Zinc uptake into synaptosomes

A model of zinc dynamics evoked by intense stimulation at the cleft of hippocampal mossy fiber synapses

Zinc is a transition metal that is particularly abundant in the mossy fibers of the hippocampal CA3 area. Despite the large number of studies about the zinc role in mossy fibers, the action of zinc in synaptic mechanisms is only partly known. The use of computational models can be a useful tool for this study. In a previous work, a model was developed to evaluate zinc dynamics at the mossy fiber synaptic cleft, following weak stimulation, insufficient to evoke zinc entry into postsynaptic neurons. For intense stimulation, cleft zinc effluxes must be considered. Therefore, the initial model was extended to include postsynaptic zinc effluxes based on the Goldman-Hodgkin-Katz current equation combined with Hodgkin and Huxley conductance changes. These effluxes occur through different postsynaptic escape routes, namely L- and N-types voltage-dependent calcium channels and NMDA receptors. For that purpose, various stimulations were assumed to induce high concentrations of cleft free zinc, named as intense (10 μM), very intense (100 μM) and extreme (500 μM). It was observed that the main postsynaptic escape routes of cleft zinc are the L-type calcium channels, followed by the NMDA receptor channels and by N-type calcium channels. However, their relative contribution for cleft zinc clearance was relatively small and decreased for higher amounts of zinc, most likely due to the blockade action of zinc in postsynaptic receptors and channels. Therefore, it can be concluded that the larger the zinc release, the more predominant the zinc uptake process will be in the cleft zinc clearance.

Zinc: Multidimensional Effects on Living Organisms

Zinc is a redox-inert trace element that is second only to iron in abundance in biological systems. In cells, zinc is typically buffered and bound to metalloproteins, but it may also exist in a labile or chelatable (free ion) form. Zinc plays a critical role in prokaryotes and eukaryotes, ranging from structural to catalytic to replication to demise. This review discusses the influential properties of zinc on various mechanisms of bacterial proliferation and synergistic action as an antimicrobial element. We also touch upon the significance of zinc among eukaryotic cells and how it may modulate their survival and death through its inhibitory or modulatory effect on certain receptors, enzymes, and signaling proteins. A brief discussion on zinc chelators is also presented, and chelating agents may be used with or against zinc to affect therapeutics against human diseases. Overall, the multidimensional effects of zinc in cells attest to the growing number of scientific research that reveal the consequential prominence of this remarkable transition metal in human health and disease.

Potential roles of zinc in the pathophysiology and treatment of major depressive disorder

Incomplete response to monoaminergic antidepressants in major depressive disorder (MDD), and the phenomenon of neuroprogression, suggests a need for additional pathophysiological markers and pharmacological targets. Neuronal zinc is concentrated exclusively within glutamatergic neurons, acting as an allosteric modulator of the N-methyl D-aspartate and other receptors that regulate excitatory neurotransmission and neuroplasticity. Zinc-containing neurons form extensive associational circuitry throughout the cortex, amygdala and hippocampus, which subserve mood regulation and cognitive functions. In animal models of depression, zinc is reduced in these circuits, zinc treatment has antidepressant-like effects and dietary zinc insufficiency induces depressive behaviors. Clinically, serum zinc is lower in MDD, which may constitute a state-marker of illness and a risk factor for treatment-resistance. Marginal zinc deficiency in MDD may relate to multiple putative mechanisms underlying core symptomatology and neuroprogression (e.g. immune dysfunction, monoamine metabolism, stress response dysregulation, oxidative/nitrosative stress, neurotrophic deficits, transcriptional/epigenetic regulation of neural networks). Initial randomized trials suggest a benefit of zinc supplementation. In summary, molecular and animal behavioral data support the clinical significance of zinc in the setting of MDD.

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.

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.

Measurement of presynaptic zinc changes in hippocampal mossy fibers

The hippocampal mossy fiber terminals of CA3 area contain high levels of vesicular zinc that is released in a calcium-dependent way, following high-frequency stimulation. However the properties of zinc release during normal synaptic transmission, paired-pulse facilitation and mossy fiber long-term potentiation are still unknown. Using the fluorescent zinc probe N-(6-methoxy-8-quinolyl)-para-toluenesulfonamide, we measured fast mossy fiber zinc changes indicating that zinc is released following single and low levels of electrical stimulation. The observed presynaptic zinc signals are maintained during the expression of mossy fiber long-term potentiation, assumed to be mediated by an increase in transmitter release, and are enhanced during paired-pulse facilitation. This zinc enhancement is, like paired-pulse facilitation, reduced during established long-term potentiation. The correlation between the paired-pulse evoked zinc and field potential responses supports the idea that zinc is co-released with glutamate.

Subunit-dependent high-affinity zinc inhibition of acid-sensing ion channels

Acid-sensing ion channels (ASICs), a novel class of ligand-gated cation channels activated by protons, are highly expressed in peripheral sensory and central neurons. Activation of ASICs may play an important role in physiological processes such as nociception, mechanosensation, and learning-memory, and in the pathology of neurological conditions such as brain ischemia. Modulation of the activities of ASICs is expected to have a significant influence on the roles that these channels can play in both physiological and/or pathological processes. Here we show that the divalent cation Zn2+, an endogenous trace element, dose-dependently inhibits ASIC currents in cultured mouse cortical neurons at nanomolar concentrations. With ASICs expressed in Chinese hamster ovary cells, Zn2+ inhibits currents mediated by homomeric ASIC1a and heteromeric ASIC1a-ASIC2a channels, without affecting currents mediated by homomeric ASIC1beta, ASIC2a, or ASIC3. Consistent with ASIC1a-specific modulation, high-affinity Zn2+ inhibition is absent in neurons from ASIC1a knock-out mice. Current-clamp recordings and Ca2+-imaging experiments demonstrated that Zn2+ inhibits acid-induced membrane depolarization and the increase of intracellular Ca2+. Mutation of lysine-133 in the extracellular domain of the ASIC1a subunit abolishes the high-affinity Zn2+ inhibition. Our studies suggest that Zn2+ may play an important role in a negative feedback system for preventing overexcitation of neurons during normal synaptic transmission and ASIC1a-mediated excitotoxicity in pathological conditions.

Abnormal benzodiazepine and zinc modulation of GABAA receptors in an acquired absence epilepsy model

Brain cholesterol synthesis inhibition (CSI) at a young age in rats has been shown to be a faithful model of acquired absence epilepsy, a devastating condition for which few therapies or models exist. We employed the CSI model to study cellular mechanisms of acquired absence epilepsy in Long-Evans Hooded rats. Patch-clamp, whole-cell recordings were compared from neurons acutely dissociated from the nucleus reticularis of thalamus (nRt) treated and untreated with a cholesterol synthesis inhibitor, U18666A. In U18666A-treated animals, 91% of rats developed EEG spike-waves (SWs). Patchclamp results revealed that although there was no remarkable change in GABAA receptor affinity, both a loss of ability of benzodiazepines to enhance GABAA-receptor responses and an increase of Zn2+ inhibition of GABAA-receptor responses of nRt neurons occurred in Long-Evans Hooded rats previously administered U18666A. This change was specific, since no significant changes were found in neurons exposed to the GABA allosteric modulator, pentobarbital. Taken collectively, these findings provide evidence for abnormalities in benzodiazepine and Zn2+ modulation of GABAA receptors in the CSI model, and suggest that decreased gamma2 subunit expression may underlie important aspects of generation of thalamocortical SWs in atypical absence seizures. The present results are also consistent with recent findings that mutation of the gamma2 subunit of the GABAA receptor changes benzodiazepine modulation in families with generalized epilepsy syndromes.

