The mammalian brain high-affinity L-proline transporter is enriched preferentially in synaptic vesicles in a subpopulation of excitatory nerve terminals in rat forebrain.

The expression of a brain-specific high-affinity Na+-dependent (and Cl--dependent) L-proline transporter (PROT) in subpopulations of putative glutamatergic neurons in mammalian brain suggests a physiological role for this carrier in excitatory neurotransmission (). To gain insights into potential sites where PROT may function, we used a C-terminal domain antipeptide antibody to determine the regional distribution and subcellular localization of PROT in rat forebrain. PROT immunoreactivity was seen in processes having a regional light microscopic distribution comparable to that of known glutamatergic projections within the cortex, caudate putamen nucleus (CPN), hippocampal formation, and other forebrain regions. In all regions examined by electron microscopy (cortex, CPN, and the stratum oriens of CA1), PROT labeling was observed primarily within subpopulations of axon terminals forming asymmetric excitatory-type synapses. Immunogold labeling for PROT was detected in close contact with membranes of small synaptic vesicles (SSVs) and more rarely with the plasma membrane in these axon terminals. Subcellular fractionation studies confirmed the preferential distribution of PROT to synaptic vesicles. The topology of PROT in synaptic vesicles was found to be inverted with respect to the plasma membrane, suggesting that PROT-containing vesicles are generated by a process involving endocytosis from the plasma membrane. Because PROT lacks any of the known characteristics of other vesicular transporters, these results suggest that certain excitatory terminals have a reserve pool of PROT associated with SSVs. The delivery of PROT to the plasma membrane by exocytosis could play a critical role in the plasticity of certain glutamatergic pathways.

Neuronal control of cardiac and hepatic macromolecule synthesis in the neonatal rat: effects of sympathectomy.

Neurotransmitters are thought to influence cell development in their target tissues. In the current study, neonatal rats were given 6-hydroxydopamine to produce permanent sympathetic denervation, and the effects on cardiac and hepatic DNA and protein synthesis were assessed. Lesioned animals showed deficits in cardiac DNA synthesis over the first 8 d postpartum, a period in which sympathetic innervation is sparse and synaptic norepinephrine concentrations are low; the effect of lesioning was also evident for protein synthesis. Subsequently, DNA synthesis in control animals declined precipitously during the second to third postnatal week, the phase associated with ingrowth of the majority of sympathetic terminals and sympathetic hyperactivity. Neonatal lesioning delayed the ontogenetic decline in DNA synthesis: this effect was not shared by protein synthesis. In the liver, a tissue whose cells, unlike the heart, maintain the ability to divide into adulthood, there was no effect of 6-hydroxydopamine on DNA synthesis and only minor changes in protein synthesis. These results suggest that neural input provides two distinct trophic signals to the developing heart: an early promotion of cell replication associated with low levels of stimulation, and a subsequent promotion of the switchover from cell replication, to cell differentiation and enlargement, associated with high levels of stimulation. In light of the precipitous rise in circulating catecholamines at parturition, and of the subsequent development of sympathetic innervation, catecholamines are likely to play a trophic role in the establishment of the proper pattern of cardiac cell development.