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.