Localization in rat brain of the trace metals, zinc and manganese, after intracerebroventricular injection

Age-Dependent Modification of Intracellular Zn2+ Buffering in the Hippocampus and Its Impact

The basal concentrations of extracellular Zn²⁺ and intracellular Zn²⁺, which are approximately 10 nM and 100 pM, respectively, in the brain, are markedly lower than those of extracellular Ca²⁺ (1.3 mM) and intracellular Ca²⁺ (100 nM), respectively, resulting in much less attention paid to Zn²⁺ than to Ca²⁺. However, intracellular Zn²⁺ dysregulation, which is closely linked with glutamate- and amyloid β-mediated extracellular Zn²⁺ influx, is more critical for cognitive decline and neurodegeneration than intracellular Ca²⁺ dysregulation. It is estimated that the age-dependent increase in the basal concentration of extracellular Zn²⁺ in the hippocampus plays a key role in cognitive decline and neurodegeneration. The characteristics of extracellular Zn²⁺ influx in the hippocampus may be modified age-dependently, probably followed by modification of intracellular Zn²⁺ buffering that is closely linked with age-related cognitive decline and neurodegeneration. Reduction of intracellular Zn²⁺-buffering capacity may be linked with the pathophysiology of progressive neurodegeneration such as Alzheimer's disease. This paper deals with age-dependent modification of intracellular Zn²⁺ buffering in the hippocampus and its impact. On the basis of the estimated impact, we propose a potential defense strategy against Zn²⁺-mediated neurodegeneration, i.e., metallothionein induction in the hippocampus.

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

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

Upregulation of zinc transporter 2 in the blood-CSF barrier following lead exposure

Zinc (Zn) is an essential element for normal brain function; an abnormal Zn homeostasis in brain and the cerebrospinal fluid (CSF) has been implied in the etiology of Alzheimer's disease (AD). However, the mechanisms that regulate Zn transport in the blood-brain interface remain unknown. This study was designed to investigate Zn transport by the blood-CSF barrier (BCB) in the choroid plexus, with a particular focus on Zn transporter-2 (ZnT2), and to understand if lead (Pb) accumulation in the choroid plexus disturbed the Zn regulatory function in the BCB. Confocal microscopy, quantitative PCR and western blot demonstrated the presence of ZnT2 in the choroidal epithelia; ZnT2 was primarily in cytosol in freshly isolated plexus tissues but more toward the peripheral membrane in established choroidal Z310 cells. Exposure of rats to Pb (single ip injection of 50 mg Pb acetate/kg) for 24 h increased ZnT2 fluorescent signals in plexus tissues by confocal imaging and protein expression by western blot. Similar results were obtained by in vitro experiments using Z310 cells. Further studies using cultured cells and a two-chamber Transwell device showed that Pb treatment significantly reduced the cellular Zn concentration and led to an increased transport of Zn across the BCB, the effect that may be due to the increased ZnT2 by Pb exposure. Taken together, these results indicate that ZnT2 is present in the BCB; Pb exposure increases the ZnT2 expression in choroidal epithelial cells by a yet unknown mechanism and as a result, more Zn ions may be deposited into the intracellular Zn pool, leading to a relative Zn deficiency state in the cytoplasm at the BCB.

Zinc induces long-term upregulation of T-type calcium current in hippocampal neurons in vivo

