Synaptic Zn2+ inhibits neurotransmitter release by promoting endocannabinoid synthesis. J Neurosci. 2013;33:9259-9272. [PMID: 23719795 DOI: 10.1523/jneurosci.0237-13.2013]

Chronic memantine disrupts spatial memory and up-regulates Htr1a gene expression in the hippocampus of GPR39 (zinc-sensing receptor) KO male mice

GPR39 is a receptor involved in zincergic neurotransmission, and its role in regulating psychological functions is an active area of research. The purported roles of GPR39 at the cellular level include regulation of inflammatory and oxidative stress response, and modulation of GABAergic and endocannabinoid neurotransmission. GPR39 knock-out (KO) mice exhibit episodic-like and spatial memory (ELM and SM, respectively) deficits throughout their lifetime, and are similar in that respect to senescent wild-type (WT) conspecifics. Since a role for zinc has been postulated in neurodegenerative disorders, in this study we investigated the possibility of a pharmacological rescue of both types of declarative memory with memantine - a noncompetitive NMDAR antagonist used for slowing down dementia; or, a putative GPR39 agonist - TC-G 1008. First, we tested adult WT and GPR39KO male mice under acute 5 mg/kg memantine or vehicle treatment in an object recognition task designed to simultaneously probe the "what?", "where?" and "when?" components of ELM. Next, we investigated the impact of chronic memantine or TC-G 1008 on ELM and SM (Morris water maze, MWM) in both WT and GPR39KO mice. Following chronic experiments, we assessed with qRT-PCR hippocampal gene expression of targets previously associated with GPR39. We report: no effects of acute memantine on ELM; a tendency to improve the "where?" component of ELM in both WT and GPR39 KO mice following 12 days of memantine; and, a disruption of SM in GPR39KO mice after 24 days of memantine treatment. The latter result was associated with upregulation of Htr1a hippocampal expression.

GPR39 Knockout Worsens Microcirculatory Response to Experimental Stroke in a Sex-Dependent Manner

No current treatments target microvascular reperfusion after stroke, which can contribute to poor outcomes even after successful clot retrieval. The G protein-coupled receptor GPR39 is expressed in brain peri-capillary pericytes, and has been implicated in microvascular regulation, but its role in stroke is unknown. We tested the hypothesis that GPR39 plays a protective role after stroke, in part due to preservation of microvascular perfusion. We generated GPR39 knockout (KO) mice and tested whether GPR39 gene deletion worsens capillary blood flow and exacerbates brain injury and functional deficit after focal cerebral ischemia. Stroke was induced in male and female GPR39 KO and WT littermates by 60-min middle cerebral artery occlusion (MCAO). Microvascular perfusion was assessed via capillary red blood cell (RBC) flux in deep cortical layers in vivo using optical microangiography (OMAG). Brain injury was assessed by measuring infarct size by 2,3,5-triphenyltetrazolium chloride staining at 24 h or brain atrophy at 3 weeks after ischemia. Pole and cylinder behavior tests were conducted to assess neurological function deficit at 1 and 3 weeks post-stroke. Male but not female GPR39 KO mice exhibited larger infarcts and lower capillary RBC flux than WT controls after stroke. Male GPR39 KO mice also exhibited worse neurologic deficit at 1 week post-stroke, though functional deficit disappeared in both groups by 3 weeks. GPR39 deletion worsens brain injury, microvascular perfusion, and neurological function after experimental stroke. Results indicate that GPR39 plays a sex-dependent role in re-establishing microvascular flow and limiting ischemic brain damage after stroke.

Synaptic zinc potentiates AMPA receptor function in mouse auditory cortex

Synaptic zinc signaling modulates synaptic activity and is present in specific populations of cortical neurons, suggesting that synaptic zinc contributes to the diversity of intracortical synaptic microcircuits and their functional specificity. To understand the role of zinc signaling in the cortex, we performed whole-cell patch-clamp recordings from intratelencephalic (IT)-type neurons and pyramidal tract (PT)-type neurons in layer 5 of the mouse auditory cortex during optogenetic stimulation of specific classes of presynaptic neurons. Our results show that synaptic zinc potentiates AMPA receptor (AMPAR) function in a synapse-specific manner. We performed in vivo 2-photon calcium imaging of the same classes of neurons in awake mice and found that changes in synaptic zinc can widen or sharpen the sound-frequency tuning bandwidth of IT-type neurons but only widen the tuning bandwidth of PT-type neurons. These results provide evidence for synapse- and cell-type-specific actions of synaptic zinc in the cortex.

A CRE/DRE dual recombinase transgenic mouse reveals synaptic zinc-mediated thalamocortical neuromodulation

Synaptic zinc is a neuromodulator that shapes synaptic transmission and sensory processing. The maintenance of synaptic zinc is dependent on the vesicular zinc transporter, ZnT3. Hence, the ZnT3 knockout mouse has been a key tool for studying the mechanisms and functions of synaptic zinc. However, the use of this constitutive knockout mouse has notable limitations, including developmental, compensatory, and brain and cell type specificity issues. To overcome these limitations, we developed and characterized a dual recombinase transgenic mouse, which combines the Cre and Dre recombinase systems. This mouse allows for tamoxifen-inducible Cre-dependent expression of exogenous genes or knockout of floxed genes in ZnT3-expressing neurons and DreO-dependent region and cell type-specific conditional ZnT3 knockout in adult mice. Using this system, we reveal a neuromodulatory mechanism whereby zinc release from thalamic neurons modulates N-methyl-d-aspartate receptor activity in layer 5 pyramidal tract neurons, unmasking previously unknown features of cortical neuromodulation.

Zinc and Central Nervous System Disorders

Zinc (Zn²⁺) is the second most abundant necessary trace element in the human body, exerting a critical role in many physiological processes such as cellular proliferation, transcription, apoptosis, growth, immunity, and wound healing. It is an essential catalyst ion for many enzymes and transcription factors. The maintenance of Zn²⁺ homeostasis is essential for the central nervous system, in which Zn²⁺ is abundantly distributed and accumulates in presynaptic vesicles. Synaptic Zn²⁺ is necessary for neural transmission, playing a pivotal role in neurogenesis, cognition, memory, and learning. Emerging data suggest that disruption of Zn²⁺ homeostasis is associated with several central nervous system disorders including Alzheimer's disease, depression, Parkinson's disease, multiple sclerosis, schizophrenia, epilepsy, and traumatic brain injury. Here, we reviewed the correlation between Zn²⁺ and these central nervous system disorders. The potential mechanisms were also included. We hope that this review can provide new clues for the prevention and treatment of nervous system disorders.

