We studied dopamine levels in three compartments of the dopaminergic synapse, including the presynaptic neuron cytosol, dopamine storage vesicles, and the synaptic gap. By considering three transport pathways (dopamine transporter (DAT), vesicular transporter (VT), and exocytosis), four simulated scenarios were investigated: homeostasis, application of cocaine, methamphetamine, and reserpine. Recent experiments show that upon cocaine administration, the Drosophila melanogaster DAT permeation rate constant is decreased by 55% and we adopted this value for the human DAT. Amphetamine and methamphetamine block DAT and VT, while reserpine blocks VT; however, their decreased permeation rate constants are not available. A system of three differential equations of dopamine levels as a function of time was developed respectively for the synaptic compartments and was solved numerically. Per computational inference, the cytosol dopamine concentration was noted to increase in the case of methamphetamine and reserpine, but was practically unchanged in the case of the cocaine administration. Accordingly, our study suggests that amphetamines and other substances that block VT, but not cocaine or substances that only block DAT, may be etiologically important in the cytosolic dopamine mediation of neurodegeneration in Parkinson disease/Parkinsonism.
Keywords: Amphetamine; Cocaine; Dopamine; Neurodegeneration; Parkinson disease; Reserpine; Synapse.