Researchers have made a new discovery around the powerful connection between the gut and the brain.
If you feel queasy before an exam or find it difficult to focus after eating, you can vouch for the connection between the gut and the brain, or ‘gut feeling’.
A growing number of scientists believe that a broad array of conditions, from depression to appetite disorders, begin in the gut.
While the links had been made, it was unclear how the messages in this so-called ‘second brain’ spread from our stomachs to the cerebrum. The prevailing theory was that hormones present in the bloodstream were the indirect channel between the brain and gut.
Gut feeling may not be hormonal after all
Researchers at Duke University have been looking into this and their findings suggest that the line of communication behind that gut feeling is more direct and faster than a hormonal diffusion. Their research was recently published in the journal Science.
Using a rabies virus that had been augmented with green fluorescence, the researchers traced a signal as it travelled from the intestines to the brainstem of mice. They were surprised to see the signal cross a single synapse in under 100 milliseconds, faster than the blink of an eye.
Prof Diego Bohórquez, postdoctoral fellow, senior author of the study and assistant professor of medicine at Duke University School of Medicine, said: “That has profound implications for our understanding of appetite. Many of the appetite suppressants that have been developed target slow-acting hormones, not fast-acting synapses. And that’s probably why most of them have failed.”
The brain takes in data from all five senses – touch, sight, hearing, smell and taste – through electrical signals. These signals travel along nerve fibres beneath our skin and muscle. They move quickly, which is why the smell of food hits you as soon as you enter the kitchen.
When it comes to the gut, researchers originally hypothesised that it delivered its messages by a multi-step, often indirect process. Nutrients in your gut were said to stimulate hormonal release, which then entered the bloodstream minutes to hours after eating, with the brain eventually feeling the effects.
Bohórquez noticed that the sensory cells lining the gut share a lot of the same features as their counterparts on the nose and tongue. In 2015, he published a study showing these cells contained nerve endings or synapses, suggesting they may be involved in a neural circuit of some description.
A direct line
The new study set out to map this circuitry. First, the team fed the mice the fluorescent rabies virus. Postdoctoral fellow Maya Kaelberer noted that the virus had labelled the vagus nerve before landing in the brainstem, showing her there was a direct circuit.
Kaelberer then recreated the gut-brain neural circuit by growing sensory gut cells of mice in the same dish with vagal neurons. She saw the neurons crawl along the surface of the dish to connect with the gut cells and begin to fire signals. The research team added sugar to the mix, which sped up the firing rate considerably. The rate the information from sugar in the gut was communicated was down to milliseconds.
This suggests that a neurotransmitter such as glutamate (which is involved in conveying other senses such as taste and smell) may be acting as the messenger. When the researchers blocked the release of glutamate in the sensory gut cells, the messages were silenced.
Bohórquez said he has data that suggests a similar process in the human body, which could open up the biological basis for a new sense. He added: “One [a new sense] that serves as the entry point for how the brain knows when the stomach is full of food and calories. It brings legitimacy to idea of the ‘gut feeling’ as a sixth sense.”