"Provocation test" as a novel approach to early diagnosis of Parkinson's disease.

Abstract

Parkinson's disease (PD) is one of the most severe socially important neurodegenerative disorders. The loss of dopaminergic neurons of the nigrostriatal sys� tem is considered to be a key aspect of the pathogene� sis of PD. The first clinical symptoms of the disease including motor dysfunction develop many years after the onset of the pathological process, which is believed to be a consequence of the brain plasticity (or com� pensatory mechanisms). However, when 50-70% of dopaminergic neurons in the substantia nigra (SN) are degenerated and the striatal dopamine (DA) is reduced by 80% those compensatory mechanisms lose the ability to mask motor impairments (1-5). Conse� quently, preclinical diagnosis of the masked patholog� ical process is of great importance because it may become the basis for preventive neuroprotective ther� apy. The therapeutic approach in this case can be aimed at slowingdown the process of neuronal loss and thus extending the preclinical stages of the disease or, in other words, the period of patient's comfortable state. Currently, the methods of preclinical diagnosis of PD including positron emission tomography and singlephoton emission computed tomography are very costly and technically challenging, which results in their limited use in mass health screening. The purpose of this study was to experimentally develop a "provocation test" that would allow to diag� nose PD at its preclinical stage. This approach is based on the temporary and reversible "impairment" of dopaminergic nigrostriatal system by pharmacological inhibition of DA synthesis to the threshold level, where the first motor symptoms can be observed. Moreover, it is necessary to use the minimal dose of the DA inhibitor that would cause motor abnormali� ties in an experimental model of presymptomatic PD, but would not affect the behavior of normal animals in the control. Experiments were performed on male C57BL/6JSto mice (2.5� to 3�monthold animals with a body weight of 22-26 g) using the model of PD at a preclinical stage that was developed in our laboratory (6). Accord� ing to this model, the mice (n = 42) were subcutane� ously injected with the neurotoxin 1�methyl�4�phe� nyl�1,2,3,6�tetrahydropyridine (MPTP) at a single dose of 12 mg/kg in 0.9% NaCl. The injection was repeated twice at a 2�h interval. Animals in the control group (n = 42) were administered with 0.9% NaCl in a similar way. The model is characterized by the absence of any motor function impairment, the loss of 26% in dopaminergic neurons SN 59% of axon terminals in the striatum, and a 60% reduction of the striatal level of DA. The locomotor and exploratory activities of the animals were assessed 15 days after MPTP or saline administration during the 6�min period in the "open� field" test using an OptoVarimex�3 system (Columbus Instruments International, United States). Based on the data obtained, all animals were grouped into two subgroups with equal average and variance values; the subgroups were designated as NaCl 1 and NaCl 2 , MPTP 1 and MPTP 2 , respectively.