TC-G 1008 facilitates epileptogenesis by acting selectively at the GPR39 receptor but non-selectively activates CREB in the hippocampus of pentylenetetrazole-kindled mice

The pharmacological activation of the GPR39 receptor has been proposed as a novel strategy for treating seizures; however, this hypothesis has not been verified experimentally. TC-G 1008 is a small molecule agonist increasingly used to study GPR39 receptor function but has not been validated using gene knockout. Our aim was to assess whether TC-G 1008 produces anti-seizure/anti-epileptogenic effects in vivo and whether the effects are mediated by GPR39. To obtain this goal we utilized various animal models of seizures/epileptogenesis and GPR39 knockout mice model. Generally, TC-G 1008 exacerbated behavioral seizures. Furthermore, it increased the mean duration of local field potential recordings in response to pentylenetetrazole (PTZ) in zebrafish larvae. It facilitated the development of epileptogenesis in the PTZ-induced kindling model of epilepsy in mice. We demonstrated that TC-G 1008 aggravated PTZ-epileptogenesis by selectively acting at GPR39. However, a concomitant analysis of the downstream effects on the cyclic-AMP-response element binding protein in the hippocampus of GPR39 knockout mice suggested that the molecule also acts via other targets. Our data argue against GPR39 activation being a viable therapeutic strategy for treating epilepsy and suggest investigating whether TC-G 1008 is a selective agonist of the GPR39 receptor.

The role of GPR39 zinc receptor in the modulation of glutamatergic and GABAergic transmission

BACKGROUND

Despite our poor understanding of the pathophysiology of depression, a growing body of evidence indicates the role of both glutamate and gamma-aminobutyric acid (GABA) signaling behind the effects of rapid-acting antidepressants (RAADs). GPR39 is a zinc-sensing receptor whose activation leads to a prolonged antidepressant-like response in mice. Both GPR39 and zinc can modulate glutamatergic and GABAergic neurotransmission, however, exact molecular mechanisms are still elusive. In this study, we aimed to research the role of glutamatergic and GABAergic system activation in TC-G 1008 antidepressant-like effects and the disruptions in this effect caused by a low-zinc diet.

METHODS

In the first part of our study, we investigated the role of joint administration of the GPR39 agonist (TC-G 1008) and ligands of the glutamatergic or GABAergic systems, in antidepressant-like response. To evaluate animal behaviour we used the forced swim test in mice. In the second part of the study, we assessed the effectiveness of TC-G 1008-induced antidepressant-like response in conditions of decreased dietary zinc intake and its molecular underpinning by conducting a Western Blot analysis of selected proteins involved in glutamatergic and GABAergic neurotransmission.

RESULTS

The TC-G 1008-induced effect was blocked by the administration of NMDA or picrotoxin. The joint administration of TC-G 1008 along with muscimol or SCH50911 showed a trend toward decreased immobility time. Zinc-deficient diet resulted in dysregulation of GluN1, PSD95, and KCC2 protein expression.

CONCLUSIONS

Our findings indicate the important role of glutamate/GABA signaling in the antidepressant-like effect of TC-G 1008 and imply that GPR39 regulates the balance between excitatory and inhibitory activity in the brain. Thus, we suggest the zinc-sensing receptor be considered an interesting new target for the development of novel antidepressants.

A genetically encoded far-red fluorescent indicator for imaging synaptically released Zn2

Synaptic zinc ion (Zn²⁺) has emerged as a key neuromodulator in the brain. However, the lack of research tools for directly tracking synaptic Zn²⁺ in the brain of awake animals hinders our rigorous understanding of the physiological and pathological roles of synaptic Zn²⁺. In this study, we developed a genetically encoded far-red fluorescent indicator for monitoring synaptic Zn²⁺ dynamics in the nervous system. Our engineered far-red fluorescent indicator for synaptic Zn²⁺ (FRISZ) displayed a substantial Zn²⁺-specific turn-on response and low-micromolar affinity. We genetically anchored FRISZ to the mammalian extracellular membrane via a transmembrane (TM) ⍺ helix and characterized the resultant FRISZ-TM construct at the mammalian cell surface. We used FRISZ-TM to image synaptic Zn²⁺ in the auditory cortex in acute brain slices and awake mice in response to electric and sound stimuli, respectively. Thus, this study establishes a technology for studying the roles of synaptic Zn²⁺ in the nervous system.

The Zinc-sensing Receptor (GPR39) Modulates Declarative Memory and Age-related Hippocampal Gene Expression in Male Mice

As a neuromodulator, zinc regulates synaptic plasticity, learning and memory. Synaptic zinc is also a crucial factor in the development of toxic forms of amyloid beta protein and, subsequently, of Alzheimer's dementia (AD). Therefore, efforts to pinpoint mechanisms underlying zinc-dependent cognitive functions might aid AD research, by providing potential novel targets for drugs. One of the most understudied proteins in this regard is a zinc-sensing metabotropic receptor: GPR39. In this study we investigated the impact of GPR39 knock-out (KO) on age-related memory decline in mice of both sexes, by comparing them to age-matched wild-type (WT) littermates. We also tested the effects of a GPR39 agonist (TC-G 1008) on declarative memory of old animals, and its disruption in adult mice. We observed episodic-like memory (ELM) and spatial memory (SM) deficits in male GPR39 KO mice, as well as intact procedural memory in GPR39 KO mice regardless of age and sex. ELM was also absent in old WT male mice, and all female mice regardless of their genotype. Acute application of TC-G 1008 (10 mg/kg) reversed a deficit in two of three ELM components in old WT male mice, and had no promnesic effect on consolidation interference of ELM in adult WT mice. We discuss the possible neurobiological mechanisms and the translational value of these results for potential add-on pharmacotherapy of AD aimed at the zinc-sensing receptor.

Neuronal signalling of zinc: from detection and modulation to function

Zinc is an essential trace element that stabilizes protein structures and allosterically modulates a plethora of enzymes, ion channels and neurotransmitter receptors. Labile zinc (Zn²⁺) acts as an intracellular and intercellular signalling molecule in response to various stimuli, which is especially important in the central nervous system. Zincergic neurons, characterized by Zn²⁺ deposits in synaptic vesicles and presynaptic Zn²⁺ release, are found in the cortex, hippocampus, amygdala, olfactory bulb and spinal cord. To provide an overview of synaptic Zn²⁺ and intracellular Zn²⁺ signalling in neurons, the present paper summarizes the fluorescent sensors used to detect Zn²⁺ signals, the cellular mechanisms regulating the generation and buffering of Zn²⁺ signals, as well as the current perspectives on their pleiotropic effects on phosphorylation signalling, synapse formation, synaptic plasticity, as well as sensory and cognitive function.

Role of GPR39 in Neurovascular Homeostasis and Disease

GPR39, a member of the ghrelin family of G protein-coupled receptors, is zinc-responsive and contributes to the regulation of diverse neurovascular and neurologic functions. Accumulating evidence suggests a role as a homeostatic regulator of neuronal excitability, vascular tone, and the immune response. We review GPR39 structure, function, and signaling, including constitutive activity and biased signaling, and summarize its expression pattern in the central nervous system. We further discuss its recognized role in neurovascular, neurological, and neuropsychiatric disorders.

