Zinc signaling in the hippocampus and its relation to pathogenesis of depression

Intracellular Zn2+ promotes extracellular matrix remodeling in dexamethasone-treated trabecular meshwork

Given the widespread application of glucocorticoids in ophthalmology, the associated elevation of intraocular pressure (IOP) has long been a vexing concern for clinicians, yet the underlying mechanisms remain inconclusive. Much of the discussion focuses on the extracellular matrix (ECM) of trabecular meshwork (TM). It is widely agreed that glucocorticoids impact the expression of matrix metalloproteinases (MMPs), leading to ECM deposition. Since Zn2+ is vital for MMPs, we explored its role in ECM alterations induced by dexamethasone (DEX). Our study revealed that in human TM cells treated with DEX, the level of intracellular Zn2+ significantly decreased, accompanied by impaired extracellular Zn2+ uptake. This correlated with changes in several Zrt-, Irt-related proteins (ZIPs) and metallothionein. ZIP8 knockdown impaired extracellular Zn2+ uptake, but Zn2+ chelation did not affect ZIP8 expression. Resembling DEX's effects, chelation of Zn2+ decreased MMP2 expression, increased the deposition of ECM proteins, and induced structural disarray of ECM. Conversely, supplementation of exogenous Zn2+ in DEX-treated cells ameliorated these outcomes. Notably, dietary zinc supplementation in mice significantly reduced DEX-induced IOP elevation and collagen content in TM, thereby rescuing the visual function of the mice. These findings underscore zinc's pivotal role in ECM regulation, providing a novel perspective on the pathogenesis of glaucoma.NEW & NOTEWORTHY Our study explores zinc's pivotal role in mitigating extracellular matrix dysregulation in the trabecular meshwork and glucocorticoid-induced ocular hypertension. We found that in human trabecular meshwork cells treated with dexamethasone, intracellular Zn2+ significantly decreased, accompanied by impaired extracellular Zn2+ uptake. Zinc supplementation rescues visual function by modulating extracellular matrix proteins and lowering intraocular pressure, offering a direction for further exploration in glaucoma management.

Evaluation of Antidepressive-like Behaviours and Oxidative Stress Parameters in Mice Receiving Imipramine-Zinc Complex Compound

The study aimed to evaluate the antidepressant-like effects of an imipramine-zinc (IMI-Zn) complex compound on mice and assess the level of oxidative stress parameters. The research also investigated whether the IMI-Zn complex showed superior antidepressant activity compared to individual treatments of both compounds at effective doses and their joint administration at subtherapeutic doses. The study was conducted on mice. Forced swim (FST), tail suspension (TST), and locomotor activity tests were used for behavioral studies. The results demonstrated the IMI-Zn complex's dose-dependent antidepressant potential when orally administered to mice. Its efficacy was similar to the separate administration of therapeutic doses of imipramine (IMI) and zinc (Zn) and their joint administration at subtherapeutic doses. Moreover, subjecting mice to acute stress did not significantly affect the activity of on glutathione peroxidase (GPX), glutathione reductase (GR), and total antioxidant status (TAS), possibly due to the short exposure time to the stress stimulus. By developing the IMI-Zn complex, it might be possible to simplify the treatment approach, potentially improving patient compliance by combining the therapeutic effects of both IMI and Zn within a single compound, thus addressing one of the contributing factors to non-compliance in depression therapy. The IMI-Zn complex could be a valuable strategy to optimize therapeutic outcomes and balance efficacy and tolerability.

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.

The Role of Zinc Status on Spatial Memory, Hippocampal Synaptic Plasticity, and Insulin Signaling in icv-STZ-Induced Sporadic Alzheimer's-Like Disease in Rats

