Researchers found two different areas of the brain’s reward centre, called the the striatum, responds to different elements of food: the ventral striatum (VS) releases dopamine in response to sweet flavours while the dorsal striatum (DS) responds to nutrients. And when pressed, the DS response to calories trumps the VS response to taste, according to the study by a team at Yale University.
Energy is brain’s top priority
“It turns out the brain actually has two segregated sets of neurons to process sweetness and energy signals. If the brain is given the choice between pleasant taste and no energy, or unpleasant taste and energy, the brain picks energy,” said Ivan de Araujo of the John B Pierce Laboratory and one of the study’s authors.
Across various experiments, the researchers tested mice with bottles containing sweet or bitter-tasting substances and then separately supplied the mice with either sugar or artificial no-calorie sweetener directly to their stomachs, via a surgically-fitted catheter. The team would release the sugar or sweetener in response to how much the mice licked the bottles, measured via a “lickometer”.
“Mice were initially tested on brief-access two-bottle preferences involving a sucralose solution and an unpalatable bitter/sweet mixture. Not surprisingly, all mice preferred to lick the sweet-only solution. Next, mice were exposed to one-bottle learning sessions in which licks for the sweet solution were paired to intra-gastric infusions of the non-nutritive sweetener,” wrote the authors in the study.
“Then, on a different day, licks for the bitter/sweet solution were paired to intra-gastric infusions of the nutritive glucose. Two-bottle preference tests were performed again on a fourth testing day. All mice markedly shifted preferences toward the unpalatable nutritive stimulus, except DS-casp mice, which continued to prefer the non-nutritive, but palatable, solution,” they added.
The “DS-casp” mice were animals which had their dorsal striatum deactivated – so their brains no longer rewarded them for choosing the calorie-rich option.
Neurone-prodding confirms difference
In addition, when the team stimulated DS neurones artificially, this had the same effect as feeding the mice sugar, even while actually feeding them artificial sweetener. They also monitored the mice’s brains, and confirmed the VS and DS areas responded differently and separately to taste and calories.
The researchers said hungry mice consumed a lot more of the bitter-tasting solution when this triggered the release of sugar, rather than artificial sweetener: “The activation of dorsal pathways was sufficient to override inhibitory signals generated by ventral pathways during the ingestion of aversive substances. Such circuit logic implemented in the striatum allows the organism to prioritise energy seeking over sensory quality.”
According to the study’s authors, the activation of the dorsal striatum should mean the consumption of calorific food is related to the body’s motor function, which is also influenced by the same brain area. But they said more research was needed to confirm this hypothesis.
Published online ahead of print: doi:10.1038/nn.4224
“Separate circuitries encode the hedonic and nutritional values of sugar”
Authors: Tellez, L A; Han, W; Zhang, X; Ferreira, T L; Perez, I O; Shammah-Lagnado, S J; van den Pol, A N; de Araujo, I E