Differential effects of zinc on glutamatergic and GABAergic neurotransmitter systems in the hippocampus

Carbonic anhydrase I, II, and VI, blood plasma, erythrocyte and saliva zinc and copper increase after repetitive transcranial magnetic stimulation

INTRODUCTION

Repetitive transcranial magnetic stimulation (rTMS) has been used to treat symptoms from many disorders; biochemical changes occurred with this treatment. Preliminary studies with rTMS in patients with taste and smell dysfunction improved sensory function and increased salivary carbonic anhydrase (CA) VI and erythrocyte CA I, II. To obtain more information about these changes after rTMS, we measured changes in several CA enzymes, proteins, and trace metals in their blood plasma, erythrocytes, and saliva.

METHODS

Ninety-three patients with taste and smell dysfunction were studied before and after rTMS in an open clinical trial. Before and after rTMS, we measured erythrocyte CA I, II and salivary CA VI, zinc and copper in parotid saliva, blood plasma, and erythrocytes, and appearance of novel salivary proteins by using mass spectrometry.

RESULTS

After rTMS, CA I, II and CA VI activity and zinc and copper in saliva, plasma, and erythrocytes increased with significant sensory benefit. Novel salivary proteins were induced at an m/z value of 21.5K with a repetitive pattern at intervals of 5K m/z.

CONCLUSIONS

rTMS induced biochemical changes in specific enzymatic activities, trace metal concentrations, and induction of novel salivary proteins, with sensory improvement in patients with taste and smell dysfunction. Because patients with several neurologic disorders exhibit taste and smell dysfunction, including Parkinson disease, Alzheimer disease, and multiple sclerosis, and because rTMS improved their clinical symptoms, the biochemical changes we observed may be relevant not only in our patients with taste and smell dysfunction but also in patients with neurologic disorders with these sensory abnormalities.

Insight into zinc signaling from dietary zinc deficiency

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

Zinc deficiency reduces neurogenesis accompanied by neuronal apoptosis through caspase-dependent and -independent signaling pathways

Dietary zinc deficiency may affect zinc homeostasis in the brain and lead to reductions of neurogenesis and neuronal survival. However, the mechanisms responsible for the effects of zinc deficiency on hippocampal neurogenesis and neuronal death remain obscure. In the present study, young CD-1 mice were fed with zinc-deficient diet (0.85 ppm) for 5 weeks. The vesicular zinc was reduced at CA1 and CA3 regions of the hippocampus in zinc-deficient mice. The significant decreased zinc ions was associated with a reduction in proliferating cells labeled with bromo-deoxyuridine (BrdU) and immature neurons labeled with doublecortin (DCX) immunoreactivity in the dentate gyrus of the hippocampus. The processes of DCX-positive neurons were shortened, and flexuously went through into the granular cell layer in zinc-deficient hippocampus. There was also a conspicuous increase in the number of TUNEL-positive cells in the hippocampus after zinc-deficient diet treatment. Meanwhile, the apoptosis proteins, including Fas, Fas ligand (FasL), apoptosis inducing factor (AIF), and caspase-3, were significantly activated in zinc-deficient mouse hippocampus. These data suggest that chronic treatment with zinc-deficient diet results in reduction in hippocampal neurogenesis and increases neuronal apoptosis, indicating that zinc deficiency is associated with destroying structural plasticity in the hippocampus.

Hippocampal zinc infusion delays the development of afterdischarges and seizures in a kindling model of epilepsy

PURPOSE

Zinc occurs in high concentration in synaptic vesicles of glutamatergic terminals including hippocampal mossy fibers. This vesicular zinc can be synaptically released during neuronal activity, including seizures. Zinc inhibits certain subtypes of N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid (GABA)(A) receptors. By blocking NMDA excitation or GABA inhibition, an excess of zinc may alter the excitability of hippocampal circuits, which contribute to the development of seizures.

METHODS

Twenty-one adult Wistar rats were implanted under anesthesia with Alzet pumps releasing vehicle, 10 microM ZnCl(2) or 1,000 microM ZnCl(2), at a rate of 0.25 microl/h continuously into the hippocampal hilus for 4 weeks. Kindling was performed by daily awake commissural stimulation at 60 Hz and afterdischarges were recorded from a dentate gyrus electrode. Development of behavioral Racine seizure stages was recorded by a blinded investigator.

RESULTS

The development of behavioral Racine seizure stages was delayed only in rats infused with 1,000 microM ZnCl(2) (p < 0.02). With completion of kindling at stimulation number 20, all groups had reached the same maximum level of behavioral seizures. The expected increased progression of afterdischarge duration was inhibited by both 10 microM ZnCl(2) and 1,000 microM ZnCl(2) infusion compared to control animals (p < 0.01). At stimulation number 18, all groups had reached the same maximum duration of afterdischarges.

DISCUSSION

We conclude that excess infused zinc delayed the development of behavioral seizures in a kindling model of epilepsy. These data support the hypothesis that zinc synaptically released during seizures may alter hippocampal excitability similar to zinc infused in our experiment.