Extracellular zinc can induce numerous acute and persistent physiological and toxic effects in neurons by acting at their plasma membrane or intracellularly following permeation or uptake into them. Zinc acutely and reversibly blocks T-type voltage-gated calcium current (I(CaT)), but the long-term effect of zinc on this current has not been studied. Because chemically induced status epilepticus (SE) results in the release of zinc into the extracellular space, as well as in a long-lasting increase in I(CaT) in CA1 pyramidal cells, we hypothesized that zinc may play a causative role in I(CaT) upregulation. We tested this hypothesis by monitoring for 18 days the effects of zinc and ibotenic acid (a neurotoxic agent serving as control for zinc), injected into the right lateral ventricle, on I(CaT) in rat CA1 pyramidal cells. Both zinc and ibotenic acid caused marked hippocampal lesions on the side of injection, but only minor damage to contralateral hippocampi. Zinc, but not ibotenic acid, caused upregulation of a nickel-sensitive I(CaT) in a subset of contralateral CA1 pyramidal cells, appearing 2 days after injection and lasting for about 2 weeks thereafter. In contrast, acute application of zinc to CA1 pyramidal cells promptly blocked I(CaT). These data indicate that extracellular zinc has a dual effect on I(CaT), blocking it acutely while causing its long-term upregulation. Through the latter effect, zinc may regulate the intrinsic excitability of principal neurons, particularly in pathological conditions associated with enhanced release of zinc, such as SE.

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 transporter 7 is located in the cis-Golgi apparatus of mouse choroid epithelial cells

The cellular localization of zinc transporter 7 protein in the mouse choroid plexus was examined in this study. Zinc transporter 7 immunoreactive cells were detected in the third, lateral, and fourth ventricles of CD-1 mouse brain. Distinct zinc transporter 7 immunoreactivity was concentrated in the perinuclear regions of the positive cells. The results from zinc autometallography showed that zinc-positive grains were also predominantly located in the perinuclear areas. Ultrastructural localization showed that zinc transporter 7 immunostaining was predominantly present in the membrane and cisternae of the cis-Golgi networks and some vesicle compartments. The results support the notion that zinc transporter 7 may participate in the transport of the cytoplasmic zinc into the Golgi apparatus, and may be involved in local packaging of zinc-binding proteins in the mouse choroid plexus.

Manganese

Manganese is an essential mineral that is found at low levels in virtually all diets. Manganese ingestion represents the principal route of human exposure, although inhalation also occurs, predominantly in occupational cohorts. Regardless of intake, animals generally maintain stable tissue manganese levels as a result of homeostatic mechanisms that tightly regulate the absorption and excretion of this metal. However, high-dose exposures are associated with increased tissue manganese levels, causing adverse neurological, reproductive and respiratory effects. In humans, manganese-induced neurotoxicity is associated with a motor dysfunction syndrome, commonly referred to as manganism or Parkinsonism, which is of paramount concern and is considered to be one of the most sensitive endpoints. This article focuses on the dosimetry of manganese with special focus on transport mechanisms of manganese into the CNS. It is not intended to be an exhaustive review of the manganese literature; rather it aims to provide a useful synopsis of contemporary studies from which the reader may progress to other research citations as desired. Specific emphasis is directed towards recent published literature on manganese transporters' systemic distribution of manganese upon inhalation exposure as well as the utility of magnetic resonance imaging in quantifying brain manganese distribution.

Zinc deficiency is associated with increased brain zinc import and LIV-1 expression and decreased ZnT-1 expression in neonatal rats

Zinc (Zn) deficiency has been associated with adverse behavioral outcomes in infants and children. However, Zn deficiency does not affect brain Zn concentration, suggesting that brain Zn homeostasis is tightly regulated. The recent identification of Zn-specific transport proteins allowed us to examine effects of low Zn intake on tissue Zn level, brain Zn uptake, and zinc transporter expression and localization in neonatal rat brain. Female rats were fed diets differing only in Zn content [7, moderately zinc deficient (ZD); 10, marginally zinc deficient (MZD); or 25 mg Zn/kg, control] and pups were killed on postnatal d 11. Plasma and brain Zn concentrations were measured, brain Zn uptake was assessed using (65)Zn, brain metallothionein-I and -III; LIV-1, zinc transporter ZnT-1, and ZnT-3 expression was measured by semiquantitative RT-PCR. LIV-1 localization in the brain was determined by immunohistochemistry; brain and hippocampi LIV-1 and ZnT-1 protein expressions were measured by Western blotting. Plasma Zn concentration was lower in MZD and ZD pups, whereas brain Zn concentration was not affected. Brain Zn uptake was higher in MZD and ZD rats compared with controls. Metallothionein-I and ZnT-1 expressions were lower and LIV-1 expression was higher in the whole brain of MZD and ZD pups. Metallothionein-III and ZnT-3 mRNA expressions were not affected. LIV-1 was localized to the plasma membrane of many brain cell types, including hippocampal pyramidal neurons and the apical membrane of the choroid plexus. Our results indicate that Zn deficiency results in alterations in Zn transporter expression, which facilitates increased brain Zn uptake and results in the conservation of brain Zn during Zn deficiency.

