The electrical response of cerebellar Purkinje neurons to simulated ischaemia

Interplay between metabotropic glutamate type 4 and adenosine type 1 receptors modulate synaptic transmission in the cerebellar cortex

The synapses between parallel fibers and Purkinje cells play a pivotal role in cerebellar function. They are intricately governed by a variety of presynaptic receptors, notably by type 4 metabotropic glutamate (mGlu4) receptors and type 1 adenosine (A1) receptors both of which curtail glutamate release upon activation. Despite their pivotal role in regulating synaptic transmission within the cerebellar cortex, functional interactions between mGlu4 and A1 receptors have remained relatively unexplored. To bridge this gap, our study delves into how mGlu4 receptor activity influences A1 receptor-mediated alterations in excitatory transmission. Employing a combination of whole-cell patch clamp recordings of Purkinje cells and parallel fiber presynaptic fluorometric calcium measurements in acute rat and mouse cerebellar cortical slices, our results reveal functional interactions between these receptor types. These findings hold implications for understanding potential roles of these presynaptic receptors in neuroprotection during pathophysiological conditions characterized by elevated glutamate and adenosine levels.

Low Temperature Delays the Effects of Ischemia in Bergmann Glia and in Cerebellar Tissue Swelling

Cerebral ischemia results in oxygen and glucose deprivation that most commonly occurs after a reduction or interruption in the blood supply to the brain. The consequences of cerebral ischemia are complex and involve the loss of metabolic ATP, excessive K⁺ and glutamate accumulation in the extracellular space, electrolyte imbalance, and brain edema formation. So far, several treatments have been proposed to alleviate ischemic damage, yet few are effective. Here, we focused on the neuroprotective role of lowering the temperature in ischemia mimicked by an episode of oxygen and glucose deprivation (OGD) in mouse cerebellar slices. Our results suggest that lowering the temperature of the extracellular 'milieu' delays both the increases in [K⁺]_e and tissue swelling, two dreaded consequences of cerebellar ischemia. Moreover, radial glial cells (Bergmann glia) display morphological changes and membrane depolarizations that are markedly impeded by lowering the temperature. Overall, in this model of cerebellar ischemia, hypothermia reduces the deleterious homeostatic changes regulated by Bergmann glia.

Multi-target action of β-alanine protects cerebellar tissue from ischemic damage

Brain ischemic stroke is among the leading causes of death and long-term disability. New treatments that alleviate brain cell damage until blood supply is restored are urgently required. The emerging focus of anti-stroke strategies has been on blood-brain-barrier permeable drugs that exhibit multiple sites of action. Here, we combine single-cell electrophysiology with live-cell imaging to find that β-Alanine (β-Ala) protects key physiological functions of brain cells that are exposed to acute stroke-mimicking conditions in ex vivo brain preparations. β-Ala exerts its neuroprotective action through several distinct pharmacological mechanisms, none of which alone could reproduce the neuroprotective effect. Since β-Ala crosses the blood-brain barrier and is part of a normal human diet, we suggest that it has a strong potential for acute stroke treatment and facilitation of recovery.

The Critical Role of Spreading Depolarizations in Early Brain Injury: Consensus and Contention

BACKGROUND

When a patient arrives in the emergency department following a stroke, a traumatic brain injury, or sudden cardiac arrest, there is no therapeutic drug available to help protect their jeopardized neurons. One crucial reason is that we have not identified the molecular mechanisms leading to electrical failure, neuronal swelling, and blood vessel constriction in newly injured gray matter. All three result from a process termed spreading depolarization (SD). Because we only partially understand SD, we lack molecular targets and biomarkers to help neurons survive after losing their blood flow and then undergoing recurrent SD.

METHODS

In this review, we introduce SD as a single or recurring event, generated in gray matter following lost blood flow, which compromises the Na⁺/K⁺ pump. Electrical recovery from each SD event requires so much energy that neurons often die over minutes and hours following initial injury, independent of extracellular glutamate.

RESULTS

We discuss how SD has been investigated with various pitfalls in numerous experimental preparations, how overtaxing the Na⁺/K⁺ ATPase elicits SD. Elevated K⁺ or glutamate are unlikely natural activators of SD. We then turn to the properties of SD itself, focusing on its initiation and propagation as well as on computer modeling.

CONCLUSIONS

Finally, we summarize points of consensus and contention among the authors as well as where SD research may be heading. In an accompanying review, we critique the role of the glutamate excitotoxicity theory, how it has shaped SD research, and its questionable importance to the study of early brain injury as compared with SD theory.