Comparison of the effects of dose-dependent zinc chloride on short-term and long-term memory in young male rats

The aim of the present study was to evaluate the effects of dose-dependent of zinc chloride on short-term and long-term memory in a shuttle box. Young Wistar rats (94+/-10 g) (age 27-30 days) consumed zinc chloride drinking water in five different doses (20, 30, 50, 70 and 100 mg kg(-1) day(-1)) for two weeks by gavage. After 14 days on experimental diets, a shuttle box used to test short- and long-term memory. Two criteria considering for behavioral test, including latency in entering dark chamber and time spent in the dark chamber. This experiment shows that after 2 weeks oral administration of ZnCl2 with (20, 30 and 50 mg kg(-1) day(-1)) doses, the rat's working (short-term) has been improved (p<0.05). Whereas ZnCl2 with 30 mg kg(-1) day(-1) dose has been more effected than other doses (p<0.001). But rat which received ZnCl2 with 100 mg kg(-1) day(-1), has been shown significant impairment in working memory (p<0.05) and there was no significant difference in reference (long-term) memory for any of groups. In general, this study has demonstrated that zinc chloride consumption with 30 mg kg(-1) day(-1) dose for two weeks was more effective than other doses on short-term memory. But consumption of ZnCl2 with 100 mg kg(-1) day(-1) dose for two week had the negative effect on short-term memory. On the other hand, zinc supplementation did not have an effect on long-term memory.

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

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

Effects of calcium channel blockers on antidepressant action of Alprazolam and Imipramine

Alprazolam is effective as an anxiolytic and in the adjunct treatment of depression. In this study, the effects of calcium channel antagonists on the antidepressant action of alprazolam and imipramine were investigated. A forced swimming maze was used to study behavioral despair in albino mice. Mice were divided into nine groups (n = 7 per group). One group received a single dose of 1% Tween 80; two groups each received a single dose of the antidepressant alone (alprazolam or imipramine); two groups each received a single dose of the calcium channel blocker (nifedipine or verapamil); four groups each received a single dose of the calcium channel blocker followed by a single dose of the antidepressant (with same doses used for either in the previous four groups). Drug administration was performed concurrently on the nine groups. Our data confirmed the antidepressant action of alprazolam and imipramine. Both nifedipine and verapamil produced a significant antidepressant effect (delay the onset of immobility) when administered separately. Verapamil augmented the antidepressant effects of alprazolam and imipramine (additive antidepressant effect). This may be due to the possibility that verapamil might have antidepressant-like effect through different mechanism. Nifedipine and imipramine combined led to a delay in the onset of immobility greater than their single use but less than the sum of their independent administration. This may be due to the fact that nifedipine on its own might act as an antidepressant but blocks one imipramine mechanism that depends on L-type calcium channel activation. Combining nifedipine with alprazolam produced additional antidepressant effects, which indicates that they exert antidepressant effects through different mechanisms.

Zinc release at the synaptic terminals of rod photoreceptors

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

Depletion of vesicular zinc in dorsal horn of spinal cord causes increased neuropathic pain in mice

Zinc enriched (ZEN) neurons and terminals are abundant in the rodent spinal cord. Zinc ions have been suggested to modulate the excitability of primary afferent fibers believed to be important in nociceptive transmission. To test the hypothesis that vesicular zinc concentration is related to neuropathic pain we applied Chung's rodent pain model on BALB/c mice, and traced zinc transporter 3 (ZnT3) proteins and zinc ions with immunohistochemistry and autometallography (AMG), respectively. Under anesthesia the left fifth lumbar spinal nerve was ligated in male mice in order to produced neuropathic pain. The animals were then sacrificed 5 days later. The ZnT3 immunoreactivity was found to have decreased significantly in dorsal horn of fourth, fifth, and sixth lumbar segments. In parallel with the depressed ZnT3 immunoreactivity the amount of vesicular zinc decreased perceptibly in superficial gray matters of especially layer I-IV of the same segments. The transection-induced reduction of vesicular zinc in ZEN terminals of the dorsal horn was synchronic to reduced pain threshold, as measured by von Frey method. In a separate study, we observed intensive zinc selenite precipitation in somata of the smaller spinal ganglion cell, but 5 days after spinal nerve transection zinc precipitation was also found in the lager ganglion cells. The present results indicate that zinc may be involved in pain mechanism in the spinal ganglion level. These results support the hypothesis that vesicular zinc might have a modulatory role for neuropathic pain. Thus, increased pain sensitivity might be related to reduce vesicular zinc level in the dorsal spinal gray matter.