Manganese-enhanced magnetic resonance imaging (MEMRI): methodological and practical considerations

Manganese-enhanced MRI (MEMRI) is being increasingly used for MRI in animals due to the unique T1 contrast that is sensitive to a number of biological processes. Three specific uses of MEMRI have been demonstrated: to visualize activity in the brain and the heart; to trace neuronal specific connections in the brain; and to enhance the brain cytoarchitecture after a systemic dose. Based on an ever-growing number of applications, MEMRI is proving useful as a new molecular imaging method to visualize functional neural circuits and anatomy as well as function in the brain in vivo. Paramount to the successful application of MEMRI is the ability to deliver Mn2+ to the site of interest at an appropriate dose and in a time-efficient manner. A major drawback to the use of Mn2+ as a contrast agent is its cellular toxicity. Therefore, it is critical to use as low a dose as possible. In the present work the different approaches to MEMRI are reviewed from a practical standpoint. Emphasis is given to the experimental methodology of how to achieve significant, yet safe, amounts of Mn2+ to the target areas of interest.

[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.

Direct CSF injection of MnCl(2) for dynamic manganese-enhanced MRI

MnCl(2) was injected intrathecally through the cisterna magna in rats, allowing infusion of divalent manganese ions (Mn(++)) into the CSF space and thence into the brain, without breaking the blood-brain barrier (BBB). Mn(++) uptake and washout dynamics in the brain were measured by serial T(1)-weighted MRI and EPI T(1) and T(2) mapping for up to 3 weeks after injection. Observations within the first 6 hr after injection demonstrated anterograde and bilateral distribution of the Mn(++) within the CSF space, from the olfactory bulb and frontal cortex to the brain stem. Enhancement increased in most brain areas up to 4 days after injection, and then slowly decreased. Relaxation maps at each time point demonstrated higher concentrations of Mn in basal ganglia. Residual concentrations were still observable after 3 weeks in all brain regions. With the use of MnCl(2) calibration phantoms, the maximum Mn concentration in the brain was estimated to be approximately 27 +/- 16 microM, corresponding to changes in relaxation rates of 0.49 +/- 0.30 s(-1) for R(1) and 3.9 +/- 2.4 s(-1) for R(2). For comparison, an intrathecal GdDTPA injection was performed. This injection showed different distribution dynamics: it remained chiefly within the CSF spaces, and was largely washed out after 1 day. This method shows promise as a means of supplying Mn(++) uniformly to the whole brain for a variety of chronic functional activation studies.

Dynamic zinc pools in mouse choroid plexus

We examined the presence of Zn-transporters (ZnT1, ZnT3, ZnT4, and ZnT6) proteins and zinc ions in rat choroid epithelium with immunohistochemistry and zinc selenide autometallography (ZnSe(AMG)). The four ZnT proteins were all expressed in the choroid epithelial cells. ZnT3 immunostaining was found in vesicle membranes in the apical part of the cells, associated to the microvillus membrane. Correspondingly, the ZnSe(AMG) technique revealed zinc ions in small vesicles, in microvilli, and multivesicular bodies in the epithelial cells. Traceable zinc ions were also found in lysosome-like organelles of fenestrated endothelial cells, but here no corresponding ZnT3 immunostaining was seen. The observations suggests that the choroid plexus is instrumental to regulation of the level of zinc ions in the cerebrospinal fluid.

