Localization of the brain-specific high-affinity L-proline transporter in cultured hippocampal neurons: Molecular heterogeneity of synaptic terminals

The expression of a brain-specific, high-affinity Na+-(and Cl--)dependent L -proline transporter in subpopulations of putative glutamatergic pathways i n mammalian brain suggests a physiological role for this carrier in excitat ory neurotransmission (Fremeau et al., Neuron 8: 915-926, 1992). To assess further the cell-type and subcellular localization of PROT, we examined its distribution in low-density cultures of embryonic rat hippocampus, PROT im munoreactivity was detected beginning at 8 days in culture in a highly punc tate pattern localizing to a subset of synaptic terminals. PROT was not det ected at GABAergic terminals but was specifically localized to a subset of excitatory nerve terminals. PROT-labeled terminals showed partial appositio n to AMPA-type and NMDA-type glutamate receptor clusters. Immunolabeling of isolated neurons grown in microisland cultures revealed that PROT was expr essed by 60% of cultured hippocampal neurons. Individual microisland cultur es were immunopositive for either PROT or glutamic acid decarboxylase, but never both. in the expressing pyramidal neurons, PROT was targeted to all p resynaptic terminals. These findings indicate that PROT contributes to the molecular heterogeneity of glutamatergic terminals and suggest a novel pres ynaptic regulatory role for PROT in excitatory transmission at specific glu tamatergic synapses.