Distribution of histochemically reactive zinc in the forebrain of the rat

A Timm-Nissl multiplane microscopic atlas of rat brain zincergic terminal fields and metal-containing glia

The ability of Timm's sulphide silver method to stain zincergic terminal fields has made it a useful neuromorphological marker. Beyond its roles in zinc-signalling and neuromodulation, zinc is involved in the pathophysiology of ischemic stroke, epilepsy, degenerative diseases and neuropsychiatric conditions. In addition to visualising zincergic terminal fields, the method also labels transition metals in neuronal perikarya and glial cells. To provide a benchmark reference for planning and interpretation of experimental investigations of zinc-related phenomena in rat brains, we have established a comprehensive repository of serial microscopic images from a historical collection of coronally, horizontally and sagittally oriented rat brain sections stained with Timm's method. Adjacent Nissl-stained sections showing cytoarchitecture, and customised atlas overlays from a three-dimensional rat brain reference atlas registered to each section image are included for spatial reference and guiding identification of anatomical boundaries. The Timm-Nissl atlas, available from EBRAINS, enables experimental researchers to navigate normal rat brain material in three planes and investigate the spatial distribution and density of zincergic terminal fields across the entire brain.

Mobile zinc as a modulator of sensory perception

Mobile zinc is an abundant transition metal ion in the central nervous system, with pools of divalent zinc accumulating in regions of the brain engaged in sensory perception and memory formation. Here, we present essential tools that we developed to interrogate the role(s) of mobile zinc in these processes. Most important are (a) fluorescent sensors that report the presence of mobile zinc and (b) fast, Zn-selective chelating agents for measuring zinc flux in animal tissue and live animals. The results of our studies, conducted in collaboration with neuroscientist experts, are presented for sensory organs involved in hearing, smell, vision, and learning and memory. A general principle emerging from these studies is that the function of mobile zinc in all cases appears to be downregulation of the amplitude of the response following overstimulation of the respective sensory organs. Possible consequences affecting human behavior are presented for future investigations in collaboration with interested behavioral scientists.

A review of the “metallome” within neurons and glia, as revealed by elemental mapping of brain tissue

It is now well established that transition metals, such as Iron (Fe), Copper (Cu), and Zinc (Zn) are necessary for healthy brain function. Although Fe, Cu, and Zn are essential to the brain, imbalances in the amount, distribution, or chemical form ("metallome") of these metals is linked to the pathology of numerous brain diseases or disorders. Despite the known importance of metal ions for both brain health and disease, the metallome that exists within specific types of brain cells is yet to be fully characterised. The aim of this mini-review is to present an overview of the current knowledge of the metallome found within specific brain cells (oligodendrocytes, astrocytes, microglia, and neurons), as revealed by direct elemental mapping techniques. It is hoped this review will foster continued research using direct elemental mapping techniques to fully characterise the brain cell metallome.

Synaptic Zn2+ potentiates the effects of cocaine on striatal dopamine neurotransmission and behavior

Cocaine binds to the dopamine (DA) transporter (DAT) to regulate cocaine reward and seeking behavior. Zinc (Zn²⁺) also binds to the DAT, but the in vivo relevance of this interaction is unknown. We found that Zn²⁺ concentrations in postmortem brain (caudate) tissue from humans who died of cocaine overdose were significantly lower than in control subjects. Moreover, the level of striatal Zn²⁺ content in these subjects negatively correlated with plasma levels of benzoylecgonine, a cocaine metabolite indicative of recent use. In mice, repeated cocaine exposure increased synaptic Zn²⁺ concentrations in the caudate putamen (CPu) and nucleus accumbens (NAc). Cocaine-induced increases in Zn²⁺ were dependent on the Zn²⁺ transporter 3 (ZnT3), a neuronal Zn²⁺ transporter localized to synaptic vesicle membranes, as ZnT3 knockout (KO) mice were insensitive to cocaine-induced increases in striatal Zn²⁺. ZnT3 KO mice showed significantly lower electrically evoked DA release and greater DA clearance when exposed to cocaine compared to controls. ZnT3 KO mice also displayed significant reductions in cocaine locomotor sensitization, conditioned place preference (CPP), self-administration, and reinstatement compared to control mice and were insensitive to cocaine-induced increases in striatal DAT binding. Finally, dietary Zn²⁺ deficiency in mice resulted in decreased striatal Zn²⁺ content, cocaine locomotor sensitization, CPP, and striatal DAT binding. These results indicate that cocaine increases synaptic Zn²⁺ release and turnover/metabolism in the striatum, and that synaptically released Zn²⁺ potentiates the effects of cocaine on striatal DA neurotransmission and behavior and is required for cocaine-primed reinstatement. In sum, these findings reveal new insights into cocaine's pharmacological mechanism of action and suggest that Zn²⁺ may serve as an environmentally derived regulator of DA neurotransmission, cocaine pharmacodynamics, and vulnerability to cocaine use disorders.

Synaptic Zinc: An Emerging Player in Parkinson's Disease

Alterations of zinc homeostasis have long been implicated in Parkinson's disease (PD). Zinc plays a complex role as both deficiency and excess of intracellular zinc levels have been incriminated in the pathophysiology of the disease. Besides its role in multiple cellular functions, Zn²⁺ also acts as a synaptic transmitter in the brain. In the forebrain, subset of glutamatergic neurons, namely cortical neurons projecting to the striatum, use Zn²⁺ as a messenger alongside glutamate. Overactivation of the cortico-striatal glutamatergic system is a key feature contributing to the development of PD symptoms and dopaminergic neurotoxicity. Here, we will cover recent evidence implicating synaptic Zn²⁺ in the pathophysiology of PD and discuss its potential mechanisms of actions. Emphasis will be placed on the functional interaction between Zn²⁺ and glutamatergic NMDA receptors, the most extensively studied synaptic target of Zn²⁺.

Nasal Delivery of D-Penicillamine Hydrogel Upregulates a Disintegrin and Metalloprotease 10 Expression via Melatonin Receptor 1 in Alzheimer's Disease Models

Alzheimer's disease (AD) is a type of neurodegenerative disease that is associated with the accumulation of amyloid plaques. Increasing non-amyloidogenic processing and/or manipulating amyloid precursor protein signaling could reduce AD amyloid pathology and cognitive impairment. D-penicillamine (D-Pen) is a water-soluble metal chelator and can reduce the aggregation of amyloid-β (Aβ) with metals in vitro. However, the potential mechanism of D-Pen for treating neurodegenerative disorders remains unexplored. In here, a novel type of chitosan-based hydrogel to carry D-Pen was designed and the D-Pen-CS/β-glycerophosphate hydrogel were characterized by scanning electron microscopy and HPLC. Behavior tests investigated the learning and memory levels of APP/PS1 mice treated through the D-Pen hydrogel nasal delivery. In vivo and in vitro findings showed that nasal delivery of D-Pen-CS/β-GP hydrogel had properly chelated metal ions that reduced Aβ deposition. Furthermore, D-Pen mainly regulated A disintegrin and metalloprotease 10 (ADAM10) expression via melatonin receptor 1 (MTNR1α) and the downstream PKA/ERK/CREB pathway. The present data demonstrated D-Pen significantly improved the cognitive ability of APP/PS1 mice and reduced Aβ generation through activating ADAM10 and accelerating non-amyloidogenic processing. Hence, these findings indicate the potential of D-Pen as a promising agent for treating AD.

