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

Intracellular zinc protects Kv7 K+ channels from Ca2+/calmodulin-mediated inhibition

Zinc (Zn) is an essential trace element; it serves as a cofactor for a great number of enzymes, transcription factors, receptors, and other proteins. Zinc is also an important signaling molecule, which can be released from intracellular stores into the cytosol or extracellular space, for example, during synaptic transmission. Amongst cellular effects of zinc is activation of Kv7 (KCNQ, M-type) voltage-gated potassium channels. Here, we investigated relationships between Kv7 channel inhibition by Ca²⁺/calmodulin (CaM) and zinc-mediated potentiation. We show that Zn²⁺ ionophore, zinc pyrithione (ZnPy), can prevent or reverse Ca²⁺/CaM-mediated inhibition of Kv7.2. In the presence of both Ca²⁺ and Zn²⁺, the Kv7.2 channels lose most of their voltage dependence and lock in an open state. In addition, we demonstrate that mutations that interfere with CaM binding to Kv7.2 and Kv7.3 reduced channel membrane abundance and activity, but these mutants retained zinc sensitivity. Moreover, the relative efficacy of ZnPy to activate these mutants was generally greater, compared with the WT channels. Finally, we show that zinc sensitivity was retained in Kv7.2 channels assembled with mutant CaM with all four EF hands disabled, suggesting that it is unlikely to be mediated by CaM. Taken together, our findings indicate that zinc is a potent Kv7 stabilizer, which may protect these channels from physiological inhibitory effects of neurotransmitters and neuromodulators, protecting neurons from overactivity.

Intracellular Zn2+ Signaling Facilitates Mossy Fiber Input-Induced Heterosynaptic Potentiation of Direct Cortical Inputs in Hippocampal CA3 Pyramidal Cells

Repetitive action potentials (APs) in hippocampal CA3 pyramidal cells (CA3-PCs) backpropagate to distal apical dendrites, and induce calcium and protein tyrosine kinase (PTK)-dependent downregulation of Kv1.2, resulting in long-term potentiation of direct cortical inputs and intrinsic excitability (LTP-IE). When APs were elicited by direct somatic stimulation of CA3-PCs from rodents of either sex, only a narrow window of distal dendritic [Ca²⁺] allowed LTP-IE because of Ca²⁺-dependent coactivation of PTK and protein tyrosine phosphatase (PTP), which renders non-mossy fiber (MF) inputs incompetent in LTP-IE induction. High-frequency MF inputs, however, could induce LTP-IE at high dendritic [Ca²⁺] of the window. We show that MF input-induced Zn²⁺ signaling inhibits postsynaptic PTP, and thus enables MF inputs to induce LTP-IE at a wide range of [Ca²⁺]_i values. Extracellular chelation of Zn²⁺ or genetic deletion of vesicular zinc transporter abrogated the privilege of MF inputs for LTP-IE induction. Moreover, the incompetence of somatic stimulation was rescued by the inhibition of PTP or a supplement of extracellular zinc, indicating that MF input-induced increase in dendritic [Zn²⁺] facilitates the induction of LTP-IE by inhibiting PTP. Consistently, high-frequency MF stimulation induced immediate and delayed elevations of [Zn²⁺] at proximal and distal dendrites, respectively. These results indicate that MF inputs are uniquely linked to the regulation of direct cortical inputs owing to synaptic Zn²⁺ signaling.SIGNIFICANCE STATEMENT Zn²⁺ has been mostly implicated in pathological processes, and the physiological roles of synaptically released Zn²⁺ in intracellular signaling are little known. We show here that Zn²⁺ released from hippocampal mossy fiber (MF) terminals enters postsynaptic CA3 pyramidal cells, and plays a facilitating role in MF input-induced heterosynaptic potentiation of perforant path (PP) synaptic inputs through long-term potentiation of intrinsic excitability (LTP-IE). We show that the window of cytosolic [Ca²⁺] that induces LTP-IE is normally very narrow because of the Ca²⁺-dependent coactivation of antagonistic signaling pairs, whereby non-MF inputs become ineffective in inducing excitability change. The MF-induced Zn²⁺ signaling, however, biases toward facilitating the induction of LTP-IE. The present study elucidates why MF inputs are more privileged for the regulation of PP synapses.

