Interaction of zinc with ionotropic and metabotropic glutamate receptors in rat hippocampal slices

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.

Zinc enhances hippocampal long-term potentiation at CA1 synapses through NR2B containing NMDA receptors

The role of zinc (Zn²⁺), a modulator of N-methyl-D-aspartate (NMDA) receptors, in regulating long-term synaptic plasticity at hippocampal CA1 synapses is poorly understood. The effects of exogenous application of Zn²⁺ and of chelation of endogenous Zn²⁺ were examined on long-term potentiation (LTP) of stimulus-evoked synaptic transmission at Schaffer collateral (SCH) synapses in field CA1 of mouse hippocampal slices using whole-cell patch clamp and field recordings. Low micromolar concentrations of exogenous Zn²⁺ enhanced the induction of LTP, and this effect required activation of NMDA receptors containing NR2B subunits. Zn²⁺ elicited a selective increase in NMDA/NR2B fEPSPs, and removal of endogenous Zn²⁺ with high-affinity Zn²⁺ chelators robustly reduced the magnitude of stimulus-evoked LTP. Taken together, our data show that Zn²⁺ at physiological concentrations enhances activation of NMDA receptors containing NR2B subunits, and that this effect enhances the magnitude of LTP.

Dietary Zinc Acts as a Sleep Modulator

While zinc is known to be important for many biological processes in animals at a molecular and physiological level, new evidence indicates that it may also be involved in the regulation of sleep. Recent research has concluded that zinc serum concentration varies with the amount of sleep, while orally administered zinc increases the amount and the quality of sleep in mice and humans. In this review, we provide an exhaustive study of the literature connecting zinc and sleep, and try to evaluate which molecular mechanism is likely to be involved in this phenomenon. A better understanding should provide critical information not only about the way zinc is related to sleep but also about how sleep itself works and what its real function is.

Zinc as a Neuromodulator in the Central Nervous System with a Focus on the Olfactory Bulb

The olfactory bulb (OB) is central to the sense of smell, as it is the site of the first synaptic relay involved in the processing of odor information. Odor sensations are first transduced by olfactory sensory neurons (OSNs) before being transmitted, by way of the OB, to higher olfactory centers that mediate olfactory discrimination and perception. Zinc is a common trace element, and it is highly concentrated in the synaptic vesicles of subsets of glutamatergic neurons in some brain regions including the hippocampus and OB. In addition, zinc is contained in the synaptic vesicles of some glycinergic and GABAergic neurons. Thus, zinc released from synaptic vesicles is available to modulate synaptic transmission mediated by excitatory (e.g., N-methyl-D aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)) and inhibitory (e.g., gamma-aminobutyric acid (GABA), glycine) amino acid receptors. Furthermore, extracellular zinc can alter the excitability of neurons through effects on a variety of voltage-gated ion channels. Consistent with the notion that zinc acts as a regulator of neuronal activity, we and others have shown zinc modulation (inhibition and/or potentiation) of amino acid receptors and voltage-gated ion channels expressed by OB neurons. This review summarizes the locations and release of vesicular zinc in the central nervous system (CNS), including in the OB. It also summarizes the effects of zinc on various amino acid receptors and ion channels involved in regulating synaptic transmission and neuronal excitability, with a special emphasis on the actions of zinc as a neuromodulator in the OB. An understanding of how neuroactive substances such as zinc modulate receptors and ion channels expressed by OB neurons will increase our understanding of the roles that synaptic circuits in the OB play in odor information processing and transmission.

Voltage-gated calcium channels: Determinants of channel function and modulation by inorganic cations

Voltage-gated calcium channels (VGCCs) represent a key link between electrical signals and non-electrical processes, such as contraction, secretion and transcription. Evolved to achieve high rates of Ca(2+)-selective flux, they possess an elaborate mechanism for selection of Ca(2+) over foreign ions. It has been convincingly linked to competitive binding in the pore, but the fundamental question of how this is reconcilable with high rates of Ca(2+) transfer remains unanswered. By virtue of their similarity to Ca(2+), polyvalent cations can interfere with the function of VGCCs and have proven instrumental in probing the mechanisms underlying selective permeation. Recent emergence of crystallographic data on a set of Ca(2+)-selective model channels provides a structural framework for permeation in VGCCs, and warrants a reconsideration of their diverse modulation by polyvalent cations, which can be roughly separated into three general mechanisms: (I) long-range interactions with charged regions on the surface, affecting the local potential sensed by the channel or influencing voltage-sensor movement by repulsive forces (electrostatic effects), (II) short-range interactions with sites in the ion-conducting pathway, leading to physical obstruction of the channel (pore block), and in some cases (III) short-range interactions with extracellular binding sites, leading to non-electrostatic modifications of channel gating (allosteric effects). These effects, together with the underlying molecular modifications, provide valuable insights into the function of VGCCs, and have important physiological and pathophysiological implications. Allosteric suppression of some of the pore-forming Cavα1-subunits (Cav2.3, Cav3.2) by Zn(2+) and Cu(2+) may play a major role for the regulation of excitability by endogenous transition metal ions. The fact that these ions can often traverse VGCCs can contribute to the detrimental intracellular accumulation of metal ions following excessive release of endogenous Cu(2+) and Zn(2+) or exposure to non-physiological toxic metal ions.

