Hypoxic changes in hippocampal neurons

Mild hypoxia-induced structural and functional changes of the hippocampal network

Hypoxia causes structural and functional changes in several brain regions, including the oxygen-concentration-sensitive hippocampus. We investigated the consequences of mild short-term hypoxia on rat hippocampus in vivo. The hypoxic group was treated with 16% O2 for 1 h, and the control group with 21% O2. Using a combination of Gallyas silver impregnation histochemistry revealing damaged neurons and interneuron-specific immunohistochemistry, we found that somatostatin-expressing inhibitory neurons in the hilus were injured. We used 32-channel silicon probe arrays to record network oscillations and unit activity from the hippocampal layers under anaesthesia. There were no changes in the frequency power of slow, theta, beta, or gamma bands, but we found a significant increase in the frequency of slow oscillation (2.1-2.2 Hz) at 16% O2 compared to 21% O2. In the hilus region, the firing frequency of unidentified interneurons decreased. In the CA3 region, the firing frequency of some unidentified interneurons decreased while the activity of other interneurons increased. The activity of pyramidal cells increased both in the CA1 and CA3 regions. In addition, the regularity of CA1, CA3 pyramidal cells' and CA3 type II and hilar interneuron activity has significantly changed in hypoxic conditions. In summary, a low O2 environment caused profound changes in the state of hippocampal excitatory and inhibitory neurons and network activity, indicating potential changes in information processing caused by mild short-term hypoxia.

Cognitive Function Mainly Shaped by Socioeconomic Status Rather Than Chronic Hypoxia in Adolescents at High Altitude

Yu, Linyang, Jinqing Feng, Chen Zhou, Xiaohan Zhu, Xiaobin Lou, Jian Yang, Haiying Qi, and Jia Li. Cognitive function mainly shaped by socioeconomic status rather than chronic hypoxia in adolescents at high altitude. High Alt Med Biol. 23:000-000, 2022. Background: The study evaluated cognitive function in relation to the changes in brain tissue oxygenation in three groups of high school students from different socioeconomic regions including Tibetans in Jiuzhi and Lhasa (both at 3,600 m), and Han in Beijing (44 m). Methods: Jiuzhi, Lhasa, and Beijing Group included 21 Tibetans, 24 Tibetans, and 23 Han, respectively. Wechsler Intelligence Scale was used for cognitive evaluation. Functional near infrared spectroscopy was used to measure the changes of oxygenated hemoglobin (oxy-Hb) during the cognitive assessment. Gross domestic product (GDP) was used to indicate the socioeconomic status. Results: All the cognitive scores were significantly lower in the two high altitude groups compared with the Beijing Group (p < 0.001). The scores in Jiuzhi Group were significantly lower compared with the Lhasa Group (p < 0.001). The changes in oxy-Hb in channels 6 and 15 in both high-altitude groups were significantly greater compared with the Beijing Group (p < 0.05), without significant difference between the two high-altitude groups. GDP was significantly correlated with all the scores (p < 0.001), but not altitude. Conclusions: Cognitive impairment occurs in adolescents at high altitude, being severer in Jiuzhi Group compared with the Lhasa Group. The lower performance in both high-altitude groups require greater brain activity over-compensated by cerebral oxygen delivery as indicated by the changes in oxy-Hb. The cognitive scores were significantly correlated with GDP rather than altitude. Cognitive function in adolescents at high altitude is not limited by chronic hypoxia, but mainly shaped by socioeconomic determinants.

Dynamic Gain Analysis Reveals Encoding Deficiencies in Cortical Neurons That Recover from Hypoxia-Induced Spreading Depolarizations

Cortical regions that are damaged by insults, such as ischemia, hypoxia, and trauma, frequently generate spreading depolarization (SD). At the neuronal level, SDs entail complete breakdown of ionic gradients, persisting for seconds to minutes. It is unclear whether these transient events have a more lasting influence on neuronal function. Here, we describe electrophysiological changes in cortical neurons after recovery from hypoxia-induced SD. When examined with standard measures of neuronal excitability several hours after recovery from SD, layer 5 pyramidal neurons in brain slices from mice of either sex appear surprisingly normal. However, we here introduce an additional parameter, dynamic gain, which characterizes the bandwidth of action potential encoding by a neuron, and thereby reflects its potential efficiency in a multineuronal circuit. We find that the ability of neurons that recover from SD to track high-frequency inputs is markedly curtailed; exposure to hypoxia did not have this effect when SD was prevented pharmacologically. Staining for Ankyrin G revealed at least a fourfold decrease in the number of intact axon initial segments in post-SD slices. Since this effect, along with the effect on encoding, was blocked by an inhibitor of the Ca²⁺-dependent enzyme, calpain, we conclude that both effects were mediated by the SD-induced rise in intracellular Ca²⁺ Although effects of calpain activation were detected in the axon initial segment, changes in soma-dendritic compartments may also be involved. Whatever the precise molecular mechanism, our findings indicate that in the context of cortical circuit function, effectiveness of neurons that survive SD may be limited.SIGNIFICANCE STATEMENT Spreading depolarization, which commonly accompanies cortical injury, entails transient massive breakdown of neuronal ionic gradients. The function of cortical neurons that recover from hypoxia-induced spreading depolarization is not obviously abnormal when tested for usual measures of neuronal excitability. However, we now demonstrate that they have a reduced bandwidth, reflecting a significant impairment of their ability to precisely encode high-frequency components of their synaptic input in output spike trains. Thus, neurons that recover from spreading depolarizations are less able to function normally as elements in the multineuronal cortical circuitry. These changes are correlated with activation of the calcium-dependent enzyme, calpain.

Aging of stimulus-driven and goal-directed attentional processes in young immigrants with long-term high altitude exposure in Tibet: An ERP study

High altitude (HA) exposure reduces the behavioral response to visual attention and the neural basis is still largely unclear. The present study explored the stimulus-driven and goal-directed factors that are hidden within this attentional behavior impairment via a visual search paradigm in young immigrants in Tibet by recording event-related potential (ERPs). We found that HA explosure significantly slowed the stimulus-driven behaviors instead of the goal-directed behaviors. Furthermore, the P1, N1, and P3 amplitudes collectively indicated the poor efficiency of entire attention behaviors, in which the P3 magnitude of resources allocation was negatively correlated with the attentional behavior response. And the P3 scalp distribution suggested a compensation for insufficient resources of sensory processing only in the goal-directed behaviors. Together, the present study made the point on how stimulus-driven and goal-directed attentional behaviors changed as a result of chronic HA environment exposure, which is similar to aging.

Acute sensitivity of astrocytes in the Substantia Nigra to oxygen and glucose deprivation (OGD) compared with hippocampal astrocytes in brain slices

The Substantia Nigra is a brainstem nucleus critical for movement control. Although its dopamine-producing neurons degenerate in Parkinsons disease, little is known of the acute effects of ischemia in this region. We recently reported that oxygen and glucose deprivation (OGD) in brain slices, an in vitro ischemia model, evokes a profound depolarization and swelling of GABAergic neurons in the Substantia Nigra pars reticulata (SNr), but not dopaminergic neurons in the Substantia Nigra pars compacta (SNc). The current study characterised the effects of OGD on nigral astrocytes, and compared these with the established responses of astrocytes in the CA1 hippocampal region. Intracellular recordings were made from astrocytes at the border between SNc and SNr subregions, in midbrain slices from postnatal day 21-23 rats. Immunoreactivity for astrocyte-specific proteins was also assessed. OGD evoked a slow, then fast depolarization of nigral astrocytes. The fast phase developed during the anoxic depolarization (indicated by a fast negative shift of extracellular DC potential and increase in light transmittance) and rapid increase in extracellular K⁺ concentration in the SNr. This biphasic response resembled the OGD-evoked depolarization of hippocampal astrocytes. However, unlike the partial repolarization seen in hippocampal cells after reperfusion with O₂ and glucose, nigral astrocytes remained depolarized near 0 mV. In addition, immunoreactivity for glial fibrillary acidic protein-positive astrocytes markedly decreased in the Substantia Nigra after OGD, while in the hippocampus remained unchanged. These data indicate an acute post-ischemic withdrawal of astrocytic support in the Substantia Nigra, but not in the hippocampus.

