Dopamine

• 1. Introduction

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  Dopamine sits at the center of some of the most urgent questions in modern medicine: why do people become addicted, how does Parkinson's disease destroy movement, and what actually happens in the brain when something feels good. Yet despite decades of research, popular culture still gets it wrong. Most people believe dopamine is simply the brain's pleasure chemical. The current scientific view tells a more complicated story. What dopamine actually does is signal motivational salience, the degree to which an outcome seems desirable or worth avoiding, and that difference between pleasure and motivation turns out to matter enormously. This documentary follows dopamine from its first synthesis in a London laboratory in 1910, through the brain pathways that shape addiction and mental illness, and out into the body's peripheral systems, where the same molecule quietly regulates kidney function, immune responses, and even insulin production.

• 2. A Molecule Built from Food

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  Tyrosine, one of the amino acids found in nearly every protein a person eats, is the raw material from which dopamine is made. The body first converts tyrosine into L-DOPA using the enzyme tyrosine hydroxylase, and then strips a carboxyl group from L-DOPA using a second enzyme, aromatic L-amino acid decarboxylase, producing dopamine. Phenylalanine, an essential amino acid that must come from food, can also begin the chain by converting first into tyrosine.

  Dopamine itself cannot cross the blood-brain barrier, the protective layer surrounding the brain, so any dopamine that enters the body through food is useless to the nervous system. The brain must manufacture its own supply locally. Once synthesized, dopamine is loaded into secretory vesicles by a transporter called VMAT2 and held there until a nerve signal triggers its release into the gap between neurons, the synapse.

  After release, the molecule binds to one of five receptor subtypes, labeled D1 through D5, each of which works through a second-messenger system inside the target cell. D1 receptors are the most numerous in the human nervous system; D2 receptors are next most common; D3, D4, and D5 are present in significantly lower numbers. Whether dopamine excites or inhibits a target neuron depends entirely on which receptor types that neuron carries, not on dopamine itself.

  When its work is done, dopamine is either pulled back into the releasing neuron by a transporter and repackaged for reuse, or broken down by enzymes including monoamine oxidase and catechol-O-methyl transferase into an end product called homovanillic acid, which the kidneys filter out and excrete in urine. In clinical research on schizophrenia, plasma levels of homovanillic acid have been used as a proxy for estimating dopamine activity in the brain.

• 3. Mapping the Brain's Dopamine Roads

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  In 1964, researchers Annica Dahlstrom and Kjell Fuxe produced the first systematic map of dopaminergic cell groups in the brain, assigning them labels starting with the letter A. Their scheme placed dopamine-producing neurons in areas A8 through A14, which correspond to the substantia nigra, the ventral tegmental area, the posterior hypothalamus, the arcuate nucleus, the zona incerta, and the periventricular nucleus.

  The substantia nigra, a small midbrain structure, sends a projection called the nigrostriatal pathway to the dorsal striatum. This pathway controls motor function and the learning of new motor skills. The neurons along this route are peculiarly fragile, and when enough of them die, the result is the movement disorder known as parkinsonism.

  The ventral tegmental area, the other major dopamine source, sends fibers along two routes: the mesocortical pathway to the prefrontal cortex, and the mesolimbic pathway to the nucleus accumbens. Together, these form the mesocorticolimbic projection, the network at the heart of reward, motivation, and addiction. The ventral tegmental area also projects to the amygdala, the hippocampus, the cingulate gyrus, and the olfactory bulb.

  In the hypothalamus, the arcuate nucleus and periventricular nucleus form the tuberoinfundibular pathway, which runs to the pituitary gland and controls prolactin secretion. Dopamine is the primary inhibitor of prolactin release from the anterior pituitary; without it, prolactin cells secrete the hormone continuously.

  One of the least-discussed dopamine locations is the retina. Amacrine cells in the eye release dopamine into the surrounding medium specifically during daylight hours, going silent at night. This retinal dopamine boosts the activity of cone cells, which handle color and contrast, while suppressing rod cells, which handle low-light vision.