Zinc released from olfactory bulb glomeruli by patterned electrical stimulation of the olfactory nerve

Zinc is a trace element with a multitude of roles in biological systems including structural and cofactor functions for proteins. Although most zinc in the central nervous system (CNS) is protein bound, the CNS contains a pool of mobile zinc housed in synaptic vesicles within a subset of neurons. Such mobile zinc occurs in many brain regions, such as the hippocampus, hypothalamus, and cortex, but the olfactory bulb (OB) contains one of the highest such concentrations in the CNS. Zinc is distributed throughout the OB, with the glomerular and granule cell layers containing the highest levels. Here, we visualize vesicular zinc in the OB using zinc-responsive fluorescent probes developed by one of us. Moreover, we provide the first demonstration that vesicular pools of zinc can be released from olfactory nerve terminals within individual glomeruli by patterned electrical stimulation of the olfactory nerve designed to mimic the breathing cycle in rats. We also provide electrophysiological evidence that elevated extracellular zinc potentiates α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated synaptic events. AMPA receptors are required for the synchronous activation of neurons within individual OB glomeruli, and zinc-mediated potentiation leads to enhanced synaptic summation.

Copper signaling in the mammalian nervous system: synaptic effects

Copper is an essential metal present at high levels in the CNS. Its role as a cofactor in mitochondrial ATP production and in essential cuproenzymes is well defined. Menkes and Wilson's diseases are severe neurodegenerative conditions that demonstrate the importance of Cu transport into the secretory pathway. In the brain, intracellular levels of Cu, which is almost entirely protein bound, exceed extracellular levels by more than 100-fold. Cu stored in the secretory pathway is released in a Ca(2+)-dependent manner and can transiently reach concentrations over 100 μM at synapses. The ability of low micromolar levels of Cu to bind to and modulate the function of γ-aminobutyric acid type A (GABA(A)) receptors, N-methyl-D-aspartate (NMDA) receptors, and voltage-gated Ca(2+) channels contributes to its effects on synaptic transmission. Cu also binds to amyloid precursor protein and prion protein; both proteins are found at synapses and brain Cu homeostasis is disrupted in mice lacking either protein. Especially intriguing is the ability of Cu to affect AMP-activated protein kinase (AMPK), a monitor of cellular energy status. Despite this, few investigators have examined the direct effects of Cu on synaptic transmission and plasticity. Although the variability of results demonstrates complex influences of Cu that are highly method sensitive, these studies nevertheless strongly support important roles for endogenous Cu and new roles for Cu-binding proteins in synaptic function/plasticity and behavior. Further study of the many roles of Cu in nervous system function will reveal targets for intervention in other diseases in which Cu homeostasis is disrupted.

Expression of zinc transporter ZnT7 in mouse superior cervical ganglion

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

Chelatable zinc modulates excitability and seizure duration in the amygdala rapid kindling model

Zinc is present in high concentration in many structures of the limbic circuitry, however the role of zinc as a neuromodulator in such synapses is still uncertain. In this work, we verified the effects of zinc chelation in an animal model of epileptogenesis induced by amygdala rapid kindling. The basolateral amygdala was electrically stimulated ten times per day for 2 days. A single stimulus was applied on the third day. Stimulated animals received injections of PBS or the zinc chelator diethildythiocarbamate acid (DEDTC) before each stimulus series. Animals were monitored with video-EEG and were perfused 3h after the last stimulus for subsequent neo-Timm and Fluoro-Jade B analysis. Zinc chelation decreased the duration of both behavioral seizures and electrical after-discharges, and also decreased the EEG spikes frequency, without changing the progression of behavioral seizure severity. These results indicate that the zinc ion may have a facilitatory role during kindling progression.

Role of zinc influx via AMPA/kainate receptor activation in metabotropic glutamate receptor-mediated calcium release

