Transient Increase in Zn2+ in Hippocampal CA1 Pyramidal Neurons Causes Reversible Memory Deficit

The translocation of synaptic Zn2+ to the cytosolic compartment has been studied to understand Zn2+ neurotoxicity in neurological diseases. However, it is unknown whether the moderate increase in Zn2+ in the cytosolic compartment affects memory processing in the hippocampus. In the present study, the moderate increase in cytosolic Zn2+ in the hippocampus was induced with clioquinol (CQ), a zinc ionophore. Zn2+ delivery by Zn-CQ transiently attenuated CA1 long-term potentiation (LTP) in hippocampal slices prepared 2 h after i.p. injection of Zn-CQ into rats, when intracellular Zn2+ levels was transiently increased in the CA1 pyramidal cell layer, followed by object recognition memory deficit. Object recognition memory was transiently impaired 30 min after injection of ZnCl2 into the CA1, but not after injection into the dentate gyrus that did not significantly increase intracellular Zn2+ in the granule cell layer of the dentate gyrus. Object recognition memory deficit may be linked to the preferential increase in Zn2+ and/or the preferential vulnerability to Zn2+ in CA1 pyramidal neurons. In the case of the cytosolic increase in endogenous Zn2+ in the CA1 induced by 100 mM KCl, furthermore, object recognition memory was also transiently impaired, while ameliorated by co-injection of CaEDTA to block the increase in cytosolic Zn2+. The present study indicates that the transient increase in cytosolic Zn2+ in CA1 pyramidal neurons reversibly impairs object recognition memory

Behavioral Abnormality Induced by Enhanced Hypothalamo-Pituitary-Adrenocortical Axis Activity under Dietary Zinc Deficiency and Its Usefulness as a Model

Dietary zinc deficiency increases glucocorticoid secretion from the adrenal cortex via enhanced hypothalamo-pituitary-adrenocortical (HPA) axis activity and induces neuropsychological symptoms, i.e., behavioral abnormality. Behavioral abnormality is due to the increase in glucocorticoid secretion rather than disturbance of brain zinc homeostasis, which occurs after the increase in glucocorticoid secretion. A major target of glucocorticoids is the hippocampus and their actions are often associated with disturbance of glutamatergic neurotransmission, which may be linked to behavioral abnormality, such as depressive symptoms and aggressive behavior under zinc deficiency. Glucocorticoid-mediated disturbance of glutamatergic neurotransmission in the hippocampus is also involved in the pathophysiology of, not only psychiatric disorders, such as depression, but also neurodegenerative disorders, e.g., Alzheimer’s disease. The evidence suggests that zinc-deficient animals are models for behavioral and psychological symptoms of dementia (BPSD), as well as depression. To understand validity to apply zinc-deficient animals as a behavioral abnormality model, this paper deals with the effect of antidepressive drugs and herbal medicines on hippocampal dysfunctions and behavioral abnormality, which are induced by enhanced HPA axis activity under dietary zinc deficiency

Amyloid β-mediated Zn2+ influx into dentate granule cells transiently induces a short-term cognitive deficit.

We examined an idea that short-term cognition is transiently affected by a state of confusion in Zn2+ transport system due to a local increase in amyloid-β (Aβ) concentration. A single injection of Aβ (25 pmol) into the dentate gyrus affected dentate gyrus long-term potentiation (LTP) 1 h after the injection, but not 4 h after the injection. Simultaneously, 1-h memory of object recognition was affected when the training was performed 1 h after the injection, but not 4 h after the injection. Aβ-mediated impairments of LTP and memory were rescued in the presence of zinc chelators, suggesting that Zn2+ is involved in Aβ action. When Aβ was injected into the dentate gyrus, intracellular Zn2+ levels were increased only in the injected area in the dentate gyrus, suggesting that Aβ induces the influx of Zn2+ into cells in the injected area. When Aβ was added to hippocampal slices, Aβ did not increase intracellular Zn2+ levels in the dentate granule cell layer in ACSF without Zn2+, but in ACSF containing Zn2+. The increase in intracellular Zn2+ levels was inhibited in the presence of CaEDTA, an extracellular zinc chelator, but not in the presence of CNQX, an AMPA receptor antagonist. The present study indicates that Aβ-mediated Zn2+ influx into dentate granule cells, which may occur without AMPA receptor activation, transiently induces a short-term cognitive deficit. Extracellular Zn2+ may play a key role for transiently Aβ-induced cognition deficits

Decrease in extracellular Zn²⁺ in the hippocampus by zinc chelators.

<p>Hippocampal slices were prepared 2 h after i.p. injection of vehicle (A, n = 7) or CQ (30 mg/kg, B, n = 5) into NER, and 3 h after i.p. injection of vehicle (C, n = 7) or TPEN (1 mg/kg, D, n = 8) into NER. The hippocampal slices were imaged with ZnAF-2 (left side). To evaluate the effects of CQ (the left image of A and B, vehicle; the right image of A and B, CQ) and TPEN (he left image of C and D, vehicle; the right image of C and D, TPEN), CQ-injected (A and B) and TPEN-injected (C and D) groups were separately measured under the exactly same condition. The measurement condition was slightly different between CQ and TPEN groups for the optimal imaging of the decrease in Zn²⁺ by fluorescence intensity. Region of interest was set the area surrounding the abbreviated letters, i.e., ML and SL, where zincergic (glutamatergic) synapses exist. ML, the molecular layer of the dentate gyrus (A and C). SL, the stratum lucidum of the CA3 (B and D). The data (mean ± SEM) represent the rate of ZnAF-2 intensity after zinc chelator injection to that after vehicle injection, which was expressed as 100% (right side).</p

Timm’s staining in the hippocampus.

<p>Hippocampal slices were prepared from Wistar rats (A) and NER (B) (n = 10). The lower panel show the density of Timm’ stain, which was measured by using Multi Gauge V3.1, in the stratum lucidum where mossy fiber terminals exist. As a representative sample (circle) in Fig. 1A, five regions of interest per slice were set in the stratum lucidum and the densities measured were averaged. Each bar and line (mean ± SEM) represents the rate (%) of the density of Timm’ stain of Wistar rats to that of NER, which was represented as 100%.</p

Suppression of CQ-induced seizures in NER by enhancing GABAergic activity.

<p>AOAA (5, 10, 20 mg/kg) or phenobarbital (PHB, 20 mg/kg) was i.p. co-injected with CQ (30 mg/kg) into NER, and then the behavior was observed for 6 h in home cage. The incidence represents the rate of seized rats, which exhibited tonic-clonic convulsion, to the total rats (vehicle, n = 22; CQ 30 mg/kg, n = 38; CQ+AOAA (5 mg/kg), n = 10; CQ+AOAA (10 mg/kg), n = 10; CQ+AOAA (20 mg/kg), n = 20; CQ+PHB (20 mg/kg), n = 18). Each bar and line represents the mean ± SEM (A). ***, p<0.001, vs. vehicle; ^#, p<0.05, ^##, p<0.01, ^###, p<0.001, vs. CQ. To check the effect of AOAA on locomotor activity, NER were subjected to the open-field test 2 h after i.p. injection of AOAA (n = 10). Each bar and line represents the mean ± SEM (B). *, p<0.05, vs. vehicle.</p