Why is dopamine so greedy?

The materials responsible for transmitting information in the nervous system are rich and diverse.
These "messengers" are called neurotransmitters
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Different neurotransmitters play different roles
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But among all neurotransmitters, dopamine occupies an almost "god" position
.

Decades of research have confirmed that this substance has important contributions to several seemingly unrelated brain functions
.

It is a well-known "desire molecule" and is closely related to the mechanism of reward and pleasure in our brains.
It is also vital to our ability to control motor behavior
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In fact, it is involved in many aspects of brain cognitive processing
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This also raises a more critical question for us: how does a neurotransmitter so "greedy" affect many aspects of brain activity, and how does it exert so many functions? Solving the mystery of the multiple roles of dopamine has always been a big challenge, partly because the advanced brains of humans and other mammals contain different types of dopamine neurons, all of which are embedded in extremely complex circuits
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In a new study, neuroscientist Vanessa Ruta and his team delved into this problem, but they cleverly chose the simpler fruit fly brain
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The neurons of this organism and their connections have been drawn in detail
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The team found that two different aspects of dopamine's role, that is, between learning and motivation, seem to be closely related, and revealed how a dopamine pathway performs two functions at the same time
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The research has been published in "Nature Neuroscience" recently
.

Odor is essential to fruit flies
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The olfactory learning center in the Drosophila brain is called the mushroom body.
Here, three types of neurons are gathered together.
They are Kenyon cells that respond to odors, and output neurons that send signals to other parts of the brain.
There are also neurons that produce dopamine
.

 The brain of Drosophila melanogaster
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The mushroom body is the part in the picture that resembles the handle shape
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｜Image source: Jenett et al.
, 2006 The mushroom is responsible for the "church" fruit flies' association of smell
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Like humans, the dopamine neurons of fruit flies can provide a learning signal that helps them associate a specific smell with a specific result
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When the fruit flies smell a smell and are rewarded by sugar, the rapid release of dopamine will change the strength of the connections between the mushroom neurons, which will help the fruit flies establish new associations and change the future of the fruit flies.
The reaction to this smell
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This can be seen as a guided "long-term teaching" activity.
For example, when fruit flies learn that apple cider vinegar contains delicious sugar, they can help shape their behavior the next time they smell the smell of apple cider vinegar
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But the team also noticed that even in the absence of rewards, dopamine continues to send signals
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The same dopamine neurons related to learning are also closely related to the behavior of animals, and they fire frequently when they move
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This brings up the question, do these neurons represent certain aspects of movement (how do the legs of the drosophila move), or are they related to other aspects (the target that the drosophila moves)? To find out, the research team developed a virtual reality system in which fruit flies can explore in an olfactory environment, walking on a small ball similar to a treadmill, while a microscope on their head monitors their brains.
Ministry activities
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 Researchers recorded the activity of neurons in their mushroom bodies when fruit flies received a drop of sucrose (below)
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｜Image source: Rockefeller University A stream of air will blow out a scent through a small tube
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When the fruit fly smells this attractive smell, such as the smell of apple cider vinegar, it redirects and starts to move against the wind toward the source
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Using this system, researchers can monitor the brain activity of fruit flies under different conditions
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They found that only when the flies were pursuing purposeful tracking, the activity of dopamine neurons closely reflected the movement that was taking place, but not when they "roamed" aimlessly
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The activity of these dopamine neurons not only encodes the mechanism of movement, but also reflects the motivation or goal behind the behavior of the fruit fly in real time
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When the researchers inhibited the activity of dopamine neurons, these little guys reduced their tracking of odors, even when they were hungry, that is, when they were more interested in food-related odors
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In contrast, activating the neurons in fruit flies that are already "satisfied with food" and are not interested in food will prompt them to actively pursue smells
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In general, the results of the study reveal the mechanism by which a dopamine pathway performs two functions: it not only provides guiding signals, but also guides future behaviors through learning
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At the same time, it also transmits motivational signals, thereby quickly shaping the ongoing behavior
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In other words, the same dopamine neuron is not only responsible for "long-term teaching", but also brings a constant reinforcement to encourage fruit flies to continue beneficial behaviors
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This result gives us a deeper understanding of how a single pathway can produce different forms of flexible behavior
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The researchers said that the next step is to understand the "cooperation" and "communication" between them and other neurons.
For example, how do other neurons know what a surge of dopamine means at any moment
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Ruta said that one possibility is that learning is a more continuous and dynamic process than generally thought
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On a short time scale, animals are constantly evaluating their behaviors as they take each step.
They not only learn the final association, but also learn every move that guides them to reach the final goal
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