Action of hypoxia on different types of calcium channels in hippocampal neurons

The effect of cerium dioxide nanoparticles on the viability of hippocampal neurons in Alzheimer’s disease modeling

The possibilities of using nanoparticle materials based on cerium dioxide (CNPs) are exciting since they are low toxic and have specific redox, antiradical properties. It can be supposed that CNPs’ biomedical use is also relevant in neurodegenerative diseases, especially Alzheimer’s disease (AD). AD is known as the pathologies leading to progressive dementia in the elderly. The factor that provokes nerve cell death and cognitive impairment in AD is the pathological accumulation of beta-amyloid peptide (Aβ) in the brain tissue. In our studies, we examined the impact of Aβ 1-42 on neuronal death and evaluated the potential neuroprotective properties of CNPs during AD modeling in cell culture. Our findings show that, under AD modeling conditions, the number of necrotic neurons increased from 9.4% in the control to 42.7% when Aβ 1-42 was used. In contrast, CNPs alone showed low toxicity, with no significant increase in the number of necrotic cells compared to control conditions. We further explored the potential of CNPs as a neuroprotective agent against Aβ-induced neuronal death. We found that introducing CNPs 24 h after Aβ 1-42 incubation or prophylactically incubating hippocampal cells with CNPs 24 h before amyloid administration significantly reduced the percentage of necrotic cells to 17.8 and 13.3%, respectively. Our results suggest that CNPs in the cultural media can significantly reduce the number of dead hippocampal neurons in the presence of Aβ, highlighting their neuroprotective properties. These findings suggest that CNPs may hold promise for developing new treatments for AD based on their neuroprotective properties.

SARS-CoV-2, Hypoxia, and Calcium Signaling: The Consequences and Therapeutic Options

Currently, COVID-19 has created difficulties in understanding the pathological mechanisms and therapeutic options for treatment. COVID-19 patients have shown to be hypoxic, and hypoxia causes alteration of the cell calcium dynamics, which leads to alterations in many signal transduction pathways and gene expression. Also, both viruses and hypoxia directly alter many pathological and biochemical pathways, such as inflammation, cytokine signaling, glycolysis, and calcium signaling. Therefore, understanding of these cellular events would be useful in finding therapeutic options.

Calcium channel blockade with nimodipine reverses MRI evidence of cerebral oedema following acute hypoxia

Acute cerebral hypoxia causes rapid calcium shifts leading to neuronal damage and death. Calcium channel antagonists improve outcomes in some clinical conditions, but mechanisms remain unclear. In 18 healthy participants we: (i) quantified with multiparametric MRI the effect of hypoxia on the thalamus, a region particularly sensitive to hypoxia, and on the whole brain in general; (ii) investigated how calcium channel antagonism with the drug nimodipine affects the brain response to hypoxia. Hypoxia resulted in a significant decrease in apparent diffusion coefficient (ADC), a measure particularly sensitive to cell swelling, in a widespread network of regions across the brain, and the thalamus in particular. In hypoxia, nimodipine significantly increased ADC in the same brain regions, normalizing ADC towards normoxia baseline. There was positive correlation between blood nimodipine levels and ADC change. In the thalamus, there was a significant decrease in the amplitude of low frequency fluctuations (ALFF) in resting state functional MRI and an apparent increase of grey matter volume in hypoxia, with the ALFF partially normalized towards normoxia baseline with nimodipine. This study provides further evidence that the brain response to acute hypoxia is mediated by calcium, and importantly that manipulation of intracellular calcium flux following hypoxia may reduce cerebral cytotoxic oedema.