Reappraisal of anoxic spreading depolarization as a terminal event during oxygen-glucose deprivation in brain slices in vitro

Anoxic spreading depolarization (aSD) has been hypothesized as a terminal event during oxygen–glucose deprivation (OGD) in submerged cortical slices in vitro. However, mechanical artifacts caused by aSD-triggered edema may introduce error in the assessment of neuronal viability. Here, using continuous patch-clamp recordings from submerged rat cortical slices, we first confirmed that vast majority of L4 neurons permanently lost their membrane potential during OGD-induced aSD. In some recordings, spontaneous transition from whole-cell to out-side out configuration occurred during or after aSD, and only a small fraction of neurons survived aSD with reperfusion started shortly after aSD. Secondly, to minimize artifacts caused by OGD-induced edema, cells were short-term patched following OGD episodes of various duration. Nearly half of L4 cells maintained membrane potential and showed the ability to spike-fire if reperfusion started less than 10 min after aSD. The probability of finding live neurons progressively decreased at longer reperfusion delays at a rate of about 2% per minute. We also found that neurons in L2/3 show nearly threefold higher resistance to OGD than neurons in L4. Our results suggest that in the OGD ischemia model, aSD is not a terminal event, and that the “commitment point” of irreversible damage occurs at variable delays, in the range of tens of minutes, after OGD-induced aSD in submerged cortical slices.

A Light-Controlled Allosteric Modulator Unveils a Role for mGlu4 Receptors During Early Stages of Ischemia in the Rodent Cerebellar Cortex

Metabotropic glutamate receptors (mGlus) are G Protein coupled-receptors that modulate synaptic transmission and plasticity in the central nervous system. Some act as autoreceptors to control neurotransmitter release at excitatory synapses and have become attractive targets for drug therapy to treat certain neurological disorders. However, the high degree of sequence conservation around the glutamate binding site makes the development of subtype-specific orthosteric ligands difficult to achieve. This problem can be circumvented by designing molecules that target specific less well conserved allosteric sites. One such allosteric drug, the photo-switchable compound OptoGluNAM4.1, has been recently employed to reversibly inhibit the activity of metabotropic glutamate 4 (mGlu₄) receptors in cell cultures and in vivo. We studied OptoGluNAM4.1 as a negative modulator of neurotransmission in rodent cerebellar slices at the parallel fiber – Purkinje cell synapse. Our data show that OptoGluNAM4.1 antagonizes pharmacological activation of mGlu₄ receptors in a fully reversible and photo-controllable manner. In addition, for the first time, this new allosteric modulator allowed us to demonstrate that, in brain slices from the rodent cerebellar cortex, mGlu₄ receptors are endogenously activated in excitotoxic conditions, such as the early phases of simulated cerebellar ischemia, which is associated with elevated levels of extracellular glutamate. These findings support OptoGluNAM4.1 as a promising new tool for unraveling the role of mGlu₄ receptors in the central nervous system in physio-pathological conditions.

Oxygen and Glucose Deprivation Induces Bergmann Glia Membrane Depolarization and Ca2+ Rises Mainly Mediated by K+ and ATP Increases in the Extracellular Space

During brain ischemia, intense energy deficiency induces a complex succession of events including pump failure, acidosis and exacerbated glutamate release. In the cerebellum, glutamate is the principal mediator of Purkinje neuron anoxic depolarization during episodes of oxygen and glucose deprivation (OGD). Here, the impact of OGD is studied in Bergmann glia, specialized astrocytes closely associated to Purkinje neurons. Patch clamp experiments reveal that during OGD Bergmann glial cells develop a large depolarizing current that is not mediated by glutamate and purinergic receptors but is mainly due to the accumulation of K⁺ in the extracellular space. Furthermore, we also found that increases in the intracellular Ca²⁺ concentration appear in Bergmann glia processes several minutes following OGD. These elevations require, in an early phase, Ca²⁺ mobilization from internal stores via P2Y receptor activation, and, over longer periods, Ca²⁺ entry through store-operated calcium channels. Our results suggest that increases of K⁺ and ATP concentrations in the extracellular space are primordial mediators of the OGD effects on Bergmann glia. In the cerebellum, glial responses to energy deprivation-triggering events are therefore highly likely to follow largely distinct rules from those of their neuronal counterparts.

Proton-gated Ca(2+)-permeable TRP channels damage myelin in conditions mimicking ischaemia

The myelin sheaths wrapped around axons by oligodendrocytes are crucial for brain function. In ischaemia myelin is damaged in a Ca(2+)-dependent manner, abolishing action potential propagation. This has been attributed to glutamate release activating Ca(2+)-permeable N-methyl-D-aspartate (NMDA) receptors. Surprisingly, we now show that NMDA does not raise the intracellular Ca(2+) concentration ([Ca(2+)]i) in mature oligodendrocytes and that, although ischaemia evokes a glutamate-triggered membrane current, this is generated by a rise of extracellular [K(+)] and decrease of membrane K(+) conductance. Nevertheless, ischaemia raises oligodendrocyte [Ca(2+)]i, [Mg(2+)]i and [H(+)]i, and buffering intracellular pH reduces the [Ca(2+)]i and [Mg(2+)]i increases, showing that these are evoked by the rise of [H(+)]i. The H(+)-gated [Ca(2+)]i elevation is mediated by channels with characteristics of TRPA1, being inhibited by ruthenium red, isopentenyl pyrophosphate, HC-030031, A967079 or TRPA1 knockout. TRPA1 block reduces myelin damage in ischaemia. These data suggest that TRPA1-containing ion channels could be a therapeutic target in white matter ischaemia.

