The "orphan" Na+/Cl--dependent transporter, Rxtl, is primarily localized within nerve endings of cortical origin in the rat striatum

Previous studies have shown that the striatum expresses very low levers of Na+/Cl--dependent "orphan" transporter Rxt1 transcripts but contains high l evels of protein. This study investigated the origin of Rxt1 expression in rat striatum. Striatal Rxt1 contents assessed by immunocytochemistry or wes tern blotting were found to be significantly reduced after corticostriatal denervation but not after striatal or thalamic lesion with kainic acid or s elective 6-hydroxydopamine-induced nigrostriatal deafferentation. Corticost riatal neurons retrogradely labeled by intrastriatal fluorogold injections were shown to express Rxt1 mRNA. Combination of anterograde biotin-dextran amine labeling of the corticostriatal pathway with Rxt1 immunogold detectio n at the ultrastructural level demonstrated the presence of Rxt1 in about o ne-third of the corticostriatal synaptic terminals and in numerous unidenti fied synaptic terminals. All the Rxt1-positive terminals formed asymmetrica l contacts on spines. These data provide evidence that striatal Rxt1 immuno reactivity is mainly of extrinsic origin and more specifically associated w ith the corticostriatal pathway. Rxt1 appears as a selective presynaptic ma rker of synapses formed by presumably excitatory amino acid afferents, but it segregates a subclass of these synapses, thereby revealing a functional heterogeneity among excitatory amino acid systems.