BDNF Deficiency Preserves Shoal Structure but Selectively Modulates Horizontal Exploration in an Adult BDNF^-/-...

Brain-derived neurotrophic factor (BDNF) is a key regulator of neural development, plasticity, and behaviour. Recent work has enabled the generation of a viable adult bdnf^-/- zebrafish line, which provides a unique opportunity to investigate how complete loss of bdnf affects social behaviour. Here, we examined three-dimensional shoaling behaviour in adult male and female AB wild-type and bdnf^-/- knock-out zebrafish to determine whether the extensive molecular and behavioural alterations previously observed in individual-based assays extend to collective contexts. The bdnf^-/- shoals showed no differences in group structure, as inter-fish distance, shoal volume, shoal area, distance to the centroid, and homogeneity index were comparable to wild-type groups. Vertical spatial distribution was also largely preserved, although bdnf^-/- fish shifted toward the upper regions of the tank earlier during the trial. By contrast, horizontal distribution revealed a clear genotype effect: bdnf^-/- shoals spent more time in peripheral regions and displayed a pronounced early peak in peripheral occupancy. These findings indicate that bdnf loss does not impair shoal formation or cohesion but selectively modulates specific components of spatial exploration. The results also highlight a dissociation between the vertical and horizontal axes of behaviour, as well as between individual- and group-based phenotypes, underscoring the importance of social context in shaping the behavioural consequences of BDNF deficiency.