Food reward entrainment increases mealtime anxiety in goldfish via a ghrelin-dependent mechanism.

Food anticipatory activity (FAA), the increase in locomotor activity prior to a scheduled feeding time, is linked not only to energy demands but also to food reward expectations. However, the mechanisms behind FAA remain unclear. We hypothesize that FAA induces anxiety due to reward anticipation. To test this, anxiety-like behavior was assessed in goldfish using open field and black-and-white preference tests under different feeding conditions and daytimes. Increased thigmotactic and scototactic behavior during FAA, compared to the post-FAA period, was observed, even in animals under constant light. Additionally, an unexpected meal did not reduce anxiety, while more hours of fasting did not increase it, suggesting that FAA is an anxiogenic state driven by a food-entrained oscillator rather than energy status. Ghrelin signaling was further explored using two antagonists, both of which reduced anxiety-like behavior when administered during FAA. Moreover, ghrelin administration induced anxiety-like behavior that was reversed by co-injection with an antagonist, supporting its anxiogenic role. These findings suggest a shared neural mechanism between FAA and anxiety, mediated by ghrelin. The possible involvement of ghrelin in activating dopamine-reward circuits in teleosts should be explored in future studies to gain further insight into the neurobiological basis of food anticipatory activity.