TrkB-mediated activation of the phosphatidylinositol-3-kinase/Akt cascade reduces the damage inflicted by oxygen-glucose deprivation in area CA3 of the rat hippocampus

The selective vulnerability of hippocampal area CA1 to ischemia-induced injury is a well-known phenomenon. However, the cellular mechanisms that confer resistance to area CA3 against ischemic damage remain elusive. Here, we show that oxygen-glucose deprivation-reperfusion (OGD-RP), an in vitro model that mimic the pathological conditions of the ischemic stroke, increases the phosphorylation level of tropomyosin receptor kinase B (TrkB) in area CA3. Slices preincubated with brain-derived neurotrophic factor (BDNF) or 7,8-dihydroxyflavone (7,8-DHF) exhibited reduced depression of the electrical activity triggered by OGD-RP. Consistently, blockade of TrkB suppressed the resistance of area CA3 to OGD-RP. The protective effect of TrkB activation was limited to area CA3, as OGD-RP caused permanent suppression of CA1 responses. At the cellular level, TrkB activation leads to phosphorylation of the accessory proteins SHC and Gab as well as the serine/threonine kinase Akt, members of the phosphoinositide 3-kinase/Akt (PI-3-K/Akt) pathway, a cascade involved in cell survival. Hence, acute slices pretreated with the Akt antagonist MK2206 in combination with BDNF lost the capability to resist the damage inflicted with OGD-RP. Consistently, with these results, CA3 pyramidal cells exhibited reduced propidium iodide uptake and caspase-3 activity in slices pretreated with BDNF and exposed to OGD-RP. We propose that PI-3-K/Akt downstream activation mediated by TrkB represents an endogenous mechanism responsible for the resistance of area CA3 to ischemic damage.

Dissipation of transmembrane potassium gradient is the main cause of cerebral ischemia-induced depolarization in astrocytes and neurons

Membrane potential (V_M) depolarization occurs immediately following cerebral ischemia and is devastating for the astrocyte homeostasis and neuronal signaling. Previously, an excessive release of extracellular K⁺ and glutamate has been shown to underlie an ischemia-induced V_M depolarization. Ischemic insults should impair membrane ion channels and disrupt the physiological ion gradients. However, their respective contribution to ischemia-induced neuronal and glial depolarization and loss of neuronal excitability are unanswered questions. A short-term oxygen-glucose deprivation (OGD) was used for the purpose of examining the acute effect of ischemic conditions on ion channel activity and physiological K⁺ gradient in neurons and glial cells. We show that a 30 min OGD treatment exerted no measurable damage to the function of membrane ion channels in neurons, astrocytes, and NG2 glia. As a result of the resilience of membrane ion channels, neuronal spikes last twice as long as our previously reported 15 min time window. In the electrophysiological analysis, a 30 min OGD-induced dissipation of transmembrane K⁺ gradient contributed differently in brain cell depolarization: severe in astrocytes and neurons, and undetectable in NG2 glia. The discrete cellular responses to OGD corresponded to a total loss of 69% of the intracellular K⁺ contents in hippocampal slices as measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). A major brain cell depolarization mechanism identified here is important for our understanding of cerebral ischemia pathology. Additionally, further understanding of the resilient response of NG2 glia to ischemia-induced intracellular K⁺ loss and depolarization should facilitate the development of future stroke therapy.

Hypometabolism as the ultimate defence in stress response: how the comparative approach helps understanding of medically relevant questions

First conceptualized from breath-hold diving mammals, later recognized as the ultimate cell autonomous survival strategy in anoxia-tolerant vertebrates and burrowing or hibernating rodents, hypometabolism is typically recruited by resilient organisms to withstand and recover from otherwise life-threatening hazards. Through the coordinated down-regulation of biosynthetic, proliferative and electrogenic expenditures at times when little ATP can be generated, a metabolism turned 'down to the pilot light' allows the re-balancing of energy demand with supply at a greatly suppressed level in response to noxious exogenous stimuli or seasonal endogenous cues. A unifying hallmark of stress-tolerant organisms, the adaptation effectively prevents lethal depletion of ATP, thus delineating a marked contrast with susceptible species. Along with disengaged macromolecular syntheses, attenuated transmembrane ion shuttling and PO₂ -conforming respiration rates, the metabolic slowdown in tolerant species usually culminates in a non-cycling, quiescent phenotype. However, such a reprogramming also occurs in leading human pathophysiologies. Ranging from microbial infections through ischaemia-driven infarcts to solid malignancies, cells involved in these disorders may again invoke hypometabolism to endure conditions non-permissive for growth. At the same time, their reduced activities underlie the frequent development of a general resistance to therapeutic interventions. On the other hand, a controlled induction of hypometabolic and/or hypothermic states by pharmacological means has recently stimulated intense research aimed at improved organ preservation and patient survival in situations requiring acutely administered critical care. The current review article therefore presents an up-to-date survey of concepts and applications of a coordinated and reversibly down-regulated metabolic rate as the ultimate defence in stress responses.

Spreading depolarization in the brain of Drosophila is induced by inhibition of the Na+/K+-ATPase and mitigated by a decrease in activity of protein kinase G

Spreading depolarization (SD) is characterized by a massive redistribution of ions accompanied by an arrest in electrical activity that slowly propagates through neural tissue. It has been implicated in numerous human pathologies, including migraine, stroke, and traumatic brain injury, and thus the elucidation of control mechanisms underlying the phenomenon could have many health benefits. Here, we demonstrate the occurrence of SD in the brain of Drosophila melanogaster, providing a model system, whereby cellular mechanisms can be dissected using molecular genetic approaches. Propagating waves of SD were reliably induced by disrupting the extracellular potassium concentration ([K(+)]o), either directly or by inhibition of the Na(+)/K(+)-ATPase with ouabain. The disturbance was monitored by recording the characteristic surges in [K(+)]o using K(+)-sensitive microelectrodes or by monitoring brain activity by measuring direct current potential. With the use of wild-type flies, we show that young adults are more resistant to SD compared with older adults, evidenced by shorter bouts of SD activity and attenuated [K(+)]o disturbances. Furthermore, we show that the susceptibility to SD differs between wild-type flies and w1118 mutants, demonstrating that our ouabain model is influenced by genetic strain. Lastly, flies with low levels of protein kinase G (PKG) had increased latencies to onset of both ouabain-induced SD and anoxic depolarization compared with flies with higher levels. Our findings implicate the PKG pathway as a modulator of SD in the fly brain, and given the conserved nature of the signaling pathway, it could likely play a similar role during SD in the mammalian central nervous system.