Shank and Zinc Mediate an AMPA Receptor Subunit Switch in Developing Neurons

During development, pyramidal neurons undergo dynamic regulation of AMPA receptor (AMPAR) subunit composition and density to help drive synaptic plasticity and maturation. These normal developmental changes in AMPARs are particularly vulnerable to risk factors for Autism Spectrum Disorders (ASDs), which include loss or mutations of synaptic proteins and environmental insults, such as dietary zinc deficiency. Here, we show how Shank2 and Shank3 mediate a zinc-dependent regulation of AMPAR function and subunit switch from GluA2-lacking to GluA2-containing AMPARs. Over development, we found a concomitant increase in Shank2 and Shank3 with GluA2 at synapses, implicating these molecules as potential players in AMPAR maturation. Since Shank activation and function require zinc, we next studied whether neuronal activity regulated postsynaptic zinc at glutamatergic synapses. Zinc was found to increase transiently and reversibly with neuronal depolarization at synapses, which could affect Shank and AMPAR localization and activity. Elevated zinc induced multiple functional changes in AMPAR, indicative of a subunit switch. Specifically, zinc lengthened the decay time of AMPAR-mediated synaptic currents and reduced their inward rectification in young hippocampal neurons. Mechanistically, both Shank2 and Shank3 were necessary for the zinc-sensitive enhancement of AMPAR-mediated synaptic transmission and act in concert to promote removal of GluA1 while enhancing recruitment of GluA2 at pre-existing Shank puncta. These findings highlight a cooperative local dynamic regulation of AMPAR subunit switch controlled by zinc signaling through Shank2 and Shank3 to shape the biophysical properties of developing glutamatergic synapses. Given the zinc sensitivity of young neurons and its dependence on Shank2 and Shank3, genetic mutations and/or environmental insults during early development could impair synaptic maturation and circuit formation that underlie ASD etiology.

Neurotoxicity Linked to Dysfunctional Metal Ion Homeostasis and Xenobiotic Metal Exposure: Redox Signaling and Oxidative Stress

SIGNIFICANCE

Essential metals such as copper, iron, manganese, and zinc play a role as cofactors in the activity of a wide range of processes involved in cellular homeostasis and survival, as well as during organ and tissue development. Throughout our life span, humans are also exposed to xenobiotic metals from natural and anthropogenic sources, including aluminum, arsenic, cadmium, lead, and mercury. It is well recognized that alterations in the homeostasis of essential metals and an increased environmental/occupational exposure to xenobiotic metals are linked to several neurological disorders, including neurodegeneration and neurodevelopmental alterations. Recent Advances: The redox activity of essential metals is key for neuronal homeostasis and brain function. Alterations in redox homeostasis and signaling are central to the pathological consequences of dysfunctional metal ion homeostasis and increased exposure to xenobiotic metals. Both redox-active and redox-inactive metals trigger oxidative stress and damage in the central nervous system, and the exact mechanisms involved are starting to become delineated.

CRITICAL ISSUES

In this review, we aim to appraise the role of essential metals in determining the redox balance in the brain and the mechanisms by which alterations in the homeostasis of essential metals and exposure to xenobiotic metals disturb the cellular redox balance and signaling. We focus on recent literature regarding their transport, metabolism, and mechanisms of toxicity in neural systems.

FUTURE DIRECTIONS

Delineating the specific mechanisms by which metals alter redox homeostasis is key to understand the pathological processes that convey chronic neuronal dysfunction in neurodegenerative and neurodevelopmental disorders. Antioxid. Redox Signal. 28, 1669-1703.

Excess influx of Zn(2+) into dentate granule cells affects object recognition memory via attenuated LTP

The influx of extracellular Zn(2+) into dentate granule cells is nonessential for dentate gyrus long-term potentiation (LTP) and the physiological significance of extracellular Zn(2+) dynamics is unknown in the dentate gyrus. Excess increase in extracellular Zn(2+) in the hippocampal CA1, which is induced with excitation of zincergic neurons, induces memory deficit via excess influx of Zn(2+) into CA1 pyramidal cells. In the present study, it was examined whether extracellular Zn(2+) induces object recognition memory deficit via excess influx of Zn(2+) into dentate granule cells. KCl (100 mM, 2 µl) was locally injected into the dentate gyrus. The increase in intracellular Zn(2+) in dentate granule cells induced with high K(+) was blocked by co-injection of CaEDTA and CNQX, an extracellular Zn(2+) chelator and an AMPA receptor antagonist, respectively, suggesting that high K(+) increases the influx of Zn(2+) into dentate granule cells via AMPA receptor activation. Dentate gyrus LTP induction was attenuated 1 h after KCl injection into the dentate gyrus and also attenuated when KCl was injected 5 min after the induction. Memory deficit was induced when training of object recognition test was performed 1 h after KCl injection into the dentate gyrus and also induced when KCl was injected 5 min after the training. High K(+)-induced impairments of LTP and memory were rescued by co-injection of CaEDTA. These results indicate that excess influx of Zn(2+) into dentate granule cells via AMPA receptor activation affects object recognition memory via attenuated LTP induction. Even in the dentate gyrus where is scarcely innervated by zincergic neurons, it is likely that extracellular Zn(2+) homeostasis is strictly regulated for cognition.