The uptake of free zinc into CA3 pyramidal cells and its significance was examined in rat hippocampal slices with ZnAF-2DA, a membrane-permeable zinc indicator. Intracellular ZnAF-2 signal in the CA3 pyramidal cell layer was increased during delivery of tetanic stimuli to the dentate granule cell layer. This increase was completely blocked in the presence of CNQX, an AMPA/kainate receptor antagonist. These results suggest that free zinc is taken up into CA3 pyramidal cells via activation of AMPA/kainate receptors. The effect of free zinc levels in the CA3 pyramidal cells on the increase in intracellular calcium via Group I metabotropic glutamate receptors was examined by regional delivery of tADA, a Group I metabotropic glutamate receptor agonist, to the stratum lucidum after blockade of AMPA/kainate receptor-mediated calcium and zinc influx. Intracellular calcium orange signal in the CA3 pyramidal cell layer was increased by tADA, whereas intracellular ZnAF-2 signal was not increased even in the presence of 100 muM zinc, suggesting that tADA induces calcium release from internal stores in CA3 pyramidal cells and is not involved in zinc uptake. The increase in calcium orange signal by tADA was enhanced by perfusion with pyrithione, a zinc ionophore that decreased basal ZnAF-2 signal in the CA3 pyramidal cell layer. It was blocked by perfusion with pyrithione and zinc that increased basal ZnAF-2 signal. The present study indicates that the increase in free calcium levels via the metabotropic glutamate receptor pathway is inversely related to free zinc levels in CA3 pyramidal cells.

Brain, aging and neurodegeneration: role of zinc ion availability

Actual fields of research in neurobiology are not only aimed at understanding the different aspects of brain aging but also at developing strategies useful to preserve brain compensatory capacity and to prevent the onset of neurodegenerative diseases. Consistent with this trend much attention has been addressed to zinc metabolism. In fact, zinc acts as a neuromodulator at excitatory synapses and has a considerable role in the stress response and in the functionality of zinc-dependent enzymes contributing to maintaining brain compensatory capacity. In particular, the mechanisms that modulate the free zinc pool are pivotal for safeguarding brain health and performance. Alterations in zinc homeostasis have been reported in Parkinson's and Alzheimer's disease as well as in transient forebrain ischemia, seizures and traumatic brain injury, but little is known regarding aged brain. There is much evidence that that age-related changes, frequently associated to a decline in brain functions and impaired cognitive performances, could be related to dysfunctions affecting the intracellular zinc ion availability. A general agreement emerges from studies of humans' and rodents' old brains about an increased expression of metallothionein (MT) isoforms I and II, but dyshomogenous results are reported for MT-III, and it is still uncertain whether these proteins maintain in aging the protective role, as it occurs in adult/young age. At the same time, there is considerable evidence that amyloid-beta deposition in Alzheimer's disease is induced by zinc, but the pathological significance and the causes of this phenomenon are still an open question. The scientific debate on the role of zinc and of some zinc-binding proteins in aging and neurodegenerative disorders, as well as on the beneficial effect of zinc supplementation in aged brain and neurodegeneration, is extensively discussed in this review.

Tetanically released zinc inhibits hippocampal mossy fiber calcium, zinc and synaptic responses

At the zinc-enriched mossy fiber synapses from hippocampal CA3 area, electrical or chemical stimulation evokes zinc release from glutamatergic synaptic vesicles that may cause different pre- or postsynaptic actions. Besides zinc that can be co-localized with glutamate and GABA, the mossy fibers contain a very high density of ATP-sensitive potassium channels that are activated by zinc. We have investigated the possibility that intensely released zinc inhibits presynaptic calcium changes and consequently zinc and glutamate release. The studies were made combining optical recording of fast presynaptic calcium and zinc signals, using the fluorescent indicators Fura-2 and N-(6-methoxy-8-quinolyl)-para-toluenesulfonamide, respectively, with measurements of field potentials. We have observed that strong tetanic stimulation caused posttetanic depressions of electrically induced presynaptic calcium and zinc signals and of synaptic responses, the depressions being blocked by zinc chelators. These results suggest that endogenously released zinc has an inhibitory role, mediated by presynaptic ATP-sensitive potassium channels and/or presynaptic calcium channels, that leads to the depression of zinc and glutamate release.

Diverse modulation of olfactory bulb AMPA receptors by zinc

Increasing evidence suggests that zinc modulates synaptic transmission in the olfactory bulb and other brain regions. We investigated the sensitivity of AMPA receptors on the bulb's two primary neuronal populations to several concentrations of zinc. Zinc (30-1000 microM) was coapplied to mitral/tufted cells and interneurons during AMPA-evoked currents, and current responses (potentiation, inhibition, no effect) were analyzed. Both neuronal populations expressed zinc-sensitive and zinc-insensitive AMPA receptors. However, the frequency and magnitude of zinc's effects varied with cell type. In addition, zinc did not always have biphasic effects at AMPA receptors (potentiation at low concentrations; inhibition at high concentrations), as reported in other brain regions. Zinc's diverse effects suggest that zinc may alter odor information processing by differential modulation of excitatory circuits.

