A search for effective reinforcers in appetitive conditioning for adult zebrafish: Ecologically relevant unconditioned stimuli

Abstract

Learning and memory related brain disorders represent a large unmet medical need. Laboratory studies with animals may model brain disorders and facilitate uncovering their mechanisms. The zebrafish has been proposed for such studies. However, numerous factors that influence performance in learning tasks have yet to be understood in zebrafish. One such factor is what motivates zebrafish. Here we introduce a novel reinforcer, an ecologically relevant unconditioned stimulus (US). We placed a photograph of gravel underneath quarter of the bottom of an experimental tank on one side and also positioned artificial plants there, the “natural” US. First, we showed that this stimulus was preferred by zebrafish. Next, we investigated whether this stimulus could serve as US for associative learning. We marked the walls of the tank on the side where the US was presented with red paper, the conditioned stimulus (CS+) we found neutral before, and we also marked the walls on the other side of the tank where no US was placed with blue paper (CS-). In addition to fish receiving this “paired” training, we also ran unpaired training with another group of zebrafish, in which the fish saw the US associated with blue and red in a random manner. After having trained the fish in this manner, we tested the performance of the paired and unpaired group of zebrafish in a memory probe trial during which no US was present, and only the CSs (blue and red walls) were shown. We found the paired group of zebrafish to show significant preference for the CS+, as they spent more time and swam closer to the red side compared to the unpaired group and compared to chance. We conclude that ecologically relevant stimuli can serve as efficient US in appetitive conditioning of zebrafish.