plasticity when the individual is faced with future life challenges. Chicks were divided into a barren environment group (BG), a litter materials group (LG), and a perches and litter materials group (PLG) until 31 days of age, and then their learning abilities were tested following further rearing in barren environments for 22 days. In response to the future life challenge, the learning ability showed no differences among the three groups. In the hippocampal KEGG pathways, the LG chicks showed the downregulation of neural-related genes neuronal growth regulator 1 (NEGR1) and neurexins (NRXN1) in the cell adhesion molecules pathway compared to the BG (p < 0.05). Immune-related genes TLR2 in Malaria and Legionellosis and IL-18 and IL18R1 in the TNF signaling pathway were upregulated in the LG compared to in the BG (p < 0.05). Compared to the BG, the PLG displayed upregulated TLR2A in Malaria (p < 0.05). The PLG showed upregulated neural-related gene, i.e., neuronal acetylcholine receptor subunit alpha-7-like (CHRNA8) in the nicotine addiction pathway and secretagogin (SCGN) gene expression, as compared to the LG (p < 0.05). In conclusion, early-life environmental complexities had limited effects on the learning ability in response to a future life challenge. Early-life perches and litter materials can improve neural- and immune-related gene expression and functional pathways in the hippocampus of chicks.
Highlights
The hippocampus region in birds is a pivotal area for learning and memory [1,2] and is highly responsive to evolutionary adaptations
The chicks were brooded in three different early-life environmental groups, which were as following: (1) a barren environment group (BG, n = 40); (2) an environment enriched with a litter materials group (LG, n = 40); and (3) an environment enriched with two perches and a litter materials group (PLG, n = 40), from post-hatching to 31 days of life
In the later barren condition challenge, no significant difference was observed among the three groups in learning ability or mRNA expression of genes in the hippocampus related to learning ability, including N-methyl-D-aspartic acid (NMDA) [36,37] and brain-derived neurotrophic factor (BDNF) [38,39]
Summary
The hippocampus region in birds is a pivotal area for learning and memory [1,2] and is highly responsive to evolutionary adaptations. Memory and learning ability have a crucial role in the adaptability and fitness of animals. Developing and maintaining learning ability is associated with neurogenesis and the establishment of neural pathways in the hippocampus [4]. Environmental complexity influences the hippocampal morphology of adult birds [7,8]. Exposure to early-life stress (e.g., a barren system or poor environment) may impact neural plasticity and impair hippocampal plasticity in birds, suggesting that birds in the wild have a complex hippocampus with more neurons than adult aviary birds [4]. These studies indicate that environmental complexity is conducive to hippocampal plasticity in birds. How early-life environmental complexity influences hippocampus gene expression and function into later life has not been wellstudied in birds
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