The hypothalamus contains populations of neurons that control food intake, and whose activity is suppressed by food consumption. The mesolimbic pathway appears to assign reward values to sugary foods, whereas the nigrostriatal pathway seems to be necessary and sufficient for the reinforcing effects of absorbing sugar. The brain’s reward circuitry contains two distinct pathways, which appear to perform different roles in learning about sugar consumption. In the brainstem, vagus nerve terminals form synaptic connections with fibers that project to the dopamine-producing reward circuitry in a subcortical structure called the basal ganglia, and also to the hypothalamus, which regulates eating behavior. This confirms that neuropod cells guide the consumption of, and preference for, sugar, but it is still unclear whether these cells are also necessary for the development or memory of this preference. This activates fibers in the vagus nerve, which signals up to the brain within milliseconds.Ī study published earlier this year showed that inhibiting neuropod cell activity or blocking glutamate receptors in the guts of mice abolishes their preference for sugar over artificial sweetener. Neuropod cells express a protein called sodium-glucose co-transporter 1 (SGLT1), and respond to glucose (the basic component of all sugars) by releasing the excitatory neurotransmitter glutamate. In the past decade, however, it has emerged that sugar-detecting cells form synaptic connections with the vagus nerve - showing that the cells are in fact specialized neurons, which were dubbed “neuropod” cells. Until very recently, it was widely believed that sugar detection in the gut is mediated by hormone-secreting endocrine cells. These proteins are also found in the pancreas, fat cells, and brain, and all of them contribute to detecting sugar before and after it has been absorbed. Sugar is detected by both the tongue and the gut by taste receptors, glucose transporter proteins, and ATP-gated potassium channels.
Later, when given the choice of the solutions alone, they chose those that had previously been paired with the eggnog. In the late 1960s, the role of gut-brain signaling was confirmed in a classic study in which rats were given differently flavored sweet solutions paired with gastric injections of eggnog or water. He fed rats a diet consisting of cellulose, which animals cannot digest, and found that they ate more but did not consume more calories.Ī few years later, further experiments showed that milk injected directly into rats’ stomachs was rewarding and led to learning.
DOPAMINE TRANSPORTER SERIES
In the 1940s, the physiologist Edward Adolph performed a series of experiments designed to test the relationship between food quantity and nutritional content.
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