Zinc transporter 3 and zinc ions in the rodent superior cervical ganglion neurons

Previous studies have revealed that zinc-enriched (ZEN) terminals are present in all parts of the CNS though with great differences in intensity. The densest populations of both ZEN terminals and ZEN somata are found in telencephalic structures, but also structures like the spinal cord demonstrate impressive ZEN systems spreading terminals several segments around the respective ZEN somata. The present study evaluates whether sympathetic neurons in the superior cervical ganglia (SCG) are ZEN neurons, i.e. contain vesicles that have zinc transporter 3 (ZnT3) proteins in their membranes and contain zinc ions. ZnT3 immunoreactivity (IR) was found in the somata and processes in the postganglionic neurons of mouse SCG. Only a small fraction of neurons (less than 5%), expressed varying degrees of ZnT3. Colchicine treatment, however, increased the number of ZnT3-positive neurons three-fold, suggesting an accumulation of ZnT3 protein in the somata. A small proportion of the postganglionic axons revealed dotted accumulations of ZnT3 IR along their courses. Double labeling showed that all ZnT3-positive neurons and axons were also tyrosine hydroxylase-positive with strong immunofluorescence, while no colocalization was found between ZnT3 and the vesicular acetylcholine transporter (VAChT) or neuropeptide Y IR. VAChT-positive preganglionic neurons were found to terminate on ZnT3 neuronal somata. 6-Methoxy 8-para toluene sulfonamide quinoline fluorescence and zinc selenium autometallography (ZnSe(AMG)) revealed that a subgroup of SCG cells contained free or loosely bound zinc ions. It is therefore concluded that ZnT3 and zinc ions are present in a subpopulation of TH-positive, NPY-negative neurons in the rodent SCG, supporting the notion that vesicular zinc ions may play a special role in the peripheral sympathetic adrenergic system.     

Changes in the vesicular zinc pattern following traumatic brain injury

The present study aims at evaluating the significance of zinc ions on the development of brain damage in a model of traumatic brain injury (TBI). The zinc ion specific autometallographic technique, the ZnSe(AMG) method, using silver enhancement of in vivo-captured zinc ions bound in zinc-selenium nanocrystals was applied to follow changes in the vesicular zinc pattern. Balb/c mice, ZnT3 knockout (ZnT3-Ko) mice, a mouse genetically knocked out for the protein ZnT3 responsible for sequestering zinc into synaptic vesicles, and littermates from the genetically un-manipulated mother type mice, wild type (Wt), were used. The Wt and the Balb/c mice exhibited instantaneously a boost in the zinc staining adjacent to the lesion involving all six neocortical layers. Ultra-structural analyses revealed that the in vivo created ZnSe nanocrystals were still confined to the vesicles of the zinc-enriched (ZEN) neurons in the neuropil. No differences between the Balb/c and Wt mice were seen at any time points. In the ZnT3-Ko mice the ZEN terminals stayed void of AMG grains, but a number of neuronal somata around the lesion became loaded with ZnSe nanocrystals. These silver-enhanced ZnSe nanocrystals were confined to the cytoplasm of the somata and their proximal dendrites. No such soma staining was seen in the Wt or Balb/c mice. We speculate that vesicular zinc may not contribute to neuronal damage following TBI.     

小鼠脑中Zn元素和ZnT3 mRNA表达的研究

目的：研究ZnT3 mRNA的表达与Zn等金属元素在脑中精细分布的相互作用和功能。方法:使用同步辐射X射线荧光法（SRXRF）测定小鼠全脑和脑切片中Zn等金属元素分布，同时使用反转录多聚酶链式反应（RT-PCR）检测小鼠各组织中的ZnT3 mRNA的表达量。结果:脑中Zn元素不是均匀分布的，主要分布在皮层、海马和齿状回部位。大脑皮层、海马和睾丸中的ZnT3 mRNA有较高丰度，而其它组织中未检出ZnT3 mRNA 。结论：ZnT3能促进胞浆内Zn富集于囊泡中，通过介导胞浆锌的跨膜转运过程，构造囊泡“锌池”。     

Altered expression and distribution of zinc transporters in APP/PS1 transgenic mouse brain