Depolarization-induced 65zinc influx into cultured cortical neurons

Toxic Zn(2+) influx may be a key mechanism underlying selective neuronal death after transient global ischemia in rats. To identify routes responsible for neuronal Zn(2+) influx, we measured the accumulation of (65)Zn(2+) into cultured murine cortical cells under depolarizing conditions (60 mM K(+)) associated with severe hypoxia-ischemia in brain tissue. Addition of 60 mM K(+) or 300 microM kainate substantially increased (65)Zn(2+) accumulation into mixed cultures of neurons and glia, but not glia alone. (65)Zn(2+) accumulation was attenuated by increasing concentrations of extracellular Ca(2+) or trypsin pretreatment, but not by late trypsinization, and corresponded to an increase in atomic Zn(2+). Confirming predominantly neuronal entry, K(+)-induced (65)Zn(2+) accumulation was reduced by prior selective destruction of neurons with NMDA. K(+)-induced (65)Zn(2+) influx was not sensitive to glutamate receptor antagonists, but was attenuated by Gd(3+) and Cd(2+) as well as 1 microM nimodipine; it was partially sensitive to 1 microM omega-conotoxin-GVIA, and insensitive to 1 microM omega-agatoxin-IVA. K(+)-induced, Gd(3+)-sensitive (45)Ca(2+) accumulation but not (65)Zn(2+) accumulation was sharply attenuated by lowering extracellular pH to 6.6.

Zinc inhibition of cAMP signaling

Zn(2+) is required as either a catalytic or structural component for a large number of enzymes and thus contributes to a variety of important biological processes. We report here that low micromolar concentrations of Zn(2+) inhibited hormone- or forskolin-stimulated cAMP production in N18TG2 neuroblastoma cells. Similarly, low concentrations inhibited hormone- and forskolin-stimulated adenylyl cyclase (AC) activity in membrane preparations and did so primarily by altering the V(max) of the enzyme. Zn(2+) also inhibited recombinant isoforms, indicating that this reflects a direct interaction with the enzyme. The IC(50) for Zn(2+) inhibition was approximately 1-2 microm with a Hill coefficient of 1.33. The dose-response curve for Zn(2+) inhibition was identical for AC1, AC5, and AC6 as well as for the C441R mutant of AC5 whose defect appears to be in one of the catalytic metal binding sites. However, AC2 displayed a distinct dose-response curve. These data in combination with the findings that Zn(2+) inhibition was not competitive with Mg(2+) or Mg(2+)/ATP suggest that the inhibitory Zn(2+) binding site is distinct from the metal binding sites involved in catalysis. The prestimulated enzyme was found to be less susceptible to Zn(2+) inhibition, suggesting that the ability of Zn(2+) to inhibit AC could be significantly influenced by the coincidence timing of the input signals to the enzyme.

pH dependence and compartmentalization of zinc transported across plasma membrane of rat cortical neurons

In this study, Zn(2+) transport in rat cortical neurons was characterized by successfully combining radioactive tracer experiments with spectrofluorometry and fluorescence microscopy. Cortical neurons showed a time-dependent and saturable transport of (65)Zn(2+) with an apparent affinity of 15-20 microM. (65)Zn(2+) transport was pH dependent and was decreased by extracellular acidification and increased by intracellular acidification. Compartmentalization of newly transported Zn(2+) was assessed with the Zn(2+)-selective fluorescent dye zinquin. Resting cortical neurons showed uniform punctate labeling that was found in cell processes and the soma, suggesting extrasynaptic compartmentalization of Zn(2+). Depletion of intracellular Zn(2+) with the membrane-permeant chelator N,N,N',N'-tetrakis(2-pyridylmethyl)-ethylenediamine (TPEN) resulted in the complete loss of punctate zinquin labeling. After Zn(2+) depletion, punctate zinquin labeling was rapidly restored when cells were placed in 30 microM Zn(2+), pH 7.4. However, rapid restoration of punctate zinquin labeling was not observed when cells were placed in 30 microM Zn(2+), pH 6.0. These data were confirmed in parallel (65)Zn(2+) transport experiments.

Atypical neural messengers

Notions of what constitutes a neurotransmitter have changed markedly with the advent in the past decade of synaptic molecules, which satisfy key neurotransmitter criteria but differ radically from classical transmitters. Thus, NO and carbon monoxide are neither stored in synaptic vesicles nor released by exocytosis. These gases do not act via traditional receptors on postsynaptic membranes. In addition, zinc, stored together with glutamate in synaptic vesicles, appears to act as an 'antagonist' co-transmitter at the NMDA receptor, and although localized exclusively to glia, D-serine fulfills most neurotransmitter criteria as an endogenous ligand for the 'glycine' site of NMDA receptors.

Release of synaptic zinc is substantially depressed by conventional brain slice preparations

Research on synaptically-released zinc is frequently done in vitro with acute brain slice preparations. We show here the in vitro hippocampal slice preparation has two major pitfalls for zinc research. First, up to 50% of the synaptic zinc is lost during slice cutting and/or the first 10 min of slice incubation, with the losses being most pronounced on the edges of the slice. Second, the release of the remaining zinc from a slice is substantially depressed (up to 50%) at the low temperatures (32 degrees C) typically used for brain slice studies. In concert, these effects reduce zinc release about 75% in vitro, compared to in vivo. Implications for research on synaptically-released zinc are discussed.

Evidence for a zinc/proton antiporter in rat brain

The data presented in this paper are consistent with the existence of a plasma membrane zinc/proton antiport activity in rat brain. Experiments were performed using purified plasma membrane vesicles isolated from whole rat brain. Incubating vesicles in the presence of various concentrations of 65Zn2+ resulted in a rapid accumulation of 65Zn2+. Hill plot analysis demonstrated a lack of cooperativity in zinc activation of 65Zn2+ uptake. Zinc uptake was inhibited in the presence of 1 mM Ni2+, Cd2+, or CO2+. Calcium (1 mM) was less effective at inhibiting 65Zn2+ uptake and Mg2+ and Mn2+ had no effect. The initial rate of vesicular 65Zn2+ uptake was inhibited by increasing extravesicular H+ concentration. Vesicles preloaded with 65Zn2+ could be induced to release 65Zn2+ by increasing extravesicular H+ or addition of 1 mM nonradioactive Zn2+. Hill plot analysis showed a lack of cooperativity in H+ activation of 65Zn2+ release. Based on the Hill analyses, the stoichiometry of transport may include Zn2+/Zn2+ exchange and Zn2+/H+ antiport, the latter being potentially electrogenic. Zinc/proton antiport may be an important mode of zinc uptake into neurons and contribute to the reuptake of zinc to replenish presynaptic vesicle stores after stimulation.