Alzheimer's disease (AD), especially its sporadic form (sAD), is of multifactorial nature. Brain insulin resistance and disrupted zinc homeostasis are two key aspects of AD that remain to be elucidated. Here, we investigated the effects of dietary zinc deficiency and supplementation on memory, hippocampal synaptic plasticity, and insulin signaling in intracerebroventricular streptozotocin (icv-STZ)-induced sAD in rats. The memory performance was evaluated by Morris water maze. The expression of hippocampal protein and mRNA levels of targets related to synaptic plasticity and insulin pathway was assessed by Western blot and real-time quantitative PCR. We found memory deficits in icv-STZ rats, which were fully recovered by zinc supplementation. Western blot analysis revealed that icv-STZ treatment significantly reduced hippocampal PSD95 and p-GSK3β, and zinc supplementation restored the normal protein levels. mRNA levels of BDNF, PSD95, SIRT1, GLUT4, insulin receptor, and ZnT3 were found to be reduced by icv-STZ and reestablished by zinc supplementation. Our data suggest that zinc supplementation improves cognitive deficits and rescues the decline in key molecular targets of synaptic plasticity and insulin signaling in hippocampus caused by icv-STZ induced sAD in rats.

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

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

Localization of Free and Bound Metal Species through X-Ray Synchrotron Fluorescence Microscopy in the Rodent Brain and Their Relation to Behavior

Biometals in the brain, such as zinc, copper, and iron, are often discussed in cases of neurological disorders; however, these metals also have important regulatory functions and mediate cell signaling and plasticity. With the use of synchrotron X-ray fluorescence, our lab localized total, both bound and free, levels of zinc, copper, and iron in a cross section of one hemisphere of a rat brain, which also showed differing metal distributions in different regions within the hippocampus, the site in the brain known to be crucial for certain types of memory. This review discusses the several roles of these metals in brain regions with an emphasis on hippocampal cell signaling, based on spatial mapping obtained from X-ray fluorescence microscopy. We also discuss the localization of these metals and emphasize different cell types and receptors in regions with metal accumulation, as well as the potential relationship between this physiology and behavior.

Supplementation with zinc in rats enhances memory and reverses an age-dependent increase in plasma copper

Zinc and copper are essential trace elements. Dyshomeostasis in these two metals has been observed in Alzheimer's disease, which causes profound cognitive impairment. Insulin therapy has been shown to enhance cognitive performance; however, recent data suggest that this effect may be at least in part due to the inclusion of zinc in the insulin formulation used. Zinc plays a key role in regulation of neuronal glutamate signaling, suggesting a possible link between zinc and memory processes. Consistent with this, zinc deficiency causes cognitive impairments in children. The effect of zinc supplementation on short- and long-term recognition memory, and on spatial working memory, was explored in young and adult male Sprague Dawley rats. After behavioral testing, hippocampal and plasma zinc and copper were measured. Age increased hippocampal zinc and copper, as well as plasma copper, and decreased plasma zinc. An interaction between age and treatment affecting plasma copper was also found, with zinc supplementation reversing elevated plasma copper concentration in adult rats. Zinc supplementation enhanced cognitive performance across tasks. These data support zinc as a plausible therapeutic intervention to ameliorate cognitive impairment in disorders characterized by alterations in zinc and copper, such as Alzheimer's disease.

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

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

Administration of Zinc Chelators Improves Survival of Mice Infected with Aspergillus fumigatus both in Monotherapy and in Combination with Caspofungin

Aspergillus fumigatus can infect immunocompromised patients, leading to high mortality rates due to the lack of reliable treatment options. This pathogen requires uptake of zinc from host tissues in order to successfully grow and cause virulence. Reducing the availability of that micronutrient could help treat A. fumigatus infections. In this study, we examined the in vitro effects of seven chelators using a bioluminescent strain of A. fumigatus 1,10-Phenanthroline and N,N,N',N'-tetrakis(2-pyridylmethyl)ethane-1,2-diamine (TPEN) proved to be the chelators most effective at inhibiting fungal growth. Intraperitoneal administration of either phenanthroline or TPEN resulted in a significant improvement in survival and decrease of weight loss and fungal burden for immunosuppressed mice intranasally infected with A. fumigatus In vitro both chelators had an indifferent effect when employed in combination with caspofungin. The use of TPEN in combination with caspofungin also significantly increased survival compared to that when using these drugs individually. Our results suggest that zinc chelation may be a valid strategy for dealing with A. fumigatus infections and that both phenanthroline and TPEN could potentially be used either independently or in combination with caspofungin, indicating that their use in combination with other antifungal treatments might also be applicable.

