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Go to Brilliant.org/SciShow to check out their course on Cryptocurrency. [♪♪♪Intro♪♪♪]. In the 1990s, doctors noticed something unusual happening in the islands of Guadeloupe, Guam and New Caledonia, as well as in Caribbean communities in the UK.

Patients were presenting with stiffness, difficulty with movement, and dementia -- as well as tremors and hallucinations. These symptoms were similar to what we see in Parkinson's disease, a progressive condition that affects motor and mental abilities. But not quite the same.

And doctors were able to tie the condition to… a fruit. These patients were diagnosed with atypical parkinsonism, a cluster of conditions that are similar to classic Parkinson's disease except that they don't respond to the same drugs, and may progress more quickly. Because atypical parkinsonism is, well, atypical compared to classic Parkinson's, doctors suspected that something in the patients' environment was to blame.

So they looked at their diets and discovered they all had a taste for a spiky green fruit called soursop. The scientific name for soursop is Annona muricata, and it goes by a number of aliases: graviola, guanábana, corossol, and Brazilian pawpaw. It's said to have a mild pineapple-y, banana-y, strawberry-y flavor.

And lots of people love it. But unfortunately, many parts of the plant and fruit contain a low dose of a neurotoxin. Scientists suspected a toxin in soursop might be harming people's dopamine-producing neurons.

Dopamine is a neurotransmitter involved in movement, and when those dopamine neurons die, it can lead to Parkinson's. So in a 2002 study, researchers put extract of soursop root bark into a culture of dopamine neurons. After 24 hours, half of the neurons were dead.

The soursop toxin had actually induced programmed cell death in those neurons. Researchers later identified the neurotoxin responsible as annonacin, and it's not just in the plant's root bark. There are about 15 milligrams of annonacin in each soursop fruit and about 36 milligrams in a can of soursop nectar.

In a 2004 study, rats given injections of annonacin developed brain lesions within a month. Though, those rats got intravenous infusions of the purified chemical, which isn't necessarily the same as eating fruit -- plus, rats don't always respond to toxins the way we do. Still, studies like these have established that this chemical isn't the safest thing for us.

There's a twist, however. Because annonacin isn't the only thing hiding out in this fruit. Scientists have identified other, totally separate chemicals in soursop that may protect dopamine neurons and help treat Parkinson's.

The chemicals are called tryptamine-derived alkaloids. They're similar in structure to melatonin, a hormone involved in sleep regulation that can have protective effects on dopamine neurons. In a series of studies, a team based in France synthesized compounds based on ones found in soursop, modified their chemistry, and tested them for their ability to protect dopamine neurons from damage.

Their most promising candidate was a molecule they called PPQ. And it helped to counteract the loss of dopamine cells in a mouse model of Parkinson's. More trials will be needed, but the researchers say PPQ is a promising candidate for Parkinson's treatment.

So soursop – a widely loved but slightly toxic food that can cause Parkinson's symptoms – could also lead us toward a treatment for Parkinson's. Talk about a frenemy. Speaking of frenemies, what's the deal with cryptocurrency?

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