Effects of environmental enrichment on recognition memory in zebrafish larvae

Zebrafish (Danio rerio) constitute a useful model for studying memory function and impairment in vertebrates and are now widely used in translational research. On the one hand, the adoption of simple, fast and reliable tests such as novel object recognition (NOR) has increased our knowledge considerable about memory mechanisms in animals. On the other hand, in many model organisms, exposure to environmental enrichment, especially during the early stages of development, affects various cognitive functions. Evidence for the effects of environmental enrichment on zebrafish has been accumulating rapidly, but most of this evidence has been collected in adult subjects. We compared larvae raised in either an enriched or barren environments and measured their memory performance at 14-days post-fertilization. Initially, subjects were allowed to familiarize with two identical novel objects (i.e., pattern of 2D-geometrical figures). After a time interval, larvae faced a two-choice task presenting the same objects paired with a new one. As a measure of recognition memory, we exploit the tendency of individuals to explore a novel object over a familiar one. Our results indicate that larvae from the barren environment spent more time exploring familiar stimuli than novel ones, showing the innate presence of recognition memory capacity in zebrafish larvae. Conversely, subjects bred in a visually enriched environment explore both familiar and novel stimuli almost equally. The increase of exploratory behavior and, consequently, the reduction of avoidance to the novel object may explain the performance shown by larvae exposed to an enrichment environment compared to the larvae bred in a barren environment. Taken together, these results confirm that early-stage zebrafish possess complex visual discrimination capacities and that rearing subjects in a structurally complex environment might hinder memory performance by reducing their neophobic response.