The earliest neuronal responses to hypoxia in the neocortical circuit are glutamate-dependent

Soon after exposure to hypoxia or ischemia, neurons in cortical tissues undergo massive anoxic depolarization (AD). This precipitous event is preceded by more subtle neuronal changes, including enhanced excitatory and inhibitory synaptic transmitter release. Here, we have used patch-in-slice techniques to identify the earliest effects of acute hypoxia on the synaptic and intrinsic properties of Layer 5 neurons, to determine their time course and to evaluate the role of glutamate receptors in their generation. Coronal slices of mouse somatosensory cortex were maintained at 36°C in an interface chamber and challenged with episodes of hypoxia. In recordings with cell-attached electrodes, the open probability of Ca(2+)-dependent BK channels began to increase within seconds of hypoxia onset, indicating a sharp rise in [Ca(2+)]i just beneath the membrane. By using a high concentration of K(+) in the pipette, we simultaneously monitored the membrane potential and showed that the [Ca(2+)]i rise was not associated with membrane depolarization. The earliest hypoxia-induced synaptic disturbance was a marked increase in the frequency of sPSCs, which also began soon after the removal of oxygen and long before AD. This synaptic effect was accompanied by depletion of the readily releasable transmitter pools, as demonstrated by a decreased response to hyperosmotic solutions. The early [Ca(2+)]i rise, the early increase in transmitter release and the subsequent AD itself were all prevented by bathing in a cocktail containing blockers of ionotropic glutamate receptors. We found no evidence for involvement of pannexin hemichannels or TRPM7 channels in the early responses to hypoxia in this experimental preparation. Our data indicate that the earliest cellular consequences of cortical hypoxia are triggered by activation of glutamate-gated channels.

Raised Intracellular Calcium Contributes to Ischemia-Induced Depression of Evoked Synaptic Transmission

Oxygen-glucose deprivation (OGD) leads to depression of evoked synaptic transmission, for which the mechanisms remain unclear. We hypothesized that increased presynaptic [Ca²⁺]_i during transient OGD contributes to the depression of evoked field excitatory postsynaptic potentials (fEPSPs). Additionally, we hypothesized that increased buffering of intracellular calcium would shorten electrophysiological recovery after transient ischemia. Mouse hippocampal slices were exposed to 2 to 8 min of OGD. fEPSPs evoked by Schaffer collateral stimulation were recorded in the stratum radiatum, and whole cell current or voltage clamp recordings were performed in CA1 neurons. Transient ischemia led to increased presynaptic [Ca²⁺]_i, (shown by calcium imaging), increased spontaneous miniature EPSP/Cs, and depressed evoked fEPSPs, partially mediated by adenosine. Buffering of intracellular Ca²⁺ during OGD by membrane-permeant chelators (BAPTA-AM or EGTA-AM) partially prevented fEPSP depression and promoted faster electrophysiological recovery when the OGD challenge was stopped. The blocker of BK channels, charybdotoxin (ChTX), also prevented fEPSP depression, but did not accelerate post-ischemic recovery. These results suggest that OGD leads to elevated presynaptic [Ca²⁺]_i, which reduces evoked transmitter release; this effect can be reversed by increased intracellular Ca²⁺ buffering which also speeds recovery.

Acyl Ghrelin Improves Synapse Recovery in an In Vitro Model of Postanoxic Encephalopathy

Comatose patients after cardiac arrest have a poor prognosis. Approximately half never awakes as a result of severe diffuse postanoxic encephalopathy. Several neuroprotective agents have been tested, however without significant effect. In the present study, we used cultured neuronal networks as a model system to study the general synaptic damage caused by temporary severe hypoxia and the possibility to restrict it by ghrelin treatment. Briefly, we applied hypoxia (pO₂ lowered from 150 to 20 mmHg) during 6 h in 55 cultures. Three hours after restoration of normoxia, half of the cultures were treated with ghrelin for 24 h, while the other, non-supplemented, were used as a control. All cultures were processed immunocytochemically for detection of the synaptic marker synaptophysin. We observed that hypoxia led to drastic decline of the number of synapses, followed by some recovery after return to normoxia, but still below the prehypoxic level. Additionally, synaptic vulnerability was selective: large- and small-sized neurons were more susceptible to synaptic damage than the medium-sized ones. Ghrelin treatment significantly increased the synapse density, as compared with the non-treated controls or with the prehypoxic period. The effect was detected in all neuronal subtypes. In conclusion, exogenous ghrelin has a robust impact on the recovery of cortical synapses after hypoxia. It raises the possibility that ghrelin or its analogs may have a therapeutic potential for treatment of postanoxic encephalopathy.

Increased excitability and excitatory synaptic transmission during in vitro ischemia in the neonatal mouse hippocampus

OBJECTIVE

The present study tested the hypothesis that exposure to in vitro hypoxia-ischemia alters membrane properties and excitability as well as excitatory synaptic transmission of CA1 pyramidal neurons in the neonatal mouse.

METHODS

Experiments were conducted in hippocampal slices in P7-P9 C57Bl/6 mice using whole-cell patch clamp in current- and voltage-clamp mode. Passive membrane potential (Vm), input resistance (Rin) and active (action potential (AP) threshold and amplitude) membrane properties of CA1 pyramidal neurons were assessed at baseline, during 10 min in vitro ischemia (oxygen-glucose deprivation (OGD)) and during reoxygenation. Spontaneous and miniature excitatory post-synaptic currents (s and mEPSCs) were studied under similar conditions.

RESULTS

OGD caused significant depolarization of CA1 pyramidal neurons as well as decrease in AP threshold and increase in AP amplitude. These changes were blocked by the application of tetrodotoxin (TTX), indicating Na(+) channels' involvement. Following 10 min of reoxygenation, significant membrane hyperpolarization was noted and it was associated with a decrease in Rin. AP threshold and amplitude returned to baseline during that stage. sEPSC and mEPSC frequency increased during both OGD and reoxygenation but their amplitude remained unchanged. Additionally, we found that OGD decreases Ih (hyperpolarization activated current) in CA1 neurons from neonatal mice and this effect persists during reoxygenation.

SIGNIFICANCE

These results indicate that in vitro ischemia leads to changes in membrane excitability mediated by sodium and potassium channels. Further, it results in enhanced neurotransmitter release from presynaptic terminals. These changes are likely to represent one of the mechanisms of hypoxia/ischemia-mediated seizures in the neonatal period.

Glycogen Fuels Survival During Hyposmotic-Anoxic Stress in Caenorhabditis elegans

Oxygen is an absolute requirement for multicellular life. Animals that are deprived of oxygen for sufficient periods of time eventually become injured and die. This is largely due to the fact that, without oxygen, animals are unable to generate sufficient quantities of energy. In human diseases triggered by oxygen deprivation, such as heart attack and stroke, hyposmotic stress and cell swelling (edema) arise in affected tissues as a direct result of energetic failure. Edema independently enhances tissue injury in these diseases by incompletely understood mechanisms, resulting in poor clinical outcomes. Here, we present investigations into the effects of osmotic stress during complete oxygen deprivation (anoxia) in the genetically tractable nematode Caenorhabditis elegans. Our findings demonstrate that nematode survival of a hyposmotic environment during anoxia (hyposmotic anoxia) depends on the nematode’s ability to engage in glycogen metabolism. We also present results of a genome-wide screen for genes affecting glycogen content and localization in the nematode, showing that nematode survival of hyposmotic anoxia depends on a large number of these genes. Finally, we show that an inability to engage in glycogen synthesis results in suppression of the enhanced survival phenotype observed in daf-2 insulin-like pathway mutants, suggesting that alterations in glycogen metabolism may serve as a basis for these mutants’ resistance to hyposmotic anoxia.

