Zinc-enriched (ZEN) terminals in mouse spinal cord: immunohistochemistry and autometallography

Metal-organic framework materials promote neural differentiation of dental pulp stem cells in spinal cord injury

Spinal cord injury (SCI) is accompanied by loss of Zn2+, which is an important cause of glutamate excitotoxicity and death of local neurons as well as transplanted stem cells. Dental pulp stem cells (DPSCs) have the potential for neural differentiation and play an immunomodulatory role in the microenvironment, making them an ideal cell source for the repair of central nerve injury, including SCI. The zeolitic imidazolate framework 8 (ZIF-8) is usually used as a drug and gene delivery carrier, which can release Zn2+ sustainedly in acidic environment. However, the roles of ZIF-8 on neural differentiation of DPSCs and the effect of combined treatment on SCI have not been explored. ZIF-8-introduced DPSCs were loaded into gelatin methacryloyl (GelMA) hydrogel and in situ injected into the injured site of SCI rats. Under the effect of ZIF-8, axon number and axon length of DPSCs-differentiated neuro-like cells were significantly increased. In addition, ZIF-8 protected transplanted DPSCs from apoptosis in the damaged microenvironment. ZIF-8 promotes neural differentiation and angiogenesis of DPSCs by activating the Mitogen-activated protein kinase (MAPK) signaling pathway, which is a promising transport nanomaterial for nerve repair.

Zinc transporter 3 modulates cell proliferation and neuronal differentiation in the adult hippocampus

The subgranular zone of the dentate gyrus is a subregion of the hippocampus that has two uniquely defining features; it is one of the most active sites of adult neurogenesis as well as the location where the highest concentrations of synaptic zinc are found, the mossy fiber terminals. Therefore, we sought to investigate the idea that vesicular zinc plays a role as a modulator of hippocampal adult neurogenesis. Here, we used ZnT3^−/− mice, which are depleted of synaptic‐vesicle zinc, to test the effect of targeted deletion of this transporter on adult neurogenesis. We found that this manipulation reduced progenitor cell turnover as well as led to a marked defect in the maturation of newborn cells that survive in the DG toward a neuronal phenotype. We also investigated the effects of zinc (ZnCl₂), n‐acetyl cysteine (NAC), and ZnCl₂ plus 2NAC (ZN) supplement on adult hippocampal neurogenesis. Compared with ZnCl₂ or NAC, administration of ZN resulted in an increase in proliferation of progenitor cells and neuroblast. ZN also rescued the ZnT3 loss‐associated reduction of neurogenesis via elevation of insulin‐like growth factor‐1 and ERK/CREB activation. Together, these findings reveal that ZnT3 plays a highly important role in maintaining adult hippocampal neurogenesis and supplementation by ZN has a beneficial effect on hippocampal neurogenesis, as well as providing a therapeutic target for enhanced neuroprotection and repair after injury as demonstrated by its ability to prevent aging‐dependent cognitive decline in ZnT3^−/− mice. Therefore, the present study suggests that ZnT3 and vesicular zinc are essential for adult hippocampal neurogenesis.

Zinc Promotes Adipose-Derived Mesenchymal Stem Cell Proliferation and Differentiation towards a Neuronal Fate

Zinc is an essential element required for cell division, migration, and proliferation. Under zinc-deficient conditions, proliferation and differentiation of neural progenitors are significantly impaired. Adipose-derived mesenchymal stem cells (AD-MSCs) are multipotent stem cells that can differentiate into neurons. The aim of this study was to evaluate the effect of zinc on AD-MSC proliferation and differentiation. We initially examined the effect of zinc on stem cell proliferation at the undifferentiated stage. AD-MSCs showed high proliferation rates on day 6 in 30 μM and 100 μM of ZnCl₂. Zinc chelation inhibited AD-MSC proliferation via downregulation of ERK1/2 activity. We then assessed whether zinc was involved in cell migration and neurite outgrowth during differentiation. After three days of neuronal differentiation, TUJ-1-positive cells were observed, implying that AD-MSCs had differentiated into early neuron or neuron-like cells. Neurite outgrowth was increased in the zinc-treated group, while the CaEDTA-treated group showed diminished, shrunken neurites. Furthermore, we showed that zinc promoted neurite outgrowth via the inactivation of RhoA and led to the induction of neuronal gene expression (MAP2 and nestin) in differentiated stem cells. Taken together, zinc promoted AD-MSC proliferation and affected neuronal differentiation, mainly by increasing neurite outgrowth.

Zinc Inhibits TRPV1 to Alleviate Chemotherapy-Induced Neuropathic Pain

Zinc is a transition metal that has a long history of use as an anti-inflammatory agent. It also soothes pain sensations in a number of animal models. However, the effects and mechanisms of zinc on chemotherapy-induced peripheral neuropathy remain unknown. Here we show that locally injected zinc markedly reduces neuropathic pain in male and female mice induced by paclitaxel, a chemotherapy drug, in a TRPV1-dependent manner. Extracellularly applied zinc also inhibits the function of TRPV1 expressed in HEK293 cells and mouse DRG neurons, which requires the presence of zinc-permeable TRPA1 to mediate entry of zinc into the cytoplasm. Moreover, TRPA1 is required for zinc-induced inhibition of TRPV1-mediated acute nociception. Unexpectedly, zinc transporters, but not TRPA1, are required for zinc-induced inhibition of TRPV1-dependent chronic neuropathic pain produced by paclitaxel. Together, our study demonstrates a novel mechanism underlying the analgesic effect of zinc on paclitaxel-induced neuropathic pain that relies on the function of TRPV1.SIGNIFICANCE STATEMENT The chemotherapy-induced peripheral neuropathy is a major limiting factor affecting the chemotherapy patients. There is no effective treatment available currently. We demonstrate that zinc prevents paclitaxel-induced mechanical hypersensitivity via inhibiting the TRPV1 channel, which is involved in the sensitization of peripheral nociceptors in chemotherapy. Zinc transporters in DRG neurons are required for the entry of zinc into the intracellular side, where it inhibits TRPV1. Our study provides insight into the mechanism underlying the pain-soothing effect of zinc and suggests that zinc could be developed to therapeutics for the treatment of chemotherapy-induced peripheral neuropathy.

Localization of zinc transporters in the spinal cord of cynomolgus monkey

Zinc is abundant in the spinal cord, where it participates in several physiological and pathophysiological processes, including neurotransmission, spinal cord injury, and amyotrophic lateral sclerosis. However, the mechanisms underlying zinc homeostasis in the spinal cord are largely unknown. Zinc transporters (ZnTs) are responsible for transporting zinc from the cytoplasm to the extracellular space or to intracellular compartments. In the present study, we examined the distribution of ZnT1-10 proteins in the spinal cord of cynomolgus monkey. Immunohistochemical studies demonstrate that all detected ZnT family members are expressed in the gray matter. ZnT1-10 immunoreactivity can be seen in both motor and sensory neurons in the dorsal and ventral horn from the cervical to sacral segments. No obvious immunostaining was found in the glia cells. The present study demonstrates that ZnT proteins are functionally important for regulating zinc metabolism in both motor and sensory functions in monkey spinal cord.

