Marjorie A. Ariano, Ph.D.
Professor of Neuroscience
Associate Dean, Undergraduate Studies
Room 1.330
Telephone (847) 578-3412

  Lise S. Eliot, Ph.D.
Associate Professor, Room 2.274
Telephone (847) 578-3416

  Robert Marr, Ph.D.
Assistant Professor, Room 2.212
Telephone (847) 578-8541

  Daniel A. Peterson, Ph.D.
Professor of Neuroscience, Room 2.217B
Telephone (847) 578-3411

 Grace E. Stutzmann (Beth), Ph.D.
Associate Professor, Room 2.216
Telephone (847) 578-8540

 Anthony R. West, Ph.D.
Associate Professor, Room 2.217A
Telephone (847) 578-8658

 Marina E. Wolf, Ph.D
Professor and Chair, Room 2.262
Telephone (847) 578-8659

Parkinson’s disease (PD) is a prevalent neurodegenerative disorder that affects millions in the US. The distinguishing symptoms of the disease include stiffness, tremors in the hands at rest, slowed movements, fewer movements and unsteady posture. PD develops spontaneously from unknown causes, but the change detected in the brain is reduction of the neurotransmitter DA due to death of these neurons that make this substance. The nigrostriatal pathway connects the substantia nigra where the DA neuron cell bodies are located to the striatum, a crucial brain nucleus involved in initiating movement. PD symptoms occur when there is an 80% or greater loss of DA within the striatum. In the first decade of PD substantia nigra DA neuron death is about 50% while DA loss in the striatum is much less. Many years may elapse before there is sufficient death of DA neurons to produce the onset of the motor symptoms that characterize PD. This time period before the onset of abnormal movements is known as the preclinical phase of the disease. This preclinical phase can be detected by using positron emission tomography (PET) imaging of DA uptake into the striatal terminations, but the diagnosis for PD is that L-DOPA treatment improves the motor signs of the disorder.

L-DOPA is the precursor for the neurotransmitter DA. Current drug therapies for PD, such as L-DOPA treatment will increase brain DA levels and effectively treat the clinical symptoms for many years, but the disease continues to progress. L-DOPA leads to incapacitating side affects that seriously influence the individual’s quality of life and eventually the effectiveness of the therapy is lost. As a consequence, new treatments seek to postpone the use of L-DOPA as long as possible, delaying the side effects. Finding alternative drugs to postpone L-DOPA treatment would be of great benefit, but presently medications that can retard or halt the death of the DA neurons in PD have not been identified. Our studies are aimed at distinguishing the underlying changes in the striatum – beyond the loss of DA terminals – that may provide an alternative therapy to use as a stand alone drug treatment, or in combination with reduced L-DOPA medication, to delay the onset of motor symptoms and provide a better quality of life for the patient with PD. To do this we have produced a rat model of early stage PD.

Depletion of DA in animal models reproduces the neurochemistry detected in the brains of PD patients and provides an opportunity to study the initial phases of PD experimentally when DA loss is modest and not reduced sufficiently to disrupt the neuronal circuits. We induce early PD in rats by injecting the neurotoxin 6-hydroxydopamine (OHDA) into the substantia nigra. This 6-OHDA injection depletes the striatal DA levels to 30-65% of normal and mimics the deficit of the PD patient in the preclinical stage of the disease. Rats are treated with 6-OHDA on one side of the brain and are monitored in a behavioral test (limb-use asymmetry), which detects the imbalance of DA levels between both striata. We compared the 6-OHDA treated animals use of their forepaws to rats that had experienced the same surgical procedure, but the neurotoxic 6-OHDA did not reach the intended target and this produced a sham group of animals. The performance of limb use asymmetry was used to predict the level of striatal DA.