Effects of blast overpressure on neurons and glial cells in rat organotypic hippocampal slice cultures

Due to recent involvement in military conflicts, and an increase in the use of explosives, there has been an escalation in the incidence of blast-induced traumatic brain injury (bTBI) among US military personnel. Having a better understanding of the cellular and molecular cascade of events in bTBI is prerequisite for the development of an effective therapy that currently is unavailable. The present study utilized organotypic hippocampal slice cultures (OHCs) exposed to blast overpressures of 150 kPa (low) and 280 kPa (high) as an in vitro bTBI model. Using this model, we further characterized the cellular effects of the blast injury. Blast-evoked cell death was visualized by a propidium iodide (PI) uptake assay as early as 2 h post-injury. Quantification of PI staining in the cornu Ammonis 1 and 3 (CA1 and CA3) and the dentate gyrus regions of the hippocampus at 2, 24, 48, and 72 h following blast exposure revealed significant time dependent effects. OHCs exposed to 150 kPa demonstrated a slow increase in cell death plateauing between 24 and 48 h, while OHCs from the high-blast group exhibited a rapid increase in cell death already at 2 h, peaking at ~24 h post-injury. Measurements of lactate dehydrogenase release into the culture medium also revealed a significant increase in cell lysis in both low- and high-blast groups compared to sham controls. OHCs were fixed at 72 h post-injury and immunostained for markers against neurons, astrocytes, and microglia. Labeling OHCs with PI, neuronal, and glial markers revealed that the blast-evoked extensive neuronal death and to a lesser extent loss of glial cells. Furthermore, our data demonstrated activation of astrocytes and microglial cells in low- and high-blasted OHCs, which reached a statistically significant difference in the high-blast group. These data confirmed that our in vitro bTBI model is a useful tool for studying cellular and molecular changes after blast exposure.

Regulation of Kv4.2 A-Type Potassium Channels in HEK-293 Cells by Hypoxia

We previously observed that A-type potassium currents were decreased and membrane excitability increased in hippocampal dentate granule cells after neonatal global hypoxia associated with seizures. Here, we studied the effects of hypoxia on the function and expression of Kv4.2 and Kv4.3 α subunit channels, which encode rapidly inactivating A-type K currents, in transfected HEK-293 cells to determine if hypoxia alone could regulate I_Ain vitro. Global hypoxia in neonatal rat pups resulted in early decreased hippocampal expression of Kv4.2 mRNA and protein with 6 or 12 h post-hypoxia. Whole-cell voltage-clamp recordings revealed that similar times after hypoxia (1%) in vitro decreased peak currents mediated by recombinant Kv4.2 but not Kv4.3 channels. Hypoxia had no significant effect on the voltage-dependencies of activation and inactivation of Kv4.2 channels, but increased the time constant of activation. The same result was observed when Kv4.2 and Kv4.3 channels were co-expressed in a 1:1 ratio. These data suggested that hypoxia directly modulates A-type potassium channels of the subfamily typically expressed in principal hippocampal neurons, and does so in a manner to decrease function. Given the role of I_A to slow action potential firing, these data are consistent with a direct effect of hypoxia to decrease I_A as a mechanism of increased neuronal excitability and promotion of seizures.

Voltage-gated potassium channels at the crossroads of neuronal function, ischemic tolerance, and neurodegeneration

Voltage-gated potassium (Kv) channels are widely expressed in the central and peripheral nervous system and are crucial mediators of neuronal excitability. Importantly, these channels also actively participate in cellular and molecular signaling pathways that regulate the life and death of neurons. Injury-mediated increased K(+) efflux through Kv2.1 channels promotes neuronal apoptosis, contributing to widespread neuronal loss in neurodegenerative disorders such as Alzheimer's disease and stroke. In contrast, some forms of neuronal activity can dramatically alter Kv2.1 channel phosphorylation levels and influence their localization. These changes are normally accompanied by modifications in channel voltage dependence, which may be neuroprotective within the context of ischemic injury. Kv1 and Kv7 channel dysfunction leads to neuronal hyperexcitability that critically contributes to the pathophysiology of human clinical disorders such as episodic ataxia and epilepsy. This review summarizes the neurotoxic, neuroprotective, and neuroregulatory roles of Kv channels and highlights the consequences of Kv channel dysfunction on neuronal physiology. The studies described in this review thus underscore the importance of normal Kv channel function in neurons and emphasize the therapeutic potential of targeting Kv channels in the treatment of a wide range of neurological diseases.

Attenuated effects of Neu2000 on hypoxia-induced synaptic activities in a rat hippocampus

Neu2000 (NEU; 2-hydroxy-5-(2,3,5,6-tetrafluoro-4-trifluoromethyl-benzylamino)-benzoic acid), a recently developed derivative of acetylsalicylic acid and sulfasalazine, potently protects against neuronal cell death following ischemic brain injury by antagonizing NMDA receptor-mediated neuronal toxicity and oxidative stress. However, it has yet to be determined whether NEU can attenuate hypoxia-induced impairment of neuronal electrical activity. In this study, we carried out extracellular recordings of hippocampal slices in order to investigate the effects of NEU on the electrical activity of neurons exposed to a hypoxic insult (oxygen and glucose deprivation). NEU prominently suppressed hypoxia-induced impairment of neuronal activity in a concentration-dependent manner. NEU, at a low dose (1 μM), competently depressed the hypoxia-induced convulsive activity in a manner similar to trolox. Furthermore, high concentrations of NEU (50 μM) markedly abolished all hypoxia-mediated impairment of neuronal activity and accelerated the slow recovery of neuronal activity more efficiently than ifenprodil and APV. These results suggest that NEU attenuates hypoxia-induced impairment of neuronal activity more potently than the antioxidant, trolox, and the NMDA receptor antagonists, ifenprodil and APV. We propose that NEU is a striking pharmacological candidate for neuroprotection against hypoxia because of its defensive action on hypoxia-mediated impairment of electrical neurotransmission as well as its neuroprotective action against neuronal cell death induced by exposure to pathological hypoxic conditions.

Relationship between asthma and cognition: the Cooper Center Longitudinal Study

BACKGROUND

Minimal data are available on the relationship between asthma and cognitive performance. In this report, we examine the relationship between asthma and cognitive performance in older adults, a subpopulation with elevated risk of cognitive impairment.

METHODS

We conducted a cross-sectional, retrospective analysis of 1380 participants age ≥55 who completed preventive health examinations at the Cooper Clinic in Dallas, TX. Cognition was assessed using the Montreal Cognitive Assessment (MoCA), a brief test for mild cognitive impairment. Data were analyzed in a multiple logistic regression using MoCA scores suggestive of cognitive impairment as the dependent variable.

RESULTS

When controlling for demographic characteristics, self-rated health status, inhaled corticosteroid use, and FEV1 /FVC, asthma were associated with 78% increased risk of cognitive impairment (P = 0.02) as defined by MoCA score.

CONCLUSIONS

In the largest sample examined to date, we have identified a significant relationship between asthma and cognitive impairment in older people.