Zinc-specific Autometallographic In Vivo Selenium Methods: Tracing of Zinc-enriched (ZEN) Terminals, ZEN Pathways, and Pools of Zinc Ions in a Multitude of Other ZEN Cells

In vivo-applied sodium selenide or sodium selenite causes the appearance of zinc-selenium nanocrystals in places where free or loosely bound zinc ions are present. These nanocrystals can in turn be silver enhanced by autometallographic (AMG) development. The selenium method was introduced in 1982 as a tool for zinc-ion tracing, e.g., in vesicular compartments such as synaptic vesicles of zinc-enriched (ZEN) terminals in the central nervous system, and for visualization of zinc ions in ZEN secretory vesicles of, e.g., somatotrophic cells in the pituitary, zymogene granules in pancreatic acinar cells, beta-cells of the islets of Langerhans, Paneth cells of the crypts of Lieberkühn, secretory cells of the tubuloacinar glands of prostate, epithelium of parts of ductus epididymidis, and osteoblasts. If sodium selenide/selenite is injected into brain, spinal cord, spinal nerves containing sympathetic axons, or intraperitoneally, retrograde axonal transport of zinc-selenium nanocrystals takes place in ZEN neurons, resulting in accumulation of zinc-selenium nanocrystals in lysosomes of the neuronal somata. The technique is, therefore, also a highly specific tool for tracing ZEN pathways. The present review includes an update of the 1982 paper and presents evidence that only zinc ions are traced with the AMG selenium techniques if the protocols are followed to the letter.

Zinc chelation reduces traumatic brain injury-induced neurogenesis in the subgranular zone of the hippocampal dentate gyrus

Numerous studies have demonstrated that traumatic brain injury (TBI) increases hippocampal neurogenesis in the rodent brain. However, the mechanisms underlying increased neurogenesis after TBI remain unknown. Continuous neurogenesis occurs in the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG) in the adult brain. The mechanism that maintains active neurogenesis in the hippocampal area is not known. A high level of vesicular zinc is localized in the presynaptic terminals of the SGZ (mossy fiber). The mossy fiber of dentate granular cells contains high levels of chelatable zinc in their terminal vesicles, which can be released into the extracellular space during neuronal activity. Previously, our lab presented findings indicating that a possible correlation may exist between synaptic zinc localization and high rates of neurogenesis in this area after hypoglycemia or epilepsy. Using a weight drop animal model to mimic human TBI, we tested our hypothesis that zinc plays a key role in modulating hippocampal neurogenesis after TBI. Thus, we injected a zinc chelator, clioquinol (CQ, 30mg/kg), into the intraperitoneal space to reduce brain zinc availability twice per day for 1 week. Neuronal death was evaluated with Fluoro Jade-B and NeuN staining to determine whether CQ has neuroprotective effects after TBI. The number of degenerating neurons (FJB (+)) and live neurons (NeuN (+)) was similar in vehicle and in CQ-treated rats at 1 week after TBI. Neurogenesis was evaluated using BrdU, Ki67 and doublecortin (DCX) immunostaining 1 week after TBI. The number of BrdU, Ki67 and DCX positive cell was increased after TBI. However, the number of BrdU, Ki67 and DCX positive cells was significantly decreased by CQ treatment. The present study shows that zinc chelation did not prevent neurodegeneration but did reduce TBI-induced progenitor cell proliferation and neurogenesis. Therefore, this study suggests that zinc has an essential role for modulating hippocampal neurogenesis after TBI.

Supplement zinc as an effective treatment for spinal cord ischemia/reperfusion injury in rats

OBJECTIVE

Brain-derived neurotrophic factor (BDNF) plays a key role in the pathophysiology process and therapy of spinal cord injury (SCI). Accordingly, zinc regulates the expression of BDNF and its receptor in the central nervous system, the mechanism of which is still unknown. The present study investigates whether supplement zinc could reduce neurological damage in a rat model, with spinal cord ischemia-reperfusion (I/R) injury and how the effect of zinc transporter 1(ZnT-1) was involved.

METHODS

100 Sprague-Dawley male rats were randomly and evenly divided into four groups. They were subjected to spinal cord ischemia by clamping the abdominal aorta for 45 min. Rats in the zinc-deficient dietary model group (ZD), zinc-adequate dietary model group (ZA), and zinc-high dietary model group (ZH) were given free access to purified diet, containing 5, 30, or 180 mg Zn/kg. Sham operation rats were subjected to laparotomy without clamping of the aorta and were fed by ZA diet (30 mg Zn/kg). Neurological function was scored by Tarlov's score. The spinal cord segments (L5) were harvested for histological examination, auto-metallographic (AMG) analysis, myeloperoxidase (MPO) activity analysis, expression of ZnT-1 and BDNF.

RESULTS

The rats in the ZH group have shown the higher neurological scores, slighter histological changes and the attenuated MPO activity, compared with those in the ZD and ZA groups at the four observation time points (p<0.05). The AMG staining density in the ZH group was significantly higher than that of ZD group in 14 days later after the operation. Compared with other groups, ZH group's expression of Zn-T1 and BDNF were significantly increased, and was positively correlated with the same time points after surgery (Spearman rho=0.403, p=0.0152.)

CONCLUSION

These findings suggest that zinc supplement can significantly reduce the spinal cord I/R injury in rats. The mechanism may be related with restraining the MPO activity and increasing of ZnT-1, which promoted the synthesis and release of BDNF.

Properties of spinal lamina III GABAergic neurons in naïve and in neuropathic mice

BACKGROUND

Nerve injury leads to Aβ-fibre-mediated mechanical allodynia that is in part due to an impaired GABAergic inhibition in the spinal cord dorsal horn. The properties and function of GABAergic neurons in spinal cord lamina III, an area where low-threshold mechanosensitive Aβ-fibres terminate are, however, largely unknown.

METHODS

We used transgenic mice, which express enhanced green fluorescent protein (EGFP) under control of the promoter GAD67. The morphology and neurochemical characteristics of GABAergic, EGFP-expressing neurons were characterized. We assessed active and passive membrane properties of spinal lamina III GABAergic neurons in naïve animals and animals with a chronic constriction injury (CCI) of the sciatic nerve.

RESULTS

EGFP-expressing neurons in lamina III were predominantly islet cells (47%), whereas non-EGFP-expressing neurons were largely inverted stalked cells (40%). EGFP-expressing neurons accounted for about 25% of GABAergic neurons in lamina III. Forty-four percent co-expressed glycine, 10% neuronal nitric oxide synthase and 3% co-expressed parvalbumin. We found costaining with protein kinase CβII in 42% of EGFP-expressing neurons but no expression of protein kinase Cγ. Membrane properties and excitability of EGFP-and non-EGFP-expressing neurons from naïve and neuropathic animals were indistinguishable. The most frequent firing pattern was tonic firing (naïve: 35%, neuropathic: 37%) followed by gap firing (naïve: 33%, neuropathic: 25%). Delayed, initial burst and single-spike firing patterns made up the remainder in both groups.

CONCLUSION

A change in membrane excitability or discharge pattern of this group of lamina III GABAergic neurons is unlikely the cause for mechanical allodynia in animals with CCI.

