Tracer membrane vesicles

After release from storage sites in presynaptic monoamine transporter vesides, DA is bound by postsynaptic DA receptors, and unbound DA is broken down by enzymes, or taken back into presynaptic neurons, called reuptake into presynaptic neurons by spedfic presynaptic protein transporters. The uptake of neurotransmitters by presynaptic neurons is a major mechanism for stopping neurotransmission. The recyded neurotransmitter is repackaged in vesicles in presynaptic neurons, and then rdeased again in response to stimulation by presynaptic electrical action potentials. Tracers have been developed to assess the vesicular membranes, as well as the specific reuptake protein transporters. The selectivity with which monoaminergic neurons store DA, 5-HT, NER, or histamine in presynaptic vesides depends on the spedficity of the membrane transporters. 

Much of the work on non-receptor-mediated endocytosis has involved the use of HRP as a tracer. In one study in rats (Cornell etal, 1971), HRP was injected into the lumen of ligated portions of the jejunum and ileum. Uptake of the HRP was comparable in the jejunum and ileum and was greatest near the apical portion of the villi, with less in the base and none in the cr3 ts. Besides being absorbed at the apical surface membrane, HRP was also present within membrane-bound cytoplasmic caimicular vesicles and vacuolar structures. HRP was also absorbed into extracellular spaces between adjacent absorptive cells and in the lamina propria. In a study of HRP infusion into ileal segments of mice (Owen, 1977), HRP was observed to be taken up more readily by M cells than by colunmar cells. HRP absorbed by M cells was released into the extracellular space, for uptake by lymphoid cells, rather than being sequestered into lysosomes, as was seen... 

---