Ischaemia Flashcards
(42 cards)
why do we need O2 for brain?
the immediate danger of hypoxia lies in the fact that oxygen is intimately coupled to the generation of ATP through OP, ATP needed for eg Na pump, the means to control volume, internal environment etc; Another problem with hypoxia is that a halt in OP means that the respiratory chain will stop pumping H+ out of the mitochondria. Thus, not only the cells but also their mitochondria may lose the membrane potential and become depolarized. This leads to the activation of apoptosis so cells may be doomed to die even if O2 suppl restored before necrosis begins; brain particularly vulnerable due to high energy use rate (ATP turnover 10x higher than average body tissue), ATP turned over every 5-10s and depleted if O2 deprived after a few secs; he brain is likely to be the first organ to lose energy charge and
depolarize when an animal is exposed to severe hypoxia or anoxia. This has two consequences. First, necrotic and apoptotic processes will be initiated rapidly in the brain of an animal that has lost its oxygen supply. Secondly, the depol brain can no longer regulate its volume, and the brain cells will start to swell. For many vertebrates, this is a particular problem because there is simply no space in the cranial cavity to allow the brain to swell. Therefore, instead of an increase in brain volume, there will be an increase in pressure in
the cranial cavity, and when this pressure rises above the blood pressure, it is no longer possible for blood to reach the brain. Even with the best health care, this is usually an irreversible situation, and consequently a lack of blood circulation
in the brain is a principal legal sign of death in many countries; fish/cold blooded animals often have much bigger cranial cavity than brain so it can swell 10%+ and be fine; humans, unconsciousness occurs and electric activity is suppressed in the brain just 6-7 s after blood flow to the brain is halted, likely to be an initial emergency response, possibly functioning to save energy by reducing ATPconsuming electrical activity. Moreover, as a standing person faints and falls, the brain moves into a lower position in relation to the heart, which increases cerebral blood pressure and thereby cerebral blood flow
selective neuronal loss
ischaemic infarct is as an area of pannecrosis, i.e., necrosis of all cell types including neurons, glia, and endothelial cells. The histologic appearance of the fully developed infarct is defined as coagulative necrosis. Injury to neurons in the rim surrounding the infarct has been termed general neuronal necrosis, since the other cell types are relatively intact. Some recent authors talk about this vulnerability as “selective neuronal loss”; where there is necrosis, there will also be inflammation, and this phenomenon may act
as a secondary insult
in SNL tissue bulk and extracellular matrix is preserved; SNL may be present in rescued penumbra after stroke, which hinders early clinical benefit and dampens long-term periinfarct plasticity; SNL can be hard to study, antibodies against neurons best as stains all meaning clear gaps where loss, but can stain acute neurons that are dead but not yet phagocytosed - nissl stains etc dont detect neurons that have been phagocytosed but stains neuropil and glia so hard to detect gaps; clinical correlates of SNL are difficult to address because in the vast majority of patients there is an associated infarct, albeit
small in most; some studies underlined their patients’ excellent-to-full neurological recovery despite sometimes extensive cortical SNL
impeding neuronal activation; plastic processes may develop over time so that the neural
functions impaired by SNL are taken over by neighboring neurons/areas, and second that standard clinical stroke scales may not capture subtle cognitive or sensorimotor impairments, as
suggested in recent studies after minor stroke or transient ischemic attacks
mechanism(s) underlying SNL in the salvaged penumbra, including why neurons die but other cells do not, are incompletely understood. Even the time course of SNL, i.e. when exactly the neurons die after reperfusion, is not well known; very high energy consumption of neurons makes them more sensitive to oxygen/glucose deprivation than other CNS cells, and hence more susceptible to ischemic injury during the occlusion phase; conversely, their very high postreperfusion mitochondrial activity would make them prone to
oxygen radical production and damage. Also, glutamate excitotoxicity or glutamate/GABA imbalance are also more likely to affect neurons than glial cells. One major hypothesis therefore is that SNL reflects irreversible cell damage ignited during the period of severe ischemia, which, if true, would imply that the degree of SNL should be proportional to both the severity and duration of
hypoperfusion. The major alternative hypothesis is that SNL is secondary to detrimental processes triggered by reperfusion—socalled ‘reperfusion injury’—such as oxygen radicals or inflammatory
processes, particularly microglia activation. A dual mechanism, requiring first the ischemic insult and then secondary reperfusion injury, may also be at play; inflammation also in SNL and maybe neurotoxic cytokines have a role
SNL likely reduces the ‘neuronal reserve’ that normally mitigates the motor, gait, and cognitive effects of age-related brain injury including small strokes, white matter damage, and neurodegenerative pathology. In this sense, among other established vascular factors, SNL following stroke and perhaps also transient ischemic attacks may contribute to poststroke cognitive decline; SNL may also occur in remote areas after focal lesion, esp thalamus and midbrain, with mechanism unknown but underpinned by combo or antero and retrograde degen; As EPND (Exofocal postischemic neuronal cell death) develops in a delayed manner, while patients are recovering from their deficits, and is uncoupled from the maturation of the primary ischemic lesion, it may be a target for (secondary) neuroprotection. At present, however, it is not entirely clear whether EPND has any functional/clinical relevance; it becomes detectable on MRI/histologically days or weeks after stroke; cortical granular atrophy or watershed territory microemboli in the cerebral cortex distal to large cerebral arterial occlusion in ICA or MCA due to athrosclerosis and SNL may occur but has not been demonstrated in humans yet
