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Papers by David Andrew

Research paper thumbnail of Structure and innervation of a crustacean neurosecretory cell

Canadian Journal of Zoology, Mar 1, 1978

The crayfish medulla terminalis X organ contains type 1 neurosecretory somata which are distingui... more The crayfish medulla terminalis X organ contains type 1 neurosecretory somata which are distinguishable by size, location, and ultrastructure and which have homologues in the eyestalks of other decapods. Neurosecretory (NS) vesicles (140 nm diameter) in these somata are structurally similar to those in the X organ–sinus gland (Xo-sg) tract and the neurohemal sinus gland. Cobalt iontophoresis into type 1 somata shows that their axons traverse the underlying neuropil of the medulla terminalis as the Xo-sg tract before emerging to the surface of the ganglion and terminating in the sinus gland. The proximal third of these type 1 axons branches extensively in the ganglionic neuropil; only in the neuropil adjacent to Xo-sg tract does electron microscopy reveal many fine postsynaptic neurites containing NS vesicles. Serial thin sections show continuity between these neurites and the Xo-sg tract. Structural evidence suggests that this fine arborization is dendritic and represents the site of informational input from the central nervous system to the Xo-sg complex. This study supports previous electrophysiological recordings within Xo somata which indicate synaptic input to these neuroendocrine cells.

Research paper thumbnail of Validating MALDI-IMS Feasibility in <i>Ex Vivo</i> Brain Slices

Journal of the American Society for Mass Spectrometry, Jul 20, 2023

Research paper thumbnail of Expression of Neuronal Na+/K+-ATPase α Subunit Isoforms in the Mouse Brain Following Genetically Programmed or Behaviourally-induced Oxidative Stress

Neuroscience, Aug 1, 2020

The Na+/K+-ATPase is a transmembrane ion pump that has a critical homeostatic role within every m... more The Na+/K+-ATPase is a transmembrane ion pump that has a critical homeostatic role within every mammalian cell; however, it is vulnerable to the effects of increased oxidative stress. Understanding how expression of this transporter is influenced by oxidative stress may yield insight into its role in the pathophysiology of neurological and neuropsychiatric diseases. In this study we investigated whether increased oxidative stress could influence Na+/K+-ATPase expression in various brain regions of mice. We utilized two different models of oxidative stress: a behavioural chronic unpredictable stress protocol and the Aldh2-/- mouse model of oxidative stress-based and age-related cognitive impairment. We identified distinct regional baseline mRNA and protein expression patterns of the Na+/K+-ATPase α1 and α3 isoforms within the neocortex, hippocampus, and brainstem of wildtype mice. Consistent with previous studies, there was a higher proportion of α3 expression relative to α1 in the brainstem versus neocortex, but a higher proportion of α1 expression relative to α3 in the neocortex versus the brainstem. The hippocampus had similar expression levels of both α1 and α3. Despite increased staining for oxidative stress in higher brain, no differences in α1 or α3 expression were noted in Aldh2-/- mice versus wildtype, or in mice exposed to a 28-day chronic unpredictable stress protocol. In both models of oxidative stress, gene and protein expression of Na+/K+-ATPase α1 and α3 isoforms within the higher and lower brain was remarkably stable. Thus, Na+/K+-ATPase function previously reported as altered by oxidative stress is not through induced changes in the expression of pump isoforms.

Research paper thumbnail of Morphometric Analysis of Hippocampal and Neocortical Pyramidal Neurons in a Mouse Model of Late Onset Alzheimer’s Disease

Journal of Alzheimer's Disease, Apr 21, 2020

The study of late-onset (sporadic) Alzheimer's disease (LOAD) has lacked animal models where impa... more The study of late-onset (sporadic) Alzheimer's disease (LOAD) has lacked animal models where impairments develop with aging. Oxidative stress promotes LOAD, so we have developed an oxidative stress-based model of age-related cognitive impairment based on gene deletion of aldehyde dehydrogenase 2 (ALDH2). This enzyme is important for the detoxification of endogenous aldehydes arising from lipid peroxidation. Compared to wildtype (WT) mice, the knockout (KO) mice exhibit a progressive decline in recognition and spatial memory and AD-like pathologies. Here we performed morphometric analyses in the dorsal and ventral hippocampal CA1 regions (dCA1 and vCA1) as well as in overlying primary sensory cortex to determine if altered neuronal structure can help account for the cognitive impairment in 12-month old KO mice. Dendritic morphology was quantitatively analyzed following Golgi-Cox staining using 9 WT mice (108 neurons) and 15 KO mice (180 neurons). Four pyramidal neurons were traced per mouse in each region, followed by branched structured analysis and Sholl analysis. Compared to WT controls, the morphology and complexity of dCA1 pyramidal neurons from KOs showed significant reductions in apical and basal dendritic length, dendrite intersections, ends, and nodes. As well, spine density along dorsal CA1 apical dendrites was significantly lower in KO versus WT. In contrast, pyramidal arborization in the vCA1 and primary sensory cortex were only minimally reduced in KO versus WT mice. These data suggest a region-specific vulnerability to oxidative stress-induced damage and/or a major and specific reduction in synaptic input to the pyramidal neurons of the dorsal hippocampus. This is in keeping with studies showing that lesions to the dorsal hippocampus impair primarily cognitive memory whereas ventral hippocampal lesions cause deficits in stress, emotion, and affect.

Research paper thumbnail of Neuronal Swelling: A Non-osmotic Consequence of Spreading Depolarization

Neurocritical Care, Sep 8, 2021

An acute reduction in plasma osmolality causes rapid uptake of water by astrocytes but not by neu... more An acute reduction in plasma osmolality causes rapid uptake of water by astrocytes but not by neurons, whereas both cell types swell as a consequence of lost blood flow (ischemia). Either hypoosmolality or ischemia can displace the brain downwards, potentially causing death. However, these disorders are fundamentally different at the cellular level. Astrocytes osmotically swell or shrink because they express functional water channels (aquaporins), whereas neurons lack functional aquaporins and thus maintain their volume. Yet both neurons and astrocytes immediately swell when blood flow to the brain is compromised (cytotoxic edema) as following stroke onset, sudden cardiac arrest, or traumatic brain injury. In each situation, neuronal swelling is the direct result of spreading depolarization (SD) generated when the ATP-dependent sodium/potassium ATPase (the Na + /K + pump) is compromised. The simple, and incorrect, textbook explanation for neuronal swelling is that increased Na + influx passively draws Cl − into the cell, with water following by osmosis via some unknown conduit. We first review the strong evidence that mammalian neurons resist volume change during acute osmotic stress. We then contrast this with their dramatic swelling during ischemia. Counter-intuitively, recent research argues that ischemic swelling of neurons is non-osmotic, involving ion/ water cotransporters as well as at least one known amino acid water pump. While incompletely understood, these mechanisms argue against the dogma that neuronal swelling involves water uptake driven by an osmotic gradient with aquaporins as the conduit. Promoting clinical recovery from neuronal cytotoxic edema evoked by spreading depolarizations requires a far better understanding of molecular water pumps and ion/water cotransporters that act to rebalance water shifts during brain ischemia.