The Zinc Ion Chelating Agent TPEN Attenuates Neuronal Death/apoptosis Caused by Hypoxia/ischemia Via Mediating the Pathophysiological Cascade Including Excitotoxicity, Oxidative Stress, and Inflammation

AIMS

We aim to determine the significant effect of TPEN, a Zn(2+) chelator, in mediating the pathophysiological cascade in neuron death/apoptosis induced by hypoxia/ischemia.

METHODS

We conducted both in vivo and in vitro experiments in this study. PC12 cells were used to establish hypoxia/ischemia model by applying oxygen-glucose deprivation (OGD). SHR-SP rats were used to establish an acute ischemic model by electrocoagulating middle cerebral artery occlusion. The effect of TPEN on neuron death/apoptosis was evaluated. In addition, the relative biomarks of excitotoxicity, oxidative stress, and inflammation reactions in hypoxia/ischemia PC12 cell model as well as in SHR-SP rat hypoxia/ischemia model were also assessed.

RESULTS

TPEN significantly attenuates the neurological deficit, reduced the cerebral infarction area and the ratio of apoptotic neurons, and increased the expression of GluR2 in the rat hypoxia/ischemia brain. TPEN also increased blood SOD activity, decreased blood NOS activity and blood MDA and IL-6 contents in rats under hypoxia/ischemia. In addition, TPEN significantly inhibited the death and apoptosis of cells and attenuated the alteration of GluR2 and NR2 expression caused by OGD or OGD plus high Zn(2+) treatments.

CONCLUSIONS

Zn(2+) is involved in neural cell apoptosis and/or death caused by hypoxia/ischemia via mediating excitotoxicity, oxidative stress, and inflammation.

Trans-synaptic zinc mobilization improves social interaction in two mouse models of autism through NMDAR activation

Genetic aspects of autism spectrum disorders (ASDs) have recently been extensively explored, but environmental influences that affect ASDs have received considerably less attention. Zinc (Zn) is a nutritional factor implicated in ASDs, but evidence for a strong association and linking mechanism is largely lacking. Here we report that trans-synaptic Zn mobilization rapidly rescues social interaction in two independent mouse models of ASD. In mice lacking Shank2, an excitatory postsynaptic scaffolding protein, postsynaptic Zn elevation induced by clioquinol (a Zn chelator and ionophore) improves social interaction. Postsynaptic Zn is mainly derived from presynaptic pools and activates NMDA receptors (NMDARs) through postsynaptic activation of the tyrosine kinase Src. Clioquinol also improves social interaction in mice haploinsufficient for the transcription factor Tbr1, which accompanies NMDAR activation in the amygdala. These results suggest that trans-synaptic Zn mobilization induced by clioquinol rescues social deficits in mouse models of ASD through postsynaptic Src and NMDAR activation.

Zinc is a nutritional factor implicated in autism spectrum disorders (ASDs), but evidence for a strong association and linking mechanism is largely lacking. Here, the authors report that trans-synaptic zinc mobilization rapidly rescues social interaction in two independent mouse models of ASD.

Corticosterone increases intracellular Zn(2+) release in hippocampal HT-22 cells

The previous studies suggested that the hippocampal zinc dyshomeostasis and high glucocorticoid level might hurt hippocampal function. However, the effect of corticosterone (CORT) on hippocampus zinc homeostasis is not fully characterized. In this study, we investigated the intracellular Zn(2+) concentration in hippocampal HT-22 cells after CORT treatment. The cells were incubated with 10μM CORT for 0h-24h, 0μM-50μM CORT for 6h and 2.5μM glucocorticoid receptor antagonist RU486 administered 30min before CORT application. The results showed that 10μM CORT increased the intracellular Zn(2+) level after 6h, which was diminished by 2.5μM RU486. Co-treatment of ZnSO4 and CORT augmented the increase in Zn(2+) level. TPEN, a membrane-permeable chelator for intracellular Zn(2+) greatly attenuated the Zn(2+) increase by CORT, while DTPA, a chelator for extracellular Zn(2+), had no same effects. CCK-8 tests demonstrated that 10μM CORT treatment for 6h had no inhibition effect on cells. However, intracellular reactive oxygen species (ROS) production increased and adenosine triphosphate (ATP) level decreased significantly after same CORT treatment, which was corrected by TPEN and aggravated by ZnSO4. It could be suggested that the increased intracellular Zn(2+) by CORT was greatly dependent on intracellular Zn(2+) release, but not extracellular Zn(2+) intake. Meanwhile, our results demonstrated that increased intracellular Zn(2+) by CORT resulted in ROS generation and decreased ATP level in cells, which have possible roles in the hippocampal function disorder induced by stress.