Zinc transport in the brain: routes of zinc influx and efflux in neurons

Studies of the routes of entry and exit for zinc in different tissues and cell types have shown that zinc can use several pathways of exit or entry. In neurons, known pathways include (1) presynaptic release along with glutamate when synaptic vesicles empty their contents into the synaptic cleft, (2) voltage-gated L-type Ca(2+) channels and glutamate-gated channels that provide an entry route when cells are depolarized and that mediate extracellular zinc toxicity and (3) a plasma membrane transporter potentially present in all neurons important for cellular zinc homeostasis. The least understood of these pathways, in terms of mechanism, is the transporter pathway. The kinetics of zinc uptake in cultured neurons under resting conditions are consistent with and suggest the existence of a saturable transporter in the plasma membrane. The proteins responsible for plasma membrane zinc transport have not yet been definitely identified. Likely candidates include two proteins identified by molecular cloning termed zinc transporter 1 and divalent cation transporter DCT1. Both proteins have been shown to be expressed in the brain, but only DCT1 is clearly demonstrated to be a transport protein, whereas zinc transporter 1 may only modulate zinc transport in association with as-yet-unidentified proteins. Understanding the mechanism and neuromodulation of plasma membrane zinc transport will be an important first step toward a complete understanding of neuronal zinc homeostasis.

Importance of zinc in the central nervous system: the zinc-containing neuron

Zinc is essential to the structure and function of myriad proteins, including regulatory, structural and enzymatic. It is estimated that up to 1% of the human genome codes for zinc finger proteins. In the central nervous system, zinc has an additional role as a neurosecretory product or cofactor. In this role, zinc is highly concentrated in the synaptic vesicles of a specific contingent of neurons, called "zinc-containing" neurons. Zinc-containing neurons are a subset of glutamatergic neurons. The zinc in the vesicles probably exceeds 1 mmol/L in concentration and is only weakly coordinated with any endogenous ligand. Zinc-containing neurons are found almost exclusively in the forebrain, where in mammals they have evolved into a complex and elaborate associational network that interconnects most of the cerebral cortices and limbic structures. Indeed, one of the intriguing aspects of these neurons is that they compose somewhat of a chemospecific "private line" of the mammalian cerebral cortex. The present review outlines (1) the methods used to discover, define and describe zinc-containing neurons; (2) the neuroarchitecture and synaptology of zinc-containing neural circuits; (3) the physiology of regulated vesicular zinc release; (4) the "life cycle" and molecular biology of vesicular zinc; (5) the importance of synaptically released zinc in the normal and pathological processes of the cerebral cortex; and (6) the role of specific and nonspecific stressors in the release of zinc.

Regionally selective blockade of GABAergic inhibition by zinc in the thalamocortical system: functional significance

The thalamocortical (TC) system is a tightly coupled synaptic circuit in which GABAergic inhibition originating from the nucleus reticularis thalami (NRT) serves to synchronize oscillatory TC rhythmic behavior. Zinc is colocalized within nerve terminals throughout the TC system with dense staining for zinc observed in NRT, neocortex, and thalamus. Whole cell voltage-clamp recordings of GABA-evoked responses were conducted in neurons isolated from ventrobasal thalamus, NRT, and somatosensory cortex to investigate modulation of the GABA-mediated chloride conductance by zinc. Zinc blocked GABA responses in a regionally specific, noncompetitive manner within the TC system. The regional levels of GABA blockade efficacy by zinc were: thalamus > NRT > cortex. The relationship between clonazepam and zinc sensitivity of GABA(A)-mediated responses was examined to investigate possible presence or absence of specific GABA(A) receptor (GABAR) subunits. These properties of GABARs have been hypothesized previously to be dependent on presence or absence of the gamma2 subunit and seem to display an inverse relationship. In cross-correlation plots, thalamic and NRT neurons did not show a statistically significant relationship between clonazepam and zinc sensitivity; however, a statistically significant correlation was observed in cortical neurons. Spontaneous epileptic TC oscillations can be induced in vitro by perfusion of TC slices with an extracellular medium containing no added Mg(2+). Multiple varieties of oscillations are generated, including simple TC burst complexes (sTBCs), which resemble spike-wave discharge activity. A second variant was termed a complex TC burst complex (cTBC), which resembled generalized tonic clonic seizure activity. sTBCs were exacerbated by zinc, whereas cTBCs were blocked completely by zinc. This supported the concept that zinc release may modulate TC rhythms in vivo. Zinc interacts with a variety of ionic conductances, including GABAR currents, N-methyl-D-aspartate (NMDA) receptor currents, and transient potassium (A) currents. D-2-amino-5-phosphonovaleric acid and 4-aminopyridine blocked both s- and cTBCs in TC slices. Therefore NMDA and A current-blocking effects of zinc are insufficient to explain differential zinc sensitivity of these rhythms. This supports a significant role of zinc-induced GABAR modulation in differential TC rhythm effects. Zinc is localized in high levels within the TC system and appears to be released during TC activity. Furthermore application of exogenous zinc modulates TC rhythms and differentially blocks GABARs within the TC system. These data are consistent with the hypothesis that endogenously released zinc may have important neuromodulatory actions impacting generation of TC rhythms, mediated at least in part by effects on GABARs.

Zinc inhibits Ca2+ transport by rat brain NA+/Ca2+ exchanger

Calcium transport by the Na+/Ca2+ exchanger was measured in plasma membranes vesicles purified from rat brain and in primary rat cortical cell culture. Sodium-loaded vesicles rapidly accumulate Ca2+ via Na+/Ca2+ exchange (Na+(i)-dependent Ca2+ uptake). Extravesicular zinc inhibited Na+/Ca2+ exchange as evidenced by a reduction of the initial velocity of Ca2+ uptake. Significant inhibition of Ca2+ uptake was seen at concentrations of zinc as low as 3 microM. Lineweaver-Burk analysis of the data was consistent with noncompetitive inhibition with respect to extravesicular Ca2+ concentration. The Ki for zinc inhibition of Ca2+ uptake determined from a Dixon plot was 14.5 microM. This is within the range of zinc concentrations thought to be obtained extracellularly after excitation. When vesicles were preloaded with Ca2+, extravesicular zinc also inhibited reversal of Na+/Ca2+ exchange (Na+(i)-dependent Ca2+ release) although its potency was much less: concentrations of > or = 30 microM zinc were required. Zinc inhibition of Ca2+ release was not Na+ dependent. Na+(i)-dependent calcium uptake by rat cortical cells in primary culture also was inhibited by zinc. The extent of inhibition was similar to that seen for inhibition of Na+(i)-dependent Ca2+ uptake in membrane vesicles, but the potency was less. The results suggest that Ca2+ transport by the Na+/Ca2+ exchanger is inhibited by concentrations of zinc thought to be attained extracellularly after excitation.