Revealing differences in the chemical form of zinc in brain tissue using K-edge X-ray absorption near-edge structure spectroscopy

Zinc is a prominent trace metal required for normal memory function. Memory loss and cognitive decline during natural ageing and neurodegenerative disease have been associated with altered brain-Zn homeostasis. Yet, the exact chemical pathways through which Zn influences memory function during health, natural ageing, or neurodegenerative disease remain unknown. The gap in the literature may in part be due to the difficulty to simultaneously image, and therefore, study the different chemical forms of Zn within the brain (or biological samples in general). To this extent, we have begun developing and optimising protocols that incorporate X-ray absorption near-edge structure (XANES) spectroscopic analysis of tissue at the Zn K-edge as an analytical tool to study Zn speciation in the brain. XANES is ideally suited for this task as all chemical forms of Zn are detected, the technique requires minimal sample preparation that may otherwise redistribute or alter the chemical form of Zn, and the Zn K-edge has known sensitivity to coordination geometry and ligand type. Herein, we report our initial results where we fit K-edge spectra collected from micro-dissected flash-frozen brain tissue, to a spectral library prepared from standard solutions, to demonstrate differences in the chemical form of Zn that exist between two brain regions, the hippocampus and cerebellum. Lastly, we have used an X-ray microprobe to demonstrate differences in Zn speciation within sub-regions of thin air-dried sections of the murine hippocampus; but, the corresponding results highlight that the chemical form of Zn is easily perturbed by sample preparation such as tissue sectioning or air-drying, which must be a critical consideration for future work.

Sample preparation with sucrose cryoprotection dramatically alters Zn distribution in the rodent hippocampus, as revealed by elemental mapping

Transition metal ions (Fe, Mn, Cu, Zn) are essential for healthy brain function, but altered concentration, distribution, or chemical form of the metal ions has been implicated in numerous brain pathologies. Currently, it is not possible to image the cellular or sub-cellular distribution of metal ions in vivo and therefore, studying brain-metal homeostasis largely relies on ex vivo in situ elemental mapping. Sample preparation methods that accurately preserve the in vivo elemental distribution are essential if one wishes to translate the knowledge of elemental distributions measured ex vivo toward increased understanding of chemical and physiological pathways of brain disease. The choice of sample preparation is particularly important for metal ions that exist in a labile or mobile form, for which the in vivo distribution could be easily distorted by inappropriate sample preparation. One of the most widely studied brain structures, the hippocampus, contains a large pool of labile and mobile Zn. Herein, we describe how sucrose cryoprotection, the gold standard method of preparing tissues for immuno-histochemistry or immuno-fluorescence, which is also often used as a sample preparation method for elemental mapping studies, drastically alters hippocampal Zn distribution. Based on the results of this study, in combination with a comparison against the strong body of published literature that has used either rapid plunge freezing of brain tissue, or sucrose cryo-protection, we strongly urge investigators in the future to cease using sucrose cryoprotection as a method of sample preparation for elemental mapping, especially if Zn is an analyte of interest.

Rapid Intramitochondrial Zn2+ Accumulation in CA1 Hippocampal Pyramidal Neurons After Transient Global Ischemia: A Possible Contributor to Mitochondrial Disruption and Cell Death

Mitochondrial Zn2+ accumulation, particularly in CA1 neurons, occurs after ischemia and likely contributes to mitochondrial dysfunction and subsequent neurodegeneration. However, the relationship between mitochondrial Zn2+ accumulation and their disruption has not been examined at the ultrastructural level in vivo. We employed a cardiac arrest model of transient global ischemia (TGI), combined with Timm's sulfide silver labeling, which inserts electron dense metallic silver granules at sites of labile Zn2+ accumulation, and used transmission electron microscopy (TEM) to examine subcellular loci of the Zn2+ accumulation. In line with prior studies, TGI-induced damage to CA1 was far greater than to CA3 pyramidal neurons, and was substantially progressive in the hours after reperfusion (being significantly greater after 4- than 1-hour recovery). Intriguingly, TEM examination of Timm's-stained sections revealed substantial Zn2+ accumulation in many postischemic CA1 mitochondria, which was strongly correlated with their swelling and disruption. Furthermore, paralleling the evolution of neuronal injury, both the number of mitochondria containing Zn2+ and the degree of their disruption were far greater at 4- than 1-hour recovery. These data provide the first direct characterization of Zn2+ accumulation in CA1 mitochondria after in vivo TGI, and support the idea that targeting these events could yield therapeutic benefits.

Zinc modulates synaptic transmission by differentially regulating synaptic glutamate homeostasis in hippocampus

A subset of presynaptic glutamatergic vesicles in the brain co-releases zinc (Zn²⁺ ) with glutamate into the synapse. However, the role of synaptically released Zn²⁺ is still under investigation. Here, we studied the effect of Zn²⁺ on glutamate homeostasis by measuring the evoked extracellular glutamate level (EGL) and the probability of evoked action potential (P_EAP ) at the Zn²⁺ -containing or zincergic mossy fiber-CA3 synapses of the rat hippocampus. We found that the application of Zn²⁺ (ZnCl₂ ) exerted bidirectional effects on both EGL and P_EAP : facilitatory at low concentration (~1 µM) while repressive at high concentration (~50 µM). To determine the action of endogenous Zn²⁺ , we also used extracellular Zn²⁺ chelator to remove the synaptically released Zn²⁺ . Zn²⁺ chelation reduced both EGL and P_EAP , suggesting that endogenous Zn²⁺ has mainly a facilitative role in glutamate secretion on physiological condition. We revealed that calcium/calmodulin-dependent protein kinase II was integral to the mechanism by which Zn²⁺ facilitated the release of glutamate. Moreover, a glutamate transporter was the molecular entity for the action of Zn²⁺ on glutamate uptake by which Zn²⁺ decreases glutamate availability. Taken together, we show a novel action of Zn²⁺ , which is to biphasically regulate glutamate homeostasis via Zn²⁺ concentration-dependent synaptic facilitation and depression. Thus, co-released Zn²⁺ is physiologically important for enhancing weak stimulation, but potentially mitigates excessive stimulation to keep synaptic transmission within optimal physiological range.

