Scott and Roy showed that a-synuclein plays a role in maintaining the overall size of the recycling pool of vesicles. We observed expression and synaptic localization of a-synuclein in excitatory neurons, but not in inhibitory neurons, although there were no obvious differences between the two types of Rapamycin neurons in expression of the synaptic markers synaptotagmin and synapsin. We also observed that activity-dependent presynaptic membrane recycling occurred independently of the presence of a-synuclein. While there have been many studies investigating the properties of postsynaptic channels including kinetics and the signaling molecules involved in synaptic transduction, less is known about the presynaptic differences between excitatory and inhibitory neurons. Recent work revealed that the sizes of both the recycling pool and total vesicular pool are more variable at glutamatergic synapses than gamma-aminobutyric acid synapses. This heterogeneity of the size of the recycling pool at glutamatergic synapses may provide a dynamic range of synaptic strength that is not present at GABAergic synapses. a-Synuclein might act as a modulator of the size of the recycling pool at excitatory synapses. a-Synuclein is also suggested to be involved in mobilization of glutamate from the reserve pool using electrophysiology of hippocampal slices. There is a possibility that the differential expression of asynuclein is due to a difference in protein turnover between excitatory and inhibitory neurons. However, treatment with inhibitors of proteasomes or lysosomes did not alter the immunostaining patterns of a-synuclein, suggesting that differential synthesis rather than degradation of a-synuclein is responsible for the distinct expression patterns in neurons. Recently, it was reported that a-synuclein promotes early neurite outgrowth in cultured primary neurons. It has also been suggested that a-synuclein plays important roles in the early development of synapses. The expression ratio of asynuclein/synaptophysin is higher during early development than in adult and aged rat brain. We demonstrated that cultured neurons exhibit differential expression of a-synuclein by 30 h after cell dissemination. At this stage, there were no synaptic connections established between neurons. In addition, expression of GAD was very weak and not strong enough to distinguish inhibitory neurons. These results suggest that a-synuclein is involved in the differentiation of neurons. Concerning the pathogenicity of a-synuclein, we observed that inhibitory neurons did not exhibit aggregate formation after treatment with preformed fibrils. This result was due to the low expression level of a-synuclein, because the expression of exogenous human a-synuclein in GAD neurons enabled them to form a-synuclein aggregates.