neuropath of globlal ischaemia
Reversible global ischaemia results in the death of certain selectively vulnerable, or regionally vulnerable neuronal populations. For example, pyramidal neurons in the CA1 subfield of the hippocampus will die after only 5 to 10 minutes of global ischaemia, while neurons in the nearby CA3 region remain intact; neurons selectively vulnerable to ischaemia include cortical pyramidal neurons (middle laminae), cerebellar Purkinje cells, hippocampal CA1 pyramidal neurons and subpopulations in the amygdala, striatum, thalamus and brain stem nuclei; depends on ischaemic sensitivity (cell type, brain region), delay interval (time after cerebral recirculation), time for irreversible injury to evolve (delayed neuron degen)
characteristics of selective neuronal necrosis (SNN) have been defined as a series of histologic
changes encompassed by the term ischaemic cell change; at least 3 time related properties of SNN: ischaemic interval required for initiating irreversible injury (ischaemic sensitivity), the time after cerebral
recirculation before irreversible injury begins (delay interval), and the time it takes for irreversible injury to evolve once it has been initiated; ischaemic sensitivity varies - neurons in CA1
succumb to periods of depolarising ischaemia lasting as little as 3 minutes, while periods of 15 to 20 minutes of severe ischaemia are needed to kill medium-sized spiny neurons in the striatum; the
interval between cerebral microcirculation and the initiation of irreversible injury differs markedly among neuronal populations. The comparatively ischaemia-resistant medium-sized striatal neurons show signs of irreversible injury within 3 hours of recirculation, while the onset of irreversible injury to the ischaemiasensitive CA1 neurons is apparently delayed by 48 to 72 hours; (smith et al with CAl/4 after 4 mins, CA3 after 6, and caudoputamen + middle laminae after 8-10)
hippocampal lesions in global ischaemia
Neurons in the central nervous system have different vulnerability to ischemic injury in a quite inhomogeneous way; some parts of HC very vulnerable meanwhile DG is close by and can tolerate considerable degree of ischaemia; In 1880, Sommer reported that one-third of epileptic patients showed extensive loss of neurons in a circumscribed portion of the hippocampus. He hypothesized that the lesion was due to repeated ischemia, provoked during the seizure attacks. This
area is now called Sommer’s sector, which corresponds approximately to the CA1 subfield; seems to explain secondary temporal lobe epilepsies in epilepsy patients; In Mongolian gerbils, bilateral carotid occlusion for 5 min always produced the hippocampal lesions
lactate transport and signalling in the brain
monocarboxylate transporters carry lactate, pyruvate, ketone bodies across brain cell membranes; this is facc diffusion; equilibrating action of MCTs also provides the basis for lactate acting as a volume transmitter that can mediate metabolic signals through the nervous tissue, as it can bind to the lactate receptor GPR81 on brain cells and cerebral blood vessels, resulting in inhibition of adenylyl cyclase; Lactate acts as a ‘buffer’ between glycolysis and oxidative
metabolism. In this process, it is exchanged as a fuel between cells and tissues with different glycolytic and oxidative rates; normally efflux from brain to blood but when blood lactate levels rise, such as in physical exercise, there is an influx of lactate from blood to brain; there is an instant decrease in lactate immediately after neuronal activation, followed after a few seconds by a rise, indicating that the cells have a latent capacity
for lactate oxidation that is rapidly exhausted; uptake cycle in which proton binds before the lactate anion, followed by a domain rearrangement during which lactate and proton pass through the channel between the extracellular and intracellular binding site to get released to the cytosol, lactate
first and then proton; lactate modifies prostaglandin action contributing to vasomotor
regulation, and influences the NADH/NAD+ ratio contributing to redox regulation; GPR81, a G-protein-coupled orphan receptor was previously
discovered to be selectively activated by lactate, downregulating cAMP through Gi; GPR81 is therefore subsequently named hydroxycarboxylic acid receptor 1 HCAR1; in adipose lactate serves autocrine role when adipocytes release high concs of lactate due to insulin dependent glucose uptake
MCT2 is concentrated at the postsynaptic membranes of glutamatergic synapses on spines of Purkinje cells in cerebellum and pyramidal cells in the hippocampus The density is lower at mossy fiber synapses, which have lower prevalent firing rate, compared with Schaffercollateral synapses. The lack of MCT2 at inhibitory type synapses on pyramidal cell somata suggests a connection to glutamatergic neurotransmission; role of lactate in synaptic plasticity is consistent with the observation that MCT2 expression is enhanced, together with postsynaptic density 95 protein and glutamate receptor subunit 2 (GluR2) protein, by BDNF, central mediator of synaptic plasticity; oligodendrocytes take up lactate and ketone bodies to synthesise myelin and after myelination has taken place, releases lactate to be used by axons
