Influx of extracellular Zn(2+) into the hippocampal CA1 neurons is required for cognitive performance via long-term potentiation

Intracellular zinc signaling influences NMDA receptor function by enhancing the interaction of ZnT1 with GluN2A

Zinc, loaded into glutamate-containing presynaptic vesicles and released into the synapse in an activity-dependent manner, modulates neurotransmission through its actions on postsynaptic targets, prominently via high-affinity inhibition of GluN2A-containing NMDA receptors. Recently, we identified a postsynaptic transport mechanism that regulates endogenous zinc inhibition of NMDARs. In this new model of zinc regulation, the postsynaptic transporter ZnT1 mediates zinc inhibition of NMDARs by binding to GluN2A. Through this interaction, ZnT1, a transporter that moves zinc from the cytoplasm to the extracellular domain, generates a zinc microdomain that modulates NMDAR-mediated neurotransmission. As ZnT1 expression is transcriptionally driven by the metal-responsive transcription factor 1 (MTF-1), we found that intracellular zinc strongly drives MTF-1 in cortical neurons in vitro and increases the number of GluN2A-ZnT1 interactions, thereby enhancing tonic zinc inhibition of NMDAR-mediated currents. Importantly, this effect is absent when the interaction between GluN2A and ZnT1 is disrupted by a cell-permeable peptide. These results suggest that zinc-regulated gene expression can dynamically regulate NMDAR-mediated synaptic processes.

[Brain Function and Pathophysiology Focused on Zn2+ Dynamics]

The basal levels of intracellular Zn²⁺ and extracellular Zn²⁺ are in the range of ～100 pM and ～10 nM, respectively, in the brain. Extracellular Zn²⁺ dynamics is involved in both cognitive performance and neurodegeneration. The bidirectional actions are linked with extracellular glutamate and amyloid-β_1-42 (Aβ_1-42). Intracellular Zn²⁺ signaling via extracellular glutamate is required for learning and memory, while intracellular Zn²⁺ dysregulation induces cognitive decline. Furthermore, human Aβ_1-42, a causative peptide in Alzheimer's disease pathogenesis captures extracellular Zn²⁺ and readily taken up into hippocampal neurons followed by intracellular Zn²⁺ dysregulation. Aβ_1-42-mediated intracellular Zn²⁺ dysregulation is accelerated with aging, because extracellular Zn²⁺ is age-relatedly increased, resulting in Aβ_1-42-induced cognitive decline and neurodegeneration with aging. On the other hand, metallothioneins, zinc-binding proteins can capture Zn²⁺ released from intracellular Zn-Aβ_1-42 complexes and serve for intracellular Zn²⁺-buffering to maintain intracellular Zn²⁺ homeostasis. This review summarizes Zn²⁺ function and its neurotoxicity in the brain, and also the potential defense strategy via metallothioneins against Aβ_1-42-induced pathogenesis.

Beneficial Effects of Spirulina Consumption on Brain Health

Spirulina is a microscopic, filamentous cyanobacterium that grows in alkaline water bodies. It is extensively utilized as a nutraceutical food supplement all over the world due to its high levels of functional compounds, such as phycocyanins, phenols and polysaccharides, with anti-inflammatory, antioxidant, immunomodulating properties both in vivo and in vitro. Several scientific publications have suggested its positive effects in various pathologies such as cardiovascular diseases, hypercholesterolemia, hyperglycemia, obesity, hypertension, tumors and inflammatory diseases. Lately, different studies have demonstrated the neuroprotective role of Spirulina on the development of the neural system, senility and a number of pathological conditions, including neurological and neurodegenerative diseases. This review focuses on the role of Spirulina in the brain, highlighting how it exerts its beneficial anti-inflammatory and antioxidant effects, acting on glial cell activation, and in the prevention and/or progression of neurodegenerative diseases, in particular Parkinson’s disease, Alzheimer’s disease and Multiple Sclerosis; due to these properties, Spirulina could be considered a potential natural drug.