Pathological accumulation of beta-amyloid peptide (Abeta) is an early and common feature of Alzheimer's disease (AD). An increased zinc concentration can initiate the deposition of Abeta. The present study aimed to study the expression and distribution patterns of six members of the zinc transporter (ZnT) family, ZnT1, ZnT3, ZnT4, ZnT5, ZnT6, and ZnT7, in the APPswe/PS1dE9 transgenic mouse brain. Our results demonstrated a statistically significant (P<0.05) increase of ZnT1, ZnT3, ZnT4, ZnT6, and ZnT7 in both hippocampus and neo-cortex using Western blot method and an abundant distribution of zinc ions in the plaques and amyloid angiopathic vessels using immersion autometallography. Furthermore, all ZnT immunoreactions were detected in most amyloid plaques and amyloid angiopathic vessels. ZnT1 and ZnT4 were extensively expressed in all parts of the plaques. ZnT3, ZnT5, and ZnT6 were expressed most prominently in the degenerating neurites in the peripheral part of the plaques, while ZnT7 was present in the core of the plaques. The amyloid angiopathic vessels showed a strong ZnT3 immunoreactivity. These results might suggest multiple roles of ZnTs in the deposition and organization of the Abeta composition.     

Inhibitory zinc-enriched terminals in mouse spinal cord

The ultrastructural localization of zinc transporter-3, glutamate decarboxylase and zinc ions in zinc-enriched terminals in the mouse spinal cord was studied by zinc transporter-3 and glutamate decarboxylase immunohistochemistry and zinc selenium autometallography, respectively.The distribution of zinc selenium autometallographic silver grains, and zinc transporter-3 and glutamate decarboxylase immunohistochemical puncta in both ventral and dorsal horns as seen in the light microscope corresponded to their presence in the synaptic vesicles of zinc-enriched terminals at ultrastructural levels. The densest populations of zinc-enriched terminals were seen in dorsal horn laminae I, III and IV, whereas the deeper laminae V and VI contained fewer terminals. At ultrastructural levels, zinc-enriched terminals primarily formed symmetrical synapses on perikarya and dendrites. Only relatively few asymmetrical synapses were observed on zinc-enriched terminals. In general, the biggest zinc-enriched terminals contacted neuronal somata and large dendritic elements, while medium-sized and small terminals made contacts on small dendrites. The ventral horn was primarily populated by big and medium-sized zinc-enriched terminals, whereas the dorsal horn was dominated by medium-sized and small zinc-enriched terminals.The presence of boutons with flat synaptic vesicles with zinc ions and symmetric synaptic contacts suggests the presence of inhibitory zinc-enriched terminals in the mammalian spinal cord, and this was confirmed by the finding that zinc ions and glutamate decarboxylase are co-localized in these terminals. The pattern of zinc-enriched boutons in both dorsal and ventral horns is compatible with evidence suggesting that zinc may be involved in both sensory transmission and motor control.     

硒酸锌金属自显影技术检测游离锌离子在小鼠卵巢的分布

目的研究游离锌离子在小鼠卵巢的定位分布。方法应用硒酸锌金属自显影技术(ZnSeAMG)检测小鼠卵巢内的游离锌离子分布。结果游离锌离子在卵巢内分布广泛,在原始卵泡、初级卵泡、次级卵泡、闭锁卵泡以及间质细胞中都有大量棕黑色的AMG染色颗粒。结论小鼠卵巢内含有丰富的游离锌离子,锌离子可能参与了卵泡的发育以及性激素的合成。     

ZnT7及游离锌离子在小鼠室管膜和脉络丛细胞的分布

目的 研究锌转运体7（ZnT7）和游离锌离子在小鼠脊髓室管膜和脉络丛上皮细胞中的分布.方法 应用ZnSe金属自显影技术（AMG）检测硒酸钠注射1.5h后小鼠脊髓室管膜细胞及脑室脉络丛上皮细胞的游离锌离子;应用免疫组织化学SABC法检测小鼠脊髓室管膜细胞及脑室脉络丛上皮细胞中ZnT7的表达.结果 光镜下观察AMG染色的切片,小鼠脊髓室管膜细胞及脑室脉络丛上皮细胞中均有游离锌离子的分布;免疫组织化学结果表明,脊髓室管膜细胞及脉络丛上皮细胞中均有ZnT7的表达,且与游离锌离子分布区域基本一致.结论 锌离子可能在脊髓室管膜细胞及脉络丛上皮细胞内发挥重要作用,脊髓室管膜细胞及脉络丛上皮细胞可能在脑脊液锌转运过程中发挥重要的作用.     

