One of the more emotional moments of the 1996 summer Olympics in Atlanta occurred at the opening ceremonies, even before the games started.
Muhammad Ali--the former world heavyweight boxing champion and a 1960 Olympic gold medal winner--took the torch that was relayed to him and, with trembling hands, determinedly lit the Olympic flame. His obvious effort reminded the world of the roll Parkinson's disease and related disorders can take on the human nervous system. Ali, who in his championship days had prided himself on his ability to "float like a butterfly, sting like a bee," now had to fight to control his body and steady his feet.
Ali's condition also highlighted the urgent need for better treatments. We cannot claim that a cure is around the corner, but we can offer a glimpse into the considerable progress investigators have made in understanding Parkinson's disease, which afflicts more than half a million people in the U.S. alone. Although still incomplete, this research has recently begun suggesting ideas not only for easing symptoms but, more important, for stopping the underlying disease process.
Parkinson's disease progressively destroys a part of the brain critical to coordinated motion. It has been recognized since at least 1817, when James Parkinson, a British physician, described its characteristic symptoms in "An Essay on the Shaking Palsy,." Early on, affected individuals are likely to display a rhythmic tremor in a hand or foot, particularly when the limb is at rest. (Such trembling has helped convince many observers that Pope John Paul II has the disorder.) As time goes by, patients may become slower and stiffer. They may also have difficulty initiating movements (especially rising from a sitting position), may lose their balance and coordination and may freeze unpredictably, as their already tightened muscles halt altogether.
Nonmotor symptoms can appear as well. These may include excessive sweating or other disturbances of the involuntary nervous system and such psychological problems as depression or, in late stages, dementia. Most of the problems, motor or otherwise. are subtle at first and worsen over time, often becoming disabling after five to 15 years. Patients typically show their first symptoms after age 60.
The motor disturbances have long been known to stem primarily from destruction of certain nerve cells that reside in the brain stem and communicate with a region underlying the cortex. More specifically, the affected neurons are the darkly pigmented ones that lie in the brain stem's substantia nigra ("black substance") and extend projections into a higher domain called the striatum (for its stripes).
As Arvid Carlsson of Gothenburg University reported in 1959, the injured neurons normally help to control motion by releasing a chemical messenger--the neurotransmitter dopamine--into the striatum. Striatal cells, in turn, relay dopamine's message through higher motion-controlling centers of the brain to the cortex, which then uses the information as a guide for determining how the muscles should finally behave. But as the dopamine-producing neurons die, the resulting decline in dopamine signaling disrupts the smooth functioning of the overall motor network and compromises the person's activity. Nonmotor symptoms apparently result mainly from the elimination of other kinds of neurons elsewhere in the brain. What remains unknown, however, is how the various neurons that are lost usually become injured.
Because damage to the substantia nigra accounts for most symptoms, investigators have concentrated on that area. Some 4 percent of our original complement of dopamine-producing neurons disappears during each decade of adulthood, as part of normal aging. But Parkinson's disease is not a normal feature of aging. A pathological process amplifies the usual cell death, giving rise to symptoms after approximately 70 percent of the neurons have been destroyed.
Whether this process is commonly triggered by something in the environment, by a genetic flaw or by some combination of the two is still unclear, although a defect on chromosome 4 has recently been implicated as a cause in some cases.
Research into the root causes of Parkinson's disease has been fueled in part by frustration over the shortcomings of the drugs available for treatment. Better understanding of the nature of the disease process will undoubtedly yield more effective agents.
The first therapeutics were found by chance. In 1867 scientists noticed that extracts of the deadly nightshade plant eased some symptoms, and so doctors began to prescribe the extracts. The finding was not explained until about a century later. By the mid-1900s pharmacologists had learned that the medication worked by inhibiting the activity in the striatuna of acetylcholine, one of the chemical molecules that carries messages between neurons. This discovery implied that dopamine released into the striatum was normally needed, at least in part, to counteract the effects of acetylcholine. Further, in the absence of such moderation, acetylcholine overexcited striatal neurons that projected to higher motor regions of the brain.
