By Giselle M. Petzinger, M.D.
This article was originally published in the Fall 2009 issue of PDF's newsletter, News & Review.
We all know that exercise is helpful to good health. We also know it is good for the heart and the muscles. But can it change the brain, and might these changes make an impact on Parkinson’s disease (PD) symptoms?
The answer to both of these questions is yes.
Can the Brain Change?
We know that in PD, neurons — the brain cells that produce the chemical transmitter dopamine — are damaged and lost. We also know that there is a lag between the time when the loss of neurons begins and the time when Parkinson’s motor symptoms start to show. In fact, by the time most people are diagnosed, nearly 80 percent of their dopamine neurons are already gone.
During this lag time, the brain is actually changing, compensating for the loss of dopamine neurons that occurs during the process of neurodegeneration. In fact, the brain reshapes itself throughout life in response to experience. As children learn motor skills, their brain cells are making connections and this process continues through adulthood.
Scientists call this ability to change and compensate exercise-dependent neuroplasticity. Exercise may have an effect on the brain by driving this compensation. On a day-to-day basis, people with PD who exercise can move more normally than those who do not. We believe that exercise may be contributing to neuroplasticity — helping the brain to maintain old connections, form new ones and restore lost ones. This may actually outweigh the effects of neurodegeneration.
But what has been proven? There is compelling scientific evidence in animal models of PD that intensive exercise can alter the way the brain works and promote recovery. Research in my lab and those of my colleagues at the University of Southern California (USC), is showing how exercise improves walking and other motor skills in people with PD. It is also shedding light on how exercise influences neuroplasticity at the molecular level.
Exercise for Improved Walking and Balance in PD
Scientists who study stroke and traumatic brain injury have identified four features of exercise that drive neuroplasticity: intensity, specificity, difficulty and complexity.
To explore whether these findings apply to PD, we investigated the effect of exercise in two ways. One study was in mice that had been made Parkinsonian. The other study, conducted in collaboration with Beth Fisher, Ph.D., P.T., at USC, was in people who had been diagnosed with PD for less than three years. The human participants were divided into three groups to compare the effects of different levels of exercise. People in the high-intensity group exercised three times a week, an hour each time, on a body-weight supported treadmill — that is, a treadmill that supported them with a harness so that they didn’t have to worry about falling. The second group did low-intensity balance and stretching exercises, which is what most previous studies of exercise in people with PD have evaluated. The third group did no organized exercise at all.
The body-weight supported treadmill combines all the features of exercise that we think are important for driving changes in the brain. The exercise is intense, meaning it moves fast and with high repetition. It is also complex. People are getting feedback from a trainer while they run, so they are simultaneously listening, processing the instructions and adjusting their pace. Exercising on the treadmill is also very task-specific: it addresses walking, a functional task that we need in our daily lives.
Over the course of 24 sessions, those in the high-intensity group walked and ran faster than those in the other groups, working up to speeds of five to eight miles an hour. They also took longer strides, and had better posture and bigger arm swings. And as they practiced on the treadmill three times a week, for an hour each time, every characteristic of their walking began to look more normal.
We also tested their balance. Though the participants were newly diagnosed and had not yet reported balance problems, we found — before they exercised — that already their balance was not normal. That is to say, when walking, they made very tight turns and they took small constrained steps. After several weeks of treadmill exercise, however, their turns became more stable. Balance, as well as gait, had improved.
How Exercise Changes the Brain
What happens in the brain to produce these visible benefits? To find out, we looked at the brains of the mice that had exercised under conditions parallel to the human study.
We found that exercising changed neither the amount of dopamine nor the amount of neurons in the animals’ brains. But in the ones that had exercised, the brain cells were using dopamine more efficiently. We found that exercise improves that efficiency by modifying the areas of the brain where dopamine signals are received — the substantia nigra and basal ganglia.
At the molecular level, at least two things are happening to make dopamine use more efficient. Dopamine travels across a space between two adjacent brain cells called a synapse. This process is called signaling and it is essential for normal functioning. To end the signal, a protein complex called the dopamine transporter normally retrieves dopamine from the synapse. The first thing we found is that animals that had exercised possessed less of the dopamine transporter, meaning that dopamine stayed in their synapses longer, and their dopamine signals lasted longer. Secondly, we found that the cells receiving the dopamine signal had more places for the dopamine to bind in animals that exercised, and so could receive a stronger signal. This binding site is called the D2 receptor. We also studied the D2 receptor in a subset of our human subjects who were within one year of diagnosis and not on any medications, using the imaging technique known as positron emission tomography (PET). We found that in humans, too, exercise increased the number of D2 receptors.
Lastly, animal studies have shown that intensive exercise may also play a role in controlling glutamate, another molecule that signals between brain cells. Dopamine is responsible for holding glutamate in check, so when dopamine levels drop in PD, a chain of events is set off that leads to a build-up of glutamate signaling. The resulting surplus of glutamate damages the cells that control body movements. But animal studies show that exercise may play a role in normalizing glutamate signaling, thereby helping the brain to function normally and to promote recovery of the ability to move.
More research is needed to understand which aspects of exercise are most important, whether the benefits are long-lasting and whether drug and other therapies influence its effects. In studying the underlying molecular mechanisms, we may find new targets for drug therapies. In the meantime, we know the following: intensive exercise can help people with PD walk and move more normally, and research is beginning to reveal how it reconditions the underlying brain circuits.
Dr. Petzinger is Assistant Professor in the Department of Neurology, Movement Disorders Division at USC. Previously, Dr. Petzinger occupied a PDF-funded fellowship in Movement Disorders at Columbia University.
This article was adapted from her presentation at a PDF webcast. To watch her presentation, please review PDF's Past Online Educational Events.
Editor's Note: As with every aspect of your care, consult your doctor before starting an exercise program.