Research paper thumbnail of Large extracellular space leads to neuronal susceptibility to ischemic injury in a Na+/K + pumps–dependent manner

Journal of Computational Neuroscience, Feb 6, 2016

The extent of anoxic depolarization (AD), the initial electrophysiological event during ischemia,... more The extent of anoxic depolarization (AD), the initial electrophysiological event during ischemia, determines the degree of brain region-specific neuronal damage. Neurons in higher brain regions exhibiting nonreversible, strong AD are more susceptible to ischemic injury as compared to cells in lower brain regions that exhibit reversible, weak AD. While the contrasting ADs in different brain regions in response to oxygen-glucose deprivation (OGD) is well established, the mechanism leading to such differences is not clear. Here we use computational modeling to elucidate the mechanism behind the brain region-specific recovery from AD. Our extended Hodgkin-Huxley (HH) framework consisting of neural spiking dynamics, processes of ion accumulation, and ion homeostatic mechanisms unveils that glial-vascular K + clearance and Na + /K +-exchange pumps are key to the cell's recovery from AD. Our phase space analysis reveals that the large extracellular space in the upper brain regions leads to impaired Na + /K +-exchange pumps so that they function at lower than normal capacity and are unable to bring the cell out of AD after oxygen and glucose is restored.

Research paper thumbnail of Spreading Depression Expands Traumatic Injury in Neocortical Brain Slices

Journal of Neurotrauma, Feb 1, 2005

Traumatic brain injury (TBI) is particularly common in young people, generating healthcare costs ... more Traumatic brain injury (TBI) is particularly common in young people, generating healthcare costs that can span decades. The cellular processes activated in the first minutes following injury are poorly understood, and the 3-4 h following trauma are crucial for reducing subsequent injury. Spreading depression (SD) is a profound inactivation of neurons and glia lasting 1-2 min that arises focally and migrates outward across gray matter. In the hours following focal stroke, the metabolic stress of energy reduction and recurring SD-like events (peri-infarct depolarizations, PIDs) interact to promote neuronal injury. Similar recurring depolarizations might evolve immediately following TBI and exacerbate neuronal damage peripheral to the impact site. To test this possibility and examine if certain drugs might limit damage by inhibiting what we term traumatic spreading depression (tSD), we developed a technique whereby a small weight was dropped onto a live slice of rat neocortex while imaging changes in light transmittance (LT). Imaging revealed a propagating front of increased LT arising at the border of the impact site. Traumatic SD significantly expanded the region of ensuing damage. Both tSD and subsequent damage were blocked by the NMDA receptor antagonist MK-801 (100 microM) or the sigma-1 receptor (sigma1R) ligands dextromethorphan (30 microM) or BD-1063 (100 microM). Co-application of the sigma1R antagonist (+)3-PPP with DM reversed the block as did lowering temperature from 35 degrees C to 32 degrees C. This study provides evidence that an event similar to peri-infarct depolarization can arise from an injury site in neocortex within seconds following impact and act to expand the region of acute neuronal damage.

Research paper thumbnail of CA3 neuron excitation and epileptiform discharge are sensitive to osmolality

Journal of Neurophysiology, Jun 1, 1993

1. The clinical signs of rapidly developing overhydration commonly include generalized tonic-clon... more 1. The clinical signs of rapidly developing overhydration commonly include generalized tonic-clonic seizure, which can be combatted by raising plasma osmolality. How cortical neurons respond to osmotic imbalance has been addressed only recently. In the CA3 cell region of hippocampal slices, lowered osmolality (-40 mOsm) rapidly swelled cells, increasing field potential amplitude over a period of 8 min and thereby elevating field effects and associated neuronal synchronization. 2. Over a longer time course (10-30 min), spontaneous excitatory postsynaptic potential (EPSP) amplitude gradually increased in 7 of 10 CA3 neurons recorded intracellularly. In nine additional CA3 cells, hyposmolality gradually induced combinations of action potential discharge, endogenous bursting, and increased synchronized synaptic input. All of these effects reversed in normosmotic ACSF. 3. Hyperosmotic artificial cerebrospinal fluid (ACSF) using mannitol reduced field potentials and dramatically lowered CA3 excitability by reducing spontaneous EPSP amplitude and associated bursting. Again, the gradual onset (10-30 min) of changes in spontaneous EPSP amplitude appeared independent of field potential changes, which were already maximal by 8 min. 4. Cutting mossy fibers did not affect the excitability changes induced by osmotic stress noted above. The EPSP/inhibitory postsynaptic potential (IPSP) sequence evoked from mossy fibers or stratum oriens was unaltered by osmotic change and so did not represent osmosensitive afferent input to CA3 neurons. Furthermore, as measured at the soma, resting membrane potential, cell input resistance, and the action potential threshold were unchanged in all cells. It followed that, because the CA3 neurons themselves were not responsive, a recurrent excitatory pathway could not represent the osmosensitive input.(ABSTRACT TRUNCATED AT 250 WORDS)

Research paper thumbnail of Endogenous bursting by rat supraoptic neuroendocrine cells is calcium dependent

