α‐Synuclein oligomers slow down action potential firing and enhance dopamine release by increasing Cav2.2 currents in midbrain dopaminergic neurons

Abstract: The spontaneous firing activity of substantia nigra (SN) dopaminergic (DA) neurons is finely tuned by the autocrine inhibition mediated by D2 DA autoreceptors (D2-ARs) that activate GIRK2 channels. Despite this regulatory mechanism, the vulnerability of SN DA neurons may nevertheless increase due to an altered spontaneous firing activity of DA neurons, Ca2+ dishomeostasis, mitochondrial stress, high dendritic arborization, aggregation of α-synuclein (α-syn), α-syn mutations, reduced levels of calbindin protein, etc. Although the intraneuronal accumulation and the spreading of misfolded α-syn is a hallmark of full-blown Parkinson's disease, the effects produced by α-syn aggregation on neuronal functionality at the early onset of neurodegeneration are still of debate. We previously reported that α-syn oligomers in the extracellular medium drastically inhibit the firing rate of midbrain neurons and significantly impair burst generation and network synchronization. Here, by combining conventional electrophysiology and cutting-edge technology of micro-graphitized diamond micro-electrode arrays, we confirm that exogenous α-syn effectively slows down the firing rate of SN DA neurons, but it also selectively upregulates Cav2.2 (N-type) Ca2+ currents and consequently Ca2+-dependent DA release. Thus, our data uncover a novel regulatory mechanism in SN DA neurons and demonstrate that exogenous α-syn alters the interplay among Ca2+ entry, spontaneous firing and DA release causing DA accumulation in the extracellular milieu and intracellular Ca2+ overload. Both processes may represent a target for future investigations to better understand the initial phases of SN DA neuron degeneration. (Figure presented.). Key points: We combined conventional electrophysiology and micro-graphitized diamond multi-electrode arrays to investigate the effect of exogeneous α-synuclein on cultured midbrain dopaminergic neurons isolated from substantia nigra. α-Synuclein oligomers slow down the firing rate of dopaminergic neurons and up-regulate Cav2.2 (N-type) Ca2+ currents. Raised Cav2.2 currents in turn increase the depolarization-evoked dopamine release and the frequency of quantal exocytotic events. Overall, this mechanism causes dopamine accumulation in the extracellular milieu and intracellular Ca2+ overload.