The Zinc-Sensing Receptor GPR39 in Physiology and as a Pharmacological Target

The G-protein coupled receptor GPR39 is abundantly expressed in various tissues and can be activated by changes in extracellular Zn²⁺ in physiological concentrations. Previously, genetically modified rodent models have been able to shed some light on the physiological functions of GPR39, and more recently the utilization of novel synthetic agonists has led to the unraveling of several new functions in the variety of tissues GPR39 is expressed. Indeed, GPR39 seems to be involved in many important metabolic and endocrine functions, but also to play a part in inflammation, cardiovascular diseases, saliva secretion, bone formation, male fertility, addictive and depression disorders and cancer. These new discoveries offer opportunities for the development of novel therapeutic approaches against many diseases where efficient therapeutics are still lacking. This review focuses on Zn²⁺ as an endogenous ligand as well as on the novel synthetic agonists of GPR39, placing special emphasis on the recently discovered physiological functions and discusses their pharmacological potential.

Interaction between zinc, the GPR39 zinc receptor and the serotonergic system in depression

Zinc signalling has a crucial impact on the proper functioning of the brain. Disturbances within the zincergic system may lead to neuropsychological disorders, including major depression. Studying this disease and designing effective treatment is hampered by its heterogeneous etiology and the diversified nature of the symptoms. Over the years, studies have shown that zinc deficiency and disturbances in the expression profile of the zinc receptor - GPR39 - might be a useful neurobiological indicator of a pathological state. Zinc levels and the zinc receptor are altered by classic antidepressant treatment, which indicates possible reciprocity between the monoaminergic system and zinc signalling. Disruptions in this specific interplay might be a cause of a pathological depressive state, and restoring balance and cooperation between those systems might be key to a successful form of pharmacotherapy. In this review, we aim to describe interactions between the serotonergic and zincergic systems and to highlight their significance in the pathophysiology and treatment of depression.

The Function and Regulation of Zinc in the Brain

Nearly sixty years ago Fredrich Timm developed a histochemical technique that revealed a rich reserve of free zinc in distinct regions of the brain. Subsequent electron microscopy studies in Timm- stained brain tissue found that this "labile" pool of cellular zinc was highly concentrated at synaptic boutons, hinting a possible role for the metal in synaptic transmission. Although evidence for activity-dependent synaptic release of zinc would not be reported for another twenty years, these initial findings spurred decades of research into zinc's role in neuronal function and revealed a diverse array of signaling cascades triggered or regulated by the metal. Here, we delve into our current understanding of the many roles zinc plays in the brain, from influencing neurotransmission and sensory processing, to activating both pro-survival and pro-death neuronal signaling pathways. Moreover, we detail the many mechanisms that tightly regulate cellular zinc levels, including metal binding proteins and a large array of zinc transporters.

Disruption of zinc transporter ZnT3 transcriptional activity and synaptic vesicular zinc in the brain of Huntington's disease transgenic mouse

Background

Huntington’s disease (HD) is a neurodegenerative disease that involves a complex combination of psychiatric, cognitive and motor impairments. Synaptic dysfunction has been implicated in HD pathogenesis. However, the mechanisms have not been clearly delineated. Synaptic vesicular zinc is closely linked to modulating synaptic transmission and maintaining cognitive ability. It is significant to assess zinc homeostasis for further revealing the pathogenesis of synaptic dysfunction and cognitive impairment in HD.

Results

Histochemical staining by autometallography indicated that synaptic vesicular zinc was decreased in the hippocampus, cortex and striatum of N171-82Q HD transgenic mice. Analyses by immunohistochemistry, Western blot and RT-PCR found that the expression of zinc transporter 3 (ZnT3) required for transport of zinc into synaptic vesicles was obviously reduced in these three brain regions of the HD mice aged from 14 to 20 weeks  and  BHK  cells  expressing  mutant  huntingtin. Significantly, dual-luciferase reporter gene and chromatin immunoprecipitation assays demonstrated that transcription factor Sp1 could activate ZnT3 transcription via its binding to the GC boxes in ZnT3 promoter. Moreover, mutant huntingtin was found to inhibit the binding of Sp1 to the promoter of ZnT3 and down-regulate ZnT3 expression, and the decline in ZnT3 expression could be ameliorated through overexpression of Sp1.

Conclusions

This is first study to reveal a significant loss of synaptic vesicular zinc and a decline in ZnT3 transcriptional activity in the HD transgenic mice. Our work sheds a novel mechanistic insight into pathogenesis of HD that mutant huntingtin down-regulates expression of ZnT3 through inhibiting binding of Sp1 to the promoter of ZnT3 gene, causing disruption of synaptic vesicular zinc homeostasis. Disrupted vesicular zinc ultimately leads to early synaptic dysfunction and cognitive deficits in HD. It is also suggested that maintaining normal synaptic vesicular zinc concentration is a potential therapeutic strategy for HD.

Micromolar concentrations of Zn2+ depress cellular excitability through a blockade of calcium current in rat adrenal slices

The present work, using chromaffin cells in rat adrenal slices (RCCs), aims to describe what type of ionic current alterations induced by zinc underlies their effects reported on synaptic transmission. Thus, Zn²⁺ blocked calcium channels of RCCs in a time- and concentration-dependent manner with an IC₅₀ of 391 μM. This blockade was partially reversed upon washout and was greater at more depolarizing holding potentials (i.e. 32 ± 5% at -110 mV, and 43 ± 6% at -50 mV, after 5 min perfusion). In ω-toxins-sensitive calcium channels (N-, P- and Q-types), Zn²⁺caused a lower blockade of I_Ca, 33.3%, than in ω-toxins-resistant ones (L-type, 55.3%; and R-type, 90%). This compound inhibited calcium current at all test potentials and shows a shift of the I-V curve to more depolarized values of about 10 mV. The sodium current was not blocked by acute application of high Zn²⁺concentrations. Voltage-dependent potassium current was marginally affected by high Zn²⁺ concentrations showing no concentration-dependence. Nevertheless, calcium- and voltage-dependent potassium current was drastically depressed in a dose-dependent manner, with an IC₅₀ of 453 μM. This blockade was related to the prevention of Ca²⁺ influx through voltage-dependent calcium channels coupled to BK channels. Under current-clamp conditions, RCCs exhibit a resting potential of -50.7 mV, firing spontaneous APs (1-2 spikes/s) generated by the opening of Na⁺ and Ca²⁺-channels, and terminated by the activation of voltage and Ca²⁺-activated K⁺-channels (BK). We found that the blockade of these ionic currents by Zn²⁺ led to a drastic alteration of cellular excitability with a depolarization of the membrane potential, the slowdown and broadening of the APs and the severe reduction of the after hyperpolarization (AHP) which led to a decrease in the APs firing frequency. Taken together, these results point to a neurotoxic action evoked by zinc that is associated with changes to cellular excitability by blocking the ionic currents responsible for both the neurotransmitter release and the action potentials firing.

