Humans have a sugar sense. Animals and humans prefer sugar over artificial sweeteners in experiments, and that could be because a specific gut sensor cell triggers one of two separate neural pathways depending on which it detects, researchers suggest in a January 13 study in Nature Neuroscience.
“It has been known for decades that animals prefer sugar to non-caloric sweeteners and that this preference relies on feedback from the gut,” Lisa Beutler, a Northwestern University endocrinologist who researches the connection between the gut and brain and was not affiliated with the new work, writes in an email to The Scientist. “This study is among the first to provide insight at the molecular level into how the gut knows the difference between sugar and non-caloric sweeteners, and how this drives preference.”
The study builds on previous research from the lab of Duke University gut-brain neuroscientist Diego Bohórquez. In 2015, Bohórquez established that endocrine cells, which were previously thought to only communicate with the nervous system indirectly through hormone secretion, can in fact have direct contact with neurons, evidenced by a video.
Then, in 2018, the Bohórquez Lab found that the gut has similar cells to those that allow for taste on the tongue and smell in the nose, and that these sensors also have direct contact with neurons. “If they are connected to neurons, they must be connected to the brain,” Bohórquez tells The Scientist. “When we ingest sugar, it stimulates cells in the gut, and these cells release glutamate and activate the vagus nerve,” Bohórquez explains of his prior research. The vagus nerve is a cranial nerve that plays a regulatory role in internal organ functions such as digestion. His team observed that these gut sensor cells, which the team dubbed “neuropods,” transmit the chemosensory information mere milliseconds after detecting sugar.
Now, he and his colleagues have delved deeper to find out whether this gut-to-vagus pathway discriminates between sugar and artificial sweeteners, and if so, the neural mechanisms underpinning this differentiation. The team anesthetized mice, perfused sucrose or sucralose directly into their guts, and then analyzed the response of neurons that make up the vagus nerve using calcium imaging. On average, 40.7 percent responded to sucrose only, 22.2 percent to sucralose only, and the remainder to neither stimulus, explaining how the vagus nerve is able to react differently to the two substances.
Performing RT-qPCR to assess the expression of molecular receptors on neuropods, the researchers also showed that neuropods sense sugars through SGLT1, a sodium glucose transporter, and artificial sweeteners through T1R3, a sweet taste receptor that when activated releases the neurotransmitter ATP. Also, blocking glutamate receptors in the gut using antagonists decreased the vagal nerve response to sucrose and silenced response to alpha-MGP, a sugar analog that is transported into the cell in the same way as glucose. However, the antagonists had no effect on sucralose, further bolstering the case for the existence of differentiated neural pathways between the two sweet tasting stimuli.
Based on these results, the authors conclude that the entry of the sugar sucrose and sugar analog alpha-MGP into a neuropod cell stimulates it to release glutamate, which activates the vagus nerve and is involved in reward signaling, while no- or low-calorie sweeteners such as sucralose cause neuropod cells to release ATP, activating a different gut-brain pathway.
But Beutler says the case isn’t yet closed, particularly on whether the neuropod-to-vagus glutamate transmission is essential to animals’ preference of sugar over sweetener. “Glutamate receptors are expressed on virtually every neuron,” and the antagonists the researchers used could have acted on neurons other than neuropods in the gut or brainstem, she writes.
The findings left open the question of whether the neuropods contribute to different behaviors in response to sugars versus artificial sweeteners. To answer that, the group turned to optogenetics, a methodology predominately used in the brain. The researchers genetically altered mice so that exposure to certain wavelengths of light would silence neuropod cells typically stimulated by sugars and sweeteners, while others would stimulate them, and yet others would have no effect and record how this affected the mice’s preferences. The problem was that light delivery traditionally requires a stiff silica fiber optic cable, which would pierce the soft tissue of the ever-churning gut. So, the researchers collaborated with engineers to develop a flexible fiber optic cable adapted to the unique biological conditions of the gut, Bohórquez says.
Adapting optogenetic manipulation to the gut is the “most exciting” part of the paper, writes Beutler, and “a technique that I think/hope will gain traction in the gut-brain science community.”
Bohórquez says that the novel optogenetics fiber his team used in their experiments is flexible and can be placed in very tiny places that are constantly in motion—like the heart and lung—potentially enabling researchers to “start to interrogate how it is that the sensory inputs from all of these organs influence our behavior.” It could “hold the key,” according to Bohórquez, to better understanding some mental health, mood, or sleep disorders that are associated with the GI tract.
In their experiments, silencing neuropods had notable effects on the mice’s behavior. Mice exposed to neuropod-silencing light lost their preference for consuming sugar over the sweetener sucralose when presented with the two options, while mice exposed to a control wavelength did not. Moreover, when a mouse’s neuropod cells were excited via light, their intake of the artificial sweetener was doubled, with the animal drinking it as if it were sugar.
The researchers also measured vagus nerve activity in the optogenetic mice, finding that neuropod-silencing light eliminated all responses from the vagus nerve, while activity was unchanged by control wavelengths.
Ivan de Araujo, a neuroscientist at Icahn School of Medicine at Mount Sinai who was not involved in the study, calls the findings “a very exciting development.” He explains that in previous research, it was unclear if sugar stimulated vagal signals or if this signaling was limited to other nutrients like fats and amino acids. “The fact that that they connected these [neuropod] cells in the gut to the brain via the vagus nerve in this context of sugar reward was a little surprising to me and very intriguing,” he says.
Beutler writes that it will take time to understand whether these findings translate to human nutrition. One potential application, she suggests, “would be to develop a non-caloric sweetener that is able to stimulate the SGLT1 transporter (normally activated by sugar) to actually trick the brain into thinking it was ingesting something caloric. Would this suppress overall calorie intake? Would it generate conditioned taste preference?”
De Araujo tells The Scientist, “One interesting question for the future is the involvement of these [neuropod] cells in the reward associated with other nutrients. Are they important for detecting the nutritional value of lipids and amino acids in the same way they are for sugar?” Bohórquez agrees it would be interesting to explore how signals from nonsugar nutrients such as fats and proteins are relayed from the gut.
“There is something in nutrients that signal to the brain in a way that is specific to each molecule. How does this operate? How does the brain know which nutrients have been ingested based on the mere presence of the nutrient in the gut in the absence of taste signaling?” de Araujo asks. “The big picture [from the study] is that we have some cells in the gut from where to start” in answering these questions. “And that’s exciting.”