S100B dysregulation during brain development affects synaptic SHANK protein networks via alteration of zinc homeostasis

Autism Spectrum Disorders (ASD) are caused by a combination of genetic predisposition and nongenetic factors. Among the nongenetic factors, maternal immune system activation and zinc deficiency have been proposed. Intriguingly, as a genetic factor, copy-number variations in S100B, a pro-inflammatory damage-associated molecular pattern (DAMP), have been associated with ASD, and increased serum S100B has been found in ASD. Interestingly, it has been shown that increased S100B levels affect zinc homeostasis in vitro. Thus, here, we investigated the influence of increased S100B levels in vitro and in vivo during pregnancy in mice regarding zinc availability, the zinc-sensitive SHANK protein networks associated with ASD, and behavioral outcomes. We observed that S100B affects the synaptic SHANK2 and SHANK3 levels in a zinc-dependent manner, especially early in neuronal development. Animals exposed to high S100B levels in utero similarly show reduced levels of free zinc and SHANK2 in the brain. On the behavioral level, these mice display hyperactivity, increased stereotypic and abnormal social behaviors, and cognitive impairment. Pro-inflammatory factors and zinc-signaling alterations converge on the synaptic level revealing a common pathomechanism that may mechanistically explain a large share of ASD cases.

Effects of Quercetin and Resveratrol on Zinc Chloride- and Sodium Metavanadate-Induced Passive Avoidance Memory Retention Deficits in Male Mice

Quercetin and resveratrol are found in a variety of fruits and vegetables and have several biological and pharmacological properties. In this study, the effects of quercetin [50 mg/kg, intraperitoneal (i.p.)] and resveratrol (50 mg/kg, i.p.) on zinc chloride (ZnCl₂; 75 mg/kg/d, 2 weeks oral gavage) and sodium metavanadate (SMV; 22.5 mg/kg/d, 2 weeks oral gavage) induced passive avoidance memory retention were investigated in step-through passive avoidance tasks. ZnCl₂ was dissolved in saline and SMV was dissolved in drinking water. Mice received ZnCl₂ or SMV orally for two weeks and were administered quercetin or resveratrol by i.p. injection on day 14, days 12 and 14, or days 10, 12, and 14. At the end of treatment, animals were trained for one day in a step-through passive avoidance task, then alterations in avoidance memory retention were evaluated after 24, 48, 96, and 168 h. Oral consumption of ZnCl₂ and SMV decreased latency time compared with control groups. Both quercetin and resveratrol (50 mg/kg, i.p.) prevented ZnCl₂- and SMV-induced avoidance memory retention impairments and did not significantly alter muscle strength, as demonstrated in rotarod tasks. No significant differences were observed between mice who received single, double, or triple doses of quercetin or resveratrol. The results suggest that quercetin and resveratrol may have preventive effects on ZnCl₂- and SMV-induced memory impairment in male mice.

Extracellular Zn2+-Dependent Amyloid-β1-42 Neurotoxicity in Alzheimer's Disease Pathogenesis

The basal level of extracellular Zn²⁺ is in the range of low nanomolar (~ 10 nM) in the hippocampus. However, extracellular Zn²⁺ dynamics plays a key role for not only cognitive activity but also cognitive decline. Extracellular Zn²⁺ dynamics is modified by glutamatergic synapse excitation and the presence of amyloid-β_1-42 (Aβ_1-42), a causative peptide in Alzheimer's disease (AD). When human Aβ_1-42 reaches high picomolar (> 100 pM) in the extracellular compartment of the rat dentate gyrus, Zn-Aβ_1-42 complexes are readily formed and taken up into dentate granule cells, followed by Aβ_1-42-induced cognitive decline that is linked with Zn²⁺ released from intracellular Zn-Aβ_1-42 complexes. Aβ_1-42-induced intracellular Zn²⁺ toxicity is accelerated with aging because of age-related increase in extracellular Zn²⁺. The recent findings suggest that Aβ_1-42 secreted continuously from neuron terminals causes age-related cognitive decline and neurodegeneration via intracellular Zn²⁺ dysregulation. On the other hand, metallothioneins (MTs), zinc-binding proteins, quickly serve for intracellular Zn²⁺-buffering under acute intracellular Zn²⁺ dysregulation. On the basis of the idea that the defense strategy against Aβ_1-42-induced pathogenesis leads to preventing the AD development, this review deals with extracellular Zn²⁺-dependent Aβ_1-42 neurotoxicity, which is accelerated with aging.