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

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

Zn2+ modulates light responses of color-opponent bipolar and amacrine cells in the carp retina

The effects of Zn(2+) on color-opponent bipolar cells (BCs) and amacrine cells (ACs) were studied in the isolated superfused carp retina using intracellular recording techniques. Bath-applied Zn(2+) (25 micro M) depolarized R(+)G(-)-type BCs and suppressed both depolarizing responses of these cells to red (680 nm) flashes and hyperpolarizing ones to green (500 nm) flashes. Following Zn(2+) application, G(+)R(-)-type BCs were hyperpolarized, which was accompanied by a potentiation of their depolarizing responses to green flashes and a suppression of hyperpolarizing ones to red flashes. Similar Zn(2+) effects were observed in R(+)G(-)- and G(+)R(-)-type ACs. The Zn(2+) effects persisted in the presence of picrotoxin and strychnine, suggesting that modulation by Zn(2+) of GABA and glycine receptors was unlikely involved. Using whole-cell recording techniques, it was found Ca(2+) currents in cone terminals were dose-dependently suppressed by Zn(2+), suggesting that Zn(2+) may reduce glutamate release from cone photoreceptors. Furthermore, lowering extracellular Ca(2+), a procedure that increases glutamate release from photoreceptors, exerted actions on R(+)G(-)- and G(+)R(-)-type BCs, almost opposite to the Zn(2+) effects on these two types of BCs. It is therefore postulated that the Zn(2+) effects reported in the present work may reflect a consequence of the changes in input resistances of color-opponent BCs and driving forces for their light responses resulted from the reduced glutamate release by Zn(2+).

Thalamic distribution of zinc-rich terminal fields and neurons of origin in the rat

Several cortico-cortical and limbic-related circuits are enriched in zinc, which is considered as an important modulator of glutamatergic transmission. While heavy metals have been detected in the thalamus, the specific presence of zinc has not been examined in this region. We have used two highly sensitive variations of the Timm method to study the zinc-rich innervation in the rat thalamus, which was compared to the distribution of acetylcholinesterase activity. The origin of some of these zinc-rich projections was also investigated by means of retrograde transport after intracerebral infusions of sodium selenium (Na2SeO3). The overall zinc staining in the thalamus was much lower than in the neocortex, striatum or basal forebrain; however, densely stained terminal fields were observed in the dorsal tip of the reticular thalamic nucleus, the anterodorsal and lateral dorsal thalamic nuclei and the zona incerta. In addition, moderately stained zinc-rich terminal fields were found in the rostral intralaminar nuclei, nucleus reuniens and lateral habenula. Intracerebral infusions of Na2SeO3 in the lateral dorsal nucleus resulted in retrogradely labeled neurons that were located in the postsubiculum, and also in the pre- and parasubiculum. These results are the first to establish the existence of a zinc-rich subicular-thalamic projection. Similar infusions in either the intralaminar nuclei or the zona incerta resulted in labeling of neurons in several brainstem structures related to the reticular formation. Our results provide morphological evidence for zinc modulation of glutamatergic inputs to highly selective thalamic nuclei, arising differentially from either cortical limbic areas or from brainstem ascending activation systems.

Adrenalectomy causes loss of zinc ions in zinc-enriched (ZEN) terminals and decreases seizure-induced neuronal death

Chelatable zinc ions from synaptic vesicles have been suggested to be involved in neuronal death caused by stroke, epilepsy and head trauma. Elevated glucocorticoid concentration exacerbates such neuron loss, while low levels protect. We have tested the notion that the neuroprotective effect of prior glucocorticoid reduction is mediated by a reduction of zinc ions contained in zinc-enriched (ZEN) synaptic vesicles. The level of vesicular zinc ions was evaluated by toluene sulfonamide quinoline (TSQ) fluorometry and zinc autometallography (ZnS(AMG)) 10 and 30 days, respectively, after adrenalectomy. The hippocampus showed significant vesicular zinc ion depletion following adrenalectomy. After the kainate injection, adrenalectomized rats showed proconvulsive seizure behavior, i.e. shortened latency to seizure onset time and increased seizure score. Additionally they showed decreased hippocampal CA3 neuronal death as compared to control animals. The present data suggest that zinc ions released from damaged ZEN terminals are involved in seizure-induced neuronal death.

Zinc-induced augmentation of excitatory synaptic currents and glutamate receptor responses in hippocampal CA3 neurons

Zinc is found throughout the CNS at synapses co-localized with glutamate in presynaptic terminals. In particular, dentate granule cells' (DGC) mossy fiber (MF) axons contain especially high concentrations of zinc co-localized with glutamate within vesicles. To study possible physiological roles of zinc, visualized slice-patch techniques were used to voltage-clamp rat CA3 pyramidal neurons, and miniature excitatory postsynaptic currents (mEPSCs) were isolated. Bath-applied zinc (200 microM) enhanced median mEPSC peak amplitudes to 153.0% of controls, without affecting mEPSC kinetics. To characterize this augmentation further, rapid agonist application was performed on perisomatic outside-out patches to coapply zinc with glutamate extremely rapidly for brief (1 ms) durations, thereby emulating release kinetics of these substances at excitatory synapses. When zinc was coapplied with glutamate, zinc augmented peak glutamate currents (mean +/- SE, 116.6 +/- 2.8% and 143.8 +/- 9.8% of controls at 50 and 200 microM zinc, respectively). This zinc-induced potentiation was concentration dependent, and pharmacological isolation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated currents (AMPAR currents) gave results similar to those observed with glutamate application (mean, 115.0 +/- 5.4% and 132.5 +/- 9.1% of controls at 50 and 200 microM zinc, respectively). Inclusion of the AMPAR desensitization blocker cyclothiazide in the control solution, however, abolished zinc-induced augmentation of glutamate-evoked currents, suggesting that zinc may potentiate AMPAR currents by inhibiting AMPAR desensitization. Based on the results of the present study, we hypothesize that zinc is a powerful modulator of both excitatory synaptic transmission and glutamate-evoked currents at physiologically relevant concentrations. This modulatory role played by zinc may be a significant factor in enhancing excitatory neurotransmission and could significantly regulate function at the mossy fiber-CA3 synapse.