In vivo detection of neuroarchitecture in the rodent brain using manganese-enhanced MRI

Visualizing brain anatomy in vivo could provide insight into normal and pathophysiology. Here it is demonstrated that neuroarchitecture can be detected in the rodent brain using MRI after systemic MnCl2. Administration of MnCl2 leads to rapid T1 enhancement in the choroid plexus and circumventricular organs, which spreads to the CSF space in ventricles and periventricular tissue. After 1 day, there was MRI enhancement throughout the brain with high intensity in the pituitary, olfactory bulb, cortex, basal forebrain, hippocampus, basal ganglia, hypothalamus, amygdala, and cerebellum. Contrast obtained enabled visualization of specific features of neuroarchitecture. The arrowhead structure of the dentate gyrus as well as the CA1-CA3 region of the hippocampus and layers in cortex, cerebellum, as well as the olfactory bulb could be readily observed. Preliminary assignments of olfactory bulb layers, cortical layers in frontal and somatosensory cortex, and cerebellum were made. Systemic MnCl2 leads to MRI visualization of neuroarchitecture nondestructively.

Manganese action in brain function

Manganese, an essential trace metal, is supplied to the brain via both the blood-brain and the blood-cerebrospinal fluid barriers. There are some mechanisms in this process and transferrin may be involved in manganese transport into the brain. A large portion of manganese is bound to manganese metalloproteins, especially glutamine synthetase in astrocytes. A portion of manganese probably exists in the synaptic vesicles in glutamatergic neurons and the manganese is dynamically coupled to the electrophysiological activity of the neurons. Manganese released into the synaptic cleft may influence synaptic neurotransmission. Dietary manganese deficiency, which may enhance susceptibility to epileptic functions, appears to affect manganese homeostasis in the brain, probably followed by alteration of neural activity. On the other hand, manganese also acts as a toxicant to the brain because this metal has prooxidant activity. Abnormal concentrations of manganese in the brain, especially in the basal ganglia, are associated with neurological disorders similar to Parkinson's disease. Understanding the movement and action of manganese in synapses may be important to clarify the function and toxicity of manganese in the brain.

Zinc homeostasis in the brain of adult rats fed zinc-deficient diet

Zinc concentration and (65)Zn uptake in the brain of rats fed zinc-deficient diet for 12 weeks were examined, based on a previous finding of the impairment of learning behavior by the zinc deprivation. Zinc concentrations in the brain, except for the hippocampal formation, did not decrease significantly in zinc-deficient rats, whereas zinc concentration in the liver of the zinc-deficient rats was approximately half that of control rats. When zinc-deficient rats were subjected to brain autoradiography with (65)Zn, (65)Zn concentration in any brain region of zinc-deficient rats was significantly higher than in control rats 6 days after injection of (65)ZnCl(2). The increase rate of (65)Zn concentration in the brain by the zinc deprivation was approximately 150%, and was similar to those in the liver and serum, suggesting that dietary zinc deprivation may cause a scarcity of zinc in the brain, in addition to the peripheral tissues such as the liver. These results indicate that the adult brain is responsive to dietary zinc deprivation. In the brain of zinc-deficient rats, the increase rate of (65)Zn concentration in the hippocampal formation seemed to be low compared to those in other brain regions. The hippocampal formation may be the most responsive to dietary zinc deprivation in the adult brain. The present finding demonstrates that zinc homeostasis in the brain is altered by chronically dietary zinc deprivation.

Zinc elevates neuropeptide Y levels in rat pheochromocytoma cells by a mechanism independent of L-channel mediated inhibition of release

Both zinc and neuropeptide Y (NPY) have been implicated as playing a role in seizures and feeding behavior. We investigated the hypothesis that zinc could regulate levels of NPY, and found that chronic exposure to 50-100 microM zinc increased levels of cellular NPY in cultured PC12 cells grown in the presence of nerve growth factor. Zinc's effect on NPY was specific, time- and concentration-dependent, and independent of inhibition of NPY release secondary to blockade of dihydropyridine-sensitive calcium channels. These results are consistent with a role for zinc in regulating hippocampal NPY following high-frequency neuronal activity.