Diverse effects of metal chelating agents on the neuronal cytotoxicity of zinc in the hippocampus

Abnormal metabolism of metal ions such as zinc may contribute to neuropathology. Complexing zinc could reduce this pathology. Thus, to examine the effectiveness of metal chelating agents in vivo, a model system was used. This involved determining the ability of chelating agents to prevent neuronal death caused by zinc chloride injected into the rat hippocampus. Significant protection against zinc toxicity was obtained with pyrithione, inositol hexakisphosphate, ethylenediamine tetraacetate (EDTA) and N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN). The affinity of these agents for zinc varied between 106 M-1 and 1018 M-1. Thus, the affinity for zinc within this range does not appear to be a major factor affecting the ability of chelators to provide neuroprotection. While almost complete protection was found with EDTA and TPEN given simultaneously with zinc chloride, poor protection was obtained if TPEN was given before or after zinc chloride. Other agents either did not protect against zinc-induced neuronal death (zincon), or exacerbated zinc toxicity (BTC-5N and about 40% of rats injected with a combination of zinc chloride and diethylenetriamine pentaacetate [DTPA]). Rats showing increased damage after zinc plus BTC-5N or DTPA suffered wet dog-like shakes (WDS), suggesting that these zinc chelate complexes can induce seizures resulting in seizure-related damage. In contrast, in the 60% of rats treated with zinc chloride and DTPA that had no WDS, there was about an 80% reduction in the size of the zinc-induced lesion. The ability of chelators to cross cell membranes was examined by determining whether Timm's staining for vesicular zinc was reduced following the injection of a chelator into the hippocampus. TPEN and pyrithione reduced Timm's staining for zinc. However, cell permeability was not necessary for a chelator to protect against zinc toxicity.

Characterization of a plasma membrane zinc transporter in rat brain

Many studies now show that zinc plays a critical and unique role in central nervous system development and function. The cellular mechanisms of zinc efflux and influx are largely unknown and few models exist that describe cellular zinc transport in the brain. This report provides convincing evidence of a zinc transporter in plasma membrane vesicles isolated from rat brain. Zinc uptake was saturable (Km = 15 microM; Vmax = 10 nmol/mg per 30 s), was seen in the absence of ATP, and was unaffected by gradients for other ions such as Na+ or K+. Increasing the ionic strength of the extravesicular media with Na+, K+, or choline+ reduced zinc uptake approximately 50%. Whereas, increasing extravesicular H+ concentration (pH = 5) resulted in near complete inhibition of zinc uptake. Intravesicular zinc was rapidly released upon lowering extravesicular concentrations of zinc with the heavy metal chelator O-phenanthroline (1 mM). The results are consistent with a freely-reversible transport of zinc across the plasma membrane of neurons.

Modulation by zinc of the glutamate transporters in glial cells and cones isolated from the tiger salamander retina

1. Zinc may be released from some presynaptic glutamatergic neurons, including hippocampal mossy fibres and retinal photoreceptors. We whole-cell-clamped glial (Müller) cells isolated from the salamander retina to investigate the effect of zinc on glutamate transporters in these cells. Glutamate-evoked currents in these cells are generated largely by carriers homologous to the mammalian GLAST/EAAT1 transporter. 2. Zinc inhibited both glutamate uptake into the cells, and glutamate release by reversal of the uptake process. The IC50 for inhibition of uptake (< 1 microM) was similar to or below the values for zinc modulating NMDA, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and GABA receptors, and 100-fold less than the calculated value for the rise in extracellular zinc concentration evoked by depolarization with potassium in area CA3 of the hippocampus. 3. Although zinc altered the apparent affinity of the transporter for glutamate and Na+, it did not act simply by binding competitively to the glutamate-, Na(+)-, K(+)- or H(+)-binding sites on the transporter. Zinc inhibited both forward and reversed glutamate transport from the outside of the cell membrane, but not from the inside. The inhibitory action of zinc on uptake was voltage independent, indicating a zinc-binding site outside the membrane field. 4. As well as inhibiting glutamate transport, zinc potentiated activation of the anion conductance in the Müller cell glutamate transporter. However, zinc reduced the current mediated by the anion conductance in the cone synaptic terminal glutamate transporter (homologous to the mammalian EAAT5), indicating that zinc has different actions on different glutamate transporter subtypes. 5. By acting on glutamate transporters, zinc may have a neuromodulatory role during synaptic transmission and a neuroprotective role during transient ischaemia.

Zinc metabolism in the brain: relevance to human neurodegenerative disorders

Zinc is an important trace element in biology. An important pool of zinc in the brain is the one present in synaptic vesicles in a subgroup of glutamatergic neurons. In this form it can be released by electrical stimulation and may serve to modulate responses at receptors for a number of different neurotransmitters. These include both excitatory and inhibitory receptors, particularly the NMDA and GABA(A) receptors. This pool of zinc is the only form of zinc readily stained histochemically (the chelatable zinc pool), but constitutes only about 8% of the total zinc content in the brain. The remainder of the zinc is more or less tightly bound to proteins where it acts either as a component of the catalytic site of enzymes or in a structural capacity. The metabolism of zinc in the brain is regulated by a number of transport proteins, some of which have been recently characterized by gene cloning techniques. The intracellular concentration may be mediated both by efflux from the cell by the zinc transporter ZrT1 and by complexing with apothionein to form metallothlonein. Metallothionein may serve as the source of zinc for incorporation into proteins, including a number of DNA transcription factors. However, zinc is readily released from metallothionein by disulfides, increasing concentrations of which are formed under oxidative stress. Metallothionein is a very good scavenger of free radicals, and zinc itself can also reduce oxidative stress by binding to thiol groups, decreasing their oxidation. Zinc is also a very potent inhibitor of nitric oxide synthase. Increased levels of chelatable zinc have been shown to be present in cell cultures of immune cells undergoing apoptosis. This is very reminiscent of the zinc staining of neuronal perikarya dying after an episode of ischemia or seizure activity. Thus a possible role of zinc in causing neuronal death in the brain needs to be fully investigated. intraventricular injections of calcium EDTA have already been shown to reduce neuronal death after a period of ischemia. Pharmacological doses of zinc cause neuronal death, and some estimates indicate that extracellular concentrations of zinc could reach neurotoxic levels under pathological conditions. Zinc is released in high concentrations from the hippocampus during seizures. Unfortunately, there are contrasting observations as to whether this zinc serves to potentiate or decrease seizure activity. Zinc may have an additional role in causing death in at least some neurons damaged by seizure activity and be involved in the sprouting phenomenon which may give rise to recurrent seizure propagation in the hippocampus. In Alzheimer's disease, zinc has been shown to aggregate beta-amyloid, a form which is potentially neurotoxic. The zinc-dependent transcription factors NF-kappa B and Sp1 bind to the promoter region of the amyloid precursor protein (APP) gene. Zinc also inhibits enzymes which degrade APP to nonamyloidogenic peptides and which degrade the soluble form of beta-amyloid. The changes in zinc metabolism which occur during oxidative stress may be important in neurological diseases where oxidative stress is implicated, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). Zinc is a structural component of superoxide dismutase 1, mutations in which give rise to one form of familiar ALS. After HIV infection, zinc deficiency is found which may be secondary to immune-induced cytokine synthesis. Zinc is involved in the replication of the HIV virus at a number of sites. These observations should stimulate further research into the role of zinc in neuropathology.

Zinc and Alzheimer's disease: is there a direct link?