After 4 weeks, 6-OHDA treated rats and sham rats were exposed to the odor of a chemical in fox urine (TMT), which induces a fear reaction in the animals. The exposure to TMT caused a further reduction in the DA levels of the striatum of both groups, but was more pronounced and statistically significant in the rats with the partial DA loss. This suggests that the DA neurons are especially sensitive to stress, and if the nigrostriatal system is in the early stages of deterioration as seen in the preclinical phase of PD, stress will accelerate their death. Clinical studies have shown that in early stages of PD, motor symptoms are intermittent and occur only during stress, but in the later part of PD tremors are worsened by anxiety, fatigue and psychological stress. Finding #1: Alleviation of stress in the preclinical PD patient may retard the DA loss in the nigrostriatal pathway. A potential remedy might be as easy as stress-reduction techniques, or could employ medications to reduce anxiety in the PD patient.

We next examined if there were neurochemical changes in the striatum between the sham and PD rats that might cause the behavioral findings. We felt that neuronal changes should be more severe in the rats exposed to the fox odor, and be much more significant in the 6-OHDA treated rats that endured the additional fox urine odor experience. Novel and unexpected changes in striatal neurons occurred in the stressed rats, and were especially prominent in one subset of neurons that connect the striatum with another nucleus of the motor system called the globus pallidus. These striatopallidal neurons in the 6-OHDA treated rats given predator odor showed dramatic increases in activated caspase-3. Activated caspase-3 is the “executioner enzyme” that starts the programmed cell death cascade, apoptosis. Apoptosis has been reported in Alzheimer’s disease, amyotrophic lateral sclerosis (ALS), following stroke, in some forms of epilepsy, and may be the cause of the death of the DAergic substantia nigra neurons. It has never been re-ported to occur in the striatal neurons, which RECEIVE the DA inputs, and certainly apoptosis is not detected in the early stages of PD, even in the substantia nigra neurons. To verify that the apoptosis pathway had been activated by the partial loss of DA in the striatopallidal neurons, we examined proteins that precede the activation of cleaved caspase-3 and are formed after caspase-3 appearance. These two proteins, cytochrome C and fractin were likewise increased within the striatopallidal neurons in the DA-impoverished striatum compared to the normal side, and these elevations in cleaved caspase-3, cytochrome C and fractin also were detected in the normal striatum of rats that were stressed by the fox urine odor, but were increased even more on the DA depleted side. We validated these changes using biochemical assays in these same four groups of rats (sham, sham-TMT exposed, DA depleted, DA depleted-TMT exposed). Finding #2: Partial loss of DA increases three components in the programmed cell death pathway (apoptosis) in a subset of striatal neurons and stress elevates these protein levels even more. This suggests that striatal neurons AS WELL as substantia nigra neurons die early in PD.

Do striatal neurons, specifically striatopallidal neurons die by apoptosis in early stages of PD? We don’t think so. We base this on making cell counts by visualizing stained neurons using both microscopic identification and biochemical assays, which clearly showed equivalent neuron numbers were present in the striatum regardless of the DA status or if the animals had been stressed by exposure to fox urine. Recent evidence shows that partial insults to neurons will activate components of the apoptosis cascade in stroke models, and in that situation, activation of caspase-3 becomes neuroprotective. We hypothesize that a similar event occurs in early PD, and this allows the striatal neurons to readjust to the loss of the critical DA neurotransmitter. Finding #3: Enzymes of the apoptosis cascade in early PD are indications of neuronal instability in the striatopallidal subset and protects them from further insults that occur as the DA neurons deteriorate. New PD therapies should expand the use of neuroprotective compounds in the striatum in addition to prescribing DA replacement drugs.

Future work will be aimed at the examination of the physiological changes in the striatal neurons after partial DA depletion to evaluate how they respond to activation of the cortical inputs to the circuits. Our preliminary experiments indicate that there is a change in the temporal (timing) response of the striatal neurons after DA loss. Another avenue of investigation is to determine whether interventions will be effective in correcting the behavioral (limb use asymmetry), neurochemical (activated caspase-3, cytochrome C, and fractin), and physiological responses within the striatum following a therapy regimen.