Scenario 28: Epilepsy Flashcards
Why must glutamate be made locally in the CNS?
It cannot cross the BBB
How is glutamate degraded?
To glutamine by glutamine synthase in astrocytes
How can glutamate be formed from the TCA cycle?
From a-ketoglurate
How is glutamate stored in vesicles?
Driven by the electrical gradient created by the difference in H+ concentrations across the membrane generated by vesicle protein pump. The vesicle is ve+ compared to cytoplasm and highly selective glutamate transporters VGLUT1-3 allow influx of glutamate.
How is glutamate taken up after it has caused an AP in the postsynaptic neurone?
By excitatory amino acid transporters (EAAT) which reduce the extracellular concentration and terminate the action of the transmitter
What are the 3 families of iontropic glutamate receptor?
NMDA, AMPA and kainate
What are the families of metabotropic glutamate receptors?
mGLUR1-8
What is the structure of iontropic glutamate receptors?
3 transmembrane spanning domains, large extra-cellular N-terminus, receptors have 4 subunits arranged in a ring with a central pore
What are the features of transmission at AMPA receptors?
Fast synaptic current, fast decay due to loose binding (low affinity)
What are the features of transmission at NMDA receptors?
Slower onset and decay then AMPA receptors due to higher affinity glutamate binding. Glutamate can only dissociate from the receptor when the channel is closed
Why is glutamate slow to activate NMDA receptors?
There is a Mg2+ block which binds at membrane potentials below -50 mV.
Why are NMDA and AMPA co-incedence receptors?
The Mg2+ block of NMDA can only be relived by the depolarisation from fast AMPA receptors (needs multiple/repetitive inputs)
What two neurotransmitters must bind to NMDA for its activation?
Glutamte to GluN2 subunit and glycine to GluN1 subunit
What is the structure of the NMDA receptor?
4 subunits: 2 x GluN1, 2 x GluN2
What are the potential sites of pharmacology of the NMDA receptor?
NMDA antagonist at glutamate binding site, channel blockers at Mg2+ binding site, glycine-site anatagonists
What are some Mg2+ channel blockers of the NMDA receptor?
Phencyclidine, ketamine, dextromethrophan
What are the functions of Mg2+ channel blockers of the NMDA receptor?
Hallucinations, cognitive defects at low conc and dissociative anaesthesia, analgesia and amnesia at high concs (dextromethrophan- cough suppressant)
What are the different families of kainate receptors?
GluK1-3 which are functional channels alone or combined
GluK4-5 which need 1-3 to be active
What is the structure of the metabotropic receptors?
7 transmembrane structure
Which receptors are in group 1 of metabotropic glutamate receptors?
mGluR1 and mGluR5
What is the mechanism of group 1 of metabotropic glutamate receptors?
Glutamate binds to Ga which activates PLC to increase IP3 and DAG concentrations. IP3 increases Ca2+ release and therefore concentration and DAG activates PKC for protein phosphorylation which modulates ion channel activity
Which receptors are in group 2 of metabotropic glutamate receptors?
mGluR2 and mGluR3
What is the mechanism of group 2 of metabotropic glutamate receptors?
Inhibit adenylate cyclase to decrease cAMP levels to modify ion channel activity
Which receptors are in group 3 of metabotropic glutamate receptors?
mGluR4 and mGluR6-8
What is the mechanism of group 3 of metabotropic glutamate receptors?
Inhibit adenylate cyclase to decrease cAMP levels to modify ion channel activity
What mediates the inhibitory effects of glutamate?
Metabotropic receptors
How can mGluRs been modulated presynaptically?
Inhibit voltage gated calcium channels to reduce NT release
How can mGluRs been modulated postsynaptically?
Inhibit K+ channels to increase excitability, increase the no. of K+ channels which decreases excitability
How can AMPA receptors be involved in learning and memory
Phosphorylated by PKC to increase channel conductance and promote membrane insertion for a more effective synapse
How can gene expression be altered to effect glutamate receptors?
CaM kinase activates small GTPases which can activate multiple kinases for gene expression
How can the site of the glutamate be altered synapse be altered?
CaM kinase changes actin cytoskeleton causing structural changes to dendritic spines
What can happen if there is too much Ca2+ influx?
Overwhelms Ca2+ and intracellular stores, activates pathological processes and enzymes like calpain which activates caspases leading to apoptosis. Cell death via oxidative stress and lysophospholipids which comprimise membrane integrity
How is GABA formed?