Synaptic zinc inhibition of NMDA receptors depends on the association of GluN2A with the zinc transporter ZnT1

The NMDA receptor (NMDAR) is inhibited by synaptically released zinc. This inhibition is thought to be the result of zinc diffusion across the synaptic cleft and subsequent binding to the extracellular domain of the NMDAR. However, this model fails to incorporate the observed association of the highly zinc-sensitive NMDAR subunit GluN2A with the postsynaptic zinc transporter ZnT1, which moves intracellular zinc to the extracellular space. Here, we report that disruption of ZnT1-GluN2A association by a cell-permeant peptide strongly reduced NMDAR inhibition by synaptic zinc in mouse dorsal cochlear nucleus synapses. Moreover, synaptic zinc inhibition of NMDARs required postsynaptic intracellular zinc, suggesting that cytoplasmic zinc is transported by ZnT1 to the extracellular space in close proximity to the NMDAR. These results challenge a decades-old dogma on how zinc inhibits synaptic NMDARs and demonstrate that presynaptic release and a postsynaptic transporter organize zinc into distinct microdomains to modulate NMDAR neurotransmission.

GPR39 protects against corticosterone-induced neuronal injury in hippocampal cells through the CREB-BDNF signaling pathway

BACKGROUND

The release of zinc from glutamatergic terminals in the hippocampal CA3 region can activate postsynaptic GPR39 receptors and regulate cognition and depression. However, the role and mechanism of GPR39 in the stress-induced depression is still poorly understood.

METHODS

In this study, hippocampal cells (HT-22) were treated with corticosterone (CORT). Then the effects of stress on the activity, mitochondrial function and apoptosis of HT-22 cells were observed. The effects of GPR39 on CORT-induced stress injury were analyzed by both siRNA and agonist (TC-G-1008).

RESULTS

Compared with the 500 nM CORT group, the cell viability, apoptosis, mitochondrial membrane potential, and expression levels of BCL-2, CREB and BDNF mRNA were significantly decreased in the GPR39 siRNA+500 nM CORT group, while the expression levels of caspase3, caspase9, AIF and BAX mRNA were significantly increased in the GPR39 siRNA+500 nM CORT group. Compared with the 1 μM CORTgroup, the cell viability, apoptosis, mitochondrial membrane potential, and expression levels of BCL-2, CREB and BDNF were significantly increased in the GPR39 agonist+1 μΜ CORT group, while the expression levels of caspase3, caspase9, AIF and BAX mRNA were significantly decreased in the GPR39 siRNA+500 nM CORT group. Compared with the control group, the mRNA and protein levels of GPR39, CREB and BDNF were significantly increased, and the mRNA and protein levels of CREB and BDNF were significantly decreased after 50 μM zinc sulfate treatment for 6 h.

CONCLUSIONS

GPR39 may play a neuroprotective role in CORT-induced cell injury via the improvement of CREB-BDNF expression, by inhibiting pro-apoptotic proteins and by upregulating anti-apoptotic proteins.

Mechanisms Underlying Long-Term Synaptic Zinc Plasticity at Mouse Dorsal Cochlear Nucleus Glutamatergic Synapses

In many brain areas, such as the neocortex, limbic structures, and auditory brainstem, synaptic zinc is released from presynaptic terminals to modulate neurotransmission. As such, synaptic zinc signaling modulates sensory processing and enhances acuity for discrimination of different sensory stimuli. Whereas sensory experience causes long-term changes in synaptic zinc signaling, the mechanisms underlying this long-term synaptic zinc plasticity remain unknown. To study these mechanisms in male and female mice, we used in vitro and in vivo models of zinc plasticity observed at the zinc-rich glutamatergic dorsal cochlear nucleus (DCN) parallel fiber synapses onto cartwheel cells. High-frequency stimulation of DCN parallel fiber synapses induced LTD of synaptic zinc signaling (Z-LTD), evidenced by reduced zinc-mediated inhibition of EPSCs. Low-frequency stimulation induced LTP of synaptic zinc signaling (Z-LTP), evidenced by enhanced zinc-mediated inhibition of EPSCs. Pharmacological manipulations of Group 1 metabotropic glutamate receptors (G1 mGluRs) demonstrated that G1 mGluR activation is necessary and sufficient for inducing Z-LTD and Z-LTP. Pharmacological manipulations of Ca²⁺ dynamics indicated that rises in postsynaptic Ca²⁺ are necessary and sufficient for Z-LTD induction. Electrophysiological measurements assessing postsynaptic expression mechanisms, and imaging studies with a ratiometric extracellular zinc sensor probing zinc release, supported that Z-LTD is expressed, at least in part, via reductions in presynaptic zinc release. Finally, exposure of mice to loud sound caused G1 mGluR-dependent Z-LTD at DCN parallel fiber synapses, thus validating our in vitro results. Together, our results reveal a novel mechanism underlying activity- and experience-dependent plasticity of synaptic zinc signaling.SIGNIFICANCE STATEMENT In the neocortex, limbic structures, and auditory brainstem, glutamatergic nerve terminals corelease zinc to modulate excitatory neurotransmission and sensory responses. Moreover, sensory experience causes bidirectional, long-term changes in synaptic zinc signaling. However, the mechanisms of this long-term synaptic zinc plasticity remain unknown. Here, we identified a novel Group 1 mGluR-dependent mechanism that causes bidirectional, long-term changes in synaptic zinc signaling. Our results highlight new mechanisms of brain adaptation during sensory processing, and potentially point to mechanisms of disorders associated with pathologic adaptation, such as tinnitus.

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.

Zinc Modulates Olfactory Bulb Kainate Receptors

Kainate receptors (KARs) are glutamate receptors with ionotropic and metabotropic activity composed of the GluK1-GluK5 subunits. We previously reported that KARs modulate excitatory and inhibitory transmission in the olfactory bulb (OB). Zinc, which is highly concentrated in the OB, also appears to modulate OB synaptic transmission via actions at other ionotropic glutamate receptors (i.e., AMPA, NMDA). However, few reports of effects of zinc on recombinant and/or native KARs exist and none have involved the OB. In the present study, we investigated the effects of exogenously applied zinc on OB KARs expressed by mitral/tufted (M/T) cells. We found that 100 µM zinc inhibits currents evoked by various combinations of KAR agonists (kainate or SYM 2081) and the AMPA receptor antagonist SYM 2206. The greatest degree of zinc-mediated inhibition was observed with coapplication of zinc with the GluK1- and GluK2-preferring agonist SYM 2081 plus SYM 2206. This finding is consistent with prior reports of zinc's inhibitory effects on some recombinant (homomeric GluK1 and GluK2 and heteromeric GluK2/GluK4 and GluK2/GluK5) KARs, although potentiation of other (GluK3, GluK2/3) KARs has also been described. It is also of potential importance given our previously reported molecular data suggesting that OB neurons express relatively high levels of GluK1 and GluK2. Our present findings suggest that a physiologically relevant concentration of zinc modulates KARs expressed by M/T cells. As M/T cells are targets of zinc-containing olfactory sensory neurons, synaptically released zinc may influence odor information-encoding synaptic circuits in the OB via actions at KARs.

