Spreading depolarization and neuronal damage or survival in mouse neocortical brain slices immediately and 12 hours following middle cerebral artery occlusion

Whereas many studies have examined the properties of the compromised neocortex in the first several days following ischemia, there is less information regarding the initial 12 h poststroke. In this study we examined live mouse neocortical slices harvested immediately and 12 h after a 30-min middle cerebral artery occlusion (MCAo). We compared nonischemic and ischemic hemispheres with regard to the propensity for tissue swelling and for generating spreading depolarization (SD), as well as evoked synaptic responses and single pyramidal neuron electrophysiological properties. We observed spontaneous SD in 7% of slices on the nonstroked side and 25% in the stroked side following the 30-min MCAo. Spontaneous SD was rare in 12-h recovery slices. The region of the ischemic core and surround in slices was not susceptible to SD induced by oxygen and glucose deprivation. At the neuronal level, neocortical gray matter is surprisingly unaltered in brain slices harvested immediately poststroke. However, by 12 h, the fields of pyramidal and striatal neurons that comprise the infarcted core are electrophysiologically silent because the majority are morphologically devastated. Yet, there remains a subset of diffusely distributed "healthy" pyramidal neurons in the core at 12 h post-MCAo that persist for days poststroke. Their intact electrophysiology and dendritic morphology indicate a surprisingly selective resilience to stroke at the neuronal level. NEW & NOTEWORTHY It is generally accepted that the injured core region of the brain resulting from a focal stroke contains no functioning neurons. Our study shows that some neurons, although surrounded by devastated neighbors, can maintain their structure and electrical activity. This surprising finding raises the possibility of discovering how these neurons are protected to pinpoint new strategies for reducing stroke injury.

Protective effects of baicalin on oxygen/glucose deprivation- and NMDA-induced injuries in rat hippocampal slices

Baicalin is a flavonoid derivative from Scutellaria baicalensis Georgi with various pharmacological effects. Recently, the neuroprotective effect of baicalin was reported. To confirm this effect and explore the possible mechanism, we have investigated the protective effect of baicalin on ischaemiclike or excitotoxic injury and the activation of protein kinase C alpha (PKC(alpha)) in rat hippocampal slices. In-vitro ischaemic-like injury was induced by oxygen/glucose deprivation (OGD) and the excitotoxic injury by N-methyl-D-aspartate (NMDA). The viability and swelling of the slices were detected by triphenyltetrazolium chloride (TTC) staining and image analysis of light transmittance (LT), respectively. The translocation of PKC(alpha) was measured by immunoblotting. Baicalin was added during both injuries. Baicalin (0.1, 1, and 10 micromol L(-1)) concentration-dependently inhibited OGD-induced viability reduction and acute neuron swelling, and inhibited the increased portion of PKC(alpha) present in the membrane fraction over the total PKC(alpha). Baicalin ameliorated NMDA-induced viability reduction (not LT elevation) and inhibited the NMDA-increased membrane portion of PKC(alpha) at 1 micromol L(-1). We concluded that baicalin had a protective effect on ischaemic-like or excitotoxic injury in rat hippocampal slices, which might have been partly related to inhibition of PKC(alpha) translocation.

Domoic acid enhances Bcl-2-calcineurin-inositol-1,4,5-trisphosphate receptor interactions and delayed neuronal death in rat brain slices

Mechanisms of neuronal death following neuronal damage due to domoic acid are not completely defined. Bcl-2, a survival protein, protects neurons from ischemia and excitotoxin-induced damage. We previously demonstrated that Bcl-2 shuttles calcineurin to its substrates and may regulate calcium release from internal stores during neuronal ischemia. We now confirm that during excitotoxicity induced by domoic acid, calcineurin-Bcl-2 and calcineurin-1,4,5-inositol-trisphosphate receptor (IP3-R) interactions increase. Furthermore, we now show that calcineurin-IP3-R interactions are mediated by Bcl-2 in brain slices following short-term treatment with domoic acid (10 microM). Domoic acid induced late neuronal death and caspase-3-like activity in organotypic cortical and hippocampal cultures. These experiments further define the mechanisms by which neurons respond to excitotoxic insults, and suggest that interactions between calcineurin and its target proteins may influence cellular responses to injury.

