Non-synaptic localization of the glutamate transporter EAAC1 in cultured hippocampal neurons

It has been postulated for several years that the high affinity neuronal glutamate uptake system plays a role in clearing glutamate from the synaptic cleft. Four different glutamate transporter subtypes are now identified, the major neuronal one being EAAC1. To be a good candidate for the reuptake of glutamate at the synaptic cleft, EAAC1 should be properly located at synapses, either at pre- or postsynaptic sites. We have investigated the distribution of EAAC1 in primary cultures of hippocampal neurons, which represent an advantageous model for the study of synaptogenesis and synaptic specializations. We have demonstrated that EAAC1 immunoreactivity is segregated in the somatodendritic compartment of fully differentiated hippocampal neurons, where it is localized in the dendritic shaft and in the spine neck, outside the area facing the active zone. No co-localization of EAAC1 immunoreactivity with the stainings produced by typical presynaptic and postsynaptic markers was ever observed, indicating that EAAC1 is not to be considered a synaptic protein. Accordingly, the developmental pattern of expression of EAAC1 was found to be different from that of typical synaptic markers. Moreover, EAAC1 was expressed in the somatodendritic compartment of hippocampal neurons already at stages preceding the formation of synaptic contacts, and was also expressed in GABAergic interneurons with identical subcellular distribution. Taken together, these data rule against a possible role for EAAC1 in the clearance of glutamate from within the cleft and in the regulation of its time in the synapse. They suggest an unconventional non-synaptic function of this high-affinity glutamate carrier, not restricted to glutamatergic fibres.