Nervous Tissue Metabolism, Nervous Tissue Metabolism, Neurotransmitters & Neurotransmitters &

Related DiseasesRelated Diseases

Energy source of the brainEnergy source of the brain

• The mass of the brain is only 2% of the total body mass, yet its energy requirement is more than seven times than that of the other organs

• Thus for brain metabolism, there is a high requirement for glucoseglucose and oxygenoxygen at steady rate.

• The main source of energy is the generation of ATP by the aerobic aerobic

metabolismmetabolism

IntroductionIntroductionMetabolism & CNS functions Metabolism & CNS functions

IntroductionIntroductionMetabolism & CNS functions Metabolism & CNS functions

NeurotransmittersNeurotransmitters:

•are chemicals released at synapses for transmission of nerve are chemicals released at synapses for transmission of nerve impulsesimpulses•can be divided structurally into two categoriescan be divided structurally into two categories: - small nitrogen-containing neurotransmitters - neuropeptides.

• A large number of cofactors are required for the synthesis of

neurotransmitters, and deficiencies of pyridoxal phosphate, thiamine pyrophosphate, and vitamin B12 result in a variety of neurologic dysfunctions

•A large number of cofactors are required for the synthesis ofneurotransmitters, and deficiencies of pyridoxal phosphate, thiamine pyrophosphate, and vitamin B12 result in a variety of neurologic dysfunctions

Glucose metabolism & neurotramsmitters Glucose metabolism & neurotramsmitters (in CNS)(in CNS)There is a relationship between the oxidation of glucose in glycolysis and the supply of precursors for the synthesis of neurotransmitters in neurons within CNS.

Accordingly, deficiencies of either glucose or oxygen (hypoglycemia or hypoxia) affect brain function because they influence: 1- ATP production for CNS neurons 2- Supply of precursors for neurotransmitter synthesis.

IntroductionIntroductionMetabolism & CNS functions Metabolism & CNS functions

Glucose Metabolism & Neurotransmitter SynthesisGlucose Metabolism & Neurotransmitter Synthesis

Clinical manifestations of hypoglycemia:Clinical manifestations of hypoglycemia:

EarlyEarly clinical signs in hypoglycemia initiated by hypothalamic sensory nuclei as sweatingsweating, palpitationspalpitations, anxietyanxiety and hungerhunger.

In latelate stages, these symptoms give way to serious manifestations of CNS disorders CNS disorders as confusionconfusion, lethargylethargy, seizuresseizures & comacoma

Clinical manifestation & Biochemistry of Clinical manifestation & Biochemistry of Hypoglycemic EncephalopathyHypoglycemic Encephalopathy

• As blood glucose falls below 45 mg/dLAs blood glucose falls below 45 mg/dL the brain attempts to use internal substrates such as glutamate and TCA cycle intermediates as fuels for ATP production. Because the pool size of these substrates is quite small, they are quickly depletedquickly depleted.

• As the blood glucose drops from 45 to 36 mg/dLAs the blood glucose drops from 45 to 36 mg/dL NONO EEG changes are observed Symptoms appear to arise from decreased synthesis of neurotransmitters decreased synthesis of neurotransmitters in particular regions of the brain (hippocampal and cortical structures)

• If blood glucose levels continue to fall below 18 mg/dLIf blood glucose levels continue to fall below 18 mg/dL EEG becomes isoelectricisoelectric Neuronal cell death Neuronal cell death ensues that may be caused by glutamate excitotoxicityglutamate excitotoxicity ?? (as result of ATP depletion)

SO, GLUTAMATE METABOLIDSM HAS TO BE UNDERSTOOD SO, GLUTAMATE METABOLIDSM HAS TO BE UNDERSTOOD

Clinical manifestation & Biochemistry of Clinical manifestation & Biochemistry of Hypoglycemic EncephalopathyHypoglycemic Encephalopathy

GlutamateGlutamate as a neurotransmitter as a neurotransmitter

• Within the CNS, glutamate is the main excitatoryexcitatory neurotransmitter neurotransmitter. • Neurons that respond to glutamate are referred to as glutaminergic neuronsglutaminergic neurons

Sources of glutamate in nerve terminals:Sources of glutamate in nerve terminals: 1- Synthesized from glucose through glucose metabolism in neurones (main

source) 2- From blood (few as no cross BBB)

Mechanism of action of glutamate as a neurotransmitter:Mechanism of action of glutamate as a neurotransmitter:1- Synthesis from glucose metabolism & concentration in vesicles (in presynapses)2- Release by exocytosis to synaptic cleft3- Uptake by postsynaptic 4- Binding to glutaminergic receptors in postsynapses5- Functional effect

Role of glutamate as a transmitter in CNS:Role of glutamate as a transmitter in CNS:

Within the CNS, glutaminergic neurons glutaminergic neurons are responsible for the mediation of many vital processes such as the encoding of information, the formation and retrieval of memories, spatial recognition and the maintenance of consciousness.