Significance of Zn(2+) signaling in cognition: insight from synaptic Zn(2+) dyshomeostasis

Zinc is concentrated in the synaptic vesicles via zinc transporter-3 (ZnT3), released from glutamatergic (zincergic) neuron terminals, and serves as a signal factor (Zn(2+) signal) in the intracellular (cytosol) compartment as well as in the extracellular compartment. Synaptic Zn(2+) signaling is dynamically linked to neurotransmission via glutamate and is involved in synaptic plasticity such as long-term potentiation (LTP) and cognitive activity. Zinc concentration in the synaptic vesicles is correlated with ZnT3 protein expression and potentially decreased under chronic zinc deficiency. Synaptic vesicle serves as a large pool for Zn(2+) signaling and other organelles might also serve as a pool for Zn(2+) signaling. ZnT3KO mice and zinc-deficient animals, which lack or reduce Zn(2+) release into the extracellular space by action potentials, are able to recognize novel or displaced objects normally. However, the amount of Zn(2+) functioning as a signal factor increases along with brain development. Exogenous Zn(2+) lowers the threshold in hippocampal CA1 LTP induction in young rat. Furthermore, ZnT3KO mice lose advanced cognition such as contextual discrimination. It is likely that the optimal range of synaptic Zn(2+) signaling is involved in cognitive activity. On the basis of the findings on the relationship between dyshomeostasis of synaptic Zn(2+) and cognition, this paper summarizes the possible involvement of intracellular Zn(2+) signaling in cognitive ability.

A specific role for hippocampal mossy fiber's zinc in rapid storage of emotional memories

We investigated the specific role of zinc present in large amounts in the synaptic vesicles of mossy fibers and coreleased with glutamate in the CA3 region. In previous studies, we have shown that blockade of zinc after release has no effect on the consolidation of spatial learning, while zinc is required for the consolidation of contextual fear conditioning. Although both are hippocampo-dependent processes, fear conditioning to the context implies a strong emotional burden. To verify the hypothesis that zinc could play a specific role in enabling sustainable memorization of a single event with a strong emotional component, we used a neuropharmacological approach combining a glutamate receptor antagonist with different zinc chelators. Results show that zinc is mandatory to allow the consolidation of one-shot memory, thus being the key element allowing the hippocampus submitted to a strong emotional charge to switch from the cognitive mode to a flashbulb memory mode. Individual differences in learning abilities have been known for a long time to be totally or partially compensated by distributed learning practice. Here we show that contextual fear conditioning impairments due to zinc blockade can be efficiently reduced by distributed learning practice.

Intracellular Zn(2+) signaling in cognition

Brain zinc homeostasis is strictly controlled under healthy conditions, indicating the importance of zinc for physiological function in the brain. A part of zinc in the brain exists in the synaptic vesicles, is released from a subclass of glutamatergic neurons (i.e., zincergic neurons), and serves as a signal factor (Zn(2+) signal) in the intracellular (cytosol) compartment as well as in the extracellular compartment. Zn(2+) signaling is dynamically linked to glutamate signaling and may be involved in synaptic plasticity, such as long-term potentiaion and cognitive activity. In zincergic synapses, intracellular Zn(2+) signaling in the postsynaptic neurons, which is linked to Zn(2+) release from zincergic neuron terminals, plays a role in cognitive activity. When nonzincergic synapses participate in cognition, on the other hand, it is possible that intracellular Zn(2+) signaling, which is due mainly to Zn(2+) release from the internal stores and/or metallothioneins, also is involved in cognitive activity, because zinc-dependent system such as zinc-binding proteins is usually required for cognitive process. Intracellular Zn(2+) dynamics may be modified via an endocrine system activity, glucocorticoid secretion in both zincergic and nonzincergic neurons, which is linked to a long-lasting change in synaptic efficacy. On the basis of the evidence of cognitive decline caused by the lack and/or the blockade of synaptic Zn(2+) signaling, this article summarizes the involvement of intracellular Zn(2+) signaling in zincergic synapses in cognition and a hypothetical involvement of that in nonzincergic synapses.

