Active avoidance conditioning in zebrafish (Danio rerio)

The zebrafish represents a potentially useful organism for studying genes involved in learning and memory function in vertebrates, because a number of genetic techniques in zebrafish have been developed to produce a wide variety of genetic mutants. While zebrafish mutants are being developed, behavioral studies on learning and memory function in zebrafish are in urgent need. The present study investigated active avoidance conditioning in normal zebrafish. Zebrafish were trained to swim from a lighted (CS) compartment to a dark compartment to avoid an electrical body shock (US) in a shuttle-box that consisted of a water-filled tank separated by an opaque barrier into two equal compartments. By varying the number of trials per training session and the duration of the intertrial interval, Experiments 1 and 2 showed that, with the CS, US, and intertrial interval being 12 s, zebrafish learned avoidance responses within a training session consisting of 30 trials and retained the avoidance responses. Experiment 3 showed that zebrafish learned avoidance responses following the association between the CS of light and the US of shock in the avoidance conditioning paradigm. Using the avoidance conditioning paradigm, Experiment 4 investigated the amnestic effects of N-methyl-d-aspartate receptor antagonist MK-801 and nitric oxide synthase inhibitor l-NAME in zebrafish. Experiment 4 showed that post-training injection of l-NAME significantly impaired retention of avoidance responses while MK-801 did not, confirming previous results with other vertebrates. The results of the present study suggest the similar involvements of neurochemicals in learning and memory among vertebrates. Thus, future studies with zebrafish mutants may identify genes involved in learning and memory in vertebrates.