Zn2+ sensitivity of high- and low-voltage activated calcium channels

The essential cation zinc (Zn2+) blocks voltage-dependent calcium channels in several cell types, which exhibit different sensitivities to Zn2+. The specificity of the Zn2+ effect on voltage-dependent calcium channel subtypes has not been systematically investigated. In this study, we used a transient protein expression system to determine the Zn2+ effect on low- and high-voltage activated channels. We found that in Ba2+, the IC50 value of Zn2+ was alpha1-subunit-dependent with lowest value for CaV1.2, and highest for CaV3.1; the sensitivity of the channels to Zn2+ was approximately ranked as CaV1.2>CaV3.2>CaV2.3>CaV2.2=CaV 2.1>or=CaV3.3=CaV3.1. Although the CaV2.2 and CaV3.1 channels had similar IC50 for Zn2+ in Ba2+, the CaV2.2, but not CaV3.1 channels, had approximately 10-fold higher IC50 to Zn2+ in Ca2+. The reduced sensitivity of CaV2.2 channels to Zn2+ in Ca2+ was partially reversed by disrupting a putative EF-hand motif located external to the selectivity filter EEEE locus. Thus, our findings support the notion that the Zn2+ block, mediated by multiple mechanisms, may depend on conformational changes surrounding the alpha1 pore regions. These findings provide fundamental insights into the mechanism underlying the inhibitory effect of zinc on various Ca2+ channel subtypes.

Fluorescence-based zinc ion sensor for zinc ion release from pancreatic cells

In this paper, we describe the synthesis and characterization of analytical properties of fluorescence-based zinc ion-sensing glass slides and their application in monitoring zinc ion release from beta pancreatic cells in cell cultures. To fabricate the sensors, the zinc ion indicator ZnAF-2 {6-[N-[N',N'-bis(2-pyridinylmethyl)-2-aminoethyl]amino-3',6'-dihydroxyspiro[isobenzofuran-1(3H),9'-[9H]xanthene]-3-one} was modified to include a sufficiently long linking aliphatic chain with a terminal carboxyl functional group. The recently synthesized ZnAF-2 zinc ion indicator provided high zinc ion selectivity in physiological solutions containing millimolar levels of calcium and other possible interfering cations. The carboxyl-modified ZnAF-2 was conjugated to the activated surface of glass slides, which then served as zinc ion sensors. It was possible to grow pancreatic cells directly on the zinc-sensing glass slide or on a membrane placed on these glass slides. The sensors were used to monitor zinc ion release events from glucose-stimulated pancreatic cells. The study showed that the zinc ion sensors responded effectively to the release of zinc ions from pancreatic cells at the nanomolar level with high selectivity and rapid subsecond response time.

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

The uptake of free zinc into CA3 pyramidal cells and its significance was examined in rat hippocampal slices with ZnAF-2DA, a membrane-permeable zinc indicator. Intracellular ZnAF-2 signal in the CA3 pyramidal cell layer was increased during delivery of tetanic stimuli to the dentate granule cell layer. This increase was completely blocked in the presence of CNQX, an AMPA/kainate receptor antagonist. These results suggest that free zinc is taken up into CA3 pyramidal cells via activation of AMPA/kainate receptors. The effect of free zinc levels in the CA3 pyramidal cells on the increase in intracellular calcium via Group I metabotropic glutamate receptors was examined by regional delivery of tADA, a Group I metabotropic glutamate receptor agonist, to the stratum lucidum after blockade of AMPA/kainate receptor-mediated calcium and zinc influx. Intracellular calcium orange signal in the CA3 pyramidal cell layer was increased by tADA, whereas intracellular ZnAF-2 signal was not increased even in the presence of 100 muM zinc, suggesting that tADA induces calcium release from internal stores in CA3 pyramidal cells and is not involved in zinc uptake. The increase in calcium orange signal by tADA was enhanced by perfusion with pyrithione, a zinc ionophore that decreased basal ZnAF-2 signal in the CA3 pyramidal cell layer. It was blocked by perfusion with pyrithione and zinc that increased basal ZnAF-2 signal. The present study indicates that the increase in free calcium levels via the metabotropic glutamate receptor pathway is inversely related to free zinc levels in CA3 pyramidal cells.

Impairment of GABAergic neurotransmitter system in the amygdala of young rats after 4-week zinc deprivation

On the basis of the evidence that the excitability of hippocampal glutamatergic neurotransmitter system is enhanced by dietary zinc deficiency, the response of amygdalar neurotransmitter system was checked in young rats fed a zinc-deficient diet for 4 weeks. Extracellular zinc concentration in the amygdala, which was measured by the in vivo microdialysis, was almost the same as that in the hippocampus and decreased by zinc deficiency. Extracellular zinc concentration in the amygdala was increased both in the control and zinc-deficient rats by stimulation with 100mM KCl, suggesting that the increase in extracellular zinc in the amygdala, as well as that in the hippocampus, is linked with neuronal depolarization. In amygdalar extracellular fluid, the basal glutamate concentration was not significantly different between the control and zinc-deficient rats and was increased to almost the same extent between them by stimulation with 100mM KCl, unlike more increase in extracellular glutamate concentration in the hippocampus in zinc deficiency. On the other hand, the basal GABA concentration in the amygdalar extracellular fluid was significantly lower in zinc-deficient rats and was not increased both in the control and zinc-deficient rats by stimulation with 100mM KCl. These results suggest that GABAergic neurotransmitter system is critically impaired in the amygdala of young rats after 4-week zinc deprivation.