Zinc-mediated Neurotransmission in Alzheimer's Disease: A Potential Role of the GPR39 in Dementia

With more people reaching an advanced age in modern society, there is a growing need for strategies to slow down age-related neuropathology and loss of cognitive functions, which are a hallmark of Alzheimer's disease. Neuroprotective drugs and candidate drug compounds target one or more processes involved in the neurodegenerative cascade, such as excitotoxicity, oxidative stress, misfolded protein aggregation and/or ion dyshomeostasis. A growing body of research shows that a G-protein coupled zinc (Zn²⁺) receptor (GPR39) can modulate the abovementioned processes.

Zn²⁺ itself has a diverse activity profile at the synapse, and by binding to numerous receptors, it plays an important role in neurotransmission. However, Zn²⁺ is also necessary for the formation of toxic oligomeric forms of amyloid beta, which underlie the pathology of Alzheimer’s disease. Furthermore, the binding of Zn²⁺ by amyloid beta causes a disruption of zincergic signaling, and recent studies point to GPR39 and its intracellular targets being affected by amyloid pathology.

In this review, we present neurobiological findings related to Zn²⁺ and GPR39, focusing on its signaling pathways, neural plasticity, interactions with other neurotransmission systems, as well as on the effects of pathophysiological changes observed in Alzheimer's disease on GPR39 function.

Direct targeting of the GPR39 might be a promising strategy for the pharmacotherapy of zincergic dyshomeostasis observed in Alzheimer’s disease. The information presented in this article will hopefully fuel further research into the role of GPR39 in neurodegeneration and help in identifying novel therapeutic targets for dementia.

Amelioration of neurobehavioral and cognitive abilities of F1 progeny following dietary supplementation with Spirulina to protein malnourished mothers

Early life adversities (stress, infection and mal/undernutrition) can affect neurocognitive, hippocampal and immunological functioning of the brain throughout life. Substantial evidence suggests that maternal protein malnutrition contributes to the progression of neurocognitive abnormalities and psychopathologies in adolescence and adulthood in offspring. Maternal malnutrition is prevalent in low and middle resource populations. The present study was therefore undertaken to evaluate the effects of dietary Spirulina supplementation of protein malnourished mothers during pregnancy and lactation on their offspring's reflex, neurobehavioral and cognitive development. Spirulina is a Cyanobacterium and a major source of protein and is being used extensively as a dynamic nutraceutical against aging and neurodegeneration. Sprague Dawley rats were switched to low protein (8% protein) or normal protein (20% protein) diet for 15 days before conception. Spirulina was orally administered (400 mg/kg/b.wt.) to subgroups of pregnant females from the day of conception throughout the lactational period. We examined several parameters including reproductive performance of dams, physical development, postnatal reflex ontogeny, locomotor behavior, neuromuscular strength, anxiety, anhedonic behavior, cognitive abilities and microglia populations in the F1 progeny. The study showed improved reproductive performance of Spirulina supplemented protein malnourished dams, accelerated acquisition of neurological reflexes, better physical appearance, enhanced neuromuscular strength, improved spatial learning and memory and partly normalized PMN induced hyperactivity, anxiolytic and anhedonic behavior in offspring. These beneficial effects of Spirulina consumption were also accompanied by reduced microglial activation which might assist in restoring the behavioral and cognitive skills in protein malnourished F1 rats. Maternal Spirulina supplementation is therefore proposed as an economical nutraceutical/supplement to combat malnutrition associated behavioral and cognitive deficits.

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

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

Is Vulnerability of the Dentate Gyrus to Aging and Amyloid-β1-42 Neurotoxicity Linked with Modified Extracellular Zn2+ Dynamics?

