Suppression of conditioned odor approach by feeding is independent of taste and nutritional value in Drosophila.

Motivation controls behavior [1]. A variety of food-related behaviors undergo motivational modulation by hunger, satiety, and other states [2-4]. Here we searched for critical satiation factors modulating approach to an odor associated with sugar reward in Drosophila melanogaster. We selectively manipulated different parameters associated with feeding, such as internal glucose levels, and determined which are required for suppressing conditioned odor approach. Surprisingly, glucose levels in the hemolymph, nutritional value, sweetness of the food, and ingested volume (above a minimal threshold) did not influence behavior suppression. Instead, we found that the total osmolarity of ingested food is a critical satiation factor. In parallel, we found that conditioned approach is transiently suppressed by artificial stimulation of adipokinetic hormone (AKH) expressing corpora cardiaca cells, which causes elevation of hemolymph carbohydrate and lipid concentrations [5, 6]. This result implies that a rise in hemolymph osmolarity, without the experience of feeding, is sufficient to satiate conditioned odor approach. AKH stimulation did not affect innate sugar preference, suggesting that multiple satiation signals control different sets of appetitive behaviors.