Decreased calcium-activated potassium channels by hypoxia causes abnormal firing in the spontaneous firing medial vestibular nuclei neurons

Vertebrobasilar insufficiency (VBI) presents complex varied clinical symptoms, including vertigo and hearing loss. Little is known, however, about how Ca(2+)-activated K(+) channel attributes to the medial vestibular nucleus (MVN) neural activity in VBI. To address this issue, we performed whole-cell patch clamp and quantitative polymerase chain reaction (qPCR) to examine the effects of hypoxia on neural activity and the changes of the large conductance Ca(2+) activated K(+) channels (BKCa channels) in the MVN neurons in brain slices of male C57BL/6 mice. Brief hypoxic stimuli of the brain slices containing MVN were administrated by switching the normoxic artificial cerebrospinal fluid (ACSF) equilibrated with 21% O2/5% CO2 to hypoxic ACSF equilibrated with 5% O2/5% CO2 (balance N2). 3-min hypoxia caused a depolarization in the resting membrane potential (RM) in 8/11 non-spontaneous firing MVN neurons. 60/72 spontaneous firing MVN neurons showed a dramatic increase in firing frequency and a depolarization in the RM following brief hypoxia. The amplitude of the afterhyperpolarization (AHPA) was significantly decreased in both type A and type B spontaneous firing MVN neurons. Hypoxia-induced firing response was alleviated by pretreatment with NS1619, a selective BKCa activator. Furthermore, brief hypoxia caused a decrease in the amplitude of iberiotoxin-sensitive outward currents and mRNA level of BKCa in MVN neurons. These results suggest that BKCa channels protect against abnormal MVN neuronal activity induced by hypoxia, and might be a key target for treatment of vertigo and hearing loss in VBI.

Leptin counteracts the hypoxia-induced inhibition of spontaneously firing hippocampal neurons: a microelectrode array study

Besides regulating energy balance and reducing body-weight, the adipokine leptin has been recently shown to be neuroprotective and antiapoptotic by promoting neuronal survival after excitotoxic and oxidative insults. Here, we investigated the firing properties of mouse hippocampal neurons and the effects of leptin pretreatment on hypoxic damage (2 hours, 3% O(2)). Experiments were carried out by means of the microelectrode array (MEA) technology, monitoring hippocampal neurons activity from 11 to 18 days in vitro (DIV). Under normoxic conditions, hippocampal neurons were spontaneously firing, either with prevailing isolated and randomly distributed spikes (11 DIV), or with patterns characterized by synchronized bursts (18 DIV). Exposure to hypoxia severely impaired the spontaneous activity of hippocampal neurons, reducing their firing frequency by 54% and 69%, at 11 and 18 DIV respectively, and synchronized their firing activity. Pretreatment with 50 nM leptin reduced the firing frequency of normoxic neurons and contrasted the hypoxia-induced depressive action, either by limiting the firing frequency reduction (at both ages) or by increasing it to 126% (in younger neurons). In order to find out whether leptin exerts its effect by activating large conductance Ca(2+)-activated K(+) channels (BK), as shown on rat hippocampal neurons, we applied the BK channel blocker paxilline (1 µM). Our data show that paxilline reversed the effects of leptin, both on normoxic and hypoxic neurons, suggesting that the adipokine counteracts hypoxia through BK channels activation in mouse hippocampal neurons.

Hypoxia enhances high-voltage-activated calcium currents in rat primary cortical neurons via calcineurin

Hypoxia regulates neuronal ion channels, sometimes resulting in seizures. We evaluated the effects of brief sustained hypoxia (1% O(2), 4h) on voltage-gated calcium channels (VGCCs) in cultured rat primary cortical neurons. High-voltage activated (HVA) Ca(2+) currents were acquired immediately after hypoxic exposure or after 48h recovery in 95% air/5% CO(2). Maximal Ca(2+) current density increased 1.5-fold immediately after hypoxia, but reverted to baseline after 48h normoxia. This enhancement was primarily due to an increase in L-type VGCC activity, since nimodipine-insensitive residual Ca(2+) currents were unchanged. The half-maximal potentials of activation and steady-state inactivation were unchanged. The calcineurin inhibitors FK-506 (in the recording pipette) or cyclosporine A (during hypoxia) prevented the post-hypoxic increase in HVA Ca(2+) currents, while rapamycin and okadaic acid did not. L-type VGCCs were the source of Ca(2+) for calcineurin activation, as nimodipine during hypoxia prevented post-hypoxic enhancement. Hypoxia transiently potentiated L-type VGCC currents via calcineurin, suggesting a positive feedback loop to amplify neuronal calcium signaling that may contribute to seizure generation.

