Anticonvulsants Flashcards
Seizure
single event of brain dysfunction with various forms, but commonly involving hypersynchrony of neuronal discharge in cortex
Epilepsy
disorder of chronic seizures; one of the oldest known brain disorders, associated with demonic possession in ancient times; prevalence is 1%
Partial seizure
involves one hemisphere
Simple seizure
sensory disturbances, 30-60 sec, retains consciousness
Complex seizure
lost consciousness, 1-2 min, aura, automatisms, amnesia after
Secondarily generalized seizure
turns into a generalize seizure, treated as such
Generalized seizure
involves both hemispheres, lost consciousness
Tonic/clonic (grand mal) seizure
stiffening (tonic) then jerking (clonic), 1-2 min
Absence
lapse in awareness, children, 100s per day
Myoclonic and Clonic seizures
brief contractions of body, local or generalized
Tonic seizures
stiffening only
Atonic seizures
loss of tone, falls
Status epilepticus
prolonged seizure (5 min), medical emergency
Seizure etiology
- stress
- head trauma
- infection
- fever
- tumors
- brain malformations
- genetic factors
What happens with neuronal firing in seizures
neurons fire in synchrony, producing large summations in current recorded from scalp electrodes
What happens with neuronal firing post-seizure
possibly less activity than pre-seizure, often termed post-ictal depression
Electrophysiological progression of a seizure
- Large excitatory synaptic current in seizure focus
- PDS induction and interictal spike/wave generation
- Secondary spikes/PDS coincide with seizure activity
- Postictal depression
What gives rise to the initial spike in the EEG?
thought to be from some form of enhanced excitatory synaptic current or diminished inhibitory current
Post-tetanic potentiation
cellular model where a tetanus (which would model a seizure) can induce a potentiated response from a synapse due to buildup of Ca2+ in the terminal during the tetanus; may be relevant to anticonvulsant drugs that manipulate Ca2+ channels
Long term potentiation
phenomenon where repeated stimulation strengthens synaptic connections, which could occur in seizures, perhaps explaining aspects of kindling and mirror foci; relevant to anti-glutamateric drugs that block NMDA and/or AMPA receptors
Paroxysmal depolarizing shift
describes cellular phenomenon that there are checks and balances in neurons that maintain proper balance of excitation/inhibition, but excessive excitation or reduced inhibition can result in excessive discharge characteristic of seizures
Epilepsy animal models
- Mirror foci
- Maximum electroshock
- Kindling
- Pentylenetetrazole
- Channelopathies
Mirror foci
a seizure focus is created on one side of the brain using various chemical or electrical methods, which is followed by spontaneous formation of another focus int he same brain area on the other hemisphere
Maximum electroshock
current applied to the brain is incrementally increased until a maximal seizure is elicited, providing a measure of seizure sensitivity; drugs are assayed for their ability to reduce the sensitivity to seizure formation, or to increase the current that is necessary to produce a seizure
Kindling
a brain area is repeatedly stimulated to produce a seizure focus, and drugs are assayed for their ability to suppress formation of this focus and/or to suppress seizure activity from this focus once formed
Pentylenetetrazole (PTZ)
a pro-convulsant drug that likely mediates its effects by blocking GABA-A receptors, thereby reducing inhibition and shifting the balance toward excitation of neurons; anticonvulsant drugs are sued to test suppression of PTZ-induced seizures
Channelopathies
genetic animal models that produce mutated ion channels to model mutations in human that are thought to produce seizure disorders
Drugs targeted to the VG Na+ channel
- Phenytoin
- Carbamazepine
- Lamotrigine
- Lacosamide
Drugs targeted to the VG Ca2+ channel
- Ethosuximide
- Lamotrigine
- Gabapentin
- Pregabalin
Drugs targeted to the K+ channels
- Retigabine
Drugs targeted to AMPA receptors
- Phenobarbital
- Topiramate
- Lamotrigine
Drugs targeted to NMDA receptors
- Felbamate
What do drugs targeted to the VG Na+ channel do?
drugs slow recovery from inactivation, preventing high frequency spiking
What do drugs targeted to the VG Ca2+ channel do?
drugs diminish excitability, especially at the synapse
What do drugs targeted to the K+ channels do?