The Journal of Physiology, Mar 1, 1987

1. Phasic bursting by magnocellular neuroendocrine cells (m.n.c.s) in vivo causes increased vasop... more 1. Phasic bursting by magnocellular neuroendocrine cells (m.n.c.s) in vivo causes increased vasopressin release from axon terminals in the neurohypophysis. In the supraoptic nucleus of the coronal hypothalamic slice thirty-two of sixty-five m.n.c.s recorded intracellularly displayed repetitive bursting, either spontaneously or during a low level of tonic current injection. 2. Of the thirty-two repetitive bursters, twenty-four received no apparent patterned synaptic input and the phasic burst behaviour was voltage dependent. The evidence for these cells being bursting pace-makers and the underlying mechanism driving bursting were further investigated. 3. Phasic bursting by m.n.c.s is usually contingent upon two depolarizing events: a slow depolarization (s.d.) between bursts that brings the membrane potential to burst threshold, and the spike depolarizing after-potential (d.a.p.). One or several d.a.p.s can initiate a burst by summing to form a plateau potential which sustains firing. 4. Of eight phasic cells exposed to tetrodotoxin (TTX) and tonically depolarized with current injection, two cells retained the phasic burst pattern and underlying plateau potentials. Of the remaining six cells in TTX, three of four cells tested regained phasic firing with plateau potentials following the addition of Sr2+, a Ca2+ agonist. Evoked post-synaptic potentials were demonstrably blocked throughout TTX exposure, firmly establishing that some m.n.c.s are bursting pace-makers. 5. The s.d., d.a.p. and plateau potential were retained in TTX or low-Na+ saline, augmented in Sr2+ and blocked in low-Ca2+ saline. All three events were activated at membrane potentials depolarized from -70 mV but steadily inactivated with increasing hyperpolarization to -90 mV. The s.d. and d.a.p. apparently represented partial activation of the same process that drives a burst, the plateau potential. 6. Hyperpolarizing pulses of constant current revealed an apparent decrease in cell conductance underlying the s.d., d.a.p. and plateau potential which was not due to membrane rectification. The plateau potential was reduced in low Na+ and eliminated in low Ca2+. However, it remained relatively unaffected by altering the external K+ concentration and it did not reverse below -90 mV, suggesting a less important role for K+ movement relative to Ca2+ or Na+. A hyperpolarizing pulse during the s.d., d.a.p. or plateau potential probably momentarily inactivated inward Ca2+ current, causing the apparent conductance decrease.(ABSTRACT TRUNCATED AT 400 WORDS)

Research paper thumbnail of Intrinsic inhibition in magnocellular neuroendocrine cells of rat hypothalamus

The Journal of Physiology, Aug 1, 1984

1. Endogenous mechanisms of inhibition in magnocellular neuroendocrine cells were studied with in... more 1. Endogenous mechanisms of inhibition in magnocellular neuroendocrine cells were studied with intracellular recordings in the rat hypothalamic slice preparation. 2. Hyperpolarizing after-potentials (duration up to 125 ms) followed single action potentials and after-hyperpolarizations (a.h.p.s) lasting hundreds of milliseconds followed brief evoked spike trains. The amplitude and duration of the a.h.p. increased after spike trains of longer duration or higher frequency. 3. The a.h.p. appears endogenous, rather than synaptically mediated from recurrent inhibition, because it persisted after pharmacological blockade of axonal conduction or of chemical synaptic transmission. 4. The reversal potential of the a.h.p. was at least 20 mV more negative than that of inhibitory post-synaptic potentials. Cl-ionophoresis did not alter the a.h.p. Chelation of intracellular Ca2+ with EGTA injection eliminated the a.h.p. 5. A Ca2+-activated K+ conductance, rather than recurrent synaptic inhibition, apparently causes the a.h.p. and is at least partly responsible for the inhibition after single spikes in magnocellular neurones. During hormone release, this endogenous mechanism may contribute to the post-burst silent period in putative oxytocinergic cells and to the interburst interval in phasic neurones, which are known to fire repetitive bursts associated with vasopressin release.

Research paper thumbnail of Seizure and acute osmotic change: Clinical and neurophysiological aspects

Journal of the Neurological Sciences, 1991

There are a number of clinical situations where overhydration may occur. If the reduction in plas... more There are a number of clinical situations where overhydration may occur. If the reduction in plasma osmolality is acute, passive water influx swells brain cells, shrinking the extracellular space around them. It is during this time that susceptibility to generalized tonic-clonic seizure dramatically increases. Common clinical examples include hastened rehydration therapy, the dialysis disequilibrium syndrome, compulsive polydipsia, the syndrome of inappropriate ADH secretion (SIADH) and post-TURP syndrome. Treatments that tend to restore normal cellular volume (dehydration, mannitol infusion) help protect against this form of seizure. Support for a correlation between plasma osmolality and seizure susceptibility is scattered amongst the literature of several medical disciplines and spans almost 70 years. However a cellular basis to explain how overhydration might promote epileptiform activity has been examined only recently. The neocortical and hippocampal brain slice preparations permit an examination of how acute osmotic change alters cortical excitability independent of vascular damage, brain compression or other factors secondary to brain swelling. Electrophysiological evidence indicates that hyposmolality promotes epileptiform activity by strengthening both excitatory synaptic communication in neocortex and field effects among the entire cortical population. Moreover there is little evidence that associated hyponatremia in itself leads to increased CNS excitability. Such findings help in understanding how rapid lowering of plasma osmolality in clinical situations can promote the hyperexcitability associated with generalized tonic-clonic seizure.

Research paper thumbnail of Glucose concentration inversely alters neocortical slice excitability through an osmotic effect

Brain Research, Jul 1, 1991

Neurological problems can develop when blood glucose levels rapidly rise or fall yet there has be... more Neurological problems can develop when blood glucose levels rapidly rise or fall yet there has been little experimentation at a cellular level to assess how neurophysiological change may be induced. Using intracellular recording in the rat neocortical slice preparation, we examined pyramidal neurons of layers II-III as saline D-glucose concentration was altered. Single cell properties, synaptic transmission and epileptiform discharges were studied in control saline and compared with corresponding data when osmolality was raised with o-glucose by 20-80 mOsm. Although single cell properties were not significantly altered, the amplitude of evoked early and late EPSPs were proportionally reduced within 5 min. A similar but more pronounced effect was observed with mannitol, whereas freely permeable dimethylsulfoxide (DMSO) was without effect. Hyposmolality increased the amplitude of evoked early and late EPSPs. Therefore the dampening of synaptic transmission by D-glucose appears osmotic in origin. D-glucose was osmotically effective only above 30 mM probably because it is cell-permeable at lower concentrations. In slices made epileptogenic by Mg2+-free saline, increasing D-glucose decreased the frequency and increased the duration of interietal bursts. Again mannitol mimicked the glucose effects and hyposmotic change gave opposing responses. The inverse relation between glucose concentration and neocortieal excitability correlates well with clinical observations that an acute reduction in blood glucose from hyperglycemic levels (as follows insulin over-administration) can evoke generalized seizure.

Research paper thumbnail of Neural shutdown under stress: an evolutionary perspective on spreading depolarization

Journal of Neurophysiology, Mar 1, 2020

Neural function depends on maintaining cellular membrane potentials as the basis for electrical s... more Neural function depends on maintaining cellular membrane potentials as the basis for electrical signaling. Yet, in mammals and insects, neuronal and glial membrane potentials can reversibly depolarize to zero, shutting down neural function by the process of spreading depolarization (SD) that collapses the ion gradients across membranes. SD is not evident in all metazoan taxa with centralized nervous systems. We consider the occurrence and similarities of SD in different animals and suggest that it is an emergent property of nervous systems that have evolved to control complex behaviors requiring energetically expensive, rapid information processing in a tightly regulated extracellular environment. Whether SD is beneficial or not in mammals remains an open question. However, in insects, it is associated with the response to harsh environments and may provide an energetic advantage that improves the chances of survival. The remarkable similarity of SD in diverse taxa supports a model systems approach to understanding the mechanistic underpinning of human neuropathology associated with migraine, stroke, and traumatic brain injury.