Oxygen-glucose deprivation increases firing of unipolar brush cells and enhances spontaneous EPSCs in Purkinje cells in the vestibulo-cerebellum

Unipolar brush cells (UBCs) are excitatory interneurons in the granular layer of the cerebellar cortex, which are predominantly distributed in the vestibulo-cerebellar region. The unique firing properties and synaptic connections of UBCs may underlie lobular heterogeneity of excitability in the granular layer and the susceptibility to ischemia-induced excitotoxicity. In this study, we investigated the effects of oxygen-glucose deprivation (OGD) on the firing properties of UBCs and granule cells and spontaneous excitatory postsynaptic currents (sEPSCs) of Purkinje cells using whole-cell recordings. Short-term OGD induced increases in spontaneous firing of UBCs by causing membrane depolarization via the activation of NMDA receptors. UBC firing indirectly affected Purkinje cells by altering parallel fiber inputs of a subset granule cells, resulting in a marked increase in sEPSCs in Purkinje cells in vestibulo-cerebellar lobules IX-X, but not in lobules IV-VI, which have fewer UBCs. Similarly, the frequency and amplitude of sEPSCs in Purkinje cells were significantly greater in lobules IX-X than in IV-VI, even in control conditions. These results reveal that UBCs play key roles in regulating local excitability in the granular layer, resulting in lobular heterogeneity in the susceptibility to ischemic insult in the cerebellum.

A lab-on-a-chip for hypoxic patch clamp measurements combined with optical tweezers and spectroscopy- first investigations of single biological cells

The response and the reaction of the brain system to hypoxia is a vital research subject that requires special instrumentation. With this research subject in focus, a new multifunctional lab-on-a-chip (LOC) system with control over the oxygen content for studies on biological cells was developed. The chip was designed to incorporate the patch clamp technique, optical tweezers and absorption spectroscopy. The performance of the LOC was tested by a series of experiments. The oxygen content within the channels of the LOC was monitored by an oxygen sensor and verified by simultaneously studying the oxygenation state of chicken red blood cells (RBCs) with absorption spectra. The chicken RBCs were manipulated optically and steered in three dimensions towards a patch-clamp micropipette in a closed microfluidic channel. The oxygen level within the channels could be changed from a normoxic value of 18% O ₂ to an anoxic value of 0.0-0.5% O ₂. A time series of 3 experiments were performed, showing that the spectral transfer from the oxygenated to the deoxygenated state occurred after about 227 ± 1 s and a fully developed deoxygenated spectrum was observed after 298 ± 1 s, a mean value of 3 experiments. The tightness of the chamber to oxygen diffusion was verified by stopping the flow into the channel system while continuously recording absorption spectra showing an unchanged deoxygenated state during 5400 ± 2 s. A transfer of the oxygenated absorption spectra was achieved after 426 ± 1 s when exposing the cell to normoxic buffer. This showed the long time viability of the investigated cells. Successful patching and sealing were established on a trapped RBC and the whole-cell access (Ra) and membrane (Rm) resistances were measured to be 5.033 ± 0.412 M Ω and 889.7 ± 1.74 M Ω respectively.

Extrasynaptic glutamate release through cystine/glutamate antiporter contributes to ischemic damage

During brain ischemia, an excessive release of glutamate triggers neuronal death through the overactivation of NMDA receptors (NMDARs); however, the underlying pathways that alter glutamate homeostasis and whether synaptic or extrasynaptic sites are responsible for excess glutamate remain controversial. Here, we monitored ischemia-gated currents in pyramidal cortical neurons in brain slices from rodents in response to oxygen and glucose deprivation (OGD) as a real-time glutamate sensor to identify the source of glutamate release and determined the extent of neuronal damage. Blockade of excitatory amino acid transporters or vesicular glutamate release did not inhibit ischemia-gated currents or neuronal damage after OGD. In contrast, pharmacological inhibition of the cystine/glutamate antiporter dramatically attenuated ischemia-gated currents and cell death after OGD. Compared with control animals, mice lacking a functional cystine/glutamate antiporter exhibited reduced anoxic depolarization and neuronal death in response to OGD. Furthermore, glutamate released by the cystine/glutamate antiporter activated extrasynaptic, but not synaptic, NMDARs, and blockade of extrasynaptic NMDARs reduced ischemia-gated currents and cell damage after OGD. Finally, PET imaging showed increased cystine/glutamate antiporter function in ischemic rats. Altogether, these data suggest that cystine/glutamate antiporter function is increased in ischemia, contributing to elevated extracellular glutamate concentration, overactivation of extrasynaptic NMDARs, and ischemic neuronal death.

The cystine/glutamate antiporter: when too much of a good thing goes bad

Glutamate excitotoxicity represents a major cellular component of ischemic brain injury. In this issue of the JCI, Soria and colleagues reveal that the cystine/glutamate exchanger is an important source of excitotoxic glutamate in response to ischemia induced by oxygen and glucose deprivation. As the exchanger is a primary determinant of both extracellular glutamate and intracellular glutathione, the findings from this study not only provide important insight into the mechanisms of brain ischemia but also demonstrate the complexity of the yin and yang of glutamate homeostasis and cellular redox balance.