Low dietary intake of magnesium is associated with increased externalising behaviours in adolescents

OBJECTIVE

Adequate Zn and Mg intakes may be beneficial for the prevention and treatment of mental health problems, such as depression, anxiety and attention-deficit hyperactivity disorder. We aimed to investigate the prospective association between dietary intakes of Zn and Mg and internalising and externalising behaviour problems in a population-based cohort of adolescents.

DESIGN

Prospective analysis (general linear mixed models) of dietary intakes of Zn and Mg assessed using a validated FFQ and mental health symptoms assessed using the Youth Self-Report (YSR), adjusting for sex, physical activity, family income, supplement status, dietary misreporting, BMI, family functioning and energy intake.

SETTING

Western Australian Pregnancy Cohort (Raine) Study.

SUBJECTS

Adolescents (n 684) at the 14- and 17-year follow-ups.

RESULTS

Higher dietary intake of Mg (per SD increase) was significantly associated with reduced externalising behaviours (β = -1.45; 95% CI -2.40, -0.50; P = 0.003). There was a trend towards reduced externalising behaviours with higher Zn intake (per SD increase; β = -0.73; 95% CI -1.57, 0.10; P = 0.085).

CONCLUSIONS

The study shows an association between higher dietary Mg intake and reduced externalising behaviour problems in adolescents. We observed a similar trend, although not statistically significant, for Zn intake. Randomised controlled trials are necessary to determine any benefit of micronutrient supplementation in the prevention and treatment of mental health problems in adolescents.

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

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

Psychological stress induced hippocampus zinc dyshomeostasis and depression-like behavior in rats

There are strong evidences showed that psychological stress (PS) could result in depression. Recently, many attentions were paid to the roles of corticosterone (CORT) and zinc dyshomeostasis in the development of depression. In this study, we investigated the zinc level in rat hippocampus after exposure to PS and accompanied behavior change. Male SD rats were randomly divided into the control and PS groups. Each group had two subgroups: 7-d group and 14-d group. A communication box was used to produce the PS model in rats. Compared to control group, the PS-treated group showed decreased total zinc levels and increased free zinc levels observed by TSQ staining in hippocampus. Meanwhile, there were significant decreases in mRNA expressions of zinc transporters including ZnT1, ZnT3 and ZIP1 and metallothionein (MT) contents in hippocampus. Moreover, the increased immobility time in forced swim test (FST), lower movement time and total movement distance and longer immobile time in spontaneous activity test were demonstrated in rats after PS exposure. These results suggested that the depression-like behavior in PS-treated rats might be correlated with zinc dyshomeostasis including decreased zinc contents and increased free zinc in hippocampus which was related to changes in zinc transporters and MT expressions.