Research paper thumbnail of Spike broadening in magnocellular neuroendocrine cells of rat hypothalamic slices

Brain Research, May 1, 1985

Research paper thumbnail of Burst Discharge in Mammalian Neuroendocrine Cells Involves an Intrinsic Regenerative Mechanism

Science, Sep 9, 1983

ureotelic metabolism can occur even among embryos developing in fully cleidoic eggs, which do not... more ureotelic metabolism can occur even among embryos developing in fully cleidoic eggs, which do not take up supplemental water from the environment. Retention of ureotely in this case cannot be explained by suggesting that embryonic softshell turtles lack the biochemical preadaptations necessary for uricotely (18) because they have the capacity to form urate (Fig. 1). Thus, our results raise the possibility that uricotely is not a necessary outcome of natural selection for mechanisms to conserve water during embryonic development (19) and indicate a need to reassess the adaptive significance of uricotely among embryos of other terrestrial vertebrates.

Research paper thumbnail of Synaptic inputs and action potentials of magnocellular neuropeptidergic cells: Intracellular recording and staining in slices of rat hypothalamus

Brain Research Bulletin, 1982

inpufs and actiun ~~fe~t~ff~~ of ~~gn~celi~~ar ~e~ro~ept~dergic cells: ~ntr~ce~i~lar recording an... more inpufs and actiun ~~fe~t~ff~~ of ~~gn~celi~~ar ~e~ro~ept~dergic cells: ~ntr~ce~i~lar recording and staining in slices of rat hypothalamus. BRAIN RES. BULL. S(1) 87-93, 1982.~-Excitatory postsynaptic potentials (EPSPs) and action potentials of magnocellular neuropep

Research paper thumbnail of Ultrastructure of neurosecretory granule exocytosis by crayfish sinus gland induced with ionic manipulations

Journal of Morphology, Sep 1, 1976

Neurosecretory granule exocytosis from axon terminals in the crayfish (Orconectes uirilis) sinus ... more Neurosecretory granule exocytosis from axon terminals in the crayfish (Orconectes uirilis) sinus gland can be effected by alterations in intracellular ionic concentrations through modification of the bathing medium or b y electrical stimulation. This may result in increased numbers of exocytotic profiles involving one granule (single exocytosis) or several granules (compound exocytosis), which we interpret as evidence of stimulated neurosecretion. Our results suggest that increased free intracellular Ca++ together with a decrease in intracellular K+ are prerequisite for and sufficient to elicit increased exocytosis from sinus gland terminals. Similar release, stimulated by horseradish peroxidase, indicates that local surface permeability changes are involved. A minor response to high-K'-Ringer, and secretion via compound exocytosis, are uncharacteristic of cells with a neural ancestry. Exocytosis is generally accepted as the mechanism whereby cells export membrane-packaged secretory products. Confirmation of this has come from biochemical and ultrastructural studies on such diverse cell types as salivary, mast, chromaffin, neuronal and neurosecretory, as outlined by Douglas ('74). In each type, calcium is essential for product release (see review by Rubin, '701, acting as a "stimulus-secretion coupler" (Douglas and Rubin, '61). Although Na' and K fluxes are major events in membrane depolarization, an increased level of intracellular calcium is essential for release of secretion granules from chromaffin cells (Douglas et al., '67; Douglas and Poisner, '67) and neurohypophysial axon endings (Douglas and Poisner, '64). Neurons appear to act similarly (see review by Hubbard, '70). Some investigation of stimulus-secretion coupling in crustaceans has been carried out on the crab pericardial organ. Electrical stimulation of these neurosecretory cells elicits a Ca+ +-dependent release of cardioexcitor hormone as determined by increased contraction of the heart in vitro (Berlind and Cooke, '68, '71).

Research paper thumbnail of Magnetic resonance imaging of neuronal and glial swelling as an indicator of function in cerebral tissue slices

Magnetic Resonance in Medicine, 2008

Here we demonstrate a new basis of signal change in magnetic resonance imaging (MRI) related to n... more Here we demonstrate a new basis of signal change in magnetic resonance imaging (MRI) related to neuronal function, independent of blood oxygenation or flow. Time series MRI data acquired from living, superfused brain slices of adult rats revealed that the signal intensity reversibly increased with depolarization evoked by briefly elevating extracellular K+. This was presumably a consequence of increased tissue water in the intracellular compartment. Reversible increases in light transmittance (LT) demonstrating a similar time course in response to K+ elevation supported cellular swelling as generating the MRI signal intensity changes. This was confirmed by reversibly swelling cells in the slice under hypoosmotic challenge, which increased both MRI and LT signals with an identical time course. Conversely, shrinking cells under hyperosmotic challenge reversibly decreased the MRI and LT signals. We propose that specific MRI of neuronal function (fMRI) signals detected under identical parameters during predominantly proton‐density‐weighted fMRI of the spinal cord can now be explained by neuronal and glial swelling in activated central nervous system (CNS) regions. These observations demonstrate the biophysical basis of the fMRI contrast mechanism that has been termed “signal enhancement by extravascular water protons,” or SEEP. Magn Reson Med 59:700–706, 2008. © 2008 Wiley‐Liss, Inc.

Research paper thumbnail of Mechanisms of spreading depolarization in vertebrate and insect central nervous systems

Journal of Neurophysiology, Sep 1, 2016

Armstrong GA, Rodgers CI, Money TG, Robertson RM. Suppression of spreading depression-like events... more Armstrong GA, Rodgers CI, Money TG, Robertson RM. Suppression of spreading depression-like events in locusts by inhibition of the NO/cGMP/ PKG pathway. J Neurosci 29: 8225-8235, 2009. Armstrong GA, Rodriguez EC, Meldrum RR. Cold hardening modulates K ϩ homeostasis in the brain of Drosophila melanogaster during chill coma.

Research paper thumbnail of <b>Spreading depression determines acute cellular damage in the hippocampal slice during oxygen/glucose deprivation</b>

European Journal of Neuroscience, Nov 1, 1998

During ischaemia neurons depolarize and release the neurotransmitter l‐glutamate, which accumulat... more 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 °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 °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.