Effects of zinc on spatial reference memory and brain dopamine (D1) receptor binding kinetics in rats

1. The present study was designed to evaluate the effects of zinc on spatial reference memory and brain dopamine (D1) receptor binding kinetics in rats. Male Sprague-Dawley rats (120-150 g), adapted 12 hour light: 12 hour dark illumination cycle were used. Treated animals were given zinc chloride (25 mg/kg, 50 mg/kg, or 100 mg/kg) by oral gavage for 15 days at 11:00 hr. Controlrats received an equivalent volume of saline. 2. Spatial reference memory was evaluated in treated and control rats on days 10 through 15 using the Morris Water Maze. The time to find the platform (latency) was significantly increased in the 50 mg/kg and 100-mg/kg zinc treated animals as compared to the controls. One hour after the last spatial reference memory testing, the animals were sacrificed by decapitation; their brains were removed and dissected into various regions. 3. D1 receptor binding kinetics were measured using the ligand [3H] SCH23390. Results obtained indicate that zinc chloride administration resulted in a statistically significant decline in the binding affinity (increased Kd) of the D1 receptors in the frontal cortex, hypothalamus, hippocampus, and midbrain. However, there was a significant increase in the D1 receptor binding capacity (Bmax) in these same brain regions following zinc chloride administration. 4. These findings clearly indicate that administration of high doses of zinc to rats resulted in spatial reference memory deficit, which may in part be explained by alterations in dopamine receptor binding kinetics.

Zinc-rich synaptic boutons in human temporal cortex biopsies

The distribution of zinc-rich synaptic boutons in biopsies of the temporal cortex from epileptic patients who had undergone surgery is described. Unfixed cryostat sections were exposed to H(2)S vapour to precipitate endogenous zinc, which was subsequently shown by silver enhancement. In the temporal cortex, the stain for zinc was arranged in bands: stain was heavy in layers II and VI, moderate-to-heavy in layers I, III and V, and low in layer IV. The white matter was virtually devoid of staining. At the electron microscope level, labelling was found in synaptic boutons that made asymmetric synaptic contacts. Immunohistochemical staining for glutamate receptor subunits GluR2/3 was observed in cell bodies in layers II, III, V and VI, coincident with the layers that showed heavy staining for zinc. Immunostaining for glutamate receptor subunit GluR1 was prominent in non-pyramidal neurons in deep cortical layers. These results support findings in other mammals and indicate that the human neocortex may contain an extensive system of zinc-rich cortico-cortical connections. This system may be altered in pathological conditions.

Modulation of the phosphorylation and activity of calcium/calmodulin-dependent protein kinase II by zinc

Calcium/calmodulin-dependent protein kinase II (CaMPK-II) is a key regulatory enzyme in living cells. Modulation of its activity, therefore, could have a major impact on many cellular processes. We found that Zn(2+) has multiple functional effects on CaMPK-II. Zn(2+) generated a Ca(2+)/CaM-independent activity that correlated with the autophosphorylation of Thr(286), inhibited Ca(2+)/CaM binding that correlated with the autophosphorylation of Thr(306), and inhibited CaMPK-II activity at high concentrations that correlated with the autophosphorylation of Ser(279). The relative level of autophosphorylation of these three sites was dependent on the concentration of zinc used. The autophosphorylation of at least these three sites, together with Zn(2+) binding, generated an increased mobility form of CaMPK-II on sodium dodecyl sulfate gels. Overall, autophosphorylation induced by Zn(2+) converts CaMPK-II into a different form than the binding of Ca(2+)/CaM. In certain nerve terminals, where Zn(2+) has been shown to play a neuromodulatory role and is present in high concentrations, Zn(2+) may turn CaMPK-II into a form that would be unable to respond to calcium signals.

