Concentration-Dependent Dual Mode of Zn Action at Serotonin 5-HT1A Receptors: In Vitro and In Vivo Studies

A systematic review of the association between zinc and anxiety

CONTEXT

The incidence of anxiety, which stems from both intrinsic and extrinsic factors, has been increasing worldwide. Various methods by which it can be treated or prevented have been reported thus far. One of the most popular and effective treatments is supplementation therapy. Zinc, which is an essential nutrient found in various plants, animal foods, and supplements, has been shown to be a potential nutrient in anxiety reduction by acting on γ-aminobutyric acid (GABA), glutamatergic, serotonergic, neurogenesis, and immune systems. It can also influence important receptors, such as GPR39. Thus, zinc has received considerable attention with respect to its potential role as a therapeutic or detrimental factor for anxiety; yet, the available evidence needs to be analyzed systematically to reach a convergent conclusion.

OBJECTIVE

The objective was to systematically review any potential connection between adult human anxiety and zinc intake.

DATA SOURCES AND EXTRACTION

Nine original human studies, of which 2 assessed the relationship between zinc consumption and anxiety (based on a questionnaire) and 7 assessed the relationship between serum zinc levels and anxiety, were included based on specific selection criteria. Studies that had been written in English and published in peer-reviewed publications with no restrictions on the date of publication were searched in the Google Scholar and PubMed databases. This project was also reported according to the PRISMA guidelines.

DATA ANALYSIS

As per the studies analyzed in this review, there was a noticeable relationship between serum zinc levels and anxiety, which means that patients with anxiety have lower levels of zinc in their serum, as compared with healthy individuals. Furthermore, zinc consumption was inversely associated with anxiety.

CONCLUSION

The results provide plausible evidence for the positive role of zinc in the treatment of patients afflicted with anxiety, albeit with some limitations.

Binary effects of fluoxetine and zinc on the biomarker responses of the non-target model organism Daphnia magna

The antidepressant effect of zinc on mammals has been documented in recent decades, and the concentration of the antidepressant fluoxetine (FLX) in aquatic environments has been rising constantly. The aim of the present study is to evaluate the combined toxicity of a serotonin reuptake inhibitor (FLX) and Zn2+ on a non-target aquatic model organism Daphnia magna. Animals were exposed to single and binary combinations of FLX (20.5 and 41 µg/L for subchronic and 41 and 82 µg/L for acute exposures) and Zn2+ (40 µg/L for subchronic and 80 µg/L for acute exposures). In vivo experiments were done for 7 days subchronic and 48 h acute exposure, while subcellular supernatants of whole Daphnia lysate (WDL) were directly treated with the same concentrations used in the acute experiments. Morphological characteristics, Ca2+-ATPase, antioxidant enzyme activities, and lipid peroxidation were examined. There was antioxidant system suppression and Ca2+-ATPase inhibition despite the diverse response patterns due to duration, concentration, and toxicant type. After acute exposure, biomarkers showed a diminishing trend compared to subchronic exposure. According to integrated biomarker response index (IBR) analysis, in vivo Zn2+ exposure was reasonably effective on the health of D. magna, whereas exposure of WDL to Zn2+ had a lesser impact. FLX toxicity increased in a concentration-dependent manner, reversed by the combined exposure. We concluded that potential pro-oxidative and adverse Ca2+-ATPase effects of FLX and Zn2+ in D. magna may also have harmful impact on ecosystem levels. Pharmaceutical exposure (FLX) should be considered along with their potential to interact with other toxicants in aquatic biota.

Caged Zn2+ Photolysis in Zebrafish Whole Brains Reveals Subsecond Modulation of Dopamine Uptake

Free, ionic zinc (Zn2+) modulates neurotransmitter dynamics in the brain. However, the sub-s effects of transient concentration changes of Zn2+ on neurotransmitter release and uptake are not well understood. To address this lack of knowledge, we have combined the photolysis of the novel caged Zn2+ compound [Zn(DPAdeCageOMe)]+ with fast scan cyclic voltammetry (FSCV) at carbon fiber microelectrodes in live, whole brain preparations from zebrafish (Danio rerio). After treating the brain with [Zn(DPAdeCageOMe)]+, Zn2+ was released by application of light that was gated through a computer-controlled shutter synchronized with the FSCV measurements and delivered through a 1 mm fiber optic cable. We systematically optimized the photocage concentration and light application parameters, including the total duration and light-to-electrical stimulation delay time. While sub-s Zn2+ application with this method inhibited DA reuptake, assessed by the first-order rate constant (k) and half-life (t1/2), it had no effect on the electrically stimulated DA overflow ([DA]STIM). Increasing the photocage concentration and light duration progressively inhibited uptake, with maximal effects occurring at 100 μM and 800 ms, respectively. Furthermore, uptake was inhibited 200 ms after Zn2+ photorelease, but no measurable effect occurred after 800 ms. We expect that application of this method to the zebrafish whole brain and other preparations will help expand the current knowledge of how Zn2+ affects neurotransmitter release/uptake in select neurological disease states.