Research paper thumbnail of Structure and innervation of a crustacean neurosecretory cell

Canadian Journal of Zoology, Mar 1, 1978

The crayfish medulla terminalis X organ contains type 1 neurosecretory somata which are distingui... more The crayfish medulla terminalis X organ contains type 1 neurosecretory somata which are distinguishable by size, location, and ultrastructure and which have homologues in the eyestalks of other decapods. Neurosecretory (NS) vesicles (140 nm diameter) in these somata are structurally similar to those in the X organ–sinus gland (Xo-sg) tract and the neurohemal sinus gland. Cobalt iontophoresis into type 1 somata shows that their axons traverse the underlying neuropil of the medulla terminalis as the Xo-sg tract before emerging to the surface of the ganglion and terminating in the sinus gland. The proximal third of these type 1 axons branches extensively in the ganglionic neuropil; only in the neuropil adjacent to Xo-sg tract does electron microscopy reveal many fine postsynaptic neurites containing NS vesicles. Serial thin sections show continuity between these neurites and the Xo-sg tract. Structural evidence suggests that this fine arborization is dendritic and represents the site of informational input from the central nervous system to the Xo-sg complex. This study supports previous electrophysiological recordings within Xo somata which indicate synaptic input to these neuroendocrine cells.

Research paper thumbnail of Validating MALDI-IMS Feasibility in <i>Ex Vivo</i> Brain Slices

Journal of the American Society for Mass Spectrometry, Jul 20, 2023

Research paper thumbnail of Expression of Neuronal Na+/K+-ATPase α Subunit Isoforms in the Mouse Brain Following Genetically Programmed or Behaviourally-induced Oxidative Stress

Neuroscience, Aug 1, 2020

The Na+/K+-ATPase is a transmembrane ion pump that has a critical homeostatic role within every m... more The Na+/K+-ATPase is a transmembrane ion pump that has a critical homeostatic role within every mammalian cell; however, it is vulnerable to the effects of increased oxidative stress. Understanding how expression of this transporter is influenced by oxidative stress may yield insight into its role in the pathophysiology of neurological and neuropsychiatric diseases. In this study we investigated whether increased oxidative stress could influence Na+/K+-ATPase expression in various brain regions of mice. We utilized two different models of oxidative stress: a behavioural chronic unpredictable stress protocol and the Aldh2-/- mouse model of oxidative stress-based and age-related cognitive impairment. We identified distinct regional baseline mRNA and protein expression patterns of the Na+/K+-ATPase α1 and α3 isoforms within the neocortex, hippocampus, and brainstem of wildtype mice. Consistent with previous studies, there was a higher proportion of α3 expression relative to α1 in the brainstem versus neocortex, but a higher proportion of α1 expression relative to α3 in the neocortex versus the brainstem. The hippocampus had similar expression levels of both α1 and α3. Despite increased staining for oxidative stress in higher brain, no differences in α1 or α3 expression were noted in Aldh2-/- mice versus wildtype, or in mice exposed to a 28-day chronic unpredictable stress protocol. In both models of oxidative stress, gene and protein expression of Na+/K+-ATPase α1 and α3 isoforms within the higher and lower brain was remarkably stable. Thus, Na+/K+-ATPase function previously reported as altered by oxidative stress is not through induced changes in the expression of pump isoforms.

Research paper thumbnail of Morphometric Analysis of Hippocampal and Neocortical Pyramidal Neurons in a Mouse Model of Late Onset Alzheimer’s Disease

Journal of Alzheimer's Disease, Apr 21, 2020

The study of late-onset (sporadic) Alzheimer's disease (LOAD) has lacked animal models where impa... more The study of late-onset (sporadic) Alzheimer's disease (LOAD) has lacked animal models where impairments develop with aging. Oxidative stress promotes LOAD, so we have developed an oxidative stress-based model of age-related cognitive impairment based on gene deletion of aldehyde dehydrogenase 2 (ALDH2). This enzyme is important for the detoxification of endogenous aldehydes arising from lipid peroxidation. Compared to wildtype (WT) mice, the knockout (KO) mice exhibit a progressive decline in recognition and spatial memory and AD-like pathologies. Here we performed morphometric analyses in the dorsal and ventral hippocampal CA1 regions (dCA1 and vCA1) as well as in overlying primary sensory cortex to determine if altered neuronal structure can help account for the cognitive impairment in 12-month old KO mice. Dendritic morphology was quantitatively analyzed following Golgi-Cox staining using 9 WT mice (108 neurons) and 15 KO mice (180 neurons). Four pyramidal neurons were traced per mouse in each region, followed by branched structured analysis and Sholl analysis. Compared to WT controls, the morphology and complexity of dCA1 pyramidal neurons from KOs showed significant reductions in apical and basal dendritic length, dendrite intersections, ends, and nodes. As well, spine density along dorsal CA1 apical dendrites was significantly lower in KO versus WT. In contrast, pyramidal arborization in the vCA1 and primary sensory cortex were only minimally reduced in KO versus WT mice. These data suggest a region-specific vulnerability to oxidative stress-induced damage and/or a major and specific reduction in synaptic input to the pyramidal neurons of the dorsal hippocampus. This is in keeping with studies showing that lesions to the dorsal hippocampus impair primarily cognitive memory whereas ventral hippocampal lesions cause deficits in stress, emotion, and affect.

Research paper thumbnail of Neuronal Swelling: A Non-osmotic Consequence of Spreading Depolarization

Neurocritical Care, Sep 8, 2021

An acute reduction in plasma osmolality causes rapid uptake of water by astrocytes but not by neu... more An acute reduction in plasma osmolality causes rapid uptake of water by astrocytes but not by neurons, whereas both cell types swell as a consequence of lost blood flow (ischemia). Either hypoosmolality or ischemia can displace the brain downwards, potentially causing death. However, these disorders are fundamentally different at the cellular level. Astrocytes osmotically swell or shrink because they express functional water channels (aquaporins), whereas neurons lack functional aquaporins and thus maintain their volume. Yet both neurons and astrocytes immediately swell when blood flow to the brain is compromised (cytotoxic edema) as following stroke onset, sudden cardiac arrest, or traumatic brain injury. In each situation, neuronal swelling is the direct result of spreading depolarization (SD) generated when the ATP-dependent sodium/potassium ATPase (the Na + /K + pump) is compromised. The simple, and incorrect, textbook explanation for neuronal swelling is that increased Na + influx passively draws Cl − into the cell, with water following by osmosis via some unknown conduit. We first review the strong evidence that mammalian neurons resist volume change during acute osmotic stress. We then contrast this with their dramatic swelling during ischemia. Counter-intuitively, recent research argues that ischemic swelling of neurons is non-osmotic, involving ion/ water cotransporters as well as at least one known amino acid water pump. While incompletely understood, these mechanisms argue against the dogma that neuronal swelling involves water uptake driven by an osmotic gradient with aquaporins as the conduit. Promoting clinical recovery from neuronal cytotoxic edema evoked by spreading depolarizations requires a far better understanding of molecular water pumps and ion/water cotransporters that act to rebalance water shifts during brain ischemia.