Elemental characterisation of the pyramidal neuron layer within the rat and mouse hippocampus

A unique combination of sensitivity, resolution, and penetration make X-ray fluorescence imaging (XFI) ideally suited to investigate trace elemental distributions in the biological context. XFI has gained widespread use as an analytical technique in the biological sciences, and in particular enables exciting new avenues of research in the field of neuroscience. In this study, elemental mapping by XFI was applied to characterise the elemental content within neuronal cell layers of hippocampal sub-regions of mice and rats. Although classical histochemical methods for metal detection exist, such approaches are typically limited to qualitative analysis. Specifically, histochemical methods are not uniformly sensitive to all chemical forms of a metal, often displaying variable sensitivity to specific "pools" or chemical forms of a metal. In addition, histochemical methods require fixation and extensive chemical treatment of samples, creating the strong likelihood for metal redistribution, leaching, or contamination. Direct quantitative elemental mapping of total elemental pools, in situ within ex vivo tissue sections, without the need for chemical fixation or addition of staining reagents is not possible with traditional histochemical methods; however, such a capability, which is provided by XFI, can offer an enormous analytical advantage. The results we report herein demonstrate the analytical advantage of XFI elemental mapping for direct, label-free metal quantification, in situ within ex vivo brain tissue sections. Specifically, we definitively characterise for the first time, the abundance of Fe within the pyramidal cell layers of the hippocampus. Localisation of Fe to this cell layer is not reproducibly achieved with classical Perls histochemical Fe stains. The ability of XFI to directly quantify neuronal elemental (P, S, Cl, K, Ca, Fe, Cu, Zn) distributions, revealed unique profiles of Fe and Zn within anatomical sub-regions of the hippocampus i.e., cornu ammonis 1, 2 or 3 (CA1, CA2 or CA3) sub-regions. Interestingly, our study reveals a unique Fe gradient across neuron populations within the non-degenerating and pathology free rat hippocampus, which curiously mirrors the pattern of region-specific vulnerability of the hippocampus that has previously been established to occur in various neurodegenerative diseases.

Effects of enriched housing on the neuronal morphology of mice that lack zinc transporter 3 (ZnT3) and vesicular zinc

In the central nervous system, certain neurons store zinc within the synaptic vesicles of their axon terminals. This vesicular zinc can then be released in an activity-dependent fashion as an intercellular signal. The functions of vesicular zinc are not entirely understood, but evidence suggests that it is important for some forms of experience-dependent plasticity in the brain. The ability of neurons to store and release vesicular zinc is dependent on expression of the vesicular zinc transporter, ZnT3. Here, we examined the neuronal morphology of mice that lack ZnT3. Brains were collected from mice housed under standard laboratory conditions and from mice housed in enriched environments - large, multilevel enclosures with running wheels, numerous objects and tunnels, and a greater number of cage mates. Golgi-Cox staining was used to visualize neurons for analysis of dendritic length and dendritic spine density. Neurons were analyzed from the barrel cortex, striatum, basolateral amygdala, and hippocampus (CA1). ZnT3 knockout mice, relative to wild type mice, exhibited increased basal dendritic length in the layer 2/3 pyramidal neurons of barrel cortex, independently of housing condition. Environmental enrichment decreased apical dendritic length in these same neurons and increased dendritic spine density on striatal medium spiny neurons. Elimination of ZnT3 did not modulate any of the effects of enrichment. Our results provide no evidence that vesicular zinc is required for the experience-dependent changes that occur in response to environmental enrichment. They are consistent, however, with recent reports suggesting increased cortical volume in ZnT3 knockout mice.

Elevated zinc transporter ZnT3 in the dentate gyrus of mast cell-deficient mice

Zinc is important in neurogenesis, but excessive levels can cause apoptosis and other pathologies leading to cognitive impairments. Mast cells are present in many brain regions including the hippocampus, an area rich in vesicular zinc. Mast cells contain zinc-rich granules and a well-developed mechanism for uptake of zinc ions; both features point to the potential for a role in zinc homeostasis. Prior work using the Timm stain supported this hypothesis, as increased labile zinc was detected in the hippocampus of mast cell-deficient mice compared to wild-type mice while no differences in total zinc were found between the two genotypes in the whole brain or other tissues. The current report further examines differences in zinc homeostasis between wild-type and mast cell-deficient mice by exploring the zinc transporter ZnT3, which transports labile zinc into synaptic vesicles. The first study used immunocytochemistry to localize ZnT3 within the mossy fibre layer of the hippocampus to determine whether there was differential expression of ZnT3 in wild-type versus mast cell-deficient mice. The second study used inductively coupled plasma mass spectrometry (ICP-MS) to determine total zinc content in the whole dentate gyrus of the two genotypes. The immunocytochemical results indicate that there are higher levels of ZnT3 localized to the mossy fibre layer of the dentate gyrus of mast cell-deficient mice than in wild-type mice. The ICP-MS data reveal no differences in total zinc in dentate gyrus as a whole. The results are consistent with the hypothesis that mast cells participate in zinc homeostasis at the level of synaptic vesicles.

Intracellular Zn2+ transients modulate global gene expression in dissociated rat hippocampal neurons

Zinc (Zn²⁺) is an integral component of many proteins and has been shown to act in a regulatory capacity in different mammalian systems, including as a neurotransmitter in neurons throughout the brain. While Zn²⁺ plays an important role in modulating neuronal potentiation and synaptic plasticity, little is known about the signaling mechanisms of this regulation. In dissociated rat hippocampal neuron cultures, we used fluorescent Zn²⁺ sensors to rigorously define resting Zn²⁺ levels and stimulation-dependent intracellular Zn²⁺ dynamics, and we performed RNA-Seq to characterize Zn²⁺-dependent transcriptional effects upon stimulation. We found that relatively small changes in cytosolic Zn²⁺ during stimulation altered expression levels of 931 genes, and these Zn²⁺ dynamics induced transcription of many genes implicated in neurite expansion and synaptic growth. Additionally, while we were unable to verify the presence of synaptic Zn²⁺ in these cultures, we did detect the synaptic vesicle Zn²⁺ transporter ZnT3 and found it to be substantially upregulated by cytosolic Zn²⁺ increases. These results provide the first global sequencing-based examination of Zn²⁺-dependent changes in transcription and identify genes that may mediate Zn²⁺-dependent processes and functions.

A Novel hAPP/htau Mouse Model of Alzheimer's Disease: Inclusion of APP With Tau Exacerbates Behavioral Deficits and Zinc Administration Heightens Tangle Pathology

The brains of those with Alzheimer's disease have amyloid and tau pathology; thus, mice modeling AD should have both markers. In this study, we characterize offspring from the cross of the J20 (hAPP) and rTg4510 (htau) strains (referred to as dual Tg). Behavior was assessed at both 3.5 and 7 months, and biochemical differences were assessed at 8 months. Additionally, mice were placed on zinc (Zn) water or standard lab water in order to determine the role of this essential biometal. Behavioral measures examined cognition, emotion, and aspects of daily living. Transgenic mice (dual Tg and htau) showed significant deficits in spatial memory in the Barnes Maze at both 3.5 and 7 months compared to controls. At 7 months, dual Tg mice performed significantly worse than htau mice (p < 0.01). Open field and elevated zero maze (EZM) data indicated that dual Tg and htau mice displayed behavioral disinhibition compared to control mice at both 3.5 and 7 months (p < 0.001). Transgenic mice showed significant deficits in activities of daily living, including burrowing and nesting, at both 3.5 and 7 months compared to control mice (p < 0.01). Dual Tg mice built very poor nests, indicating that non-cognitive tasks are also impacted by AD. Overall, dual Tg mice demonstrated behavioral deficits earlier than those shown by the htau mice. In the brain, dual Tg mice had significantly less free Zn compared to control mice in both the dentate gyrus and the CA3 of the hippocampus (p < 0.01). Dual Tg mice had increased tangles and plaques in the hippocampus compared to htau mice and the dual Tg mice given Zn water displayed increased tangle pathology in the hippocampus compared to htau mice on Zn water (p < 0.05). The dual Tg mouse described here displays pathology reminiscent of the human AD condition and is impaired early on in both cognitive and non-cognitive behaviors. This new mouse model allows researchers to assess how both amyloid and tau in combination impact behavior and brain pathology.

