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Parkinson's Disease & the Paradox of Dopamine



Dopamine gets talked about constantly, frequently in the context of ADHD, as a"reward" or "motivation" chemical, and/or in the context of "dopamine-seeking." Similarly, Parkinson's Disease is frequently considered a dopamine disease. After exploring inflammation in part 1 of this series, this may be starting to appear as a shorthand that's so oversimplified, it's basically wrong. I will offer there is a reason each of these have come to life, but, especially for Parkinson's disease (PD) there are some interesting things that come up when you look closer at this picture. The same molecule that dopaminergic neurons exist to produce can, under the wrong conditions, become the very thing that damages them.


Here's how dopamine gets made and where the process can go sideways.


What Dopaminergic Neurons Actually Do

Dopaminergic neurons do three distinct jobs, and it's worth separating them because each one is a different point where things can go wrong.


They synthesize dopamine.

This happens in two steps, both in the cytosol (the fluid interior of the cell, outside the vesicles and mitochondria). Tyrosine hydroxylase, the rate-limiting enzyme of the pathway, first converts the amino acid tyrosine into L-DOPA, the bottleneck step that determines how fast the whole process runs. Second, another enzyme (aromatic amino acid decarboxylase, AADC) converts L-DOPA into dopamine itself.


They package it.

This is the step most people never hear about, and it turns out to be the most important one for understanding disease risk. Newly made dopamine doesn't just float around inside the cell. It gets pulled into synaptic vesicles by a transporter called VMAT2 (vesicular monoamine transporter 2), essentially tiny storage containers. Dopamine molecules spend most of their lifespan stored in these vesicles, waiting to be released, and very little time loose in the cytosol or outside the cell. That detail matters more than it sounds like it should.


They release it.

When the neuron fires, vesicles move to the cell membrane and release their dopamine into the synapse, where it can bind receptors on the next neuron and do its actual signaling job.


Dopaminergic neurons make dopamine, store it, and release it. All three functions live in the same cell.

From Tyrosine to Dopamine Release

So let's summarize, because Part 3 and 4 will be treatment focused!

Dopamine is made in dopaminergic neurons.


  1. We start from Tyrosine (Get your protein!)

  2. Tyrosine is converted L-DOPA via Tyrosine Hydroxylase

    • Recall, the I Love Lucy principle (Not official terminology)-- if the Lucy and Ethel can't handle the amount of chocolate, i.e. Tyrosine Hydroxylase can't keep up, more Tyrosine isn't going to help.


  3. L-DOPA is converted to Dopamine via Aromatic L-amino acid decarboxylase (AADC), also called DOPA decarboxylase (DDC)

    • Note, the DDC activity is markedly lower in PD, and tracks with progression of the illness. Note, this is because there are less dopaminergic neurons in general.

    • Not less DDC within the dopaminergic neurons that exist. If anything, neurons in folks with PD likely have more DDC activity in trying to compensate.

  4. Dopamine is Trafficked

    • This is a key component, because the emerging narrative over years of research is that free floating dopamine can be toxic to dopaminergic neurons. AND

    • Vesicular function in PD is impaired. You're not storing/ trafficking well.

      • Genetics reinforces this as causal, not incidental. In part 1, we talked about genetics. LRRK2 mutations (one of the major familial PD genes) disrupt vesicle trafficking directly.

      • Think of VMAT2 as the vesicle filler. VMAT2 is more for storage but it interfaces with trafficking, too. Certain VMAT2 gene variants that increase its own expression (more VMAT2) are associated with lower PD risk-- Think of it as making the most of the system you have. If you have a faulty LRRK2, the whole trafficking system will be diminished. If you have faulty LRRK2 and VMAT2, it would be worse. So it should come as no surprise that variants that reduce VMAT2 raise risk. That's a dose-response relationship in the genetics that argues this is a real causal lever, not just a downstream marker of disease.

  5. Dopamine is Stored for Use.

    • So imagine you have a big area to organize all or vesicles of dopamine. What do you do? You'd probably want shelves. You think of a mail room-- you have shelves. But what if someone brought you shelves... and then more shelves that are kind of crappy... okay, let's just take away the crappy and keep the good ones. But what if no one could take away the crappy shelves. Instead, you just get more and more and more shelves... and suddenly your scaffolding starts to drown you. When your body has a GBA1 mutation, your cells' lysosomes struggle to take away the old shelving, and you end up with excess alpha-synuclein (Lewy bodies).

  6. Technically Trafficking Again to the Membrane This is still LRRK2 territory.

  7. Dopamine is Released

    • Release is not generally an issue in Parkinson's minus the storage issue.

  8. Cleanup Crew

    • It's already implied above, but in the case of GBA1, your cleanup crew isn't functioning properly.

    • The reactive byproducts and the shelves start to sit. Now you have a hoarder-house situation we'll talk more about below.

