Leptin regulates the reward value of nutrient.

We developed an assay for quantifying the reward value of nutrient and used it to analyze the effects of metabolic state and leptin. In this assay, mice chose between two sippers, one of which dispensed water and was coupled to optogenetic activation of dopaminergic (DA) neurons and the other of which dispensed natural or artificial sweeteners. This assay measured the reward value of sweeteners relative to lick-induced optogenetic activation of DA neurons. Mice preferred optogenetic stimulation of DA neurons to sucralose, but not to sucrose. However, the mice preferred sucralose plus optogenetic stimulation versus sucrose. We found that food restriction increased the value of sucrose relative to sucralose plus optogenetic stimulation, and that leptin decreased it. Our data suggest that leptin suppresses the ability of sucrose to drive taste-independent DA neuronal activation and provide new insights into the mechanism of leptin's effects on food intake.