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

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

Modulation of Gpr39, a G-protein coupled receptor associated with alcohol use in non-human primates, curbs ethanol intake in mice

Alcohol use disorder (AUD) is a chronic condition with devastating health and socioeconomic effects. Still, pharmacotherapies to treat AUD are scarce. In a prior study aimed at identifying novel AUD therapeutic targets, we investigated the DNA methylome of the nucleus accumbens core (NAcc) of rhesus macaques after chronic alcohol use. The G-protein coupled receptor 39 (GPR39) gene was hypermethylated and its expression downregulated in heavy alcohol drinking macaques. GPR39 encodes a Zn2+-binding metabotropic receptor known to modulate excitatory and inhibitory neurotransmission, the balance of which is altered in AUD. These prior findings suggest that a GPR39 agonist would reduce alcohol intake. Using a drinking-in-the-dark two bottle choice (DID-2BC) model, we showed that an acute 7.5 mg/kg dose of the GPR39 agonist, TC-G 1008, reduced ethanol intake in mice without affecting total fluid intake, locomotor activity or saccharin preference. Furthermore, repeated doses of the agonist prevented ethanol escalation in an intermittent access 2BC paradigm (IA-2BC). This effect was reversible, as ethanol escalation followed agonist "wash out". As observed during the DID-2BC study, a subsequent acute agonist challenge during the IA-2BC procedure reduced ethanol intake by ~47%. Finally, Gpr39 activation was associated with changes in Gpr39 and Bdnf expression, and in glutamate release in the NAcc. Together, our findings suggest that GPR39 is a promising target for the development of prevention and treatment therapies for AUD.

Long-lasting antidepressant-like activity of the GPR39 zinc receptor agonist TC-G 1008

BACKGROUND

The discovery of the zinc-sensing receptor, has provided new possibilities for explaining the neurobiology of zinc. Recent studies indicate that the GPR39 zinc receptor may play an important role in the pathogenesis of depression as well as in the antidepressant mechanism of action.

METHODS

In this study we evaluated the time-course of the antidepressant response of the GPR39 agonist (TC-G 1008), imipramine, ZnCl₂ and MK-801 in the forced swim test in mice 30 min, 3 h, 6 h and 24 h after acute drug administration as well as after 14-day treatment. Zinc level was measured in serum of mice. BDNF protein level was evaluated in hippocampus following both acute and chronic TC-G 1008 treatment.

RESULTS

A single administration of the GPR39 agonist caused an antidepressant-like effect lasting up to 24 h following the injection, which is longer than the effect of imipramine, ZnCl₂ and MK-801. Chronic treatment with these compounds caused a decrease in immobility time in the FST. Serum zinc concentrations showed an increased level following chronic ZnCl₂ administration, but not following administration of TC-G 1008, imipramine or MK-801. We also observed some tendencies for increased BDNF following acute TC-G 1008 treatment.

LIMITATIONS

TC-G 1008 is new drug designed to study GPR39 therefore additional pharmacodynamic and pharmacokinetic properties in preclinical studies are required.

CONCLUSION

This study shows for the first time the long-lasting antidepressant effect of the GPR39 agonist in comparison with imipramine, ZnCl2 and MK-801. Our findings suggest that GPR39 should be considered as a target in efforts to develop new antidepressant drugs.

Fine Control of Sound Frequency Tuning and Frequency Discrimination Acuity by Synaptic Zinc Signaling in Mouse Auditory Cortex

Neurons in the auditory cortex are tuned to specific ranges of sound frequencies. Although the cellular and network mechanisms underlying neuronal sound frequency selectivity are well studied and reflect the interplay of thalamocortical and intracortical excitatory inputs and further refinement by cortical inhibition, the precise synaptic signaling mechanisms remain less understood. To gain further understanding on these mechanisms and their effects on sound-driven behavior, we used in vivo imaging as well as behavioral approaches in awake and behaving female and male mice. We discovered that synaptic zinc, a modulator of neurotransmission and responsiveness to sound, sharpened the sound frequency tuning of principal and parvalbumin-expressing neurons and widened the sound frequency tuning of somatostatin-expressing inhibitory neurons in layer 2/3 of the primary auditory cortex. In the absence of cortical synaptic zinc, mice exhibited reduced acuity for detecting changes in sound frequencies. Together, our results reveal that cell-type-specific effects of zinc contribute to cortical sound frequency tuning and enhance acuity for sound frequency discrimination.SIGNIFICANCE STATEMENT Neuronal tuning to specific features of sensory stimuli is a fundamental property of cortical sensory processing that advantageously supports behavior. Despite the established roles of synaptic thalamocortical and intracortical excitation and inhibition in cortical tuning, the precise synaptic signaling mechanisms remain unknown. Here, we investigated these mechanisms in the mouse auditory cortex. We discovered a previously unknown signaling mechanism linking synaptic zinc signaling with cell-specific cortical tuning and enhancement in sound frequency discrimination acuity. Given the abundance of synaptic zinc in all sensory cortices, this newly discovered interaction between synaptic zinc and cortical tuning can provide a general mechanism for modulating neuronal stimulus specificity and sensory-driven behavior.

Zinc signaling and epilepsy

Evidence from both preclinical and clinical studies suggest the importance of zinc homeostasis in seizures/epilepsy. Undoubtedly, zinc, via modulation of a variety of targets, is necessary for maintaining the balance between neuronal excitation and inhibition, while an imbalance between excitation and inhibition underlies seizures. However, the relationship between zinc signaling and seizures/epilepsy is complex as both extracellular and intracellular zinc may produce either protective or detrimental effects. This review provides an overview of preclinical/behavioral, functional and molecular studies, as well as clinical data on the involvement of zinc in the pathophysiology and treatment of seizures/epilepsy. Furthermore, the potential of targeting elements associated with zinc signaling or homeostasis and zinc levels as a therapeutic strategy for epilepsy is discussed.

How cellular Zn2+ signaling drives physiological functions

Zinc is an essential micronutrient affecting many aspects of human health. Cellular Zn²⁺ homeostasis is critical for cell function and survival. Zn²⁺, acting as a first or second messenger, triggers signaling pathways that mediate the physiological roles of Zn²⁺. Transient changes in Zn²⁺ concentrations within the cell or in the extracellular region occur following its release from Zn²⁺ binding metallothioneins, its transport across membranes by the ZnT or ZIP transporters, or release of vesicular Zn²⁺. These transients activate a distinct Zn²⁺ sensing receptor, ZnR/GPR39, or modulate numerous proteins and signaling pathways. Importantly, Zn²⁺ signaling regulates cellular physiological functions such as: proliferation, differentiation, ion transport and secretion. Indeed, novel therapeutic approaches aimed to maintain Zn²⁺ homeostasis and signaling are evolving. This review focuses on recent findings describing roles of Zn²⁺ and its transporters in regulating physiological or pathological processes.