Zinc chelation reduces hippocampal neurogenesis after pilocarpine-induced seizure

Several studies have shown that epileptic seizures increase hippocampal neurogenesis in the adult. However, the mechanism underlying increased neurogenesis after seizures remains largely unknown. Neurogenesis occurs in the subgranular zone (SGZ) of the hippocampus in the adult brain, although an understanding of why it actively occurs in this region has remained elusive. A high level of vesicular zinc is localized in the presynaptic terminals of the SGZ. Previously, we demonstrated that a possible correlation may exist between synaptic zinc localization and high rates of neurogenesis in this area after hypoglycemia. Using a lithium-pilocarpine model, we tested our hypothesis that zinc plays a key role in modulating hippocampal neurogenesis after seizure. Then, we injected the zinc chelator, clioquinol (CQ, 30 mg/kg), into the intraperitoneal space to reduce brain zinc availability. Neuronal death was detected with Fluoro Jade-B and NeuN staining to determine whether CQ has neuroprotective effects after seizure. The total number of degenerating and live neurons was similar in vehicle and in CQ treated rats at 1 week after seizure. Neurogenesis was evaluated using BrdU, Ki67 and doublecortin (DCX) immunostaining 1 week after seizure. The number of BrdU, Ki67 and DCX positive cell was increased after seizure. However, the number of BrdU, Ki67 and DCX positive cells was significantly decreased by CQ treatment. Intracellular zinc chelator, N,N,N0,N-Tetrakis (2-pyridylmethyl) ethylenediamine (TPEN), also reduced seizure-induced neurogenesis in the hippocampus. The present study shows that zinc chelation does not prevent neurodegeneration but does reduce seizure-induced progenitor cell proliferation and neurogenesis. Therefore, this study suggests that zinc has an essential role for modulating hippocampal neurogenesis after seizure.

Chemical coding of zinc-enriched neurons in the intramural ganglia of the porcine jejunum

Zinc ions in the synaptic vesicles of zinc-enriched neurons (ZEN) seem to have an important role in normal physiological and pathophysiological processes in target organ innervation. The factor directly responsible for the transport of zinc ions into synaptic vesicles is zinc transporter 3 (ZnT3), a member of the divalent cation zinc transporters and an excellent marker of ZEN neurons. As data concerning the existence of ZEN neurons in the small intestine is lacking, this study was designed to disclose the presence and neurochemical coding of such neurons in the porcine jejunum. Cryostat sections (10 mμ thick) of porcine jejunum were processed for routine double- and triple-immunofluorescence labeling for ZnT3 in various combinations with immunolabeling for other neurochemicals including pan-neuronal marker (PGP9.5), substance P (SP), somatostatin (SOM), vasoactive intestinal peptide (VIP), nitric oxide synthase (NOS), leu-enkephalin (LENK), vesicular acetylcholine transporter (VAChT), neuropeptide Y (NPY), galanin (GAL), and calcitonin-gene related peptide (CGRP). Immunohistochemistry revealed that approximately 39%, 49%, and 45% of all PGP9.5- positive neurons in the jejunal myenteric (MP), outer submucous (OSP), and inner submucous (ISP) plexuses, respectively, were simultaneously ZnT3⁺. The majority of ZnT3⁺ neurons in all plexuses were also VAChT-positive. Both VAChT-positive and VAChT-negative ZnT3⁺ neurons co-expressed a variety of active substances with diverse patterns of co-localization depending on the plexus studied. In the MP, the largest populations among both VAChT-positive and VAChT-negative ZnT3⁺ neurons were NOS-positive cells. In the OSP and ISP, substantial subpopulations of ZnT3⁺ neurons were VAChT-positive cells co-expressing SOM and GAL, respectively. The broad-spectrum of active substances that co-localize with the ZnT3⁺ neurons in the porcine jejunum suggests that ZnT3 takes part in the regulation of various processes in the gut, both in normal physiological and during pathophysiological processes.

Neurochemical characterization of zinc transporter 3-like immunoreactive (ZnT3(+)) neurons in the intramural ganglia of the porcine duodenum

The SLC30 family of divalent cation transporters is thought to be involved in the transport of zinc in a variety of cellular pathways. Zinc transporter 3 (ZnT3) is involved in the transport of zinc into synaptic vesicles or intracellular organelles. As the presence of ZnT3 immunoreactive neurons has recently been reported in both the central and peripheral nervous systems of the rat, the present study was aimed at disclosing the presence of a zinc-enriched neuron enteric population in the porcine duodenum to establish a preliminary insight into their neurochemical coding. Double- and triple-immunofluorescence labeling of the porcine duodenum for ZnT3 with the pan-neuronal marker (PGP 9.5), substance P, somatostatin, vasoactive intestinal peptide (VIP), nitric oxide synthase (NOS), leu-enkephalin, vesicular acetylcholine transporter (VAChT), neuropeptide Y, galanin (GAL), and calcitonin gene-related peptide were performed. Immunohistochemistry revealed that approximately 35, 43, and 48 % of all PGP9.5-postive neurons in the myenteric (MP), outer submucous (OSP), and inner submucous (ISP) plexuses, respectively, of the porcine duodenum were simultaneously ZnT3⁺. In the present study, ZnT3⁺ neurons coexpressed a broad spectrum of active substances, but co-localization patterns unique to the plexus were studied. In the ISP, all ZnT3⁺ neurons were VAChT positive, and the largest populations among these cells formed ZnT3⁺/VAChT⁺/GAL⁺ and ZnT3⁺/VAChT⁺/VIP⁺ cells. In the OSP and MP, the numbers of ZnT3⁺/VAChT⁺ neurons were two times smaller, and substantial subpopulations of ZnT3⁺ neurons in both these plexuses formed ZnT3⁺/NOS⁺ cells. The large population of ZnT3⁺ neurons in the porcine duodenum and a broad spectrum of active substances which co-localize with this peptide suggest that ZnT3 takes part in the regulation of various processes in the gut both in normal physiology and during pathological processes.

Exercise-induced motor improvement after complete spinal cord transection and its relation to expression of brain-derived neurotrophic factor and presynaptic markers

BACKGROUND

It has been postulated that exercise-induced activation of brain-derived neurotrophic factor (BDNF) may account for improvement of stepping ability in animals after complete spinal cord transection. As we have shown previously, treadmill locomotor exercise leads to up-regulation of BDNF protein and mRNA in the entire neuronal network of intact spinal cord. The questions arise: (i) how the treadmill locomotor training, supplemented with tail stimulation, affects the expression of molecular correlates of synaptic plasticity in spinal rats, and (ii) if a response is related to BDNF protein level and distribution. We investigated the effect of training in rats spinalized at low thoracic segments on the level and distribution of BDNF immunoreactivity (IR) in ventral quadrants of the lumbar segments, in conjunction with markers of presynaptic terminals, synaptophysin and synaptic zinc.

RESULTS

Training improved hindlimb stepping in spinal animals evaluated with modified Basso-Beattie-Bresnahan scale. Grades of spinal trained animals ranged between 5 and 11, whereas those of spinal were between 2 and 4. Functional improvement was associated with changes in presynaptic markers and BDNF distribution. Six weeks after transection, synaptophysin IR was reduced by 18% around the large neurons of lamina IX and training elevated its expression by over 30%. The level of synaptic zinc staining in the ventral horn was unaltered, whereas in ventral funiculi it was decreased by 26% postlesion and tended to normalize after the training. Overall BDNF IR levels in the ventral horn, which were higher by 22% postlesion, were unchanged after the training. However, training modified distribution of BDNF in the processes with its predominance in the longer and thicker ones. It also caused selective up-regulation of BDNF in two classes of cells (soma ranging between 100-400 microm2 and over 1000 microm2) of the ventrolateral and laterodorsal motor nuclei.