K+ channel openers; prolong refractory period and attenuate high frequency spiking
What do drugs targeted to the AMPA receptors do?
block the receptors; blocking will diminish excitability
What do drugs targeted to the NMDA receptors do?
block the receptors; block will diminish excitability, potentially inhibit LTP
Why do anticonvulsants appear to selectively suppress firing in parts of the brain that exhibit rapid firing and depolarized membrane potentials?
because they interact with specific states of the voltage-gated sodium channels
Describe the normal neuronal firing rate (frequency)
Lower frequency; drugs completely dissociate between spikes; activity is preserved
Describe epileptic-style firing rates (frequency)
High frequency; drugs do not completely dissociate between spikes; block accumulates each spike; activity is suppressed
Describe the normal neuronal membrane potential
Hyperpolarized Vm; drugs completely dissociates between spikes; activity is preserved
Describe a depolarized neuron membrane potential
depolarized Vm prolongs drugs interaction with channel; high percentage of channels are always blocked; activity is suppressed
What kind of block does lidocaine exhibit?
Frequency-dependent and voltage-dependent block
What happens when you block GAT-1?
blocking transporter will permit GABA to build-up, decreasing excitability
GAT-1 blocker
Tiagabine
What happens when you block GABA-T?
transaminase breaks down GABA, so block will increase levels
GABA-T blocker
Vigabatrin
How do benzodiazepines relate to seizures?
benzos act at allosteric site in GABAA receptor, augmenting Cl- current and decreasing excitability
Standard seizure drugs
- Phenytoin
- Carbamazepine
- Ethosuximide
- Phenobarbital
- Benzodiazepines
- Valproate
New seizure drugs
- Gabapentin
- Lamotrigine
- Tiagabine
- Topirimate
- Felbamate
- Levetiracetam
- Zonisamide
- Vigabatrin
Phenytoin useful for
partial and generalized tonic/clonic seizures
Phenytoin blocks
voltage-gated sodium channels, perhaps preferentially blocking repetitive spiking that occurs in seizures
Carbamazine useful for
partial and generalized tonic/clonic seizures
Carbamazine very similar to
similar in activity and clinical usefulness to phenytoin
Ethosuximide blocks
specifically blocks T-type Ca2+ channels
Ethosuximide useful for
only useful for absence seizures
Benzodiazepines useful for which types of seizures
Status epilepticus and types of seizure clusters
Two main benzodiazepines useful for seizures
Diazepam and Lorazepam
MOA of phenobarbital
potentiates GABA at GABA-A receptors and at higher concentrations can directly activate the receptor
Phenobarbital is useful for
partial and generalized tonic/clonic seizures, though rarely used due to sedation and safety concerns
MOA of gabapentin
block of presynaptic voltage-gated Ca2+ channels, and enhancement of GABA levels
Gabapentin is useful for
partial and generalized tonic/clonic seizures
MOA of lamotrigine
block voltage-gated sodium channels as well as L-type Ca2+ channels, suppressing excitatory activity
Lamotrigine is useful for
partial and myoclonic seizures and some usefulness in absence seizures
MOA of tiagabine
blocks GAT-1, which will result in increased synaptic levels of GABA and therefore increased inhibition of postsynaptic cells
Tiagabine used for
often used as adjunct or addition to those not responsive to monotherapy
Adverse effects of tiagabine
can cause seizures in those taking it for other indications (non-epileptic patients), such as psychiatric disorders; somnolence, headache, depression
MOA of topiramate
blocks VG Na+ channels, potentiates GABA, and antagonizes glutamate receptors; blocks repetitive firing, so likely an effect on VG sodium channels
Use of topiramate
used in partial and generalized T/C seizures
Drug of choice for unambiguous idiopathic generalized epilepsy
Valproate
MOA of valproate
blocks repetitive firing of neurons (VG Na+ channels), blocks NMDA current, enhances GABa levels
Valproate used in
absence and generalized T/C seizures
Adverse effects of valproate
teratogenic (spina bifida); should be avoided in patients with liver disease; GI (nausea/vomiting); weight gain
Adverse effects of phenytoin
gingival hyperplasia, diplopia, ataxia, hirsuitism, teratogenic, low toxicity but many drug interactions
Adverse effects of carbamazepine
nausea, ataxia, teratogenic
Adverse effects of ethosuximide
GI, sleep disturbances, drowsiness, depression
Adverse effects of benzodiazepines
sedation and tolerance
Adverse effects of gabapentin
mild/moderate sedation, ataxia fatigue
Adverse effects of lamotrigine
serious rash (with valproate), myoclonus
Adverse effects of phenobarbitol
sedation, teratogenic
Adverse effects of topiramate
sedation and somnolence
Pharmacokinetic considerations with anticonvulsants
- Monitor plasma concentrations
- Be aware of dose-dependent elimination
- Induction and inhibition of hepatic enzymes
Which anticonvulsants induce hepatic enzymes?
carbamazepine, phenytoin, phenobarbital
Which anticonvulsants inhibit hepatic enzymes?
Valproate and topiramate