Phyto and endocannabinoids exert complex actions on calcium and zinc signaling in mouse cortical neurons

Live-cell imaging experiments were performed with the fluorescent Ca²⁺ and Zn²⁺ probes Fluo-4 and FluoZin-3 on cultured cortical neurons dissociated from embryonic mice to investigate the effects of the cannabinoids anandamide (AEA), cannabidiol (CBD), and N-arachidonoyl glycine (NAGly) on neuronal store-operated Ca²⁺ entry (SOCE). When tested individually AEA, CBD or NAGly inhibited SOCE. CBD and NAGly also released Ca²⁺ from the endoplasmic reticulum. Furthermore, NAGly mobilized Zn²⁺ from a store distinct from the endoplasmic reticulum and mitochondria, and up-regulated the thapsigargin-evoked Ca²⁺ release. All these effects developed in a cannabinoid receptor CB1/2 independent manner via an intracellular pathway sensitive to the GPR55 antagonist ML193. Evidence is presented that cannabinoids influence Ca²⁺ and Zn²⁺ signaling in central nervous system neurons. The lipid sensing receptor GPR55 seems to be a central actor governing these responses. In addition, the alteration of the cytosolic Zn²⁺ levels produced by NAGly provides support for the existence of a connection between endocannabinoids and Zn²⁺ signaling in the brain.

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

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

Orphan G protein-coupled receptors: The role in CNS disorders

There are various types of receptors in the central nervous system (CNS). G protein-coupled receptors (GPCRs) have the highest expression with a wide range of physiological functions. A newer sub group of these receptors namely orphan GPCRs have been discovered. GPR3, GPR6, GPR17, GPR26, GPR37, GPR39, GPR40, GPR50, GPR52, GPR54, GPR55, GPR85, GPR88, GPR103, and GPR139 are the selected orphan GPCRs for this article. Their roles in the central nervous system have not been understood well so far. However, recent studies show that they may have very important functions in the CNS. Hence, in the present study, we reviewed most recent findings regarding the physiological roles of the selected orphan GPCRs in the CNS. After a brief presentation of each receptor, considering the results from genetic and pharmacological manipulation of the receptors, their roles in the pathophysiology of different diseases and disorders including anxiety, depression, schizophrenia, epilepsy, Alzheimer's disease, Parkinson's disease, and substance abuse will be discussed. At present, our knowledge regarding the role of GPCRs in the brain is very limited. However, previous limited studies show that orphan GPCRs have an important place in psychopharmacology and these receptors are potential new targets for the treatment of major CNS diseases.

Signaling by Synaptic Zinc is Required for Whisker-Mediated, Fine Texture Discrimination

Zinc-containing terminals are found throughout the neocortex, concentrated predominantly in layers II/III, V, and VI. Synaptic zinc is a potent neurotransmitter/modulator and, therefore, may mediate inter- or intra-cortical integration of sensory information. We have previously shown that levels of synaptic zinc are rapidly modulated in somatosensory (barrel) cortex, in an experience- and activity-dependent manner. Zinc transporter 3 (ZnT3) knockout (KO) mice lack synaptic zinc and provide us with a good model to examine the contribution of synaptic zinc to barrel cortex-dependent behavior. In the present study, we show that ZnT3 KO mice display a marked decrease in acuity for whisker-dependent texture discrimination. ZnT3 KO mice were not able to discriminate between textures having an average particle diameter less than 300 μm while control mice were able to discriminate between textures having particle diameters separated by as little as 25 μm. This loss of texture discrimination acuity in ZnT3 KO mice was whisker-dependent and was observed in young (2 months-of-age) and older mice (12 months-of-age). These results show that zincergic signaling is necessary for the normal integration of somatosensory information.

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.

Cell-specific gain modulation by synaptically released zinc in cortical circuits of audition

In many excitatory synapses, mobile zinc is found within glutamatergic vesicles and is coreleased with glutamate. Ex vivo studies established that synaptically released (synaptic) zinc inhibits excitatory neurotransmission at lower frequencies of synaptic activity but enhances steady state synaptic responses during higher frequencies of activity. However, it remains unknown how synaptic zinc affects neuronal processing in vivo. Here, we imaged the sound-evoked neuronal activity of the primary auditory cortex in awake mice. We discovered that synaptic zinc enhanced the gain of sound-evoked responses in CaMKII-expressing principal neurons, but it reduced the gain of parvalbumin- and somatostatin-expressing interneurons. This modulation was sound intensity-dependent and, in part, NMDA receptor-independent. By establishing a previously unknown link between synaptic zinc and gain control of auditory cortical processing, our findings advance understanding about cortical synaptic mechanisms and create a new framework for approaching and interpreting the role of the auditory cortex in sound processing.

Context-Dependent Modulation of Excitatory Synaptic Strength by Synaptically Released Zinc

Synaptically released zinc inhibits baseline excitatory neurotransmission; however, the role of this neuromodulator on short-term plasticity during different levels of synaptic activity remains largely unknown. This lack of knowledge prevents our understanding of information transfer across zinc-releasing synapses, including 50% of excitatory synapses in cortical areas. We used in vitro electrophysiology in mouse brain slices and discovered that the effects of zinc on excitatory postsynaptic current (EPSC) amplitudes are context-dependent. At lower frequencies of activity, synaptically released zinc reduces EPSC amplitudes. In contrast, at higher stimulation frequencies and vesicular release probability (Pr), zinc inhibits EPSC amplitudes during the first few stimuli but leads to enhanced steady-state EPSC amplitudes during subsequent stimuli. This paradoxical enhancement is due to zinc-dependent potentiation of synaptic facilitation via the recruitment of endocannabinoid signaling. Together, these findings demonstrate that synaptically released zinc is a modulator of excitatory short-term plasticity, which shapes information transfer among excitatory synapses.