CONCLUSION

Our results show that it is not BDNF deficit that determines lack of functional improvement in spinal animals. They indicate selectivity of up-regulation of BDNF in distinct subpopulations of cells in the motor nuclei which leads to changes of innervation targeting motoneurons, tuned up by locomotor activity as indicated by a region-specific increase of presynaptic markers.

Zinc: the brain's dark horse

Zinc is a life-sustaining trace element, serving structural, catalytic, and regulatory roles in cellular biology. It is required for normal mammalian brain development and physiology, such that deficiency or excess of zinc has been shown to contribute to alterations in behavior, abnormal central nervous system development, and neurological disease. In this light, it is not surprising that zinc ions have now been shown to play a role in the neuromodulation of synaptic transmission as well as in cortical plasticity. Zinc is stored in specific synaptic vesicles by a class of glutamatergic or "gluzinergic" neurons and is released in an activity-dependent manner. Because gluzinergic neurons are found almost exclusively in the cerebral cortex and limbic structures, zinc may be critical for normal cognitive and emotional functioning. Conversely, direct evidence shows that zinc might be a relatively potent neurotoxin. Neuronal injury secondary to in vivo zinc mobilization and release occurs in several neurological disorders such as Alzheimer's disease and amyotrophic lateral sclerosis, in addition to epilepsy and ischemia. Thus, zinc homeostasis is integral to normal central nervous system functioning, and in fact its role may be underappreciated. This article provides an overview of zinc neurobiology and reviews the experimental evidence that implicates zinc signals in the pathophysiology of neuropsychiatric diseases. A greater understanding of zinc's role in the central nervous system may therefore allow for the development of therapeutic approaches where aberrant metal homeostasis is implicated in disease pathogenesis.

Axonal transport of zinc transporter 3 and zinc containing organelles in the rodent adrenergic system

Zinc is the second most abundant trace metal (after iron) in mammalian tissues, and it is an essential element for growth, development, DNA synthesis, immunity, and other important cellular processes. A considerable amount of zinc in the brain exists as a pool of free or loosely bound zinc ions in synaptic vesicles with zinc transporter 3 (ZnT3) in their membranes. Here we demonstrate that also in the peripheral sympathetic nervous system zinc handling neurons exist. In autonomic ganglia of rats and mice a subset of neuronal cell bodies contain zinc, visualized by the autometallographic (AMG) and TSQ histochemical methods. The Zn-transporter 3 is, as shown by immunofluorescence, also present in tyrosine hydroxylase (TH)-positive neurons, but rarely in cell bodies with neuropeptide Y (NPY)-immunoreactivity (IR). In axons of crush-operated sciatic nerves a rapid bidirectional accumulation of AMG granules occurred. Also ZnT3-IR was found to accumulate rapidly in anterograde as well as retrograde direction, colocalized with TH-IR. So far nerve terminals with ZnT3-IR have not been observed. The functional significance of zinc ions in the sympathetic system is not known.

Expression of zinc transporter ZnT7 in mouse superior cervical ganglion

The superior cervical ganglion (SCG) neurons contain a considerable amount of zinc ions, but little is known about the zinc homeostasis in the SCG. It is known that zinc transporter 7 (ZnT7, Slc30a7), a member of the Slc30 ZnT family, is involved in mobilizing zinc ions from the cytoplasm into the Golgi apparatus. In the present study, we examined the expression and localization of ZnT7 and labile zinc ions in the mouse SCG using immunohistochemistry, Western blot and in vivo zinc selenium autometallography (AMG). Our immunohistochemical analysis revealed that the ZnT7 immunoreactivity in the SCG neurons was predominately present in the perinuclear region of the neurons, suggesting an affiliation to the Golgi apparatus. The Western blot results verified that ZnT7 protein was expressed in the mouse SCGs. The AMG reaction product was shown to have a similar distribution as ZnT7 immunoreactivity. These observations support the notion that ZnT7 may participate in zinc transport, storage, and incorporation of zinc into zinc-binding proteins in the Golgi apparatus of mouse SCG neurons.

Acid sensing ion channels in dorsal spinal cord neurons

Acid-sensing ion channels (ASICs) are broadly expressed in the CNS, including the spinal cord. However, very little is known about the properties of ASICs in spinal cord neurons compared with brain. We show here that ASIC1a and ASIC2a are the most abundant ASICs in mouse adult spinal cord and are coexpressed by most neurons throughout all the laminas. ASIC currents in cultured embryonic day 14 mouse dorsal spinal neurons mainly flow through homomeric ASIC1a (34% of neurons) and heteromeric ASIC1a plus 2a channels at a ratio of 2:1 (83% of neurons). ASIC2b only has a minor contribution to these currents. The two channel subtypes show different active pH ranges and different inactivation and reactivation kinetics supporting complementary functional properties. One striking property of native dorsal spinal neuron currents and recombinant currents is the pH dependence of the reactivation process. A light sustained acidosis induces a threefold slow-down of the homomeric ASIC1a (from pH 7.4 to pH 7.3) and heteromeric ASIC1a plus 2a (from pH 7.4 to pH 7.2) current reactivation (T(0.5) increasing from 5.77 to 16.84 s and from 0.98 to 3.2 s, respectively), whereas a larger acidosis to pH 6.6 induces a 32-fold slow-down of the ASIC1a plus 2a current reactivation (T(0.5) values increasing to 31.30 s). The pH dependence of ASIC channel reactivation is likely to modulate neuronal excitability associated with repetitive firing in response to extracellular pH oscillations, which can be induced, for example, by intense synaptic activity of central neurons.

Zinc distribution and expression pattern of ZnT3 in mouse brain

To explore the relationship between the zinc distribution and zinc transporter 3 (ZnT3) mRNA expression in the mouse brain, zinc contents and its distribution were determined by synchrotron radiation x-ray fluorescence (SRXRF), and ZnT3 mRNA expression was examined by reverse-transcription polymerase chain reaction and in situ hybridization. The results showed that the zinc contents were not distributed evenly in various brain tissues. The zinc contents in cerebral cortex and hippocampus were nearly 5-10 times higher than that in other neural locations. Correspondingly, ZnT3 mRNA expression was observed in high abundance in the cerebral cortex, hippocampus, and testis, but was not detected in other organs and tissues. In the nervous system, ZnT3 mRNA was detected mainly in hippocampus, cerebral cortex, and spinal ganglion. The present results show the coincident distribution of zinc and ZnT3 mRNA in mouse brain. The high zinc contents might be determined by the high expression of ZnT3. More meaningfully, the results showed the feasibility of applying of SRXRF in examining the distribution of minerals in different organs and tissues. In addition, it was observed for the first time that ZnT3 mRNA was expressed in the facial nucleus. The function of ZnT3 in facial nucleus awaits further study.