In vitro hypoxia and excitotoxicity in human brain induce calcineurin-Bcl-2 interactions

Although pathogenesis of neuronal ischemia is incompletely understood, evidence indicates apoptotic neuronal death after ischemia. Bcl-2, an anti-apoptotic and neuroprotective protein, interacts with calcineurin in non-neuronal tissues. Activation of calcineurin, which is abundant in the brain, may play a role in apoptosis. Using co-immunoprecipitation experiments in biopsy-derived, fresh human cortical and hippocampal slices, we examined possible interactions between calcineurin and Bcl-2. Calcineuin-Bcl-2 interactions increased after exposure in vitro to excitotoxic agents and conditions of hypoxia/aglycia. This interaction may shuttle calcineurin to substrates such as the inositol-1,4,5-tris-phosphate receptor because under these experimental conditions interactions between calcineurin and inositol-1,4,5-tris-phosphate receptor also increased. A specific calcineurin inhibitor, FK-520, attenuated insult-induced increases in calcineurin-Bcl-2 interactions and augmented caspase-3 like activity. These data suggest that Bcl-2 modulates neuroprotective effects of calcineurin and that calcineurin inhibitors increase ischemic neuronal damage.

Functional imaging of the retinal layers by laser scattering: an approach for the study of Leão's spreading depression in intact tissue

This paper presents a novel optical approach for the study of spreading depression in isolated retina. The method makes it possible to register the laser light scattered from each layer of the tissue, yielding a functional image of the retina during spreading depression. The tissue is kept intact, since histological cuts are not necessary. Measurements of other variables, such as extracellular potential, are also allowed by the described method. This is done simultaneously with the functional image in a high spatial resolution, with the positioning of the microelectrode tip being easily monitored. The information about temporal and spatial evolution of light was compacted in a single image. The image-processing technique used here enables the visualization of the light scattered by the inner plexiform layer (IPL), which is the most prominent scatter layer during spreading depression. The wavefront velocity and its increase as two wavefronts approach each other can then be determined, and it is also possible to observe the thickness variation of the tissue during the wave travel. The relationship between two peaks of light-scattering sequence during the phenomenon was studied at two wavelengths (632.8 and 543.5 nm). This relationship is shown to be dependent on the wavelength.

Protection by rapid chemical preconditioning of stressed hippocampal slice: a study of cellular swelling using optical scatter imaging

It has been demonstrated that anoxic preconditioning protects against a subsequent 'lethal' injury in the hippocampal slice. The goal of this paper was to test the hypothesis that chemical preconditioning could help reduce the cellular swelling observed in excitotoxically injured hippocampal slices. The control slice was given a 10-min insult of 100 microM N-methyl-D-aspartate (NMDA) to simulate ischemic injury, followed by 30-min perfusion of standard Ringers solution. Cellular swelling was observed with a microscope designed to image light scatter changes resulting from cellular swelling. After the control NMDA injury, the average peak scatter change for CA1, CA3 and DG regions was 31.0 +/- 3.4, 22.4 +/- 4.8 and 27.6 +/- 4.6%, respectively. The peak scatter change of the overall slice was 26.0 +/- 3.6%. The experimental slices were preconditioned by three short 100 microM NMDA insults of 15 s each separated by 10 min of standard Ringers solution perfusion. The slices then received 10 min of 'lethal' injury by 100 microM NMDA. It was observed that the overall scatter signal, as a measure of cellular swelling, was reduced by 8.0% (P<0.05, n=11) after preconditioning. A regional heterogeneity in the responses was also observed. Cellular swelling in CA1, CA3 and DG were reduced by 9.8% (P<0.001, n=11), 9.2% (P<0.005, n=11) and 7.7% (P<0.05, n=11), respectively, when compared to the control. This study presents experimental evidence that short episodes of preconditioning may protect against acute cellular swelling under ischemic conditions.