Zinc is an essential trace element in human biology, but is neurotoxic at high concentrations. Several studies show that zinc promotes aggregations of beta-amyloid protein, the main component of the senile plaques typically found in Alzheimer's disease brains. In other neurological disorders where neurons appear to be dying by apoptosis (gene-directed cell death), chelatable zinc accumulates in the perikarya of neurons before, or during degeneration. As there is evidence for apoptotic death of neurons in Alzheimer's disease, an involvement of zinc in this process needs to be investigated. Zinc interacts with enzymes and proteins, including transcription factors, which are critical for cell survival and could be linked to apoptotic processes. While controversial, some studies indicate that total tissue zinc is markedly reduced in several brain regions of Alzheimer's patients. At face value, it seems that a paradox exists between reports of a decrease in zinc in the Alzheimer's brain and the putative link to aberrant high zinc levels promoting plaque formation. An hypothesis to explain this inconsistency is presented. Neuropathological changes mediated by endogenous or exogenous stressors may be relevant factors affecting abnormal zinc metabolism. This paper reviews current investigations that suggest a role of zinc in the etiology of Alzheimer's disease.

Metallothioneins in brain--the role in physiology and pathology

A symposium on the role of brain metallothioneins (MTs) in physiology and pathology was held at the 1996 Annual Society of Toxicology Meeting in Anaheim, California. The objectives of this symposium were to: (1) review the physiologic function of MTs, (2) examine the distribution of brain MTs with particular emphasis on cell-specific localization (neurons vs neuroglia), (3) discuss MT gene responsiveness upon toxic insult with metals, and (4) discuss the potential role of MTs in the etiology of neurodegenerative disorders. Dr. Cherian discussed the biochemical properties of the MTs, emphasizing structural similarities and differences between the MTs. Dr. Klaassen addressed the expression and distribution of the MTs in brains with special reference to the cell-specific localization of MTs. Dr. Aschner provided data illustrating a potential role for MTs in attenuating the cytotoxicity caused by methylmercury (MeHg) in cultured neonatal astrocytes. Dr. Palmiter discussed the properties of MT-III and the increased sensitivity of MT-III knockout mice to kainate-induced seizures. Cerebral zinc metabolism, its relationship to MT homeostasis, and its pathogenic potential in Alzheimer's disease was addressed by Dr. Bush.

Biological half-lives of zinc and manganese in rat brain

The brains of rats injected intravenously with 65ZnCl2 or 54MnCl2 were subjected to high-resolution autoradiography. The distribution of 65Zn and 54Mn in each brain region gradually decreased from 6 days to 42 days for 65Zn and from 15 days to 60 days for 54Mn after the injection. The biological half-lives of Zn in each region studied were in the range of 16-43 days; the longest was observed in the amygdaloid nuclei. The regions where the long biological half-life was observed were consistent with the ones with the high density of Zn-containing neuron terminals reported previously. The biological half-lives of Mn in each region were 51-74 days; the longest were those in the hypothalamic nuclei and thalamus.

Permeation by zinc of bovine chromaffin cell calcium channels: relevance to secretion

Zn2+ increased the rate of spontaneous release of catecholamines from bovine adrenal glands. This effect was Ca2+ independent; in fact, in the absence of extracellular Ca2+, the secretory effects of Zn2+ were enhanced. At low concentrations (3-10 microM), Zn2+ enhanced the secretory responses to 10-s pulses of 100 microM 1,1-dimethyl-4-phenylpiperazinium (DMPP, a nicotinic receptor agonist) or 100 mM K+. In the presence of DMPP, secretion was increased 47% above controls and in high-K+ solutions, secretion increased 54% above control. These low concentrations of Zn2+ did not facilitate the whole-cell Ca2+ (ICa) or Ba2+ (IBa) currents in patch-clamped chromaffin cells. Higher Zn2+ concentrations inhibited the currents (IC50 values, 346 microM for ICa and 91 microM for IBa) and blocked DMPP- and K(+)-evoked secretion (IC50 values, 141 and 250 microM, respectively). Zn2+ permeated the Ca2+ channels of bovine chromaffin cells, although at a much slower rate than other divalent cations. Peak currents at 10 mM Ba2+, Ca2+, Sr2+ and Zn2+ were 991, 734, 330 and 7.4 pA, respectively. Zn2+ entry was also evidenced using the fluorescent Ca2+ probe fura-2. This was possible because Zn2+ causes an increase in fura-2 fluorescence at the isosbestic wave-length for Ca2+, i.e. 360 nm. There was a slow resting entry of Zn2+ which was accelerated by stimulation with DMPP or high-K+ solution. The entry of Zn2+ was concentration dependent, slightly antagonized by 1 mM Ca2+ and completely blocked by 5 mM Ni2+. The entry of Ca2+ evoked by depolarization with high-K+ solution was antagonized by Zn2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of zinc, copper and glucocorticoids on metallothionein levels of cultured neurons and astrocytes from rat brain

The knowledge of brain metallothionein (MT) regulation and especially of MT presence in specific cell types is scarce. Therefore, the effect of several well-known MT inducers, measured by radioimmunoassays using antibodies that cross-react with MT-I and MT-II or specific for MT-I and which do not cross-react with human growth inhibitory factor (GIF or MT-III), has been studied in primary cultures of neurons or astrocytes obtained from rat cerebrum. MT-I levels in glial cells were about ten times higher than those in neuronal cells (538 +/- 194 vs. 49 +/- 16 pg MT-I/micrograms protein, mean +/- S.D. from three separate cell preparations). Increasing the concentration of Zn in the bovine serum albumin (BSA)-containing culture medium up to 50 microM significantly increased MT-I levels by up to 3.5-fold in neurons and 2.5-fold in astrocytes. In contrast, Cu up to 50 microM increased MT-I levels in a saturable manner in both neurons (up to 5-fold) and astrocytes (up to 1.5-fold), the maximum effect occurring at 5 microM Cu. In general, the combination of Zn and Cu further increased MT-I levels. The effect of the metals on MT-I appeared to reflect metal uptake, since MT-I induction was less marked when the BSA concentration in the medium was increased from 2 to 10 mg/ml. Dexamethasone increased MT-I levels in both neurons and astrocytes in vitro in a concentration-dependent manner. Endotoxin, IL-1 and IL-6 did not have a significant effect on glial MT levels at the concentrations studied. The administration of dexamethasone to rats increased MT-I levels in non-frontal cortex, cerebellum, pons+medulla, midbrain and hippocampus, but not in hypothalamus, frontal cortex and striatum. Endotoxin increased liver but not brain MT-I levels. Immunocytochemical studies in adult rat brain preparations with a polyclonal antibody that cross-reacts with MT-I and MT-II indicated that immunostaining was always nuclear in glial cells, whereas in neurons it was nuclear in the cerebral cortex, hippocampus and the granular layer of the cerebellum, and nuclear plus cytoplasmic in Purkinje cells in the cerebellum, hypothalamic nuclei and gigantocellular reticular nucleus in the brain stem. Meninges, choroidal plexus, ependymal and endothelial cells were also MT-immunoreactive.