Temporal and region-dependent changes in muscarinic M4 receptors in the hippocampus and entorhinal cortex of adrenalectomized rats

Long-term adrenalectomy induces a dramatic loss of cells in the dentate gyrus and CA1-CA4 fields of the hippocampus resulting in an impairment of cognitive functions such as spatial learning, memory and exploratory behaviour. Muscarinic M₁ and M₄ receptor levels in the hippocampus and entorhinal cortex of adult male Wistar rats were examined 3, 14, 30, 90, and 150 days after adrenalectomy. Receptor levels in the entorhinal cortex and the hippocampus were determined by quantitative autoradiography using ¹²⁵I-M₁-toxin-1 and ¹²⁵I-M₄-toxin-1, M₁ and M₄ subtype selective antagonists, respectively. Moreover, the level of hippocampal M₁ and M₄ muscarinic receptors were evaluated 1 month after adrenalectomy by immunoblot analysis. Adrenalectomy induced apoptotic processes were examined by analysing apoptotic markers using Western blot analysis. No significant changes were observed in the level of muscarinic M₁ receptors in the entorhinal cortex, the dentate gyrus and in the different CA fields of the hippocampus of adrenalectomized (ADX) rats. However, M₄ receptors showed a significant decrease in the entorhinal cortex (at 3 days), dentate gyrus and CA4 (at 14 days), CA3 (at 30 days), and CA2 and CA1 (at 90 days) after adrenalectomy. Moreover, a decrease in the level of M₄ receptors was detected in ADX rats 1 month after adrenalectomy as compared with sham groups using M₄ specific antibody. Apoptotic markers such as PARP and p53 were significantly increased whereas Bcl-2 marker was decreased in ADX rat brain homogenates compared to controls. Our results show that M₁ and M₄ receptors are differentially affected by adrenalectomy and indicate that these subtypes have different functions in the hippocampus. Our data on time and region-dependent decreases in hippocampal M₄ receptors indicate that the M₄ receptor subtype is influenced by adrenal hormones and suggest that the M₄ receptor might be linked to memory function in the hippocampus.     

Amyloid plaques arise from zinc-enriched cortical layers in APP/PS1 transgenic mice and are paradoxically enlarged with dietary zinc deficiency

The ZnT3 zinc transporter is uniquely expressed in cortical glutamatergic synapses where it organizes zinc release into the synaptic cleft and mediates beta-amyloid deposition in transgenic mice. We studied the association of zinc in plaques in relation to cytoarchitectural zinc localization in the APP/PS1 transgenic mouse model of Alzheimer's disease. The effects of low dietary zinc for 3 months upon brain pathology were also studied. We determined that synaptic zinc distribution within cortical layers is paralleled by amyloid burden, which is heaviest for both in layers 2-3 and 5. ZnT3 immunoreactivity is prominent in dystrophic neurites within amyloid plaques. Low dietary zinc caused a significant 25% increase in total plaque volume in Alzheimer's mice using stereological measures. The level of oxidized proteins in brain tissue did not changed in animals on a zinc-deficient diet compared with controls. No obvious changes were observed in the autometallographic pattern of zinc-enriched terminals in the neocortex or in the expression levels of zinc transporters, zinc importers or metallothioneins. A small decrease in plasma zinc induced by the low-zinc diet was consistent with the subclinical zinc deficiency that is common in older human populations. While the mechanism remains uncertain, our findings indicate that subclinical zinc deficiency may be a risk factor for Alzheimer's pathology.     

