Skeptics are silenced
In 1960, however, the medical community was not yet ready to accept that dopamine was anything but a boring intermediate between levodopa and noradrenaline. At a prestigious London symposium, Dr. Carlsson and his two colleagues presented their hypothesis that dopamine may be a neurotransmitter, thus implicating it in Parkinson’s disease. They were met with harsh criticism. Some of the experts said levodopa was nothing more than a poison. Dr. Carlsson later recalled facing “a profound and nearly unanimous skepticism regarding our points of view.”
That would soon change. Dr. Hornykiewicz, the biochemist who had earlier discovered dopamine’s BP-lowering effects, tested Dr. Carlsson’s ideas using the postmortem brains of Parkinson’s disease patients. It appeared Dr. Carlsson was right: Unlike in healthy brains, the striatum of patients with Parkinson’s disease contained almost no dopamine whatsoever. Beginning in 1961, in collaboration with neurologist Walther Birkmayer, MD, Hornykiewicz injected levodopa into 20 patients with Parkinson’s disease and observed a “miraculous” (albeit temporary) amelioration of rigidity, motionlessness, and speechlessness.
By the late 1960s, levodopa and dopamine were making headlines. A 1969 New York Times article described similar stunning improvements in patients with Parkinson’s disease who were treated with levodopa. A patient who had arrived at a hospital unable to speak, with hands clenched and rigid expression, was suddenly able to stride into his doctor’s office and even jog around. “I might say I’m a human being,” he told reporters. Although the treatment was expensive – equivalent to $210 in 2022 – physicians were deluged with requests for “dopa.” To this day, levodopa remains a gold standard in the treatment of Parkinson’s disease.
Still misunderstood
The history of dopamine, however, is not only about Parkinson’s disease but extends to the treatment of schizophrenia and addiction. When in the1940s a French military surgeon started giving a new antihistamine drug, promethazine, to prevent shock in soldiers undergoing surgery, he noticed a bizarre side effect: the soldiers would become euphoric yet oddly calm at the same time.
After the drug was modified by adding a chlorine atom and renamed chlorpromazine, it fast became a go-to treatment for psychosis. At the time, no one made the connection to dopamine. Contemporary doctors believed that it calmed people by lowering body temperature (common treatments for mental illness back in the day included swaddling patients in cold, wet sheets). Yet just like reserpine, chlorpromazine produced range of nasty side effects that closely mimicked Parkinson’s disease. This led a Dutch pharmacologist, Jacques van Rossum, to hypothesize that dopamine receptor blockade could explain chlorpromazine’s antipsychotic effects – an idea that remains widely accepted today.
In the 1970s, dopamine was linked with addiction through research on rodents, and this novel idea caught people’s imagination over the coming decades. A story on dopamine titled, “How We Get Addicted,” made the cover of Time in 1997.
Yet as the dopamine/addiction connection became widespread, it also became oversimplified. According to a 2015 article in Nature Reviews Neuroscience, a wave of low-quality research followed – nonreplicated, insufficient – which led the authors to conclude that we are “addicted to the dopamine theory of addiction.” Just about every pleasure under the sun was being attributed to dopamine, from eating delicious foods and playing computer games to sex, music, and hot showers. As recent science shows, however, dopamine is not simply about pleasure – it’s about reward prediction, response to stress, memory, learning, and even the functioning of the immune system. Since its first synthesis in the early 20th century, dopamine has often been misunderstood and oversimplified – and it seems the story is repeating itself now.
In one of his final interviews, Dr. Carlsson, who passed away in 2018 at the age of 95, warned about playing around with dopamine and, in particular, prescribing drugs that have an inhibitory action on this neurotransmitter. “Dopamine is involved in everything that happens in our brains – all its important functions,” he said.
We should be careful how we handle such a delicate and still little-known system.
A version of this article first appeared on Medscape.com.