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The Complex Connection Between Food Intake and Brain Signals

Explore how serotonin and insulin influence our cravings and hunger.

― 7 min read


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Food intake is a tricky business. It’s all about balance. Just like Goldilocks and her porridge, animals need to find the right amount of food-not too much and not too little. When an organism is low on energy, it starts feeling hungry, like that annoying growl your stomach makes during a long meeting. This feeling pushes them to go find food, turning them into expert food detectives. If they smell something delicious, they become extra alert, like a cat hearing a can opener.

Appetite vs. Hunger

Now, there’s a difference between feeling hungry and having an appetite. Hunger is that basic urge to eat when you’re running low on energy. Appetite, on the other hand, is that desire to munch, even if you're not starving. Think of it like wanting dessert after a hearty meal; it’s not about hunger but about craving something specific. When people lose control over their appetite, it can lead to unintended overeating and health issues like obesity and diabetes. Not fun!

In both animals with backbones (like us) and those without (like fruit flies), a special chemical called Serotonin plays a key role in regulating appetite and hunger. However, the exact ways serotonin affects our food choices are still a bit of a mystery.

Serotonin: The Food Regulator

For vertebrates, more serotonin usually means less desire to eat. However, in the world of fruit flies, serotonin has a mixed bag of effects. It can increase food intake when the flies are full but can also cut back on eating when they’re hungry. Imagine being told to eat dessert when you’re not only full but also craving salad instead. This confusion leads to varying food preferences based on their energy levels and hunger state.

Interestingly, different Neurons, or tiny messengers in the brain, react to the sweetness of things like sugar or the bitterness of vegetables. Sweet-loving neurons release signals that tell the body to store energy, while the bitter-loving ones help with digestion. So, the brain is like a traffic cop, directing food choices based on what’s pleasing or potentially harmful.

The Switcheroo of Serotonin

There’s a neat trick to keeping serotonin around longer. By blocking its reuptake, serotonin sticks around in the brain, leading to reduced food intake in both rats and humans. Think of it as a party where no one leaves-everyone just keeps mingling. In human medicine, this idea has been turned into drugs to help with weight loss. If only it worked as easily for that slice of cake we all want!

Scientists have found that this serotonin game isn’t just a fly thing; it’s quite similar in other living beings too. When fruit flies lack the serotonin transporter, a device responsible for managing serotonin's presence, they also eat less. It seems serotonin and its transporter are best buddies in regulating how we consume food.

The Insulin Connection

Food intake doesn’t just depend on serotonin. There’s also insulin, a hormone that plays a significant role in how our cells use sugar. In both vertebrates and flies, insulin helps manage glucose levels. In flies, they even have their own version of insulin called insulin-like peptides. These nifty little guys can influence how much food flies eat.

In flies, specific insulin-like peptides inspire a sugar craving after a period of starvation. However, when they have plenty of food, the flies might overdo it on sugar or protein. It’s a balancing act between wanting food and figuring out how much to eat.

Investigating Food Cravings

To understand how flies regulate their sweet and protein Appetites, scientists ran a series of experiments. They trained flies to respond to specific food types, watching how serotonin affected their cravings. Using the clever technique of genetic manipulation, they played around with the serotonin transporter and insulin receptor to see how these changes affected what the flies wanted to eat.

The scientists discovered that a group of neurons specifically manages the appetite for sugar. When they turned up serotonin signaling in these flies, their sweet cravings dropped. It’s like being on a diet where you can’t stop thinking about cookies, but suddenly they don’t seem so appealing anymore.

The Hungry vs. the Satiated

These experiments also showed something interesting: the same neurons that control sugar cravings behave differently depending on whether the flies were hungry or full. When the flies were well-fed, activating certain neurons made them want more food, but when they were hungry, the same activation reduced their desire to eat. Talk about a brain with mood swings!

In studying how these neurons work, the researchers wanted to identify which ones managed sugar cravings and which ones managed protein hunger. It turned out that different sets of neurons were responsible for each type of hunger. It’s like having different channels on a TV to watch your favorite shows-if you want to watch sports, but the cooking show is on, you’ll need to change the channel!

