Methods for Investigating the Motivation of Mice to Explore and Access Food Rewards.

The emotional state of domestic animals is an essential component of the assessment of their welfare. In addition, sensitivity to various rewards can be a valuable indicator when investigating these states. We aimed to design an exploration test and a contrast test that did not evoke fear and anxiety in C57BL/6N mice but that instead were perceived as positive experiences and that might be used to assess sensitivity to various rewards. The exploratory arena had a larger central area and 8 smaller sections containing various objects. Motivation (measured as latency to enter the arena under conditions of increasing weight of the entrance door), anticipation (measured as latency to enter the arena under conditions of increasing delay in opening the entrance door), and the numbers of visits to the different sections were evaluated during a 5-min session in the arena. In the contrast test, after traversing a runway, half of the mice received a tasty reward (hazelnut cream), whereas the others received a neutral reward (food pellet) at the far end. Latency to reach the reward was recorded. After baseline training, rewards were swapped for half of the mice from each category for 3 d, to establish a negative and positive contrast. Mice were both motivated and showed anticipation to enter the exploration arena; after entering, they were active and visited many sections. In the contrast test, latency during the baseline period was longer for mice given the neutral reward compared with the tasty reward. Compared with baseline, latency during the postshift phase decreased for the positive-contrast group (neutral-tasty reward pattern) but did not differ for the negative-contrast group (tasty-neutral reward pattern). Overall, both tests seemed to be positive experiences for the mice and showed potential for use to investigate reward sensitivity.