Regional distribution of sodium-dependent excitatory amino acid transporters in rat spinal cord

Background/Objective: The excitatory amino acid transporters (EAATs), or sodium-dependent glutamate transporters, provide the primary mechanism for glutamate removal from the synaptic cleft. EAAT distribution has been determined in the rat brain, but it is only partially characterized in the spinal cord. Methods: The regional anatomic distribution of EAATs in spinal cord was assessed by radioligand autoradiography throughout cervical, thoracic, and lumbar cord levels in female Sprague-Dawley rats. EAAT subtype regional distribution was evaluated by inclusion of pharmacologic transport inhibitors in the autoradiography assays and by immunohistochemistry using subtype-specific polyclonal antibodies to rat GLT1 (EAAT2), GLAST (EAAT1), and EAAC1 (EAAT3) rat transporter subtypes. Results: [H-3]-D-Aspartate binding was distributed throughout gray matter at the 3 spinal cord levels, with negligible binding in white matter. Inclusion of pharmacologic transport inhibitors indicates that the EAAT2/GLT1 subtype represents 21% to 40% of binding. Both EAAT1 /GLAST and EAAT3/EAAC1 contributed the remainder of binding. Immunoreactivity to subtype-specific antibodies varied, depending on cord level, and was present in both gray and white matter. All 3 subtypes displayed prominent immunoreactivity in the dorsal horn. EAAT3/EAAC1 and to a lesser extent EAAT1/GLAST immunoreactivity also occurred in a punctate pattern in the ventral horn. Conclusions: The results indicate heterogeneity of EAAT distribution among spinal cord levels and regions. The presence of these transporters throughout rat spinal cord suggests the importance of their contributions to spinal cord function.