The basal levels of extracellular Zn²⁺ are in the range of low nanomolar concentrations in the hippocampus and perhaps increase age-dependently. Extracellular Zn²⁺ dynamics is critical for cognitive activity and excess influx of extracellular Zn²⁺ into hippocampal neurons is a known cause of cognitive decline. The dentate gyrus is vulnerable to aging in the hippocampus and affected in the early stage of Alzheimer's disease (AD). The reasons remain unclear. Neurogenesis-related apoptosis may induce non-specific neuronal depolarization by efflux of intracellular K⁺ in the dentate gyrus and be markedly increased along with aging. Extracellular Zn²⁺ influx into dentate granule cells via high K⁺-induced perforant pathway excitation leads to cognitive decline. Modified extracellular Zn²⁺ dynamics in the dentate gyrus of aged rats is linked with vulnerability to cognitive decline. Amyloid-β_1-42 (Aβ_1-42) is a causative candidate for AD pathogenesis. When Aβ_1-42 concentration reaches picomolar in the extracellular compartment in the dentate gyrus, Zn-Aβ_1-42 is formed in the extracellular compartment and rapidly taken up into dentate granule cells, followed by Aβ_1-42-induced cognitive decline that is due to Zn²⁺ released from Aβ_1-42, suggesting that dentate granule cells are sensitive to extracellular Zn²⁺-dependent Aβ_1-42 toxicity. This paper deals with proposed vulnerability of the dentate gyrus to aging and Aβ_1-42 neurotoxicity.

Rapid Intracellular Zn2+ Dysregulation via Membrane Corticosteroid Receptor Activation Affects In Vivo CA1 LTP

Involvement of membrane mineralocorticoid (MC) and glucocorticoid (GC) receptors in synaptic Zn²⁺ dynamics remains unclear. Here, we tested whether synaptic plasticity is affected by rapid intracellular Zn²⁺ dysregulation via membrane MC and GC receptor activation, in comparison with intracellular Ca²⁺ dysregulation. In anesthetized rats, extracellular Zn²⁺ level was increased under local perfusion of the hippocampal CA1 with 500 ng/ml corticosterone. In vivo CA1 long-term potentiation (LTP) at Schaffer collateral-CA1 pyramidal cell synapses was attenuated by the pre-perfusion with corticosterone prior to tetanic stimulation, and the attenuation was canceled by co-perfusion with CaEDTA, an extracellular Zn²⁺ chelator, suggesting that corticosterone-induced increase in extracellular Zn²⁺ is involved in the subsequent attenuation of LTP. In rat brain slices, corticosterone-induced increases in extracellular and intracellular Zn²⁺ were blocked in the presence of spironolactone, a MC receptor antagonist that canceled corticosterone-induced attenuation of LTP. Mifepristone, a GC receptor antagonist, which canceled corticosterone-induced attenuation of LTP, also blocked corticosterone-induced increase in intracellular Zn²⁺, but not extracellular Zn²⁺. Moreover, corticosterone-induced decrease in phosphorylated CaMKII was restored in the presence of CaEDTA or spironolactone. These results indicate that glucocorticoid rapidly induces the increase in intracellular Zn²⁺, which occurs via membrane MC and GC receptor activations, and decreases phosphorylated CaMKII level, resulting in attenuating LTP. Membrane MC and GC receptors induce intracellular Zn²⁺ dysregulation via differential mechanisms. In contrast, glucocorticoid-induced intracellular Ca²⁺ dysregulation is not crucial for affecting LTP.

Zinc Signal in Brain Diseases

The divalent cation zinc is an integral requirement for optimal cellular processes, whereby it contributes to the function of over 300 enzymes, regulates intracellular signal transduction, and contributes to efficient synaptic transmission in the central nervous system. Given the critical role of zinc in a breadth of cellular processes, its cellular distribution and local tissue level concentrations remain tightly regulated via a series of proteins, primarily including zinc transporter and zinc import proteins. A loss of function of these regulatory pathways, or dietary alterations that result in a change in zinc homeostasis in the brain, can all lead to a myriad of pathological conditions with both acute and chronic effects on function. This review aims to highlight the role of zinc signaling in the central nervous system, where it may precipitate or potentiate diverse issues such as age-related cognitive decline, depression, Alzheimer's disease or negative outcomes following brain injury.