• 4. Wanting Versus Liking

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  A 1998 study published in Nature used positron emission tomography scans and a radiolabeled compound, 11C-labelled raclopride, to show that playing video games releases dopamine in the human striatum. Dopamine release was positively correlated with how well participants performed the task, and was greatest in the ventral striatum. That study was the first to demonstrate the behavioral conditions under which dopamine is released in humans.

  What those findings illuminate is a distinction researchers call incentive salience, the "wanting" versus "liking" model. Dopamine drives approach behavior, the urge to seek a reward, but it does not directly produce pleasure from consuming one. Animals in which the ventral tegmental dopamine system has been rendered inactive will not seek food and will starve if left alone; but if food is placed directly in their mouths, they consume it and show signs of pleasure. Wanting and liking can come completely apart.

  In human drug addiction, this split becomes clinically visible. Psychostimulants such as methamphetamine and cocaine flood the synapse with dopamine, producing intense increases in wanting and approach behavior, but do not greatly alter expressions of pleasure or change satiation levels. Opiate drugs such as heroin and morphine, by contrast, increase both wanting and liking. As tolerance builds with repeated stimulant use, wanting intensifies while pleasure from the drug actually decreases.

  A clinical study from January 2019 examined this directly by giving participants levodopa (a dopamine precursor), risperidone (a dopamine antagonist), or a placebo, then measuring their pleasure responses to music, including the physical response of musical chills. The study found that manipulating dopamine neurotransmission bidirectionally regulated the hedonic impact of music in human subjects, confirming that dopamine does play a role in some forms of pleasure, even if it is not pleasure's sole driver.

  Microelectrode recordings from animals have added another layer: dopamine neurons in the ventral tegmental area and substantia nigra respond not just to rewards themselves, but to reward prediction errors, signals about whether outcomes were better or worse than expected. Rewards that arrive as predicted produce no extra dopamine pulse; unexpected rewards trigger a burst; and the omission of an expected reward causes dopamine release to drop below baseline. This prediction-error signal drew attention from computer scientists because it closely mirrors a computational method called temporal difference learning.

• 5. When the System Breaks

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  Parkinson's disease results from the loss of dopamine-secreting neurons in the substantia nigra pars compacta, specifically cell groups A8 and A9. The disease is characterized by stiffness, slowing of movement, and trembling of limbs at rest; in advanced stages it progresses to dementia and death. Most cases are idiopathic, meaning no cause can be identified. A variety of insults can produce a similar syndrome, including encephalitis, as depicted in the book and film Awakenings, repeated sports-related concussions, and chemical poisoning by a compound called MPTP.

  The standard treatment, L-DOPA, works because it can cross the blood-brain barrier that dopamine itself cannot penetrate. Once inside the brain, it is converted to dopamine by surviving neurons, partially compensating for the deficit. It is usually co-administered with carbidopa or benserazide, inhibitors that prevent conversion of L-DOPA to dopamine in the body's periphery, so that more of the dose reaches the brain. Long-term use eventually produces side effects including dyskinesia, involuntary movements. In advanced disease the remaining neurons become too few to convert enough L-DOPA regardless of dose.

  Schizophrenia presented a different puzzle. Psychiatrists in the early 1950s found that a class of drugs called typical antipsychotics, beginning with chlorpromazine (Thorazine), reduced psychotic symptoms and led to the release of many patients from institutions. By the 1970s, researchers understood that these drugs worked as antagonists at D2 receptors, giving rise to the dopamine hypothesis of schizophrenia, which proposed that the illness stemmed from excessive dopamine activity. The hypothesis drew support from the observation that dopamine-enhancing stimulants like methamphetamine could induce psychosis in healthy people. Later observations complicated the picture; patients with schizophrenia do not typically show measurably elevated brain dopamine levels, and the hypothesis has evolved toward subtler and more complex formulations. Psychopharmacologist Stephen M. Stahl suggested in 2018 that in many cases of psychosis, three interconnected networks based on dopamine, serotonin, and glutamate contribute to an overexcitation of dopamine D2 receptors in the ventral striatum.