Intrinsic optical signals in respiratory brain stem regions of mice: neurotransmitters, neuromodulators, and metabolic stress

In the rhythmic brain stem slice preparation, spontaneous respiratory activity is generated endogenously and can be recorded as output activity from hypoglossal XII rootlets. Here we combine these recordings with measurements of the intrinsic optical signal (IOS) of cells in the regions of the periambigual region and nucleus hypoglossus of the rhythmic slice preparation. The IOS, which reflects changes of infrared light transmittance and scattering, has been previously employed as an indirect sensor for activity-related changes in cell metabolism. The IOS is believed to be primarily caused by cell volume changes, but it has also been associated with other morphological changes such as dendritic beading during prolonged neuronal excitation or mitochondrial swelling. An increase of the extracellular K(+) concentration from 3 to 9 mM, as well as superfusion with hypotonic solution induced a marked increase of the IOS, whereas a decrease in extracellular K(+) or superfusion with hypertonic solution had the opposite effect. During tissue anoxia, elicited by superfusion of N(2)-gassed solution, the biphasic response of the respiratory activity was accompanied by a continuous rise in the IOS. On reoxygenation, the IOS returned to control levels. Cells located at the surface of the slice were observed to swell during periods of anoxia. The region of the nucleus hypoglossus exhibited faster and larger IOS changes than the periambigual region, which presumably reflects differences in sensitivities of these neurons to metabolic stress. To analyze the components of the hypoxic IOS response, we investigated the IOS after application of neurotransmitters known to be released in increasing amounts during hypoxia. Indeed, glutamate application induced an IOS increase, whereas adenosine slightly reduced the IOS. The IOS response to hypoxia was diminished after application of glutamate uptake blockers, indicating that glutamate contributes to the hypoxic IOS. Blockade of the Na(+)/K(+)-ATPase by ouabain did not provoke a hypoxia-like IOS change. The influences of K(ATP) channels were analyzed, because they contribute significantly to the modulation of neuronal excitability during hypoxia. IOS responses obtained during manipulation of K(ATP) channel activity could be explained only by implicating mitochondrial volume changes mediated by mitochondrial K(ATP) channels. In conclusion, the hypoxic IOS response can be interpreted as a result of cell and mitochondrial swelling. Cell swelling can be attributed to hypoxic release of neurotransmitters and neuromodulators and to inhibition of Na(+)/K(+)-pump activity.

Optical approaches to embryonic development of neural functions in the brainstem

The ontogenetic approach to physiological events is a useful strategy for understanding the functional organization/architecture of the vertebrate brainstem. However, conventional electrophysiological techniques are difficult or impossible to employ in the early embryonic central nervous system. Optical techniques using voltage-sensitive dyes have made it possible to monitor neural activities from multiple regions of living systems, and have proven to be a useful tool for analyzing the embryogenetic expression of brainstem neural function. This review describes recent progress in optical studies made on embryonic chick and rat brainstems. Several technical issues concerning optical recording from the embryonic brainstem preparations are discussed, and characteristics of the optical signals evoked by cranial nerve stimulation or occurring spontaneously are described. Special attention is paid to the chronological analyses of embryogenetic expression of brainstem function and to the spatial patterning of the functional organization/architecture of the brainstem nuclei. In addition, optical analyses of glutamate, GABA, and glycine receptor functions during embryogenesis are described in detail for the chick nucleus tractus solitarius. This review also discusses intrinsic optical signals associated with neuronal depolarization. Some emphases are also placed on the physiological properties of embryonic brainstem neurons, which may be of interest from the viewpoint of developmental neurobiology.