The Impact of Synaptic Zn2+ Dynamics on Cognition and Its Decline

The basal levels of extracellular Zn²⁺ are in the range of low nanomolar concentrations and less attention has been paid to Zn²⁺, compared to Ca²⁺, for synaptic activity. However, extracellular Zn²⁺ is necessary for synaptic activity. The basal levels of extracellular zinc are age-dependently increased in the rat hippocampus, implying that the basal levels of extracellular Zn²⁺ are also increased age-dependently and that extracellular Zn²⁺ dynamics are linked with age-related cognitive function and dysfunction. In the hippocampus, the influx of extracellular Zn²⁺ into postsynaptic neurons, which is often linked with Zn²⁺ release from neuron terminals, is critical for cognitive activity via long-term potentiation (LTP). In contrast, the excess influx of extracellular Zn²⁺ into postsynaptic neurons induces cognitive decline. Interestingly, the excess influx of extracellular Zn²⁺ more readily occurs in aged dentate granule cells and intracellular Zn²⁺-buffering, which is assessed with ZnAF-2DA, is weakened in the aged dentate granule cells. Characteristics (easiness) of extracellular Zn²⁺ influx seem to be linked with the weakened intracellular Zn²⁺-buffering in the aged dentate gyrus. This paper deals with the impact of synaptic Zn²⁺ signaling on cognition and its decline in comparison with synaptic Ca²⁺ signaling.

Involvement of intracellular Zn2+ signaling in LTP at perforant pathway-CA1 pyramidal cell synapse

Physiological significance of synaptic Zn²⁺ signaling was examined at perforant pathway-CA1 pyramidal cell synapses. In vivo long-term potentiation (LTP) at perforant pathway-CA1 pyramidal cell synapses was induced using a recording electrode attached to a microdialysis probe and the recording region was locally perfused with artificial cerebrospinal fluid (ACSF) via the microdialysis probe. Perforant pathway LTP was not attenuated under perfusion with CaEDTA (10 mM), an extracellular Zn²⁺ chelator, but attenuated under perfusion with ZnAF-2DA (50 μM), an intracellular Zn²⁺ chelator, suggesting that intracellular Zn²⁺ signaling is required for perforant pathway LTP. Even in rat brain slices bathed in CaEDTA in ACSF, intracellular Zn²⁺ level, which was measured with intracellular ZnAF-2, was increased in the stratum lacunosum-moleculare where perforant pathway-CA1 pyramidal cell synapses were contained after tetanic stimulation. These results suggest that intracellular Zn²⁺ signaling, which originates in internal stores/proteins, is involved in LTP at perforant pathway-CA1 pyramidal cell synapses. Because the influx of extracellular Zn²⁺ , which originates in presynaptic Zn²⁺ release, is involved in LTP at Schaffer collateral-CA1 pyramidal cell synapses, synapse-dependent Zn²⁺ dynamics may be involved in plasticity of postsynaptic CA1 pyramidal cells.

In vitro and in vivo physiology of low nanomolar concentrations of Zn2+ in artificial cerebrospinal fluid

Artificial cerebrospinal fluid (ACSF), i.e., brain extracellular medium, which includes Ca²⁺ and Mg²⁺, but not other divalent cations such as Zn²⁺, has been used for in vitro and in vivo experiments. The present study deals with the physiological significance of extracellular Zn²⁺ in ACSF. Spontaneous presynaptic activity is suppressed in the stratum lucidum of brain slices from young rats bathed in ACSF containing 10 nM ZnCl₂, indicating that extracellular Zn²⁺ modifies hippocampal presynaptic activity. To examine the in vivo action of 10 nM ZnCl₂ on long-term potentiation (LTP), the recording region was perfused using a recording electrode attached to a microdialysis probe. The magnitude of LTP was not modified in young rats by perfusion with ACSF containing 10 nM ZnCl₂, compared to perfusion with ACSF without Zn²⁺, but attenuated by perfusion with ACSF containing 100 nM ZnCl₂. Interestingly, the magnitude of LTP was not modified in aged rats even by perfusion with ACSF containing 100 nM ZnCl₂, but enhanced by perfusion with ACSF containing 10 mM CaEDTA, an extracellular Zn²⁺ chelator. The present study indicates that the basal levels of extracellular Zn²⁺, which are in the range of low nanomolar concentrations, are critical for synaptic activity and perhaps increased age-dependently.