MCT1 knockout caused pathology similar to ALS and ALS patients show reduced MCT1 expression failure in the supply of lactate (generated in oligodendrocytes by glysolysis) could be involved in ALS and other degen diseases; lower cAMP levels may serve to regulate multiple cellular processes, including curbing glycolysis and glycogenolysis to save energy and limit acidification when glucose breakdown is faster than oxidation through mitochondria. This may counteract damage in hypoxic conditions,
but may even have a role in the first moments of cell activation, when glucose uptake and its conversion to lactate increase earlier than oxygen uptake; lowering cAMP using lactate could help preserve working memory (action of cAMP on HCN); Lactate stimulates production of BDNF by
astrocytes and neuronal cells, and BDNF helps regulate plasticity: possible mechanism for how exercise boosts the brain
pasteur effect and comparative anoxia tolerance
in anoxia inc’d need for anaerobic glycolysis which has thus inc’d requirement for glucose (but even maximal rate of glycolysis insufficient)
some indications that the marine mammalian brain has an enhanced glycolytic capacity. The glycogen content of the seal brain is some three times higher than that of the land mammal; Furthermore, 20-25% of the brain glucose uptake of the Weddell seal is released as lactate even when breathing air, compared to less than 10% for the human brain; Man will suffocate if breathing is stopped for more than 3-4 minutes; Weddell seals can remain submerged, actively swimming, for an hour or so; lutz et al 2004, brain of freshwater can withstand anoxia for days due to a fascinating, interlocking
cluster of adaptations that produce a state of deep
hypometabolism, the most effective of all hypoxia defense strategies; turtle brain is remarkable in having glycogen concentrations about fivefold greater than that of the rat which provides an immediately accessible store of glucose for anaerobic glycolysis until adequate glucose
supplies are liberated (recruited) from the large glycogen stores in the liver; Three distinct phases are involved in fully surviving anoxia:
(1) a drastic and immediate downregulation of ATP demand processes, (2) long-term maintenance at basal levels of ATP expenditure and (3) a rapid upregulation when oxygen becomes available; in mammlas NF-kB levels inc’d after stroke rapidly, turtle it translocates much slower as part of a preemptive defense mechanism against reoxygenation ROS damage; in the anoxic turtle brain gene transcription of Kv1 channels reduced to 18.5% of normoxic levels; almost full suppression in turtle brain electrical activity during anoxia; Neuronal network integrity is preserved through
the continued operation of core activities. These include periodic electrical activity, an increased release of GABA and a continued release of glutamate and dopamine
hypoxia and the brain
at PaO2 50 mm Hg there is an increase in the [lactate]/[pyruvate] ratio and a decrease in brain tissue pH. Increased lactate production is
accompanied by a rise in the cerebral metabolism of glucose, indicating accelerated glycolysis. The
cytosolic redox potential (measured by the NADH:NAD+ ratio) is shifted toward a more reduced state at PaO2 below 45 mm Hg. Phosphocreatine (PCr) concentration starts to decline and then fall precipitously below a PaO2 20 mm Hg. The steep fall in PCr coincides with
a decrease in ATP and reflects failure of mit respiration and OP; studies demonstrate that
cerebral energy metabolism remains normal when mild to moderate hypoxia (PaO2 = 25- 40 mm Hg) results in severe cognitive dysfunction in human volunteers. In mild hypoxia, CBF increases in order to maintain oxygen delivery to the brain. CBF can increase only about twofold; beyond this, the
cerebral metabolic rate for oxygen starts to fall and symptoms of hypoxia occur; In normal individuals exposed acutely to high altitudes there are the following problems. First, at altitudes of around 10,000 feet, as PaO2 falls below 45 mm Hg, difficulties with complex learning tasks and short-term memory appear. Second, above 20,000 feet, where PaO2 = 30mm Hg, cognitive disturbances and problems with motor co-ordination appear. Acute hypoxia of less than 20 mm Hg generally results in coma. Graded hypoxia in experimental animals reveals that brain ATP levels remain normal at PaO2 20 to 25 mm Hg, despite EEG slowing and a 50% reduction in PCr; so 50mmHg inc lac:pyru, inc glyco, no functional effects, 45mmHg inc CBF, inc redox state reduction, learning difficulty and memory loss, 30mmHg motor incoordination, 20mmHg PCR falls (starts falling from 40mmHg ish) but ATP preserved, coma
hypoxia inducible factor 1
Heterodimeric transcription factor alpha normally degraded (is hydroxylated at conserved proline residues allowing ubiquination then proteasome degradation, enzyme that adds OH needs O2 as substrate), stabilised due to hypoxia and binds to beta then translocates to nucleus; Adaptive response to hypoxia with time course 2 to 4 hours and induces glycolysis; HIF-1a accumulates in hypoxia; upregs genes involved in glycolsis, as well as VEGF (angiogenesis) by binding to HIF response elements
safeguarding against underperfusion
Cerebral perfusion is safeguarded through brainstem regulation of other circulations with first control of BP through neural regulation of CO and TPR, then when necessary the perfusion of
peripheral organs (except heart) is sacrificed; Baroreceptor unloading leads to tachycardia, increased contractility, vaso and venocontriction etc; intravascular injection to visualise vessels; striking density of intracortical vascular network; moody et al: few arteriole to arteriole anastomoses but there is a continuous cap network allowing weak anastomotic flow; A multiple supply is found in certain brain regions where adjacent arterioles arise from two or three widely separated surface (pial) arterial sources.