Brainstem neurons survive the identical ischemic stress that kills higher neurons: insight to the persistent vegetative state

Global ischemia caused by heart attack, pulmonary failure, near-drowning or traumatic brain injury often damages the higher brain but not the brainstem, leading to a ‘persistent vegetative state’ where the patient is awake but not aware. Approximately 30,000 U.S. patients are held captive in this condition but not a single research study has addressed how the lower brain is preferentially protected in these people. In the higher brain, ischemia elicits a profound anoxic depolarization (AD) causing neuronal dysfunction and vasoconstriction within minutes. Might brainstem nuclei generate less damaging AD and so be more resilient? Here we compared resistance to acute injury induced from simulated ischemia by ‘higher’ hippocampal and striatal neurons versus brainstem neurons in live slices from rat and mouse. Light transmittance (LT) imaging in response to 10 minutes of oxygen/glucose deprivation (OGD) revealed immediate and acutely damaging AD propagating through gray matter of neocortex, hippocampus, striatum, thalamus and cerebellar cortex. In adjacent brainstem nuclei, OGD-evoked AD caused little tissue injury. Whole-cell patch recordings from hippocampal and striatal neurons under OGD revealed sudden membrane potential loss that did not recover. In contrast brainstem neurons from locus ceruleus and mesencephalic nucleus as well as from sensory and motor nuclei only slowly depolarized and then repolarized post-OGD. Two-photon microscopy confirmed non-recoverable swelling and dendritic beading of hippocampal neurons during OGD, while mesencephalic neurons in midbrain appeared uninjured. All of the above responses were mimicked by bath exposure to 100 µM ouabain which inhibits the Na⁺/K⁺ pump or to 1–10 nM palytoxin which converts the pump into an open cationic channel.

Therefore during ischemia the Na⁺/K⁺ pump of higher neurons fails quickly and extensively compared to naturally resilient hypothalamic and brainstem neurons. The selective survival of lower brain regions that maintain vital functions will support the persistent vegetative state.

Optogenetic countering of glial acidosis suppresses glial glutamate release and ischemic brain damage

The brain demands high-energy supply and obstruction of blood flow causes rapid deterioration of the healthiness of brain cells. Two major events occur upon ischemia: acidosis and liberation of excess glutamate, which leads to excitotoxicity. However, cellular source of glutamate and its release mechanism upon ischemia remained unknown. Here we show a causal relationship between glial acidosis and neuronal excitotoxicity. As the major cation that flows through channelrhodopsin-2 (ChR2) is proton, this could be regarded as an optogenetic tool for instant intracellular acidification. Optical activation of ChR2 expressed in glial cells led to glial acidification and to release of glutamate. On the other hand, glial alkalization via optogenetic activation of a proton pump, archaerhodopsin (ArchT), led to cessation of glutamate release and to the relief of ischemic brain damage in vivo. Our results suggest that controlling glial pH may be an effective therapeutic strategy for intervention of ischemic brain damage.

Cortical GABAergic neurons are more severely impaired by alkalosis than acidosis

Background

Acid–base imbalance in various metabolic disturbances leads to human brain dysfunction. Compared with acidosis, the patients suffered from alkalosis demonstrate more severe neurological signs that are difficultly corrected. We hypothesize a causative process that the nerve cells in the brain are more vulnerable to alkalosis than acidosis.

Methods

The vulnerability of GABAergic neurons to alkalosis versus acidosis was compared by analyzing their functional changes in response to the extracellular high pH and low pH. The neuronal and synaptic functions were recorded by whole-cell recordings in the cortical slices.

Results

The elevation or attenuation of extracellular pH impaired these GABAergic neurons in terms of their capability to produce spikes, their responsiveness to excitatory synaptic inputs and their outputs via inhibitory synapses. Importantly, the dysfunction of these active properties appeared severer in alkalosis than acidosis.

Conclusions

The severer impairment of cortical GABAergic neurons in alkalosis patients leads to more critical neural excitotoxicity, so that alkalosis-induced brain dysfunction is difficultly corrected, compared to acidosis. The vulnerability of cortical GABAergic neurons to high pH is likely a basis of severe clinical outcomes in alkalosis versus acidosis.

