Knockout in zebrafish suggests a role of BDNF in behavioural and cognitive plasticity

An adaptive mechanism observed in virtually all living animals is the phenotypic plasticity, a phenomenon by which different phenotypes can develop from the same genotype depending on the environment experienced. Phenotypic plasticity can involve a range of life-history, physiology, morphology, and also behavioural and cognitive traits. For example, individuals exposed to complex and enriched environments generally show greater learning abilities compared to individuals raised in simpler or barren environments. Several studies suggest that brain-derived neurotrophic factor (BDNF) may be involved in this plasticity, as it is often differentially expressed in individuals exposed to different environments. We investigated this possibility in fish, taking advantage of a knockout zebrafish line lacking the gene coding for BDNF (bdnf). Zebrafish from both the knockout (bdnf ^-/-) and control (bdnf ^+/+) lines were raised in either barren or enriched environments, and their behavioural and cognitive phenotypes were analysed. bdnf ^-/- zebrafish exhibited higher behavioural and cognitive plasticity compared to bdnf ^+/+ controls, as evidenced by a pronounced increase in activity and learning scores following exposure to environmental enrichment. These results suggest that BDNF may play a modulatory role in phenotypic plasticity, although broader organisational alterations resulting from the absence of BDNF during development in knockout zebrafish cannot be ruled out. An additional exploratory analysis of the expression of other neuroplasticity-related genes identified potential pathways that may be involved in this effect.