1nterdigitation describes an overlapping and interpenetration of adjacent arteriolar territories; the imaginary boundaries of perfusion responsibility in the capillary bed between these
arterioles fit together like a jigsaw puzzle rather than smoothly; last 2 occur rarely in human brains and seem to give inc’d protection; regions in the human cerebrum with interdigitating arterioles arising from different parent arteries on the surface
of the brain are the subcortical U fibers and the external capsule area. All other areas of
the cerebrum have arterioles from adjacent sources with capillary beds that each nourish a cylinder of brain, and other arterioles usually do not penetrate that particular unique cylinder
interdigitation of arterioles from separate sources offers favored regions of the brain unique protection from hypotensive events due to better collateral flow from caps; cortex and corpus callosum offered some protection from hypotension by the short distance from the pial plexus, and by having an afferent supply solely from arterioles (dont usually get atherosclerosis); centrum semiovale, basal ganglia, and thalamus are supplied by long arteries and arterioles and do not have interdigitating arteriolar fields. These regions are the most frequent sites for small ischemic infarcts, their arteries are subject to luminal narrowing from atherosclerosis (especially in hypertension and diabetes mellitus) and other degenerative vessel wall conditions; cortex is vulnerable to circulatory hypoperfusion in those areas in which distal middle and anterior (or posterior) cerebral artery distributions meet due to watershed infarct; cortex and BG/thalamus at risk of anoxia, centrum, BG, thalamus at risk of hypoperfusion (and cortex at watershed)
cerebral bloodflow autoreg
Over a normal range of blood pressure (mean between 50 and 140 mm Hg) CBF is kept constant by alteration in cerebrovascular resistance. Partial pressure of carbon dioxide and oxygen also influence CBF - increased by rising PaCO2 or falling PaO2 This effect is absent in hypotension; autoregulatory curve may be shifted to the right under conditions of increased sympathetic activity; upstream resistance vessels have increased tone, because these have sympathetic innervation, the effect is less capacity for downstream resistance vessels to increase flow by even maximal vasodilation (i.e., the socalled waterfall effect of two resistances in series)
capacity of an organ to regulate its blood supply in accordance with metabolic needs, or for brain: capacity of CBF to remain constant despite variation in BP; CBF first alters passively with BP
changes but then returns to initial values despite maintained change in BP (30s to 3 mins); Large cerebral arteries account for ~50% vascular resistance and receive PS (ACh, VIP) and symp (NA, NPY) innervation, as well as innervation from nociceptive c-fibres; the microcirculation contributes 50% of vascular resistance and don’t have symp reg but do account for CO2 (pH)
and O2 responsiveness; Lower limit of autoreg: Arteries and Arterioles dilate with falling CPP until Critical Pressure and Upper limit of Autoregulation
Arteries (> 400 µm), not Arterioles, constrict as CPP rises to Critical Pressure; thus autoreg works between these two limits
neurovasc coupling
state where regional CBF is increased to meet regional changes in brain tissue metabolism or activity. NVC consists of three brain cell types: neurons, supporting cells (astrocytes), and vascular cells (vascular smooth muscle, pericyte, and endothelial cells). These cells can be grouped into three conceptual components: neurons, the message senders associated with information processing; supporting cells, the potential transmission sites that mediate vasoactive signals
in response to the neuronal messages; and vascular cells, the recipients of the signals. After evoked neural activation, vasoactive signals are transmitted directly and indirectly via supporting cells to the vascular cells, which cause redistribution of the local CBF; substantial evidence for the role of astrocytes in NVC and functional hyperaemia (FH, increase in CBF with increased activity); 1) astrocytes must be activated in some way by neuronal signals that cause FH; 2) removing astrocytic signalling specifically in time and spatial location must perturb or abolish increased CBF by increased neural activity; 3) specifically activating astrocytic signals in the absence of neuronal activity should lead to FH. Of these, the first requirement has significant experimental support, but the last two have not been fully addressed
role of astrocytes in neurovasc coupling
signaling from neurons in activated brain regions
to local penetrating arterioles (and possibly also capillaries) and a coordinated response of surface vessels, are necessary for local CBF to increase during neuronal activation; changes in hemodynamics can appear within 1-3 s of increased neural activity, while metabolic changes occur more slowly than this (so not just CO2, O2 etc) and oxygen consumption occurs in
a much smaller area than the subsequent CBF increase; ionotropic glutamate receptor activation may mediate functional hyperemia by calcium-activated synthesis of nitric oxide (NO), prostaglandins etc in neurons; stimulation
of cortical astrocytes, either directly or through nearby neurons, triggers an intraastrocytic calcium surge and a subsequent dilation or constriction of neighboring arterioles. Vasodilation was triggered by activation of astrocytic metabotropic glutamate
receptors (mGluR) and either cyclooxygenase products or K channels on astrocytes and smooth muscles; astrocytes also release precursors for 20-HETE to make vasoconstriction, and cause changes in vascular tone by NO levels too; a study found astrocytic calcium elevations in
somatosensory cortex in awake mice, which appeared 1-2 s after the onset of voluntary running; astrocytic Ca++ changes observed after FH but maybe astrocytes maintain and eg NO triggers initial FH; ischaemia and global hypoxia reduce FH; CBF inc’s during spreading depol (migraine aura); Astrocytes swell under ischemia, and because of their proximity to arterioles
and capillaries, this edema may contribute to the CBF reduction in the microcirculation after stroke (but they are neuroprotective); also participate in
spreading neocortical depolarizations but unknown to what level they influence CBF here
glutamate and glut transporters
Glutamate and aspartate are non-essential amino acids that do not cross the blood-brain barrier. They are synthesised from glucose and a variety of other precursors within the brain. Synthetic and metabolic enzymes for glutamate and aspartate have been localised to neurons and glial cells. (Glutamic acid is in a metabolic pool with a-ketoglutaric acid and glutamine.) A large fraction of the glutamate released from nerve terminals probably is taken up into glial cells, where it is converted into glutamine. Glutamine then cycles back to nerve terminals, where it participates in the transmitter pools of glutamate and GABA; transmitter pool of glutamate is stored in synaptic vesicles that actively accumulate glutamate through a Mg2+/ATP-dependent process. Substances that destroy the electrochemical gradient inhibit this uptake mechanism. The concentration of glutamate within synaptic