Zinc-rich afferents to the rat neocortex: projections to the visual cortex traced with intracerebral selenite injections

Infusion of sodium selenite to the occipital cortex of the rat was used for the specific tracing of zinc-rich pathways. Large numbers of labeled somata were found ipsilaterally in the visual, orbital and frontal cortices, and contralaterally in homotopic and heterotopic visual areas. Labeled neurons were also found ipsilaterally in the retrosplenial, parietal, sensory-motor, temporal and perirhinal cortex. In contrast to the cortico-cortical connections, ascending afferents to the visual cortex were not zinc-rich except for a few labeled neurons in the claustrum. Additional injections showed reciprocal zinc-rich connections between the visual cortex and the orbital and frontal cortices. The latter cortices also received ascending zinc-rich afferents from the claustrum. Selenite injections revealed the layered distribution and the morphology of these labeled neurons in the neocortex. Zinc-rich neurons were found in layers II-III, V and VI. However, none was found in layer IV. Zinc-rich somata appeared as pyramidal and inverted neurons. The contrasting chemical properties of cortical and subcortical visual afferents may account for the functional differences between these systems.

Effect of metal chelating agents on the direct and seizure-related neuronal death induced by zinc and kainic acid

The ability of metal chelating agents to affect seizure-induced neuronal death caused by intra-amygdaloid injections of kainic acid was investigated. N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), diethyldithiocarbamate (DEDTC) and diphenylthiocarbazone (dithizone), administered simultaneously or within 30 min of a kainate injection, all failed to affect the amount of neuronal loss in the ipsilateral hippocampus. This failure was not due to an inability to complex endogenous zinc as all these chelating agents quenched staining for endogenous zinc by the Timm method. However, the period for which this quenching occurred was short for DEDTC and dithizone (a maximum of 1.5 h) although it lasted for 8 h with TPEN. TPEN, but not DEDTC or dithizone prevented the neuronal loss caused by intra-hippocampal injections of zinc chloride. In the presence of diazepam to prevent seizures, co-injection of TPEN and kainate into the hippocampus also failed to prevent the direct cytotoxicity of kainate. Endogenous zinc, released from mossy fibres in the hippocampus by seizure activity, does not appear to modify seizure activity sufficiently to alter the extent of the resulting neuronal death.

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.

Distribution of synaptic zinc in the developing mouse somatosensory barrel cortex

Histochemical localization of synaptic zinc was examined in the somatosensory (SI) barrel cortex of mouse. The laminar distribution and distribution within the barrel field were described. At postnatal day 3 (P3) and 5 (P5), very faint and uniform zinc staining was present in the lower part of the subplate. At P6, subtle laminar variations emerged. At P8, these variations were clearly observed. Intense zinc staining was found in layers I, II, III, and V. Layers IV and VI showed a weaker staining. From this postnatal age to adult, uneven patchy distribution of synaptic zinc in layer IV could be distinguished in coronal sections. In tangential sections through layer IV, zinc staining showed a barrel-like pattern due to a higher zinc concentration in septa and the surrounding cortex. Barrel sides revealed a lower zinc concentration compared with the barrel hollow. With brain maturation, the zinc staining increased more intensely outside the barrel field, thus producing a progressively higher contrast between the barrel field and adjacent cortical regions. The differences in zinc staining between the barrel side and barrel hollow diminished with age but were still visible at P70. The changes in synaptic zinc distribution probably reflect the process of synaptic maturation of glutamatergic terminals projecting to the SI cortex. The time course of postnatal changes in terminal zinc distribution suggests that synaptic zinc is not involved in the mechanisms of barrel formation.

Postnatal development of zinc-containing cells and neuropil in the hippocampal region of the mouse

The present study describes the postnatal development of zinc-containing boutons and their neurons of origin in the hippocampal region of the mouse. Ages investigated for the development of zinc-containing neuropil were postnatal days 0 (P0), P3, P7, P11, P15, P21, and P28. For zinc-containing cell bodies P7, P15, P21, and P28 were studied. In the area dentata, zinc-containing neuropil appeared first by P3 adjacent to the suprapyramidal limb of the granule cell layer and extended later toward the infrapyramidal limb. By P15, inter- and intralaminar gradients corresponded to those seen in adult animals. The appearance of labeled granule cells followed closely, although temporally delayed, the pattern of granule cell neurogenesis. All granule cells were labeled by P28. In the hippocampus proper, zinc-containing neuropil was seen by P0, but staining of the incipient mossy fiber zone was first visible by P3. Staining pattern and intensity developed gradually until they reached their mature appearance by P15. The distribution of labeled cells was identical to that seen in mature animals by P7 in CA3, but first by P21 in CA1. In the subiculum, neuropil staining first appeared proximally by P7, included all of this area by P11, and appeared mature by P21. A few labeled cells were seen in the proximal subiculum at all ages at which labeled cells were present in CA1. Labeled cells which extended further distally became first visible by P21. Their number and labeling intensity reached mature levels by P28. In the presubiculum, retrosplenial area 29e, and parasubiculum, neuropil staining first appeared by P3. The retrosplenial area 29e could be distinguished by P11. This area and the presubiculum reached their adult appearance by P21. This occurred first by P28 in the parasubiculum due to the late maturation of the parasubiculum a. Labeled cells were first seen by P7 in layer III of the presubiculum and by P15 in the retrosplenial area 29e and the parasubiculum. Cell labeling appeared mature by the same times as the neuropil staining. In the entorhinal areas a very light neuropil stain was apparent in the deeper layers by P0. A distinct rise in staining intensity was first observed by P7 in layers I-III. Thereafter, mature characteristics developed gradually and were attained by P21. Cell labeling was not seen in the medial entorhinal area. A few labeled cells were apparent by P7 in the lateral entorhinal area. After a slight increase by P15, numerous labeled cells were found in layer II and layer VI by P21. Their distribution and labeling intensity appeared mature by P28. Zinc-containing cells appear to represent cells formed late in the course of neurogenesis in all areas aside from the lateral entorhinal area. As far as intrinsic connections are concerned, it is the development of projections from this subset of neurons which is monitored in this study. We suggest that the appearance of zinc may contribute via its different effects on N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors to the end of a developmental phase that is permissive to changes in synaptic efficacy. Species differences and alternative functions of zinc are considered.

