| Summary: | For several reasons, such as easy maintenance, rapid generation times, and increasingly mapped genomes, teleost fishes are emerging as an alternative to small mammals in biomedical, neural, and behavioral research. Behavioral, genetic, and physiological screening of high numbers of individuals across treatments and generations is one particularly attractive feature of fish model systems. Both animal welfare considerations and fundamental scientific questions regarding the evolution of learning and memory have directed particular attention towards possible cognitive and emotional processes in fishes. Here we show that the omission of expected reward (OER) leads to increased aggression towards conspecifics in classically conditioned Atlantic salmon (Salmo salar). Furthermore, in response to an acute stressor, OER fish displayed increased dopaminergic (DA) neurotransmission compared to controls. There was also a general downregulation of dopamine receptor D1 gene expression in the telencephalon of OER groups, which suggests a coping mechanism in response to unbalanced DA metabolism. These results indicate that animals subjected to unpredictable reward conditions develop a senzitation of the DA signalling system, manifest as a potentiated response to novel, stressful stimuli. Similarities between fish and mammals in this response to unpredictability illustrates a role for teleost fish as models to understand the development of different types of DA dysfunction
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