Although the acetylcholine inhibitors helped somewhat, they did not eliminate most symptoms of Parkinson's disease; moreover, their potential side effects included such disabling problems as blurred vision and memory impairment. Hence, physicians were delighted when, in the 1960s, the more effective drug levodopa, or L-dopa, proved valuable. This agent, which is still a mainstay of therapy, became available thanks largely to the research efforts of Walter Birkmayer of the Geriatric Hospital Lainz-Vienna, Oleh Hornykiewicz of the University of Vienna, Theodore L. Sourkes and Andre Barbeau of McGill University and George Cotzias of the Rockefeller University.
These and other workers developed L-dopa specifically to compensate for the decline of dopamine in the brain of Parkinson's patients. They knew that dopamine-producing neurons manufacture the neurotransmitter by converting the amino acid tyrosine to L-dopa and then converting L-dopa into dopamine.
Dopamine itself cannot be used as a drug, because it does not cross the bloodbrain barrier--the network of specialized blood vessels that strictly controls which substances will be allowed into the central nervous system. But L-dopa crosses the barrier readily. It is then convetted to dopamine by dopamine-making neurons that survive in the substantia nigra and by nonneuronal cells, called astrocytes and microgila, in the striatum. When L-dopa was introduced, it was hailed for its ability to control symptoms. But over time physicians realized it was far from a cure-all.
After about four years, most patients experience a wearing-off phenomenon: they gradually lose sensitivity to the compound, which works for shorter and shorter increments. Also, side effects increasingly plague many people--among them, psychological disturbances and a disabling "on-off" phenomenon, in which episodes of immobility, or freezing, alternate unpredictably with episodes of normal or involuntary movements. Longer-acting preparations that more closely mimic dopamine release from neurons are now available, and they minimize some of these effects.
As scientists came to understand that L-dopa was not going to be a panacea, they began searching for additional therapies. By 1974 that quest had led Donald B. Calne and his co-workers at the National Institutes of Health to begin treating patients with drugs that mimic the actions of dopamine (termed dopamine agonists). These agents can avoid some of the fluctuations in motor control that accompany extended use of L-dopa, but they are more expensive and can produce unwanted effects of their own, including confusion, dizziness on standing and involuntary motion.
In 1975 our own work resulted in the introduction of selegiline (also called deprenyl) for treatment of Parkinson's disease. This substance, invented by a Hungarian scientist, had failed as a therapy for depression and was almost forgotten. But it can block the breakdown of dopamine, thus preserving its availability in the striatum. Dopamine can be degraded by the neurons that make it as well as by astrocyres and microglia that reside near the site of its release. Selegiline inhibits monoamine oxidase B, the enzyme that breaks down dopamine in the astrocytes and microgila.
Selegiline has some very appealing properties, although it, too, falls short of ideal. For example, it augments thc effects of L-dopa and allows the dose of that drug to be reduced. It also sidesteps the dangers of related drugs that can block dopamine degradation. Such agents proved disastrous as therapies for depression, because they caused potentially lethal disturbances in patients who ate certain foods, such as cheese. In fact, we began exploring selegiline as a treatment for Parkinson's disease partly because studies in animals had implied it would avoid this so-called cheese effect.
Tantalizingly, some of our early findings suggested that selegiline could protect people afflicted with Parkinson's disease from losing their remaining dopamine-producing neurons. A massive study carried out several years ago in the U.S. (known as DATATOP) was unable to confirm or deny this effect, but animal research continues to be highly supportive. Whether or not selegiline itself turns out to be protective, exploration of that possibility has already produced at least two important benefits. It has led to the development of new kinds of enzyme inhibitors as potential treatments not only for Parkinson's disease but also for Alzheimer's disease and depression. And the work has altered the aims of many who study Parkinson's disease, causing them to seek new therapies aimed at treating the underlying causes instead of at merely increasing the level or activitv of dopamine in the striatum (approaches that relieve symptoms but do not prevent neurons from degenerating).
Of course, the best way to preserve neurons is to halt one or more key steps in the sequence of events that culminates in their destruction--if those events can be discerned. In the case of Parkinson's disease, the collected evidence strongly implies (though does nor vet prove) that the neurons that die are, to a great extent, doomed by the excessive accumulation of highly reactive molecules known as oxygen free radicals. Free radicals are destructive because they lack an electron. This state makes them prone to snatching electrons from other molecules, a process known as oxidation. Oxidation is what rusts metal and spoils butter. In the body the radicals are akin to biological bullets, in that they can injure whatever they hit--be it fatrv cell membranes, genetic material or critical proteins. Equally disturbing, by taking electrons from other molecules, one free radical often creates many others, thus amplifying the destruction.