Primate cerebellar granule cells exhibit a tonic GABAAR conductance that is not affected by alcohol: a possible cellular substrate of the low level of response phenotype

In many rodent brain regions, alcohol increases vesicular release of GABA, resulting in an increase in the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) and the magnitude of tonic GABAA receptor (GABAAR) currents. A neglected issue in translating the rodent literature to humans is the possibility that phylogenetic differences alter the actions of alcohol. To address this issue we made voltage-clamp recordings from granule cells (GCs) in cerebellar slices from the non-human primate (NHP), Macaca fascicularis. We found that similar to Sprague Dawley rats (SDRs), NHP GCs exhibit a tonic conductance generated by α6δ subunit containing GABAARs, as evidenced by its blockade by the broad spectrum GABAAR antagonist, GABAzine (10 μM), inhibition by α6 selective antagonist, furosemide (100 μM), and enhancement by THDOC (10-20 nM) and THIP (500 nM). In contrast to SDR GCs, in most NHP GCs (~60%), application of EtOH (25-105 mM) did not increase sIPSC frequency or the tonic GABAAR current. In a minority of cells (~40%), EtOH did increase sIPSC frequency and the tonic current. The relative lack of response to EtOH was associated with reduced expression of neuronal nitric oxide synthase (nNOS), which we recently reported mediates EtOH-induced enhancement of vesicular GABA release in rats. The EtOH-induced increase in tonic GABAAR current was significantly smaller in NHPs than in SDRs, presumably due to less GABA release, because there were no obvious differences in the density of GABAARs or GABA transporters between SDR and NHP GCs. Thus, EtOH does not directly modulate α6δ subunit GABAARs in NHPs. Instead, EtOH enhanced GABAergic transmission is mediated by enhanced GABA release. Further, SDR GC responses to alcohol are only representative of a subpopulation of NHP GCs. This suggests that the impact of EtOH on NHP cerebellar physiology will be reduced compared to SDRs, and will likely have different computational and behavioral consequences.

A distinct boundary between the higher brain's susceptibility to ischemia and the lower brain's resistance

Higher brain regions are more susceptible to global ischemia than the brainstem, but is there a gradual increase in vulnerability in the caudal-rostral direction or is there a discrete boundary? We examined the interface between `higher` thalamus and the hypothalamus the using live brain slices where variation in blood flow is not a factor. Whole-cell current clamp recording of 18 thalamic neurons in response to 10 min O₂/glucose deprivation (OGD) revealed a rapid anoxic depolarization (AD) from which thalamic neurons do not recover. Newly acquired neurons could not be patched following AD, confirming significant regional thalamic injury. Coinciding with AD, light transmittance (LT) imaging during whole-cell recording showed an elevated LT front that initiated in midline thalamus and that propagated into adjacent hypothalamus. However, hypothalamic neurons patched in paraventricular nucleus (PVN, n= 8 magnocellular and 12 parvocellular neurons) and suprachiasmatic nucleus (SCN, n= 18) only slowly depolarized as AD passed through these regions. And with return to control aCSF, hypothalamic neurons repolarized and recovered their input resistance and action potential amplitude. Moreover, newly acquired hypothalamic neurons could be readily patched following exposure to OGD, with resting parameters similar to neurons not previously exposed to OGD. Thalamic susceptibility and hypothalamic resilience were also observed following ouabain exposure which blocks the Na⁺/K⁺ pump, evoking depolarization similar to OGD in all neuronal types tested. Finally, brief exposure to elevated [K⁺]_o caused spreading depression (SD, a milder, AD-like event) only in thalamic neurons so SD generation is regionally correlated with strong AD. Therefore the thalamus-hypothalamus interface represents a discrete boundary where neuronal vulnerability to ischemia is high in thalamus (like more rostral neocortex, striatum, hippocampus). In contrast hypothalamic neurons are comparatively resistant, generating weaker and recoverable anoxic depolarization similar to brainstem neurons, possibly the result of a Na/K pump that better functions during ischemia.

Similar cation channels mediate protection from cerebellar exitotoxicity by exercise and inheritance

Exercise and inherited factors both affect recovery from stroke and head injury, but the underlying mechanisms and interconnections between them are yet unknown. Here, we report that similar cation channels mediate the protective effect of exercise and specific genetic background in a kainate injection model of cerebellar stroke. Microinjection to the cerebellum of the glutamatergic agonist, kainate, creates glutamatergic excito-toxicity characteristic of focal stroke, head injury or alcoholism. Inherited protection and prior exercise were both accompanied by higher cerebellar expression levels of the Kir6.1 ATP-dependent potassium channel in adjacent Bergmann glia, and voltage-gated KVbeta2 and cyclic nucleotide-gated cation HCN1 channels in basket cells. Sedentary FVB/N and exercised C57BL/6 mice both expressed higher levels of these cation channels compared to sedentary C57BL/6 mice, and were both found to be less sensitive to glutamate toxicity. Moreover, blocking ATP-dependent potassium channels with Glibenclamide enhanced kainate-induced cell death in cerebellar slices from the resilient sedentary FVB/N mice. Furthermore, exercise increased the number of acetylcholinesterase-positive fibres in the molecular layer, reduced cerebellar cytokine levels and suppressed serum acetylcholinesterase activity, suggesting anti-inflammatory protection by enhanced cholinergic signalling. Our findings demonstrate for the first time that routine exercise and specific genetic backgrounds confer protection from cerebellar glutamatergic damages by similar molecular mechanisms, including elevated expression of cation channels. In addition, our findings highlight the involvement of the cholinergic anti-inflammatory pathway in insult-inducible cerebellar processes. These mechanisms are likely to play similar roles in other brain regions and injuries as well, opening new venues for targeted research efforts.