vesicles is thought to be very high >20mM; glut/asp are zwitterions so cant cross PM; Uptake
mechanisms have an important role in regulating the extracellular concentrations of glutamate and aspartate in the brain. At least two families of
glutamate transporter have been localised to the plasma membrane of neurons and astrocytes. Only the Na+-dependent glutamate transporter is coupled to the electrochemical gradient that
permits transport of glutamate and aspartate against their concentration gradients; n the CNS, glutamate transporter-1 (GLT-1) and glutamate-aspartate transporter (GLAST) are expressed preferentially in glial cells, GLT-1 has its highest expression in the thalamus and cerebellum, with lower levels in the hippocampus, cortex and striatum. GLAST immunoreactivity is found
predominantly in the cerebellum and less so in the forebrain. Excitatory amino acid carrier-1 (EAAC1) is
expressed predominantly in neurons, most prominently in the hippocampus and not in glial cells; glut in brain tissue at 1-2mM
glut transporters major role limit glut/asp conc in ecf to prevent excessive GLUTr stim; Net transport of glutamate is increased by high intracellular K+; upon dissociation of glutamate and Na+ from the transport machinery, cytoplasmic K+ binds to
the protein to be recycled into the extracellular compartment - 3Na, 1H, 1 glut in and 1 K out; OH or HCO3 accompanies K out
glutamate neurotoxicity
first recognised as a neurotoxin in the late 1960s; Glutamate, and other amino acids, given to immature animals induced acute neurodegen in the areas of the brain not well protected by the blood-brain barrier - notably the hypothalamic arcuate nucleus. The mechanisms of neurodegen are divergent, and activation of all classes of ionotropic glutamate receptors has been implicated; Olney J. Brain lesions, obesity, and other disturbances in mice treated with
monosodium glutamate. Science 1969;164:719-21: In newborn mice subcutaneous injections of msg
induced acute neuronal necrosis in several regions of developing brain including the hypothalamus; treated mice became obese and females were infertile; MSG usually safe to eat, small numbers of people may have headaches, flushing, inc HR etc from glut (chinese restaurant syndrome - outdated term) but most people getting this will be experiencing nocebo
glut neurotox in disease states
Neurolathyrism is a spastic disorder occurring in East Africa and India. There is degeneration of
lower and upper motor neurons. The condition is associated with the dietary consumption of the
legume Latyrus sativus (grass/white/indian pea). This plant contains a toxin b-N-oxalyl-a,b-diaminopropionic acid; bridges et al 1989, compound is potent non-NMDAr agonist, inc KA, AP5 doesnt affect the compounds toxicity , thus non-NMDAr have role in excitotoxicity; the receptor it most binds to is AMPAr; moorhem et al 2011; susceptibility of motor neurons in this condition plus maybe ALS (sensory neurons spared) for excitotoxicity may in part relate to their relatively high expression of AMPA receptors, esp Ca++ permeable ones; beta odap also inhibs Na dep glut transport, increasing ecf levels further
Rasmussen’s encephalitis: Antibodies to the GluR3 receptor function as agonists and induce
seizure-like activity in rabbits. Anti-GluR3 receptor reactivity had been found in the sera of children
with a particularly severe form of epilepsy called Rasmussen’s encephalitis, as well as in people
with other forms of severe epilepsy; twyman 1997: studies demonstrate that antisera and purified IgG
antibodies to GluR3 directly activate a subpop of GluRs; receptor activation was blocked by the competitive antagonist CNQX, thus indicating that the antibody epitope interacted with the AMPA/KA binding or an agonist binding site of the receptor. Blockade of antibody activity by CNQX also suggests that the antibody activation of the receptor was not due nonspecific interactions with the receptor channel protein; Ig dosnt usually cross BBB in large amounts, possibility that focal disruption of the blood-brain barrier, perhaps following trauma or infection, in a region that
provides access of autoantibodies to the appropriate epitope on the target receptor would seem feasible. This scenario would account for the focal nature of the immune lesion and would also suggest that in other regions of the brain where the blood brain barrier is intact, particularly
the other brain hemisphere, autoantibody fails to gain access to the target antigen and is spared. Consequently, at least three independent events could be hypothesized to initiate and perpetuate the disease state: first, disruption of the blood brain barrier by trauma or possibly infection;
second, the presence or ability to produce the GluR3B or related autoantibodies; and third, the presence or display of GluR3 on the neuron in a form that can be accessed by the immune system. A “three-hit” model would also be consistent with the rare incidence of this disease
Anti-NMDA-receptor encephalitis is an acute form of encephalitis, potentially lethal but with high
probability for recovery, caused by autoimmune reaction against NR1- and NR2-subunits of the
glutamate NMDA receptor. Different descriptions and syndromal designations for this disease
existed in the medical literature before 2007, when the causal associations were established the
condition received its current name. The disease is associated with tumours, mostly teratomas of
the ovaries, and thus is considered a paraneoplastic syndrome. However, there are a substantial number of cases with no detectable cancerous tissue; Domoic acid poisoning s an acute form of hippocampal (CA3) toxicity that was
reported in an outbreak of elderly subjects exposed to shellfish, domoate is a kainate like agonist that algae makes and accumulates in shellfish, anchovies, and sardines though illnes in humans only seen from shellfish consumption; AMPA and KA activation damages HC and amyg with short term memory loss, possibly seizures and death, and motor weakness
processes following energy failure
anoxic insult for >few secs produces predictable anoxic depol: depletion of energy stores within
neurons and glia with a concomitant acidosis, and release of free radicals. Depletion of energy stores
affects cellular metabolism, energy dependent ionic pumps, and the ability of cells to maintain resting membrane potential; Within seconds of an
ischaemic insult, normal brain electrical activity
ceases due to the activation of membrane K+ channels and widespread neuronal hyperpol. The
hyperpol may be due to opening of K+
channels responding to acute changes in local
concentrations of ATP, H+ or Ca2+, or it may reflect
altered non-heme metalloprotein associated