GPR39: An orphan receptor begging for ligands

Progress in the understanding of the receptor GPR39 is held up by inconsistent pharmacological data. First, the endogenous ligand(s) remain(s) contentious. Data pointing to zinc ions (Zn2+) and/or eicosanoids as endogenous ligands are a matter of debate. Second, there are uncertainties in the specificity of the widely used synthetic ligand (agonist) TC-G 1008. Third, activation of GPR39 has been often proposed as a novel treatment strategy, but new data also support that inhibition might be beneficial in certain disease contexts. Constitutive activity/promiscuous signaling suggests the need for antagonists/inverse agonists in addition to (biased) agonists. Here, we scrutinize data on the signaling and functions of GPR39 and critically assess factors that might have contributed to divergent outcomes and interpretations of investigations on this important receptor.

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.

Are Essential Trace Elements Effective in Modulation of Mental Disorders? Update and Perspectives

The emergence of mental disorders is associated with several risk factors including genetic and environmental susceptibility. A group of nutrients serves an especially important role in a number of essential neurodevelopmental processes through brain areas promoting the high degree of brain metabolism during early life, although almost all nutrients are needed. These include macronutrients and micronutrients (e.g., iron, magnesium, zinc, copper, selenium). Numerous nutritional psychiatry trials have been performed to examine the correlation of many individual nutrients with mental health, such as essential trace elements. The increased accumulation or lack of such components will facilitate an alternative metabolic pathway that can lead to many diseases and conditions of neurodevelopment. Mental functions have biochemical bases, so the impairment of such neurochemical mechanisms due to lack of trace elements can have mental effects. In psychological conditions such as depression, anxiety, schizophrenia, and autism, scientific studies demonstrate the putative role of trace element deficiency. Therefore, given the critical roles played by essential trace elements in the neurodevelopment and mental health, the effect of these elements' intake on the modulation of psychological functioning is reviewed.

Zinc as an adjunct to antidepressant medication: a meta-analysis with subgroup analysis for different levels of treatment response to antidepressants

RESULTS

We performed random effects meta-analysis We computed a weighted effect size (Hedges' g) for the difference of the absolute scores of the Hamilton Depression Rating Scale between the group given zinc and the group given placebo after 6 and after 12 weeks. We performed subgroup analysis according to whether not it was stated in the original studies that patients in a clinical trial or subgroup within a trial had not reached remission with previous treatment with a standard antidepressant during the same illness episode. After 6 weeks, we obtained the following effect sizes: Overall effect size: Hedges' g = -0.67, p = 0.03. Subgroup with documented non-remission: Hedges' g = - 0.97, p = 0.001. Group without documented non-remission -0.35, p = 0.24. After 12 weeks: Overall effect size: Hedges' g = -1.0, p = 0.03. Subgroup with documented non-remission: Hedges' g = - 1.46, p < 0.001. Group without documented non-remission: Hedges' g = - 0.54, p = 0.07.

CONCLUSIONS

We provide preliminary evidence for the efficacy of zinc vs. placebo as an adjunct to antidepressant medication.

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.

Differential Roles of Extracellular Histidine Residues of GPR68 for Proton-Sensing and Allosteric Modulation by Divalent Metal Ions

GPR68, an orphan G-protein coupled receptor, senses protons, couples to multiple G-proteins, and is also activated or inhibited by divalent metal ions. It has seven extracellular histidine residues, although it is not clear how these histidine residues play a role in both proton-sensing and metal ion modulation. Here we demonstrate that divalent metal ions are allosteric modulators that can activate or inhibit proton activity in a concentration- and pH-dependent manner. We then show that single histidine mutants have differential and varying degrees of effects on proton-sensing and metal ion modulation. Some histidine residues play dual roles in proton-sensing and metal ion modulation, while others are important in one or the other but not both. Two extracellular disulfide bonds are predicted to constrain histidine residues to be spatially close to each other. Combining histidine mutations leads to reduced proton activity and resistance to metal ion modulation, while breaking the less conserved disulfide bond results in a more severe reduction in proton-sensing over metal modulation. The small-molecule positive allosteric modulators (PAMs) ogerin and lorazepam are not affected by these mutations and remain active at mutants with severely reduced proton activity or are resistant to metal ion modulation. These results suggest GPR68 possesses two independent allosteric modulation systems, one through interaction with divalent metal ions at the extracellular surface and another through small-molecule PAMs in the transmembrane domains. A new GPR68 model is developed to accommodate the findings which could serve as a template for further studies and ligand discovery by virtual ligand docking.