SVHRP对Aβ_(1-40)处理后大鼠海马突触形态学改变的抑制作用

目的:探索蝎毒耐热蛋白(SVHRP)对淀粉样β蛋白(Aβ)神经毒性的抑制作用。方法:在大鼠海马内每侧注射Aβ1-40(10μg/2μl)后1d,给予腹腔SVHRP(0.5～2μg/100g),1次/d,连续10次。建模16d后分别进行海马部位的突触体素免疫组织化学分析和突触超微结构计量观察。结果:与对照组比较,Aβ组大鼠突触体素免疫反应强度和电镜下突触密度明显下降(P0.01),小突触丢失为主,突触终末可见突触小泡大量集聚,突触活性区平均长度增加。SVHRP干预组大鼠实触体素免疫反应强度和电镜下突触密度明显高于Aβ组大鼠(P0.01),突触终末内未见突触小泡过量集聚,但小突触比例显著增加(P0.01)。结论:以上结果强有力的提示,Aβ是突触退变的始动因素,SVHRP可抑制Aβ引起的突触退变,有可能成为治疗Alzheimer病(AD)的一种药物。     

Annexin 7-immunoreactive microglia in the hippocampus of control and adrenalectomized rats

Annexin 7 (ANX7), also termed synexin, is a member of the annexin family of calcium-binding proteins. In the present study, we examined the distribution and cellular localization of ANX7-immunoreactivity in the rat hippocampus and its response to adrenalectomy (ADX). ANX7 was co-localized with OX42 in microglia distributed throughout the hippocampus of both control and ADX animals. ANX7-immunoreactivity was not detected in GFAP-positive astrocytes or in hippocampal neurons. At 1-week and 4-weeks following ADX, we observed a population of large, ameboid, ANX7-immunopositive microglia ("reactive microglia") which were largely confined to the granule cell layer of the dentate gyrus throughout its rostrocaudal extent. No reactive microglia were present in the hippocampus of sham-ADX or ADX + corticosterone treated animals. In 4-weeks ADX animals but not 1-week ADX, ANX7-immunostaining was significantly increased in the mossy fiber layer of CA3, due to the presence of many small, dark-staining "activated microglia". Our results show that ANX7 is abundantly expressed in the rat hippocampus by different microglial forms (e.g., ramified, activated and reactive microglia), suggesting an important role for this calcium-binding protein in microglial Ca2+-dependent processes.     

Localization and glucocorticoid regulation of 11beta-hydroxysteroid dehydrogenase type 1 mRNA in the male mouse forebrain

The enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) converts the inactive 11-dehydrocorticosterone into the active glucocorticoid corticosterone. There is accumulating evidence indicating widespread expression of 11beta-HSD1 in the brain. However, there is little information about regulation of 11beta-HSD1 expression in this tissue. Using in situ hybridization involving use of 35S-labeled cRNA probe, we have studied the distribution of cells expressing 11beta-HSD1 mRNA in the male mouse forebrain as well as the effects of adrenalectomy (ADX) and acute administration of corticosterone (3 and 24 h) on 11beta-HSD1 mRNA levels. Cells expressing 11beta-HSD1 mRNA were mostly detected in the cerebral cortex, hippocampus, amygdala and medial preoptic area, with the highest expression in the cerebral cortex (retrosplenial granular area) and hippocampus (CA3 and granular layer of the gyrus dentatus). Seven days following ADX, 11beta-HSD mRNA levels were increased by 50% in the gyrus dentatus, by 100% in the CA3 area, and 105% in the cerebral cortex. Administration of corticosterone to ADX mice induced a significant decrease in mRNA, in both the hippocampus and cerebral cortex so that, at the 24 h time interval, the levels were similar to those observed in intact mice. These results clearly indicate that circulating corticosterone is downregulating the expression of 11beta-HSD1 mRNA in the two forebrain areas studied. This downregulation might contribute to maintain low intracellular corticosterone levels in central regions and then prevent the deleterious effects induced by high glucocorticoid levels.     

Synaptic vesicle recycling at the neuromuscular junction in the presence of a presynaptic membrane marker

Staining of the presynaptic axonal membrane of the neuromuscular junction with horseradish peroxidase-labeled α-bungarotoxin was utilized as a marker for observing directly the fate of this membrane during the process of synaptic vesicle release and recycling. The neuromuscular junctions of frog sartorius-sciatic nerve preparations were stained with horseradish peroxidase-α-bungarotoxin and stimulated by electrical stimulation of the nerve, high concentration of external potassium ions, and black widow spider venom. Some preparations were stimulated in the presence of exogenous horseradish peroxidase tracer after incubation in the conjugate and were found to contain horseradish peroxidase within many synaptic vesicles, indicating that the conjugate did not affect the process of synaptic vesicle recycling. Stimulation was followed by depletion of synaptic vesicles and appearance of axolemmal infoldings and membranous cisternae. With rest after electrical and potassium stimulation, synaptic vesicles were reconstituted and terminals assumed a more normal appearance. Membrane staining after stimulation occurred in the axolemmal infoldings, some of the intra-axonal cisternae, and in a few coated vesicles. However, all synaptic vesicles were unreactive, in either rested or unrested terminals. Thus, axonal membrane labeled with horseradish peroxidase-α-bungarotoxin did not become incorporated into new synaptic vesicles.These observations support a mechanism of recycling of synaptic vesicles by specific retrieval of vesicle membrane or constituents from the axolemma.     