Zinc release in the lateral nucleus of the amygdala by stimulation of the entorhinal cortex

Zinc release in the lateral nucleus of the amygdala was examined using rat brain slices. The lateral and basolateral nuclei in the amygdala were evidently stained by Timm's sulfide-silver staining method. When the amygdala including both the nuclei was stimulated with 100 mM KCl by means of in vivo microdialysis, extracellular zinc concentration was increased significantly. Zinc release in the lateral nucleus of the amygdala innervated by the entorhinal cortex was next examined in brain slices double-stained with zinc and calcium indicators. Extracellular zinc signal (ZnAF-2) in the lateral nucleus was increased with intracellular calcium signal (calcium orange) during delivery of tetanic stimuli to the entorhinal cortex. Both the increases were completely inhibited by addition of 1 micro M tetrodotoxin, a sodium channel blocker. Furthermore, calcium signal in the lateral nucleus during delivery of tetanic stimuli to the entorhinal cortex was increased in the presence of 10 micro M CNQX, an AMPA/KA receptor antagonist, and this increase was facilitated by addition of 1 mM CaEDTA, a membrane-impermeable zinc chelator. The present study suggested that zinc is released in the lateral nucleus of the amygdala by depolarization of the entorhinal neurons. In the lateral nucleus, zinc released may suppress the increase in presynaptic calcium signal.

Responsiveness to kainate in young rats after 2-week zinc deprivation

On the basis of the evidence of the enhanced susceptibility to kainate-induced seizures in young rats fed a zinc-deficient diet for 4 weeks, the relationship between zinc release from hippocampal neuron terminals and seizure susceptibility was studied in young rats fed the zinc-deficient diet for 2 weeks. Timm's stain, with which histochemically reactive zinc in the presynaptic vesicle is detected, was not attenuated in mossy fibers and other areas in the hippocampus after 2-week zinc deprivation, whereas the attenuation was observed after 4-week zinc deprivation. Extracellular zinc concentration was not also decreased after 2-week zinc deprivation, unlike the case after 4-week zinc deprivation. To check the capacity for zinc release from neuron terminals after 2-week zinc deprivation, the hippocampus was excessively stimulated with 100 mM KCl. The increase in extracellular zinc concentration of zinc-deficient group was significantly more than that of control group. These results suggest that zinc release from hippocampal neuron terminals is not affected by 2-week zinc deprivation. On the other hand, the latency in myoclonic jerks of zinc-deficient group was significantly shorter than in the control group after treatment with kainate, while the latency in clonic convulsions was not different between the two groups. Intracellular fura-2 signal, a calcium indicator, was significantly higher in the hippocampal CA3 areas of zinc-deficient group 4 s after delivery of kainate to dentate granule cells. These results suggest that susceptibility to kainate-induced seizures is altered prior to the decrease in extracellular zinc concentration and zinc release from neuron terminals in zinc-deficient young rats. The alteration of calcium signaling seems to be involved in the susceptibility in zinc deficiency.

Region-specific loss of zinc in the brain in pentylentetrazole-induced seizures and seizure susceptibility in zinc deficiency

The hippocampus is thought to be an epileptic focus in human temporal lobe epilepsy. Kainate-induced seizures decrease zinc concentrations in the hippocampus, which is also decreased in young mice fed a zinc-deficient diet for 4 weeks, and is enhanced by zinc deficiency. To understand zinc movement in the brain in epileptic seizures, zinc concentrations in the brain were measured in young mice after administration of pentylentetrazole, a GABAA receptor antagonist. Zinc concentration in the hippocampus and Timm's stain, with which histochemically reactive zinc in the presynaptic vesicle is detected, were decreased after the administration, suggesting that excessive excitation of zinc-containing glutamatergic neurons is induced in the hippocampus with pentylentetrazole. To clarify whether the decrease in zinc concentration in the hippocampus in zinc deficiency alter seizure susceptibility, furthermore, susceptibility to pentylentetrazole-induced seizures was examined in young mice fed the zinc-deficient diet for 4 weeks. The susceptibility, unlike susceptibility to kainate-induced seizures, was not appreciably enhanced by zinc deficiency. These results suggest that the decrease in zinc concentration in the hippocampus in zinc deficiency does not influence susceptibility to pentylentetrazole-induced seizures.