Neuroprotective effects of mesenchymal stem cell transplantation in animal model of cerebellar degeneration

BACKGROUND

The cerebellum has been considered a key structure for the processes involved in sensorimotor integration ultimately leading to motor planning and execution of coordinated movement. Thus, motor deficits and behavioral changes can be associated with cerebellar degeneration.

METHODS

Here, the chemical neurotoxin pyridine-2,3-dicarboxylic acid (quinolinic acid, QA) used to create partially cerebellar degeneration in adult Wistar rats suitable for use in stem cell transplantation studies. Stereotaxicaly administration of QA (0.2 mmol) in the right cerebellar hemisphere (folia VI) caused noticeable motor disturbance in all treated animals. Forty-eights hours after causing lesion, rat bone marrow-derived mesenchymal stem cells (MSCs) were transplanted into damaged cerebellar hemisphere. We investigated the role of MSC transplantation in forms of motor and non-motor learning that involves the cerebellum and its neuroprotective effects in Purkinje cells loss.

RESULTS

CM-Dil labeling showed that the transplanted MSCs survived and migrated in the cerebellum 6 weeks after transplantation. The MSC-transplanted group showed markedly improved functional performance on the rotating rod test (P≤0.0001) and beam walking test (P≤0.0001) during 6 weeks compared with the controls. For non-motor learning, we used passive avoidance learning test in 3 weeks after transplantation. The results showed that MSC transplantation prevented the development of memory deficit caused by cerebellar degeneration (P≤0.001). Stereological analysis in 6 weeks after transplantation showed that QA significantly decreases Purkinje cells in vehicle-treated rats and MSC transplantation is neuroprotective and decreases Purkinje cell loss in MSC-treated rats (P≤0.0001).

CONCLUSION

The results indicate that transplantation of MSCs can significantly reduce the behavioral and neuroanatomical abnormalities of these animals during 6 weeks after engraftment. According to results of this assay, cell therapy by means of bone marrow-derived adult stem cells promises for treatment of cerebellar diseases.

Genetic ablation of Pannexin1 protects retinal neurons from ischemic injury

Pannexin1 (Panx1) forms large nonselective membrane channel that is implicated in paracrine and inflammatory signaling. In vitro experiments suggested that Panx1 could play a key role in ischemic death of hippocampal neurons. Since retinal ganglion cells (RGCs) express high levels of Panx1 and are susceptible to ischemic induced injury, we hypothesized that Panx1 contributes to rapid and selective loss of these neurons in ischemia. To test this hypothesis, we induced experimental retinal ischemia followed by reperfusion in live animals with the Panx1 channel genetically ablated either in the entire mouse (Panx1 KO), or only in neurons using the conditional knockout (Panx1 CKO) technology. Here we report that two distinct neurotoxic processes are induced in RGCs by ischemia in the wild type mice but are inactivated in Panx1KO and Panx1 CKO animals. First, the post-ischemic permeation of RGC plasma membranes is suppressed, as assessed by dye transfer and calcium imaging assays ex vivo and in vitro. Second, the inflammasome-mediated activation of caspase-1 and the production of interleukin-1β in the Panx1 KO retinas are inhibited. Our findings indicate that post-ischemic neurotoxicity in the retina is mediated by previously uncharacterized pathways, which involve neuronal Panx1 and are intrinsic to RGCs. Thus, our work presents the in vivo evidence for neurotoxicity elicited by neuronal Panx1, and identifies this channel as a new therapeutic target in ischemic pathologies.

An impairment of cortical GABAergic neurons is involved in alkalosis-induced brain dysfunctions

Acid-base imbalance leads to pathological cognition and behaviors in the clinical practices. In the comparison with acidosis, the cellular mechanisms underlying alkalosis-induced brain dysfunction remain unclear. By using electrophysiological approach, we investigated the influences of high extracellular pH environment on cortical GABAergic neurons in terms of their responsiveness to synaptic inputs and their ability to produce action potentials. Artificial cerebral spinal fluid in high pH impairs excitatory synaptic transmission and spike initiation in cortical GABAergic neurons. The alkalosis-induced dysfunction of GABAergic neurons is associated with the decrease of receptor responsiveness and the increases of spike refractory periods and threshold potentials. Our studies reveal that alkalosis impairs cortical GABAergic neurons and subsequently deteriorate brain functions. The molecular targets for alkalosis action include glutamate receptor-channels and voltage-gated sodium channels on GABAergic neurons.

Acidosis and alkalosis impair brain functions through weakening spike encoding at cortical GABAergic neurons

Acidosis and alkalosis, associated with metabolic disorders, lead to the pathological changes of cognition and behaviors in clinical practices of neurology and psychology. Cellular mechanisms for these functional disorders in the central nervous system remain unclear. We have investigated the influences of acidosis and alkalosis on the functions of cortical GABAergic neurons. Both acidosis and alkalosis impair the ability of encoding sequential spikes at these GABAergic neurons. The impairments of their spiking are associated with the increases of refractory periods, threshold potential and afterhyperpolarization. Our studies reveal that acidosis and alkalosis impair cortical GABAergic neurons and in turn deteriorate brain functions, in which their final targets may be voltage-gated channels of sodium and potassium.