Modulation of GABA-mediated synaptic transmission by endogenous zinc in the immature rat hippocampus in vitro

1. Intracellular recordings from postnatal 2- to 12-day-old (P2-12) rat hippocampal CA3 pyramidal neurones exhibited spontaneous synaptic potentials mediated by GABAA receptors. These potentials can be separated on the basis of amplitude into two classes which are referred to as small and large. 2. The large depolarizing potentials were reversibly inhibited by the Zn2+ chelator 1,2-diethyl-3-hydroxypyridin-4-one (CP94). The small inhibitory postsynaptic potentials. (IPSPs) were apparently unaffected. 3. Stimulation of the mossy fibre pathway evoked composite excitatory postsynaptic potentials (EPSPs) and IPSPs. Threshold stimulus-evoked synaptic potentials were mediated by GABAA receptors and were reversibly blocked by CP94. The responses evoked by suprathreshold stimulation and persisting in the presence of bicuculline or CP94 were partially inhibited by 2-amino-5-phosphonopropionic acid (AP5) and were completely blocked with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 4. L-Histidine, which preferentially forms complexes with Cu2+ > Zn2+ > Fe2+ > Mn2+, inhibited both naturally occurring spontaneous and evoked GABAA-mediated large synaptic potentials without affecting the neuronal resting membrane properties. Exogenously applied Zn2+ induced large spontaneous synaptic potentials and prolonged the duration of the evoked potentials. These effects were reversibly blocked by histidine. 5. The metal chelating agent diethyldithiocarbamate had little effect on the large amplitude synaptic potentials. 6. The transition metal divalent cations Fe2+ and Mn2+ did not initiate large synaptic potentials in CA3 neurones; however, Cu2+ depolarized the membrane and enhanced both excitatory and inhibitory synaptic transmission, resulting in a transient increase in the frequency of the large amplitude events. In comparison, zinc increased the frequency of the large potentials and also induced such events in neurons (P4-21) where innate potentials were absent. The postsynaptic response to ionophoretically applied GABA was either unaffected or slightly enhanced by Zn2+. 7. Under conditions favouring the activation of non-NMDA receptors, excitatory synaptic transmission was unaffected by CP94 but was depressed by Zn2+. Responses to ionophoretically applied glutamate were not inhibited by Zn2+, indicating that Zn2+ affects excitatory synaptic transmission via a presynaptic mechanism. 8. We conclude that the naturally occurring large synaptic potentials in young CA3 neurones are apparently induced by endogenous Zn2+ which can promote or synchronize the release of GABA in the immature hippocampus.

Brain uptake of trace metals, zinc and manganese, in rats

The brains of rats injected intravenously with 65ZnCl2 or 54MnCl2 were subjected to high resolution autoradiography. 65Zn and 54Mn were largely concentrated in choroid plexus 1 h after injection and then gradually decreased, with increases in other brain regions, suggesting that both metals were taken up gradually into brain mainly via cerebrospinal fluid in the choroid plexus. By 3 days after injection, relatively high level of 65Zn was seen in the dentate gyrus and CA3 region of the hippocampus and in the cerebral cortex. The level of 54Mn was also high in the former, while relatively low in the latter.

The mechanism of zinc uptake by cultured rat liver cells

1. The initial rate of 65Zn uptake into cultured rat hepatocytes has been measured over a range of Zn2+ concentrations from 3 x 10(-10) M to 5 x 10(-6) M. Histidine and albumin were used to buffer Zn2+ ions at concentrations below 1 x 10(-6) M. 2. The results suggest there are two mechanisms for Zn2+ uptake; a high-affinity, saturable pathway, with a maximum velocity (Vmax) of 20-30 pmol (mg protein)-1 min-1 and a Michaelis-Menten constant (Km) of about 2 x 10(-9) M Zn2+ (with histidine), and a low-affinity, linear pathway, that only makes a significant contribution to Zn2+ uptake at Zn2+ concentrations above 1 x 10(-6) M. 3. Transport via the high-affinity pathway is dependent on the concentration of Zn2+ ions and not on the concentrations of Zn(2+)-ligand complexes, suggesting that Zn2+ is the transported species. 4. The affinity of the saturable pathway for Zn2+ is slightly lower in the presence of albumin, with a Km of about 1.3 x 10(-8) M. The reason for this is uncertain.

Effect of CH3HgCl and several transition metals on the dopamine neuronal carrier; peculiar behaviour of Zn2+

CH3Hg+ and metal ions inhibited the specific binding of (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenyl-2-[1-3H]propenyl) piperazine) ([3H]GBR 12783) to the dopamine neuronal carrier present in membranes from rat striatum with a general rank order of potency CH3Hg+ > Cu2+ > Cd2+ > Zn2+ > Ni2+ = Mn2+ = Co2+, suggesting that -SH groups are chiefly involved in this inhibition. Five millimolar dithiothreitol reversed the rather stable block of the specific binding produced by Cd2+ or Zn2+. An increase in the concentration of Na+, or addition of either K+ or Ca2+ reduced the inhibitory effects of metal cations, except Cu2+. Zn2+ (3 microM) reduced the inhibitory potency of Cd2+ on the binding but was ineffective against CH3Hg+ and Cu2+. Zn2+ at 0.3 to 10 microM significantly enhanced the specific binding of [3H]GBR 12783 and [3H]cocaine by 42 to 146%. Zn2+ (3 microM) increased the affinity of all pure uptake inhibitors tested and of the majority of the substrates for the [3H]GBR 12783 binding site. Dissociation experiments revealed that Zn2+ both inhibited and enhanced the [3H]GBR 12783 binding by recognizing amino acids located close to or in the radioligand binding site. Micromolar concentrations of Zn2+ noncompetitively blocked the [3H]dopamine uptake but they did not modify the block of the transport provoked by pure uptake inhibitors. These findings suggest that Na+, K+, Ca2+ and metal ions could recognize some -SH groups located in the [3H]GBR 12783 binding site; low concentrations of Zn2+ could allow a protection of these -SH groups.

Measurement of free Zn2+ ion concentration with the fluorescent probe mag-fura-2 (furaptra)

The fluorescent probe mag-fura-2, previously used to measure [Mg2+], can also be used to measure [Zn2+]. The peak in the excitation spectrum occurs at 323 nm for Zn2+, compared with 335 nm for Ca2+ and Mg2+. This allows simultaneous measurements of [Zn2+] and either [Ca2+] or [Mg2+], by using 3 excitation wavelengths. The dissociation constant for Zn2+ is 20 nM at pH 7.0-7.8, ionic strength 0.15 and 37 degrees C. This allows [Zn2+] to be measured in the range from 0.5 nM to 1 microM. Mag-fura-2 was used to measure [Zn2+] in Zn2+/albumin and Zn2+/histidine mixtures in a physiological buffer at 37 degrees C and pH 7.4. The data obtained enable one to formulate Zn(2+)-buffers for the 1 to 100 nM Zn2+ range.