Oxygen/Glucose Deprivation and Reperfusion Cause Modifications of Postsynaptic Morphology and Activity in the CA3 Area of Organotypic Hippocampal Slice Cultures

Brain ischemia leads to overstimulation of N-methyl-D-aspartate (NMDA) receptors, referred as excitotoxicity, which mediates neuronal cell death. However, less attention has been paid to changes in synaptic activity and morphology that could have an important impact on cell function and survival following ischemic insult. In this study, we investigated the effects of reperfusion after oxygen/glucose deprivation (OGD) not only upon neuronal cell death, but also on ultrastructural and biochemical characteristics of postsynaptic density (PSD) protein, in the stratum lucidum of the CA3 area in organotypic hippocampal slice cultures. After OGD/reperfusion, neurons were found to be damaged; the organelles such as mitochondria, endoplasmic reticulum, dendrites, and synaptic terminals were swollen; and the PSD became thicker and irregular. Ethanolic phosphotungstic acid staining showed that the density of PSD was significantly decreased, and the thickness and length of the PSD were significantly increased in the OGD/reperfusion group compared to the control. The levels of PSD proteins, including PSD-95, NMDA receptor 1, NMDA receptor 2B, and calcium/calmodulin-dependent protein kinase II, were significantly decreased following OGD/reperfusion. These results suggest that OGD/reperfusion induces significant modifications to PSDs in the CA3 area of organotypic hippocampal slice cultures, both morphologically and biochemically, and this may contribute to neuronal cell death and synaptic dysfunction after OGD/reperfusion.

Pathophysiology of diabetic erectile dysfunction: potential contribution of vasa nervorum and advanced glycation endproducts

Erectile dysfunction (ED) due to diabetes mellitus remains difficult to treat medically despite advances in pharmacotherapeutic approaches in the field. This unmet need has resulted in a recent re-focus on the pathophysiology, in order to understand the cellular and molecular mechanisms leading to ED in diabetes. Diabetes-induced ED is often resistant to PDE5 inhibitor treatment, thus there is a need to discover targets that may lead to novel approaches for a successful treatment. The aim of this brief review is to update the reader in some of the latest development on that front, with a particular focus on the role of impaired neuronal blood flow and the formation of advanced glycation endproducts.

Ischemic cerebral damage: an appraisal of synaptic failure

In the human brain, ≈30% of the energy is spent on synaptic transmission. Disappearance of synaptic activity is the earliest consequence of cerebral ischemia. The changes of synaptic function are generally assumed to be reversible and persistent damage is associated with membrane failure and neuronal death. However, there is overwhelming experimental evidence of isolated, but persistent, synaptic failure resulting from mild or moderate cerebral ischemia. Early failure results from presynaptic damage with impaired transmitter release. Proposed mechanisms include dysfunction of adenosine triphosphate-dependent calcium channels and a disturbed docking of glutamate-containing vesicles resulting from impaired phosphorylation. We review energy distribution among neuronal functions, focusing on energy usage of synaptic transmission. We summarize the effect of ischemia on neurotransmission and the evidence of long-lasting synaptic failure as a cause of persistent symptoms in patients with cerebral ischemia. Finally, we discuss the implications of synaptic failure in the diagnosis of cerebral ischemia, including the limited sensitivity of diffusion-weighted MRI in those cases in which damage is presumably limited to the synapses.

Energy deprivation transiently enhances rhythmic inhibitory events in the CA3 hippocampal network in vitro

Oxygen glucose deprivation (OGD) leads to rapid suppression of synaptic transmission. Here we describe an emergence of rhythmic activity at 8 to 20 Hz in the CA3 subfield of hippocampal slice cultures occurring for a few minutes prior to the OGD-induced cessation of evoked responses. These oscillations, dominated by inhibitory events, represent network activity, as they were abolished by tetrodotoxin. They were also completely blocked by the GABAergic antagonist picrotoxin, and strongly reduced by the glutamatergic antagonist NBQX. Applying CPP to block NMDA receptors had no effect and neither did UBP302, an antagonist of GluK1-containing kainate receptors. The gap junction blocker mefloquine disrupted rhythmicity. Simultaneous whole-cell voltage-clamp recordings from neighboring or distant CA3 pyramidal cells revealed strong cross-correlation of the incoming rhythmic activity. Interneurons in the CA3 area received similar correlated activity. Interestingly, oscillations were much less frequently observed in the CA1 area. These data, together with the observation that the recorded activity consists primarily of inhibitory events, suggest that CA3 interneurons are important for generating these oscillations. This transient increase in inhibitory network activity during OGD may represent a mechanism contributing to the lower vulnerability to ischemic insults of the CA3 area as compared to the CA1 area.

Oxygen and glucose deprivation-induced changes in astrocyte membrane potential and their underlying mechanisms in acute rat hippocampal slices

Accumulating evidence indicates a significant astrocytic involvement in cerebral ischemia neuropathology, but little is known about the immediate astrocytic responses to ischemia insults in terms of electrophysiology and their pathologic implications. We show that astrocytes in acute rat hippocampal slices responded reversibly to more than 30 mins oxygen and glucose deprivation (OGD) treatment with depolarized membrane potentials (V(m)) in whole-cell current clamp recording. This depolarization was multiphasic, showing an initial approximately 11 mins small-amplitude depolarization plateau, followed by a 6-mins accelerated depolarization, and then a second plateau. Oxygen and glucose deprivation-induced astrocyte V(m) depolarization was only marginally inhibited, approximately 10%, by inhibition of ionotropic glutamate, gamma-aminobutyric acid, purinergic receptors, and glutamate transporters presumed to be present on astrocytes in situ, suggesting increase in extracellular [K(+)] was primarily responsible for the astrocytic V(m) change. The V(m) depolarization was five-fold greater when glycolysis was inhibited by iodoacetate in a short 8 mins OGD treatment, suggesting glycolytic ATP is critical in maintaining extracellular K(+) homeostasis in the early phase of OGD. Addition of oxidative metabolism inhibitors had much less effect. Cessation of OGD was always followed by a rapid and transient 9 mV astrocyte V(m) hyperpolarization relative to the control V(m) that was inhibited by ouabain, indicating a reactively enhanced Na(+)/K(+)-ATPase activity in post-OGD reperfusion. Altogether, hippocampal astrocytes appear to be electrophysiologically more resistant to acute ischemia insults as compared with neurons, and this should allow astrocytes to rescue endangered neurons in the face of acute ischemia insults via their various homeostatic functions.

Hypoxia tolerance in mammals and birds: from the wilderness to the clinic

All mammals and birds must develop effective strategies to cope with reduced oxygen availability. These animals achieve tolerance to acute and chronic hypoxia by (a) reductions in metabolism, (b) the prevention of cellular injury, and (c) the maintenance of functional integrity. Failure to meet any one of these tasks is detrimental. Birds and mammals accomplish this triple task through a highly coordinated, systems-level reconfiguration involving the partial shutdown of some but not all organs. This reconfiguration is achieved through a similarly complex reconfiguration at the cellular and molecular levels. Reconfiguration at these various levels depends on numerous factors that include the environment, the degree of hypoxic stress, and developmental, behavioral, and ecological conditions. Although common molecular strategies exist, the cellular and molecular changes in any given cell are very diverse. Some cells remain metabolically active, whereas others shut down or rely on anaerobic metabolism. This cellular shutdown is temporarily regulated, and during hypoxic exposure, active cellular networks must continue to control vital functions. The challenge for future research is to explore the cellular mechanisms and conditions that transform an organ or a cellular network into a hypometabolic state, without loss of functional integrity. Much can be learned in this respect from nature: Diving, burrowing, and hibernating animals living in diverse environments are masters of adaptation and can teach us how to deal with hypoxia, an issue of great clinical significance.