Antidepressant, Anxiolytic and Neuroprotective Activities of Two Zinc Compounds in Diabetic Rats

Behavioral disorders affect most diabetic patients and Zinc (Zn) has been used among adjuvant therapies for involvement in the etiology of depression and anxiety, however, the results are still controversial. The objective of this study was to compare the antidepressant, anxiolytic and neuroprotective activity of the supplementation of two Zn compounds in an animal model of Diabetes Mellitus type 1 (DM1). Thirty-eight (38) adult rats were randomized into four groups: Control (C; n = 8); Diabetic (D; n = 10); Diabetic Zn Sulfate Supplement (DSZ; n = 10) and Diabetic Zn Gluconate Supplement (DGZ; n = 10). The DSZ group received Zn sulfate supplementation and the DGZ group received Zn gluconate supplementation at a dose of 15 mg/kg for 4 weeks. Data (mean ±SEM) were analyzed by the Mann–Whitney test with a significance level of p < 0.05. The results indicate that Zn gluconate supplementation in diabetic animals presented an antidepressant effect demonstrated through the results obtained in the Forced Swim Test, and neuroprotective effect by attenuating alterations in the cerebral cortex; while Zn sulfate supplementation in diabetic animals showed an anxiolytic effect demonstrated by the results obtained in the open field test and the elevated plus maze test. Considering the set of results, supplementation with both zinc compounds showed neurobehavioral benefits in diabetic animals with different effects depending on the type of anion associated with Zn.

Zinc, Magnesium, Selenium and Depression: A Review of the Evidence, Potential Mechanisms and Implications

Micronutrient deficiency and depression are major global health problems. Here, we first review recent empirical evidence of the association between several micronutrients—zinc, magnesium, selenium—and depression. We then present potential mechanisms of action and discuss the clinical implications for each micronutrient. Collectively, empirical evidence most strongly supports a positive association between zinc deficiency and the risk of depression and an inverse association between zinc supplementation and depressive symptoms. Less evidence is available regarding the relationship between magnesium and selenium deficiency and depression, and studies have been inconclusive. Potential mechanisms of action involve the HPA axis, glutamate homeostasis and inflammatory pathways. Findings support the importance of adequate consumption of micronutrients in the promotion of mental health, and the most common dietary sources for zinc and other micronutrients are provided. Future research is needed to prospectively investigate the association between micronutrient levels and depression as well as the safety and efficacy of micronutrient supplementation as an adjunct treatment for depression.

Involvement of extracellular signal-regulated kinase (ERK) in the short and long-lasting antidepressant-like activity of NMDA receptor antagonists (zinc and Ro 25-6981) in the forced swim test in rats

Short and long acting NMDA receptor (NMDAR) antagonists exert their antidepressant-like effects by activating signaling pathways involved in the synthesis of synaptic proteins and formation of new synaptic connections in the prefrontal cortex (PFC) of rats. The blockade of the ERK pathway abolishes ketamine and Ro 25-6981 antidepressant potency. However, the role of ERK in the antidepressant-like activity of short acting NMDAR antagonists is still unclear. More puzzling is the fact that the precise role of ERK in the short and long lasting effects of long-acting NMDAR antagonists is unknown. In this study, we show that zinc, (Zn) a short-acting NMDAR antagonist evokes only transient ERK activation, which is observed 7 min after its administration in the PFC of rats. In contrast to Zn, the long acting NMDAR antagonist Ro 25-6981 produces persistent ERK activation lasting up to 24 h. Pretreatment with the MAPK/ERK inhibitor (U0126) totally abolished Zn and Ro 25-6981 antidepressant-like activities in the forced swim test in rats. However, when U0126 is administered 15 min after Zn or Ro 25-6981 both compounds maintain their short-lasting antidepressant-like activity. On the other hand, posttreatment with U0126 significantly attenuated the long lasting antidepressant-like activity of Ro 25-6981. These results indicate that the activation of ERK is crucial for the short- and long lasting antidepressant-like activity observed in the FST in rats.

