Synaptic transmission and excitability during hypoxia with inflammation and reoxygenation in hippocampal CA1 neurons

Electrochemical sensing of transient ascorbate fluctuation under hypoxic stress in live rat brain

Hypoxia, a common cause of programmed cell death or apoptosis, represents a neuropathological process. Although certain response proteins to hypoxic stress and their effects on cell status and fate have been identified, the real-time quantification of smaller neurochemicals to understand pathogenic mechanism in live rat brain during such stress remains unexplored. In this study, by employing a cutting-edge electrochemical tool developed with carbon nanotube-sheathed carbon fiber microelectrode that offers remarkable selectivity and temporal/spatial resolution for monitoring ascorbate, we observed a substantial efflux of ascorbate in response to hypoxic stress in live rat brain. Furthermore, using a small molecule compound as channel inhibitor to investigate the behavior of ascorbate efflux, we found that this efflux is closely correlated with N-methyl-D-aspartic acid receptor-induced neuronal excitability. Notably, antagonistic actions on volume-sensitive anion channel can suppress ascorbate efflux evoked by hypoxic stress, further revealing that ascorbate fluctuation is volume-sensitive anion channel-dependent. This research not only facilitates a greater understanding of the neurochemical mechanism in hypoxia but also uncovers a potential biomarker for future closed-loop therapies.

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.

Time-course effects and mechanisms of hypobaric hypoxia on nervous system in mice

OBJECTIVE

The aim of this study was to investigate the effect of time course on neurological impairment after acute hypobaric hypoxia exposure in mice and clarify the mechanism of acclimatization, so as to provide a suitable mice model and identify potential target against hypobaric hypoxia for further drug research.

METHOD

Male C57BL/6J mice were exposed to hypobaric hypoxia at a simulated altitude of 7000 m for 1, 3, and 7 days (1HH, 3HH and 7HH respectively). The behavior of the mice was evaluated by novel object recognition (NOR) and morris water maze test (MWM), then, the pathological changes of mice brain tissues were observed by H&E and Nissl staining. In addition, RNA sequencing (RNA-Seq) was performed to characterize the transcriptome signatures, and enzyme-linked immunosorbent assay (ELISA), Real-time polymerase chain reaction (RT-PCR), and western blot (WB) were used to verify the mechanisms of neurological impairment induced by hypobaric hypoxia.

RESULT

The hypobaric hypoxia condition resulted in impaired learning and memory, decreased new object cognitive index, and increased escape latency to the hidden platform in mice, with significant changes seen in the 1HH and 3HH groups. Bioinformatic analysis of RNA-seq results of hippocampal tissue showed that 739 differentially expressed genes (DEGs) appeared in the 1HH group, 452 in the 3HH group, and 183 in the 7HH group compared to the control group. There were 60 key genes overlapping in three groups which represented persistent changes and closely related biological functions and regulatory mechanisms in hypobaric hypoxia-induced brain injuries. DEGs enrichment analysis showed that hypobaric hypoxia-induced brain injuries were associated with oxidative stress, inflammatory responses, and synaptic plasticity. ELISA and WB results confirmed that these responses occurred in all hypobaric hypoxic groups while attenuated in the 7HH group. VEGF-A-Notch signaling pathway was enriched by DEGs in hypobaric hypoxia groups and was validated by RT-PCR and WB.

CONCLUSION

The nervous system of mice exposed to hypobaric hypoxia exhibited stress followed by gradual habituation and thus acclimatization over time, which was reflected in the biological mechanism involving inflammation, oxidative stress, and synaptic plasticity, and accompanied by activation of the VEGF-A-Notch pathway.

A Neurospheroid-Based Microrobot for Targeted Neural Connections in a Hippocampal Slice

Functional restoration by the re-establishment of cellular or neural connections remains a major challenge in targeted cell therapy and regenerative medicine. Recent advances in magnetically powered microrobots have shown potential for use in controlled and targeted cell therapy. In this study, a magnetic neurospheroid (Mag-Neurobot) that can form both structural and functional connections with an organotypic hippocampal slice (OHS) is assessed using an ex vivo model as a bridge toward in vivo application. The Mag-Neurobot consists of hippocampal neurons and superparamagnetic nanoparticles (SPIONs); it is precisely and skillfully manipulated by an external magnetic field. Furthermore, the results of patch-clamp recordings of hippocampal neurons indicate that neither the neuronal excitabilities nor the synaptic functions of SPION-loaded cells are significantly affected. Analysis of neural activity propagation using high-density multi-electrode arrays shows that the delivered Mag-Neurobot is functionally connected with the OHS. The applications of this study include functional verification for targeted cell delivery through the characterization of novel synaptic connections and the functionalities of transported and transplanted cells. The success of the Mag-Neurobot opens up new avenues of research and application; it offers a test platform for functional neural connections and neural regenerative processes through cell transplantation.