with regulation of specific K+ channels. This response (presumably protective) fails, however, to preserve high-energy phosphate levels in tissue as concentrations of PCr and ATP fall within minutes after onset; fall in PO2 during ischaemia enhances lactic acid production as cells undergo a Pasteur shift from a dependence on aerobic metabolism to a dependence on glycolysis. The resulting lactic acidosis decreases the pH of the ischaemic tissue from the normal 7.3 to intra-ischaemic values ranging 6.8 to 6.2 depending, in part, on the pre-ischaemic quantities of glycogen and glucose available for conversion to lactic acid. In addition, efflux of K+ from depolarising neurons results in prolonged elevations in extracellular [K+] and
massive cellular depolarisation. Rapid inactivation of O2-sensitive K+ channels by decreased PO2, may represent one mechanism whereby neurons put a brake on this ongoing K+ efflux; intracellular Na+ and Ca2+ rise and intracellular Mg2+ falls
subsequent phases of anoxic depol
phase II: extracellular concentrations of many neurotransmitters are increased. Depol-induced entry of Ca2+ via voltage-sensitive Ca2+ channels stimulates release of vesicular neurotransmitter pools, including the excitatory amino acid (EAA) neurotransmitter glutamate. At the same time, Na+-dependent uptake of certain neurotransmitters, including glutamate, is impaired. Highcapacity uptake of glutamate by the glutamate transporter couples the uptake of one glutamate and two
Na+ with the export of one K+ and one HCO3
- (or OH-); discharge of ion gradients means driving force for glut uptake lost; n addition, glutamate uptake by the widely expressed astrocyte high-affinity glutamate transporter (GLT-1), or EAA transporter-2 (EAAT2), and the
neuronal transporter EAAT3, can be down-regulated by free radical-mediated oxidation of a redox site on the transporter. Furthermore, since the transporter is electrogenic (i.e., normally transferring a positive charge inward), membrane depolarisation can lead to reversal of the transporter, producing glutamate efflux. Thus, both impaired glutamate uptake and enhanced glutamate release contribute to sustained
elevations of extracellular glutamate in the ischaemic brain. Microdialysis of ischaemic rat brain has detected an increase from the resting extracellular glutamate concentrations of 1 to 2 µM up to concentrations in the high µM to low mM range. However, not all of this is from the neurotransmitter pool: likely that most comes from the metabolic pool
mechanisms of glut release in brain ischaemia
Four release mechanisms have been postulated: vesicular release dependent on external calcium2 or Ca2+ released from intracellular stores; release through swelling-activated anion channels; an indomethacin-sensitive process in astrocytes;
and reversed operation of glutamate transporters. Here we have mimicked severe ischaemia in hippocampal slices and monitored glutamate release as a receptor-gated current in the CA1 pyramidal cells that are killed preferentially in ischaemic hippocampus. Using blockers of the different release mechanisms, we demonstrate that glutamate release is largely by reversed operation of neuronal glutamate transporters, and that it plays a key role in generating the anoxic depol that abolishes information processing in the central nervous system a few minutes after the start of ischaemia; most ischaemia current from glut release not K influx as higher [k]o only produced small inward current reduced by AP5 whereas applying glut gave large transient inwards current; ATP/GTP levels probably fall too low for much vesicular release so Ca++ unlikely to have big role, shown by replacing Ca++ with Mg++ in solution and adding Ca++ chelater; astrocyte swelling can activate channels leading to glut release but blocking this process didnt have sig effect on anoxic depol current; glut hom fails dramatically and transporters are the major source of the extracellular glutamate that triggers the death of neurons; modelling supported this conclusion
pannexin hemichannels in anoxic depol of hippocampal pyramidal cells
notion that this inward current reflects ion entry through glutamate-gated channels has been challenged by the discovery that, in isolated CA1 pyramidal cells, ischaemia evokes a large inward current even in the presence of glutamate receptor blockers; This current is inhibited by carbenoxolone and lanthanum (La3+) (Thompson et al., 2006), which block gap junction hemichannels. Hemichannels are made of connexin or pannexin proteins, they function as ion channels in neuronal and glial membranes and they are expressed at the post-synaptic densities of hippocampal pyramidal cells; activation of NMDA receptors is also reported to evoke a
current component mediated by hemichannels, which is blocked by carbenoxolone and by 10PanX (Thompson et al., 2008), i.e. blockers of hemichannels formed by pannexin-1, Thus, in ischaemia, pannexins might generate a large inward current, producing the anoxic depol, either because of a direct activating effect of ischaemia on pannexin hemichannels as seen in isolated pyramidal cells), or as a consequence of secondary hemichannel opening produced by ischaemia-evoked glutamate release activating NMDA receptors; used HC slice technique; in their expt, pannexin blockers didnt affect anoxic depol, and when glut-r blockers applied didnt block the slow inwards current that remained; Thus, pannexin-1 hemichannels do not contribute to the small and slow inward current, ruling out the
possibility that, when glutamate receptors are blocked, there is a significant activation of pannexin-1 hemichannels in the first 25 min of ischaemia
by varying ATP conc found magnitude and time course of the anoxic depolarization current do not depend on the availability of intracellular ATP and, whether or not ATP is present, they are not affected by hemichannel blockers; brain slices and therefore we cannot rule out the possibility that pannexins may have some effect on the response to stroke in vivo - eg maybe they have role in penumbra; Neuronal gap junctional hemichannels, composed of pannexin-1 subunits, have been suggested to play a crucial role in epilepsy
and brain ischaemia. After a few minutes of anoxia or ischaemia, neurons in brain slices show a rapid depolarization to 20 mV, called the anoxic depolarization. Glutamate receptor blockers can prevent the anoxic depolarization, suggesting
that it is produced by a cation influx through glutamate-gated channels. However, in isolated hippocampal pyramidal cells, simulated ischaemia evokes a large inward current and an increase in permeability to large molecules, mediated by the opening of pannexin-1 hemichannels. N-methyl-D-aspartate is also reported to open these hemichannels, suggesting that the activation
of N-methyl-D-aspartate receptors, which occurs when glutamate is released in ischaemia, might cause the anoxic depolarization by evoking a secondary ion flux through pannexin-1 hemichannels. We tested the contribution of pannexin hemichannels to the anoxic depol in CA1 pyramidal cells in the more physiological environment of hippocampal slices. Three independent inhibitors of hemichannels—carbenoxolone, lanthanum and mefloquine—had no significant effect on the current generating the
anoxic depolarization, while a cocktail of glutamate and gamma-aminobutyric acid class A receptor blockers abolished it. We conclude that pannexin hemichannels do not generate the large inward current that underlies the anoxic depol.