Cognitive decline due to excess synaptic Zn(2+) signaling in the hippocampus

Zinc is an essential component of physiological brain function. Vesicular zinc is released from glutamatergic (zincergic) neuron terminals and serves as a signal factor (Zn²⁺ signal) in both the intracellular (cytosol) compartment and the extracellular compartment. Synaptic Zn²⁺ signaling is dynamically linked to neurotransmission and is involved in processes of synaptic plasticity such as long-term potentiation and cognitive activity. On the other hand, the activity of the hypothalamic–pituitary–adrenal (HPA) axis, i.e., glucocorticoid secretion, which can potentiate glutamatergic neuron activity, is linked to cognitive function. HPA axis activity modifies synaptic Zn²⁺ dynamics at zincergic synapses. An increase in HPA axis activity, which occurs after exposure to stress, may induce excess intracellular Zn²⁺ signaling in the hippocampus, followed by hippocampus-dependent memory deficit. Excessive excitation of zincergic neurons in the hippocampus can contribute to cognitive decline under stressful and/or pathological conditions. This paper provides an overview of the ``Hypothesis and Theory'' of Zn²⁺-mediated modification of cognitive activity.

Disruption of calmodulin-dependent protein kinase II α/brain-derived neurotrophic factor (α-CaMKII/BDNF) signalling is associated with zinc deficiency-induced impairments in cognitive and synaptic plasticity

Maternal dietary Zn deficiency during fetal development induces substantial cognitive dysfunctions in the resultant offspring. The mechanism underlying this effect is unclear. The present study evaluated whether the impairments caused by gestational and lactational Zn deficiency are mediated by the hippocampal calmodulin-dependent protein kinase II α (α-CaMKII)/brain-derived neurotrophic factor (BDNF) signalling pathway as well as whether they can be restored by postnatal Zn supplementation. Rats were randomly divided into four groups on the first day of pregnancy (n 12): control (CO) group; pair-fed (PF) group; Zn-deprived (ZD) group; orally Zn-supplemented group. The spatial memory of the offspring was tested at postnatal day 35 using the Morris water maze. Long-term potentiation (LTP) in the rat hippocampal medial perforant path–dentate gyrus pathway was evaluated simultaneously, and α-CaMKII and BDNF protein levels were examined by Western blot analysis. The results demonstrated that the ZD group exhibited a significantly longer latency period in the Morris water maze as well as a significantly decreased LTP amplitude compared with the CO and PF groups. α-CaMKII and BDNF protein expression in the hippocampus was significantly reduced in the ZD group. Postnatal Zn supplementation restored the cognitive dysfunction induced by gestational Zn deficiency but could not completely reverse the decreased LTP and α-CaMKII/BDNF protein levels. Our findings suggest that the α-CaMKII/BDNF signalling pathway may be involved in Zn deficiency-induced cognitive and synaptic impairments.

Differential needs of zinc in the CA3 area of dorsal hippocampus for the consolidation of contextual fear and spatial memories

One peculiarity of the hippocampal CA3 mossy fiber terminals is the co-release of zinc and glutamate upon synaptic transmission. How these two players act on hippocampal-dependent memories is still unclear. To decipher their respective involvement in memory consolidation, a pharmacological approach was chosen. Using two hippocampal-dependent behavioral paradigms (water maze and contextual fear conditioning) we now report that glutamate at CA3 synapses is necessary and sufficient for the spatial learning consolidation process, whereas glutamate and zinc released by mossy fibers are both mandatory and exert cumulative effects on contextual fear consolidation, a form of learning with a strong emotional component.