Hypoxia. 4. Hypoxia and ion channel function

The ability to sense and respond to oxygen deprivation is required for survival; thus, understanding the mechanisms by which changes in oxygen are linked to cell viability and function is of great importance. Ion channels play a critical role in regulating cell function in a wide variety of biological processes, including neuronal transmission, control of ventilation, cardiac contractility, and control of vasomotor tone. Since the 1988 discovery of oxygen-sensitive potassium channels in chemoreceptors, the effect of hypoxia on an assortment of ion channels has been studied in an array of cell types. In this review, we describe the effects of both acute and sustained hypoxia (continuous and intermittent) on mammalian ion channels in several tissues, the mode of action, and their contribution to diverse cellular processes.

The effect of mechanical strain or hypoxia on cell death in subpopulations of rat dorsal root ganglion neurons in vitro

Spinal nerves and their associated dorsal root ganglion (DRG) cells can be subject to mechanical deformation and hypoxia associated with pathology such as disc herniation, spinal stenosis and spine trauma. There is very limited information on the response of adult DRG neurons to such stressors. In this study we used an in vitro approach to examine the response of adult DRG cells to (a) mechanical, hypoxic, and combined injuries; and (b) to compare the effects on injury on nociceptive and non-nociceptive neurons, as well as on non-neuronal cells. Mechanical injury (20% tensile strain) led to significant neuronal cell death (assessed by ethidium homodimer-1 labelling), which was proportional to strain duration (5 min, 1 h, 6 h or 18 h). Hypoxia (2% O(2) for 24 h) also promoted death of DRG neurons, and was further enhanced when mechanical strain and hypoxia were combined. Both mechanical strain and hypoxia significantly decreased the maximum neurite length. Conversely, death of non-neuronal cells was only increased by hypoxia and not by mechanical strain. Total cell death in response to mechanical injury or hypoxia was similar in both non-nociceptive (neurofilament, NF-200 immunoreactive) and nociceptive (calcitonin gene-related peptide, CGRP immunoreactive) neurons, but apoptosis (assessed by activated caspase-3 immunostaining) was significantly higher in CGRP than NF-200 neurons. Surprisingly, cell death of non-peptidergic nociceptors (identified by Griffonia simplicifolia IB4 lectin binding) was already high in control cultures, and was not increased further by either mechanical stretch or hypoxia. These results provide detailed information on the response of adult DRG subpopulations to hypoxia and mechanical strain, and describe in vitro models that could be useful for screening potential neuroprotective agents.

[Inhibition effect of IL-1beta on calcium channels currents in cultured cortical neurons of rat]

Interleukin-1beta (IL-1beta) is an important proinflammatory cytokine that exert wide biological functions in the central nervous system (CNS). Voltage-gated Ca2+ channels in plasma membrane are essential components of Ca2+ signal whose change on pathological conditions is closely related to processes of diseases. Although both IL-1beta and Ca2+ channels have key roles in injury and disease of the brain, so far their relations have been reported in few studies. The effects of IL-1beta on Ca2+ channels in cultured cortical neurons of rats were investigated using patch-clamp recording in present research. Our results showed that both 10 and 50 ng/mL of IL-1beta treatment inhibit the Ca2+ currents in time and dose dependent manner, without the change of activation properties of voltage-gated Ca2+ channels (VGCC)s.