Amyloid plaques arise from zinc-enriched cortical layers in APP/PS1 transgenic mice and are paradoxically enlarged with dietary zinc deficiency

The ZnT3 zinc transporter is uniquely expressed in cortical glutamatergic synapses where it organizes zinc release into the synaptic cleft and mediates beta-amyloid deposition in transgenic mice. We studied the association of zinc in plaques in relation to cytoarchitectural zinc localization in the APP/PS1 transgenic mouse model of Alzheimer's disease. The effects of low dietary zinc for 3 months upon brain pathology were also studied. We determined that synaptic zinc distribution within cortical layers is paralleled by amyloid burden, which is heaviest for both in layers 2-3 and 5. ZnT3 immunoreactivity is prominent in dystrophic neurites within amyloid plaques. Low dietary zinc caused a significant 25% increase in total plaque volume in Alzheimer's mice using stereological measures. The level of oxidized proteins in brain tissue did not changed in animals on a zinc-deficient diet compared with controls. No obvious changes were observed in the autometallographic pattern of zinc-enriched terminals in the neocortex or in the expression levels of zinc transporters, zinc importers or metallothioneins. A small decrease in plasma zinc induced by the low-zinc diet was consistent with the subclinical zinc deficiency that is common in older human populations. While the mechanism remains uncertain, our findings indicate that subclinical zinc deficiency may be a risk factor for Alzheimer's pathology.

Depletion of vesicular zinc in dorsal horn of spinal cord causes increased neuropathic pain in mice

Zinc enriched (ZEN) neurons and terminals are abundant in the rodent spinal cord. Zinc ions have been suggested to modulate the excitability of primary afferent fibers believed to be important in nociceptive transmission. To test the hypothesis that vesicular zinc concentration is related to neuropathic pain we applied Chung's rodent pain model on BALB/c mice, and traced zinc transporter 3 (ZnT3) proteins and zinc ions with immunohistochemistry and autometallography (AMG), respectively. Under anesthesia the left fifth lumbar spinal nerve was ligated in male mice in order to produced neuropathic pain. The animals were then sacrificed 5 days later. The ZnT3 immunoreactivity was found to have decreased significantly in dorsal horn of fourth, fifth, and sixth lumbar segments. In parallel with the depressed ZnT3 immunoreactivity the amount of vesicular zinc decreased perceptibly in superficial gray matters of especially layer I-IV of the same segments. The transection-induced reduction of vesicular zinc in ZEN terminals of the dorsal horn was synchronic to reduced pain threshold, as measured by von Frey method. In a separate study, we observed intensive zinc selenite precipitation in somata of the smaller spinal ganglion cell, but 5 days after spinal nerve transection zinc precipitation was also found in the lager ganglion cells. The present results indicate that zinc may be involved in pain mechanism in the spinal ganglion level. These results support the hypothesis that vesicular zinc might have a modulatory role for neuropathic pain. Thus, increased pain sensitivity might be related to reduce vesicular zinc level in the dorsal spinal gray matter.

The effect of zinc on glycinergic inhibitory postsynaptic currents in rat spinal dorsal horn neurons

The effect of zinc on glycinergic spontaneous inhibitory postsynaptic currents (IPSCs) was investigated using the whole-cell patch-clamp technique in mechanically dissociated rat spinal dorsal horn neurons. Zinc at a concentration of 10 microM reversibly increased the spontaneous IPSC frequency without changing the current amplitudes, suggesting that zinc increases spontaneous glycine release from presynaptic nerve terminals. At a low concentration of 1 microM, on the other hand, zinc potentiated the amplitude of spontaneous IPSCs but had no effect on the frequency. At a high concentration of 100 microM, zinc increased the spontaneous IPSC frequency while it inhibited the IPSC amplitude. The current evoked by exogenously applied glycine was potentiated and inhibited by low and high concentrations of zinc, respectively. The increase in spontaneous IPSC frequency by 10 microM zinc was inhibited by blocking the voltage-dependent Ca(2+) channels in the presence of both omega-conotoxin-MVIIC and nifedipine. The facilitatory effect of zinc on spontaneous IPSC frequency was also inhibited in the presence of tetrodotoxin. In the slice preparation, 30 microM zinc potentiated the evoked IPSC amplitude and decreased the paired pulse ratio. These results suggest that, in addition to an action on the postsynaptic glycine receptors, zinc may depolarize the presynaptic nerve terminals, leading to an activation of voltage-dependent Na(+) and Ca(2+) channels that in turn increases glycine release. Since dorsal horn neurons receive nociceptive inputs, zinc may play an important role in the regulation of sensory transmission.

A musculoskeletal model of low grade connective tissue inflammation in patients with thyroid associated ophthalmopathy (TAO): the WOMED concept of lateral tension and its general implications in disease

Background

Low level connective tissue inflammation has been proposed to play a role in thyroid associated ophthalmopathy (TAO). The aim of this study was to investigate this postulate by a musculoskeletal approach together with biochemical parameters.

Methods

13 patients with TAO and 16 controls were examined. Erythrocyte levels of Zn, Cu, Ca²⁺, Mg, and Fe were determined. The musculoskeletal evaluation included observational data on body posture with emphasis on the orbit-head region. The angular foot position in the frontal plane was quantified following gait observation. The axial orientation of the legs and feet was evaluated in an unloaded supine position. Functional propioceptive tests based on stretch stimuli were done by using foot inversion and foot rotation.

Results

Alterations in the control group included neck tilt in 3 cases, asymmetrical foot angle during gait in 2, and a reaction to foot inversion in 5 cases. TAO patients presented facial asymmetry with displaced eye fissure inclination (mean 9.1°) as well as tilted head-on-neck position (mean 5.7°). A further asymmetry feature was external rotation of the legs and feet (mean 27°). Both foot inversion as well as foot rotation induced a condition of neuromuscular deficit. This condition could be regulated by gentle acupressure either on the lateral abdomen or the lateral ankle at the acupuncture points gall bladder 26 or bladder 62, respectively. In 5 patients, foot rotation produced a phenomenon of moving toes in the contra lateral foot. In addition foot rotation was accompanied by an audible tendon snapping. Lower erythrocyte Zn levels and altered correlations between Ca²⁺, Mg, and Fe were found in TAO.

Conclusion

This whole body observational study has revealed axial deviations and body asymmetry as well as the phenomenon of moving toes in TAO. The most common finding was an arch-like displacement of the body, i.e. eccentric position, with foot inversion and head tilt to the contra lateral side and tendon snapping. We propose that eccentric muscle action over time can be the basis for a low grade inflammatory condition. The general implications of this model and its relations to Zn and Se will be discussed.

Hyperekplexia Phenotype of Glycine Receptor α1 Subunit Mutant Mice Identifies Zn2+ as an Essential Endogenous Modulator of Glycinergic Neurotransmission

Zn(2+) is thought to modulate neurotransmission by affecting currents mediated by ligand-gated ion channels and transmitter reuptake by Na(+)-dependent transporter systems. Here, we examined the in vivo relevance of Zn(2+) neuromodulation by producing knockin mice carrying the mutation D80A in the glycine receptor (GlyR) alpha1 subunit gene (Glra1). This substitution selectively eliminates the potentiating effect of Zn(2+) on GlyR currents. Mice homozygous for Glra1(D80A) develop a severe neuromotor phenotype postnatally that resembles forms of human hyperekplexia (startle disease) caused by mutations in GlyR genes. In spinal neurons and brainstem slices from Glra1(D80A) mice, GlyR expression, synaptic localization, and basal glycinergic transmission were normal; however, potentiation of spontaneous glycinergic currents by Zn(2+) was significantly impaired. Thus, the hyperekplexia phenotype of Glra1(D80A) mice is due to the loss of Zn(2+) potentiation of alpha1 subunit containing GlyRs, indicating that synaptic Zn(2+) is essential for proper in vivo functioning of glycinergic neurotransmission.

