Early α-synuclein aggregation decreases corticostriatal glutamate drive and synapse density.

Neuronal inclusions of α-synuclein (α-syn) are pathological hallmarks of Parkinson's disease (PD) and Dementia with Lewy Bodies (DLB). α-Syn pathology accumulates in cortical neurons which project to the striatum. To understand how α-syn pathology affects cortico-striatal synapses at early time points before significant dopamine neuron loss, pre-formed α-syn fibrils (PFF) were injected into the striatum to induce endogenous α-syn aggregation in corticostriatal-projecting neurons. Electrophysiological recordings of striatal spiny projection neurons (SPNs) from acute slices found a significant decrease in evoked corticostriatal glutamate release and corticostriatal synaptic release sites in mice with PFF-induced aggregates compared to monomer injected mice. Expansion microscopy, confocal microscopy and Imaris reconstructions were used to identify VGLUT1 positive presynaptic terminals juxtaposed to Homer1 positive postsynaptic densities, termed synaptic loci. Quantitation of synaptic loci density revealed an early loss of corticostriatal synapses. Immunoblots of the striatum showed reductions in expression of pre-synaptic proteins VGLUT1, VAMP2 and Snap25, in mice with α-syn aggregates compared to controls. Paradoxically, a small percentage of remaining VGLUT1+ synaptic loci positive for pS129-α-syn aggregates showed enlarged volumes compared to nearby synapses without α-syn aggregates. Our combined physiology and high-resolution imaging data point to an early loss of corticostriatal synapses in mice harboring α-synuclein inclusions, which may contribute to impaired basal ganglia circuitry in PD and DLB.