Elimination of vesicular zinc alters the behavioural and neuroanatomical effects of social defeat stress in mice

Chronic stress can have deleterious effects on mental health, increasing the risk of developing depression or anxiety. But not all individuals are equally affected by stress; some are susceptible while others are more resilient. Understanding the mechanisms that lead to these differing outcomes has been a focus of considerable research. One unexplored mechanism is vesicular zinc – zinc that is released by neurons as a neuromodulator. We examined how chronic stress, induced by repeated social defeat, affects mice that lack vesicular zinc due to genetic deletion of zinc transporter 3 (ZnT3). These mice, unlike wild type mice, did not become socially avoidant of a novel conspecific, suggesting resilience to stress. However, they showed enhanced sensitivity to the potentiating effect of stress on cued fear memory. Thus, the contribution of vesicular zinc to stress susceptibility is not straightforward. Stress also increased anxiety-like behaviour but produced no deficits in a spatial Y-maze test. We found no evidence that microglial activation or hippocampal neurogenesis accounted for the differences in behavioural outcome. Volumetric analysis revealed that ZnT3 KO mice have larger corpus callosum and parietal cortex volumes, and that corpus callosum volume was decreased by stress in ZnT3 KO, but not wild type, mice.

Mitochondrial Zn2+ Accumulation: A Potential Trigger of Hippocampal Ischemic Injury

Ischemic stroke is a major cause of death and disabilities worldwide, and it has been long hoped that improved understanding of relevant injury mechanisms would yield targeted neuroprotective therapies. While Ca²⁺ overload during ischemia-induced glutamate excitotoxicity has been identified as a major contributor, failures of glutamate targeted therapies to achieve desired clinical efficacy have dampened early hopes for the development of new treatments. However, additional studies examining possible contributions of Zn²⁺, a highly prevalent cation in the brain, have provided new insights that may help to rekindle the enthusiasm. In this review, we discuss both old and new findings yielding clues as to sources of the Zn²⁺ that accumulates in many forebrain neurons after ischemia, and mechanisms through which it mediates injury. Specifically, we highlight the growing evidence of important Zn²⁺ effects on mitochondria in promoting neuronal injury. A key focus has been to examine Zn²⁺ contributions to the degeneration of highly susceptible hippocampal pyramidal neurons. Recent studies provide evidence of differences in sources of Zn²⁺ and its interactions with mitochondria in CA1 versus CA3 neurons that may pertain to their differential vulnerabilities in disease. We propose that Zn²⁺-induced mitochondrial dysfunction is a critical and potentially targetable early event in the ischemic neuronal injury cascade, providing opportunities for the development of novel neuroprotective strategies to be delivered after transient ischemia.

The Zinc Sensing Receptor, ZnR/GPR39, in Health and Disease

A distinct G-protein coupled receptor that senses changes in extracellular Zn²⁺, ZnR/GPR39, was found in cells from tissues in which Zn²⁺ plays a physiological role. Most prominently, ZnR/GPR39 activity was described in prostate cancer, skin keratinocytes, and colon epithelial cells, where zinc is essential for cell growth, wound closure, and barrier formation. ZnR/GPR39 activity was also described in neurons that are postsynaptic to vesicular Zn²⁺ release. Activation of ZnR/GPR39 triggers Gαq-dependent signaling and subsequent cellular pathways associated with cell growth and survival. Furthermore, ZnR/GPR39 was shown to regulate the activity of ion transport mechanisms that are essential for the physiological function of epithelial and neuronal cells. Thus, ZnR/GPR39 provides a unique target for therapeutically modifying the actions of zinc in a specific and selective manner.

Modulation of the chelatable Zn pool in the brain by diethyldithiocarbamate is associated with behavioral impairment in adult zebrafish

DEDTC leads to a buildup of DEDTC in the brain with consequent chelation of reactive Zn and behavioral impairment of zebrafish.

Effect of insulin-induced hypoglycaemia on the central nervous system: evidence from experimental studies

Insulin-induced hypoglycaemia (IIH) is a major acute complication in type 1 as well as in type 2 diabetes, particularly during intensive insulin therapy. The brain plays a central role in the counter-regulatory response by eliciting parasympathetic and sympathetic hormone responses to restore normoglycaemia. Brain glucose concentrations, being approximately 15-20% of the blood glucose concentration in humans, are rigorously maintained during hypoglycaemia through adaptions such as increased cerebral glucose transport, decreased cerebral glucose utilisation and, possibly, by using central nervous system glycogen as a glucose reserve. However, during sustained hypoglycaemia, the brain cannot maintain a sufficient glucose influx and, as the cerebral hypoglycaemia becomes severe, electroencephalogram changes, oxidative stress and regional neuronal death ensues. With particular focus on evidence from experimental studies on nondiabetic IIH, this review outlines the central mechanisms behind the counter-regulatory response to IIH, as well as cerebral adaption to avoid sequelae of cerebral neuroglycopaenia, including seizures and coma.

Zinc histochemistry reveals circuit refinement and distinguishes visual areas in the developing ferret cerebral cortex

A critical question in brain development is whether different brain circuits mature concurrently or with different timescales. To characterize the anatomical and functional development of different visual cortical areas, one must be able to distinguish these areas. Here, we show that zinc histochemistry, which reveals a subset of glutamatergic processes, can be used to reliably distinguish visual areas in juvenile and adult ferret cerebral cortex, and that the postnatal decline in levels of synaptic zinc follows a broadly similar developmental trajectory in multiple areas of ferret visual cortex. Zinc staining in all areas examined (17, 18, 19, 21, and Suprasylvian) is greater in the 5-week-old than in the adult. Furthermore, there is less laminar variation in zinc staining in the 5-week-old visual cortex than in the adult. Despite differences in staining intensity, areal boundaries can be discerned in the juvenile as in the adult. By 6 weeks of age, we observe a significant decline in visual cortical synaptic zinc; this decline was most pronounced in layer IV of areas 17 and 18, with much less change in higher-order extrastriate areas during the important period in visual cortical development following eye opening. By 10 weeks of age, the laminar pattern of zinc staining in all visual areas is essentially adultlike. The decline in synaptic zinc in the supra- and infragranular layers in all areas proceeds at the same rate, though the decline in layer IV does not. These results suggest that the timecourse of synaptic zinc decline is lamina specific, and further confirm and extend the notion that at least some aspects of cortical maturation follow a similar developmental timecourse in multiple areas. The postnatal decline in synaptic zinc we observe during the second postnatal month begins after eye opening, consistent with evidence that synaptic zinc is regulated by sensory experience.