Cognitive fatigue due to exercise under normobaric hypoxia is related to hypoxemia during exercise

We previously found that a 10-min bout of moderate-intensity exercise (50% maximal oxygen uptake) under normobaric and hypoxic conditions (fraction of inspired oxygen [[Formula: see text]] = 0.135) reduced executive performance and neural activity in the left dorsolateral prefrontal cortex (DLPFC). To examine whether this cognitive fatigue is due to a decrease in SpO₂ during exercise, we compared executive performance and related prefrontal activation between two experimental conditions, in which the participants inhaled normobaric hypoxic gas ([Formula: see text]= 0.135) (hypoxic exercise [HE]) or hypoxic gas adjusted so that SpO₂ during exercise remained at the resting level (milder hypoxic exercise [ME]). ME condition showed that reaction time in executive performance decreased (t[13] = 2.228, P < 0.05, d = 0.34, paired t-test) and left DLPFC activity increased (t[13] = -2.376, P < 0.05, d = 0.63, paired t-test) after exercise compared with HE condition. These results showed that the HE-induced reductions in the left DLPFC activity and executive performance were both suppressed in the ME condition, supporting the hypothesis that exercise-induced cognitive fatigue under hypoxic environment is due to hypoxemia during exercise. This may lead to the development of a method of coping with cognitive fatigue due to exercise that causes hypoxemia.

Maternal Hypoxia Increases the Excitability of Neurons in the Entorhinal Cortex and Dorsal Hippocampus of Rat Offspring

Prenatal hypoxia is a widespread condition that causes various disturbances in later life, including aberrant central nervous system development, abnormalities in EEG rhythms, and susceptibility to seizures. Hypoxia in rats on the 14th day of embryogenesis (E14) disrupts cortical neuroblast radial migration, mainly affecting the progenitors of cortical glutamatergic neurons but not GABAergic interneurons or hippocampal neurons. Thus, hypoxia at this time point might affect the development of the neocortex to a greater extent than the hippocampus. In the present study, we investigated the long-term effects of hypoxia on the properties of the pyramidal neurons in the hippocampus and entorhinal cortex (EC) in 3-week-old rats subjected to hypoxia on E14. We observed a reduction in the total number of NeuN-positive neurons in EC but not in the CA1 field of the hippocampus, indicating an increased cell loss in EC. However, the principal neuron electrophysiological characteristics were altered in the EC and hippocampus of animals exposed to hypoxia. The whole-cell patch-clamp recordings revealed a similar increase in input resistance in neurons from the hippocampus and EC. However, the resting membrane potential was increased in the EC neurons only. The recordings of field postsynaptic potentials (fPSPs) in the CA1 hippocampal area showed that both the threshold currents inducing fPSPs and population spikes were lower in hypoxic animals compared to age-matched controls. Using the dosed electroshock paradigm, we found that seizure thresholds were lower in the hypoxic group. Thus, the obtained results suggest that maternal hypoxia during the generation of the pyramidal cortical neurons leads to the increased excitability of neuronal circuitries in the brain of young rats. The increased excitability can be attributed to the changes in intrinsic neuronal properties.

Hypoxia with inflammation and reperfusion alters membrane resistance by dynamically regulating voltage-gated potassium channels in hippocampal CA1 neurons

Hypoxia typically accompanies acute inflammatory responses in patients and animal models. However, a limited number of studies have examined the effect of hypoxia in combination with inflammation (Hypo-Inf) on neural function. We previously reported that neuronal excitability in hippocampal CA1 neurons decreased during hypoxia and greatly rebounded upon reoxygenation. We attributed this altered excitability mainly to the dynamic regulation of hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels and input resistance. However, the molecular mechanisms underlying input resistance changes by Hypo-Inf and reperfusion remained unclear. In the present study, we found that a change in the density of the delayed rectifier potassium current (IDR) can explain the input resistance variability. Furthermore, voltage-dependent inactivation of A-type potassium (IA) channels shifted in the depolarizing direction during Hypo-Inf and reverted to normal upon reperfusion without a significant alteration in the maximum current density. Our results indicate that changes in the input resistance, and consequently excitability, caused by Hypo-Inf and reperfusion are at least partially regulated by the availability and voltage dependence of KV channels. Moreover, these results suggest that selective KV channel modulators can be used as potential neuroprotective drugs to minimize hypoxia- and reperfusion-induced neuronal damage.