From glutamate via glutamic acid decarboxylate
How is GABA broken down?
By GABA transaminase to succinic semialdehyde
What is vigabatrine?
A synthetic GABA analogue used to treat epilepsy in multi drug resistant cases. It blocks GABA transaminase to increase GABA concentration in the brain
What can be exacerbated by GABA analogues?
Absence seizures
What are the side effects of vigabatrine?
Depression or psychotic episodes
How is GABA stored in vesicles?
Like glutamate, the proton gradient allows uptake of GABA into vesicles via ATP dependant proton pump
How is GABA taken up in astrocytes?
By transporters using energy from the Na+ gradient (co-transport)
What is tiagabine?
A drug which inhibits GABA uptake and increases GABA concentration to potentiate inhibition
What are the two classes of GABA receptors?
GABAa iontropic receptors and GABAb metabotropic receptors
What is the structure of GABAa receptors?
4 transmembrane spanning domains, 5 subunits, 2 alpha subunits, 2 beta and 1 gamma with GABA binding site at AB interface
How do GABAa receptors cause inhibition when activated?
Activate Cl- selective ion channels and cause fast inhibition by hyperpolarising neurone or clamping volatge at resting potential to inhibit depolarisation response to excitatory inputs
What is a GABAa agonist?
Muscimul from hallucinogenic mushrooms
What is a GABAa antagonist?
Convulsant alkaloid bicuclline blocks binding site, picrotoxin blocks Cl- channel
How do benzodiazepines potentiate GABA effects?
Bind to accessory site at alpha gamma subunit interaction, facilitates GABA binding by allosteric acton and enhances effect of GABA
When is diazepam usually used in epilepsy?
In status epilepticus I.V. as it is rapidly acting
When are lorazepam and temazepam used clinically?
As short acting sleeping aids that won’t make you sleepy when you wake up
What is used in a benzodiazepine overdose?
Flumazenil to antagonise
What can benzodiazepines be used for clinically?
Anxiety, sedative, reduce muscle tone, anti-convulsant, amnesia
When is midazolam used?
As an amnesia-inducing drug for gastroscopy and dental surgery
What are the side effects of benzodiazepines?
Drowsiness, confusion, amnesia, impaired coordination, tolerance and dependance issues
When are benzodiazepines toxic?
Generally safe but risk of respiratory depression if mixed with alcohol
What are the symptoms of benzodiazepine withdrawal?
Anxiety, irritability, aggression, tremor and dizziness
How do barbiturates work?
Bind to the GABAa receptor and potentiate GABA therefore Cl- influx
What happens in barbiturate overdose?
Cardiac and respiratory depression
What are the side effects of barbiturates?
Sedation, impaired cognitive and motor coordination
Why are barbiturates hepatotoxic?
Broken down by P250
When is phenobarbital used?
Status epilepsy, sleeping pills
What is a barbiturate anaesthetic?
Thiopental
When is there increased synthesis of neurosteroids?
Pregnancy, stress, alcohol consumption
Name a neurosteroid
Allopregnanolone
What do neurosteroids do?
Potentiate GABA by binding to 2 binding sites on alpha subunit
What happens if neurosteroid production is disrupted?
Panic disorder, depression, schizophrenia, epilepsy
Where does ethanol have an effect in the CNS?
At GABAa receptors similar to effects of benzodiazepines
What G protein GABAb receptors coupled to?
Gi
What is the function of GABAb receptors?
Inhibit adenylate cyclase
What are the subtypes of GABAb receptors?
GABAbR1 and 2
Which GABAb receptor subtype is not directly activated by GABA?
GABAbR2 only helps GABAbR1 get to the plasma membrane
Which GABAbR1 and 2 isoform is found on presynaptic neurones?
1a
Which GABAbR1 and 2 isoform is found on postsynaptic neurones?
1b
How do GABAb receptors inhibit the presynaptic neurone?
The betagamma subunit inhibits voltage gated calcium channels and synaptic vesicle release. The Ai subunit inhibits adenylate cyclase
How do GABAb receptors inhibit the postsynaptic neurone?
Increase K+ channel opening by betagamma subunits to reduce firing in the post synaptic neurone and inhibit adenylate cyclase to reduce cAMP levels and PKA activity. This reduces phosphorylation of other proteins including ion channels to modulate this firing
What is baclofen?