Sensor specific imaging of proteomic Zn2+ with zinquin and TSQ after cellular exposure to N-ethylmaleimide

The impact of the thiol binding reagent N-ethylmaleimide (NEM) on proteomic Zn(2+) availability was investigated in rat glioma cells. Zinquin (ZQ) or TSQ, two related fluorescent sensors, were used to observe reactive Zn(2+). Control cells contained proteomic Zn(2+) but no detectable low molecular weight (LMW) Zn(2+). With either sensor, basal cellular fluorescence emission centered near 470 nm, indicative of sensor-Zn-proteins. ZQ sequestered 13% of proteomic Zn(2+) as Zn(ZQ)(2); TSQ reacted only with the Zn-proteome. NEM (100 μM) abolished LMW thiols, including glutathione (GSH) and lowered proteomic sulfhydryl content about 30%. In ZQ-treated cells, NEM exposure enhanced fluorescent intensity and the formation of Zn(ZQ)(2) (λ(MAX), 492 nm). Cells incubated with TSQ and NEM also displayed increased fluorescence without a spectral shift in wavelength maximum, consistent with increased formation of TSQ-Zn-protein adducts but not Zn(TSQ)(2). In neither experiment was Zn(2+) lost from cells. NEM altered Zn(2+) accessibility to sensors in membrane-nuclear and cytosolic fractions, but Zn(ZQ)(2) was only generated in the cytosol. Similar results were obtained when cell supernatant replaced cells. In contrast, when isolated proteome was reacted with ZQ and 100 μM NEM in the absence of GSH, 70% of the proteomic thiols underwent reaction. As a consequence, most of the ZQ-Zn-protein adducts were converted to Zn(ZQ)(2). Substituting TSQ for ZQ, only increased TSQ-Zn-proteins were observed. Evidently, the results of imaging cells with Zn(2+) sensors are dependent upon the specific chemical properties of the sensors and can only be understood after detailed chemical analysis.

Dependence of the histofluorescently reactive zinc pool on zinc transporter-3 in the normal brain

In the brain, free zinc levels are under the exquisite control of a variety of zinc-regulating systems, in which zinc transporter (ZnT) proteins play a central role. ZnT3, which is prominently expressed in the brain, facilitates the concentration of free zinc in pre-synaptic vesicles. In addition to histochemical staining methods, a variety of zinc-specific fluorescence dyes has been developed to image or analyze zinc in brain tissue. In this study, we demonstrate the close correlations between histofluorescently reactive zinc and ZnT3. We examined the overlapping distribution of the zinc-specific fluorescent dye, N-(6-methoxy-8-quinolyl)-p-toluenesulfonamide (TSQ)-, and ZnT3-immunoreactive fluorescence throughout the normal brain. TSQ and ZnT3-antibody intensely stained the hippocampus, cortex and amygdala, highlighting the characteristic laminar organization of these regions by variably staining the different layers. TSQ fluorescence and ZnT3 immunoreactivity were roughly co-localized with synaptophysin along the neuropil, but were absent in the neuronal soma. However, albeit relatively faint, TSQ fluorescence was also found throughout the brains of ZnT3-knockout mice. Although these results may indicate the presence of very small cerebral free zinc pools distinct from synaptic vesicle zinc, the synaptic vesicle zinc pool is predominant, accounting for more than 95% of the entire histofluorescently reactive zinc pool in the hippocampus and cortex. Thus, the physiological activity of free zinc in the normal brain might largely depend on the pool of synaptic vesicle zinc that is determined by ZnT3.

TSQ (6-methoxy-8-p-toluenesulfonamido-quinoline), a common fluorescent sensor for cellular zinc, images zinc proteins

Zn(2+) is a necessary cofactor for thousands of mammalian proteins. Research has suggested that transient fluxes of cellular Zn(2+) are also involved in processes such as apoptosis. Observations of Zn(2+) trafficking have been collected using Zn(2+) responsive fluorescent dyes. A commonly used Zn(2+) fluorophore is 6-methoxy-8-p-toluenesulfonamido-quinoline (TSQ). The chemical species responsible for TSQ's observed fluorescence in resting or activated cells have not been characterized. Parallel fluorescence microscopy and spectrofluorometry of LLC-PK(1) cells incubated with TSQ demonstrated punctate staining that concentrated around the nucleus and was characterized by an emission maximum near 470 nm. Addition of cell permeable Zn-pyrithione resulted in greatly increased, diffuse fluorescence that shifted the emission peak to 490 nm, indicative of the formation of Zn(TSQ)(2). TPEN (N,N,N'N'-tetrakis(-)[2-pyridylmethyl]-ethylenediamine), a cell permeant Zn(2+) chelator, largely quenched TSQ fluorescence returning the residual fluorescence to the 470 nm emission maximum. Gel filtration chromatography of cell supernatant from LLC-PK(1) cells treated with TSQ revealed that TSQ fluorescence (470 nm emission) eluted with the proteome fractions. Similarly, addition of TSQ to proteome prior to chromatography resulted in 470 nm fluorescence emission that was not observed in smaller molecular weight fractions. It is hypothesized that Zn-TSQ fluorescence, blue-shifted from the 490 nm emission maximum of Zn(TSQ)(2), results from ternary complex, TSQ-Zn-protein formation. As an example, Zn-carbonic anhydrase formed a ternary adduct with TSQ characterized by a fluorescence emission maximum of 470 nm and a dissociation constant of 1.55 × 10(-7) M. Quantification of TSQ-Zn-proteome fluorescence indicated that approximately 8% of cellular Zn(2+) was imaged by TSQ. These results were generalized to other cell types and model Zn-proteins.

Ethanol reduces zincosome formation in cultured astrocytes

AIMS

Zinc is an ion that participates in basic cellular and tissular functions. Zinc deficiency is present in many physiological and health problems affecting most body organs, including the brain. Among the circumstances involved in zinc deficiency, ethanol consumption is probably one of the most frequent. A dietary zinc supplement has been proposed as possibly being an efficient method to palliate zinc deficiency. Astrocytes form part of the hematoencephalic barrier, and they are apparently implicated in the homeostasis of the neuronal medium. In this work, we analyze the effect of ethanol on extracellular zinc management by rat astrocytes in culture.

METHODS

Intracellular levels of 'free zinc ions', in controls and 30 mM ethanol-treated astrocytes, were visualized by using the zinc fluorochrome TSQ. Cytoplasmic fluorescence and zincosome formation were measured after adding extracellular 50 µM ZnSO(4) to cell monolayers. Zincosomes were also observed at the electron microscopy level.

RESULTS

Exposure to ethanol for 7 days lowered the basal zinc levels of astrocytes by ∼30%. This difference was consistently maintained after the zinc pulse. Zinc ions were confined to bright fluorescent particles, the 'zincosomes', which appeared to be formed by the endocytic pathway. Zincosomes were less abundant in alcohol-treated cells, indicating a deficit in endocytoses as the origin of low zinc intake in astrocytes after ethanol treatment.

CONCLUSIONS

Ethanol reduces both intracellular ionic zinc levels and extracellular zinc uptake, resulting in poorer zincosome formation. Given the endocytic nature of zincosomes, the effect of ethanol on membrane trafficking is apparently the origin of this deficit.