65Zn localization in rat brain after intracerebroventricular injection of 65Zn-histidine

Previous studies have shown that 65Zn uptake in the brain expressed relative to plasma 65Zn level is enhanced by histidine infusion into the blood vessel. To study the effect of histidine on zinc uptake in the brain parenchyma via the CSF, the brains of rats injected intracerebroventricularly with 65Zn-His were subjected to autoradiography. Six days after injection, the radioactivity from 65Zn-His was distributed in the major part of the brain parenchyma higher than that from 65ZnCl(2), and relatively concentrated in the hippocampal formation, globus pallidus and hypothalamus. The radioactivity of the aqueduct was also higher in the 65Zn-His group, indicating that CSF clearance of the 65Zn-His group may be lower than that of the 65ZnCl(2) group. These results suggest an enhancement by histidine on zinc uptake in the brain parenchyma via the CSF.

Influence of transferrin on manganese uptake in rat brain

To evaluate the influence of transferrin (Tf) on manganese (Mn) uptake in the brain, pH 8.6 buffer-treated (54)MnCl(2), which has a higher affinity for Tf than untreated (54)MnCl(2), and Tf-bound (54)Mn were prepared. When pH 8.6 buffer-treated (54)MnCl(2) and untreated (54)MnCl(2) were incubated with apo-Tf in Tris (2-amino-2-hydroxymethylpropane-1,3-diol)-HCl buffer, the percentage of the total (54)MnCl(2) bound to Tf was approximately 85% and 10%, respectively. One hour after intravenous (iv) injection of pH 8.6 buffer-treated (54)MnCl(2) and untreated (54)MnCl(2), both tracers were concentrated similarly in the choroid plexus in the ventricles and distributed in other brain regions. Six days after iv injection, both pH 8.6 buffer-treated (54)MnCl(2) and untreated (54)MnCl(2) tracers were concentrated in the superior olivary complex, inferior colliculi, and red nuclei, although the former radioactivity was lower than the latter. Moreover, Tf-bound (54)Mn was prepared and injected iv into rats. The radioactivity from Tf-bound (54)Mn, which was also concentrated in the same regions, e.g., the superior olivary complex, was the lowest of all three traces. Tf-bound (54)Mn was stable during incubation with serum for 1 hr. It is likely that more Mn is transported into the brain when Mn is not bound to Tf. When Tf-bound (54)Mn and (54)MnCl(2) were unilaterally injected into the lateral ventricle, radioactivity was distributed only around the ipsilateral ventricle in the Tf-bound (54)Mn group 7 days after injection, whereas it was distributed more extensively in the (54)MnCl(2) group. It is likely that Tf-bound Mn in the CSF is less readily transported into the brain parenchymal cells than the non-Tf-bound form. These results suggest that Mn is transported into the brain efficiently via a Tf-independent uptake system.

109Cd transport in rat brain

The brain distribution of 109CdCl2 following administration into either the tail vein, the lateral ventricle or the olfactory bulb was studied to clarify permeability of the brain barrier system to cadmium (Cd) and Cd movement in the cerebrospinal fluid (CSF) and the brain extracellular fluid. One hour after intravenous (i.v.) injection, 109Cd was largely concentrated in the choroid plexus, and 109Cd concentration in the major part of the brain parenchyma, except for the circumventricular organs such as the pineal gland and the regions around them, was low. Six days after i.v. injection, 109Cd concentration in the choroid plexus was still high, and 109Cd was also detected highly in the pineal gland and small part around the median eminence. 109Cd concentration in the major part of the brain parenchyma was decreased in parallel with that in the blood. In the case of injection of 109CdCl2 into the lateral ventricle, a large portion of 109Cd was detected in the ventricular system 6 days after injection, and 109Cd concentration in the major part of the brain parenchyma was less than the detection limit. These results suggest that Cd cannot easily get into the brain and is blocked not only by the blood- brain and the blood-CSF barriers, but also by the ependymal and pial surfaces. In the case of injection of 109CdCl2 into the olfactory bulb, a large portion of 109Cd was detected in the injected area 24 h after injection, and, the next 24 h later, 109Cd distribution in the brain was not changed appreciably. These results suggest that Cd cannot easily move in the brain extracelular space, and is taken up into the brain parenchyma.

