A hypothesis for the superior sensitivity of wheat germ agglutinin as a neuroanatomical probe

Delayed reentry of recycling vesicles into the fusion-competent synaptic vesicle pool in synaptojanin 1 knockout mice

Synaptojanin 1 is a polyphosphoinositide phosphatase implicated in synaptic vesicle recycling. We used FM1-43 imaging and electron microscopy in cultured cortical neurons from control and synaptojanin 1 knockout mice to study how the absence of this protein affects specific steps of the synaptic vesicle cycle. Exoendocytosis after a moderate stimulus was unchanged. However, during prolonged stimulation, the regeneration of fusion-competent synaptic vesicles was severely impaired. In stimulated nerve terminals, there was a persistent accumulation of clathrin-coated vesicles and a backup of newly reformed vesicles in the cytomatrix-rich area around the synaptic vesicle cluster. These findings demonstrate that synaptojanin 1 function is needed for the progression of recycling vesicles to the functional synaptic vesicle pool.

Axonal transport and MR imaging: prospects for contrast agent development

Axonal transport plays a critical role in the physiology and pathology of neurons, yet there have been virtually no clinical tools for its evaluation in human subjects. A wide variety of molecules that can act as axonal transport facilitators have been discovered and, in many cases, used to deliver labels detectable with histologic methods. Recently a number of investigators have reported preliminary success in developing intraneural contrast agents based on various versions of dextran-coated magnetite that may render magnetic resonance imaging capable of depicting axonal transport. It is not yet clear whether any clinically useful agents will eventually be developed, but there has been considerable progress in identifying design factors for such a pharmaceutical agent.

Axonal and dendritic endocytic pathways in cultured neurons

The endocytic pathways from the axonal and dendritic surfaces of cultured polarized hippocampal neurons were examined. The dendrites and cell body contained extensive networks of tubular early endosomes which received endocytosed markers from the somatodendritic domain. In axons early endosomes were confined to presynaptic terminals and to varicosities. The somatodendritic but not the presynaptic early endosomes were labeled by internalized transferrin. In contrast to early endosomes, late endosomes and lysosomes were shown to be predominantly located in the cell body. Video microscopy was used to follow the transport of internalized markers from the periphery of axons and dendrites back to the cell body. Labeled structures in both domains moved unidirectionally by retrograde fast transport. Axonally transported organelles were sectioned for EM after video microscopic observation and shown to be large multivesicular body-like structures. Similar structures accumulated at the distal side of an axonal lesion. Multivesicular bodies therefore appear to be the major structures mediating transport of endocytosed markers between the nerve terminals and the cell body. Late endocytic structures were also shown to be highly mobile and were observed moving within the cell body and proximal dendritic segments. The results show that the organization of the endosomes differs in the axons and dendrites of cultured rat hippocampal neurons and that the different compartments or stages of the endocytic pathways can be resolved spatially.

Membrane trafficking in neurons

Neurons possess an unusually extensive Golgi apparatus and exhibit a variety of active endocytic-like processes. The Golgi apparatus and the endocytic phenomena both contribute, probably in multiple overlapping ways, to the genesis and fate of the membrane systems in axons and terminals.

Anterograde neuroanatomical tract tracing with central nervous system injections of immunoglobulin G: a light and electron microscopic evaluation

Normal rabbit serum (NRS) was pressure injected into the forebrain of rats to be tested as an anterograde neuroanatomical tracing substance. Undiluted NRS was stereotaxically injected into the bed nucleus of the stria terminalis (BST) with a 1-microliter Hamilton syringe. Postinjection survival times ranged from 24 h to 14 days. An immunohistochemical method utilizing goat anti-rabbit IgG antibody was used to detect the rabbit IgG within vibratome sections. Visualization of the final reaction product (diaminobenzidine, DAB) was enhanced by a silver/gold postintensification (SGI) method. Rabbit IgG-containing neural structures were examined at both light and electron microscopic (EM) levels. At the injection site neuronal soma, dendrites and axons were filled homogeneously with the SGI-DAB at 24 h, 48 h, 7 days and 14 days indicating local neuronal uptake, storage and transport of rabbit IgG. In the hypothalamus many anterogradely filled axons were present and displayed short collateral branches and terminals. EM examination revealed synaptic terminals containing IgG, without signs of transsynaptic transport after 14 days. Signs of retrograde transport of IgG were never observed. A propensity of neurons to take up, sequester and anterogradely transport immunoglobulin G is indicated.