In vivo imaging with a cell-permeable porphyrin-based MRI contrast agent

Magnetic resonance imaging (MRI) with molecular probes offers the potential to monitor physiological parameters with comparatively high spatial and temporal resolution in living subjects. For detection of intracellular analytes, construction of cell-permeable imaging agents remains a challenge. Here we show that a porphyrin-based MRI molecular imaging agent, Mn-(DPA-C(2))(2)-TPPS(3), effectively penetrates cells and persistently stains living brain tissue in intracranially injected rats. Chromogenicity of the probe permitted direct visualization of its distribution by histology, in addition to MRI. Distribution was concentrated in cell bodies after hippocampal infusion. Mn-(DPA-C(2))(2)-TPPS(3) was designed to sense zinc ions, and contrast enhancement was more pronounced in the hippocampus, a zinc-rich brain region, than in the caudate nucleus, which contains relatively little labile Zn(2+). Membrane permeability, optical activity, and high relaxivity of porphyrin-based contrast agents offer exceptional functionality for in vivo imaging.

Prevention of hypoglycemia-induced neuronal death by hypothermia

Hypothermia reduces neuronal damage after cerebral ischemia and traumatic brain injury, while hyperthermia exacerbates damage from these insults. Previously we have shown that temperature-dependent modulation of excitotoxic neuronal death is mediated in part by temperature-dependent changes in the synaptic release/translocation of Zn(2+). In this study, we hypothesize that brain temperature also affects hypoglycemia-induced neuronal death by modulation of vesicular Zn(2+) release from presynaptic terminals. To test our hypothesis, we used a rat model of insulin-induced hypoglycemia. Here we found that hypoglycemia-induced neuronal injury was significantly affected by brain temperature, that is, hypothermia inhibited while hyperthermia aggravated neuronal death. To investigate the mechanism of temperature-dependent neuronal death after hypoglycemia, we measured zinc release/translocation, reactive oxygen species (ROS) production, and microglia activation. Here we found that hypoglycemia-induced Zn(2+) release/translocation, ROS production, and microglia activation were inhibited by hypothermia but aggravated by hyperthermia. Even when the insult was accompanied by hyperthermic conditions, zinc chelation inhibited ROS production and microglia activation. Zinc chelation during hyperthermia reduced neuronal death, superoxide production, and microglia activation, which was comparable to the protective effects of hypothermia. We conclude that neuronal death after hypoglycemia is temperature-dependent and is mediated by increased Zn(2+) release, superoxide production, and microglia activation.

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.

Detection of zinc translocation into apical dendrite of CA1 pyramidal neuron after electrical stimulation

Translocation of the endogenous cation zinc from presynaptic terminals to postsynaptic neurons after brain insult has been implicated as a potential neurotoxic event. Several studies have previously demonstrated that a brief electrical stimulation is sufficient to induce the translocation of zinc from presynaptic vesicles into the cytoplasm (soma) of postsynaptic neurons. In the present work I have extended those findings in three ways: (i) providing evidence that zinc translocation occurs into apical dendrites, (ii) presenting data that there is an apparent translocation into apical dendrites when only a zinc-containing synaptic input is stimulated, and (iii) presenting data that there is no zinc translocation into apical dendrite of ZnT3 KO mice following electrical stimulation. Hippocampal slices were preloaded with the "trappable" zinc fluorescent probe, Newport Green. After washout, a single apical dendrite in the stratum radiatum of hippocampal CA1 area was selected and focused on. Burst stimulation (100Hz, 500microA, 0.2ms, monopolar) was delivered to either the adjacent Schaffer-collateral inputs (zinc-containing) or to the adjacent temporo-ammonic inputs (zinc-free) to the CA1 dendrites. Stimulation of the Schaffer collaterals increased the dendritic fluorescence, which was blocked by TTX, low-Ca medium, or the extracellular zinc chelator, CaEDTA. Stimulation of the temporo-ammonic pathway caused no significant rise in the fluorescence. Genetic depletion of vesicular zinc by ZnT3 KO showed no stimulation-induced apical dendrite zinc rise. The present study provides evidence that synaptically released zinc translocates into postsynaptic neurons through the apical dendrites of CA1 pyramidal neurons during physiological synaptic activity.

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.

Copper and zinc promote interactions between membrane-anchored peptides of the metal binding domain of the prion protein

The prion protein (PrP) has been identified as a metalloprotein capable of binding multiple copper ions and possibly zinc. Recent studies now indicate that prion self-recognition may be an important factor in both the normal function and misfunction of this protein. We have developed fluorescently labeled models of the prion protein that allow prion-prion interactions and metal binding to be investigated on the molecular level. Peptides encompassing the full metal binding region were anchored to the surface of small unilamellar vesicles, and PrP-PrP interactions were monitored by fluorescence spectroscopy as a function of added metal. Both Cu2+ and Zn2+ were found to cause an increase in the level of PrP-PrP interactions, by 117 and 300%, respectively, whereas other metals such as Ni2+, Co2+, and Ca2+ had no effect. The binding of either of these cofactors appears to act as a switch that induces PrP-PrP interactions in a reversible manner. Both glutamine and tryptophan residues, which occur frequently in the metal binding region of PrP, were found to be important in mediating PrP-PrP interactions. Experiments demonstrate that tryptophan residues are also responsible for the low level of PrP-PrP interactions observed in the absence of Cu2+ and Zn2+, and this is further supported by molecular modeling. Overall, our results indicate that PrP may be a bifunctional molecule capable of responding to fluctuations in both neuronal Cu2+ and Zn2+ levels.

Transient synaptic zinc-positive thalamocortical terminals in the developing barrel cortex

In rat barrel cortex, layer 4 has a transiently high density of zinc-positive terminations from postnatal day (P)9 to P12 [P.W. Land & L. Shamalla-Hannah (2002)J. Comp. Neurol., 447, 43-56]. These terminations have been proposed to originate from cortico-cortical connections, but their exact origin is unknown. To determine their sources, we injected sodium selenite into the barrel cortex of two adult rats and 32 pups, from P5 to P28. As predicted, abundant zinc-positive cortically projecting neurons were visible around the injection sites and in distant cortical areas. From P9 to P13, however, neurons retrogradely labeled by zinc selenite occurred in the thalamus, in topographically appropriate regions of the ventroposterior medial (VPM) and posterior nuclei (Po). Because there are no previous reports of zinc-positive sensory thalamocortical connections, we sought corroboration of this unexpected finding by electron microscopy. This revealed a subset of boutons in layers 4 and 1, positive for both zinc and vesicular glutamate transporter 2, a protein used by thalamocortical terminations. Finally, in an additional nine rats, we carried out in situ hybridization for zinc transporter 3 mRNA. Moderate signal was detected in VPM and Po at P10, but this disappeared by P28. In contrast, a strong signal was apparent in the anterodorsal nucleus, which projects to limbic areas, and this persisted at P28. The timing of the transient zinc-positive terminations in the sensory thalamus roughly coincides with the onset of exploratory and whisking behavior in the middle of the second postnatal week; and this suggests zinc is important for activity-related refinement of circuitry.