Optical light scatter imaging of cellular and sub-cellular morphology changes in stressed rat hippocampal slices

Optical imaging, such as transmission imaging, is used to study brain tissue injury. Transmission imaging detects cellular swelling via an increase in light transmitted by tissue slices due to a decrease in scattering particle concentration. Transmission imaging cannot distinguish sub-cellular particle size changes from cellular swelling or shrinkage. We present an optical imaging method, based on Mie scatter theory, to detect changes in sub-cellular particle size and concentration. The system uses a modified inverted microscope and a 16-bit cooled CCD camera to image tissue light scatter at two angles. Dual-angle scatter ratio imaging successfully discriminated latex microsphere suspensions of differing sizes (0.6, 0.8, 1 and 2 microm) and concentrations. We applied scatter imaging to hippocampal slices treated with 100 microM N-methyl-D-aspartate (NMDA) to model excitotoxic injury or -40 mOsm hypotonic perfusion solution to cause edema injury. We detected light scatter decreases similar to transmission imaging in the CA1 region of the hippocampus for both treatments. Using our system, we could distinguish between NMDA and hypotonic treatments on the basis of statistically significant (P<0.0003) differences in the scatter ratio measured in CA1. Scatter imaging should be useful in studying tissue injuries or activity resulting in brain tissue swelling as well as morphological changes in sub-cellular organelles such as mitochondrial swelling.

Glutamate does not mediate acute neuronal damage after spreading depression induced by O2/glucose deprivation in the hippocampal slice

This study argues that, in contrast to accepted excitotoxicity theory, O2/glucose deprivation damages neurons acutely by eliciting ischemic spreading depression (SD), a process not blocked by glutamate antagonists. In live rat hippocampal slices, the initiation, propagation, and resolution of SD can be imaged by monitoring wide-band changes in light transmittance (i.e., intrinsic optical signals). Oxygen/glucose deprivation for 10 minutes at 37.5 degrees C evokes a propagating wave of elevated light transmittance across the slice, representing the SD front. Within minutes, CA1 neurons in regions undergoing SD display irreversible damage in the form of field potential inactivation, swollen cell bodies, and extensively beaded dendrites, the latter revealed by single-cell injection of lucifer yellow. Importantly, glutamate receptor antagonists do not block SD induced by O2/glucose deprivation, nor do they prevent the resultant dendritic beading of CA1 neurons. However, CA1 neurons are spared if SD is suppressed by reducing the temperature to 35 degrees C during O2/glucose deprivation. This supports previous electrophysiologic evidence in vivo that SD during ischemia promotes acute neuronal damage and that glutamate antagonists are not protective of the metabolically stressed tissue. The authors propose that the inhibition of ischemic SD should be targeted as an important therapeutic strategy against stroke damage.

A neurophysiological method of rapid detection and analysis of marine algal toxins

We have examined the effectiveness of the in vitro rat hippocampal slice preparation as a means of rapidly and specifically detecting the marine algal toxins saxitoxin, brevetoxin, and domoic acid and have identified toxin-specific electrophysiological signatures for each. Brevetoxin (PbTX3, 50-200 nM) produced a significant reduction in orthodromic population spike amplitude which was quick to reverse during a 50 min wash-out, while antidromic population spikes and field EPSPs exhibited only slight reductions, and fibre spiof orthodrokes showed no change at all. Domoic acid (100 nM) produced a robust, reversible increase in amplitude mic spikes, and the appearance of multiple spikes (i.e., epileptiform activity) within minutes of toxin wash-in. Other notable features of the domoic acid signature included a significant decrease in amplitude of the field EPSPs, and a complete absence of effect on either antidromic or fibre spikes. Fifty nanomolar saxitoxin (PSP) abolished all responses in all slices. Only antidromic spikes showed any recovery during wash-out. Field EPSP and fiber spike analysis further demonstrated that the preparation is capable of reliably detecting saxitoxin in a linearly responsive fashion at toxin concentrations of 25-200 nM, and tests of naturally contaminated shellfish confirmed the utility of this assay as a screening method for PSP. Our findings suggest that the in vitro hippocampal slice preparation has potential in the detection and analysis of three marine algal toxins important to the shellfish industry.