A GABAb agonist which reduces muscle spasticity and spasm in MS and spinal cord injury patients
What are the side effects of baclofen?
Drowsiness, dizziness, nausea, confusion
What are some GABAb antagonists?
Saclofen, phaclofen, SCH-50911
What is a GABAb agonist which is endogenous?
GHB (exogenously used as date rape drug)
What are the uses of GHB?
Treat nacrolepsy
What is the major inhibitory neurotransmitter (not GABA) in the CNS?
Glycine
How is glycine made?
Serine is converted to glycine via the enzyme serine hydromethyltransferase
How is GABA converted to GHB?
Via succinic semialdehyde dehydrogenase
How is glycine packaged into vesicles?
By H+ dependant amino acid transport (VIAAT)
Which transporters uptake released glycine?
GLYT1 and GLYT2
What is the structure of a glycine receptor?
6 subunits: 4 alpha and 2 beta, 4 transmembrane spaning domains
What is a glycine receptor antagonist?
Strychnine
Where does glycine bind on the glycine receptor?
Alpha subunit§
What is a non-competitive inhibitor of glycine receptor alpha subunits?
Picrotoxin
What is a mutation in the glycine receptor called and how does it manifest itself clinically?
Hyperplexia which causes an increased startle reflex due to muscle spasm at unexpected stimuli. Infants are also hypertonic.
How can we define epilepsy?
A disorder of the CNS characterised by recurrent sudden increases in electrical activity which may be localised or generalised
How does an EEG work?
Records electrical activity in the brain by measuring the electrical difference between electrodes on placed in a set formation on the skull
How can seizures be classified?
As partial, primary or secondary generalised
What is a partial seizure?
(localised, focal) seizure is restricted to a location in the CNS
What is a primary generalised seizure?
When most of the CNS is involved in the seizure and there is no focus
What is a secondary generalised seizure?
When eventually most of the CNS is involved but it has spread from an initial focus point
How can partial seizures be further classified?
Into simple (if the patient is conscious) and complex (if the patient loses consciousness
How is an absence seizure characterised?
A generalised seizure which is common in children, there is a sudden loss of awareness which can last up to 30 seconds. It can be hard to spot.
What is an aura?
A feeling or smell which precedes the seizure and gives a warning to the patient
How is a tonic-clonic seizure characterised?
Often what a lay person will think of when they think of a ‘seizure’. Muscle tone increases and this can cause a fall, followed by ‘clonic’ jerking movements
How is an simple partial seizure characterised?
A focal cortical seizure. May be sensory or motor. The motor seizures can be Jacksonian where there is a Jacksonian march (jerking starts in extremities and moves proximally)
How is an temporal lobe epilepsy characterised?
Partial seizure of temporal lobe that may be a simple partial seizure characterised by emotional, sensory or memory related phenomena or a complex partials seizure which spreads through the lobe and impaired consciousness. May produce tonic-clonic seizure by impairing consciousness
What is status epilepticus?
Where seizures go on for more than 30 min. Life threatening emergency
What could be some causes of a non-epileptic seizure ?
Hypoglycaemia, dehydration, alcohol or drug withdrawal, cardiac defects
What are some of the possible causes of epilepsy?
Idopathic, cryptogenic (can’t be proven), some genetic link and some following trauma (tumour, head injury
Which genes are mutated in benign febrile epilepsy?
KCNQ2 and KCNQ3 which encode potassium channels
Which genes are mutated in familial generalised epilepsy with febrile seizures?
SCN1b a gene which encodes the accessory subunit of the voltage gated sodium channel
How can we induce a seizure in animals for testing?
Kainic acid injection to the hippocampus which activates the glutamate receptor or by high frequency low intensity electrical stimulation of the brain- Kindling
What is a paroxysmal depolarising shift?
Cellular manifestation of how an epileptic seizure initiates. Spontaneous depolarisation which returns to resting potential after a burst of action potentials
What does PDS activation and repolarisation depend on?
Activation of iontropic glutamate receptors and opening of voltage gated Ca2+ channels. Curtailment of the PDS depends and repolarisation depends on opening of voltage gated K+ channels and activation of iontropic GABA receptors
How do seizures spread from PDSs?
Failure of inhibitory feedback from local interneurones to prevent PDS spreading so neighbouring neurones fire when the neurone fires
What is surround inhibition?