Glutamate receptor channels remain the main candidate for generating the large inward current that produces the anoxic depolarization
blood flow alteration in neuroprotection following ischaemia
During acute ischaemic stroke, the early restoration of oxygen and glucose to the ischaemic region is the best ‘neuroprotective therapy’. This is currently provided clinically by thrombolysis; Ischaemia has profound effects on CBF levels. Antegrade blood flow ceases during arterial occlusion, but collateral vessels may sustain cerebral perfusion in the arterial
bed, Ischaemic damage occurs when
collaterals fail to provide adequate perfusion leading to symptom onset; autoreg lost so mABP has large effect: inducing hypertension improves CBF and reduces injury after mca occlusion; upon reperfusion, significant hyperaemia within the ischaemic region immediately occurs but this is
followed by a post-ischaemic hypoperfusion which can last for hours. This is described as the ‘no-reflow phenomenon’ and has been attributed to the narrowing of capillaries and loss of both arteriolar dilating mechanisms and cerebrovascular
reactivity. Pericytes are susceptible to ischaemic injury resulting in contraction of capillaries
causing attenuated CBF, even after reperfusion; also reperfusion injury; for the nagel study, Laser Doppler flowmetry was used to measure relative CBF over the right somatosensory cortex of a male Wistar rat. Baseline CBF was normalized to 100% blood flow units (BFU). Upon temporary common carotid artery (CCA) ligation, CBF was reduced to 60% BFU; this went to 20% when MCA occluded, then hyperaemia to 80% followed by hypoperfusion of 40% seen
summary of energy failure and anoxic depol
What is the relationship between ischaemia- and
glutamate receptor-mediated neurotoxicity?; energy failure: Phase 1 Lengthen - KATP or hypothermia Phase 2 Slowed with hyperglycaemia or temperature Feature: voltage-sensitive Ca2+-
channels Phase 3 Features: energy failure, High [K+]o; how these contribute and interact: Depolarisation *Vesicular Glutamate release; Anoxia *Oxygen-sensitive metabolism blocked so Glutamine to Glutamate; High [K+]o *Remove Mg2+ block of NMDA receptor *Reversed uptake of Glutamate
depolarization process is energy-linked (ATP level); The link to ATP levels involves functioning of the Na+/K+-ATPase (inhibition under normoxia results in rapid depolarization similar to AD; Complete failure of Na+/K+-ATPase cannot account for the rate of change in extracellular ion concentrations. A change in membrane ion permeability must also occur; Blockade of GLUT-receptors alone or use of tetrodotoxin (Na+-channel) does not prevent AD; Low PO2 can directly affect the activity of membrane ion channels, but Pannexin channels do not contribute significantly to AD; AD appears to be a Ca2+-dependent process
In brain in absence of O2, glutamine degrades to glut and then to GABA; So eg CA1 vulnerability could be due to high NMDA conc (due to memory/learning); MK-801 is site for Mg++ binding where antags can bind (noncompetitive) or glut binding site (competitive); Glut is denditotoxic (blebbing seen on dendrites not soma); Toxicity matches receptor distribution (CA3 knocked out and CA1 spared by kainate, NMDA agonists spare CA3, take out CA1 and DG)
features of focal ischaemia
rim of the ischaemic core is recruited in to the
final infarct - how and why does this occur? In other words, what does/could the ischaemic penumbra tell us about focal ischaemia?; pharmacological approach was used to dissect
potential mechanisms. Consider the results of the
‘neuroprotection’ studies in a model of focal
ischaemia where infarct volume is used as the
experimental endpoint; NBQX gave no protection, MK801 did when given up to 8hrs after lesion; In focal ischaemia infarct volume is reduced when MK-801 is given to the animal, even as late as 8 hours after vessel occlusion. There is no protection with NBQX. What accounts for this time course and what does it tell us about the underlying mechanism of neurotoxicity? The most parsimonious hypothesis is that, after vessel occlusion in focal ischaemia, an NMDA-receptor-mediated (and not non-NMDA receptor mediated) form of neurotoxicity is on-going for some 8 hours. This phenomenon turned out to be cortical spreading depression; normal brain pH 7.2
is their sustained depol allowing mk-801 binding (by mg++ removal) for 8 hrs post lesion - is neuronal death delayed or is this secondary insult - the latter, and the insult is CSD; in penumbra there is increased glucose utilisation and O2 extraction
cortical spreading depression
SD is a transient depolarisation of the neuropil, which slowly spreads over the entire hemisphere, that can be triggered by a variety of pathologic events. When SD is propagated through normal cerebral cortex it does not produce major histopathological changes. SD is propagated by the release of glutamate, and the ionic changes including Ca2+ influx are similar to those evoked by anoxicischaemic depolarisations. Ion homeostasis
after SD is rapidly restored by marked activation
of ion transport that, in turn, is coupled to an
increase of metabolic activity and blood flow; SD-like depolarisation also occurs in the penumbra of focal ischaemia but the pathologic significance is