Postsynaptic glutaminergic neurons perform their roles through:Postsynaptic glutaminergic neurons perform their roles through:

1- Ionotropic receptors Ionotropic receptors that bind glutamate released from presynaptic neurons referred to as kainatekainate, 2-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid (AMPAAMPA) and N-methyl-D-aspartate (NMDANMDA) receptors.

2- Metabotropic glutamate receptorsMetabotropic glutamate receptors that are members of the G-protein coupled

receptor (GPCR) family.

GlutamateGlutamate as a neurotransmitter as a neurotransmitter

Ionotropic Glutamate ReceptorsIonotropic Glutamate Receptors

Non- NMDANon- NMDA

AMPAAMPAKAINATEKAINATE

NMDANMDA

AMPA and Kainate receptors AMPA and Kainate receptors generally allow the passage of generally allow the passage of

NaNa++ and and KK++

NMDA receptors allows the NMDA receptors allows the passage of both passage of both NaNa++ andand CaCa++++

ions. (More permeable to ions. (More permeable to CaCa++++ ))

Role of astrocytes in glutamate metabolism in CNS:Role of astrocytes in glutamate metabolism in CNS:

Glutamate released during neurotransmission is taken up taken up primarily by neighboring astrocytesastrocytes.A fraction of astrocyte glutamate is converted to glutamine by glutamine synthetase, which is abundant in astrocytes and absent from neurons. Glutamine is released from astrocytes & is taken up by neurons. In neurones, glutamine is deaminated to glutamate by glutaminase in neurons.

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GlutamateGlutamate as a neurotransmitter as a neurotransmitter

Glutamate metabolism in hyperammonemia:Glutamate metabolism in hyperammonemia:

•During hyperammonemia, ammonia can diffuse into the brain from the blood. •The ammonia is able to inhibit the neural glutaminaseinhibit the neural glutaminase, thereby decreasing formation of glutamate in presynaptic neurones.

This effect of ammonia might contribute to the lethargy associated with the hyperammonemiahyperammonemia found in patients with hepatic disease.

(hepatic encephalopathy)

GlutamateGlutamate as a neurotransmitter as a neurotransmitter

Glutamate ExcitotoxicityGlutamate Excitotoxicity

Exciotoxicity is the pathological process by which nerve cells are damaged and killed by glutamate (and similar substances).

This occurs when receptors for glutamate such as the NMDA & AMPA receptor are over activated (overstimulated).over activated (overstimulated).

Excessive excitation of glutamate receptors has been associated with hypoglycemia & strokehypoglycemia & stroke (cases in which there is lack of (cases in which there is lack of glucoseglucose or or oxygenoxygen ending in lack of energy ending in lack of energy production in CNS) production in CNS)

Glutamate excitotoxicity occurs when the cellular energy reserves are depleted cellular energy reserves are depleted

(as in hypoglycemia or stroke, etc )

Failure of the energy-dependent reuptake pumps of glutamate

Accumulation of glutamate in the synaptic cleft

Overstimulation of the postsynaptic glutamate receptors

Prolonged glutamate receptor activation leads to prolonged opening of the receptor ion channel and the influx of lethal amounts of Ca2 ionsinflux of lethal amounts of Ca2 ions, which can

activate cytotoxic intracellular pathways in the postsynaptic neurons

Glutamate ExcitotoxicityGlutamate Excitotoxicity

Cerebral IschemiaCerebral Ischemia

Cerebral ischemiaCerebral ischemia•It is the potentially reversible altered state of brain physiology and biochemistry

that occurs when substrate delivery is cut off or substantially reduced by vascular

stenosis or occlusion

StrokeStroke• is defined as “an acute neurologic dysfunction of vascular origin with sudden

(within seconds) or at least rapid (within hours) occurence of symptoms and signs

corresponding to the involvement of focal areas in the brain”

(Goldstein et al, 1989)

Pathophysiology of Cerebral IschemiaPathophysiology of Cerebral Ischemia

↓↓ ATPATP

Induction

Amplification

Expression

InductionInduction PhasePhase

Lack of oxygen supply to ischemic neuroneLack of oxygen supply to ischemic neuronessThe cell switches to anaerobic metabolism, producing lactic acidlactic acid.