Facilitation of granule cell epileptiform activity by mossy fiber-released zinc in the pilocarpine model of temporal lobe epilepsy

Recurrent mossy fiber synapses in the dentate gyrus of epileptic brain facilitate the synchronous firing of granule cells and may promote seizure propagation. Mossy fiber terminals contain and release zinc. Released zinc inhibits the activation of NMDA receptors and may therefore oppose the development of granule cell epileptiform activity. Hippocampal slices from rats that had experienced pilocarpine-induced status epilepticus and developed a recurrent mossy fiber pathway were used to investigate this possibility. Actions of released zinc were inferred from the effects of chelation with 1 mM calcium disodium EDTA (CaEDTA). When granule cell population bursts were evoked by mossy fiber stimulation in the presence of 6 mM K(+) and 30 microM bicuculline, CaEDTA slowed the rate at which evoked bursting developed, but did not change the magnitude of the bursts once they had developed fully. The effects of CaEDTA were then studied on the pharmacologically isolated NMDA receptor- and AMPA/kainate receptor-mediated components of the fully developed bursts. CaEDTA increased the magnitude of NMDA receptor-mediated bursts and reduced the magnitude of AMPA/kainate receptor-mediated bursts. CaEDTA did not affect the granule cell bursts evoked in slices from untreated rats by stimulating the perforant path in the presence of bicuculline and 6 mM K(+). These results suggest that zinc released from the recurrent mossy fibers serves mainly to facilitate the recruitment of dentate granule cells into population bursts.

Protective effect of zinc against ischemic neuronal injury in a middle cerebral artery occlusion model

In this study, we investigated the effect of vesicular zinc on ischemic neuronal injury. In cultured neurons, addition of a low concentration (under 100 microM) of zinc inhibited both glutamate-induced calcium influx and neuronal death. In contrast, a higher concentration (over 150 microM) of zinc decreased neuronal viability, although calcium influx was inhibited. These results indicate that zinc exhibits biphasic effects depending on its concentration. Furthermore, in cultured neurons, co-addition of glutamate and CaEDTA, which binds extra-cellular zinc, increased glutamate-induced calcium influx and aggravated the neurotoxicity of glutamate. In a rat transient middle cerebral artery occlusion (MCAO) model, the infarction volume, which is related to the neurotoxicity of glutamate, increased rapidly on the intracerebral ventricular injection of CaEDTA 30 min prior to occlusion. These results suggest that zinc released from synaptic vesicles may provide a protective effect against ischemic neuronal injury.

Zinc release from Schaffer collaterals and its significance

On the basis of the evidence that approximately 45% of Schaffer collateral boutons are zinc-positive, zinc release from Schaffer collaterals and its action were examined in hippocampal slices. When zinc release from Schaffer collaterals was examined using ZnAF-2, a membrane-impermeable zinc indicator, ZnAF-2 signal in the stratum radiatum of the CA1 was increased by tetanic stimuli at 100 Hz for 1s, suggesting that zinc is released from Schaffer collaterals in a calcium- and impulse-dependent manner. An in vivo microdialysis experiment indicated that the perfusion with 10 microM zinc significantly decreases extracellular glutamate concentration in the CA1. When tetanic stimuli at 100 Hz for 5s were delivered to the dentate granule cells, the increase in calcium signal in the stratum radiatum of the CA1, as well as in the stratum lucidum of the CA3, was attenuated by addition of 10 microM zinc, while enhanced by addition of 1mM CaEDTA, a membrane-impermeable zinc chelator. The increase in calcium signal in the CA1, in which Schaffer collateral synapses exist, during delivery of tetanic stimuli at 100 Hz for 1s to the Schaffer collateral-commissural pathway was also significantly enhanced by addition of 1mM CaEDTA. These results suggest that zinc released from Schaffer collaterals suppressively modulates presynaptic and postsynaptic calcium signaling in the CA1, followed by the suppression of glutamate release.

Inhibition of presynaptic activity by zinc released from mossy fiber terminals during tetanic stimulation

Zinc exists in high densities in the giant boutons of hippocampal mossy fibers. On the basis of the evidence that zinc decreases extracellular glutamate concentration in the hippocampus, the presynaptic action of zinc released from mossy fibers during high-frequency (tetanic) stimulation was examined using hippocampal slices. The increase in zinc-specific fluorescent signals was observed in both extracellular and intracellular compartments in the mossy fiber terminals during the delivery of tetanic stimuli (100 Hz, 1 sec) to the dentate granule cell layer, suggesting that zinc released from mossy fibers is immediately retaken up by mossy fibers. When mossy fiber terminals were preferentially double-stained with zinc and calcium indicators and tetanic stimuli (100 Hz, 1 sec) were delivered to the dentate granule cell layer, the increase in calcium orange signal during the stimulation was enhanced in mossy fiber terminals by addition of CaEDTA, a membrane-impermeable zinc chelator, and was suppressed by addition of zinc. The decrease in FM4-64 signal (vesicular exocytosis) during tetanic stimulation (10 Hz, 180 sec), which induced mossy fiber long-term potentiation, was also enhanced in mossy fiber terminals by addition of CaEDTA and was suppressed by addition of zinc. The present study demonstrates that zinc released from mossy fibers may be a negative-feedback factor against presynaptic activity during tetanic stimulation.

