Neurotransmitters and the Gut Microbiome

Everything you need to know about the belly-to-brain communication network

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The Nervous System - Please Stay on the Line!

We can think of the nervous system almost like an endless game of telephone -its goal is to get the right messages to the right places.

But how does that happen?

The answer is pretty complex, but a good way to get started with understanding how messages get from your brain to your body (or your body to your brain) is to learn a little about the brain’s chemical messengers -the neurotransmitters.

Neurotransmitters: Just the Messengers

To know what neurotransmitters are, we first need to understand neurons. Imagine sending something out in the mail. Neurons are like distribution centers that process the shipment on its way to its address. Neurotransmitters are the postal vehicles that transport them from these centers to their final destination. Depending on how far the mail needs to travel, and what’s in the package, there can be multiple transport vehicles (neurotransmitters) and distribution centers (neurons) on its route.

In short, neurotransmitters are chemical messengers, which pass on messages from neurons to other cells --including other neurons. Without them, many vital processes that we take for granted, like movement, cognition, and muscle contraction would be impossible.

The Neurotransmission MVPs

There are many kinds of neurotransmitters (at least 500), but neuroscientists generally agree that the following 5 are the most important:

Acetylcholine

Have you ever done exceptionally well in a class? Maybe you found yourself able to be super focused and to absorb all the material quickly. If you’ve ever had that sort of experience, acetylcholine was probably helping you out!

This neurotransmitter is responsible for maintaining several crucial bodily functions, such as thinking, working memory, muscle movement, focus, and overall abilities to learn. Acetylcholine also regulates mood and is necessary for making DNA.

Dopamine

When you win a game, it feels good. Sometimes it feels so good that you want to play again. That good feeling comes from dopamine.

When your brain is anticipating a reward, it releases dopamine. Dopamine regulates the cycle of motivation to action. It makes you feel good when you succeed, and it also makes you feel bad when you don’t. This is why we’re motivated to do things we’re good at again and again, and why we usually want to move on from things we’re not so good at.

Aside from the “feel good” and “feel bad” cycle of the reward system, dopamine regulates sleep, general mood, cognition, memory, pain processing, and motor control, among other vital bodily functions.

GABA and Glutamate

Your body is all about balance, which is why we have both excitatory and inhibitory neurotransmitters. Despite the name, excitatory neurotransmitters don’t make you feel excited. Instead, they encourage your network of neurons to be more active. For example, glutamate, the most common neurotransmitter in the central nervous system, is excitatory. It helps messages get across the neural network from your brain out to the rest of your body and also helps strengthen the connections between neurons. This makes glutamate important for memory and learning.

On the other hand, gamma-aminobutyric acid (GAH-muh uh-me-no-byoo-TEE-ric a-sud), or GABA for short, is an inhibitory neurotransmitter. It’s there to help balance excitatory neurotransmitters like glutamate. If neurons are too active, you can feel anxious or even have seizures. GABA keeps the neurons from “firing”—or sending messages—too often, so your brain can do its job without overwhelming your senses.

Serotonin

Last but not least, we have what has been coined as the “happiness hormone.” Serotonin is a neurotransmitter that plays a role in regulating our mood, appetite, cognition, sleep, and even our bowel movements. For example, when serotonin levels are low, some people develop depression. All five of these neurotransmitters do not act alone. In fact, they all work together in unison to ensure that the brain and body are maintaining homeostasis --that is, stability and balance.

Going With Your Gut -- The Gut-Brain Communication Network

Now that we’ve spoken about the nervous system and its messengers, let’s talk about the digestive tract--or “gut” for short. You may think that the gut is totally unrelated to the nervous system, but it is actually integral to it.

Have you ever wondered why thinking about food can make you hungry? Or maybe why you get butterflies in your stomach when you see your crush? The brain and your gut have a close and unique communicative relationship, and much of this relationship is determined by your “gut flora.” In other words, the kinds of bacteria in your gut determine whether its communication with your brain is good or bad. This can affect your mood and even your brain’s ability to function.

You have trillions of human cells inside of you right now! We used to think that we only had 1 bacteria cell inside us for every 10 human cells. New research suggests that we have an equal number of bacterial cells living inside of us, and they’re collectively known as your “microbiome.” We have about 160 different kinds of gut bacteria--of which trillions reside in each of our guts--and the exact makeup of your microbiome is affected by factors such as your lifestyle, geography, and genetics. Finally, the gut contains the enteric nervous system (ENS), which is a large part of the peripheral nervous system that controls gastrointestinal behavior without the input of the brain or spinal cord.

Gut floras overly populated by “bad” bacteria are linked to stress, anxiety, and depression. By contrast, a gut flora colonized by ‘good’ bacteria may contribute to resolving these symptoms. Finally, some of these conditions may bring some gastrointestinal symptoms due to their connection to the enteric nervous system. How could this be? This brings us back to neurotransmitters.

The microbiome can produce the 5 major neurotransmitters and others, such as norepinephrine. The importance of this fact is crucial for many reasons. For example, tryptophan is an amino acid found in foods like pumpkin seeds, milk, or turkey; tryptophan is an essential amino acid, meaning that the body cannot produce it on its own, and therefore, it needs to be consumed through diet. When consumed, tryptophan can either be converted into serotonin or another neurotransmitter known as kynurenine; either way, these conversions are conducted by gut bacteria.

Research shows that those with depression more readily convert tryptophan into kynurenine than into serotonin, which makes sense because, as we just learned, depression can be caused by low serotonin levels. More research needs to be done to fully explore this, but it’s possible that having gut flora with a higher population of “good” bacteria could be connected to improving symptoms of depression. In other words, depression is closely connected to the gut microbiome. This makes sense, considering roughly 90-95% of the body’s serotonin resides in the gut.

In short, the connection between the gut and brain’s communication and neurotransmitters consists of the gut’s production and regulation of neurotransmitters, the gut’s communication to the brain to do the same, and the gut’s ability to moderate the brain’s stress responses. Of course, more research needs to be done to understand the extent of all of this. Still, it is fascinating to think that bacterial cells --as opposed to human cells, which are outnumbered by bacteria --have such a profound influence on our brains and physical well-being. To maintain a healthy gut microbiome, you should maintain a high fiber and high-probiotic diet, including food such as yogurt and kimchi and other fermented things, such as kombucha.