ATP depletionATP depletion

malfunctioning of membrane ion system

Depolarisation of neurones

Influx of calciumInflux of calcium

Release of neurotransmitters as glutamate glutamate ((causing glutamate excitotoxicity) glutamate excitotoxicity)

Accumulation of more intracellular Accumulation of more intracellular levels of Ca2+ which causes levels of Ca2+ which causes

additional release of glutamate additional release of glutamate (viscious cycle)(viscious cycle)

Amplification Amplification PhasePhase

ExpressionExpression Phase Phase

1-overexcites cells and causes the generation of harmful chemicals like free radicals ( causing free radicals ( causing oxidative stressoxidative stress))2- Activation of calcium-dependent enzymes such as: calpain ( causing calpain ( causing apoptosisapoptosis)) phospholipases (causing phospholipases (causing membrane breakdownmembrane breakdown))3- Calcium can also cause the release of more glutamate (glutamate excitotoxicityglutamate excitotoxicity)

The cell's membrane is broken down by phospholipasesphospholipases

Cell membrane becomes more permeable, and more ions and harmful chemicals flow into the cell.

+ + Mitochondria membrane break down, releasing toxins and apoptoticapoptotic factors

into the cell

CaCa2+2+

lactic acid lactic acid is produced in excess in ischemiais produced in excess in ischemia

In cerebral ischemia, lack of oxygen switches metabolism of glucose to the anaerobic pathway & lactic acid production

Lactic acid contribute to the pathophysiology of ischemia asLactic acid contribute to the pathophysiology of ischemia as: 1- It decreases pH that may injure and inactivate It decreases pH that may injure and inactivate mitochondriamitochondria. 2- Lactic acid degradation of Lactic acid degradation of NADHNADH (which is needed for ATP synthesis) may also interfere with adequate post-ischemic recovery of ATP levels. 3- Lactic acid increase the amount of Lactic acid increase the amount of free-radical mediated injuryfree-radical mediated injury. Lactic acid in neurons acidosis promotes the pro-oxidant effect ↑ the rate of conversion of O2

- to H2O2 or to hydroxyperoxyl radical

What is meant by ROS?What is meant by ROS?Reactive oxygen species (ROS) are formed from partial reduction of molecular O2 i.e.

adding electrons to oxygen leading to the formation of superoxide, hydrogen peroxide & hydroxyl radical.

Generally, ROS ROS cause damage to DNA, protein and unsaturated lipids of the cellscause damage to DNA, protein and unsaturated lipids of the cells.

What is meant by oxidative stressWhat is meant by oxidative stressA condition in which cells are subjected to excessive levels of ROSROS (free radicals) &

they are unable to counterbalance their deleterious effects with antioxidantsantioxidants

Oxidative stress Oxidative stress is caused by ischemiais caused by ischemia

Oxidative StressWhy the Brain is vulnerable to Oxidative Stress• High metabolic requirement for oxygen;• Prevalent concentration of oxidizable polyunsaturated fatty acids (PUFAs) in

membranes; • Enrichment in redox active metals, most notably iron • The presence of potentially phagocytic microglia which can be a major source of

ROS and free radicals when activated by injury. • Low levels of protective antioxidants

Cerebral Vascular Effects of Oxygen RadicalsCerebral Vascular Effects of Oxygen Radicals• Vasodilation• Alteration in reactivity to CO2 and endothelium-dependent vasodilators • Increased endothelial permeability• Focal destructive lesions of endothelial cell membranes

Oxidative stress Oxidative stress

Cellular Effects of Reactive Oxygen Species in CNSCellular Effects of Reactive Oxygen Species in CNS

• Nitric oxide is over produced and turns to be a neurotoxic Nitric oxide is over produced and turns to be a neurotoxic mediator as it reacts with superoxide anions to generate mediator as it reacts with superoxide anions to generate toxic peroxynitrite which leads to production of more potent toxic peroxynitrite which leads to production of more potent neurotoxin such as hydroxyl radicalsneurotoxin such as hydroxyl radicals

• Lipid peroxidationLipid peroxidation• Inactivation of enzymesInactivation of enzymes• Nucleic acid (DNA & RNA) damageNucleic acid (DNA & RNA) damage• Release of calcium ions from intracellular stores with more Release of calcium ions from intracellular stores with more

damage to neuronesdamage to neurones• Damage to cytoskeletonDamage to cytoskeleton

Apoptosis & necrosis are caused by ischemiaApoptosis & necrosis are caused by ischemia

• NecrosisNecrosis: is commonly observed early after severe ischemic insults

• ApoptosisApoptosis:occurs with more mild insults and with longer survival periods

• The mechanism of cell death involves calcium-induced calpain-calpain-mediated proteolysismediated proteolysis of brain tissue

• Substrates for calpain include:– Cytoskeletal proteins– Membrane proteins– Regulatory and signaling proteins

ApoptosisApoptosis

Broughton et al., 2009; Stroke

Mitochondria break down, releasing toxins and apoptotic factors into the cell. The caspase-dependent apoptosis cascade is initiated, causing cells to "commit suicide."

Broughton et al., 2009; Stroke

Caplains are cytosolic proteinasesWhose irreversible proteolytic activityis against cytoskeleton and regulatory proteins

Broughton et al., 2009; Stroke