Zinc transport from the striatum and substantia nigra

65Zn was unilaterally injected into the striatum or substantia nigra of rats to see the transport of intracerebral zinc (Zn). In the case of intrastriatal injection, 65Zn was densely distributed in the ipsilateral medial forebrain bundle (MFB) and the substantia nigra. On unilateral colchicine injection into the MFB, 65Zn distribution in the ipsilateral substantia nigra decreased significantly compared to that of the contralateral one after 65Zn injection into the bilateral striata. These results suggest the presence of axonal transport of 65Zn taken up by striatonigral gamma-aminobutyric acid (GABA)-ergic and/or nigrostriatal dopaminergic neurons. On the other hand, in the case of intranigral injection, 65Zn was distributed in the ipsilateral MFB, striatum, globus pallidus, pontine reticular nuclei, and pontine nuclei. 65Zn distribution in the pons 1 day after intranigral injection was very similar to that 6 days after intrastriatal injection, suggesting that, in the case of intrastriatal injection of 65Zn, nigral 65Zn, which was transported anterogradely and/or retrogradely from the striatum, was transported to the postsynaptic neurons through the synapse and then transported to the pons.

Manganese concentration in rat brain: manganese transport from the peripheral tissues

54Mn distribution in the brain and peripheral tissues was studied with the course of time after intravenous injection of 54MnCl2 to see manganese (Mn) transport from the peripheral tissues, i.e. the liver, to the brain. One hour after injection, 54Mn concentrations in the brain were 0.15-0.25% dose/g, and 54Mn was largely concentrated in the choroid plexus. One day after injection, 54Mn in the choroid plexus decreased remarkably. 54Mn in other brain regions increased gradually after then, and reached 0.30-0.40% dose/g 6 days after injection. This increase of 54Mn was due to the redistribution from the peripheral tissues such as liver and pancreas, in which 54Mn was maintained at high levels (2.0-4.0% dose/g). The increment of 54Mn 1 h to 6 days after injection was the largest in the hippocampus, but not in the striatum. These results suggest that the delivery of Mn from the liver to the brain is not involved in preferential Mn accumulation in the basal nuclei under physiological condition. This delivery may be important for brain function.

Manganese transport in the neural circuit of rat CNS

To study manganese (Mn) transport in the neural circuit of rat CNS, brain isotope distribution after 54Mn injection into the brain was analyzed by autoradiography. One day after 54MnCl2 injection into the striatum, 54Mn was highly distributed in the ipsilateral thalamus, hypothalamus, and substantia nigra. When 54MnCl2 was bilaterally injected into the striata after unilateral treatment with colchicine or vehicle into the medial forebrain bundle, 54Mn was distributed in both sides of the substantia nigra of vehicle-treated rats. On the other hand, unilateral colchicine treatment caused a decrease of 54Mn distribution in the ipsilateral substantia nigra, suggesting that Mn is subjected to axonal transport in the striatonigra and/or nigrostriatal pathways. In the case of unilateral injection of 54MnCl2 into the olfactory bulb, 54Mn was distributed in the ipsilateral piriform, amygdaloid areas (the primary olfactory cortex), and entorhinal area (the secondary olfactory cortex). These results suggest that Mn is subject to widespread axonal transport in the neural circuits. Moreover, Mn may be taken up by the piriform neurons (the third olfactory neuron) after release from the secondary olfactory neuron terminals and transported to the entorhinal area.