Interpretation of intrinsic optical signals and calcein fluorescence during acute excitotoxic insult in the hippocampal slice

Immediate (acute) neuronal damage in response to overstimulation of glutamate receptors results from toxic exposure to food poisons acting as glutamate analogues. Glutamate agonist application evokes dramatic intrinsic optical signals (IOSs) in the rat hippocampal slice preparation, particularly in the CA1 region. Theoretically IOSs are generated by alterations to neuronal and glial structure that change light transmittance (LT) in live brain tissue. To better understand such signals, IOSs evoked by the glutamate agonist N-methyl-D-aspartate were imaged in the rat hippocampal slice. We correlated these excitotoxic signals with: (1) biophysical principles governing light transport, (2) tissue volume changes as measured using a free intracellular fluorophore (calcein), (3) dendritic morphology visualized by dye injection, and (4) standard histopathology. In theory LT elevation evoked during acute excitotoxic swelling is generated by change to subcellular structure that reduces light scattering during cell swelling. However, in responsive dendritic regions, initial LT elevation caused by cell swelling was overridden by the formation of dendritic beads, a conformation that increased light scattering (thereby reducing LT) even as the calcein signal demonstrated that the tissue continued to swell. Thus IOS imaging reveals acute somatic and dendritic damage during excitotoxic stress that can be monitored across slices of brain tissue in real time.

Potential sources of intrinsic optical signals imaged in live brain slices

Changes in how light is absorbed or scattered in biological tissue are termed intrinsic optical signals (IOSs). Imaging IOSs in the submerged brain slice preparation provides insight into brain activity if it involves significant water movement between intracellular and extracellular compartments. This includes responses to osmotic imbalance, excitotoxic glutamate agonists, and oxygen/glucose deprivation, the latter leading to spreading depression. There are several misconceptions regarding these signals. (1) IOSs are not generated by glial swelling alone. Although neuronal and glia sources cannot yet be directly imaged, several lines of evidence indicate that neurons contribute significantly to the changes in light transmittance. (2) Excitotoxic swelling and osmotic swelling are physiologically different, as are their associated IOSs. Hyposmotic swelling involves no detectable neuronal depolarization of cortical pyramidal neurons, only the passive drawing in of water from a dilute medium across the cell membrane. In contrast excitotoxic swelling involves sustained membrane depolarization associated with inordinate amounts of Na+ and Cl- entry followed by water. IOSs demonstrate substantial damage in the latter case. (3) Osmotic perturbations do not induce volume regulatory mechanisms as measured by IOSs. The osmotic responses measured by IOSs in brain slices are passive, without the compensatory mechanisms that are assumed to be active on a scale suggested by studies of cultured brain cells under excessive osmotic stress. (4) Spreading depression (SD) can cause neuronal damage. Innocuous during migraine aura, SD induces acute neuronal damage in brain slices that are metabolically compromised by oxygen/glucose deprivation, as demonstrated by IOSs. Neighboring tissue where SD does not spread remains relatively healthy as judged by a minimal reduction in light transmittance. IOSs show that the metabolic stress of SD combined with the compromise of energy resources leads to acute neuronal damage that is resistant to glutamate antagonists. (5) While hyperosmotic conditions reduce LT by causing cells to shrink, excitotoxic conditions reduce LT by causing dendritic beading. This conformational change increases light scattering even as the tissue continues to swell.

Neurochemistry and pharmacology of the major hippocampal transmitter systems: synaptic and nonsynaptic interactions

Hippocampus plays a crucial role in important brain functions (e.g. memory, learning) thus in the past two decades this brain region became a major objective of neuroscience research. During this period large number of anatomical, neurochemical and electrophysiological data have been accumulated. While excellent reviews have been published on the anatomy and electrophysiology of hippocampal formation, the neurochemistry of this area has not been thoroughly surveyed. Therefore the aim of this review is to summarize the neurochemical and pharmacological data on the release of the major neurotransmitters found in the hippocampal region: glutamate (GLU), gamma-amino butyric acid (GABA), acetylcholine (ACh), noradrenaline (NA) and serotonin (5-HT). In addition, this review analyzes the synaptic and nonsynaptic interactions between hippocampal neuronal elements and overviews how auto- and heteroreceptors are involved in the presynaptic modulation of transmitter release. The presented data clearly show that transmitters released from axon terminals without synaptic contact play an important role in the fine tuning of communication between neurons within a neuronal circuit.