Mediated by interneurones through feedback pathways, limits spread of input signalling so has a focussing effect
What controls sleep wake cycles?
Thalamo-cortical networks. Thalamic neurones arouse cortical neurones which feed back. This is active randomly and sporadically when awake and rhythmic and lower frequency when asleep
Why are benzodiazepines usually not suitable for prophylactic treatment of epilepsy?
Too sedative for every day use
What kind of epilepsy are tiagabrin and vigabatrin contradicted in?
Absence seizures
Give three examples of voltage dependant sodium channel blocking drugs
Carbamazepine, phenyton, lacoasmide
How do voltage dependant sodium channel blocking drugs work?
Reduce levels of action potentials at high frequencies but not at low frequencies. State-dependant blocking of the channel (don’t bind in resting state but during in refractory period to hold it there)
What drugs can be used in absence seizures?
Ethosuximide, lamotrigine, sodium valproate
How does ethosuximide work?
Uncertain mechanism, may block t-type calcium channels in thalamic neurones. Important in generation of rhythmic activity
What is lamotrigine?
A use dependant sodium channel blocker
What are some voltage gated calcium channel blockers?
Gabapentin, pregabalin
How does retrigabine work?
Open KCNQ potassium channels and stabilise resting state
How does perampanel work?
Antagonises AMPA receptors
How does levertiracetam work?
Binds SV2A (synaptic vesicle protein) to effect neurotransmission
What are some non-pharmacological treatments for epilepsy?
Ketogenic diet, vagal stimulation and surgery (removal of focal tissue)
What is a channelopathy?
A dysfunction in any ion channel
What can cause mutations in a gene?
Radiation (UV, X-ray, microwaves) infections (viruses) chemicals or toxins (tobacco) germline (DNA replication error) somatic (developmental error)
What is a point mutation?
A single base mutation which can lead to a different amino acid being in the chain
What is a nonsense mutation?
When a point mutation leads to there being a stop codon which stops the chain early
What is a frameshift mutation?
When there are added bases so the whole sequence shifts making most amino acids wrong
What is an autosomal dominant genetic disease?
Where the disease is expressed in the heterozygote offspring (only one copy needed to produce disease)
What is an autosomal recessive genetic disease?
Where the disease is only expressed in the homozygote offspring
What is a dominant negative genetic disease?
Where the mutant protein disrupts function of the normal protein e.g. when there is an ion channel with many subunits and one is mutated it will affect the function of the whole channel
What is haploinsufficiency?
Where having only one copy of the gene is not enough to give full function
Why do some genetic diseases mostly appear in males?
Because the Y chromosome is missing certain genes which can only be inherited from the X. In males, they only inherit one X so have half the chance of inheriting the correct gene. Females have 2 chances to inherit the correct gene.
What is the genetic mutation that leads to hyperplexia? And what does this mean?
5q32 meaning chromosome 5, the petit chromosome, 32 across from the centromere
What does a point mutation R271Q mean?
A mutation at 271 leading to expression of glutamate instead of arginine
Which receptor is the mutation that causes hyperplexia on?
GLRA1- glycine receptor alpha subunit 1
Where is the R271Q mutation found on GLRA1?
In the second extracellular loop
What is the mechanism of hyperplexia?
No negative feedback loop from Renshaw cells with cholinergic lower motor neurones
What is erythromelalgia?
Burning pain in extremities in response to warm stimuli or moderate excercise
What is the mutation for erythromelalgia?
In gene SCN9A which codes for Nav1.7 channel’s alpha subunit
What is the point mutation for erythromelalgia?
F1449V phenylalanine to valine
What are the features of the Nav1.7 channel?
6 transmembrane spanning domains and a p loop (each domain repeated 4 times) and the 4th TMS domain has a voltage centre
Where are Nav1.7 channels found?
In small dorsal root ganglion cells- nociceptive Ad and C fibres
What are the symptoms of generalised epilepsy with febrile seizures?
Convulsions associated with fever
What is the chromosome associated with generalised epilepsy with febrile seizures?
19q13.7
What protein is affected in generalised epilepsy with febrile seizures?
SCNIB subunit
What is the point mutation associated with generalised epilepsy with febrile seizures?
C121W (cysteine to tryptophan)
What is the function of the SCNIB subunit?