The Emerging Role for Zinc in Depression and Psychosis

Zinc participation is essential for all physiological systems, including neural functioning, where it participates in a myriad of cellular processes. Converging clinical, molecular, and genetic discoveries illuminate key roles for zinc homeostasis in association with clinical depression and psychosis which are not yet well appreciated at the clinical interface. Intracellular deficiency may arise from low circulating zinc levels due to dietary insufficiency, or impaired absorption from aging or medical conditions, including alcoholism. A host of medications commonly administered to psychiatric patients, including anticonvulsants, oral medications for diabetes, hormones, antacids, anti-inflammatories and others also impact zinc absorption. Furthermore, inefficient genetic variants in zinc transporter molecules that transport the ion across cellular membranes impede its action even when circulating zinc concentrations is in the normal range. Well powered clinical studies have shown beneficial effects of supplemental zinc in depression and it important to pursue research using zinc as a potential therapeutic option for psychosis as well. Meta-analyses support the adjunctive use of zinc in major depression and a single study now supports zinc for psychotic symptoms. This manuscript reviews the biochemistry and bench top evidence on putative molecular mechanisms of zinc as a psychiatric treatment.

Allosteric Inhibition of Serotonin 5-HT7 Receptors by Zinc Ions

The allosteric regulation of G protein-coupled receptors (GPCRs) is a well-known phenomenon, but there are only a few examples of allosteric modulation within the metabotropic serotonergic receptor family. Recently, we described zinc non-competitive interactions toward agonist binding at serotonin 5-HT_1A receptors, in which biphasic effects, involving potentiation at sub-micromolar concentrations (10 μM) and inhibition at sub-millimolar concentrations (500 μM) of Zn²⁺ in radioligand binding assays, were consistent with both the agonist and antagonist-like effects of zinc ions observed in in vivo studies. Here, we showed new data demonstrating zinc allosteric inhibition of both agonist and antagonist binding at human recombinant 5-HT₇ receptors stably expressed in HEK293 cells as observed by radioligand binding studies as well as zinc neutral antagonism displayed by the concentration of 10 μM in the functional LANCE assay. The allosteric nature of the effect of Zn on 5-HT₇ receptors was confirmed (1) in saturation studies in which zinc inhibited the binding of potent orthosteric 5-HT₇ receptor radioligands, the agonist [³H]5-CT, and the two antagonists [³H]SB-269970 and [³H]mesulergine, showing ceiling effect and differences in the magnitude of negative cooperativity (α = 0.15, 0.06, and 0.25, respectively); (2) in competition experiments in which 500 μM of zinc inhibited all radioligand displacements by non-labeled orthosteric ligands (5-CT, SB-269970, and clozapine), and the most significant reduction in affinity was observed for the 5-CT agonist (4.9–16.7-fold) compared with both antagonists (1.4–3.9-fold); and (3) in kinetic experiments in which 500 μM zinc increased the dissociation rate constants for [³H]5-CT and [³H]mesulergine but not for [³H]SB-269970. Additionally, in the functional LANCE test using the constitutively active HEK293 cell line expressing the 5-HT₇ receptor, 10 μM zinc had features of neutral antagonism and increased the EC₅₀ value of the 5-CT agonist by a factor of 3.2. Overall, these results showed that zinc can act as a negative allosteric inhibitor of 5-HT₇ receptors. Given that the inhibiting effects of low concentrations of zinc in the functional assay represent the most likely direction of zinc activity under physiological conditions, among numerous zinc-regulated proteins, the 5-HT₇ receptor can be considered a serotonergic target for zinc modulation in the CNS.

Zinc in the Monoaminergic Theory of Depression: Its Relationship to Neural Plasticity

Preclinical and clinical studies have demonstrated that zinc possesses antidepressant properties and that it may augment the therapy with conventional, that is, monoamine-based, antidepressants. In this review we aim to discuss the role of zinc in the pathophysiology and treatment of depression with regard to the monoamine hypothesis of the disease. Particular attention will be paid to the recently described zinc-sensing GPR39 receptor as well as aspects of zinc deficiency. Furthermore, an attempt will be made to give a possible explanation of the mechanisms by which zinc interacts with the monoamine system in the context of depression and neural plasticity.

