alpha-Synuclein (alpha-Syn) is a presynaptic protein implicated in Parkinson's disease (PD). Mice overexpressing human wildtype (WT) alpha-Syn under the Thy1 promoter show high levels of alpha-Syn in cortical and subcortical regions, exhibit progressive sensorimotor anomalies, as well as non-motor abnormalities and are considered models of pre-manifest PD as there is little evidence of early loss of dopaminergic (DA) neurons. We used whole-cell patch clamp recordings from visually identified striatal medium-sized spiny neurons (MSSNs) in slices from alpha-Syn and WT littermate control mice at 35, 90 and 300 days of age to examine corticostriatal synaptic function. MSSNs displayed significant decreases in the frequency of spontaneous excitatory postsynaptic currents (EPSCs) in alpha-Syn mice at all ages. This difference persisted in the presence of tetrodotoxin, indicating it was independent of action potentials. Stimulation thresholds for evoking EPSCs were significantly higher and responses were smaller in alpha-Syn mice. These data suggest a decrease in neurotransmitter release at the corticostriatal synapse. At 90 days the frequency of spontaneous GABA(A) receptor-mediated synaptic currents was decreased in MSSNs but increased in cortical pyramidal neurons. These observations indicate that high levels of expression of alpha-Syn alter corticostriatal synaptic function early and they provide evidence for early synaptic dysfunction in a pre-manifest model of PD. Of importance, these changes are opposite to those found in DA-depletion models, suggesting that before degeneration of DA neurons in the substantia nigra synaptic adaptations occur at the corticostriatal synapse that may initiate subtle preclinical manifestations.