In vitro dopaminergic neurotoxicity of pesticides; a link with neurodegeneration?

Abstract

Ever since people culture crops for food- and feed production, they use chemical compounds to destroy and repel plagues threatening production. Based on their primary target these compounds are classified as e.g. insecticides, herbicides or fungicides. In epidemiological studies, pesticide exposure in humans has been linked to the development of neurodegenerative disorders including Parkinson’s disease (PD). However, mechanisms behind pesticide-induced PD are largely unknown. The large majority (90 - 95%) of PD cases is not related to inheritable genetic mutations and is therefore considered idiopathic PD. Idiopathic PD is characterized by bradykinesia, resting tremor and muscle rigidity. This clinical image is attributed to a gradual loss of dopaminergic neurons in the basal ganglia that are responsible for regulation of movement and motor coordination. The disorder is considered as complex and multifactorial with limited involvement of genetic defects. This thesis aimed at identifying in vitro pathways of toxicity that may link pesticide exposure to neuronal dysfunction and parameters of neurodegeneration. Therefore, neurotoxic effects of different classes of pesticides have been assessed in PC12 cells, a rat cell model that resembles mature dopaminergic neurons. It was demonstrated that the organochlorine insecticides lindane and dieldrin exert differential effects on the intracellular calcium homeostasis including membrane depolarization and inhibition of voltage-gated calcium channels (VGCCs). In addition, it was demonstrated that in vitro exposure to dinitrophenolic herbicides results in activation of apoptosis and an increase in intracellular alpha-synuclein levels through release of calcium from the ER. Furthermore, dinitrophenolic herbicides induced differential effects on GABAA-receptor function. Organochlorine insecticides and dinitrophenolic herbicides are both phased-out. Therefore, also pesticides that are currently in use have been assessed for effects on calcium homeostasis. It was demonstrated that widely-used azole fungicides also disturb intracellular calcium through inhibition of VGCCs. As humans are often exposed to a (complex) mixture of pesticides, mixture effects have been assessed on selected endpoints. Thus, it was demonstrated that additivity may apply upon exposure to (complex) mixtures. This indicates that the current approach in risk assessment of mixtures, which is based on the individual rather than cumulative effects, questionable. Therefore, there is a need for (further) development of (software) tools for risk assessment that take into account the cumulative risk. Life and death of dopaminergic neurons in the basal ganglia rely heavily on strict maintenance of the intracellular calcium homeostasis. Therefore, pesticide-induced changes in intracellular calcium resulting in a disturbance of dopaminergic neurotransmission and potentially cell death may relate to the aetiology of PD. Consequently, neurotoxicity should be included in human risk assessment. The role of chemicals and pesticides in particular in the pathogenesis of PD is an immense jigsaw. The research presented in this thesis added some new pieces to the puzzle, demonstrating that several classes of pesticides can induce in vitro (dopaminergic) neuronal dysfunction that may lead to degenerative processes.