Psychological stress-induced lower serum zinc and zinc redistribution in rats

In humans, long-term exposure to uncontrollable and unpredictable life stressors is a major precipitant in the development of depressive disorders. There are strong evidences that depression is accompanied by lower serum zinc. The aim of present study is to assess the effects of repeated psychological stress (PS) on the zinc metabolism in rat. The rats were divided into control group and PS group which were subdivided into three subgroups: 7-day group, 14-day group, and recovery group (ten rats in each subgroup). PS model was created by a communication box which contains room A and room B. Rats in room A were only exposed to the responses of rats which were randomly given electrical shock for 30 min in room B. PS was given to rats for 30 min every morning for 14 days. The serum corticosterone (CORT), zinc in serum and tissues, and zinc apparent absorption after PS exposure were investigated. The results showed that the serum CORT increased and serum zinc decreased after 7 and 14 days of PS treatment. The zinc concentration in the liver was increased by 14 days PS exposure, whereas its concentration in the hippocampus was decreased by 7 and 14 days of PS exposure. There were no significant changes in zinc concentration in the heart, spleen, kidney, duodenum, cortex, and cerebellum. A decrease in the zinc apparent absorption was observed in the 7- and 14-day PS groups. The increased serum CORT and liver zinc concentrations and decreased serum zinc and apparent absorption of zinc recovered to normal concentrations 7 days away from PS exposure. The results suggest that PS could induce lower serum zinc, which might be correlated with decreased zinc absorption in the small intestine and increased liver zinc accumulation after PS exposure. The consequent effects of decreased hippocampal and serum zinc and increased CORT concentration after PS exposure on stress-related diseases await further research.

New alternately colored FRET sensors for simultaneous monitoring of Zn²⁺ in multiple cellular locations

Genetically encoded sensors based on fluorescence resonance energy transfer (FRET) are powerful tools for reporting on ions, molecules and biochemical reactions in living cells. Here we describe the development of new sensors for Zn²⁺based on alternate FRET-pairs that do not involve the traditional CFP and YFP. Zn²⁺ is an essential micronutrient and plays fundamental roles in cell biology. Consequently there is a pressing need for robust sensors to monitor Zn²⁺ levels and dynamics in cells with high spatial and temporal resolution. Here we develop a suite of sensors using alternate FRET pairs, including tSapphire/TagRFP, tSapphire/mKO, Clover/mRuby2, mOrange2/mCherry, and mOrange2/mKATE. These sensors were targeted to both the nucleus and cytosol and characterized and validated in living cells. Sensors based on the new FRET pair Clover/mRuby2 displayed a higher dynamic range and better signal-to-noise ratio than the remaining sensors tested and were optimal for monitoring changes in cytosolic and nuclear Zn²⁺. Using a green-red sensor targeted to the nucleus and cyan-yellow sensor targeted to either the ER, Golgi, or mitochondria, we were able to monitor Zn²⁺ uptake simultaneously in two compartments, revealing that nuclear Zn²⁺ rises quickly, whereas the ER, Golgi, and mitochondria all sequester Zn²⁺ more slowly and with a delay of 600-700 sec. Lastly, these studies provide the first glimpse of nuclear Zn²⁺ and reveal that nuclear Zn²⁺ is buffered at a higher level than cytosolic Zn²⁺.

Effect of 17ß-estradiol on zinc content of hippocampal mossy fibers in ovariectomized adult rats

Sex hormones such as estrogen (17ß-estradiol) may modulate the zinc content of the hippocampus during the female estrous cycle. The mossy fiber system is highly plastic in the adult brain and is influenced by multiple factors including learning, memory, and stress. However, whether 17ß-estradiol is able to modulate the morphological plasticity of the mossy fibers throughout the estrous cycle remains unknown. Ovariectomized (Ovx) female 70- to 90-day-old Sprague-Dawley rats without or with estrogen supplement (OvxE) were compared with control rats in three stages of the estrous cycle: diestrus, proestrus, and estrus. The brain tissue from each of the five groups was processed with Timm's silver sulfide technique using the Image J program to measure the mossy fiber area in the stratum lucidum of CA3. Total zinc in the hippocampus was measured using Graphite Furnace Atomic Absorption Spectrophotometry. Two additional (Ovx and OvxE) groups were examined in spatial learning and memory tasks using the Morris water maze. Similar increases in total zinc content and mossy fiber area were observed. The mossy fiber area decreased by 26 ± 2 % (difference ± SEM percentages) in Ovx and 23 ± 4 % in estrus as compared to the proestrus group and by 18 ± 2 % in Ovx compared to OvxE. Additionally, only the OvxE group learned and remembered the task. These results suggest that estradiol has a significant effect on zinc content in hippocampal CA3 during the proestrus stage of the estrous cycle and is associated with correct performance in learning and memory.