Properties of GABA-mediated synaptic potentials induced by zinc in adult rat hippocampal pyramidal neurones

1. Intracellular recording techniques were used to study the actions of the transition ion, zinc, on CA1 and CA3 pyramidal neurones in adult rat hippocampal slices. 2. Zinc (300 microM) hyperpolarized pyramidal neurones, increased the membrane excitability and also induced periodic, spontaneous giant depolarizing potentials associated with a conductance increase mechanism. 3. The occurrence of spontaneous giant depolarizations was dependent on the zinc concentration (10 microM-1 mM) with an apparent dissociation constant of 98 microM. The frequency of zinc-induced depolarizations was unaffected by the membrane potential from -50 to -100 mV. 4. Stimulation of the Schaffer collaterals or mossy fibre pathways evoked an excitatory and inhibitory synaptic potential complex. In the presence of zinc, nerve fibre stimulation evoked, in an all-or-none fashion, a giant depolarizing potential with an increased membrane conductance. Both spontaneous and evoked depolarizations were inhibited by 1 microM tetrodotoxin. 5. Evoked giant depolarizations were labile with too frequent stimulation resulting in a failure of generation. A minimum time of 140 s was required between stimuli to ensure successive giant depolarizations. 6. Spontaneous and evoked zinc-induced depolarizing potentials were inhibited by bicuculline (10 microM) or picrotoxin (40 microM) and enhanced by pentobarbitone (100 microM) or flurazepam (10 microM), suggesting that these potentials are mediated by activation of gamma-aminobutyric acidA (GABAA) receptors. 7. Ionophoretic application of GABA produced biphasic responses at -60 mV membrane potential. The reversal potentials for the depolarizing and hyperpolarizing GABA responses were -56 +/- 5 and -66 +/- 8 mV respectively. The giant depolarizations induced by zinc reversed at -57 +/- 4 mV. This suggests a dendritic location for the generation of these potentials. 8. Excitatory amino acid antagonists, 2-amino-5-phosphonovalerate (APV, 40 microM) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) did not affect the amplitude but slightly reduced the frequency of the giant depolarizations. 9. It is concluded that zinc induces a synchronized release of GABA, quite independent of intact excitatory synaptic transmission, which acts on GABAA receptors producing large depolarizing synaptic potentials. This increased level of GABA release may be of physiological and pathological importance since zinc is a naturally occurring metal ion endogenous to the central nervous system.

Zinc modulates GABAB binding in rat brain

The effects of ZnCl2 on [3H]GABA binding to GABAA and GABAA binding sites were investigated using receptor autoradiography. At concentrations exceeding 100 microM, zinc non-competitively inhibited GABAB binding in a dose dependent fashion. GABAA binding was not inhibited significantly by zinc eliminating the possibility of a non-specific effect of zinc. Increased calcium concentrations up to 10 mM enhanced total GABAB binding but did not prevent zinc induced inhibition of GABAB binding, indicating a separate site of action for these cations at the GABAB binding site. In some regions, zinc modulates GABAB binding in a biphasic manner as concentrations of 10-100 microM zinc significantly enhanced GABAB binding in the hippocampus and the molecular layer of the cerebellum but not in the thalamus. These results provide further evidence for a neuromodulatory role for zinc in the central nervous system.

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Neurons of origin of zinc-containing pathways and the distribution of zinc-containing boutons in the hippocampal region of the rat

Recent methods allow the study of neurons that contain zinc in synaptic vesicles of their boutons (Timm-stainable boutons) by the intravital precipitation (local or throughout the CNS) of the vesicular zinc with selenium compounds and its subsequent retrograde transport to the parent neurons, where the precipitate can be silver enhanced. The present study is a description of the distribution of zinc-containing neurons, their possible connections and their terminal fields within the hippocampal region of the rat. Problems inherent to the methods are addressed. Finally, based on the results and a review of literature, the possible function of zinc in the hippocampal region is considered. Neurons which contain silver-enhanced precipitates were observed in layers II, V and VI of the lateral entorhinal area and in layers V and VI of the medial entorhinal area. In the parasubiculum, labeled cells were seen in layer II/III of the parasubiculum a and in layer V. Labeled cells in the presubiculum were concentrated in layers III and V, in the hippocampal pyramidal cell layer and the dentate granule cell layer, but neurons containing precipitates were largely absent from the subiculum. Zinc-containing axonal boutons defined subpopulations within principal hippocampal neuron populations. Within layer II of the lateral entorhinal cortex and the pyramidal cell layer for regio inferior deeply situated neurons were labeled, whereas superficially placed pyramidal cells were labeled in regio superior. The neuropil staining described in the present study corresponded to that found in earlier studies. However, glial and vascular staining or unspecific background were largely absent, and the neuropil staining could unequivocally be identified light microscopically. Methodological problems are most prominently reflected in unstained mossy fibers in some animals. Based on series from animals treated with decreasing doses of sodium selenite and increased survival times, this problem can be related to small amounts of circulating reactive selenium and a competition of zinc compartments (vesicles) for the selenium. Staining will fail where the competition prevents individual compartments from reaching a threshold amount of zinc precipitate for silver amplification. A guide to evaluate histological material is provided. The distribution of zinc-containing boutons and their cells of origin indicate that zinc-containing and zinc-negative projections are not organized as parallel pathways. The mossy fibers provide an example of a pure zinc-containing pathway. Projections from regio superior to the dorsal presubiculum are likely to be zinc-negative while projections from the same area to the subiculum are zinc-containing.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of amino acids on zinc transport in rat erythrocytes

1. A significant proportion of plasma zinc exists complexed with amino acids. The effect of amino acids on the accumulation of radioactive zinc by rat erythrocytes was studied in vitro, to investigate the hypothesis that zinc might be transported into cells as an amino acid-zinc complex. 2. L-Histidine (500 microM-10 mM) stimulated 65Zn uptake; 50 mM-L-histidine gave a slight inhibition of uptake. D-Histidine (500 microM-10 mM) inhibited uptake in a dose-dependent manner. A non-zinc-binding amino acid, L-alanine, did not affect 65Zn uptake. 3. The effect of L-histidine was sodium dependent and temperature dependent, but was DIDS insensitive. These properties suggest that zinc is being transported as a zinc-histidine complex, utilizing an amino acid carrier system. Uptake of zinc in the presence of L-histidine differed from the previously described ionic mechanism, and may represent a physiological route of uptake. 4. L-Histidine stimulated efflux of 65Zn from pre-loaded cells. 5. The relevance of transport of a zinc-histidine complex is discussed with reference to histidinaemia, and as a significant zinc transport system in the presence of the very low ionic zinc concentrations found in plasma.

Zinc containing projections to the bed nucleus of the stria terminalis

A retrograde tracing method that selectively labels the perikarya of zinc-containing neurons was used to identify the neurons that supply zinc-containing fibers to the bed nucleus of the stria terminalis in the rat. In agreement with prior lesion studies, retrograde tracing indicates that neurons in amygdalar and periamygdalar regions are the major sources of the zinc-containing innervation of the bed nucleus complex. Zinc-containing neurons in the presubiculum and prosubiculum were also retrogradely labeled from the BNST, whereas cells of the subiculum proper did not label. Light and occasional retrograde labeling of some CA1 and CA2 neurons and limbic cortical neurons was also observed, but the possibility of transport from regions bordering BNST injections (septum, caudate-putamen) could not be excluded in the latter cases.

A physiological role for endogenous zinc in rat hippocampal synaptic neurotransmission

The mammalian central nervous system (CNS) contains an abundance of the transition metal zinc, which is highly localized in the neuronal parenchyma. Zinc is actively taken up and stored in synaptic vesicles in nerve terminals, and stimulation of nerve fibre tracts that contain large amounts of zinc, such as the hippocampal mossy fibre system, can induce its release, suggesting that it may act as a neuromodulator. The known interaction of zinc with the major excitatory and inhibitory amino-acid neurotransmitter receptors in the CNS supports this notion. That zinc has a role in CNS synaptic transmission, however, has so far not been shown. Here we report a physiological role for zinc in the young rat hippocampus (postnatal, P3-P14 days). Our results indicate that naturally occurring spontaneous giant depolarizing synaptic potentials (GDPs) in young CA3 pyramidal neurones, mediated by the release of GABA (gamma-aminobutyric acid), are induced by endogenously released zinc. These synaptic potentials are inhibited by specific zinc-chelating agents. GDPs are apparently generated by an inhibitory action of zinc on both pre- and postsynaptic GABAB receptors in the hippocampus. Our study implies that zinc modulates synaptic transmission in the immature hippocampus, a finding that may have implications for understanding benign postnatal seizures in young children suffering with acute zinc deficiency.