Alcohol-dose-dependent DNA methylation and expression in the nucleus accumbens identifies coordinated regulation of synaptic genes

Alterations in DNA methylation have been associated with alcohol exposure and proposed to contribute to continued alcohol use; however, the molecular mechanisms involved remain obscure. We investigated the escalating effects of alcohol use on DNA methylation, gene expression and predicted neural effects in the nucleus accumbens of rhesus macaques that self-administered 4% alcohol for over 12 months. Using an exploratory approach to identify CpG-rich regions, followed by bisulfite sequencing, the methylation levels of 2.7 million CpGs were compared between seven low-binge drinkers and nine heavy-very heavy drinking subjects. We identified 17 significant differential methylation regions (DMRs), including 14 with methylation levels that were correlated with average daily alcohol consumption. The size of the DMRs ranged from 29 to 158 bp (mean=63.7), included 4-19 CpGs per DMR (mean=8.06) and spanned a range of average methylation values from 5 to 34%. Eight of the DMRs mapped to genes implicated in modulating synaptic plasticity. Six of the synaptic genes have not previously been linked to alcohol use. Validation studies of these eight DMRs using bisulfite amplicon sequencing and an expanded set of 30 subjects confirmed the significant alcohol-dose-associated methylation of the DMRs. Expression analysis of three of the DMR-associated genes, LRP5, GPR39 and JAKMIP1, revealed significant correlations between DMR methylation and whole-gene or alternative transcript expression, supporting a functional role in regulating gene expression. Together, these studies suggest that alcohol-associated synaptic remodeling may be regulated and coordinated at the level of DNA methylation.

The Role of Elements in Anxiety

Elements (bioelements) are necessary factors required for the physiological function of organisms. They are critically involved in fundamental processes of life. Extra- and intracellular message and metabolic pathway factors as well as structural components include one or many elements in their functional structure. Recent years have seen an intensification in terms of knowledge gained about the roles of elements in anxiety disorders. In this chapter we present a review of the most important current data concerning the involvement of zinc, magnesium, copper, lithium, iron, and manganese, and their deficiency, in the pathophysiology and treatment of anxiety.

Zinc in the Glutamatergic Theory of Depression

Depression is a serious psychiatric illness that affects millions of people worldwide. Weeks of antidepressant therapy are required to relieve depressive symptoms, and new drugs are still being extensively researched. The latest studies have shown that in depression, there is an imbalance between the main excitatory (glutamatergic) and inhibitory (GABAergic) systems. Administration of antagonists of the glutamatergic system, including zinc, has shown an antidepressant effect in preclinical as well as clinical studies. Zinc inhibits the NMDA receptor via its binding site located on one of its subunits. This is thought to be the main mechanism explaining the antidepressant properties of zinc. In the present review, a link between zinc and the glutamatergic system is discussed in the context of depressive disorder.

Metalloneurochemistry and the Pierian Spring: 'Shallow Draughts Intoxicate the Brain'

Metal ions perform critical and diverse functions in nervous system physiology and pathology. The field of metalloneurochemistry aims to understand the mechanistic bases for these varied roles at the molecular level. Here, we review several areas of research that illustrate progress toward achieving this ambitious goal and identify key challenges for the future. We examine the use of lithium as a mood stabilizer, the roles of mobile zinc and copper in the synapse, the interplay of nitric oxide and metals in retrograde signaling, and the regulation of iron homeostasis in the brain. These topics were chosen to demonstrate not only the breadth of the field, but also to highlight opportunities for discovery by studying such complex systems in greater detail. We are beginning to uncover the principles by which receptors and transmitters utilize metal ions to modulate neurotransmission. These advances have revealed exciting new insights into the intricate mechanisms that give rise to learning, memory, and sensory perception, while opening many new avenues for further exploration.

Presynaptic D1 heteroreceptors and mGlu autoreceptors act at individual cortical release sites to modify glutamate release

The aim of this work was to study release of glutamic acid (GLU) from one-axon terminal or bouton at-a-time using cortical neurons grown in vitro to study the effect of presynaptic auto- and heteroreceptor stimulation. Neurons were infected with release reporters SypHx2 or iGluSnFR at 7 or 3 days-in-vitro (DIV) respectively. At 13-15 DIV single synaptic boutons were identified from images obtained from a confocal scanning microscope before and after field electrical stimulation. We further stimulated release by raising intracellular levels of cAMP with forskolin (10µM). Forskolin-mediated effects were dependent on protein kinase A (PKA) and did not result from an increase in endocytosis, but rather from an increase in the size of the vesicle readily releasable pool. Once iGluSnFR was confirmed as more sensitive than SypHx2, it was used to study the participation of presynaptic auto- and heteroreceptors on GLU release. Although most receptor agonizts (carbamylcholine, nicotine, dopamine D2, BDNF) did not affect electrically stimulated GLU release, a significant increase was observed in the presence of metabotropic D1/D5 heteroreceptor agonist (SKF38393 10µM) that was reversed by PKA inhibitors. Interestingly, stimulation of group II metabotropic mGLU2/3 autoreceptors (LY379268 50nM) induced a decrease in GLU release that was reversed by the specific mGLU2/3 receptor antagonist (LY341495 1µM) and also by PKA inhibitors (KT5720 200nM and PKI14-22 400nM). These changes in release probability at individual release sites suggest another level of control of the distribution of transmitter substances in cortical tissue.

Effects of maternal mild zinc deficiency and different ways of zinc supplementation for offspring on learning and memory

Background

The effect of different ways of zinc supplementation on spatial learning and memory remains unclear.

Objectives

This study aims to assess the effectiveness of two ways of zinc supplementation – oral use and intravenous transfusion – in zinc-deficient offspring rats on learning and memory.

Design

Rats were randomly divided into six groups on the first day of pregnancy (n=12): control (CO), pair fed (PF), zinc deprived (ZD), oral zinc supplementation (OZS), injection zinc supplementation (IZS), and injection control. The offspring's spatial learning and memory were tested at postnatal day 35 using Morris water maze (MWM). Maternal rats’ serum zinc was measured at postnatal day 21, while pups’ serum zinc was measured at postnatal day 35.

Results

Compared with the CO and PF groups, pups in ZD group spent more time finding the latent platform and swam longer distances (p<0.05). Compared with ZD groups, pups in OZS group significantly decreased the time used for finding the platform and the swimming distance (p<0.05) and were similar to that of CO and PF groups (p>0.05). However, compared with ZD groups, pups in IZS did not show any improvement in the indexes of MWM (p>0.05) although their zinc serum concentration increased significantly (p<0.05).

Conclusions

These results indicate that mild zinc deficiency during pregnancy and lactation leads to the impairment of learning and memory function in offspring, and that OZS, instead of intravenous transfusion zinc supplementation, can recover the impairment of spatial learning and memory function.