Silver enhancement of quantum dots resulting from (1) metabolism of toxic metals in animals and humans, (2) in vivo, in vitro and immersion created zinc–sulphur/zinc–selenium nanocrystals, (3) metal ions liberated from metal implants and particles

Autometallographic (AMG) silver enhancement is a potent histochemical tool for tracing a variety of metal containing nanocrystals, e.g. pure gold and silver nanoclusters and quantum dots of silver, mercury, bismuth or zinc, with sulphur and/or selenium. These nanocrystals can be created in many different ways, e.g. (1) by manufacturing colloidal gold or silver particles, (2) by treating an organism in vivo with sulphide or selenide ions, (3) as the result of a metabolic decomposition of bismuth-, mercury- or silver-containing macromolecules in cell organelles, or (4) as the end product of histochemical processing of tissue sections. Such nano-sized AMG nanocrystals can then be silver-amplified several times of magnitude by being exposed to an AMG developer, i.e. a normal photographic developer enriched with silver ions. The present monograph attempts to provide a review of the autometallographic silver amplification techniques known today and their use in biology. After achieving a stronghold in histochemistry by Timm's introduction of the "silver-sulphide staining" in 1958, the AMG technique has evolved and expanded into several different areas of research, including immunocytochemistry, tracing of enzymes at LM and EM levels, blot staining, retrograde axonal tracing of zinc-enriched (ZEN) neurons, counterstaining of semithin sections, enhancement of histochemical reaction products, marking of phagocytotic cells, staining of myelin, tracing of gold ions released from gold implants, and visualization of capillaries. General technical comments, protocols for the current AMG methods and a summary of the most significant scientific results obtained by this wide variety of AMG histochemical approaches are included in the present article.

Adrenalectomy-induced ZnT3 downregulation in mouse hippocampus is followed by vesicular zinc depletion

The effects of adrenalectomy (ADX) on the vesicular zinc content of zinc-enriched (ZEN) terminals in mouse hippocampus were investigated at light microscopic levels using zinc transporter-3 immunohistochemistry (ZnT3IHC) and zinc selenium autometallography (ZnSeAMG). ZnT3 resides in the synaptic vesicle membranes of ZEN neurons and is believed to move zinc ions into the vesicles. ZnT3IHC staining closely corresponds to the ZnSeAMG staining, but in the present study we present evidence of a delayed decrease of ZEN zinc, as compared to downregulation of the ZnT3 protein following ADX. Twenty-four hours after adrenalectomy the level of ZnT3IHC was visibly reduced while the ZnSeAMG staining intensity seemed unchanged. After 10 and 30 days, however, downregulation of ZnT3 was paralleled by a distinct reduction in ZnSeAMG staining. The total protein concentration of ZnT3 was reduced by about 53%, and the total zinc concentration in the hippocampus of the same mice was reduced by 43-64%, 30 days after the adrenalectomy. The present results support previous results suggesting that ZnT3 is responsible for transport of zinc ions into a pool of synaptic vesicles in ZEN terminals.

Abundant expression of zinc transporters in Bergman glia of mouse cerebellum

Zinc transporters (ZnTs) are membrane proteins involved in zinc ion transportation in mammalian cells. Seven members of ZnT family, ZnT1-7, have been cloned and characterized. These transporter proteins have different cellular and sub-cellular locations, suggesting that they may play different roles in zinc homeostasis in normal and pathological conditions in different tissues. Cerebellum is one of the most zinc-enriched regions in the central nervous system, but little is known about zinc metabolism in the cerebellum. In the present study, we investigated the detailed distributions of four members (ZnT1, ZnT3, ZnT4 and ZnT6) of the ZnT family, in the mouse cerebellum. Immunostaining and confocal microscopic observations revealed a similar staining pattern of ZnTs in the molecular layer and the Purkinje cell layer. Double labeling with anti-S-100beta or anti-MAP2 and anti-ZnTs clearly showed that the Bergman glial cell bodies in the Purkinje cell layer and their radial processes in the molecular layer exhibited strong immunofluorescence of all the tested ZnTs. However, the somata of the Purkinje cells contained a moderate immunostaining for ZnT1, but virtually lack of other three ZnTs. In the granular layer, ZnTs appeared with different immunostaining patterns. ZnT1 was expressed in a small number of neuronal cell bodies and their primary dendrites, whereas ZnT3 and ZnT4 were present in nerve terminals but not in the neuronal somata. ZnT6 was undetectable in either the cell bodies or processes in the granular layer. The present results indicate that the Bergman glial cells may play an important role in zinc metabolism in the mouse cerebellar cortex.

Zn2+ transporters and Zn2+ homeostasis in neurons

Although the presence of Zn2+ in the brain has been known for nearly half a century, only recently has its precise location and potential roles as a neuromodulator and signaling molecule as well as neurotoxic agent come to the forefront. Unfortunately, our understanding of Zn2+ homeostatic mechanisms lags far behind. The recent identification of presumed Zn2+ transporters has opened new approaches to studying Zn2+ homeostatic mechanisms in neurons. Zn2+ transporters are involved in separate Zn2+ influx and efflux pathways in neurons. However, we are only beginning to understand the mechanism of Zn2+ transport and much more research needs to be done. We are only beginning to understand the transcriptional control and cellular location of Zn2+ transporters, as well. Finally, this review presents a working model of neuronal Zn2+ homeostasis and discusses the experimental evidence for the proposed roles that Zn2+ transporters might play.

Zinc transporter 3 and zinc ions in the rodent superior cervical ganglion neurons

Previous studies have revealed that zinc-enriched (ZEN) terminals are present in all parts of the CNS though with great differences in intensity. The densest populations of both ZEN terminals and ZEN somata are found in telencephalic structures, but also structures like the spinal cord demonstrate impressive ZEN systems spreading terminals several segments around the respective ZEN somata. The present study evaluates whether sympathetic neurons in the superior cervical ganglia (SCG) are ZEN neurons, i.e. contain vesicles that have zinc transporter 3 (ZnT3) proteins in their membranes and contain zinc ions. ZnT3 immunoreactivity (IR) was found in the somata and processes in the postganglionic neurons of mouse SCG. Only a small fraction of neurons (less than 5%), expressed varying degrees of ZnT3. Colchicine treatment, however, increased the number of ZnT3-positive neurons three-fold, suggesting an accumulation of ZnT3 protein in the somata. A small proportion of the postganglionic axons revealed dotted accumulations of ZnT3 IR along their courses. Double labeling showed that all ZnT3-positive neurons and axons were also tyrosine hydroxylase-positive with strong immunofluorescence, while no colocalization was found between ZnT3 and the vesicular acetylcholine transporter (VAChT) or neuropeptide Y IR. VAChT-positive preganglionic neurons were found to terminate on ZnT3 neuronal somata. 6-Methoxy 8-para toluene sulfonamide quinoline fluorescence and zinc selenium autometallography (ZnSe(AMG)) revealed that a subgroup of SCG cells contained free or loosely bound zinc ions. It is therefore concluded that ZnT3 and zinc ions are present in a subpopulation of TH-positive, NPY-negative neurons in the rodent SCG, supporting the notion that vesicular zinc ions may play a special role in the peripheral sympathetic adrenergic system.