Neo-Timm staining in the thalamus of chronically epileptic rats

The thalamus is an important modulator of seizures and is severely affected in cholinergic models of epilepsy. In the present study, chronically epileptic rats had their brains processed for neo-Timm and acetylcholinesterase two months after the induction of status epilepticus with pilocarpine. Both controls and pilocarpine-treated animals presented neo-Timm staining in the anterodorsal nucleus, laterodorsal nucleus, reticular nucleus, most intralaminar nuclei, nucleus reuniens, and rhomboid nucleus of the thalamus, as well as in the zona incerta. The intensity of neo-Timm staining was similar in control and pilocarpine-treated rats, except for the nucleus reuniens and the rhomboid nucleus, which had a lower intensity of staining in the epileptic group. In animal models of temporal lobe epilepsy, zinc seems to modulate glutamate release and to decrease seizure activity. In this context, a reduction of neo-Timm-stained terminals in the midline thalamus could ultimately result in an increased excitatory activity, not only within its related nuclei, but also in anatomical structures that receive their efferent connections. This might contribute to the pathological substrate observed in chronic pilocarpine-treated epileptic animals.

Brain, aging and neurodegeneration: role of zinc ion availability

Actual fields of research in neurobiology are not only aimed at understanding the different aspects of brain aging but also at developing strategies useful to preserve brain compensatory capacity and to prevent the onset of neurodegenerative diseases. Consistent with this trend much attention has been addressed to zinc metabolism. In fact, zinc acts as a neuromodulator at excitatory synapses and has a considerable role in the stress response and in the functionality of zinc-dependent enzymes contributing to maintaining brain compensatory capacity. In particular, the mechanisms that modulate the free zinc pool are pivotal for safeguarding brain health and performance. Alterations in zinc homeostasis have been reported in Parkinson's and Alzheimer's disease as well as in transient forebrain ischemia, seizures and traumatic brain injury, but little is known regarding aged brain. There is much evidence that that age-related changes, frequently associated to a decline in brain functions and impaired cognitive performances, could be related to dysfunctions affecting the intracellular zinc ion availability. A general agreement emerges from studies of humans' and rodents' old brains about an increased expression of metallothionein (MT) isoforms I and II, but dyshomogenous results are reported for MT-III, and it is still uncertain whether these proteins maintain in aging the protective role, as it occurs in adult/young age. At the same time, there is considerable evidence that amyloid-beta deposition in Alzheimer's disease is induced by zinc, but the pathological significance and the causes of this phenomenon are still an open question. The scientific debate on the role of zinc and of some zinc-binding proteins in aging and neurodegenerative disorders, as well as on the beneficial effect of zinc supplementation in aged brain and neurodegeneration, is extensively discussed in this review.

Subtype-Specific GABA Transporter Antagonists Synergistically Modulate Phasic and Tonic GABAA Conductances in Rat Neocortex

GABAergic inhibition in the brain can be classified as either phasic or tonic. γ-Aminobutyric acid (GABA) uptake by GABA transporters (GATs) can limit the time course of phasic currents arising from endogenous and exogenous GABA, as well as decrease a tonically active GABA current. GABA transporter subtypes 1 and 3 (GAT-1 and GAT-3) are the most heavily expressed of the four known GAT subtypes. The role of GATs in shaping GABA currents in the neocortex has not been explored. We obtained patch-clamp recordings from layer II/III pyramidal cells and layer I interneurons in rat sensorimotor cortex. We found that selective GAT-1 inhibition with NO711 decreased the amplitude and increased the decay time of evoked inhibitory postsynaptic currents (IPSCs) but had no effect on the tonic current or spontaneous IPSCs (sIPSCs). GAT-2/3 inhibition with SNAP-5114 had no effect on IPSCs or the tonic current. Coapplication of NO711 and SNAP-5114 substantially increased tonic currents and synergistically decreased IPSC amplitudes and increased IPSC decay times. sIPSCs were not resolvable with coapplication of NO711 and SNAP-5114. The effects of the nonselective GAT antagonist nipecotic acid were similar to those of NO711 and SNAP-5114 together. We conclude that synaptic GABA levels in neocortical neurons are controlled primarily by GAT-1, but that GAT-1 and GAT-2/3 work together extrasynaptically to limit tonic currents. Inhibition of any one GAT subtype does not increase the tonic current, presumably as a result of increased activity of the remaining transporters. Thus neocortical GAT-1 and GAT-2/3 have distinct but overlapping roles in modulating GABA conductances.