The notion that oxidation could help account for Parkinson's disease was first put forward in the early 1970s by Gerald Cohen and the late Richard E. Heikkila of the Mount Sinai School of Medicine. Studies by others had shown that a synthetic toxin sometimes used in scientific experiments could cause parkinsonian symptoms in animals and that it worked by inducing the death of dopamine-producing neurons in the substantia nigra. Cohen and Heikkila discovered that the drug poisoned the neurons by inducing formation of at least two types of free radicals.
Some of the most direct proof that free radicals are involved in Parkinson's disease comes from examination of the brains of patients who died from the disorder. We and others have looked for "fingerprints" of free radical activity in the substantia nigra, measuring the levels of specific chemical changes the radicals are known to effect in cellular components. Many of these markers are highly altered in the brains of Parkinson's patients. For instance, we found a significant increase in the levels of compounds that form when fatty components of cell membranes are oxidized.
Circumstantial evidence is abundant as well. The part of the substantia nigra that deteriorates in Parkinson's patients contains above-normal levels of substances that promote free radical formation. (A notable example, which we have studied intensively, is iron.) At the same time, the brain tissue contains unusually low levels of antioxidants, molecules involved in neutralizing flee radicals or preventing their formation.
Researchers have also seen a decline in the activity of an enzyme known as complex I in the mitochondria of the affected neurons. Mitochondria are the power plants of cells, and complex I is part of the machinery by which mitochondria generate the energy required by cells. Cells use the energy for many purposes, including ejecting calcium and other ions that can facilitate oxidative reactions. When complex I is faulty, energy production drops, free radical levels rise, and the levels of some antioxidants fall--all of which can combine to increase oxidation and exacerbate any other cellular malfunctions caused by an energy shortage.
What sequence of events might account for oxidative damage and related changes in the brains of people who suffer from Parkinson's disease?
Several ideas have been proposed. One of the earliest grew out of research following up on what has been called "The Case of the Frozen Addicts."
In 1982 J. William Langston, a neurologist at Stanford University, was astonished to encounter several heroin addicts who had suddenly become almost completely immobile after taking the drug. It was as if they had developed severe Parkinson's disease overnight. While he was exploring how the heroin might have produced this effect, a toxicologist pointed him to an earlier, obscure report on a similar case in Bethesda, Md. In that instance, a medical student who was also a drug abuser had become paralyzed by a homemade batch of meperidine (Demerol) that was found, by Irwin J. Kopin and Sanford P. Markey of the NIH, to contain an impurity called MPTP. This preparation had destroyed dopamine-making cells of his substantia nigra. Langston, who learned that the drug taken by his patients also contained MPTP, deduced that the impurity accounted for the parkinsonism of the addicts.
His hunch proved correct and raised the possibility that a more common substance related to MPTP was the triggering cause in classical cases of Parkinson's disease. Since then, exploration of how MPTP damages dopamine-rich neurons has expanded understanding of the disease process in general and has uncovered at least one pathway by which a toxin could cause the disease.
Scientists now know that MPTP would be harmless if it were not altered by the body. It becomes dangerous after passing into the brain and being taken up by astrocytes and microgila. These cells feed the drug into their mitochondria, where it is converted (by monoamine oxidase B) to a more reactive molecule and then released to do mischief in dopamine-making neurons of the substantia nigra. Part of this understanding comes from study in monkeys of selegiline, the monoamine oxidase B inhibitor. By preventing MPTP from being altered, the drug protects the animals from parkinsonism. In the absence of a protective agent, altered MPTP will enter nigral neurons, pass into their mitochondria and inhibit the complex I enzyme.
This action will result, as noted earlier, in an energy deficit, an increase in free radical production and a decrease in antioxidant activity--and, in turn, in oxidative damage of the neurons. In theory, then, an MPTP-like chemical made naturally by some people or taken up from the environment could cause Parkinson's disease through a similar process. Many workers have sought such chemicals with little success. Most recently, for instance, brain chemicals known as beta carbolines have attracted much attention as candidate neurotoxins, but their levels in the brains of Parkinson's patients appear to be too low to account for the disease. Given that years of study have not yet linked any known toxin to the standard form of Parkinson's disease, other theories may more accurately describe the events that result in excessive oxidation in patients with this disorder.