Experience-dependent regulation of synaptic zinc is impaired in the cortex of aged mice

Zinc plays an important role in synaptic signaling in the mammalian cerebral cortex. Zinc is sequestered into presynaptic vesicles of subpopulations of glutamatergic neurons and is released by depolarization, in a calcium-dependent manner. As the majority of mechanisms that have been suggested to participate in experience-dependent alterations in synaptic strength in the cerebral cortex implicate signaling by glutamate, it stands to reason that zincergic signaling might also be crucial. Here we show that synaptic zinc is rapidly and dynamically modulated in relation to alterations in sensory input and that this response is highly age-dependent. Juvenile, adult, and aged mice were subjected to whisker removal and levels of staining for synaptic zinc in deprived and non-deprived cortical barrels were quantitatively assessed at post-deprivation times ranging from 3 h to 21 days. In the first 12 h, zinc levels increased slightly, but significantly, in all groups. At later time points, zinc levels increased robustly (23%) in the youngest group by 24 h and remained elevated through 7 days. By contrast, deprivation-induced changes in zinc staining in aged animals, achieved their maximal levels at 12 h (approximately 10%) and steadily declined thereafter. Adult animals revealed a biphasic, intermediate change with time. In all age groups, levels of zinc staining returned to baseline by 21 days after whisker plucking. However, only in juvenile and adult mice did we observe that the level of zinc staining in deprived barrel hollows, was correlated with the length of whiskers as they regrew. Our data suggest that alterations in the regulation of synaptic zinc may be involved with decrements of synaptic plasticity that accompany senescence.     

Synaptic reorganization by mossy fibers in human epileptic fascia dentata.

This study was designed to identify whether synaptic reorganizations occur in epileptic human hippocampus which might contribute to feedback excitation. In epileptic hippocampi, (n = 21) reactive synaptogenesis of mossy fibers into the inner molecular layer of the granule cell dendrites was demonstrated at the light microscopic and electron microscopic levels. There was no inner molecular layer staining for mossy fibers in autopsy controls (n = 4) or in controls with neocortex epilepsy having no hippocampal sclerosis (n = 2). Comparing epileptics to controls, there were statistically significant correlations between Timm stain density and hilar cell loss. Since hilar neurons are the origin of ipsilateral projections to the inner molecular layer, this suggests that hilar deafferentation of this dendritic zone precedes mossy fiber reafferentation. Quantitative Timm-stained electron microscopy revealed large, zinc-labelled vesicles in terminals with asymmetric synapses on dendrites in the inner molecular and granule cell layers. Terminals in the middle and outer molecular layers did not contain zinc, were smaller and had smaller vesicles. These histochemical and ultrastructural data suggest that in damaged human epileptic hippocampus, mossy fiber reactive synaptogenesis may result in monosynaptic recurrent excitation of granule cells that could contribute to focal seizure onsets.     

Response to kainic acid injections: changes in staining for zinc, FOS, cell death and glial response in the rat forebrain