The beta subunit is associated with the alpha subunit’s ability to do it’s job- modifies inactivation
How does the C121W mutation of SCNIB affect it?
Removal of disulphide bridge changes structure dramatically causing slower inactivation, persistent inward Na+ current, cell excitable for longer, more DP
What are benign familial neonate seizures?
Seizures which begin in the first 3 days post natal and cease by 3 months
What is the gene affected in benign familial neonate seizures?
20q13.3
What protein is affected in benign familial neonate seizures?
KCNQ2 potassium channel
What kind of mutation is involved in benign familial neonate seizures?
Frameshift mutation leading to stop codon and loss of 300 amino acids. Due to haploinsufficency- in early life we need both copies but cease to need them
What is the structure of KCNQ ion channels?
6 x TM domains and a P loop
Where are KCNQ ion channels mostly found?
At high conc. in the axon hillock and nodes of Ranvier
Where does spike initiation occur?
In the axon hillock and nodes of Ranvier
What controls repetitive firing in neurones? How?
The M current of KCNQ channels works by stopping firing and without it there is high spike firing (tonic)
Which anaesthetics are most commonly currently in use?
Seroflurone, isoflurone, nitrous oxide, propfol
What happens in stage one of Guedal’s stages of anaesthesia?
Analgesia, loss of consciousness, pupil dilation
What happens in stage two of Guedal’s stages of anaesthesia?
Increased abdo-thoracic movements, decrease in pupil size and muscle tone, disinhibited response. State of delirium
What happens in stage three of Guedal’s stages of anaesthesia?
Decreased muscle tone, eye movements, reflexes. Surgical anaesthesia, loss of swallowing reflex
What happens in stage four of Guedal’s stages of anaesthesia?
Overdose, medullary depression, respiratory and circulatory failure
What is the changes in the appearance of an EEG during the different stages of anaesthesia?
Increase amplitude in stage two and early stage three then decreases through the rest of stage 3 and 4. Mean frequency gradually decreases with increasing depth of anaesthesia
What is the appearance of an EEG when awake?
Low amplitude and high frequency
What is the appearance of an EEG when under surgical anaesthesia?
Low frequency and maximum amplitude
What happens to thalamo-cortical connections as we fall asleep?
Move from many random connections to a slowed down rhythmic connection
How do general anaesthetics affect thalamic networks?
Affect cortical and arousal nuclei as well as thalamic networks= less corticothalamic feedback resulting in slowing and synchronisation of this pathway
What is the generally accepted theory of the structure of anaesthetics?
Protein with a lipophilic pocket
How do some general anaesthetics affect GABA?
Enhancement/activation of GABAa receptor activity directly activates Cl- channels causing loss of consciousness and deactivating thalamic switch to isolate cortex from sensory input
How do some general anaesthetics affect NMDA receptors?
Block them causing analgesia by blockage of nociceptive sensory input at the level of the spinal cord
How do some general anaesthetics affect 2PKC receptors?
2 pore domain K channels are activated for general inhibitory affect
Which anaesthetics potentiate GABA and 2PKC and inhibit NMDA?
Halothane, desflurone, isoflurone, seroflurone
Which anaesthetics potentiate GABA and inhibit NMDA?
Thiopental, propofol, etomidate
Which anaesthetics potentiate 2PKC and inhibit NMDA?
Nitrous oxide and xenon
Which anaesthetics decrease NMDA only?
Ketamine, emergency and paediatric use
Why are intravenous anaesthetics usually unsuitable for long procedures?
Most collect in muscle and fat and are only slowly metabolised
What must intravenous anaesthetics overcome?
Must be lipophilic to overcome BBB
What must inhaled anaesthetics overcome?
Blood-alveolar interface and BBB
Why inhaled anaesthetics which are less soluble in the blood get to the brain more quickly?
Because the drug wants to leave the blood quickly as it is ‘uncomfortable’ due to low solubility
Why inhaled anaesthetics which are more soluble in the blood get to the brain more slowly?
Will leave blood to get to brain less easily
What do we give prior to an anaesthetic in surgery?
Narcotic (e.g. opioid) and anxiolytic/sedative/amnesic, muscarinic antagonist to reduce secretions
How is anaesthesia usually induced?
By IV propofol
How is anaesthesia usually maintained?
Seroflurone/ NOS and muscle relaxant
What is given in recovery from surgery?
Analgesic, anti-emetic, anticholinesterase