Research paper thumbnail of Large extracellular space leads to neuronal susceptibility to ischemic injury in a Na+/K + pumps–dependent manner

Journal of Computational Neuroscience, Feb 6, 2016

The extent of anoxic depolarization (AD), the initial electrophysiological event during ischemia,... more The extent of anoxic depolarization (AD), the initial electrophysiological event during ischemia, determines the degree of brain region-specific neuronal damage. Neurons in higher brain regions exhibiting nonreversible, strong AD are more susceptible to ischemic injury as compared to cells in lower brain regions that exhibit reversible, weak AD. While the contrasting ADs in different brain regions in response to oxygen-glucose deprivation (OGD) is well established, the mechanism leading to such differences is not clear. Here we use computational modeling to elucidate the mechanism behind the brain region-specific recovery from AD. Our extended Hodgkin-Huxley (HH) framework consisting of neural spiking dynamics, processes of ion accumulation, and ion homeostatic mechanisms unveils that glial-vascular K + clearance and Na + /K +-exchange pumps are key to the cell's recovery from AD. Our phase space analysis reveals that the large extracellular space in the upper brain regions leads to impaired Na + /K +-exchange pumps so that they function at lower than normal capacity and are unable to bring the cell out of AD after oxygen and glucose is restored.

Research paper thumbnail of Spreading Depression Expands Traumatic Injury in Neocortical Brain Slices

Journal of Neurotrauma, Feb 1, 2005

Traumatic brain injury (TBI) is particularly common in young people, generating healthcare costs ... more Traumatic brain injury (TBI) is particularly common in young people, generating healthcare costs that can span decades. The cellular processes activated in the first minutes following injury are poorly understood, and the 3-4 h following trauma are crucial for reducing subsequent injury. Spreading depression (SD) is a profound inactivation of neurons and glia lasting 1-2 min that arises focally and migrates outward across gray matter. In the hours following focal stroke, the metabolic stress of energy reduction and recurring SD-like events (peri-infarct depolarizations, PIDs) interact to promote neuronal injury. Similar recurring depolarizations might evolve immediately following TBI and exacerbate neuronal damage peripheral to the impact site. To test this possibility and examine if certain drugs might limit damage by inhibiting what we term traumatic spreading depression (tSD), we developed a technique whereby a small weight was dropped onto a live slice of rat neocortex while imaging changes in light transmittance (LT). Imaging revealed a propagating front of increased LT arising at the border of the impact site. Traumatic SD significantly expanded the region of ensuing damage. Both tSD and subsequent damage were blocked by the NMDA receptor antagonist MK-801 (100 microM) or the sigma-1 receptor (sigma1R) ligands dextromethorphan (30 microM) or BD-1063 (100 microM). Co-application of the sigma1R antagonist (+)3-PPP with DM reversed the block as did lowering temperature from 35 degrees C to 32 degrees C. This study provides evidence that an event similar to peri-infarct depolarization can arise from an injury site in neocortex within seconds following impact and act to expand the region of acute neuronal damage.

Research paper thumbnail of CA3 neuron excitation and epileptiform discharge are sensitive to osmolality

Journal of Neurophysiology, Jun 1, 1993

1. The clinical signs of rapidly developing overhydration commonly include generalized tonic-clon... more 1. The clinical signs of rapidly developing overhydration commonly include generalized tonic-clonic seizure, which can be combatted by raising plasma osmolality. How cortical neurons respond to osmotic imbalance has been addressed only recently. In the CA3 cell region of hippocampal slices, lowered osmolality (-40 mOsm) rapidly swelled cells, increasing field potential amplitude over a period of 8 min and thereby elevating field effects and associated neuronal synchronization. 2. Over a longer time course (10-30 min), spontaneous excitatory postsynaptic potential (EPSP) amplitude gradually increased in 7 of 10 CA3 neurons recorded intracellularly. In nine additional CA3 cells, hyposmolality gradually induced combinations of action potential discharge, endogenous bursting, and increased synchronized synaptic input. All of these effects reversed in normosmotic ACSF. 3. Hyperosmotic artificial cerebrospinal fluid (ACSF) using mannitol reduced field potentials and dramatically lowered CA3 excitability by reducing spontaneous EPSP amplitude and associated bursting. Again, the gradual onset (10-30 min) of changes in spontaneous EPSP amplitude appeared independent of field potential changes, which were already maximal by 8 min. 4. Cutting mossy fibers did not affect the excitability changes induced by osmotic stress noted above. The EPSP/inhibitory postsynaptic potential (IPSP) sequence evoked from mossy fibers or stratum oriens was unaltered by osmotic change and so did not represent osmosensitive afferent input to CA3 neurons. Furthermore, as measured at the soma, resting membrane potential, cell input resistance, and the action potential threshold were unchanged in all cells. It followed that, because the CA3 neurons themselves were not responsive, a recurrent excitatory pathway could not represent the osmosensitive input.(ABSTRACT TRUNCATED AT 250 WORDS)

Research paper thumbnail of Endogenous bursting by rat supraoptic neuroendocrine cells is calcium dependent

