Autoradiographic localization of proline uptake in excitatory hippocampal pathways

An autoradiographic method was developed to localize sites of high-affinity, Na(+)-dependent proline uptake in the rat hippocampal formation. Hippocampal slices were incubated with [3H]proline, fixed with a glutaraldehyde/carbodiimide mixture, and cut into frozen sections. The sections were coated with photographic emulsion and autoradiograms were prepared. Autoradiographic grain densities were highest over the inner and outer thirds of the dentate molecular layer, followed by stratum lacunosum-moleculare of area CA3. Stratum oriens and stratum radiatum of area CA1 and CA3 were fairly intensely labeled. The pyramidal and granule cell body layers, stratum lucidum of area CA3, and middle third of the dentate molecular layer were lightly labeled. Effects of surgical and kainic acid lesions suggested that the lateral perforant path, associational-commissural fibers in the fascia dentata, and Schaffer collateral-commissural-ipsilateral stratum oriens fibers have considerable proline uptake capacity. In contrast, the medial perforant path and the mossy fibers appear to accumulate little or no proline. These results suggest that high-affinity, Na(+)-dependent uptake of proline is expressed by a subset of hippocampal glutamate pathways. The relative capacities of glutamate terminal populations to transport glutamate and proline varies widely. Proline was previously shown to possess neuroexcitatory and excitotoxic properties in the rat hippocampal formation. Taken together, these findings argue that proline plays a role in excitatory transmission. In elucidating this role, comparisons between medial and lateral divisions of the perforant path may prove especially advantageous.