1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a systemic neurotoxin commonly used to produce partial loss of at least the dopamine (DA) input from the substantia nigra pars compacta (SNpc) to the caudate/putamen (CPu). This also alters the glutamate input to the CPu and substantia nigra (SN). Following MPTP, there is a decrease in the basal extracellular glutamate levels in the CPu and an increase in the SN. Injection of a glutamate receptor agonist into the CPu leads to parkinsonism, suggesting that glutamate synapses within at least the CPu play an important role in the development of the movement problems associated with PD. The imbalance between DA and glutamate within the CPu plays a critical role in the movement problems associated with PD and is partially reversed following treadmill exercise in acute MPTP treated mice. Forced treadmill exercise, starting 1 day after the last dose of subacute MPTP, resulted in partial recovery of DA neurons and markers in the SN and CPu, and improved gait/physical activity. However, this subacute model is limited due to a non-progressive 50% loss of DA neurons in the SNpc. We have developed a new progressive model of increased DA loss and motor dysfunction via weekly increased dosing of MPTP over 4 weeks. The 62% decrease in DA neurons in the SNpc after 4 weeks of MPTP is similar to that reported in patients with PD. There is an increase in the density of nerve terminal glutamate immuno-gold labeling in the CPu following progressive MPTP treatment, suggesting a buildup of glutamate and a decrease in release. We also find MPTP-induced changes in several glutamate transporter markers within the CPu ( VGLUT 1, EAAC1, GLAST) and SN ( VGLUT 1/2, EAAC1, GLAST, GLT-1) that would be consistent with the decrease or increase in extracellular glutamate, respectively, as measured by in vivo microdialysis. In addition, treadmill exercise in naive mice results in a decrease in CPu extracellular glutamate. The overall goal of this project is to determine the effects of treadmill and voluntary running wheel exercise initiated during (i.e., intervention) or following (i.e., restoration) progressive dosing of MPTP on alterations in glutamate within the CPu/SNpc and motor function in both young and aged mice. The overarching hypothesis of this proposal is that the effects of MPTP on CPu glutamate will be augmented following exercise, but will be reversed by exercise in the SN, leading to recovery of motor function and partial restoration of DA markers in the CPu/SNpc. These changes in glutamate and DA markers, and motor function will occur to a greater extent following treadmill versus running wheel exercise and in younger compared to older mice.
The specific aims of this proposal are to 1.)!determine the effect of forced treadmill versus voluntary running wheel exercise, beginning 2 weeks after the start of progressive MPTP administration (i.e., intervention) with continued MPTP for 2 more weeks and exercise for 4 weeks, on glutamate and DA markers in the CPu/SN, and motor function in both young and aged mice, 2.) determine the effect of forced treadmill versus voluntary running wheel exercise beginning 4 weeks after the start of progressive MPTP administration (i.e., restoration) with continued exercise for 4 weeks, on glutamate and DA markers in the CPu/SN, and motor function in both young and aged mice, and 3.) determine if increasing CPu glutamate levels during treadmill exercise will block the exercise-induced recovery of DA neurons in the SNpc and motor function following MPTP. To mimic the effect of treadmill exercise, glutamate receptors will be blocked in the SN during MPTP treatment.
Investigation of movement disorders, especially Parkinson's disease, is a high priority research area within the Department of Veterans Affairs. Because of the high incidence of this disease in the general population over the age of 50 and the fact that the age of the veteran population is slowly increasing, this movement disorder is affecting more and more of our veterans. Since treadmill exercise in Parkinsonian patients and in animal models of this disease appears to be effective in terms of augmenting motor behavior, this type of non-pharmacological approach needs further investigation. The results from this project have potentially wide implications of great significance to the population cared for by the Department of Veteran Affairs and by the fact that six Parkinson's Disease Research, Education and Care Centers (PADRECCs) currently are funded (one here at the Portland VA Medical Center), to carry out clinical research projects focusing on Parkinson's disease.