Another hypothesis that makes a great deal of sense places microglia--the brain's immune cells--high up in the destructive pathway. This concept derives in part from the discovery, by Patrick L. McGeer of the University of British Columbia and our own groups, that the substantia nigra of Parkinson's patients often contains unusually active microglia. As a rule, the brain blocks microglia from becoming too active, because in their most stimulated state, microgila produce flee radicals and behave in other ways that can be quite harmful to neurons [see "The Brain's Immune System," by Wolfgang J. Streit and Carol A. Kincaid-Colton; SCIENTIFIC AMERICAN, November 1995]. But if something, perhaps an abnormal elevation of certain cytokines (chemical messengers of the immune system), overcame that restraint in the substantia nigra, neurons there could well be hurt. Studies of dopamine-making neurons conducted by a number of laboratories have recently converged with research on microgila to suggest various ways that activated microglia in the substantia nigra could lead to oxidative damage in neurons of the region. Most of these ways involve production of the free radical nitric oxide.
For example, overactive microglia are known to produce nitric oxide, which can escape from the cells, enter nearby neurons and participate in reactions that generate other radicals; these various radicals can then disrupt internal structures [see "Biological Roles of Nitric Oxide," by Solomon H. Snyder and David S. Bredt; SCIENTIFIC AMERICAN, May 1992]. Further, nitric oxide itself is able to inhibit the complex I enzyme in mitochondria; it can thus give rise to the same oxidative injury that an MPTP-like toxin could produce.
If these actions of nitric oxide were not devastating enough, we have found that both nitric oxide and another free radical (superoxide) emitted by overactive microglia can free iron from storehouses in the brain--thereby triggering additional oxidative cascades. We have also demonstrated that iron, regardless of its source, can react with dopamine and its derivatives in at least two ways that can further increase free radical levels in dopamine-synthesizing cells.
In one set of reactions, iron helps dopamine to oxidize itself. Oxidation of dopamine converts the molecule into a new substance that nigral cells use to construct their dark pigment, neuromelanin. When iron levels are low, neuromelanin serves as an antioxidant. But it becomes an oxidant itself and contributes to the formation of free radicals when it is bound by transition metals, especially iron. In support of the possibility that the interaction of iron and neuromelanin contributes to Parkinson's disease, we and our colleagues have shown that the pigment is highly decorated with iron in brains of patients who died from the disease: in contrast, the pigment lacks iron in brains of similar individuals who died from other causes.
In the other set of dopamine-related reactions, iron disrupts the normal sequence by which the neurotransmitter is broken down to inert chemicals. Neurons and microgila usually convert dopamine to an inactive substance and hydrogen peroxide, the latter of which becomes water. When iron is abundant, though, the hydrogen peroxide is instead broken down into molecular oxygen and a free radical. Dopamine's ability to promote free radical synthesis may help explain why dopamine-making neurons are particularly susceptible to dying from oxidation. This ability has also contributed to suspicion that l-dopa, which increases dopamine levels and eases symptoms, may, ironically, damage nigral neurons. Scientists are hotly debating this topic, although we suspect the concern is overblown.
In brief, then, overactive microgila could engender the oxidative death of dopamine-producing neurons in the substantia nigra by producing nitric oxide, thereby triggering several destructive sequences of reactions. And iron released by the nitric oxide or other free radicals in the region could exacerbate the destruction. As we have noted, brain cells do possess molecules capable of neutralizing free radicals. They also contain enzymes that can repair oxidative damage. But the protective systems are less extensive than those elsewhere in the body and, in any case, are apparently ill equipped to keep up with an abnormally large onslaught of oxidants. Consequently, if the processes we have described were set off in the substantia nigra, one would expect to see ever more neurons fade from the region over time, until finally the symptoms of Parkinson's disease appeared and worsened.
Actually, any trigger able to induce an increase in nitric oxide production or iron release or a decrease in complex I activity in the substantia nigra would promote Parkinson's disease. Indeed, a theory as plausible as the microgila hypothesis holds that excessive release of the neurotransminer glutamate by neurons feeding into the striatum and substantia nigra could stimulate nitric oxide production and iron release. Excessive glutamate activity could thus set off the same destructive cascade hypothetically induced by hyperactive microgila. Overactive glutamate release has been implicated in other brain disorders, such as stroke. No one vet knows whether glutamate-producing neurons are overactive in Parkinson's disease, but circumstantial evidence implies they are.