Immunocytochemical localization of zinc transporter 3 in the ependyma of the mouse spinal cord

We report, for the first time, the light microscopical and ultrastructural appearance of ZnT3-immunoreactivities in the ependymal cells of the central canal of the mouse spinal cord. Light microscopy revealed the presence of ZnT3-immunoreactive (Ir) ependymal cells in 1 microm thick epon sections stained by the ABC method. The ZnT3-Ir cells were observed at all levels of the spinal cord, but were a little more numerous in lumbosacral segments than in cervicothoracic segments. The ZnT3-Ir cells had large, ovoid nuclei with abundant cytoplasm, and protruded into the lumen of the central canal. Our ultrastructural findings suggest that the ZnT3-Ir ependymal cells possess secretory activity directed towards the central canal. We propose that they may play a role in the trans-ependymal mechanism responsible for zinc homeostasis between cerebrospinal fluid and the central area of the gray matter.

Zinc in the spinal cord of a mutant SOD1 mouse model of ALS

Both decreases and increases in zinc have been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). We therefore examined the distribution of zinc in transgenic mutant superoxide dismutase 1 (SOD1) mice, a model for ALS. Frozen sections of spinal cord from these mice were stained for free zinc with autometallography. Zinc granules in the spinal anterior horn surrounded motor neuron cell bodies and their processes. The same distribution of zinc was seen in wildtype mice. The onset of weakness in the mutant SOD1 mice did not alter the zinc distribution. Changes in the tissue distribution of free zinc do not appear to play a role in the pathogenesis of mutant SOD1-associated ALS.

Potentiation of inhibitory glycinergic neurotransmission by Zn2+: a synergistic interplay between presynaptic P2X2 and postsynaptic glycine receptors

The divalent cation zinc is known to modulate chloride currents carried by native and recombinant mammalian glycine receptors (GlyRs). To unravel the effect of Zn2+ on glycinergic neurotransmission, inhibitory postsynaptic currents (IPSC) of rat spinal neurons grown in culture were analysed in the absence and presence of Zn2+. Low concentrations of Zn2+ (0.5 and 5 micro m) augmented the mean amplitude of miniature IPSCs by approximately 40% over values obtained in the absence of zinc, whereas higher concentrations of Zn2+ (50 micro m) significantly decreased mean amplitude values. Remarkably, low concentrations of Zn2+ also significantly increased the mean frequency of miniature IPSCs. This effect was blocked by the P2X receptor antagonists PPADS and suramin, indicating the presence of Zn2+-sensitive presynaptic P2X receptors on glycinergic terminals. Immunostaining with antibodies against different P2X receptor subtypes revealed that P2X2 receptors partially colocalize with the GlyR. Potentiating concentrations of Zn2+ also affected the kinetics of miniature and evoked IPSCs by significantly prolonging their decay time constants. Electrophysiological analysis of heterologously expressed glycine transporters (GlyT) revealed for GlyT2 zero, and for GlyT1 a modest (< 20%), reduction of glycine uptake in the presence of 5 micro m Zn2+, indicating that prolongation of glycinergic IPSCs by Zn2+ is not due to inhibition of glycine removal from the synaptic cleft. Together, these results suggest that Zn2+ is a potent modulator of glycinergic synaptic transmission which increases in a synergistic manner the agonist affinity of both presynaptic P2X2 receptors and postsynaptic GlyRs.

Inhibitory zinc-enriched terminals in the mouse cerebellum: double-immunohistochemistry for zinc transporter 3 and glutamate decarboxylase

In the present study, we showed for the first time the presence of inhibitory zinc-enriched neuron terminals in the mouse cerebellar cortex by means of double-immunohistochemistry for zinc transporter 3 (ZnT3) and glutamate decarboxylase (GAD). The co-localization of ZnT3 and GAD in the cerebellar cortex was analyzed by confocal microscopy. Strong, punctuate ZnT3-immunoreactivity (Ir) was predominantly distributed in the granule cell layer, while GAD-Ir was seen throughout the cerebellar cortical layers. All of the ZnT3-immunoreactive structures were also immunopositive to GAD, but not vice versa. Based on size and position, these double-labeled elements were axonal terminals of the Golgi and basket cells, in the granule cell and molecular layers, respectively. Observations by electron microscopy revealed that ZnT3-immunoreactive terminals showed typical characteristics of the inhibitory synapses like the following: (1) presynaptic terminals containing flat vesicles; and (2) symmetrical synaptic contacts with dendritic elements. The present results indicate that a zinc-containing GABAergic system exists in the mouse cerebellar cortex.

Localization of zinc-enriched neurons in the mouse peripheral sympathetic system

Growing evidence supports the notion that zinc ions located in the synaptic vesicles of zinc-enriched neurons (ZEN) play important physiological roles and are involved in certain pathological changes in the central nervous system. Here we present data revealing the distribution of zinc ions and the co-localization of zinc transporter 3 (ZnT3) and tyrosine hydroxylase (TH) in crush-operated sciatic nerves and lumbar sympathetic ganglia of mice, using zinc selenide autometallography (ZnSe(AMG)) and ZnT3 immunofluorescence combined with confocal scanning microscopy, respectively. Six hours after the crush operation, ZnSe(AMG) grains and ZnT3 immunoreactivity were predominantly present in a subpopulation of thin unmyelinated sciatic nerve axons. In order to identify the type(s) of ZEN axons involved, double labeling with ZnT3 and (1) TH, (2) vesicular acetylcholine transporter (VAChT), (3) calcitonin gene-related peptide (CGRP), and (4) neuropeptide Y (NPY) was performed. Confocal microscopic observations showed that ZnT3 was located in a subpopulation of sciatic axons in distended parts proximal and distal to the crush site. Most, if not all, ZnT3-positive axons contained TH immunofluorescence, a few showed co-localization of ZnT3 and VAChT with very weak immunostaining, while no congruence was observed between ZnT3 and CGRP or NPY. Studies of the lumbar sympathetic ganglia showed that not more than 5% of the neurons were ZnT3-positive and that almost all of these were TH-positive. Furthermore, approximately 5% of total lumbar sympathetic ganglionic cells were ZnSe(AMG) positive, 48 h after a local injection of sodium selenide into the sciatic nerve. The present data support the notion that a subgroup of mouse sympathetic postganglionic neurons are ZEN neurons.

Inhibitory zinc-enriched terminals in mouse spinal cord

The ultrastructural localization of zinc transporter-3, glutamate decarboxylase and zinc ions in zinc-enriched terminals in the mouse spinal cord was studied by zinc transporter-3 and glutamate decarboxylase immunohistochemistry and zinc selenium autometallography, respectively. The distribution of zinc selenium autometallographic silver grains, and zinc transporter-3 and glutamate decarboxylase immunohistochemical puncta in both ventral and dorsal horns as seen in the light microscope corresponded to their presence in the synaptic vesicles of zinc-enriched terminals at ultrastructural levels. The densest populations of zinc-enriched terminals were seen in dorsal horn laminae I, III and IV, whereas the deeper laminae V and VI contained fewer terminals. At ultrastructural levels, zinc-enriched terminals primarily formed symmetrical synapses on perikarya and dendrites. Only relatively few asymmetrical synapses were observed on zinc-enriched terminals. In general, the biggest zinc-enriched terminals contacted neuronal somata and large dendritic elements, while medium-sized and small terminals made contacts on small dendrites. The ventral horn was primarily populated by big and medium-sized zinc-enriched terminals, whereas the dorsal horn was dominated by medium-sized and small zinc-enriched terminals. The presence of boutons with flat synaptic vesicles with zinc ions and symmetric synaptic contacts suggests the presence of inhibitory zinc-enriched terminals in the mammalian spinal cord, and this was confirmed by the finding that zinc ions and glutamate decarboxylase are co-localized in these terminals. The pattern of zinc-enriched boutons in both dorsal and ventral horns is compatible with evidence suggesting that zinc may be involved in both sensory transmission and motor control.