Distribution of zinc in the substantia nigra of rats treated with 6-hydroxydopamine

To study the relationship between tissue accumulation of Zinc (Zn) and neurodegeneration in the nigrostriatal dopaminergic pathway, 65Zn distribution in this pathway was examined after unilateral injection of 6-hydroxydopamine (6-OHDA) into the substantia nigra of rats. When 65ZnCl2 was intravenously injected 4 days after treatment with 6-OHDA, 65Zn was concentrated in the ipsilateral substantia nigra 6 days after 65Zn injection. On the other hand, 19 d after treatment with 6-OHDA, 65Zn distribution in the ipsilateral substantia nigra was decreased to the level of the contralateral one. When NH4(99)TcO4, which cannot go through the blood-brain barrier, was injected into rats 4 d after treatment with 6-OHDA, 99Tc was concentrated in the ipsilateral substantia nigra 30 min after 99Tc injection, but no longer detectable 6 d after injection. These results suggest that Zn is necessary for a repair process called replacement gliosis after the death of neurons and that excess Zn does not accumulate in the lesion after completion of the gliosis.

Zinc distribution in the brain of Nagase analbuminemic rat and enlargement of the ventricular system

65ZnCl2 was intravenously injected into Nagase analbuminemic rats (NAR), which have a genetical mutation affecting albumin mRNA processing and lack serum albumin, to test the hypothesis that albumin is necessary for zinc (Zn) transport into the brain. One hour after injection, 65Zn was largely concentrated in the choroid plexus of NAR as well as normal parental Sprague-Dawley rats (SDR). Six days after injection, in both groups, the 65Zn concentration in the choroid plexus decreased, with increases in other brain regions. The finding that there was no significant difference in brain distribution of 65Zn between NAR and SDR suggests that Zn transport into the brain and its distribution through the blood-cerebrospinal fluid barrier as well as the blood-brain barrier are not dependent on serum albumin. A most interesting observation was that the cerebral ventricles were considerably enlarged in NAR.

Zn2+-stimulated sphingomyelinase is secreted by many cell types and is a product of the acid sphingomyelinase gene

Mammalian sphingomyelinases have been implicated in many important physiological and pathophysiological processes. Although several mammalian sphingomyelinases have been identified and studied, one of these, an acidic Zn2+-stimulated sphingomyelinase (Zn-SMase) originally found in fetal bovine serum, has received little attention since its first and only report 7 years ago. We now show that Zn-SMase activity is secreted by human and murine macrophages, human skin fibroblasts, microglial cells, and several other cells in culture and is markedly up-regulated during differentiation of human monocytes to macrophages. Remarkably, peritoneal macrophages from mice in which the acid SMase gene had been disrupted by homologous recombination secreted no Zn-SMase activity, indicating that this enzyme and the intracellular lysosomal SMase, which is Zn-independent, arise from the same gene. Furthermore, skin fibroblasts from patients with types A and B Niemann-Pick disease, which are known to lack lysosomal SMase activity, also lack Zn-SMase activity in their conditioned media. Chinese hamster ovary cells stably transfected with a cDNA encoding lysosomal SMase massively overexpress both cellular lysosomal SMase and secreted Zn-SMase activities. Thus, Zn-SMase arises independently of alternative splicing, suggesting a post-translational process. In summary, a wide variety of cell types secrete Zn-SMase activity, which arises from the same gene as lysosomal SMase. This secreted enzyme may play roles in physiological and pathophysiological processes involving extracellular sphingomyelin hydrolysis.

Biological half-lives of zinc and manganese in rat brain

The brains of rats injected intravenously with 65ZnCl2 or 54MnCl2 were subjected to high-resolution autoradiography. The distribution of 65Zn and 54Mn in each brain region gradually decreased from 6 days to 42 days for 65Zn and from 15 days to 60 days for 54Mn after the injection. The biological half-lives of Zn in each region studied were in the range of 16-43 days; the longest was observed in the amygdaloid nuclei. The regions where the long biological half-life was observed were consistent with the ones with the high density of Zn-containing neuron terminals reported previously. The biological half-lives of Mn in each region were 51-74 days; the longest were those in the hypothalamic nuclei and thalamus.