The effect of sulfur dioxide inhalation on active avoidance learning, antioxidant status and lipid peroxidation during aging

The effect of SO2 was examined on active avoidance learning, thiobarbituric acid reactive substances (TBARS), and the activities of Cu, Zn superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) in young (3 months), middle-age (12 months ), and old (24 months) Swiss male albino rats. Ten ppm SO2 was administered to the animals of SO2 groups in an exposure chamber for 1 h/day x 7 days/week x 6 weeks while control groups were exposed to filtered air in the same condition. The most prominent effect of aging on active performance was also observed in the older group. SO2 exposure significantly decreased the active avoidance learning in the young group, but it had no effect on this parameter in the middle-aged and the older group compared with their corresponding control groups. SO2 exposure resulted in increased levels of Cu, Zn-SOD activity while decreased level of GSH-Px activity in all experimental groups compared with their corresponding control groups. CAT activities were unaltered. TBARS levels of all SO2 exposed groups were significantly increased compared with their respective control groups. In conclusion, results from the present research showed that SO2 exposure resulted in an increase in the lipid peroxidation and caused alterations in antioxidant enzyme activities. Additionally, SO2 exposure impaired cognitive function only in the young rats during the acquisition phase of active avoidance learning.

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.

Oxygen and glucose deprivation induces major dysfunction in the somatosensory cortex of the newborn rat

The mechanisms and functional consequences of ischemia-induced injury during perinatal development are poorly understood. Subplate neurons (SPn) play a central role in early cortical development and a pathophysiological impairment of these neurons may have long-term detrimental effects on cortical function. The acute and long-term consequences of combined oxygen and glucose deprivation (OGD) were investigated in SPn and compared with OGD-induced dysfunction of immature layer V pyramidal cortical neurons (PCn) in somatosensory cortical slices from postnatal day (P)0-4 rats. OGD for 50 min followed by a 10-24-h period of normal oxygenation and glucose supply in vitro or in culture led to pronounced caspase-3-dependent apoptotic cell death in all cortical layers. Whole-cell patch-clamp recordings revealed that the majority of SPn and PCn responded to OGD with an initial long-lasting ischemic hyperpolarization accompanied by a decrease in input resistance (R(in)), followed by an ischemic depolarization (ID). Upon reoxygenation and glucose supply, the recovery of the membrane potential and R(in) was followed by a Na+/K+-ATPase-dependent postischemic hyperpolarization, and in almost half of the investigated SPn and PCn by a postischemic depolarization. Whereas neither a moderate (2.5 mm) nor a high (4.8 mm) increase in extracellular magnesium concentration protected the SPn from OGD-induced dysfunction, blockade of NMDA receptors with MK-801 led to a significant delay and decrease of the ID. Our data demonstrate that OGD induces apoptosis and a profound dysfunction in SPn and PCn, and underline the critical role of NMDA receptors in early ischemia-induced neuronal damage.

The inhibitory effect of ouabain on the response of slowly adapting pulmonary stretch receptors to hyperinflation in the rabbit

The combined effects of ouabain (Na(+)-K(+) ATPase inhibitor) and hyperinflation (inflation volume=three tidal volumes) on slowly adapting pulmonary stretch receptors (SARs) were studied before and after administration of nifedipine (an L-type Ca(2+) channel blocker) and KB-R7943 (a reverse-mode Na(+)-Ca(2+) exchanger blocker) in anesthetized, artificially ventilated rabbits after bilateral vagotomy. Before ouabain administration, hyperinflation stimulated SAR activity. After 20 min of ouabain administration (30 microg/kg) the SARs increased discharge rates in normal inflation. Under these conditions, hyperinflation initially stimulated SAR activity but subsequently inhibited the activity at peak inflation. Additional administration of 60 microg/kg ouabain (total dose=90 microg/kg) caused a further stimulation of SAR activity, but 20 min later both normal inflation and hyperinflation resulted in a greater inhibition of the receptor activity. The hyperinflation-induced SAR inhibition in the presence of ouabain (30 microg/kg) was not significantly altered by administration of either nifedipine (2 and 4 mg/kg) or KB-R7943 (1 and 3 mg/kg). In another series of experiments, we further examined the combined effects of ouabain and hyperinflation in veratridine (a Na(+) channel opener, 40 microg/kg)-treated animals. After recovery from the veratridine effect on SAR activity, which vigorously stimulated the receptor activity, ouabain treatment (30 microg/kg) that silenced the receptor activity at peak inflation greatly inhibited hyperinflation-induced SAR stimulation. These results suggest that hyperinflation-induced SAR inhibition in the presence of ouabain may be related to a Na(+) overload, but not to a Ca(2+) influx via activation of L-type Ca(2+) channels, in the SAR endings.

Alterations of potassium currents in ischemia-vulnerable and ischemia-resistant neurons in the hippocampus after ischemia

CA1 pyramidal neurons in the hippocampus die 2-3 days following transient forebrain ischemia, whereas CA3 pyramidal neurons and granule cells in the dentate gyrus remain viable. Excitotoxicity is the major cause of ischemic cell death, and potassium currents play important roles in regulating the neuronal excitability. The present study compared the changes of potassium currents in acutely dissociated hippocampal neurons at different intervals after ischemia. In CA1 neurons, the amplitude of rapid inactivating potassium currents (I(A)) was significantly increased at 14 h and returned to control levels at 38 h after ischemia; the rising slope and decay time constant of I(A) were accordingly increased after ischemia. The activation curve of I(A) in CA1 neurons shifted to the depolarizing direction at 38 h after ischemia. In granule cells, the amplitude and rising slope of I(A) were significantly increased at 38 h after ischemia; the inactivation curves of I(A) shifted toward the depolarizing direction accordingly at 38 h after ischemia. The I(A) remained unchanged in CA3 neurons after ischemia. The amplitudes of delayed rectifier potassium currents (I(Kd)) in CA1 neurons were progressively increased after ischemia. No significant difference in I(Kd) was detected in CA3 and granule cells at any time points after reperfusion. These results indicated that the voltage dependent potassium currents in hippocampal neurons were differentially altered after cerebral ischemia. The up-regulation of I(A) in dentate granule cells might have protective effects. The increase of I(Kd) in CA1 neurons might be associated with the neuronal damage after ischemia.

Effects of sodium metabisulfite on potassium currents in acutely isolated CA1 pyramidal neurons of rat hippocampus

The effects of sodium metabisulfite (SMB), a food preservative mostly used in food and drug industries, on voltage-dependent potassium currents in acutely isolated hippocampal CA1 pyramidal neurons of rat were studied using the whole-cell patch-clamp techniques. SMB increased transient outward potassium current (IA) and delayed rectifier potassium current (IK) in a concentration-dependent manner. 10 microM SMB shifted the steady-state activation curve of IK to more negative potentials, and the steady-state inactivation curves of IA and IK to more positive potentials. Time to peak of IA was not affected, but the decay of IA was delayed by SMB. However, the activation and inactivation time constants of IK were both decreased by SMB. These results suggested that SMB differently affected IA and IK, and it would decrease the excitability of hippocampal neuron by increasing potassium currents.

Protective role of neuronal KATP channels in brain hypoxia

During severe arterial hypoxia leading to brain anoxia, most mammalian neurons undergo a massive depolarisation terminating in cell death. However, some neurons of the adult brain and most immature nervous structures tolerate extended periods of hypoxia-anoxia. An understanding of the mechanisms underlying this tolerance to oxygen depletion is pivotal for developing strategies to protect the brain from consequences of hypoxic-ischemic insults. ATP-sensitive K(+) (K(ATP)) channels are good subjects for this study as they are activated by processes associated with energy deprivation and can counteract the terminal anoxic-ischemic neuronal depolarisation. This review summarises in vitro analyses on the role of K(ATP) channels in hypoxia-anoxia in three distinct neuronal systems of rodents. In dorsal vagal neurons, blockade of K(ATP) channels with sulfonylureas abolishes the hypoxic-anoxic hyperpolarisation. However, this does not affect the extreme tolerance of these neurons to oxygen depletion as evidenced by a moderate and sustained increase of intracellular Ca(2+) (Ca(i)). By contrast, a sulfonylurea-induced block of K(ATP) channels shortens the delay of occurrence of a major Ca(i) rise in cerebellar Purkinje neurons. In neurons of the neonatal medullary respiratory network, K(ATP) channel blockers reverse the anoxic hyperpolarisation associated with slowing of respiratory frequency. This may constitute an adaptive mechanism for energy preservation. These studies demonstrate that K(ATP) channels are an ubiquituous feature of mammalian neurons and may, indeed, play a protective role in brain hypoxia.