Other questions remain as well. Researchers are still in the dark as to whether Parkinson's disease can arise by different pathways in different individuals. Just as the engine of a car can fail through any number of routes, a variety of processes could presumably lead to oxidative or other damage to neurons of the substantia nigra. We also have few clues to the initial causes of Parkinson's disease--such as triggers that might, say, elevate cytokine levels or cause glutamate-emitting cells to be hyperactive.
In spite of the holes, ongoing research has suggested intriguing ideas for new therapies aimed at blocking oxidation or protecting neurons in other ways.
If the scenarios we have discussed do occur alone or together, it seems reasonable to expect that agents able to quiet microglia or inhibit glutamate release in the substantia nigra or striatum would protect neurons in at least some patients. The challenge is finding compounds that are able to cross the blood-brain barrier and produce the desired effects without, at the same time, disturbing other neurons and causing severe side effects. One of us (Riederer) and his colleague Johannes Kornhuber of the University of Wurzburg have recently demonstrated that amantadine, a long-standing anti-Parkinson's drug whose mechanism of action was not known, can block the effects of glutamate. This result suggests that the compound might have protective merit. Another glutamate blocker -- dextromethorphan -- is in clinical trials at the NIH.
Drugs could also be protective if they halted other events set in motion by the initial triggers of destruction. Iron chelators (which segregate iron and thus block many oxidative reactions), inhibitors of nitric oxide formation and anti-oxidants are all being considered. Such agents have been shown to protect dopamine-producing neurons of the substantia nigra from oxidative death in animals. On the other hand, the same human DATATOP trial that cast doubt on selegiline's protective effects found that vitamin E, an antioxidant, was ineffective. But vitamin E may have failed because very little of it crosses the blood-brain barrier or because the doses tested were too low. Antioxidants that can reach the brain deserve study; at least one such compound is in clinical trials at the NIH. Regardless of the cause of the neuronal destruction, drugs that were able to promote regeneration of lost neurons would probably be helpful as well.
Sudies of animals suggest that such substances could, indeed, be effective in the human brain. Researchers at several American facilities are now testing putting a molecule called glial-derived neurotrophic factor (GDNF) directly into the brain of patients. Efforts are also under wav to find smaller molecules that can be delivered more conveniently (via pill or injection) yet would still activate neuronal growth factors and neuronal growth in the brain. One agent, Rasagiline, has shown promise in animal trials and is now being tested in humans. Some studies imply that the nicotine in tobacco might have a protective effect, and nicotinelike drugs are being studied in the laboratory as potential therapies. Patients, however, would be foolish to take up smoking to try to slow disease progression. Data on the value of smoking to retard the death of dopamine neurons are equivocal, and the risks of smoking undoubtedly far outweigh any hypothetical benefit.
As work on protecting neurons advances, so does research into compensating for their decline. One approach being perfected is the implantation of dopamine-producing cells. Some patients have been helped. But the results are variable, and cells available for transplantation are in short supply. Further, the same processes that destroyed the original brain cells may well destroy the implants. Other approaches include surgically destroying parts of the brain that function abnormally when dopamine is lost. This surgery was once unsafe but is now being done more successfully. The true aim of therapy for Parkinson's disease must ultimately be to identify the disease process long before symptoms arise, so that therapy can be given in time to forestall the brain destruction that underlies patients' discomfort and disability. No one can say when early detection and neural protection will become a reality, but we would not be surprised to see great strides made on both fronts within a few years. In any case, researchers cannot rest easy until those dual objectives are met.
Moussa B. H. Youdim and Peter Riederer have collaborated since 1974. Youdim, a pioneer in the development of monoamine oxidase inhibitors for the treatment of Parkinson's disease and depression, is professor of pharmacology at Technion-Israel Institute of Technology in Haifa, Israel.
He is also director of the Eve Topf and U.S. National Parkinson's Disease Foundation's Centers of Excellence for Neurodegenerative Diseases, both at Technion, and a Fogarty Scholar in Residence at the U.S. National Institutes of Health, where he spends three months every year. Riederer heads the Laboratory of Clinical Neurochemistry and is professor of clinical neurochemistry at the University of Wurzburg in Germany. The authors shared the Claudius Galenus Gold Medal for the development of the anti-Parkinson's drug selegiline.