Extra-junctional localization of glutamate transporter EAAT4 at excitatory Purkinje cell synapses.

We used silver-enhanced immunogold electron microscopy to reveal synaptic localization of the glutamate transporter EAAT4 in mouse cerebellar Purkinje cells (PCs). Gold-silver particles representing the EAAT4 were densely localized on extra-junctional membrane, but not on junctional membrane of PC spines in contact with parallel fiber or climbing fiber terminals. No particle accumulations were observed at inhibitory synapses formed on cell body and dendritic shafts of PCs. Therefore, the EAAT4 is selectively targeted to the extra-junctional site of excitatory PC synapses. The finding suggests that the EAAT4 transports glutamate or its related amino acids from outside the synaptic cleft, which would facilitate glutamate diffusion from the synaptic cleft to the extrasynaptic space and restrict glutamate spillover to adjacent synapses.