Involvement of unusual glutamate release in kainate-induced seizures in zinc-deficient adult rats

Enhanced susceptibility to kainate-induced seizures is linked to a lack of increase in GABA release in zinc-deficient young rats. In the present study, susceptibility to kainate-induced seizures was examined in adult (12-week-old) rats fed a zinc-deficient diet for 4 weeks to evaluate the relationship between the dysfunction of GABAergic neurotransmitter system and seizure susceptibility in zinc deficiency. Susceptibility to kainate-induced seizures was enhanced in zinc-deficient adult rats. Extracellular glutamate concentration in the hippocampus was markedly (>2 times) higher in zinc-deficient adult rats than in the control before and after treatment with kainate. Extracellular zinc concentration in the hippocampus was also higher in zinc-deficient adult rats. Interestingly, it was decreased in both zinc-deficient and control rats after treatment with kainate. In some zinc-deficient adult rats, extracellular GABA concentration in the hippocampus was increased with extracellular glutamate concentration after treatment with kainate. When the capacity for GABA release was examined by excessive stimulation with 100mM KCl, extracellular GABA concentrations were significantly increased even in zinc-deficient adult rats. These results suggest that GABA can be excessively released in zinc-deficient adult rats. An unusual glutamate release may be involved in the enhanced susceptibility to kainate-induced seizures in zinc-deficient adult rats.

Zinc inhibition of gamma-aminobutyric acid transporter 4 (GAT4) reveals a link between excitatory and inhibitory neurotransmission

gamma-Aminobutyric acid (GABA) transporters (GATs) play an important role in inhibitory neurotransmission by clearing synaptically released GABA and by maintaining low resting levels of GABA in synaptic and extrasynaptic regions. In certain brain regions, vesicular zinc is colocalized and coreleased with glutamate and modulates the behavior of a number of channels, receptors, and transporters. We examined the effect of zinc on expressed GATs (GAT1, GAT2, GAT3, and GAT4) in Xenopus laevis oocytes by using tracer flux and electrophysiological methods. We show that zinc is a potent inhibitor of GAT4 (K(i) of 3 muM). Immunolocalization of GAT4 in the hippocampus revealed dense localization in the CA1 and CA3 regions of the hippocampus, regions which are known to be heavily populated by zinc-containing glutamatergic neurons. The results suggest a physiological role of synaptically released zinc in the hippocampus, because zinc released from hyperactive glutamatergic neurons may simultaneously bring about elevated GABAergic inhibition. Therefore, this mode of zinc function signifies a link between excitatory and inhibitory neurotransmission and may play a neuroprotective role against glutamate-induced excitotoxicity.

Xenon reduces glutamate-, AMPA-, and kainate-induced membrane currents in cortical neurones

BACKGROUND

The anaesthetic, analgesic, and neuroprotective effects of xenon (Xe) are believed to be mediated by a block of the NMDA (N-methyl-D-aspartate) receptor channel. Interestingly, the clinical profile of the noble gas differs markedly from that of specific NMDA receptor antagonists. The aim of this study was, therefore, to investigate whether Xe might be less specific, also inhibiting the two other subtypes of glutamate receptor channels, such as the alpha-amino-3-hydroxy-5-methyl-4-isoxazolole propionate (AMPA) and kainate receptors.

METHODS

The study was performed on voltage-clamped cortical neurones from embryonic mice and SH-SY5Y cells expressing GluR6 kainate receptors. Drugs were applied by a multi-barreled fast perfusion system.

RESULTS

Xe, dissolved at approximately 3.45 mM in aqueous solution, diminished the peak and even more the plateau of AMPA and glutamate induced currents. At the control EC(50) value for AMPA (29 microM) these reductions were by about 40 and 56% and at 3 mM glutamate the reductions were by 45 and 66%, respectively. Currents activated at the control EC(50) value for kainate (57 microM) were inhibited by 42%. Likewise, Xe showed an inhibitory effect on kainate-induced membrane currents of SH-SY5Y cells transfected with the GluR6 subunit of the kainate receptor. Xe reduced kainate-induced currents by between 35 and 60%, depending on the kainate concentration.

CONCLUSIONS

Xe blocks not only NMDA receptors, but also AMPA and kainate receptors in cortical neurones as well as GluR6-type receptors expressed in SH-SY5Y cells. Thus, Xe seems to be rather non-specific as a channel blocker and this may contribute to the analgesic and anaesthetic potency of Xe.

Enhanced excitability of hippocampal mossy fibers and CA3 neurons under dietary zinc deficiency

On the basis of the evidence that susceptibility to kainate-induced seizures is enhanced by zinc deficiency and that glutamate concentrations in hippocampal extracellular fluid are excessively increased during seizures, excitability of hippocampal mossy fibers and CA3 neurons was examined using hippocampal slices, which were prepare from mice fed a zinc-deficient diet for 4 weeks. The spatio-temporal dynamics of zinc and calcium was monitored using their indicators, membrane-impermeable ZnAF-2 and membrane-permeable fura-2 AM, respectively. When the molecular layer of dentate gyrus was stimulated with 100mM KCl for 1s, the increased percentages of extracellular zinc in the stratum lucidum and CA3 pyramidal cell layer were higher in zinc-deficient mice than in the control mice, implying that glutamate release from the mossy fibers of the dentate granular cells is enhanced by zinc deficiency. Judging from the increased percentages, however, the amount of zinc released was estimated to be less in zinc-deficient mice. On the other hand, the basal calcium concentrations in the stratum lucidum and CA3 pyramidal cell layer detected with fura-2 were higher in zinc-deficient mice than in the control mice, indicating that hippocampal calcium homeostasis is affected by zinc deficiency. Furthermore, the increased percentage of intracellular calcium in the stratum lucidum by stimulation with high K+ was enhanced by the zinc deficiency. The alteration of hippocampal calcium homeostasis seems to enhance excitability of dentate granular cells in zinc deficiency, following by an enhanced excitability of postsynaptic structures in CA3 neurons.