Distribution of terminal fields stained for zinc in the neocortex of the rat

Staining for zinc in terminal fields of the rat neocortex was studied by applying the sulphide/silver histochemical method. The stain was arranged in a distinct layered pattern. Two bands of heavy reaction were found in deep layer 1 plus layers 2-3 and in upper layer 5. A band of moderate-to-heavy reaction was found in layer 6. Three bands of lighter staining were found in upper layer 1, layer 4 and deep layer 5. The layers of reaction showed variations in width and intensity of staining from area to area. In the frontal and cingulate cortices and in the association cortices, the heavily stained bands were dominant over the narrow, inconspicuous, lightly stained bands. In contrast, in primary sensory regions (Par1, Oc1 and Te1), the lightly stained bands were wide and prominent. The sulphide/silver method gives a straightforward delimitation of many cortical areas and reveals a clear distinction between (A) allocortical and isocortical areas, and (B) primary sensory areas, secondary or association areas, and prefrontal plus motor areas. The presence of vivid bands of staining for zinc appears to be linked to areas with prominent pyramidal layers.

Regional zinc staining in postmortem hippocampus from schizophrenic patients

Schizophrenia-associated deficits in learning and memory have been associated with a decrease in the volume of the hippocampus, but the specific nature of the neuronal deficit remains unknown. Many critical afferent pathways in the hippocampus contain ionic zinc. Alterations of these pathways could be manifest as a decrease in ionic zinc levels within hippocampal afferent pathways. This possibility was examined in postmortem hippocampal tissue taken from schizophrenic patients, patients with other psychiatric disorders and matched, non-mentally ill subjects using a modified Timm's silver staining method. The three groups exhibited similar patterns of zinc staining within the hippocampal formation as well as similar levels of zinc within the mossy fiber projection system. A greater prevalence of zinc staining within the inner molecular layer of the dentate gyrus was observed in female as compared to male donors and in older as compared to younger donors. The results of the present study demonstrate that loss of ionic zinc within the hippocampus does not appear to be part of the pathology of schizophrenia.

Heterogeneous and compartmental distribution of zinc in the striatum and globus pallidus of the rat

The distribution of vesicular or chelatable zinc was analysed in the dorsal and ventral subdivisions of the striatum and globus pallidus of the rat with Danscher's selenium method. Acetylcholinesterase and Calbindin-D28k were used as striatal and pallidal markers in order to analyse the possible compartmentalization of the distribution of zinc in the striatum and globus pallidus. The main findings of this study are the following: (1) The distribution of vesicular zinc in the dorsal striatum was heterogeneous. A peripheral rim of tissue heavily stained for zinc was detected in the medial, dorsal and lateral striatal areas, along most of the rostrocaudal extent of the striatum. addition, patch-like zones intensely stained for zinc were prominent in the rostral half of the caudate-putamen complex. (2) In some regions of the rostral half of the caudate-putamen complex, the staining for zinc appeared to follow the well-known striatal patches (striosomes)/matrix organization. However, in other regions of the rostral half of the striatum such a relation was not detected. (3) The ventral striatum also showed a heterogeneous staining for zinc. Thus, in the most ventral part of the caudate-putamen complex, both subdivisions of the nucleus accumbens and parts of the olfactory tubercle displayed different patterns of compartmentalized distribution of zinc. In the dorsal half of the shell of the nucleus accumbens, some patches with an intense reaction for zinc seemed to overlap with acetylcholinesterase-poor patches. (4) There was a remarkable absence of staining for zinc in the globus pallidus. This histochemical study illustrates, on the one hand, the high content of vesicular zinc in the dorsal and ventral subdivisions of the striatum, which was distributed following different patterns of chemical compartmentalization, and on the other hand, the absence of vesicular zinc in the globus pallidus of the rat.