Hypoxic excitability changes and sodium currents in hippocampus CA1 neurons

1. The objective of the present study was to distinguish if inhibition of neuronal activity by hypoxia is related to a block of voltage-gated Na+ channels. 2. The effect of chemical hypoxia induced by cyanide (0.5 mM, 10 min perfusion) was studied with patch-clamp technique in visualized intact CA1 pyramidal neurons in rat brain slices. Action potentials were elicited in whole cell current-clamp recordings and the threshold was estimated by current pulses of 50-ms duration and incremental amplitudes (n = 31). The effect of cyanide on the Na+ current and conductance was studied in voltage clamp recordings from cell-attached patches (n = 13). 3. Cyanide perfusion during 10 min increased the threshold for excitation by 73 +/- 79 pA (p = 0.001), which differed from the effect in control cells (11 +/- 41 pA, ns). The change in current threshold was correlated to a change in membrane potential (r = -0.88, p < 0.0001). Cyanide had no significant effect on the peak amplitude, duration, or rate of rise of the action potential. 4. Cyanide perfusion did not change the Na+ current size, but caused a small decrease in ENa (-17 +/- 22 mV, ns) and a slight increase in Na+ conductance (+14 +/- 26%, ns), which differed (p = 0.045) from controls (-19 +/- 23 %, ns). 5. In conclusion, chemical hypoxia does not cause a decrease in Na+ conductance. The decreased excitability during hypoxia can be explained by an increase in the current threshold, which is correlated with the effect on the membrane potential.

Protective effect of rhynchophylline and isorhynchophylline on in vitro ischemia-induced neuronal damage in the hippocampus: putative neurotransmitter receptors involved in their action

Rhynchophylline and isorhynchophylline are major tetracyclic oxindole alkaloid components of Uncaira species, which have been long used as medicinal plants. In this study we examined the protective effects of rhynchophylline and isorhynchophylline on in vitro ischemia-induced neuronal damage in the hippocampus and interaction of these alkaloids with neurotransmitter receptors in a receptor expression model of Xenopus oocytes. In vitro ischemia was induced by exposing the hippocampal slices to oxygen- and D-glucose-deprived medium over 8 min. The resultant neuronal damage was elucidated as deterioration of population spike (PS) amplitudes evoked trans-synaptically by electrical stimulation of Schaffer collaterals and recorded in the CA1 area. Rhynchophylline and isorhynchophylline, as well as the N-methyl-D-aspartate (NMDA) antagonist (+/-)-2-amino-5-phosphono-valeric acid (APV), the muscarinic M1 receptor antagonist pirenzepine, and the 5-HT2 receptor antagonist ketanserin, attenuated the in vitro ischemia-induced neuronal damage in a concentration-dependent manner. There was no difference in the extent of protection against the neuronal damage between rhynchophylline and isorhynchophylline treatment. In Xenopus oocytes expressing the rat brain receptors encoded by total RNA, both rhynchophylline and isorhynchophylline reduced muscarinic receptor- and 5-HT2 receptor-mediated current responses in a competitive manner. Together with our previous findings that rhynchophylline and isorhynchophylline have a non-competitive antagonistic effect on the NMDA-type ionotropic glutamate receptors, the present results suggest that these alkaloids exert their protective action against ischemia-induced neuronal damage by preventing NMDA, muscarinic M1, and 5-HT2 receptors-mediated neurotoxicity during ischemia.

Novel method for quantification of brain cell swelling in rat hippocampal slices

We have developed a novel device for the quantification of edematous morphology changes in acute brain slices. We can also carry out real-time monitoring of detailed hippocampal cells. The device we developed is based on infrared differential interference contrast microscopy (IR-DIC) and a custom-made real-time computerized image-analysis system for quantification of the morphological dynamics of cells in slice preparations. We applied the coefficient of variation (CV) of light intensity in IR-DIC images to evaluate the change in morphological dynamics. We examined three kinds of edema in the CA1 region of rat hippocampal acute slices under conditions of hypotonic, strong excitation, and experimental ischemia, together with field excitatory postsynaptic potential (fEPSP) recording from radiatum in CA1 the region. There were notable close relationships among the edema formations, the light transmittance, the extent of changes in CV, and features of fEPSP during the three different insults. The present results indicate that CV is a reliable quantification index for edema formation in brain tissue and confirm that applying CV for the analysis in addition to the light transmittance analysis presents additional important information on brain tissue swelling.

Effects of Mannitol on Ischemia-Induced Degeneration in Rat Hippocampus

Although mannitol has been used as an osmotherapeutic drug on brain injury, the clinical efficiency of the drug are still controversial. In the present study, we examined the effects of mannitol on the edema in a hippocampal slice due to brief ischemia. To evaluate the effects, we employed an image analysis system that consists of an infrared-differential interference contrast (IR-DIC) microscope, an infrared CCD camera, and a computer with custom-made software. By this system, severity of the edema can be quantified as the coefficient of variation (CV) of digitalized slice images. The dose-dependent improvement on the deteriorated hippocampal slices could be obtained by administration of mannitol (10, 50, and 100 mM) after 10-min ischemia. However, field excitatory postsynaptic potentials (fEPSP) in CA1 stratum radiatum, which disappeared during 10-min ischemia, were never recovered by mannitol after more than 20-min treatment. fEPSP were blocked by the effective dose of mannitol for morphological recovery, but the effects found to be reversible. Although we failed to find positive rescuing effects of mannitol on the synaptic activities after ischemia, the protective effects of the drug on ischemic edema may rescue the secondary damages around the infarct area.

The effect of acute hypoxemia and hypotension on adenosine-mediated depression of evoked hippocampal synaptic transmission

The present study was designed to investigate the relative contributions of arterial P(O(2)), local cerebral blood flow, and oxygen delivery to the adenosine A(1) receptor-mediated depression of evoked synaptic transmission recorded in the rat hippocampus. Urethane-anesthetized rats were given a unilateral common carotid artery occlusion and then placed in a stereotaxic apparatus for stimulation and recording of bilateral hippocampal field excitatory postsynaptic potentials (fEPSPs). Arterial blood gases, mean arterial blood pressure (MAP), and bilateral hippocampal blood flow (HBF) were also measured. Arterial P(O(2)), HBF, and oxygen delivery were manipulated using normoxic hypotension, hypoxic hypotension, and hypoxic normotension. Both hypoxic hypotension and normoxic hypotension resulted in decreased HBF, decreased oxygen delivery, and a depression of the evoked fEPSP limited to the hippocampus ipsilateral to the occlusion. The enhanced HBF and oxygen delivery associated with increased MAP resulted in a restoration and maintenance of hippocampal fEPSPs despite sustained hypoxemia. The adenosine A(1) receptor-mediated depression of the fEPSP was more strongly correlated with changes in HBF and oxygen delivery than with arterial P(O(2)). We propose that adenosine plays an important role mediating the depression of neuronal activity associated with reduced oxygen delivery characteristically observed in ischemic brain tissue.