Taste Buds and Neurons

Interestingly, serotonin also interacts with other neurons that can influence how flies taste food. The scientists found that cells specifically tuned to detect sweetness are close to the ones that serotonin interacts with. This close relationship means serotonin can modulate how the flies experience sweetness, lowering their sugar cravings when serotonin is activated.

The Insulin and Serotonin Tango

At the cellular level, both insulin and serotonin play roles in the appetite for sugar. When flies express active insulin receptors, they tend to eat less sugar. Conversely, when insulin receptors are blocked, they tend to eat more. It’s a see-saw effect!

Insulin signaling also affects how serotonin is processed within the brain. The insulin receptor can mess with the serotonin transporter, changing how serotonin behaves in the body, meaning more or less sugar consumption based on the state of insulin.

The Role of Neurons

A special group of neurons called SE1 neurons plays a significant role in this process. They help manage both serotonin and insulin signaling. When scientists looked closely at these neurons, they found that serotonin transporter levels and neuron branching were impacted by insulin. This means that insulin not only affects how much flies eat but can also change the structure of their neurons.

Getting Deeper into the Brain

The researchers went even further, using advanced imaging techniques to visualize these neurons and how they change in response to insulin signaling. They found that when insulin receptors were active, the serotonin transporters in the SE1 neuron didn’t spread out as much, which suggests that insulin might be telling serotonin where to go and how much to express.

The Sweet Science of Learning

To see if changes in sugar appetite impacted behavior, scientists tested how flies learned to associate certain smells with sugar rewards. Flies that had reduced sugar cravings didn’t form the positive associations as well as those that had normal cravings. Their memory for that sugary reward suffered, meaning they might not react eagerly to sugary smells in the future. It was like teaching someone only to forget everything right after the first class.

Summary: A Complex Web of Signals

In conclusion, the relationship between how much food we want and how our body regulates sugar is a complex web of signals. Serotonin and insulin work together to balance our cravings and hunger, with specific neurons managing different food desires. Watching a fruit fly navigate its food choices can provide surprising insights into how appetite, taste, and memory intertwine.

So next time you feel a strong urge for that piece of cake, think about the invisible dance between hormones and neurons happening inside you. It’s not just your taste buds at work; it’s a full-on brain party with serotonin and insulin as the DJ, spinning tracks to keep your appetite just right. Now, if only they could help with those late-night cravings!

Original Source

Title: Different serotonergic neurons regulate appetite for sucrose and hunger for proteins

Abstract: For the organism it is important to replenish internal energy storages selectively and selective appetite for nutrients might uncover internal energy requirements. How is the selective uptake of a specific nutrient regulated? Here we show that in Drosophila melanogaster different sets of serotonergic neurons regulate appetite for sucrose and hunger for proteins. Increased neuronal activity in specific subsets of serotonergic neurons and interfering with serotonin reuptake using a mutated serotonin transporter reduced the appetite for sucrose selectively, but not the hunger for proteins. The insulin receptor together with the serotonin transporter regulates the selective sucrose appetite. We provide evidence that the cellular location of the serotonin transporter depends on the insulin receptor. This mechanism might allow optimizing nutrient intake and in turn might prevent overconsumption by repressing appetite for sucrose. Given the conserved nature of the molecules involved it is likely that the mechanism is conserved in higher organisms.

Authors: Katharina Dorn, Magdalena Gompert, Jianzheng He, Henrike Scholz

Last Update: Oct 31, 2024

Language: English

Source URL: https://www.biorxiv.org/content/10.1101/2024.10.30.621076

Source PDF: https://www.biorxiv.org/content/10.1101/2024.10.30.621076.full.pdf

Licence: https://creativecommons.org/licenses/by/4.0/

Changes: This summary was created with assistance from AI and may have inaccuracies. For accurate information, please refer to the original source documents linked here.

Thank you to biorxiv for use of its open access interoperability.

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