Spreading depression determines acute cellular damage in the hippocampal slice during oxygen/glucose deprivation

During ischaemia neurons depolarize and release the neurotransmitter L-glutamate, which accumulates extracellularly and binds to postsynaptic receptors. This initiates a sequence of events thought to culminate in immediate and delayed neuronal death. However, there is growing evidence that during ischaemia the development of spreading depression (SD) can be an important determinant of the degree and extent of ischaemic damage. In contrast, SD without metabolic compromise (as occurs in migraine aura) causes no discernible damage to brain tissue. SD is a profound depolarization of neurons and glia that propagates like a wave across brain tissue. Brain cell swelling, an early event of both the excitotoxic process and of SD, can be assessed by imaging associated intrinsic optical signals (IOSs). We demonstrate here that IOS imaging clearly demarcates the ignition site and migration of SD across the submerged hippocampal slice of the rat. If SD is induced by elevating [K+]O, the tissue fully recovers, but in slices that are metabolically compromised at 37.5 degrees C by oxygen/glucose deprivation (OGD) or by ouabain exposure, cellular damage develops only where SD has propagated. Specifically, the evoked CA1 field potential is permanently lost, the cell bodies of involved neurons swell and their dendritic regions increase in opacity. In contrast to OGD, bath application of L-glutamate (6-10 mM) at 37.5 degrees C evokes a non-propagating LT increase in CA1 that reverses without obvious cellular damage. Moreover, application of 2-20 mM glutamate or various glutamate agonists fail to evoke SD in the submerged hippocampal slice. We propose that SD and OGD together (but not alone) constitute a 'one-two punch', causing acute neuronal death in the slice that is not replicated by elevated glutamate. These findings support the proposal that SD generation during stroke promotes and extends acute ischaemic damage.

GABA-Induced intrinsic light-scattering changes associated with voltage-sensitive dye signals in embryonic brain stem slices: coupling of depolarization and cell shrinkage

We have found new evidence for gamma-aminobutyric acid (GABA)-induced intrinsic optical changes associated with a voltage-sensitive dye signal in the early embryonic chick brain stem slice. The slices were prepared from 8-day-old embryos, and they were stained with a voltage-sensitive dye (NK2761). Pressure ejection of GABA to one site within the preparation elicited optical changes. With 580-nm incident light, two components were identified in the GABA-induced optical change. The first component was wavelength dependent, whereas the second, slower change was independent of wavelength. Comparison with the known action spectrum of the dye indicates that the first component reflects a depolarization of the membrane and that the second, slow component is a light-scattering change resulting from cell shrinkage coupled with the depolarization. Similar optical changes also were induced by glycine, although the amplitude of both the first and second signals was much smaller than for GABA. The optical changes induced by GABA persisted in the presence of picrotoxin and 2-hydroxysaclofen, suggesting that these optical responses include a novel GABA response, which has been termed GABAD in our previous reports.

Intrinsic optical signaling denoting neuronal damage in response to acute excitotoxic insult by domoic acid in the hippocampal slice