Differential effects of zinc on hyperpolarizing and depolarizing GABAA synaptic potentials in hippocampal slice cultures

We have examined the changes in GABAA-mediated synaptic potentials recorded from CA3 pyramidal neurons in hippocampal slice cultures following application of zinc (Zn2+). Unlike 4-AP, Zn2+ did not enhance fast hyperpolarizing potentials but primarily enhanced depolarizing GABAA potentials. Zn2+ did not alter the postsynaptic response of pyramidal neurons to pressure applied GABA, consistent with previous reports that Zn2+ enhances the release of GABA from presynaptic terminals. To examine the role of local circuitry in the production of Zn2+ responses, we recorded from cultures maintained for 7-10 days following removal of the dentate and hilus to allow complete degeneration of the mossy fibers (DGX cultures). Zn2+ produced giant depolarizing potentials (GDPs) in DGX cultures that were identical to those in intact cultures. In contrast, the 4-AP response was dramatically altered in DGX cultures. In DGX cultures, Zn2+ co-applied with 4-AP appeared to inhibit the production of fast hyperpolarizing GABAA synaptic potentials produced by 4-AP alone. This inhibition of fast hyperpolarizing potentials suggests that Zn2+ may reduce the release of GABA onto pyramidal cell somata. These observations suggest that Zn2+ enhances GABA release from local circuit neurons that synapse onto pyramidal cell dendrites, and inhibits GABA release onto pyramidal cell somata.

Multiple forms of long-term potentiation and multiple regulatory sites of N-methyl-D-aspartate receptors: role of the redox site

Long-term potentiation (LTP) is a form of synaptic plasticity thought to be involved in learning and memory. Although extensively studied, mainly in the CA1 region of the hippocampus, the mechanisms underlying the induction and expression of LTP are poorly elucidated. This is probably due to the fact that LTP is not a unique process and indeed recent studies have shown that several forms of LTP could be generated depending on the experimental conditions. Furthermore, LTP is generally associated with a long-lasting increase of the synaptic efficacy of AMPA receptors but an increasing number of data also suggested that NMDA receptors could be potentiated as well. NMDA receptor responses are modulated by a large number of extracellular and intracellular events, providing additional possibilities for the generation of LTP. The role of these different modulatory sites of the NMDA receptor and their relation with LTP are reviewed with a particular attention to the redox site which seems to be a selective target to distinguish between AMPA and NMDA-LTP.

AMPA/Zn(2+)-induced neurotoxicity in rat primary cortical cultures: involvement of L-type calcium channels

Zn2+ is believed to be an endogenous modulator of glutamatergic excitation. It has been shown to attenuate NMDA receptor-mediated excitation and to increase AMPA-induced excitatory transmission. The dual activity of Zn2+ on ionotropic excitatory neurotransmission suggests that Zn2+ plays a role in the modulation of excitatory neurodegenerative events. Stimulation of rat primary cortical cultures with the combination of 50 microM AMPA and 300 microM Zn2+ for 30 min induced approximately 50% cell death compared with only approximately 20% cell death induced by AMPA alone. The degree of neurotoxicity 48 h after the incubation was reproducible and was attenuated by CNQX, EDTA, EGTA, diltiazem and DHP-type Ca2+ channel blockers but not by MK-801. These findings suggest that an initial depolarization induced by AMPA and a subsequent influx of Ca2+ and Zn2+ ions through voltage-operated L-type Ca2+ channels are crucial events which finally lead to neuronal death. Racemic nimodipine and its (+)- and (-)-enantiomers had remarkable in vitro neuroprotective efficacies, the IC50 values being 4 nM for the racemate, 11 nM for the (+)- and 1 nM for the (-)-enantiomer. This suggests a possible therapeutic role for Ca2+ channel blockers in neurodegenerative diseases which are characterized by a disturbance of cellular Ca2+ homeostasis.

Modulation of long-term potentiation in rat hippocampal pyramidal neurons by zinc

The phenomenon of long-term potentiation is frequently promulgated as an example of learning and memory mechanisms at the synaptic level in the mammalian central nervous system. In the CA3 region of the hippocampus there is an abundance of zinc, which is located in presynaptic mossy fibre nerve terminals. Stimulation of these fibres can cause the release of zinc, which interacts with excitatory amino acid receptors and may therefore modulate long-term potentiation. We now demonstrate in CA1 and CA3 neurons that zinc (100-300 microM) enhances non-N-methyl-D-aspartate-receptor-mediated responses whilst reducing excitatory synaptic transmission and inhibiting long-term potentiation. However, by using zinc-chelating agents, endogenously released zinc following high-frequency stimulation in the stratum lucidum does not appear to have any modulatory role in excitatory synaptic transmission and long-term potentiation. These results indicate that an increase in the level of extracellular zinc can limit excitatory synaptic transmission in the CA1 or CA3 region and further suggests that pathologies that can be related to excessive levels of endogenous zinc may have implications for synaptic plasticity in CA3 neurons.