  ADHD involves decreased dopamine activity and impaired cognitive control, affecting attention, inhibitory control, and working memory. Methylphenidate, marketed as Ritalin and Concerta, and amphetamine formulations such as Adderall and Dexedrine treat ADHD by increasing dopamine and norepinephrine in the prefrontal cortex, specifically through indirect activation of D1 and adrenoceptor alpha-2 receptors. One key differentiating factor between these drugs and addictive stimulants is time course. Cocaine can take effect in seconds when inhaled or injected, with effects lasting 5 to 90 minutes. Methylphenidate taken as a pill can take two hours to reach peak blood levels, with effects lasting up to 12 hours depending on formulation.

• 6. Beyond the Brain

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  In the bloodstream, over 95% of the dopamine present in humans is bound up as dopamine sulfate, a conjugate produced by an enzyme acting on free dopamine. Most of this sulfate form is made in the mesenteric organs, and plasma levels typically rise more than fifty-fold after a meal. Dopamine sulfate has no known biological function and is excreted in urine. It is thought to serve as a detoxification mechanism for dopamine absorbed from food.

  The kidneys contain their own dopamine-producing system, located in the cells of the nephron. Tubule cells synthesize dopamine and discharge it locally, where it increases blood supply to the kidneys, raises the glomerular filtration rate, and promotes sodium excretion. Defects in this renal dopamine system reduce sodium excretion and can lead to high blood pressure, and there is strong evidence linking renal dopamine dysfunction to oxidative stress, edema, and hypertension.

  In the pancreas, the situation is layered. The exocrine part secretes dopamine into the small intestine, where it may protect the intestinal lining and slow the rate at which contents move through the digestive system. The endocrine part, specifically the beta cells in the pancreatic islets that produce insulin, contains dopamine receptors; dopamine acts on these receptors to reduce insulin release.

  In the immune system, dopamine acts on receptors on lymphocytes, with the main effect being a reduction in their activation level. Dopamine can also be synthesized and released by immune cells themselves, suggesting a possible channel of communication between the nervous system and immune function.

  As a manufactured drug, dopamine is sold under trade names including Intropin, Dopastat, and Revimine, and appears on the World Health Organization's List of Essential Medicines. It is given intravenously for severe low blood pressure, slow heart rate, and cardiac arrest. Its half-life in plasma is approximately one minute in adults, two minutes in newborn infants, and up to five minutes in preterm infants, so it must be administered as a continuous drip rather than a single injection.

• 7. Discovery and Its Aftermath

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  Dopamine was first synthesized in 1910 by George Barger and James Ewens at Wellcome Laboratories in London. Its name was derived from its chemical structure: it is a monoamine whose precursor in the Barger-Ewens synthesis is 3,4-dihydroxyphenylalanine, abbreviated L-DOPA. The molecule sat largely unexamined for decades before Katharine Montagu identified it in the human brain in 1957.

  The following year, Arvid Carlsson and Nils-Ake Hillarp at the Laboratory for Chemical Pharmacology of the National Heart Institute of Sweden demonstrated that dopamine was not merely an intermediate step on the way to producing norepinephrine and epinephrine, but a neurotransmitter in its own right. Carlsson received the Nobel Prize in Physiology or Medicine in 2000 for this work.

  A separate line of discovery emerged from an unexpected source. Research into adhesive polyphenolic proteins found in mussels led, in 2007, to the finding that a wide variety of materials, when placed in a slightly basic solution containing dopamine, become coated with a layer of polymerized dopamine known as polydopamine. This spontaneous oxidation reaction produces a material that is formally a type of melanin. Polydopamine coatings have been explored for protection against light damage, as capsules for drug delivery, and as substrates for biosensors. Their adhesive properties arise from the same chemistry that lets mussels attach to wet surfaces, and the structure of polydopamine itself remains unknown.

  Dopamine's evolutionary history extends well beyond humans. Dopamine has been reported in the nervous systems of cnidarians including jellyfish, hydra, and some corals, placing the emergence of dopamine as a neurotransmitter at the earliest appearance of the nervous system, over 500 million years ago in the Cambrian Period. There is even a proposal that animals originally acquired the enzymes for dopamine synthesis from bacteria, via horizontal gene transfer that may have occurred when bacteria were incorporated into eukaryotic cells to form what became mitochondria.