Using the seafood contaminant domoic acid (an AMPA/kainate receptor agonist), we demonstrate a distinct excitotoxic sequence of events leading to acute neuronal damage in the hippocampal slice as measured by (1) loss of the evoked CA1 field potential, (2) irreversible changes in light transmittance, (3) histopathology, and (4) lucifer yellow injection of single CA1 pyramidal neurons. Change in light transmittance (LT) through the submerged slice indirectly measures altered cell volume, both neuronal and glial. At 37 degrees C, a 1-min superfusion of 10 mu M domoate induced a prolonged reversible increase in LT, primarily in the dendritic regions of CA1 and dentate granule cells (GC), but not in the CA3 region. Spectral analysis (400-800 nm) revealed a wide-band transmittance increase, indicating cell swelling as a major source of the intrinsic signal. The evoked field potential recorded in the CA1 cell body region (PYR) was lost as LT peaked, but completely recovered upon return to the baseline LT level. Increasing domoate exposure to 10 min elicited a different and distinct LT sequence in CA1 and dentate regions. An initial LT increase in dendritic regions evolved in an irreversible decrease in LT. At the same time, LT irreversibly increased in cell body regions (CA1 PYR and GC) and the evoked field potential was irretrievably lost. Also, there was histological damage to cell body and dendritic regions of CA1 and granule cells. Injection of lucifer yellow into single CA1 neurons in slices displaying the irreversible LT sequence revealed extensive dendritic beading, whereas CA1 cells in control slices displayed a smoothly contoured arbor. Consistent with acute neuronal damage, the optical changes generated by domoate did not require extracellular Ca2+, and lowering the temperature protected the slice from irreversible damage to CA1 and GC regions. Although glial changes may also occur, we conclude that imaging light transmittance reveals dynamic and compartmentalized excitotoxic changes in neuronal volume. Beading of the dendritic arbor increases light scatter, thereby decreasing LT and highlighting damaged dendritic regions.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Optical illustration of glutamate-induced cell swelling coupled with membrane depolarization in embryonic brain stem slices

Using intrinsic- and voltage-sensitive dye optical recordings, we have elucidated coupling of glutamate-induced depolarization and neuronal swelling in early embryonic chick brain stem slices. Twenty-four slices were prepared from 8-day old chick embryos, and stained with a voltage-sensitive merocyanine-rhodanine dye (NK2761). The pressure ejection of glutamate to one site within the preparation evoked changes in transmitted light intensity. With 700 nm incident light, three components were identified in glutamate-induced optical changes. The first component was wavelength dependent, while the second and third components were independent of the wavelength. With reference to the action spectrum of the merocyanine-rhodanine dye and osmotic changes in optical properties, we concluded that the first component reflects glutamate-induced depolarization of the membrane, and that the second component is an intrinsic light-scattering change resulting from neural cell swelling coupled with the membrane depolarization.

Evidence against volume regulation by cortical brain cells during acute osmotic stress

The cell bodies of neurons and glia examined in culture respond to severe osmotic stress (100 to 200 mOsm) by passive volume change that is followed within several minutes by volume regulation, even in the face of maintained osmotic change. However, in clinical situations, the brain does not experience such precipitous and severe changes in brain hydration. In this study we examined if there is evidence from the hippocampal slice preparation supporting the type of volume regulation observed in cultured brain cells. Within the CA1 region we imaged changes in light transmittance (LT), recorded the evoked field potential, and monitored tissue resistance (all measures of cell volume change) during the first hour of osmotic stress to search for evidence of volume regulation. During superfusion of hypo-osmotic aCSF (-40 mOsm), LT increased 24 to 28% in the dendritic regions of CA1 neurons. The LT reached a plateau which was maintained throughout a 45-min application interval, more than enough time to reveal a regulatory volume decrease. Upon return to control saline, LT immediately returned to baseline and settled there. Hypo-osmolality reversibly increased the relative tissue resistance (RREL) measured across the CA1 region with a time course identical to the increase in LT. Conversely, hyperosmotic aCSF (mannitol, +40 mOsm) decreased both RREL by 8% and LT by 15.5% with no indication of a regulatory volume increase. The CA1 cell body layer showed only slight hypo-osmotic swelling whereas exposure to the glutamate agonist quinolinic acid caused pronounced swelling in this region. Even when osmolality was decreased by 120 mOsm for 20 min, dendritic regions responded passively with no regulatory volume decrease. However, when aCSF Cl- was substituted, the CA1 dendritic regions displayed immediate swelling followed by a dramatic volume reduction under normosmotic conditions, indicating that such behavior can be evoked by extreme aCSF dilution. We conclude that in the brain slice preparation, the cortical cells do not exhibit classic volume regulation in response to sudden physiological changes in osmolality. Moreover it is the dendritic region, not the cell body region, that displays dynamic volume change during osmotic challenge.