Maintained LTP and Memory Are Lost by Zn2+ Influx into Dentate Granule Cells, but Not Ca2+ Influx

The idea that maintained LTP and memory are lost by either increase in intracellular Zn²⁺ in dentate granule cells or increase in intracellular Ca²⁺ was examined to clarify significance of the increases induced by excess synapse excitation. Both maintained LTP and space memory were impaired by injection of high K⁺ into the dentate gyrus, but rescued by co-injection of CaEDTA, which blocked high K⁺-induced increase in intracellular Zn²⁺ but not high K⁺-induced increase in intracellular Ca²⁺. High K⁺-induced disturbances of LTP and intracellular Zn²⁺ are rescued by co-injection of 6-cyano-7-nitroquinoxakine-2,3-dione, an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist, but not by co-injection of blockers of NMDA receptors, metabotropic glutamate receptors, and voltage-dependent calcium channels. Furthermore, AMPA impaired maintained LTP and the impairment was also rescued by co-injection of CaEDTA, which blocked increase in intracellular Zn²⁺, but not increase in intracellular Ca²⁺. NMDA and glucocorticoid, which induced Zn²⁺ release from the internal stores, did not impair maintained LTP. The present study indicates that increase in Zn²⁺ influx into dentate granule cells through AMPA receptors loses maintained LTP and memory. Regulation of Zn²⁺ influx into dentate granule cells is more critical for not only memory acquisition but also memory retention than that of Ca²⁺ influx.

Innervation from the entorhinal cortex to the dentate gyrus and the vulnerability to Zn2

Hippocampal Zn²⁺ homeostasis is critical for cognitive activity and hippocampus-dependent memory. Extracellular Zn²⁺ signaling is linked to extracellular glutamate signaling and leads to intracellular Zn²⁺ signaling, which is involved in cognitive activity. On the other hand, excess intracellular Zn²⁺ signaling that is induced by excess glutamate signaling is involved in cognitive decline. In the hippocampal formation, the dentate gyrus is the most vulnerable to aging and is thought to contribute to age-related cognitive decline. The layer II of the entorhinal cortex is the most vulnerable to neuronal death in Alzheimer's disease. The perforant pathway provides input from the layer II to the dentate gyrus and is one of the earliest affected pathways in Alzheimer's disease. Medial perforant pathway-dentate granule cell synapses are vulnerable to either excess intracellular Zn²⁺ or β-amyloid (Aβ)-bound zinc, which induce transient cognitive decline via attenuation of medial perforant pathway LTP. However, it is unknown whether the vulnerability to excess intracellular Zn²⁺ is involved in region-specific vulnerability to aging and Alzheimer's disease. To discover a strategy to prevent short-term cognitive decline in normal aging process and the pre-dementia stage of Alzheimer's disease, the present paper deals with vulnerability of medial perforant pathway-dentate granule cell synapses to intracellular Zn²⁺ dyshomeostasis and its possible involvement in differential vulnerability to aging and Alzheimer's disease in the hippocampal formation.

Significance of synaptic Zn2+ signaling in zincergic and non-zincergic synapses in the hippocampus in cognition