different from that in the intact brain. They come and go at irregular rates between 1 and 10 per hour, and they last much longer than the events in intact-non-injured brain. In focal ischaemia there is an absence of an appropriate metabolic-haemodynamic coupling (neurovascular coupling from earlier); increased oxygen requirements are not coupled to an increase in blood flow and, therefore, result in transient episodes of tissue hypoxia; the [k]o and Em of AD and SD differ, with AD more prolonged
how delayed CSD related to infarct growth
SpD may be triggered by high [K ]o and glutamate in the ischemic core and actively propagates tissue depolarization, ionic imbalances, and glutamate release into adjacent tissue. In the injury penumbra, where blood supply is compromised, SpD waves cause further reduction in tissue PO2 and exacerbate metabolic stress-failure, also called peri-infarct depols in thiz context; PIDs in the initial hours after injury have been shown to cause a step-wise increase in the ischemic tissue volume; MCAo filament occlusion (so genuine ischaemia induced); PIDs began soon after MCAo and recurred periodically over the subsequent 2 hr. After reperfusion, activity ceased and did not recur for the following 6 hr. At ~8 hr after injury, however, PID activity re-emerged in a delayed secondary phase. During this phase, which lasts 12 hr,PIDs recurred continuously and often in rapid succession; To explore the timing of infarct growth
relative to the secondary phase of PIDs, a
separate group of animals was killed at different postinjury times, and infarct areas of coronal sections were quantified; Rates of infarct growth at the caudal level changed with a time course nearly identical to that for PID frequency; NMDA receptor-mediated currents are involved in the
propagation of SpD/PID, and their antagonism has been shown to reduce PID occurrence; con-g (NMDAr antag) group had a significantly reduced incidence of PIDs associated with a 37% decrease in core infarct volume at 24 hr; The relative latency of DC deflections on electrodes over frontal and parietal regions varied for different PID events, implicating more than a single source for their initiation; First, latencies in the secondary phase were initially quite short, indicating more rapid
propagation speed, but grew progressively longer through subsequent events. Second, latencies were not randomly distributed but instead clustered around particular values. Third, the sequential order in which latency values occurred was also nonrandom. Ordered sequences included series of consecutive latencies with similar values or alternations between two values; topographic maps were constructed from multichannel recordings obtained in four animals
The most common PID pattern observed was a wave of negative DC deflections initiating in frontal regions and propagating caudally, PID waves also initiated in parieto-occipital regions and propagated to frontal areas, yielding negative latencies; PID early phase lower incidence in previous studies is likely attributable to the continuous use of anesthesia, which is neuroprotective and slows brain metabolism, and particularly halothane, which
reduces SpD/PID frequencies; However, as is the case with previous studies on PIDs during the ischemic phase, it is unknown whether reducing PID occurrences provides neuroprotection, or vice versa; e multiple foci for PID development
may reflect different mechanisms of initiation. Nedergaard and Hansen (1993) showed that the initiation of depolarization waves in MCAo can occur with characteristics of either SpD, as provoked by high [K ]o and glutamate, or ischemic depolarization resulting from hypoperfusion
in ischemic border zones. These processes
may occur in different cortical regions
Additionally, rostrocaudal waves may initiate in the striatum and propagate to the cerebral cortex, rather than initiate within the cortex itself. The rat brain is unique in that the claustrum and nucleus accumbens provide a rostral bridge of nearly continuous gray matter between the striatum and neocortex, and Vinogradova et al. (1991) demonstrated that SpD, occurring in the striatum, can traverse this bridge. Thus, the striatal infarct core might serve as an independent source of PIDs, which propagate to the cortex and contribute to its worsening pathology. This
additional source may, in part, account for the high incidence of PIDs in MCAo of the rat; in penumbra, PID metabolic challenge depletes ATP, induces permanent cell swelling, and reduces local blood flow; PID may be major form of damage in penumbra with glut release/excitotoxicity a secondary consequence: has been argued that the neuroprotective effects of glutamate receptor antagonists in brain injury models do not necessarily imply the occurrence of excitotoxic processes. In reducing PID frequency, admin of glutamate receptor antagonists reduces both infarct volume and ATP depletion (glut receptors needed for PID to spread); SPD-like events have been demonstrated in vivo in humans eg fMRI of migraine blood flow, application of KCl, although direct evidence is missing (as of 2003); PID inhib may be important part of neuroprotection; then fabricius et al 2005 provided direct electrophysiological evidence for the existence of PIDs and hence a penumbra in the human brain, by recording and finding SD post-acute brain injury (though not in everyone); late onset of PID phase 2 gives smaller infarct