Prion protein facilitates uptake of zinc into neuronal cells

Zinc is released into the synaptic cleft upon exocytotic stimuli, although the mechanism for its reuptake into neurons is unresolved. Here we show that the cellular prion protein enhances the uptake of zinc into neuronal cells. This prion-protein-mediated zinc influx requires the octapeptide repeats and amino-terminal polybasic region in the prion protein, but not its endocytosis. Selective antagonists of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors block the prion protein-mediated zinc uptake, and the prion protein co-immunoprecipitates with both GluA1 and GluA2 AMPA receptor subunits. Zinc-sensitive intracellular tyrosine phosphatase activity is decreased in cells expressing prion protein and increased in the brains of prion-protein-null mice, providing evidence of a physiological consequence of this process. Prion protein-mediated zinc uptake is ablated in cells expressing familial associated mutants of the protein and in prion-infected cells. These data suggest that alterations in the cellular prion protein-mediated zinc uptake may contribute to neurodegeneration in prion and other neurodegenerative diseases.

Transient increase in Zn2+ in hippocampal CA1 pyramidal neurons causes reversible memory deficit

The translocation of synaptic Zn²⁺ to the cytosolic compartment has been studied to understand Zn²⁺ neurotoxicity in neurological diseases. However, it is unknown whether the moderate increase in Zn²⁺ in the cytosolic compartment affects memory processing in the hippocampus. In the present study, the moderate increase in cytosolic Zn²⁺ in the hippocampus was induced with clioquinol (CQ), a zinc ionophore. Zn²⁺ delivery by Zn-CQ transiently attenuated CA1 long-term potentiation (LTP) in hippocampal slices prepared 2 h after i.p. injection of Zn-CQ into rats, when intracellular Zn²⁺ levels was transiently increased in the CA1 pyramidal cell layer, followed by object recognition memory deficit. Object recognition memory was transiently impaired 30 min after injection of ZnCl₂ into the CA1, but not after injection into the dentate gyrus that did not significantly increase intracellular Zn²⁺ in the granule cell layer of the dentate gyrus. Object recognition memory deficit may be linked to the preferential increase in Zn²⁺ and/or the preferential vulnerability to Zn²⁺ in CA1 pyramidal neurons. In the case of the cytosolic increase in endogenous Zn²⁺ in the CA1 induced by 100 mM KCl, furthermore, object recognition memory was also transiently impaired, while ameliorated by co-injection of CaEDTA to block the increase in cytosolic Zn²⁺. The present study indicates that the transient increase in cytosolic Zn²⁺ in CA1 pyramidal neurons reversibly impairs object recognition memory.

Zinc signaling through glucocorticoid and glutamate signaling in stressful circumstances

Humans and animals are constantly exposed to environmental stress. The hypothalamic-pituitary-adrenal (HPA) axis responds to stress, followed by glucocorticoid secretion from the adrenal glands. This response serves to maintain homeostasis in the living body through energy mobilization or to restore it. The brain is an important target for glucocorticoids. The hippocampus participates in the regulation of the HPA axis. Stress activates glutamatergic neurons in the hippocampus, and serious stress induces dyshomeostasis of extracellular glutamate. This dyshomeostasis, which is potentiated by glucocorticoids, modifies cognitive and emotional behavior. On the other hand, zinc is necessary for glucocorticoid signaling and is released from glutamatergic (zincergic) neurons to modulate synaptic glutamate signaling. Stress also induces dyshomeostasis of extracellular zinc, which may be linked to dyshomeostasis of extracellular glutamate. Thus, glucocorticoid signaling might also contribute to dyshomeostasis of extracellular zinc. It is likely that zinc signaling participates in cognitive and emotional behavior through glucocorticoid and glutamate signaling under stressful circumstances. This Mini-Review analyzes the relationship among signals of glucocorticoid, glutamate, and zinc under stressful circumstances to elucidate the significance of the zinc signaling in response to stress.

Insight into glutamate excitotoxicity from synaptic zinc homeostasis

Zinc is released from glutamatergic (zincergic) neuron terminals in the hippocampus, followed by the increase in Zn(2+) concentration in the intracellular (cytosol) compartment, as well as that in the extracellular compartment. The increase in Zn(2+) concentration in the intracellular compartment during synaptic excitation is mainly due to Zn(2+) influx through calcium-permeable channels and serves as Zn(2+) signaling as well as the case in the extracellular compartment. Synaptic Zn(2+) homeostasis is important for glutamate signaling and altered under numerous pathological processes such as Alzheimer's disease. Synaptic Zn(2+) homeostasis might be altered in old age, and this alteration might be involved in the pathogenesis and progression of Alzheimer's disease; Zinc may play as a key-mediating factor in the pathophysiology of Alzheimer's disease. This paper summarizes the role of Zn(2+) signaling in glutamate excitotoxicity, which is involved in Alzheimer's disease, to understand the significance of synaptic Zn(2+) homeostasis in the pathophysiology of Alzheimer's disease.