A portion of zinc concentrates in the synaptic vesicles in the brain and is released from glutamatergic (zincergic) neuron terminals. It serves as a signaling factor (in a form of free Zn²⁺). Both extracellular Zn²⁺ signaling, which predominantly originates in Zn²⁺ release from zincergic neuron terminals, and intracellular Zn²⁺ signaling, which is often linked to extracellular Zn²⁺ signaling, are involved in hippocampus-dependent memory. At mossy fiber-CA3 pyramidal cell synapses and Schaffer collateral-CA1 pyramidal cell synapses, which are zincergic, extracellular Zn²⁺ signaling leads to intracellular Zn²⁺ signaling and is involved in learning and memory. At medial perforant pathway-dentate granule cell synapses, which are non-zincergic, intracellular Zn²⁺ signaling, which originates in the internal stores containing Zn²⁺, is involved in learning and memory. The blockade of Zn²⁺ signaling with Zn²⁺ chelators induces memory deficit, while the optimal amount range of Zn²⁺ signaling is unknown. It is possible that the degree and frequency of Zn²⁺ signaling, which determine the increased Zn²⁺ levels, modulates learning and memory as well as intracellular Ca²⁺ signaling. To understand the precise role of synaptic Zn²⁺ signaling in the hippocampus, the present paper summarizes the current knowledge on Zn²⁺ signaling at zincergic and non-zincergic synapses in the hippocampus in cognition and involvement of zinc transporters and zinc-binding proteins in synaptic Zn²⁺ signaling.

Selective Inducible Nitric Oxide Synthase Inhibitor Reversed Zinc Chloride-Induced Spatial Memory Impairment via Increasing Cholinergic Marker Expression

Zinc, an essential micronutrient and biochemical element of the human body, plays structural, catalytic, and regulatory roles in numerous physiological functions. In the current study, the effects of a pretraining oral administration of zinc chloride (10, 25, and 50 mg/kg) for 14 consecutive days and post-training bilateral intra-hippocampal infusion of 1400W as a selective inducible nitric oxide synthase (iNOS) inhibitor (10, 50, and 100 μM/side), alone and in combination, on the spatial memory retention in Morris water maze (MWM) were investigated. Animals were trained for 4 days and tested 48 h after completion of training. Also, the molecular effects of these compounds on the expression of choline acetyltransferase (ChAT), as a cholinergic marker in the CA1 region of the hippocampus and medial septal area (MSA), were evaluated. Behavioral and molecular findings of this study showed that a 2-week oral administration of zinc chloride (50 mg/kg) impaired spatial memory retention in MWM and decreased ChAT expression. Immunohistochemical analysis of post-training bilateral intra-hippocampal infusion of 1400W revealed a significant increase in ChAT immunoreactivity. Furthermore, post-training bilateral intra-hippocampal infusion of 1400W into the CA1 region of the hippocampus reversed zinc chloride-induced spatial memory impairment in MWM and significantly increased ChAT expression in comparison with zinc chloride-treated animals. Taken together, these results emphasize the role of selective iNOS inhibitors in reversing zinc chloride-induced spatial memory deficits via modulation of cholinergic marker expression.

GluN2B-containing NMDA receptors contribute to the beneficial effects of hydrogen sulfide on cognitive and synaptic plasticity deficits in APP/PS1 transgenic mice

Alzheimer's disease (AD) is the most common type of clinical dementia. Previous studies have demonstrated that hydrogen sulfide (H2S) is implicated with the pathology of AD, and exogenous H2S attenuates spatial memory impairments in AD animal models. However, the molecular mechanism by which H2S improves cognition in AD has not been fully explored. Here, we report that chronic administration of sodium hydrosulfide (NaHS, a H2S donor) elevated hippocampal H2S levels and enhanced hippocampus-dependent contextual fear memory and novel object recognition in amyloid precursor protein (APP)/presenilin-1 (PS1) transgenic mice. In parallel with these behavioral results, treating transgenic mice with NaHS reversed impaired hippocampal long-term potentiation (LTP), which is deemed as the neurobiological basis of learning and memory. At the molecular level, we found that treatment with NaHS did not affect the expression of the GluN1 and GluN2A subunits of NMDA receptor (NMDAR), but did prevent the downregulation of GluN2B subunit and restored its synaptic abundance, response and downstream signaling in the hippocampus in transgenic mice. Moreover, applying Ro 25-6981, a specific GluN2B antagonist, abolished the beneficial effects of NaHS on cognitive performance and hippocampal LTP in transgenic mice. Collectively, our results indicate that H2S can reverse cognitive and synaptic plasticity deficits in AD model mice by restoring surface GluN2B expression and the function of GluN2B-containing NMDARs.