2024-09-21

The Variabilities of Dopamine (₯) - PART III: CL:0000700 & SIO:000823

Fig 1: dopaminergic neurons (DAN) & curiosity
In the complex drama of the brain, there are many characters, but dopaminergic neurons (DAN) take the lead role. These neurons are always on the lookout for new things and solving puzzles, like a brainy Sherlock Holmes. Dopamine, the neurotransmitter, is their trusty sidekick, helping them stay curious and active.

But, like in any good story, there's a twist. Over time, these once-energetic neurons start to lose their zest for new experiences. This decline in curiosity mirrors our own aging. It raises an important question: what happens in the brain to cause this loss? What makes our inner Sherlock Holmes lose interest in the unknown?

Fig 2: underlying neural mechanisms of curiosity
Did you know? Inside your brain, there's a cell that looks like a thin ginseng root with branches that resemble ginseng whiskers. This fascinating cell is the dopaminergic neuron (DAN). When we see something new, it lights up the brain's circuits, making us want to explore, learn, and discover. DAN is like a detective in the brain, always drawn to new and unknown things, eager for excitement and new knowledge.

However, as we get older, this curiosity fades. Like a firework losing its spark, older people gradually lose interest in new things, both social and inanimate. Why is DAN so fascinated by new experiences? And as we age, what changes in DAN make it lose its curiosity?

It turns out that dopamine, a very versatile neurotransmitter, is not only linked to our happiness, desires, and goals but also closely connected to our "epistemic curiosity" and "empathic curiosity". As you read this, your brain is reacting, just like it would if you were really thirsty or hungry. The dopamine-sensitive areas in your midbrain light up—a phenomenon we call "epistemic curiosity." This curiosity drives us to explore and understand the world around us.

Fig 3: The knowledge tree of curiosity: SIO:000823

Another type of curiosity happens when we interact with other people. When we feel comfortable in social settings and try to understand what others are thinking and feeling, our brains release a lot of dopamine. This is called "empathic curiosity." Today, we are exploring how our curious detective, DAN, and dopamine play a role in triggering these two kinds of curiosity.

The Chinese version of Curiosity’s Knowledge Tree was added after the article was published on 2024/09/06. At the time of writing, popular science articles PART I and II based on the ontology hadn’t been published yet, so A.H. wasn’t sure if readers would like them. That’s why she wrote the articles on dopamine and curiosity using a traditional style. After Parts I and II were published, and readers loved them, she added information about curiosity by the Semanticscience Integrated Ontology (SIO) as shown in Figure 3. Curiosity is described as a positive emotion, related to desire.

To understand the connection, we need to look at dopamine, a chemical released by special neurons (called DAN) in the brain areas known as the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). Studies using brain scans have shown that high levels of curiosity and new experiences increase activity in these areas. A 2023 study published in “Communications Biology” (a sub-journal of “Nature”) titled “Reduction in the activity of VTA/SNc dopaminergic neurons underlies aging-related decline in novelty seeking. ” looked at the brain mechanisms behind curiosity. It explained why our interest in new things decreases as we age. Through experiments with mice, the study showed that the reduction in activity of VTA and SNc neurons is a key factor in the decline of curiosity due to aging.

This
Fig 4: experiment on social and inanimate curiosity
study used mice to explore the relationship between brain neurons and curiosity, showing how these neurons affect curiosity. Figure 4 shows the results of testing curiosity about social interactions (making new mouse friends) and inanimate objects (like shiny toys) in young mice and older rats. Scientists observed that young mice are more curious and explore these new experiences more frequently than older rats. Over time, young mice's interest in new friends increases, while older rats show stable but lower curiosity levels.

In general, older rats are less curious compared to young mice. Researchers measured and analyzed the activity of dopamine neurons in the brain, finding that older rats have less spontaneous neuron activity.

They also used specific drugs to control dopamine release in the brain. When they reduced dopamine activity in older rats, the rats became less curious. When they increased dopamine activity in young mice, the mice became more curious. The study concluded that the number of dopamine neurons decreases with age, leading to less dopamine and lower curiosity in older individuals. In humans, we lose about 6% of certain neurons every decade, which reduces dopamine availability.


In simple terms, young and healthy dopaminergic neurons (DAN) can send signals on their own, without needing extra input. This means they can regularly fire up spontaneously. As we get older, these neurons become less active and send fewer signals, which is linked to a decrease in curiosity. It's like the aging brain dims the lights of curiosity. However, by boosting the activity of these neurons and increasing dopamine release, we might be able to prevent or improve the decline in curiosity and thinking skills caused by aging or Alzheimer's disease.

In the 1950s, when Einstein was 63, he wrote to a friend saying, “People like you and I, though mortal of course like everyone else, do not grow old no matter how long we live...[We] never cease to stand like curious children before the great mystery into which we were born.” While few can keep Einstein's level of curiosity, which drives exploration and learning, we now know that curiosity is closely linked to the chemical dopamine and the curious DAN neurons.

With this new knowledge, maybe the detective in our brains won't retire early. Instead, we might grow old gracefully, keeping our curiosity alive. This understanding gives us hope that our inner detective will keep looking for new and interesting things, allowing us to age with wisdom and wonder.

Fig 5: Different representative signals of dopamine, in addition to happiness, desire, motivational goals, etc., there is also curiosity

 

 

 

 

 

 

 

 

 




 

 

 

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