Differential effects of zinc influx via AMPA/kainate receptor activation on subsequent induction of hippocampal CA1 LTP components

Zinc potentiates the induction of NMDA receptor-dependent hippocampal CA1 long-term potentiation (LTP) at low micromolar concentrations, while excessive zinc attenuates it. Homeostasis of synaptic zinc is critical for LTP induction. In the present study, LTP at hippocampal CA1 synapses was analyzed focused on the timing and level of zinc influx into hippocampal cells in hippocampal slices from young rats. Zinc (100 microM) perfusion increased intracellular zinc level and subsequently attenuated CA1 LTP induced by tetanic stimuli at 100 Hz for 1s, which was completely inhibited in the presence of 50 microM APV, an NMDA receptor antagonist. When 10 microM CNQX, an AMPA/kainate receptor antagonist, which reduced zinc influx into hippocampal cells, was perfused prior to the zinc perfusion, the attenuation of CA1 LTP by the zinc perfusion was restored. These results suggest that facilitated zinc influx into hippocampal cells via AMPA/kainate receptor activation is an event to attenuate subsequent induction of NMDA receptor-dependent CA1 LTP. On the other hand, the zinc pre-perfusion also attenuated CA1 LTP induced by 200-Hz tetanus, but not NMDA receptor-independent CA1 LTP induced by 200-Hz tetanus in the presence of APV, suggesting that the induction of NMDA receptor-independent CA1 LTP is less susceptibility to the facilitated zinc influx into hippocampal CA1 cells. Zinc influx via AMPA/kainate receptor activation may differentially act on subsequent induction of CA1 LTP components.

Disruption of the CaMKII/CREB signaling is associated with zinc deficiency-induced learning and memory impairments

Many studies have shown that zinc deficiency not only retards growth, but also affects several brain functions, including learning and memory. However, the underlying mechanism of impaired hippocampus-dependent learning and memory under zinc deficiency is poorly understood. In this study, young mice were fed a zinc-deficient diet (0.85 ppm) for 5 weeks. Morris water maze result showed that zinc deficiency results in spatial learning impairment. We then examined whether zinc depletion-induced learning and memory defects are associated with changes in signaling molecules essential for the expression of long-term potentiation. Immunoblot results showed that the protein levels of calmodulin (CaM), phosphorylated CaM-dependent protein kinase II (CaMKII), and phosphorylated cAMP-responsive element binding protein (CREB) were significantly reduced, whereas the total protein levels of CaMKII and CREB did not change in the zinc-deficient hippocampus. Thus, we provide a previously unrecognized mechanism whereby zinc deficiency impairs hippocampal learning and memory, at least in part, through disruption of the CaM/CaMKII/CREB signaling pathway.

Insight into zinc signaling from dietary zinc deficiency

Zinc is necessary for not only brain development but also brain function. Zinc homeostasis in the brain is tightly regulated by the brain barrier system and is not easily disrupted by dietary zinc deficiency. However, histochemically reactive zinc as revealed by Timm's staining is susceptible to zinc deficiency, suggesting that the pool of Zn(2+) can be reduced by zinc deficiency. The hippocampus is also susceptible to zinc deficiency in the brain. On the other hand, zinc deficiency causes abnormal glucocorticoid secretion from the adrenal cortex, which is observed prior to the decrease in extracellular zinc concentration in the hippocampus. The hippocampus is enriched with glucocorticoid receptors and hippocampal functions are changed by abnormal glucocorticoid secretion. Zinc deficiency elicits neuropsychological symptoms and affects cognitive performance. It may also aggravate glutamate excitotoxicity in neurological diseases. Abnormal glucocorticoid secretion is associated with these symptoms in zinc deficiency. Furthermore, the decrease in Zn(2+) pool may cooperate with glucocorticoid action in zinc deficiency. Judging from susceptibility of Zn(2+) pool in the brain to zinc deficiency, it is possible that the decrease in Zn(2+) pool in the peripheral tissues triggers abnormal glucocorticoid secretion. To understand the importance of zinc as a signaling factor, this paper analyzes the relationship among the changes in hippocampal functions, abnormal behavior and pathophysiological changes in zinc deficiency, based on the data from experimental animals.