Reaction-Based Probes for Imaging Mobile Zinc in Live Cells and Tissues

Chelatable, or mobile, forms of zinc play critical signaling roles in numerous biological processes. Elucidating the action of mobile Zn(II) in complex biological environments requires sensitive tools for visualizing, tracking, and manipulating Zn(II) ions. A large toolbox of synthetic photoinduced electron transfer (PET)-based fluorescent Zn(II) sensors are available, but the applicability of many of these probes is limited by poor zinc sensitivity and low dynamic ranges owing to proton interference. We present here a general approach for acetylating PET-based probes containing a variety of fluorophores and zinc-binding units. The new sensors provide substantially improved zinc sensitivity and allow for incubation of live cells and tissue slices with nM probe concentrations, a significant improvement compared to the μM concentrations that are typically required for a measurable fluorescence signal. Acetylation effectively reduces or completely quenches background fluorescence in the metal-free sensor. Binding of Zn(II) selectively and quickly mediates hydrolytic cleavage of the acetyl groups, providing a large fluorescence response. An acetylated blue coumarin-based sensor was used to carry out detailed analyses of metal binding and metal-promoted acetyl hydrolysis. Acetylated benzoresorufin-based red-emitting probes with different zinc-binding sites are effective for sensing Zn(II) ions in live cells when applied at low concentrations (∼50-100 nM). We used green diacetylated Zinpyr1 (DA-ZP1) to image endogenous mobile Zn(II) in the molecular layer of mouse dorsal cochlear nucleus (DCN), confirming that acetylation is a suitable approach for preparing sensors that are highly specific and sensitive to mobile zinc in biological systems.

AMPA receptor inhibition by synaptically released zinc

The vast amount of fast excitatory neurotransmission in the mammalian central nervous system is mediated by AMPA-subtype glutamate receptors (AMPARs). As a result, AMPAR-mediated synaptic transmission is implicated in nearly all aspects of brain development, function, and plasticity. Despite the central role of AMPARs in neurobiology, the fine-tuning of synaptic AMPA responses by endogenous modulators remains poorly understood. Here we provide evidence that endogenous zinc, released by single presynaptic action potentials, inhibits synaptic AMPA currents in the dorsal cochlear nucleus (DCN) and hippocampus. Exposure to loud sound reduces presynaptic zinc levels in the DCN and abolishes zinc inhibition, implicating zinc in experience-dependent AMPAR synaptic plasticity. Our results establish zinc as an activity-dependent, endogenous modulator of AMPARs that tunes fast excitatory neurotransmission and plasticity in glutamatergic synapses.

Regulation of neuronal pH by the metabotropic Zn(2+)-sensing Gq-coupled receptor, mZnR/GPR39

Synaptically released Zn(2+) acts as a neurotransmitter, in part, by activating the postsynaptic metabotropic Zn(2+)-sensing Gq protein-coupled receptor (mZnR/GPR39). In previous work using epithelial cells, we described crosstalk between Zn(2+) signaling and changes in intracellular pH and/or extracellular pH (pHe). As pH changes accompany neuronal activity under physiological and pathological conditions, we tested whether Zn(2+) signaling is involved in regulation of neuronal pH. Here, we report that up-regulation of a major H(+) extrusion pathway, the Na(+)/H(+) exchanger (NHE), is induced by mZnR/GPR39 activation in an extracellular-regulated kinase 1/2-dependent manner in hippocampal neurons in vitro. We also observed that changes in pHe can modulate neuronal mZnR/GPR39-dependent signaling, resulting in reduced activity at pHe 8 or 6.5. Similarly, Zn(2+)-dependent extracellular-regulated kinase 1/2 phosphorylation and up-regulation of NHE activity were absent at acidic pHe. Thus, our results suggest that when pHe is maintained within the physiological range, mZnR/GPR39 activation can up-regulate NHE-dependent recovery from intracellular acidification. During acidosis, as pHe drops, mZnR/GPR39-dependent NHE activation is inhibited, thereby attenuating further H(+) extrusion. This mechanism may serve to protect neurons from excessive decreases in pHe. Thus, mZnR/GPR39 signaling provides a homeostatic adaptive process for regulation of intracellular and extracellular pH changes in the brain. We show that the postsynaptic metabotropic Zn(2+)-sensing Gq protein-coupled receptor (mZnR/GPR39) activation induces up-regulation of a major neuronal H(+) extrusion pathway, the Na(+)/H(+) exchanger (NHE), thereby enhancing neuronal recovery from intracellular acidification. Changes in extracellular pH (pHe), however, modulate neuronal mZnR/GPR39-dependent signaling, resulting in reduced activity at pHe 8 or 6.5. This mechanism may serve to protect neurons from excessive decreases in pHe during acidosis. Hence, mZnR/GPR39 signaling provides a homeostatic adaptive process for regulation of intracellular and extracellular pH changes in the brain.

Homeostatic regulation of KCC2 activity by the zinc receptor mZnR/GPR39 during seizures

The aim of this study was to investigate the role of the synaptic metabotropic zinc receptor mZnR/GPR39 in physiological adaptation to epileptic seizures. We previously demonstrated that synaptic activation of mZnR/GPR39 enhances inhibitory drive in the hippocampus by upregulating neuronal K(+)/Cl(-) co-transporter 2 (KCC2) activity. Here, we first show that mZnR/GPR39 knockout (KO) adult mice have dramatically enhanced susceptibility to seizures triggered by a single intraperitoneal injection of kainic acid, when compared to wild type (WT) littermates. Kainate also substantially enhances seizure-associated gamma oscillatory activity in juvenile mZnR/GPR39 KO hippocampal slices, a phenomenon that can be reproduced in WT tissue by extracellular Zn(2+) chelation. Importantly, kainate-induced synaptic Zn(2+) release enhances surface expression and transport activity of KCC2 in WT, but not mZnR/GPR39 KO hippocampal neurons. Kainate-dependent upregulation of KCC2 requires mZnR/GPR39 activation of the Gαq/phospholipase C/extracellular regulated kinase (ERK1/2) signaling cascade. We suggest that mZnR/GPR39-dependent upregulation of KCC2 activity provides homeostatic adaptation to an excitotoxic stimulus by increasing inhibition. As such, mZnR/GPR39 may provide a novel pharmacological target for dampening epileptic seizure activity.

Does Short-Term Dietary Omega-3 Fatty Acid Supplementation Influence Brain Hippocampus Gene Expression of Zinc Transporter-3?

Dietary omega-3 fatty acids have been recognized to improve brain cognitive function. Deficiency leads to dysfunctional zinc metabolism associated with learning and memory impairment. The objective of this study is to explore the effect of short-term dietary omega-3 fatty acids on hippocampus gene expression at the molecular level in relation to spatial recognition memory in mice. A total of 24 male BALB/c mice were randomly divided into four groups and fed a standard pellet as a control group (CTL, n = 6), standard pellet added with 10% (w/w) fish oil (FO, n = 6), 10% (w/w) soybean oil (SO, n = 6) and 10% (w/w) butter (BT, n = 6). After 3 weeks on the treatment diets, spatial-recognition memory was tested on a Y-maze. The hippocampus gene expression was determined using a real-time PCR. The results showed that 3 weeks of dietary omega-3 fatty acid supplementation improved cognitive performance along with the up-regulation of α-synuclein, calmodulin and transthyretin genes expression. In addition, dietary omega-3 fatty acid deficiency increased the level of ZnT3 gene and subsequently reduced cognitive performance in mice. These results indicate that the increased the ZnT3 levels caused by the deficiency of omega-3 fatty acids produced an abnormal zinc metabolism that in turn impaired the brain cognitive performance in mice.