Zinc-enriched GABAergic terminals in mouse spinal cord

Electrophysiological experiments have shown that zinc ions modulate glutamate and GABA receptors in brain slices. All the zinc-enriched neuronal pathways in the brain analyzed up until now have been found to be glutaminergic. Many years ago, zinc-enriched terminals with flat vesicles and symmetric synapses were found to be present in rat spinal cord by Henrik Daa Schrøder, and recently these findings have been supported by immunohistochemical and electron microscopical data in lamprey, mouse and rat. In the present study we expanded these observations by revealing a colocalization of zinc ions, zinc transporter-3 (ZnT3) and glutamic acid decarboxylase (GAD) in synaptic vesicles of zinc-enriched terminals throughout the mouse spinal cord. Confocal analysis of ZnT3 and GAD immunofluorescence was used at light microscopical levels, and a combination of zinc selenium autometallography and GAD immunocytochemistry at electron microscopic levels. Zinc-enriched/GABAergic terminals were observed in all laminae of the spinal gray matter, but most densely populated were laminae I and III in the dorsal horn. In the lateral and ventral funiculi of the white matter, rows of inhibitory zinc-enriched boutons were seen radiating from the gray matter. Ultrastructurally, colocalization of zinc ions and GAD immunoreactivity was seen in a pool of presynaptic terminals in the above locations. Some zinc-enriched terminals were not GAD-positive and some GAD-positive terminals were void of zinc ions. The majority of the zinc-enriched, not GABAergic terminals could be classified as excitatory based on their morphology, i.e. round clear vesicles and symmetric synapses. We conclude that a majority of the spinal cord zinc-enriched terminals are GABAergic. The zinc-enriched terminals with excitatory morphology are most likely glutaminergic, a few have an inhibitory morphology but are not GABAergic. These are most likely glycinergic.

The effects of Zn2+ on long-term potentiation of C fiber-evoked potentials in the rat spinal dorsal horn

Tetanic stimuli of peripheral C fibers produces long-term potentiation (LTP) in the spinal cord, which may contribute to sensitization of spinal pain-sensitive neurons. Zn2+ is widely distributed in the central nervous system and has blocked (LTP) in the hippocampus. The present study examined the effects of Zn2+ on the induction and maintenance of C fiber-evoked LTP in the deep dorsal horn of spinalized rats in vivo. The sciatic nerve was stimulated by tetanic stimuli for inducing LTP. (1) Topical administration of Zinc chloride (15 microM) to the spinal cord 15 min before tetanic stimulation completely blocked the induction of LTP, but not the baseline C responses. When Zn2+ was given 2 h after induction of LTP, no significant effect occurred. (2) Chelation of Zn2+ by disodium calcium ethylene diaminetelraacetate (CaEDTA) (500 microM) resulted in no effect on LTP. (3) Coadministration of Zn2+ (15 microM) and N-methyl-D-aspartic acid (NMDA) (5 microM) significantly attenuated C fiber-evoked potentials, which was prevented by the NMDA receptor antagonist AP-5 (100 microM). The present results showed that Zn2+ may contribute to the modulation of the formation, but not the maintenance, of spinal LTP. NMDA receptors may be involved in Zn2+-induced modulation.

Is the postganglionic sympathetic neuron zinc-enriched? A stop-flow nerve crush study on rat sciatic nerve

Axonal transport of endogenous zinc ions in the rat sciatic nerve was studied by a stop-flow/nerve crush technique combined with zinc selenide autometallography (ZnSeAMG) at light and electron microscopic levels. Distinct accumulations of ZnSeAMG grains were detected, in particular proximal but also distal to the crushes, 1.5 h after the operation, and the amounts of zinc ions increased further in the following 3-8 h. Ultrastructurally, ZnSeAMG grains were located predominantly in unmyelinated axons. The data suggest that a subpopulation of sciatic nerve axons contains and transports zinc ions both antero- and retrogradely, indicating that the second neuron in the sympathetic nervous system is zinc enriched (ZEN).

Retrograde tracing of zinc-enriched (ZEN) neuronal somata in rat spinal cord

The zinc selenide autometallographic (ZnSeAMG) technique for tracing the retrograde axonal transport of zinc ions in zinc-enriched (ZEN) neurons was used to map the distribution of ZEN neuronal somata in rat spinal cord. After a local injection of sodium selenide into the dorsal or ventral horn, ZnSeAMG-labeled ZEN neurons appeared in Rexed's laminae V, VII and X while laminae I and II were void. A few scattered ZEN somata were observed in the remaining laminae. The labeled neurons differed in shape and size, and the relatively high level of labeled somata around the injection site suggests that many ZEN neurons have relatively short axons or boutons en passage close to the neuronal origin. Ultrastructurally, the retrogradely transported zinc selenide clusters were found in the lysosomes of ZEN somata and proximal dendrites. Electron microscopic studies also revealed two different kinds of ZEN terminals: (1) terminals with flat synaptic vesicles making symmetric synaptic contacts; and (2) terminals with round vesicles making asymmetric synaptic contacts. The present study suggests the existence of propriospinal systems of ZEN neurons comprising both segmental and intersegmental ZEN connections and having either inhibitory or excitatory ZEN terminals. The ZEN neurons seem to form a vast network of terminals located primarily in the gray matter, but also contacting dendrites radiating into the white matter. Important functions of this rather massive system of ZEN terminals can not be deduced from our present knowledge, but the systems appear to be involved in both motor and sensory functions.

Removing zinc from synaptic vesicles does not impair spatial learning, memory, or sensorimotor functions in the mouse

Zinc-enriched (ZEN) neurons are distributed widely throughout the brain and spinal cord. Synaptic vesicle zinc in these neurons is thought to function as a neuromodulator upon its release into the synaptic cleft. Consistent with this possibility, zinc or zinc chelators can alter spatial learning, working memory, and nociception in rodents. Here we use zinc transporter-3 (ZnT3) knockout mice, which are depleted of synaptic vesicle zinc, to assess the consequences of removing this potential neuromodulator on the behavior of adult mice. ZnT3 knockout mice performed equally as well as wild-type mice in the rotarod, pole, and cagetop tests of motor coordination. They exhibited normal thermal nociception in the hot-plate and tail-flick tests, and had similar olfactory, auditory and sensorimotor gating capabilities as wild-type mice. ZnT3 knockout mice behaved similarly as wild-type mice in the open field test and in the elevated plus maze test of anxiety. They exhibited normal learning and memory in the passive avoidance, Morris water maze, and fear conditioning tasks, and normal working and reference memory in a water version of the radial arm maze. We conclude that synaptic vesicle zinc is not essential for mice to be able to perform these tasks, despite the abundance of ZEN neurons in the relevant regions of the CNS. Either the neuromodulatory effects of zinc are not relevant for the tasks tested here, or mice are able to compensate easily for the absence of synaptic vesicle zinc.