A pool of zinc is present in synaptic vesicles in a population of glutamatergic neurones. Zinc appears to modulate synaptic transmission and cause neuronal death. The status of vesicular zinc, neuronal death and glial reaction in the rat forebrain was analysed after a systemic injection of kainic acid in order to establish a model for future studies on zinc function. Rats received a systemic injection of kainic acid (10 mg/kg) and were killed 3, 6, 12, 24 and 48 h post-treatment. Timm's method and zinquin staining were used to detect zinc. Immunostaining for Fos-like proteins and staining with Fluoro-Jade B were used to detect cell reaction and degeneration, respectively. Glial fibrillary acidic protein and tomato lectin were used as glial markers. Zinquin staining for zinc rose transitorily in neuronal somata 6 h after injection (not observed at 24-48 h) in the piriform and entorhinal cortices, amygdala and hippocampus. In contrast sulphide/silver staining for zinc showed virtually no rise in cytoplasmic zinc (except in cornus ammonis field 1 of the hippocampus) 6 h after injection, and a decrease (bleaching) in some terminal fields starting 12 h after injection and increasing at 24-48 h. The areas most affected by the zinc bleaching were the olfactory bulb, piriform and entorhinal cortices, endopiriform and amygdaloid nuclei. Transitory Fos immunostaining (within neuronal nuclei) was observed between 3 and 12 h after kainate treatment in many telencephalic areas: olfactory bulb, cortex (piriform, hippocampal and neocortex) and amygdaloid nuclei. This was accompanied by changes in glial markers starting 3 h after injection. Fluoro-Jade B staining in neurones (degeneration) appeared 6 h after treatment and increased later. Degenerating areas generally coincided with those showing Fos immunoreactivity. Zinquin and sulphide/silver methods revealed various pools of zinc after kainate injection: a cytoplasmic pool and a terminal field (or vesicular) pool. Cytoplasmic zinc (zinquin) was coincident, in time and location, with cell degeneration, thus implicating zinc in cell death. This zinc may not have come from presynaptic stores, since no bleaching (sulphide/silver method) was observed 6 h after injection. Future experiments altering zinc pools (e.g. by chelating agents) may elucidate the function of zinc.     

Differential response of synaptic zinc levels to sensory deprivation in the barrel cortex of young and adult mice

The distribution of synaptic zinc after short-term (up to 48 h) tactile deprivation of vibrissae was investigated in the barrel cortex of mice using histochemical staining. In adult mice, 12 h after trimming selected rows of vibrissae, an increase in zinc staining in the deprived barrels was observed. This increase was still present 48 h after trimming. These results indicate that the level of synaptic zinc is rapidly regulated by neuronal activity in adult mice. In young (8-day-old) mice, the short-term deprivation did not alter zinc staining and only chronic sensory deprivation produced an increase in zinc staining. However, after long-term deprivation no changes were found in adult mice. These results suggest that different mechanisms might be involved in functional reorganization of zinc containing terminals in young and fully mature cerebral cortex.     

Studies on the fine structural localization of zinc iodide-osmium reaction in the brain. III. Some characteristics of localization in the synaptosomes

Summary Synaptosomes from rat cerebral cortex were impregnated in the zinc iodide-osmium (ZIO) solution. The fine structural localization of the ZIO impregnation product was studied and, in addition, the function-dependent features of the reaction were examined after electrical stimulation or potassium chloride treatment. It was revealed that: (i) Aldehyde prefixation resulted in an increase in the number of reactive synaptic vesicles in all types of synaptosomes; (ii) Electrical stimulation decreased the number of reactive vesicles in a voltage dependent manner; (iii) Potassium chloride treatment also reduced the reactivity of vesicles; the reduction was dependent on the concentration of potassium and duration of treatment; (iv) Experimental interventions leading to the release of neurotransmitters from the synaptic vesicles and to fatigue of the nerve terminals also resulted in a decrease of the ZIO-reaction product of synaptic vesicles in a manner proportional to the strength of stimuli.     

Extracellular chelation of zinc does not affect hippocampal excitability and seizure-induced cell death in rats

In the nervous system, zinc can influence synaptic responses and at extreme concentrations contributes to epileptic and ischaemic neuronal injury. Zinc can originate from synaptic vesicles, the extracellular space and from intracellular stores. In this study, we aimed to determine which of these zinc pools is responsible for the increased hippocampal excitability observed in zinc-depleted animals or following zinc chelation. Also, we investigated the source of intracellularly accumulating zinc in vulnerable neurons. Our data show that membrane-permeable and membrane-impermeable zinc chelators had little or no effect on seizure activity in the CA3 region. Furthermore, extracellular zinc chelation could not prevent the accumulation of lethal concentrations of zinc in dying neurons following epileptic seizures. At the electron microscopic level, zinc staining significantly increased at the presynaptic membrane of mossy fibre terminals in kainic acid-treated animals. These data indicate that intracellular but not extracellular zinc chelators could influence neuronal excitability and seizure-induced zinc accumulation observed in the cytosol of vulnerable neurons.