Assessment of the effect of selected African plants on an in vitro model of Parkinson's disease

Abstract

Parkinson’s disease (PD) is an incurable, progressive disease characterised by loss of dopaminergic neurons in the substantia nigra of the brain. The main cause of dopaminergic neuron loss is attributed to oxidative stress and mitochondrial dysfunction. Although treatments are available, focus is placed on symptomatic relief, and thus over time disease progression still occurs. Herbal remedies offer a wide range of chemical entities that may prove beneficial in treating neurodegeneration. The aim of this study was to evaluate the neuroprotective effects of selected African medicinal plants using 6- hydroxydopamine (6-OHDA)-induced cytotoxicity in the SH-SY5Y human neuroblastoma cell line as model of PD. Eight plants, used ethnomedicinally for the treatment of neurological disorders, were extracted using methanol and acetone ultrasonic maceration. Neurotoxicity was induced by exposing cells to 33.3 μM 6- OHDA for 2 h, followed by 24 h incubation with the crude extracts. Neuroprotection was initially assessed using the sulforhodamine B staining assay. Plants that displayed neuroprotective activity (Acokanthera oppositifolia, Boophane disticha and Xysmalobium undulatum) were further assessed (0.5- 15 μg/mL) using mechanistic assays. Reduced glutathione (GSH) content, mitochondrial membrane potential, reactive oxygen species (ROS) levels, intracellular calcium (Ca2+) flux and adenosine triphosphate (ATP) levels were assessed using the monochlorobamine adduct formation, JC-1 ratiometric, dihydrodichlorofluorescein cleavage assays, Fura-2 AM and a bioluminescence assay kit, respectively. Cell morphology was visualized using phase contrast and polarisation-optical transmitted light differential interference contrast (PlasDIC) microscopy. 6-Hydroxydopamine reduced cell density by 91% and increasing ROS (3-fold) and GSH (2-fold) levels. Mitochondrial depolarisation (2-fold) was evident, most likely due to blockage of mitochondrial complex I with subsequent ROS leakage. Reduced glutathione levels increased adaptively possibly in response to the ensuing oxidation. Crude extracts attenuated cytotoxicity by reducing ROS, sustaining ATP production and maintaining threshold intracellular Ca2+ effects. This was confirmed by microscopic analyses. A trend for greater protection at lower concentrations was observed. Results suggest that intermediate-polarity extracts of A. oppositifolia, B. disticha and X. undulatum may assist with reducing the detrimental effects associated with PD. In conclusion, extracts of A. oppositifolia, B. disticha and X. undulatum offer in vitro neuroprotective effects.