Sodium‐Dependent Proline Uptake in the Rat Hippocampal Formation: Association with Ipsilateral‐Commissural Projections of CA3 Pyramidal

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Abstract
Na+‐dependent uptake of L‐[3H]proline was measured in a crude synaptosomal preparation from the entire rat hippocampal formation or from isolated hippo‐campal regions. Among hippocampal regions, Na+‐depen‐dent proline uptake was significantly greater in areas CA1 and CA2‐CA3‐CA4 than in the fascia dentata, whereas there was no marked regional difference in the distribution of Na+‐dependent γ‐[14C]aminobutyric acid ([14C] GABA) uptake. A bilateral kainic acid lesion, which destroyed most of the CA3 hippocampal pyramidal cells, reduced Na+‐de‐pendent proline uptake by an average of 41% in area CA1 and 52% in area CA2‐CA3‐CA4, without affecting the Na+‐dependent uptake of GABA. In the fascia dentata, neither proline nor GABA uptake was significantly altered. Kinetic studies suggested that hippocampal synaptosomes take up proline by both a high‐affinity (KT=6.7 μM) and a low‐affinity (KT= 290 μM) Na+‐dependent process, whereas L‐[14C]glutamate is taken up predominantly by a high‐affinity (KT=6.1 μM) process. A bilateral kainic acid lesion reduced the Vmax of high‐affinity proline uptake by an average of 72%, the Vmax of low‐affinity proline uptake by 44%, and the Vmax of high affinity glutamate Uptake by 43%, without significantly changing the affinity of the transport carriers for substrate. Ipsilateral‐commissural projections of CA3 hippocampal pyramidal cells Appear to possess nearly as great a capacity for taking up proline as for taking up glutamate, a probable transmitter of these pathways. Therefore proline may play an important role in transmission at synapses made by the CA3‐derived Schaffer collateral, commissural, and ipsilateral associational fibers.