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

delta-, but not mu-, opioid receptor stabilizes K(+) homeostasis by reducing Ca(2+) influx in the cortex during acute hypoxia

Past work has shown that delta-opioid receptor (DOR) activation by [D-Ala(2),D-Leu(5)]-enkephalin (DADLE) attenuated the disruption of K(+) homeostasis induced by hypoxia or oxygen-glucose deprivation (OGD) in the cortex, while naltrindole, a DOR antagonist blocked this effect, suggesting that DOR activity stabilizes K(+) homeostasis in the cortex during hypoxic/ischemic stress. However, several important issues remain unclear regarding this new observation, especially the difference between DOR and other opioid receptors in the stabilization of K(+) homeostasis and the underlying mechanism. In this study, we asked whether DOR is different from micro-opioid receptors (MOR) in stabilizing K(+) homeostasis and which membrane channel(s) is critically involved in the DOR effect. The main findings are that (1) similar to DADLE (10 microM), H-Dmt-Tic-NH-CH (CH(2)--COOH)-Bid (1-10 microM), a more specific and potent DOR agonist significantly attenuated anoxic K(+) derangement in cortical slice; (2) [D-Ala(2), N-Me-Phe(4), glycinol(5)]-enkephalin (DAGO; 10 microM), a MOR agonist, did not produce any appreciable change in anoxic disruption of K(+) homeostasis; (3) absence of Ca(2+) greatly attenuated anoxic K(+) derangement; (4) inhibition of Ca(2+)-activated K(+) (BK) channels with paxilline (10 microM) reduced anoxic K(+) derangement; (5) DADLE (10 microM) could not further reduce anoxic K(+) derangement in the Ca(2+)-free perfused slices or in the presence of paxilline; and (6) glybenclamide (20 microM), a K(ATP) channel blocker, decreased anoxia-induced K(+) derangement, but DADLE (10 microM) could further attenuate anoxic K(+) derangement in the glybenclamide-perfused slices. These data suggest that DOR, but not MOR, activation is protective against anoxic K(+) derangement in the cortex, at least partially via an inhibition of hypoxia-induced increase in Ca(2+) entry-BK channel activity.

Redox control of calcium channels: from mechanisms to therapeutic opportunities

Calcium plays an integral role in cellular function. It is a well-recognized second messenger necessary for signaling cellular responses, but in excessive amounts can be deleterious to function, causing cell death. The main route by which calcium enters the cytoplasm is either from the extracellular compartment or internal addistores via calcium channels. There is good evidence that calcium channels can respond to pharmacological compounds that reduce or oxidize thiol groups on the channel protein. In addition, reactive oxygen species such as hydrogen peroxide and superoxide that can mediate oxidative pathology also mediate changes in channel function via alterations of thiol groups. This review looks at the structure and function of calcium channels, the evidence that changes in cellular redox state mediate changes in channel function, and the role of redox modification of channels in disease processes. Understanding how redox modification of the channel protein alters channel structure and function is providing leads for the design of therapeutic interventions that target oxidative stress responses.

Rapid pH and PO2changes in the tissue recording chamber during stoppage of a gas-equilibrated perfusate: effects on calcium currents in ventral horn neurons

In vitro studies often use bicarbonate-buffered saline solutions to mimic the normal extracellular environment of tissues. These solutions are typically equilibrated with gaseous O2 and CO2, the latter interacting with bicarbonate ions to maintain a physiological pH. In vitro tissue chambers, like those used for electrophysiology, are usually continually perfused with the gassed buffer, but stopping the perfusion to add expensive chemicals or acquire imaging data is a common practice. The present study demonstrates that this procedure leads to rapid (< 30 s) increases in pH and decreases in PO2 of the detained solution in the tissue chamber. During the first 200 s, pH increased by 0.4 units and resulted in a 25% PO2 reduction of the detained solution. The rates of these changes were dependent on the volume of solution in the chamber. In experiments using acute transverse slices from the lumbar spinal cord of neonatal (postnatal day 0-10) mice, perfusion stoppage of the same duration was accompanied by a 34.7% enhancement of the peak voltage-gated calcium current recorded from ventral horn neurons. In these cells both low voltage-activated and high voltage-activated currents were affected. These currents were unaffected by decreasing PO2 when a CO2-independent buffer was used, suggesting that changes in pH were responsible for the observed effects. It is concluded that the procedure of stopping a bicarbonate/CO2-buffered perfusate results in rapid changes in pH and PO2 of the solution detained in the tissue chamber, and that these changes have the potential to covertly influence experimental results.