Concentrations of extracellular free zinc (pZn)e in the central nervous system during simple anesthetization, ischemia and reperfusion

"Free Zn2+" (rapidly exchangeable Zn2+) is stored along with glutamate in the presynaptic terminals of specific specialized (gluzinergic) cerebrocortical neurons. This synaptically releasable Zn2+ has been recognized as a potent modulator of glutamatergic transmission and as a key toxin in excitotoxic neuronal injury. Surprisingly (despite abundant work on bound zinc), neither the baseline concentration of free Zn2+ in the brain nor the presumed co-release of free Zn2+ and glutamate has ever been directly observed in the intact brain in vivo. Here, we show for the first time in dialysates of rat and rabbit brain and human CSF samples from lumbar punctures that: (i) the resting or "tonic" level of free Zn2+ signal in the extracellular fluid of the rat, rabbit and human being is approximately 19 nM (95% range: 5-25 nM). This concentration is 15,000-fold lower than the "300 microM" concentration which is often used as the "physiological" concentration of free zinc for stimulating neural tissue. (ii) During ischemia and reperfusion in the rabbit, free zinc and glutamate are (as has often been presumed) released together into the extracellular fluid. (iii) Unexpectedly, Zn2+ is also released alone (without glutamate) at a variable concentration for several hours during the reperfusion aftermath following ischemia. The source(s) of this latter prolonged release of Zn2+ is/are presumed to be non-synaptic and is/are now under investigation. We conclude that both Zn2+ and glutamate signaling occur in excitotoxicity, perhaps by two (or more) different release mechanisms.

Zinc-rich transient vertical modules in the rat retrosplenial cortex during postnatal development

The rat retrosplenial cortex is part of a heavily interconnected limbic circuit, considered to have an important role in spatial memory. Interestingly, the granular retrosplenial cortex has an exceptionally distinct system of dendritic bundles, originating from callosally projecting pyramidal neurons in layer II. These can be detected as early as postnatal day 5; and, although their functional significance remains to be elucidated, the existence of these bundles makes the granular retrosplenial cortex an attractive model system for a wide range of development and functional investigations. Here, we report four results concerning the development of modularity in the granular retrosplenial cortex in rats as investigated by neurochemical markers associated to cortico-cortical and thalamo-cortical connections. Emphasis is placed on zinc, an activity-related substance associated with glutamatergic, non-thalamic terminations. 1) Zinc shows a transient strong expression during early postnatal development, but later than the appearance of the upper layer bundles (at postnatal day 5). By postnatal day 11 to postnatal day 15 staining for zinc achieved its most complex pattern; such that layer I had an elaborate organization both in the tangential and radial dimensions. Three sublaminae were distinguished (layers Ia-c): a superficial, thin tier (Ia) with patchy, moderate staining which periodically intruded into the underlying layer Ib ("funnel" modules), a middle band of variable width and light staining (Ib), and a deep, thin band with heavy and patchy staining (Ic) which, at rostral levels, spread upward into layer Ib (as "dome-like" modules). 2) At postnatal day 15, immunohistochemical methods showed that layers Ia, b zinc-funnels were co-localized with glutamate receptor subunits 2/3, GABA receptor type A alpha1 subunit and the thalamo-cortical marker, vesicular glutamate transporter 2. Layer Ic and the zinc dome-like modules were co-labeled for the cortico-cortical marker, vesicular glutamate transporter 1 and calretinin. 3) The spatial coincidence between zinc funnels in layers Ia, b and vesicular glutamate transporter 2 was further investigated by electron microscopy, which demonstrated co-localization of zinc and vesicular glutamate transporter 2 in synaptic boutons. The unusual co-localization of zinc and thalamo-cortical terminations was confirmed by retrograde transport of zinc to neurones in the anterodorsal thalamic nucleus at postnatal day 9 and postnatal day 13, and can thus be considered a transient zinc expression in thalamo-cortical boutons. This was not observed at postnatal day 28 or later. 4) After postnatal day 18, zinc staining started to fade in all layers. Before postnatal day 21, the heavy staining for zinc in the domes had completely disappeared. Zinc staining in layer Ia and the funnels virtually disappeared after postnatal day 28. A transient expression of zinc is reported in at least one other cortical area (layer IV of barrel cortex from postnatal day 5 to postnatal day 14, maximal at postnatal days 9-11). We conclude that the transient expression of zinc can occur in both limbic and sensory areas, and that down-regulation of zinc in cortical modules might be related to synaptic plasticity and remodeling during development.

Neo-Timm staining in the thalamus of chronically epileptic rats

The thalamus is an important modulator of seizures and is severely affected in cholinergic models of epilepsy. In the present study, chronically epileptic rats had their brains processed for neo-Timm and acetylcholinesterase two months after the induction of status epilepticus with pilocarpine. Both controls and pilocarpine-treated animals presented neo-Timm staining in the anterodorsal nucleus, laterodorsal nucleus, reticular nucleus, most intralaminar nuclei, nucleus reuniens, and rhomboid nucleus of the thalamus, as well as in the zona incerta. The intensity of neo-Timm staining was similar in control and pilocarpine-treated rats, except for the nucleus reuniens and the rhomboid nucleus, which had a lower intensity of staining in the epileptic group. In animal models of temporal lobe epilepsy, zinc seems to modulate glutamate release and to decrease seizure activity. In this context, a reduction of neo-Timm-stained terminals in the midline thalamus could ultimately result in an increased excitatory activity, not only within its related nuclei, but also in anatomical structures that receive their efferent connections. This might contribute to the pathological substrate observed in chronic pilocarpine-treated epileptic animals.

Zinc-enriched amygdalo- and hippocampo-cortical connections to the inferotemporal cortices in macaque monkey

Synaptic zinc (Zn), a co-factor in some glutamatergic synapses, has been implicated in plasticity effects, as well as in several excitotoxic and other pathophysiological conditions. In this study, we provide information about the distribution of Zn in inferotemporal cortex, a region at the interface of the visual and hippocampal networks. In brief, we found a lateral to medial increase in Zn, where TEad, a unimodal visual area, showed low levels of Zn; TEav, intermediate levels; and perirhinal cortex, a multimodal limbic area, high levels. The distribution of parvalbumin, a calcium binding protein, showed a reverse gradient to that of Zn. The neurons of origin of the Zn+ termination were identified by making intracortical injections sodium selenite (Na2SeO3). This substance interacts with Zn to form precipitates of ZnSe and in this form is transported retrogradely to the soma. A mixed population of labeled neurons was visualized, which included Zn+ neurons in CA1 of the hippocampus and in several amygdala subnuclei. In CA1, Zn+ neurons were restricted to the upper part of stratum pyramidale. Zn is thought to contribute to activity-dependent synaptic plasticity. The specifically high level in perirhinal cortex, and its origin from neurons in CA1 and the amygdala, may relate to cellular events involved in visual long-term memory formation.

Metals and amyloid-beta in Alzheimer's disease

Mounting evidence is demonstrating roles for the amyloid precursor protein (APP) and its proteolytic product Abeta in metal homeostasis. Furthermore, aberrant metal homeostasis is observed in patients with Alzheimer's disease (AD), and this may contribute to AD pathogenesis, by enhancing the formation of reactive oxygen species and toxic Abeta oligomers and facilitating the formation of the hallmark amyloid deposits in AD brain. Indeed, zinc released from synaptic activity has been shown to induce parenchymal and cerebrovascular amyloid in transgenic mice. On the other hand, abnormal metabolism of APP and Abeta may impair brain metal homeostasis as part of the AD pathogenic process. Abeta and APP expression have both been shown to decrease brain copper (Cu) levels, whereas increasing brain Cu availability results in decreased levels of Abeta and amyloid plaque formation in transgenic mice. Lowering Cu concentrations can downregulate the transcription of APP, strengthening the hypothesis that APP and Abeta form part of the Cu homeostatic machinery in the brain. This is a complex pathway, and it appears that when the sensitive metal balance in the brain is sufficiently disrupted, it can lead to the self-perpetuating pathogenesis of AD. Clinical trials are currently studying agents that can remedy abnormal Abeta-metal interactions.