The Journal of Physiology, Mar 1, 1987

1. Phasic bursting by magnocellular neuroendocrine cells (m.n.c.s) in vivo causes increased vasop... more 1. Phasic bursting by magnocellular neuroendocrine cells (m.n.c.s) in vivo causes increased vasopressin release from axon terminals in the neurohypophysis. In the supraoptic nucleus of the coronal hypothalamic slice thirty-two of sixty-five m.n.c.s recorded intracellularly displayed repetitive bursting, either spontaneously or during a low level of tonic current injection. 2. Of the thirty-two repetitive bursters, twenty-four received no apparent patterned synaptic input and the phasic burst behaviour was voltage dependent. The evidence for these cells being bursting pace-makers and the underlying mechanism driving bursting were further investigated. 3. Phasic bursting by m.n.c.s is usually contingent upon two depolarizing events: a slow depolarization (s.d.) between bursts that brings the membrane potential to burst threshold, and the spike depolarizing after-potential (d.a.p.). One or several d.a.p.s can initiate a burst by summing to form a plateau potential which sustains firing. 4. Of eight phasic cells exposed to tetrodotoxin (TTX) and tonically depolarized with current injection, two cells retained the phasic burst pattern and underlying plateau potentials. Of the remaining six cells in TTX, three of four cells tested regained phasic firing with plateau potentials following the addition of Sr2+, a Ca2+ agonist. Evoked post-synaptic potentials were demonstrably blocked throughout TTX exposure, firmly establishing that some m.n.c.s are bursting pace-makers. 5. The s.d., d.a.p. and plateau potential were retained in TTX or low-Na+ saline, augmented in Sr2+ and blocked in low-Ca2+ saline. All three events were activated at membrane potentials depolarized from -70 mV but steadily inactivated with increasing hyperpolarization to -90 mV. The s.d. and d.a.p. apparently represented partial activation of the same process that drives a burst, the plateau potential. 6. Hyperpolarizing pulses of constant current revealed an apparent decrease in cell conductance underlying the s.d., d.a.p. and plateau potential which was not due to membrane rectification. The plateau potential was reduced in low Na+ and eliminated in low Ca2+. However, it remained relatively unaffected by altering the external K+ concentration and it did not reverse below -90 mV, suggesting a less important role for K+ movement relative to Ca2+ or Na+. A hyperpolarizing pulse during the s.d., d.a.p. or plateau potential probably momentarily inactivated inward Ca2+ current, causing the apparent conductance decrease.(ABSTRACT TRUNCATED AT 400 WORDS)

Research paper thumbnail of Intrinsic inhibition in magnocellular neuroendocrine cells of rat hypothalamus

The Journal of Physiology, Aug 1, 1984

1. Endogenous mechanisms of inhibition in magnocellular neuroendocrine cells were studied with in... more 1. Endogenous mechanisms of inhibition in magnocellular neuroendocrine cells were studied with intracellular recordings in the rat hypothalamic slice preparation. 2. Hyperpolarizing after-potentials (duration up to 125 ms) followed single action potentials and after-hyperpolarizations (a.h.p.s) lasting hundreds of milliseconds followed brief evoked spike trains. The amplitude and duration of the a.h.p. increased after spike trains of longer duration or higher frequency. 3. The a.h.p. appears endogenous, rather than synaptically mediated from recurrent inhibition, because it persisted after pharmacological blockade of axonal conduction or of chemical synaptic transmission. 4. The reversal potential of the a.h.p. was at least 20 mV more negative than that of inhibitory post-synaptic potentials. Cl-ionophoresis did not alter the a.h.p. Chelation of intracellular Ca2+ with EGTA injection eliminated the a.h.p. 5. A Ca2+-activated K+ conductance, rather than recurrent synaptic inhibition, apparently causes the a.h.p. and is at least partly responsible for the inhibition after single spikes in magnocellular neurones. During hormone release, this endogenous mechanism may contribute to the post-burst silent period in putative oxytocinergic cells and to the interburst interval in phasic neurones, which are known to fire repetitive bursts associated with vasopressin release.

Research paper thumbnail of Seizure and acute osmotic change: Clinical and neurophysiological aspects

Journal of the Neurological Sciences, 1991

There are a number of clinical situations where overhydration may occur. If the reduction in plas... more There are a number of clinical situations where overhydration may occur. If the reduction in plasma osmolality is acute, passive water influx swells brain cells, shrinking the extracellular space around them. It is during this time that susceptibility to generalized tonic-clonic seizure dramatically increases. Common clinical examples include hastened rehydration therapy, the dialysis disequilibrium syndrome, compulsive polydipsia, the syndrome of inappropriate ADH secretion (SIADH) and post-TURP syndrome. Treatments that tend to restore normal cellular volume (dehydration, mannitol infusion) help protect against this form of seizure. Support for a correlation between plasma osmolality and seizure susceptibility is scattered amongst the literature of several medical disciplines and spans almost 70 years. However a cellular basis to explain how overhydration might promote epileptiform activity has been examined only recently. The neocortical and hippocampal brain slice preparations permit an examination of how acute osmotic change alters cortical excitability independent of vascular damage, brain compression or other factors secondary to brain swelling. Electrophysiological evidence indicates that hyposmolality promotes epileptiform activity by strengthening both excitatory synaptic communication in neocortex and field effects among the entire cortical population. Moreover there is little evidence that associated hyponatremia in itself leads to increased CNS excitability. Such findings help in understanding how rapid lowering of plasma osmolality in clinical situations can promote the hyperexcitability associated with generalized tonic-clonic seizure.

Research paper thumbnail of Glucose concentration inversely alters neocortical slice excitability through an osmotic effect

Brain Research, Jul 1, 1991

Neurological problems can develop when blood glucose levels rapidly rise or fall yet there has be... more Neurological problems can develop when blood glucose levels rapidly rise or fall yet there has been little experimentation at a cellular level to assess how neurophysiological change may be induced. Using intracellular recording in the rat neocortical slice preparation, we examined pyramidal neurons of layers II-III as saline D-glucose concentration was altered. Single cell properties, synaptic transmission and epileptiform discharges were studied in control saline and compared with corresponding data when osmolality was raised with o-glucose by 20-80 mOsm. Although single cell properties were not significantly altered, the amplitude of evoked early and late EPSPs were proportionally reduced within 5 min. A similar but more pronounced effect was observed with mannitol, whereas freely permeable dimethylsulfoxide (DMSO) was without effect. Hyposmolality increased the amplitude of evoked early and late EPSPs. Therefore the dampening of synaptic transmission by D-glucose appears osmotic in origin. D-glucose was osmotically effective only above 30 mM probably because it is cell-permeable at lower concentrations. In slices made epileptogenic by Mg2+-free saline, increasing D-glucose decreased the frequency and increased the duration of interietal bursts. Again mannitol mimicked the glucose effects and hyposmotic change gave opposing responses. The inverse relation between glucose concentration and neocortieal excitability correlates well with clinical observations that an acute reduction in blood glucose from hyperglycemic levels (as follows insulin over-administration) can evoke generalized seizure.

Research paper thumbnail of Neural shutdown under stress: an evolutionary perspective on spreading depolarization

Journal of Neurophysiology, Mar 1, 2020

Neural function depends on maintaining cellular membrane potentials as the basis for electrical s... more Neural function depends on maintaining cellular membrane potentials as the basis for electrical signaling. Yet, in mammals and insects, neuronal and glial membrane potentials can reversibly depolarize to zero, shutting down neural function by the process of spreading depolarization (SD) that collapses the ion gradients across membranes. SD is not evident in all metazoan taxa with centralized nervous systems. We consider the occurrence and similarities of SD in different animals and suggest that it is an emergent property of nervous systems that have evolved to control complex behaviors requiring energetically expensive, rapid information processing in a tightly regulated extracellular environment. Whether SD is beneficial or not in mammals remains an open question. However, in insects, it is associated with the response to harsh environments and may provide an energetic advantage that improves the chances of survival. The remarkable similarity of SD in diverse taxa supports a model systems approach to understanding the mechanistic underpinning of human neuropathology associated with migraine, stroke, and traumatic brain injury.