A novel O2-sensing mechanism in rat glossopharyngeal neurones mediated by a halothane-inhibitable background K+ conductance

Modulation of K+ channels by hypoxia is a common O2-sensing mechanism in specialised cells. More recently, acid-sensitive TASK-like background K+ channels, which play a key role in setting the resting membrane potential, have been implicated in O2-sensing in certain cell types. Here, we report a novel O2 sensitivity mediated by a weakly pH-sensitive background K+ conductance in nitric oxide synthase (NOS)-positive neurones of the glossopharyngeal nerve (GPN). This conductance was insensitive to 30 mM TEA, 5 mM 4-aminopyridine (4-AP) and 200 microM Cd2+, but was reversibly inhibited by hypoxia (O2 tension (PO2) = 15 mmHg), 2-5 mM halothane, 10 mM barium and 1 mM quinidine. Notably, the presence of halothane occluded the inhibitory effect of hypoxia. Under current clamp, these agents depolarised GPN neurones. In contrast, arachidonic acid (5-10 microM) caused membrane hyperpolarisation and potentiation of the background K+ current. This pharmacological profile suggests the O2-sensitive conductance in GPN neurones is mediated by a class of background K+ channels different from the TASK family; it appears more closely related to the THIK (tandem pore domain halothane-inhibited K+) subfamily, or may represent a new member of the background K+ family. Since GPN neurones are thought to provide NO-mediated efferent inhibition of the carotid body (CB), these channels may contribute to the regulation of breathing during hypoxia via negative feedback control of CB function, as well as to the inhibitory effect of volatile anaesthetics (e.g. halothane) on respiration.

Extrusion of intracellular calcium ion after in vitro ischemia in the rat hippocampal CA1 region

Simultaneous recordings of intracellular Ca(2+) ([Ca(2+)](i)) signal and extracellular DC potential were obtained from the CA1 region in 1-[6-amino-2-(5-carboxy-2-oxazolyl)-5-benzofuranyloxy]-2-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid penta-acetoxymethyl ester (Fura-2/AM)-loaded rat hippocampal slices. Superfusion with oxygen- and glucose-deprived medium (in vitro ischemia) for 5-6 min produced a rapid rise of the [Ca(2+)](i) level in the stratum radiatum (rising phase of the [Ca(2+)](i) signal), which occurred simultaneously with a rapid negative DC potential (rapid negative potential). When oxygen and glucose were reintroduced, the increased [Ca(2+)](i) signal diminished rapidly (falling phase of the [Ca(2+)](i) signal) during the generation of a slow negative DC potential (slow negative potential), which occurred within 1 min from the onset of the reintroduction. Thereafter, the [Ca(2+)](i) signal partially and the slow negative potential completely returned to the preexposure level approximately 6 min after the reintroduction. The changes in [Ca(2+)](i) signal during and after in vitro ischemia were very similar to the changes in the membrane potential of glial cells. The rising and falling phases of [Ca(2+)](i) signal corresponded to the rapid depolarization and a depolarizing hump, respectively, in the repolarizing phase of glial cells. A prolonged application of in vitro ischemia or a reintroduction of either glucose or oxygen suppressed the falling phase after ischemic exposure. The application of ouabain (30 microM) generated both a rapid negative potential and a rapid elevation of [Ca(2+)](i), but no slow negative potential or rapid reduction in [Ca(2+)](i) were observed. When oxygen and glucose were reintroduced to slices in the Na(+)-free or ouabain- or Ni(2+)-containing medium, the falling phase was suppressed. The falling phase was significantly accelerated in Ca(2+)- and Mg(2+)-free with EGTA-containing medium. In contrast, the falling phase was significantly slower in the Ca(2+)-free with high Mg(2+)- and EGTA-containing medium. The falling phase of the [Ca(2+)](i) signal after ischemic exposure is thus considered to be primarily dependent on the reactivation of Na(+), K(+)-ATPases, while the extrusion of cytosolic Ca(2+) via the forward-mode operation of Na(+)/Ca(2+) exchangers in glial cells is thought to be directly involved in the rapid reduction of [Ca(2+)](i) after ischemic exposure.

Cycloheximide Reduces the Effects of Anoxic Insult In Vivo and In Vitro

In vivo and in vitro techniques were utilized to examine the influence of a protein synthesis blocker, cycloheximide (CHX), on the damaging effects of anoxia in the rat. CHX administered 1 h before transient (30 min) forebrain ischaemia increased the survival of animals, decreased body weight loss and reduced the occurrence of delayed degeneration in the CA1 pyramidal region. The same dose of CHX injected 1 h after ischaemia induced status epilepticus, a decrease in survival rate, and did not reduce weight loss or CA1 damage in any of the surviving rats. Electrophysiological techniques were then used to determine the effects of various periods of anoxia and aglycaemia (AA) on CA1 field excitatory postsynaptic potentials (EPSPs) in hippocampal slices incubated in the presence or absence of CHX. In CHX-treated slices, recuperation of EPSP amplitude (45 +/- 16%) was significantly greater than in control slices (9 +/- 9%) following an AA episode of 3 min 45 s. No difference was seen in the percent recuperation of EPSPs in the control and CHX-treated slices after shorter or longer episodes of AA. From these studies, it appears that CHX protects against the damaging effect of ischaemia in vivo or AA in vitro.

Chemical anoxia activates ATP-sensitive and blocks Ca(2+)-dependent K(+) channels in rat dorsal vagal neurons in situ

The contribution of subclasses of K(+) channels to the response of mammalian neurons to anoxia is not yet clear. We investigated the role of ATP-sensitive (K(ATP)) and Ca(2+)-activated K(+) currents (small conductance, SK, big conductance, BK) in mediating the effects of chemical anoxia by cyanide, as determined by electrophysiological analysis and fluorometric Ca(2+) measurements in dorsal vagal neurons of rat brainstem slices. The cyanide-evoked persistent outward current was abolished by the K(ATP) channel blocker tolbutamide, but not changed by the SK and BK channel blockers apamin or tetraethylammonium. The K(+) channel blockers also revealed that ongoing activation of K(ATP) and SK channels counteracts a tonic, spike-related rise in intracellular Ca(2+) ([Ca(2+)](i)) under normoxic conditions, but did not modify the rise of [Ca(2+)](i) associated with the cyanide-induced outward current. Cyanide depressed the SK channel-mediated afterhyperpolarizing current without changing the depolarization-induced [Ca(2+)](i) transient, but did not affect spike duration that is determined by BK channels. The afterhyperpolarizing current and the concomitant [Ca(2+)](i) rise were abolished by Ca(2+)-free superfusate that changed neither the cyanide-induced outward current nor the associated [Ca(2+)](i) increase. Intracellular BAPTA for Ca(2+) chelation blocked the afterhyperpolarizing current and the accompanying [Ca(2+)](i) increase, but had no effect on the cyanide-induced outward current although the associated [Ca(2+)](i) increase was noticeably attenuated. Reproducing the cyanide-evoked [Ca(2+)](i) transient with the Ca(2+) pump blocker cyclopiazonic acid did not evoke an outward current. Our results show that anoxia mediates a persistent hyperpolarization due to activation of K(ATP) channels, blocks SK channels and has no effect on BK channels, and that the anoxic rise of [Ca(2+)](i) does not interfere with the activity of these K(+) channels.

ATP-sensitive K(+) channels in the brain: sensors of hypoxic conditions

Rapid minimization of energy consumption in excitable tissues is effective protection from lethal effects of extreme metabolic stress. The ATP-sensitive K(+) channels in the brain respond in ATP-depleted metabolic states such as hypoxia and may be involved in the protection mechanism against energy-consuming generalized seizure.