Oligomeric alpha-synuclein causes early synaptic dysfunction of the corticostriatal pathway associated with non-motor symptoms.

In synucleinopathies, α-synuclein oligomers (OSyn) appear to be associated with neurodegeneration, neurotoxicity, and proinflammatory responses, even at low concentrations, suggesting their pivotal role in the pathogenesis of Parkinson's disease (PD). We utilized a rat model of synucleinopathy induced by intrastriatal injection of OSyn, aiming to elucidate events preceding the formation of fibrillary α-syn aggregates. Electrophysiological assessments and behavioral assays revealed several early alterations in OSyn rats, evident as early as 12 weeks post-OSyn injection. These included mild and variable reduction of motor activity, anxiety-like behavior, impaired bidirectional striatal long-term synaptic plasticity, and diminished spontaneous excitatory neurotransmission in the striatum. Furthermore, p-α-syn aggregates were detected in the cortex but not in the substantia nigra (SN). Confocal microscopy analysis revealed reduced vesicular glutamate transporter 1 (VGluT1) expression at striatal glutamatergic terminals. Chronic administration of the ampakine Tulrampator to OSyn animals prevented impairment of long-term depression (LTD), spontaneous striatal neurotransmission, and VGluT1 levels. Tulrampator also ameliorated the anxiety-related behavioral phenotype, albeit without attenuating motor deficits, demonstrating its efficacy in mitigating early synaptic and emotional deficits induced by OSyn. These findings provide a basis for a novel drug treatment strategy aimed at mitigating or delaying early damage at cortico-striatal terminals induced by OSyn, thereby counteracting the pathophysiological processes underlying the onset of early non-motor symptoms in PD.