The neurobiology of zinc in health and disease

The use of zinc in medicinal skin cream was mentioned in Egyptian papyri from 2000 BC (for example, the Smith Papyrus), and zinc has apparently been used fairly steadily throughout Roman and modern times (for example, as the American lotion named for its zinc ore, 'Calamine'). It is, therefore, somewhat ironic that zinc is a relatively late addition to the pantheon of signal ions in biology and medicine. However, the number of biological functions, health implications and pharmacological targets that are emerging for zinc indicate that it might turn out to be 'the calcium of the twenty-first century'.

Zn2+ ions: modulators of excitatory and inhibitory synaptic activity

The role of Zn(2+) in the CNS has remained enigmatic for several decades. This divalent cation is accumulated by specific neurons into synaptic vesicles and can be released by stimulation in a Ca(2+)-dependent manner. Using Zn(2+) fluorophores, radiolabeled Zn(2+), and selective chelators, the location of this ion and its release pattern have been established across the brain. Given the distribution and possible release under physiological conditions, Zn(2+) has the potential to act as a modulator of both excitatory and inhibitory neurotransmission. Excitatory N-methyl-D-aspartate (NMDA) receptors are directly inhibited by Zn(2+), whereas non-NMDA receptors appear relatively unaffected. In contrast, inhibitory transmission mediated via GABA(A)receptors can be potentiated via a presynaptic mechanism, influencing transmitter release; however, although some tonic GABAergic inhibition may be suppressed by Zn(2+), most synaptic GABA receptors are unlikely to be modulated directly by this cation. In the spinal cord, glycinergic transmission may also be affected by Zn(2+) causing potentiation. Recently, the penetration of synaptically released Zn(2+) into neurons suggests that this ion has the potential to act as a direct transmitter, by affecting postsynaptic signaling pathways. Taken overall, present studies are broadly supportive of a neuromodulatory role for Zn(2+) at specific excitatory and inhibitory synapses.

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.

Distribution of synaptic zinc in the macaque monkey amygdala

We have mapped the macaque amygdala for the distribution of synaptic zinc (Zn), a co-factor of glutamate implicated in plasticity, as well as in several excitotoxic and other pathophysiological conditions. In brief, we found that the amygdala is Zn enriched in all nuclear groups (i.e., basolateral and cortical groups, as well as central and medial nuclei) but with marked differences in density. By comparing parallel tissue series histologically reacted for Zn and parvalbumin (PV), we further found that regions high in Zn are typically low in PV neuropil. In the basolateral group, there is a particularly distinct dorsoventral gradation such that Zn levels are most dense ventrally, i.e., in the paralaminar nucleus, the ventral division of the lateral nucleus, and the parvicellular divisions of both the basal nucleus and the accessory basal nucleus. PV levels are least dense in these same regions. For the central and medial nuclei, there is a slight mediolateral gradient, with Zn levels being higher medially. PV is low overall in these nuclei. Electron microscopic results confirmed that Zn is contained in synaptic boutons. These form asymmetrical, presumably excitatory, synapses, and the postsynaptic targets are mainly spines of projection neurons. The inhomogeneous distribution of Zn in the monkey amygdala may be related to different types or degrees of plasticity among the amygdaloid subnuclei. The complementary distribution with PV parallels that of several other substances and is interesting in the context of subnuclear vulnerability for human neuronal disease, such as seizure and Alzheimer's disease.

Zinc release contributes to hypoglycemia-induced neuronal death

Neurons exposed to zinc exhibit activation of poly(ADP-ribose) polymerase-1 (PARP-1), an enzyme that normally participates in DNA repair but promotes cell death when extensively activated. Endogenous, vesicular zinc in brain is released to the extracellular space under conditions causing neuronal depolarization. Here, we used a rat model of insulin-induced hypoglycemia to assess the role of zinc release in PARP-1 activation and neuronal death after severe hypoglycemia. Zinc staining with N-(6-methoxy-8-quinolyl)-para-toluenesulfonamide (TSQ) showed depletion of presynaptic vesicular zinc from hippocampal mossy fiber terminals and accumulation of weakly bound zinc in hippocampal CA1 cell bodies after severe hypoglycemia. Intracerebroventricular injection of the zinc chelator calcium ethylene-diamine tetraacetic acid (CaEDTA) blocked the zinc accumulation and significantly reduced hypoglycemia-induced neuronal death. CaEDTA also attenuated the accumulation of poly(ADP-ribose), the enzymatic product of PARP-1, in hippocampal neurons. These results suggest that zinc translocation is an intermediary step linking hypoglycemia to PARP-1 activation and neuronal death.

Histochemical and histofluorescence tracing of chelatable zinc in the developing mouse

Zinc is an essential element in mammalian development. However, little is known about concentrations of zinc in specific regions/organs in the embryo. We have employed selenite autometallography (AMG) and TSQ histofluoroscence to detect histochemically reactive (chelatable) zinc in whole midsagittal embryos and sections from neonatal mice. Chelatable zinc exhibited a broad distribution, being particularly localized to rapidly proliferating tissues, such as skin and gastrointestinal epithelium. Zinc was also observed in various types of tissues such as bone and liver. In the perinatal central nervous system, zinc was present almost exclusively in choroid plexus. The two methods used demonstrated generally similar distributions with some exceptions, e.g., in liver and blood. The ubiquity of zinc in the embryo, particularly in rapidly proliferating tissues, suggests a widespread role in fetal physiology.

Neuronal zinc exchange with the blood vessel wall promotes cerebral amyloid angiopathy in an animal model of Alzheimer's disease

Cerebral amyloid angiopathy (CAA) is common in Alzheimer's disease (AD) and may contribute to dementia and cerebral hemorrhage. Parenchymal beta-amyloid deposition is dependent on the activity of zinc transporter 3 (ZnT3), a neocortical synaptic vesicle membrane protein that causes enrichment of exchangeable Zn2+ in the vesicle, which is externalized on neurotransmission. However, the contribution of zinc to vascular beta-amyloid deposition remains unclear. Here, we identify for the first time an exchangeable pool of Zn2+ in the cerebrovascular wall of normal mice. This histochemically reactive Zn2+ is enriched in CAA in a transgenic mouse model of AD (Tg2576), and a dramatic reduction of CAA occurs after targeted disruption of the Znt3 gene in these mice. Also, in Znt3 knock-out mice, the amount of exchangeable Zn2+ [detected by N-(6-methoxy-8-quinolyl)-p-carboxybenzoylsulphonamide (TFL-Zn)] in the perivascular space was significantly decreased in the neocortex but not in peripheral organs. ZnT3 was not detected in the cerebral vessel walls or in blood components of wild-type mice. Thus, synaptic ZnT3 activity may promote CAA by indirectly raising exchangeable Zn2+ concentrations in the perivascular spaces of the brain.