Enhancement of hippocampal mossy fiber activity in zinc deficiency and its influence on behavior

The extracellular concentration of glutamate in the hippocampus is increased by hippocampal perfusion with CaEDTA, a membrane-impermeable zinc chelator, suggesting that the activity of glutamatergic neurons in the hippocampus are influenced by the extracellular concentrations of zinc. In the present study, the relationship between the extracellular concentrations of zinc and mossy fiber activity in the hippocampus was examined in mice and rats fed a zinc-deficient diet for 4 weeks. Timm's stain, by which histochemically reactive zinc in the presynaptic vesicles is detected, was attenuated in the hippocampus in zinc deficiency. The extracellular signal of ZnAF-2, a membrane-impermeable zinc indicator, was also lower in the hippocampal CA3, suggesting that the basal extracellular concentrations of zinc are lower maintained in zinc deficiency. To check mossy fiber activity after 4-week zinc deprivation, the decrease in the signal of FM4-64, an indicator of presynaptic activity (exocytosis), at mossy fiber synapses was measured under the condition of spontaneous depolarization. The decrease was significantly facilitated by zinc deficiency, suggesting that the basal exocytosis at mossy fiber synapses is enhanced by zinc deficiency. On the other hand, the increase in anxiety-like behavior was observed in the open-field test after 4-week zinc deprivation. The present study demonstrates that the decrease in the basal extracellular concentrations of zinc may be linked to the enhancement of the basal mossy fiber activity in zinc deficiency. This decrease seems to be also involved in neuropsychological behavior in zinc deficiency.

Zip6 (LIV-1) regulates zinc uptake in neuroblastoma cells under resting but not depolarizing conditions

Impaired zinc homeostasis is implicated in many cases of brain injury and pathogenesis. While several routes of zinc influx have been identified in neurons under depolarizing conditions, zinc uptake mechanisms during resting conditions are unknown. We have previously detected Zip6 at the plasma membrane of rat neurons, suggesting a role for Zip6 in neuronal zinc uptake. Zinc uptake under resting and depolarizing membrane potentials was measured in SH-SY5Y neuroblastoma cells using 65Zn. Zinc uptake was higher under depolarizing conditions, compared with resting conditions, and could be reduced by high extracellular calcium, gadolinium, or nimodipine, which suggests that L-type calcium channels are significant routes of zinc uptake under depolarizing membrane potential. In contrast, zinc uptake under resting conditions was not affected by calcium or calcium channel antagonists. Zip6 was localized to the plasma membrane in SH-SY5Y cells, and siRNA-mediated down-regulation of Zip6 expression reduced zinc uptake during resting, but not depolarizing conditions. Zinc treatment (100 microM Zn) reduced zinc uptake under resting, but not depolarizing conditions, which was associated with lower plasma membrane-associated and total Zip6 protein abundance. These results demonstrate that Zip6 functions as a zinc import protein in neuroblastoma cells, that zinc influx during resting and depolarizing conditions occurs via distinctly different processes in these cells, and suggest that neuronal zinc uptake may be down-regulated by excess zinc levels, but only under resting conditions.

Zinc release at the synaptic terminals of rod photoreceptors

The presence of reactive zinc (Zn2+) within photoreceptor terminals, and evidence that exogenous zinc affects the electrophysiological activity of the distal retina, led to the suggestion that its co-release with glutamate could play an essential role in the modulation of information at the first synapse in the visual pathway. Although we had shown previously that zinc release could be visualized in the region of the outer synaptic layer of a retinal slice preparation, it could not be ascertained with certainty that the release sites were at the presynaptic terminal rather than from the mitochondria-rich inner segment or from zinc within the distal processes of photoreceptors and Müller cells. Using membrane permeant and membrane impermeant forms of a fluorescent zinc indicator (Newport green), we show both the intracellular distribution of Zn2+ and its depolarization-dependent discharge from the terminals of isolated zebrafish photoreceptors in culture. Zinc release could be detected in the dark-adapted preparation, and was further enhanced by brief exposures to black widow spider venom or high K+. Synaptically released zinc may significantly influence neural processing in the vertebrate retina by modulating the activity of excitatory and/or inhibitory receptors as well as intracellular signaling proteins.