Google the word “dopamine” and you will learn that its nicknames are the “happy hormone” and the “pleasure molecule” and that it is among the most important chemicals in our brains. With The Guardian branding it “the Kim Kardashian of neurotransmitters,” dopamine has become a true pop-science darling – people across the globe have attempted to boost their mood with dopamine fasts and dopamine dressing.
A century ago, however, newly discovered dopamine was seen as an uninspiring chemical, nothing more than a precursor of noradrenaline. It took several stubborn and hardworking scientists to change that view.
Levodopa: An indifferent precursor
When Casimir Funk, PhD, a Polish biochemist and the discoverer of vitamins, first synthesized the dopamine precursor levodopa in 1911, he had no idea how important the molecule would prove to be in pharmacology and neurobiology. Nor did Markus Guggenheim, PhD, a Swiss biochemist, who isolated levodopa in 1913 from the seeds of a broad bean, Vicia faba. Dr. Guggenheim administered 1 g of levodopa to a rabbit, with no apparent negative consequences. He then prepared a larger dose (2.5 g) and tested it on himself. “Ten minutes after taking it, I felt very nauseous, I had to vomit twice,” he wrote in his paper. In the body, levodopa is converted into dopamine, which may act as an emetic – an effect Dr. Guggenheim didn’t understand. He simply abandoned his human study, erroneously concluding, on the basis of his animal research, that levodopa is “pharmacologically fairly indifferent.”
Around the same time, several scientists across Europe successfully synthesized dopamine, but those discoveries were shelved without much fanfare. For the next 3 decades, dopamine and levodopa were pushed into academic obscurity. Just before World War II, a group of German scientists showed that levodopa is metabolized to dopamine in the body, while another German researcher, Hermann Blaschko, MD, discovered that dopamine is an intermediary in the synthesis of noradrenaline. Even these findings, however, were not immediately accepted.
The dopamine story picked up pace in the post-war years with the observation that the hormone was present in various tissues and body fluids, although nowhere as abundantly as in the central nervous system. Intrigued, Dr. Blaschko, who (after escaping Nazi Germany, changing his name to Hugh, and starting work at Oxford [England] University) hypothesized that dopamine couldn’t be an unremarkable precursor of noradrenaline – it had to have some physiologic functions of its own. He asked his postdoctoral fellow, Oheh Hornykiewicz, MD, to test a few ideas. Dr. Hornykiewicz soon confirmed that dopamine lowered blood pressure in guinea pigs, proving that dopamine indeed had physiologic activity that was independent of other catecholamines.
Reserpine and rabbit ears
While Dr. Blaschko and Dr. Hornykiewicz were puzzling over dopamine’s physiologic role in the body, across the ocean at the National Heart Institute in Maryland, pharmacologist Bernard Brodie, PhD and colleagues were laying the groundwork for the discovery of dopamine’s starring role in the brain.
Spoiler alert: Dr. Brodie’s work showed that a new psychiatric drug known as reserpine was capable of fully depleting the brain’s stores of serotonin and – of greatest significance, as it turned out – mimicking the neuromuscular symptoms typical of Parkinson’s disease. The connection to dopamine would be made by new lab colleague Arvid Carlsson, MD, PhD, who would go on to win a Nobel Prize.
Derived from Rauwolfia serpentina (a plant that for centuries has been used in India for the treatment of mental illness, insomnia, and snake bites), reserpine was introduced in the West as a treatment for schizophrenia.
It worked marvels. In 1954, the press lauded the “dramatic” and seemingly “incredible”: results in treating “hopelessly insane patients.” Reserpine had a downside, however. Reports soon changed in tone regarding the drug’s severe side effects, including headaches, dizziness, vomiting, and, far more disturbingly, symptoms mimicking Parkinson’s disease, from muscular rigidity to tremors.
Dr. Brodie observed that, when reserpine was injected, animals became completely immobile. Serotonin nearly vanished from their brains, but bizarrely, drugs that spur serotonin production did not reverse the rabbits’ immobility.
Dr. Carlsson realized that other catecholamines must be involved in reserpine’s side effects, and he began to search for the culprits. He moved back to his native Sweden and ordered a spectrophotofluorimeter. In one of his experiments, Carlsson injected a pair of rabbits with reserpine, which caused the animals to become catatonic with flattened ears. After the researchers injected the animals with levodopa, within 15 minutes, the rabbits were hopping around, ears proudly vertical. “We were just as excited as the rabbits,” Dr. Carlsson later recalled in a 2016 interview. Dr. Carlsson realized that, because there was no noradrenaline in the rabbits’ brains, dopamine depletion must have been directly responsible for producing reserpine’s motor inhibitory effects.