Effect of hypoxia on hyperpolarization-activated current in mouse dorsal root ganglion neurons

The properties of hyperpolarization-activated current (I(h)) in mouse dorsal root ganglion (DRG) neurons and the effect of hypoxia on the current have been studied using whole-cell configuration of the patch clamp technique. Under voltage-clamp mode, I(h), blocked by 1 mM extracellular CsCl, was present in 75.5% of mouse DRG neurons. The distribution rate increased as the neurons become larger, 5.3%, 79.8% and 94.2% in small, medium and large neurons, respectively. Both I(h) density and the rate of I(h) activation increased in response to more hyperpolarized potential. The activation of I(h) current in larger neuron was faster than in smaller neuron, there was a significant correlation between the time constant of I(h) activation and neuron's size. However, I(h) density did not show any correlation with neuron's size. Under current-clamp mode, 'depolarizing sag' was observed in all neurons with I(h) current. The reversal potential (V(rev)) and the maximal conductance density of I(h) (G(h.max-density)) were -31.0 +/- 4.8 mV and 0.17 +/- 0.02 nS/pF, with a half-activated potential (V(0.5) = -99.4 +/- 1.1 mV) and a slope factor (kappa = -10.2 +/- 0.3 mV). There was a correlation between neuron's size and G(h.max-density) only. According to the effect of hypoxia on resting membrane potential, there were hypoxia-sensitive and hypoxia-insensitive neurons. In the hypoxia-sensitive neurons, I(h) was fully abolished by hypoxia, although the resting membrane potential was hyperpolarized. V(0.5) and V(rev) were shifted about 30 mV toward hyperpolarization, whereas G(h.max-density) and kappa were not affected by hypoxia. We suggest that the kinetics and voltage-dependent characteristics of I(h) are varied in mouse DRG neurons with different size. Hypoxia inhibits I(h) in the hypoxia-sensitive neurons by shifting its activation potential to a more hyperpolarized level.

Insulin receptor signaling in long-term memory consolidation following spatial learning

Evidence has shown that the insulin and insulin receptor (IR) play a role in cognitive function. However, the detailed mechanisms underlying insulin's action on learning and memory are not yet understood. Here we investigated changes in long-term memory-associated expression of the IR and downstream molecules in the rat hippocampus. After long-term memory consolidation following a water maze learning experience, gene expression of IR showed an up-regulation in the CA1, but a down-regulation in the CA3 region. These were correlated with a significant reduction in hippocampal IR protein levels. Learning-specific increases in levels of downstream molecules such as IRS-1 and Akt were detected in the synaptic membrane accompanied by decreases in Akt phosphorylation. Translocation of Shc protein to the synaptic membrane and activation of Erk1/2 were also observed after long-term memory formation. Despite the clear memory-correlated alterations in IR signaling pathways, insulin deficits in experimental diabetes mellitus (DM) rats induced by intraperitoneal injections of streptozotocin resulted in only minor memory impairments. This may be due to higher glucose levels in the DM brain, and to compensatory mechanisms from other signaling pathways such as the insulin-like growth factor-1 receptor (IGF-1R) system. Our results suggest that insulin/IR signaling plays a modulatory role in learning and memory processing, which may be compensated for by alternative pathways in the brain when an insulin deficit occurs.