[Essential trace metals and brain function]

Trace metals such as zinc, manganese, and iron are necessary for the growth and function of the brain. The transport of trace metals into the brain is strictly regulated by the brain barrier system, i.e., the blood-brain and blood-cerebrospinal fluid barriers. Trace metals usually serve the function of metalloproteins in neurons and glial cells, while a portion of trace metals exists in the presynaptic vesicles and may be released with neurotransmitters into the synaptic cleft. Zinc and manganese influence the concentration of neurotransmitters in the synaptic cleft, probably via the action against neurotransmitter receptors and transporters and ion channels. Zinc may be an inhibitory neuromodulator of glutamate release in the hippocampus, while neuromodulation by manganese might mean functional and toxic aspects in the synapse. Dietary zinc deficiency affects zinc homeostasis in the brain, followed by an enhanced susceptibility to the excitotoxicity of glutamate in the hippocampus. Transferrin may be involved in the physiological transport of iron and manganese into the brain and their utilization there. It is reported that the brain transferrin concentration is decreased in neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease and that brain iron metabolism is also altered. The homeostasis of trace metals in the brain is important for brain function and also for the prevention of brain diseases.

Release of amino acids by zinc in the hippocampus

Zinc exists in the synaptic vesicles of hippocampal mossy fibers in high concentrations. On the basis of inhibitory zinc action against glutamate release in the hippocampus, the role of zinc in release of several amino acids were studied in rat hippocampus by using in vivo microdialysis. When the hippocampal CA3 region was perfused with 10 microM ZnCl(2), the concentrations of glutamine, serine, arginine, aspartate, and glycine in the perfusate were significantly increased, whereas the concentrations of amino acids except for glycine were not increased by perfusion with 30 microM ZnCl(2). Chelation of endogenous zinc with 50 microM CaEDTA significantly decreased the concentrations of amino acids in the perfusate except for glycine. In the CA1 region, on the other hand, the concentrations of these five amino acids were not increased by perfusion with 10 microM ZnCl(2) and the concentrations of glutamine and glycine were decreased significantly. The present study suggests that zinc enhances release of glutamine, serine, arginine, and aspartate in the CA3 region and attenuates release of glutamine and glycine in the CA1 region. Zinc seems to modulate glutamatergic synapses multifunctionally in the hippocampus, because glutamine, serine, aspartate, and glycine are involved in synaptic neurotransmission.

Involvement of amygdalar extracellular zinc in rat behavior for passive avoidance

On the basis of the evidence that zinc is released from glutamatergic neuron terminals in the amygdala, the effect of chelation of amygdalar extracellular zinc on glutamate release from the neuron terminals was studied by using in vivo microdialysis. When the amygdala was perfused with 100 microM CaEDTA to chelate extracellular zinc, glutamate concentration in the perfusate was decreased significantly, whereas that tended to be increased by perfusion with 100 microM ZnEDTA as a control. The effect of CaEDTA on extracellular glutamate levels was different between the amygdala and hippocampus, implying that modulation of glutamate signaling by zinc is different between them. To evaluate chelation of zinc in rat behavior, perfusion of the amygdala with CaEDTA was started 40 min before behavioral test for passive avoidance. The behavior for passive avoidance was impaired during perfusion with CaEDTA. On the other hand, the behavior during perfusion with ZnEDTA was more rapidly developed than that with vehicle only. These results suggest that amygdalar extracellular zinc is involved in the behavior for passive avoidance.

Inhibitory function of zinc against excitation of hippocampal glutamatergic neurons

The function of zinc released from the neuron terminals is poorly understood. Here, the action of zinc in excitatory neurotransmission in rat hippocampal CA1 was studied by using in vivo microdialysis. Glutamate concentration in the perfusate was significantly decreased by perfusion with 10-300 microM ZnCl2, suggesting that presynaptic release of glutamate is inhibited by zinc in the CA1. While gamma-amino butyric acid (GABA) concentration in the perfusate was increased by perfusion with zinc. Furthermore, to study the action of zinc in postsynaptic response, the response of the CA1 pyramidal cells in the presence of 50 microM zinc was examined in the entorhinal cortex, which is connected with CA1 pyramidal cells. Perfusion of the hippocampal CA1 with zinc decreased glutamate concentration not only in the CA1, but also in the entorhinal cortex. The increase in glutamate concentration in the entorhinal cortex during perfusion of the CA1 with 50 microM glutamate was inhibited by the addition of zinc in the CA1. Zinc seems to be an inhibitory neuromodulator of glutamate release.