Research paper thumbnail of Spike broadening in magnocellular neuroendocrine cells of rat hypothalamic slices

Brain Research, May 1, 1985

Research paper thumbnail of Burst Discharge in Mammalian Neuroendocrine Cells Involves an Intrinsic Regenerative Mechanism

Science, Sep 9, 1983

ureotelic metabolism can occur even among embryos developing in fully cleidoic eggs, which do not... more ureotelic metabolism can occur even among embryos developing in fully cleidoic eggs, which do not take up supplemental water from the environment. Retention of ureotely in this case cannot be explained by suggesting that embryonic softshell turtles lack the biochemical preadaptations necessary for uricotely (18) because they have the capacity to form urate (Fig. 1). Thus, our results raise the possibility that uricotely is not a necessary outcome of natural selection for mechanisms to conserve water during embryonic development (19) and indicate a need to reassess the adaptive significance of uricotely among embryos of other terrestrial vertebrates.

Research paper thumbnail of Synaptic inputs and action potentials of magnocellular neuropeptidergic cells: Intracellular recording and staining in slices of rat hypothalamus

Brain Research Bulletin, 1982

inpufs and actiun ~~fe~t~ff~~ of ~~gn~celi~~ar ~e~ro~ept~dergic cells: ~ntr~ce~i~lar recording an... more inpufs and actiun ~~fe~t~ff~~ of ~~gn~celi~~ar ~e~ro~ept~dergic cells: ~ntr~ce~i~lar recording and staining in slices of rat hypothalamus. BRAIN RES. BULL. S(1) 87-93, 1982.~-Excitatory postsynaptic potentials (EPSPs) and action potentials of magnocellular neuropep

Research paper thumbnail of Ultrastructure of neurosecretory granule exocytosis by crayfish sinus gland induced with ionic manipulations

Journal of Morphology, Sep 1, 1976

Neurosecretory granule exocytosis from axon terminals in the crayfish (Orconectes uirilis) sinus ... more Neurosecretory granule exocytosis from axon terminals in the crayfish (Orconectes uirilis) sinus gland can be effected by alterations in intracellular ionic concentrations through modification of the bathing medium or b y electrical stimulation. This may result in increased numbers of exocytotic profiles involving one granule (single exocytosis) or several granules (compound exocytosis), which we interpret as evidence of stimulated neurosecretion. Our results suggest that increased free intracellular Ca++ together with a decrease in intracellular K+ are prerequisite for and sufficient to elicit increased exocytosis from sinus gland terminals. Similar release, stimulated by horseradish peroxidase, indicates that local surface permeability changes are involved. A minor response to high-K'-Ringer, and secretion via compound exocytosis, are uncharacteristic of cells with a neural ancestry. Exocytosis is generally accepted as the mechanism whereby cells export membrane-packaged secretory products. Confirmation of this has come from biochemical and ultrastructural studies on such diverse cell types as salivary, mast, chromaffin, neuronal and neurosecretory, as outlined by Douglas ('74). In each type, calcium is essential for product release (see review by Rubin, '701, acting as a "stimulus-secretion coupler" (Douglas and Rubin, '61). Although Na' and K fluxes are major events in membrane depolarization, an increased level of intracellular calcium is essential for release of secretion granules from chromaffin cells (Douglas et al., '67; Douglas and Poisner, '67) and neurohypophysial axon endings (Douglas and Poisner, '64). Neurons appear to act similarly (see review by Hubbard, '70). Some investigation of stimulus-secretion coupling in crustaceans has been carried out on the crab pericardial organ. Electrical stimulation of these neurosecretory cells elicits a Ca+ +-dependent release of cardioexcitor hormone as determined by increased contraction of the heart in vitro (Berlind and Cooke, '68, '71).

Research paper thumbnail of Magnetic resonance imaging of neuronal and glial swelling as an indicator of function in cerebral tissue slices

Magnetic Resonance in Medicine, 2008

Here we demonstrate a new basis of signal change in magnetic resonance imaging (MRI) related to n... more Here we demonstrate a new basis of signal change in magnetic resonance imaging (MRI) related to neuronal function, independent of blood oxygenation or flow. Time series MRI data acquired from living, superfused brain slices of adult rats revealed that the signal intensity reversibly increased with depolarization evoked by briefly elevating extracellular K+. This was presumably a consequence of increased tissue water in the intracellular compartment. Reversible increases in light transmittance (LT) demonstrating a similar time course in response to K+ elevation supported cellular swelling as generating the MRI signal intensity changes. This was confirmed by reversibly swelling cells in the slice under hypoosmotic challenge, which increased both MRI and LT signals with an identical time course. Conversely, shrinking cells under hyperosmotic challenge reversibly decreased the MRI and LT signals. We propose that specific MRI of neuronal function (fMRI) signals detected under identical parameters during predominantly proton‐density‐weighted fMRI of the spinal cord can now be explained by neuronal and glial swelling in activated central nervous system (CNS) regions. These observations demonstrate the biophysical basis of the fMRI contrast mechanism that has been termed “signal enhancement by extravascular water protons,” or SEEP. Magn Reson Med 59:700–706, 2008. © 2008 Wiley‐Liss, Inc.

Research paper thumbnail of Mechanisms of spreading depolarization in vertebrate and insect central nervous systems

Journal of Neurophysiology, Sep 1, 2016

Armstrong GA, Rodgers CI, Money TG, Robertson RM. Suppression of spreading depression-like events... more Armstrong GA, Rodgers CI, Money TG, Robertson RM. Suppression of spreading depression-like events in locusts by inhibition of the NO/cGMP/ PKG pathway. J Neurosci 29: 8225-8235, 2009. Armstrong GA, Rodriguez EC, Meldrum RR. Cold hardening modulates K ϩ homeostasis in the brain of Drosophila melanogaster during chill coma.

Research paper thumbnail of <b>Spreading depression determines acute cellular damage in the hippocampal slice during oxygen/glucose deprivation</b>

European Journal of Neuroscience, Nov 1, 1998

During ischaemia neurons depolarize and release the neurotransmitter l‐glutamate, which accumulat... more 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 °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 °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.