Cortical GABAergic neurons and cerebellar Purkinje cells respond to ischemia-pathogenic factors differently

GABAergic neurons in the central nervous system are vulnerable to hazard situations, such as ischemia and toxic substances, under which their dysfunction results in neuronal excitotoxicity and subsequently cell death. How ischemia-related pathogenic factors influence the functions of different GABAergic neurons remains to be documented. We investigated this issue at cortical GABAergic neurons and cerebellar Purkinje cells in brain slices by whole-cell recordings. Our results demonstrate that ischemia, cellular Ca(2+)-overload and acidosis lower the spike capacity of cortical GABAergic neurons, but elevate that of cerebellar Purkinje cells. These changes of spike encoding at two types of GABAergic cells are associated with the different effects of three factors on spike refractory periods and threshold potentials, which are mediated by voltage-gated sodium channels. Mechanisms underlying such differences are discussed.

The role of pannexin hemichannels in the anoxic depolarization of hippocampal pyramidal cells

Neuronal gap junctional hemichannels, composed of pannexin-1 subunits, have been suggested to play a crucial role in epilepsy and brain ischaemia. After a few minutes of anoxia or ischaemia, neurons in brain slices show a rapid depolarization to ∼−20 mV, called the anoxic depolarization. Glutamate receptor blockers can prevent the anoxic depolarization, suggesting that it is produced by a cation influx through glutamate-gated channels. However, in isolated hippocampal pyramidal cells, simulated ischaemia evokes a large inward current and an increase in permeability to large molecules, mediated by the opening of pannexin-1 hemichannels. N-methyl-d-aspartate is also reported to open these hemichannels, suggesting that the activation of N-methyl-d-aspartate receptors, which occurs when glutamate is released in ischaemia, might cause the anoxic depolarization by evoking a secondary ion flux through pannexin-1 hemichannels. We tested the contribution of pannexin hemichannels to the anoxic depolarization in CA1 pyramidal cells in the more physiological environment of hippocampal slices. Three independent inhibitors of hemichannels—carbenoxolone, lanthanum and mefloquine—had no significant effect on the current generating the anoxic depolarization, while a cocktail of glutamate and gamma-aminobutyric acid class A receptor blockers abolished it. We conclude that pannexin hemichannels do not generate the large inward current that underlies the anoxic depolarization. Glutamate receptor channels remain the main candidate for generating the large inward current that produces the anoxic depolarization.

Vesicular GABA release delays the onset of the Purkinje cell terminal depolarization without affecting tissue swelling in cerebellar slices during simulated ischemia

Neurosteroids that can enhance GABA(A) receptor sensitivity protect cerebellar Purkinje cells against transient episodes of global brain ischemia, but little is known about how ischemia affects GABAergic transmission onto Purkinje cells. Here we use patch-clamp recording from Purkinje cells in acutely prepared slices of rat cerebellum to determine how ischemia affects GABAergic signaling to Purkinje cells. In voltage-clamped Purkinje cells, exposing slices to solutions designed to simulate brain ischemia caused an early, partial suppression of the frequency of spontaneous inhibitory post synaptic currents (sIPSCs), but after 5-8 min GABA accumulated in the extracellular space around Purkinje cells, generating a large (approximately 17 nS), sustained GABA(A) receptor-mediated conductance. The sustained GABA(A) conductance occurred in parallel with an even larger (approximately 117 nS) glutamate receptor-mediated conductance, but blocking GABA(A) receptors did not affect the timing or magnitude of the glutamate conductance, and blocking glutamate receptors did not affect the timing or magnitude of the GABA(A) conductance. Despite the lack of interaction between GABA and glutamate, blocking GABA(A) receptors significantly accelerated the onset of the Purkinje cell "ischemic" depolarization (ID), as assessed with current-clamp recordings from Purkinje cells or field potential recordings in the dendritic field of the Purkinje cells. The Purkinje cell ID occurred approximately 2 min prior to the sustained glutamate release under control conditions and a further 1-2 min earlier when GABA(A) receptors were blocked. Tissue swelling, as assessed by monitoring light transmittance through the slice, peaked just after the ID, prior to the sustained glutamate release, but was not affected by blocking GABA(A) receptors. These data indicate that ischemia induces the Purkinje cell ID and tissue swelling prior to the sustained glutamate release, and that blocking GABA(A) receptors accelerates the onset of the ID without affecting tissue swelling. Taken together these data may explain why Purkinje cells are one of the most ischemia sensitive neurons in the brain despite lacking NMDA receptors, and why neurosteroids that enhance GABA(A) receptor function protect Purkinje cells against transient episodes of global brain ischemia.