The serum zinc concentration as a potential biological marker in patients with major depressive disorder

Despite many clinical trials assessing the role of zinc in major depressive disorder (MDD), the conclusions still remain ambiguous. The aim of the present clinical study was to determine and comparison the zinc concentration in the blood of MDD patients (active stage or remission) and healthy volunteers (controls), as well as to discuss its potential clinical usefulness as a biomarker of the disease. In this study 69 patients with current depressive episode, 45 patients in remission and 50 controls were enrolled. The zinc concentration was measured by electrothermal atomic absorption spectrometry (ET AAS). The obtained results revealed, that the zinc concentration in depressed phase were statistically lower than in the healthy volunteers [0.89 vs. 1.06 mg/L, respectively], while the zinc level in patients achieve remission was not significantly different from the controls [1.07 vs. 1.06 mg/L, respectively]. Additionally, among the patients achieve remission a significant differences in zinc concentration between group with and without presence of drug-resistance in the previous episode of depression were observed. Also, patients in remission demonstrated correlation between zinc level and the average number of depressive episodes in the last year. Serum zinc concentration was not dependent on atypical features of depression, presence of psychotic symptoms or melancholic syndrome, age, age of onset or duration of disease, number of episodes in the life time, duration of the episode/remission and severity of depression measured by the Hamilton Rating Scale for Depression (HDRS), and the Montgomery-Asberg Depression Rating Scale (MADRS). Concluding, our findings confirm the correlation between zinc deficit present in the depressive episode, and are consistent with the majority of previous studies. These results may also indicate that serum zinc concentration might be considered as a potential biological marker of MDD.

Antidepressant activity of zinc: Further evidence for the involvement of the serotonergic system

BACKGROUND

The present study sought to further evaluate the role of the serotonergic system especially the postsynaptic 5-HT1A receptors (5-HT1AR) in the mechanism of antidepressant action of zinc.

METHODS

Messenger RNA (mRNA), protein level, and 5-HT1AR density as well as the rate of monoamine (dopamine, DA, and serotonin) metabolism in the prefrontal cortex (PFC) and hippocampus (Hp) of rats subjected to acute and chronic (21days) zinc (5mg Zn/kg) treatment were measured.

RESULTS

Acute or chronic zinc treatment did not induce any changes in 5-HT1AR mRNA levels in the PFC or Hp of rats. However, chronic zinc treatment induced increases in both 5-HT1AR protein levels and density of 5-HT1A receptor binding sites in the Hp of rats. Chronic zinc treatment also increased tissue levels of serotonin metabolite and turnover in the rat Hp. On the other hand, DA, DOPAC, HVA tissue levels increased while DOPAC/DA and 3MT/DA decreased in the PFC of rats after chronic zinc treatment. Acute treatment induced increases only in tissue levels of DOPAC, and DOPAC/DA.

CONCLUSIONS

Our results confirm that the antidepressant effects of zinc are mediated in concert with the modulation of the serotonergic system including postsynaptic 5-HT1ARs and allude to a possible involvement of dopaminergic neurotransmission in this action.

Discovery and Characterization of Novel GPR39 Agonists Allosterically Modulated by Zinc

In this study, we identified two previously described kinase inhibitors-3-(4-chloro-2-fluorobenzyl)-2-methyl-N-(3-methyl-1H-pyrazol-5-yl)-8-(morpholinomethyl)imidazo[1,2-b]pyridazin-6-amine (LY2784544) and 1H-benzimidazole-4-carboxylic acid, 2-methyl-1-[[2-methyl-3-(trifluoromethyl)phenyl]methyl]-6-(4-morpholinyl)- (GSK2636771)-as novel GPR39 agonists by unbiased small-molecule-based screening using a β-arrestin recruitment screening approach (PRESTO-Tango). We characterized the signaling of LY2784544 and GSK2636771 and compared their signaling patterns with a previously described "GPR39-selective" agonist N-[3-chloro-4-[[[2-(methylamino)-6-(2-pyridinyl)-4- pyrimidinyl]amino]methyl]phenyl]methanesulfonamide (GPR39-C3) at both canonical and noncanonical signaling pathways. Unexpectedly, all three compounds displayed probe-dependent and pathway-dependent allosteric modulation by concentrations of zinc reported to be physiologic. LY2784544 and GS2636771 at GPR39 in the presence of zinc were generally as potent or more potent than their reported activities against kinases in whole-cell assays. These findings reveal an unexpected role of zinc as an allosteric potentiator of small-molecule-induced activation of GPR39 and expand the list of potential kinase off-targets to include understudied G protein-coupled receptors.