Age-related changes of sulphide-silver staining in the rat hippocampus

Zinc signaling through glucocorticoid and glutamate signaling in stressful circumstances

Humans and animals are constantly exposed to environmental stress. The hypothalamic-pituitary-adrenal (HPA) axis responds to stress, followed by glucocorticoid secretion from the adrenal glands. This response serves to maintain homeostasis in the living body through energy mobilization or to restore it. The brain is an important target for glucocorticoids. The hippocampus participates in the regulation of the HPA axis. Stress activates glutamatergic neurons in the hippocampus, and serious stress induces dyshomeostasis of extracellular glutamate. This dyshomeostasis, which is potentiated by glucocorticoids, modifies cognitive and emotional behavior. On the other hand, zinc is necessary for glucocorticoid signaling and is released from glutamatergic (zincergic) neurons to modulate synaptic glutamate signaling. Stress also induces dyshomeostasis of extracellular zinc, which may be linked to dyshomeostasis of extracellular glutamate. Thus, glucocorticoid signaling might also contribute to dyshomeostasis of extracellular zinc. It is likely that zinc signaling participates in cognitive and emotional behavior through glucocorticoid and glutamate signaling under stressful circumstances. This Mini-Review analyzes the relationship among signals of glucocorticoid, glutamate, and zinc under stressful circumstances to elucidate the significance of the zinc signaling in response to stress.

Insight into glutamate excitotoxicity from synaptic zinc homeostasis

Zinc is released from glutamatergic (zincergic) neuron terminals in the hippocampus, followed by the increase in Zn(2+) concentration in the intracellular (cytosol) compartment, as well as that in the extracellular compartment. The increase in Zn(2+) concentration in the intracellular compartment during synaptic excitation is mainly due to Zn(2+) influx through calcium-permeable channels and serves as Zn(2+) signaling as well as the case in the extracellular compartment. Synaptic Zn(2+) homeostasis is important for glutamate signaling and altered under numerous pathological processes such as Alzheimer's disease. Synaptic Zn(2+) homeostasis might be altered in old age, and this alteration might be involved in the pathogenesis and progression of Alzheimer's disease; Zinc may play as a key-mediating factor in the pathophysiology of Alzheimer's disease. This paper summarizes the role of Zn(2+) signaling in glutamate excitotoxicity, which is involved in Alzheimer's disease, to understand the significance of synaptic Zn(2+) homeostasis in the pathophysiology of Alzheimer's disease.

Cytochemical techniques for zinc and heavy metals localization in nerve cells

Zinc is one of the most abundant oligoelements in the living cell. It appears tightly bound to metallothioneins, loosely bound to some metalloproteins and nucleic acids, or even as free ion. Small amounts of zinc ions (in the nanomolar range) regulate a plentitude of enzymatic proteins, receptors, and transcription factors; thus, cells need accurate homeostasis of zinc ions. Some neurons have developed mechanisms to accumulate zinc in specific membrane compartments ("vesicular zinc") which can be revealed using histochemical techniques. This article is a short report on the different direct-indirect experimental approaches for zinc and heavy metal detection in neurons. Substances giving a bright color or emitting fluorescence when in contact with divalent metal ions are currently used to detect them inside cells; their use leads to the so called "direct" methods. The fixation and precipitation of metal ions as insoluble salt precipitates, their maintenance along the histological process, and their demonstration after autometallographic development are essential steps for other methods, the so-called "indirect methods" (Timm and Danscher Neo-Timm methods).

Deficiencies of hippocampal Zn and ZnT3 accelerate brain aging of Rat

We examined the link of hippocampal Zn to the functional impairments with aging using senescence-accelerated mouse prone 10 (SAMP10) with deficits in learning and memory. Zn in hippocampal mossy fiber pathway was less distributed in aged SAMP10 than that in the age-matched control. Furthermore, expression of Zn transporter 3, ZnT3, which plays to accumulate Zn in synaptic vesicles in the mossy fiber pathway, was markedly reduced in the hippocampal region even in young SAMP10. Moreover, excessive presynaptic release of glutamate as well as glycine and expression of glial fibrillary acidic protein, a marker of neuronal cell injury, were observed in the hippocampus of aged SAMP10 compared to the control. The present results suggest that age-dependent deficiencies of Zn in synaptic vesicles of the mossy fiber pathway induced by low expression of ZnT3 cause glutamatergic excitotoxicity in the hippocampal neurons and the deterioration of learning and memory in SAMP10.