Simulated ischaemia induces Ca2+-independent glutamatergic vesicle release through actin filament depolymerization in area CA1 of the hippocampus

Transient, non-catastrophic brain ischaemia can induce either a protected state against subsequent episodes of ischaemia (ischaemic preconditioning) or delayed, selective neuronal death. Altered glutamatergic signalling and altered Ca(2+) homeostasis have been implicated in both processes. Here we use simultaneous patch-clamp recording and Ca(2+) imaging to monitor early changes in glutamate release and cytoplasmic [Ca(2+)] ([Ca(2+)](c)) in an in vitro slice model of hippocampal ischaemia. In slices loaded with the Ca(2+)-sensitive dye Fura-2, ischaemia leads to an early increase in [Ca(2+)](c) that precedes the severe ischaemic depolarization (ID) associated with pan necrosis. The early increase in [Ca(2+)](c) is mediated by influx through the plasma membrane and release from internal stores, and parallels an early increase in vesicular glutamate release that manifests as a fourfold increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs). However, the increase in mEPSC frequency is not prevented by blocking the increase in [Ca(2+)](c), and the early rise in [Ca(2+)](c) is not affected by blocking ionotropic and metabotropic glutamate receptors. Thus, the increase in [Ca(2+)](c) and the increase in glutamate release are independent of each other. Stabilizing actin filaments with jaspamide or phalloidin prevented vesicle release induced by ischaemia. Our results identify several early cellular cascades triggered by ischaemia: Ca(2+) influx, Ca(2+) release from intracellular stores, actin filament depolymerization, and vesicular release of glutamate that depends on actin dynamics but not [Ca(2+)](c). All of these processes precede the catastrophic ID by several minutes, and thus represent potential target mechanisms to influence the outcome of an ischaemic episode.

Young age and low temperature, but not female gender delay ATP loss and glutamate release, and protect Purkinje cells during simulated ischemia in cerebellar slices

Excessive activation of glutamate receptors contributes to Purkinje cell (PC) damage during brain ischemia, but the mechanisms of glutamate release are contentious. Age, gender and temperature all strongly influence ischemic brain damage, but the mechanisms underlying their influence are not fully understood. We determined how age, gender and temperature influence ATP loss, glutamate release, glutamate receptor activation and PC damage during cerebellar ischemia. We used voltage-clamped PCs to monitor glutamate release during simulated ischemia in slices of cerebellum of different ages and genders, and at different temperatures. While gender did not affect ischemic glutamate release, both young age and low temperature dramatically delayed the onset of glutamate release without affecting its magnitude. Glutamate receptor and transporter density were similar around young and old PCs, but the rate of ATP decline during ischemia was dramatically slowed in young animals and by lowered temperature. Bypassing the ischemia-induced loss of ATP, and disrupting ionic gradients directly by pharmacologically inhibiting the Na(+)/K(+)-ATPase, reduced the difference in timing of glutamate release in newborn and mature cerebellum. Ischemic damage in newborn and mature cerebellum paralleled ATP loss and glutamate release, but blocking glutamate receptors did not prevent ischemic damage. Thus, protection against brain ischemia provided by young age or lowered temperature is due to slower consumption and hence delayed loss of ATP, with a corresponding delay in glutamate release and other undetermined damage mechanisms. The protection afforded by female gender must occur downstream of ATP decline, glutamate release, and activation of glutamate receptors on PCs.

A role for glutamate and glia in the fast network oscillations preceding spreading depression

The mechanism of the propagation of spreading depression is unclear. Classical theories proposed a self-maintained cycle fed by elevated potassium and/or glutamate in the extracellular space. Earlier we found in vivo a characteristic oscillatory field activity that is synchronous in a strip of tissue ahead of the oncoming wave of neuron depolarization and that occurs before the extracellular potassium level begins to rise [Herreras O, Largo C, Ibarz JM, Somjen GG, Marrín del Río R (1994) Role of neuronal synchronizing mechanisms in the propagation of spreading depression in the in vivo hippocampus. J Neurosci 14:7087-7098]. We investigated here the possible participation of glutamate and the role of glia in the prodromal field oscillations using extra and intracellular recordings and pharmacological manipulations in rat hippocampal slices. As earlier shown in vivo, field oscillations propagated ahead of the negative potential shift covering distances of up to 1 mm. The oscillatory prodromals were initially subthreshold but then each wave became crowned by a population spike. The frequency of the oscillatory prodromals was variable among slices (80-115 Hz), but constant in individual slices. The blockade of ionotropic glutamate receptors decreased the frequency of prodromal oscillations, retarded spreading depression propagation, and shortened the duration of depolarization. Blocking the glutamate membrane transport increased the oscillatory frequency. The selective metabolic poisoning of astrocytes led to gradual disorganization of prodromal oscillations whose frequency first increased and then decreased. Also, the amplitude of the population spikes within the burst diminished as individual cells fired fewer action potentials, although still phase-locked with population spikes. The effects of glial metabolic impairment were observed within the period when neuron electrical properties were still normal, and were blocked by glutamate receptor antagonists. These data suggest that glutamate released from glial cells and possibly also from neurons has a role in the generation of oscillations and neuron firing synchronization that precede the spreading depression-related depolarization, but additional mechanisms are required to fully explain the onset and propagation of spreading depression.