 

Friendly fun homework assignment. I tried to over some of the top-topics in the first post. Go back and have a look at those supplements. OR Chillax, watch I Love Lucy, and we'll talk about them again later. :)


Dopamine as a Double-Edged Molecule

Here's the part that doesn't make it into most explainers. Properly packaged dopamine, sitting inside a vesicle, is inert and safe. The problem starts when dopamine ends up loose in the cytosol instead, which can either be because it hasn't been packaged into a vesicle yet or because it's leaked back out of one.


Free cytosolic dopamine is chemically unstable. It readily oxidizes, either spontaneously or through breakdown by the enzyme monoamine oxidase (MAO), and that oxidation cascade is where the trouble starts. The process generates a sequence of reactive byproducts: dopamine quinones, then a specific one called aminochrome, and eventually neuromelanin, the dark pigment that gives the substantia nigra its color. Aminochrome formation is a completely normal, harmless part of healthy neuron function, it's not inherently pathological. The problem is what happens when the cell's protective systems for handling it get overwhelmed or fail.


Reactive By-Products

One, they react. This is an important chemistry lesson. Being reactive makes a molecule a good signaler. It's like choosing between an introvert or an extrovert as the person who hypes your business. Sooner or later, someone needs to talk to someone. So reactivity is not always bad. Sometimes it's just what you want. And... sometimes it's too much. It's too many shelves and it's too much reactivity,


Dopamine o-quinone forms adducts that inactivate mitochondrial complex I and III of the electron transport chain, the same complexes implicated in the paraquat and MPTP mechanisms discussed elsewhere in this series. This is a direct, mechanistic link between "dopamine handling gone wrong" and "mitochondrial dysfunction" as not two separate problems, the same damage pathway.


These oxidation products also promote alpha-synuclein misfolding into the specific toxic oligomer forms implicated in Lewy body pathology. This is the second bridge: dopamine oxidation connects directly to the alpha-synuclein aggregation story as well, not just the mitochondrial story. It's not just a lot of perfectly good shelves in the way anymore. They are becoming Lewy bodies.


The evidence here conveys a genuine causal mechanism, not just something that correlates with disease. It comes from engineered mouse models. Mice engineered to lack VMAT2, meaning dopamine can't be properly packaged into vesicles, develop severe motor deficits and profound neurodegeneration purely from that packaging failure, accompanied by clear markers of increased dopamine oxidation. And in a separate line of research, combining elevated dopamine levels with alpha-synuclein expression in aged mice caused progressive neurodegeneration that neither factor caused on its own, a genuine synergy between the two mechanisms rather than two independent problems running in parallel... in engineered mice.


So my other caveat is there's a saying among biologists: "What is true in clean mice is true in clean mice." Meaning, it doesn't absolutely mean it will be true in humans. Not useless information.


Why This Matters for the Bigger Picture

This gives you a fourth answer to the recurring question in this series: why the substantia nigra specifically? Dopaminergic neurons there aren't just handling unusually high metabolic demand and low antioxidant buffering, they're also handling a neurotransmitter that is actively toxic to them if it's not perfectly compartmentalized. No other neuron type carries quite this particular structural risk, because no other neuron type has to manage this molecule at this scale.


It also reframes something worth sitting with: dopamine oxidation and neuromelanin formation happen in every healthy person's dopaminergic neurons, all the time, as a normal part of aging.



That's a meaningfully different, more precise story than "dopamine builds up and that's bad." What you want to ensure is your body has the ability maintain the balance.


 

Dr. Diane Stanley is a doctor of acupuncture and Chinese medicine. Blog content is for informational and educational purposes only and does not constitute medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making changes to your health routine.

 


References

  1. Segura-Aguilar, J., Paris, I., Muñoz, P., et al. Protective and toxic roles of dopamine in Parkinson's disease. Journal of Neurochemistry, 129(6), 898-915 (2014). https://doi.org/10.1111/jnc.12686

  2. Alter, S., Lenzi, G. M., Bernstein, A. I., & Miller, G. W. Vesicular Integrity in Parkinson's Disease. Current Neurology and Neuroscience Reports, 13(7) (2013). https://doi.org/10.1007/s11910-013-0362-3

  3. Herrera Ronda, A., Muñoz, P., Steinbusch, H. W. M., & Segura-Aguilar, J. Are Dopamine Oxidation Metabolites Involved in the Loss of Dopaminergic Neurons in the Nigrostriatal System in Parkinson's Disease? ACS Chemical Neuroscience, 8(4), 702-711 (2017). https://doi.org/10.1021/acschemneuro.7b00034

  4. Zhang, S., Wang, R., & Wang, G. Impact of Dopamine Oxidation on Dopaminergic Neurodegeneration. ACS Chemical Neuroscience, 10(2), 945-953 (2018). https://doi.org/10.1021/acschemneuro.8b00454

  5. Mor, D. E., Daniels, M. J., & Ischiropoulos, H. The usual suspects, dopamine and alpha-synuclein, conspire to cause neurodegeneration. Movement Disorders, 34(2), 167-179 (2019). https://doi.org/10.1002/mds.27607

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