Axonal Transport Of Zinc Ions In Sympathetic Trunk And The Morphological Changes In Sympathetic Trunk After Zinc Deficiency

PrefaceZinc (Zn) is an important trace element for humans and required for normal growth, gene expression, protein metabolism, and immune function. In recent years, people have investigated the relationship between zinc and the nervous system, and found that zinc deficiency could cause many diseases of the nervous system. In the central nervous system (CNS), zinc ions bound with proteins, where it exerts structural functions or acts as a cofactor, however, a particular pool of zinc is present as free or loosely-bound ions accumulated in the synaptic vesicles of zinc-enriched neurons (ZEN). The zinc ions and neurotransmitters may be co-released into the synaptic cleft, and involved in learning and memory formation, the development of the nervous system, and the regulation of motor and sensory function. There are zinc ions in the peripheral nervous system (PNS). It has been reported that almost all of the superior cervical ganglion (SCG) cells have zinc positive reaction product distribution. Electron microscopic observations reveal that free zinc ions gathered in the Golgi complex and vesicles of the superior cervical ganglion sympathetic postganglionic neurons. Zinc ions may be involved in zinc-containing protein synthesis and assembling, and may exert structural functions of certain proteins. In the spinal dorsal ganglion and the sciatic nerve, however, the transport mechanisms and functions of zinc ions in the PNS are not clear. In this study, we focused on the axonal transport of zinc ions in the sympathetic trunk and superior cervical ganglion. Furthermore, the morphologic changes of sympathetic trunk after zinc deficiency were also analyzed in detail.Materials and Methods1. Wistar rats, weighing about 150 g, were randomly divided into three groups. (1) The first group performed sympathetic trunk clamp unilateral surgery, and the opposite sides without operating as a control. Perfuse and obtain the specimen 2 h, 4 h and 6 h after selenium sodium injection. Apply autometallography (AMG) for checking the distribution of zinc ions in the sympathetic trunk.(2) The second group applied the same approach. AMG electron microscopy was used for checking the axonal transport of zinc ions in the sympathetic trunk.(3) The third group clamped sympathetic trunk unilaterally, and inject selenium sodium near the top of clamping point. The opposite sides without operating as a control. After 24 h, both sides of ganglia were removed, and AMG was used to observe the retrograde transport of zinc ions in the sympathetic trunk.2. Three weeks old CD-1 mice were randomly divided into ZnD group and the normal group, and given zinc deficient and normal diet, respectively. Five weeks later, the ultrastructural changes of the sympathetic trunk utilizing were observed with electron microscopy, and the volume changes of myelinated nerve fibers and nerve fiber between the two groups were compared utilizing t-test.Results1. After sympathetic stem clamp, experimental result of AMG methods showed the accumulation of AMG grains on both proximal and distal sides to the two crushes, and the grains were mainly located close to the crush sites, but were absent in the segment between two crushes. The concentration of AMG grains increased with time.2. AMG electron microscope results showed the zinc ions in the myelinated nerve fibers.3. The experimental results of the retrograde transport in the sympathetic trunk showed that only a few neurons were positive stained under the light microscope, and the zinc ions were distributed diffusely in the cytoplasm of neurons with deeper staining around the negative stained nuclei. AMG positive grains were also found in some neuronal processes.4. Zinc deficiency induced ultrastructural changes of the sympathetic trunk, incloding degeneration, irregularity, unclear boundary in myelin sheaths, extensive axonal damage and the increase of the number of abnormal-shaped myelinated nerve fibers. Our results also showed the increaing of maylinated nerve fibers and the decrease of unmaylinated nerve fiber under the ligh microscope.DiscussionIt has been reported that the zinc particles were accumulated on the both sides of clamping point after the sciatic nerve clamp by AMG, which indicating that there was zinc ions transport in the sciatic nerve, agreeing with our results. Electron microscopic observation showed the degenerate and decreation of the number and the surface area of the sciatic nerve fibers compared with control groups, indicating that zinc may play a role to the PNS. Therefore, we speculate that there is zinc ions transport in the sympathetic trunk, and zinc may play key a role to maintain the structure and function of PNS. Using AMG and electron microscopy methods, we found the accumulation of AMG grains on both proximal and distal sides to the two crushes, and the concentration of AMG grains increased with time, indicating that almost all the zinc enriched neurons transported the zinc ions to the axson terminals to regulate some physiological functions through anterograde, and the zinc ions also could be transport from the axon terminals to the cell bodies through retrograde axonal transport.Conclusion1. The accumulation of zinc ions showed that there are anterograde and retrograde transport of zinc ions in sympathetic trunk.2. After zinc deficiency, the structure alteration or the number changes of the myelinated nerve fibers and the unmyelinated nerve fibers showed that zinc ions play a key role to maintain the structure and function of peripheral nerve.