Differential effect of zinc on the vertebrate GABAA-receptor complex

1. gamma-Aminobutyric acid (GABA) responses were recorded from rat superior cervical ganglia (SCG) in culture using the whole cell recording technique. 2. Zinc (50-300 microM) reversibly antagonized the GABA response in embryonic and young post-natal neurones, while neurones cultured from adult animals were far less sensitive and occasionally resistant to zinc blockade. Cadmium (100-300 microM) also antagonised the GABA response, while barium (100 microM-2 mM) was ineffective. 3. The differential blocking effect of zinc on cultured neurones of different ages also occurred in intact SCG tissue. 4. The GABA log dose-response curve constructed with foetal or adult cultured neurones was reduced in a non-competitive manner by zinc. This inhibition was minimally affected by the membrane potential. 5. The GABA response recorded intracellularly from guinea-pig pyriform cortical slices was enhanced by zinc (300-500 microM), which occurred concurrently with a decrease in the input conductance of the cell. The enhancement was unaffected by prior blockade of the GABA uptake carrier by 1 mM nipecotic acid. This phenomenon could be reproduced by barium (300 microM) and cadmium (300 microM). 6. We conclude that the vertebrate neuronal GABAA-receptor becomes less sensitive to zinc with neural (GABAA-receptor?) development, and the enhanced GABA response recorded in the CNS is a consequence of the reduction in the input conductance and not due to a direct effect on the receptor complex.

Effects of subcutaneous injections of zinc chloride on seizures induced by noise and by kainic acid

Several lines of evidence implicate zinc in the pathogenesis of epileptic seizures, and administration of zinc salts has been shown to affect seizure susceptibility. In the present work, we studied the effects of subcutaneous (s.c.) injections of ZnCl2 on seizures induced by intraperitoneal (i.p.) kainic acid (10 mg/kg) in rats and by noise (80-120 dB) in the DBA/2J mouse. Previous administration of zinc salt (20-200 mg/kg) substantially reduced the frequency of noise-induced running fits, clonic and tonic seizures, and deaths in mice, but had no significant effect on the incidence or severity of kainic acid-induced seizures in rats. Together with findings in the literature, our results suggest that zinc plays multiple, sometimes antagonistic roles in seizure development.

Submicromolar concentrations of zinc irreversibly reduce a calcium-dependent potassium current in rat hippocampal neurons in vitro

The action of the endogenous divalent cation zinc on Ca2+ and Ca2(+)-dependent currents was studied in rat hippocampal CA1 and CA3 neurons in vitro, by means of a single electrode voltage clamp technique. Bath application of zinc (0.5-1 microM) produced a small membrane depolarization associated with an increase in synaptic noise and cell excitability and a depression of the afterhyperpolarization following a train of action potentials. The effects on the afterhyperpolarization, could not be reversed on washout. In voltage-clamped neurons, zinc induced a steady inward current and reduced, at resting membrane potential, the peak amplitude of the outward current underlying the afterhyperpolarization, IAHP. In caesium loaded neurons (in the presence of tetrodotoxin and tetraethylammonium), zinc reduced the slow inactivating Ca2+ current activated from a holding potential of -40 mV. Similar results were observed with nickel and cobalt at comparable concentrations, with Zn2+ greater than Ni2+ greater than Co2+, in their order of potency. In contrast to nickel and cobalt the effects of zinc did not reverse on washout. These results suggest that low concentrations of zinc enhance cell excitability by reducing IAHP. In addition, zinc reduces the slow inactivating voltage-dependent Ca2+ current. The irreversible effect of this metal ion is compatible with a toxic, intracellular site of action.

Allosteric modulation of N-methyl-D-aspartate receptors

In this review we have attempted to describe the basis for current models of the NMDA receptor, and justify the need for the various binding sites that have been proposed. The NMDA receptor is clearly a complex molecule with a number of modulatory sites, any of which may have great functional significance. From the data presented above it is apparent that the NMDA recognition site is closely coupled with the glycine site, and can also be regulated by Zn2+. The glycine site is reciprocally coupled to the NMDA site, and may also be coupled to a divalent-cation site outside the channel. However, the glycine site is insensitive to Zn2+. The Zn2+ site is probably not inside the channel to any degree, but can profoundly affect the ability of NMDA site ligands to operate the channel. However, the determination of reciprocal effects at the Zn2+ site await the development of a suitably potent and selective ligand for this site. Several lines of evidence suggest that the phencyclidine and channel-blocking Mg2+ site are located within the NMDA-operated ion channel. Glutamate, glycine, and Zn2+ alter the binding of ligands to these sites. However, this is most likely to be due to alteration of access of the ligands to their sites rather than a direct allosteric coupling. It does appear that phencyclidine site drugs and Mg2+ bind to separate sites within the channel, and that these separate sites are allosterically coupled. This complex series of interactions, many of which are mediated by endogenous agents, may allow very fine control over the expression of NMDA receptor-mediated synaptic transmission. In addition to these ligand-produced modulatory effects, there may also be covalent modification of the channel by receptor phosphorylation. Furthermore, the voltage sensitivity of some of the effects allows control of NMDA receptor-mediated signaling by alteration of the membrane potential in the postsynaptic cell, which can be achieved in a wide variety of ways. The level of sophistication possible in adjusting the responsiveness of this receptor seems entirely appropriate given its central involvement in a wide variety of fundamental neurobiological events, and underscores the deleterious pathological sequelae of the system tilting out of balance. At the same time, the wide array of possible therapeutic targets raises hopes that it may soon be possible to treat effectively some severely debilitating and currently untreatable diseases.

NMDA receptor antagonists that bind to the strychnine-insensitive glycine site and inhibit NMDA-induced Ca2+ fluxes and [3H]GABA release

We have examined the actions of putative antagonists of the strychnine-insensitive glycine-mediated modulation of the N-methyl-D-aspartate (NMDA) receptor using [3H]MK801 binding, Ca2+ influx and [3H]GABA release assays. Kynurenic acid and HA-966 inhibited [3H]MK801 binding, NMDA and glycine induced Ca2+ influx measured using fura-2 and NMDA and glycine simulated [3H]GABA release. The effects of kynurenic acid could be partially overcome by the addition of excess glutamate and glycine, indicating limited selectivity for the glycine binding site. In addition, a component of the action of kynurenic acid was insensitive to agonist concentration, indicating a third action of kynurenic acid at high concentrations. In contrast, HA-966 was 100-fold selective for the glycine compared to the NMDA site. HA-966 only partially inhibited [3H]MK801 binding (IC50 19.7 microM), NMDA-induced Ca2+ influx and neurotransmitter release. The failure of HA-966 to completely block NMDA responses, even at high concentrations, suggests that glycine may not be an absolute requirement for the activation of NMDA receptors under these experimental conditions.