The α7 and β2 nicotinic acetylcholine receptor subunits regulate apoptosis in the infant hippocampus, and in sudden infant death syndrome (SIDS)

Apoptosis is increased in the hippocampus of infants who died of sudden infant death syndrome (SIDS), yet it is not known via which mechanism this has occurred. Following existing support for a role of the α7 and β2 nicotinic acetylcholine receptor (nAChR) subunits in apoptotic regulation, we aimed to determine whether these subunits are altered in the SIDS hippocampus and if they are correlated with cell death markers of active caspase-3 (Casp-3) and TUNEL. Further analyses were run according to the presence of major SIDS risk factors related to hypoxia (bed-sharing and prone sleeping), infection (presence of an upper respiratory tract infection (URTI)), cigarette smoke exposure and gender. Immunohistochemical expression of the markers was studied in 4 regions of the hippocampus (Cornu Ammonis (CA)1, CA2, CA3, CA4) and subiculum amongst 52 infants (aged 1-7 months) who died suddenly and unexpectedly (SUDI) and for whom the cause of death was explained (eSUDI; n = 9), or not and characterised as SIDS I (n = 8) and SIDS II (n = 35) according to the San Diego diagnostic criteria. Results showed that SIDS II infants had widespread increases in TUNEL compared with eSUDI and SIDS I infants, as well as increased α7 and Casp-3 in CA2 compared to eSUDI infants, although these changes were predominant amongst infants who did not bed-share. Cigarette smoke exposure had minimal effects on the markers, while an URTI was associated with changes in all markers (after accounting for bed-sharing). Our findings support the role of nAChRs in regulating apoptosis in the SIDS hippocampus, and highlight the need for separate analysis according to risk factors.

Isorhamnetin Alleviates High Glucose-Aggravated Inflammatory Response and Apoptosis in Oxygen-Glucose Deprivation and Reoxygenation-Induced HT22 Hippocampal Neurons Through Akt/SIRT1/Nrf2/HO-1 Signaling Pathway

This study is aimed at exploring the potential of isorhamnetin in protection against diabetes-exacerbated ischemia/reperfusion-induced brain injury and elucidating its action mechanism. After establishment of the model of high glucose (HG)-aggravated oxygen-glucose deprivation and reoxygenation (OGD/R), HT22 cell viability was detected by CCK-8. Lactate dehydrogenase (LDH) activity, casapase-3 activity, and oxidative stress-related markers in HT22 cells were detected by corresponding commercial kits. The apoptosis of HG-treated HT22 cells following OGD/R was observed with TUNEL staining. The level of pro-inflammatory cytokines was examined by ELISA. The expression of Akt/SIRT1/Nrf2/HO-1 signaling-related proteins was assayed by Western blot. The results showed that HG noticeably worsened the OGD/R-induced apoptosis of HT22 cells. Isorhamnetin relieved the HG-aggravated OGD/R-induced apoptosis, inflammatory response, and oxidative stress of HT22 cells. Isorhamnetin alleviated the HG-aggravated OGD/R injury in HT22 cells through Akt/SIRT1/Nrf2/HO-1 signaling pathway. Meanwhile, treatment with Akt inhibitor LY294002 reversed the protective effects of isorhamnetin against HG-aggravated OGD/R injury in HT22 cells. In a conclusion, Isorhamnetin alleviates HG-aggravated OGD/R in HT22 hippocampal neurons through Akt/SIRT1/Nrf2/HO-1 signaling pathway.

Oral microbiota and autism spectrum disorder (ASD)

Autism spectrum disorder (ASD) is associated with several oropharyngeal abnormalities, including dysbiosis in the oral microbiota. Since the oral cavity is the start of the gastrointestinal tract, this strengthens and extends the notion of a microbial gut-brain axis in ASD and even raises the question whether a microbial oral-brain axis exists. It is clear that oral bacteria can find their way to the brain through a number of pathways following routine dental procedures. A connection between the oral microbiota and a number of other brain disorders has been reported. As the evidence so far for an association between the oral microbiota and ASDs rests on a few reports only, further studies in this field are necessary. The current review discusses a possible relationship between oral bacteria and the biologic and symptomologic aspects of ASD, focusing on the clinical implications for diagnostic and therapeutic development.