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

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. 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.