Effects on Ion Channels
A critical mechanism for seizure prevention is the stabilization of neuronal membranes through the blockade of voltage-gated ion channels.
Sodium Channel Blockade Drugs act on the presynaptic membrane, preventing the propagation of pathological excitation and suppressing the generation of high-frequency discharges. This group includes phenytoin, carbamazepine, lamotrigine, and, partially, valproic acid.
Calcium Channel Modulation Drugs can act at both presynaptic and postsynaptic levels:
- T-type channels are located postsynaptically (especially in thalamic neurons, which is critical for the generation of absence seizures). They are specifically blocked by ethosuximide and valproic acid.
- High-voltage-activated (HVA) channels are located on the presynaptic membrane. Their blockade reduces calcium influx into the terminal and decreases the release of the excitatory neurotransmitter glutamate. A key drug with this mechanism is gabapentin, which binds to the specific $\alpha_2\delta$ subunit of the channel.
Interference with Neurotransmitter Transmission
The second major strategy involves altering the balance between inhibitory and excitatory central nervous system systems.
Enhancement of GABAergic Transmission GABA (gamma-aminobutyric acid) is the primary inhibitory neurotransmitter. Its activity can be pharmacologically enhanced via three pathways:
- Direct receptor action: Allosteric modulation of $\text{GABA}_\text{A}$ receptors on the postsynaptic membrane enhances chloride influx into the cell, causing hyperpolarization. This is how benzodiazepines and phenobarbital work.
- Reuptake blockade: Inhibition of GABA transporters increases the concentration of the neurotransmitter in the synaptic cleft (tiagabine).
- Metabolism inhibition: Irreversible blockade of the enzyme GABA transaminase (GABA-T) in glial cells prevents the degradation of the inhibitory neurotransmitter (vigabatrin).
Suppression of Glutamatergic Transmission Drugs can directly reduce the activity of the excitatory system. For example, felbamate acts as an antagonist at postsynaptic NMDA receptors, blocking the action of glutamate and glycine.
Drug Selection Based on Seizure Type
The clinical use of antiepileptic drugs strictly depends on the type of epilepsy:
- Generalized tonic-clonic seizures: Major agents are used (carbamazepine, valproic acid, phenytoin, lamotrigine), barbiturates (phenobarbital, primidone), as well as topiramate and vigabatrin.
- Absence seizures: Ethosuximide is the specific drug of choice. Broad-spectrum agents (valproic acid, lamotrigine) and clonazepam are also effective.
- Myoclonic epilepsy: Valproic acid, clonazepam, lamotrigine.
- Partial (focal) seizures: Broad-spectrum agents matching those used for generalized seizures are used, plus adjunctive drugs (gabapentin, topiramate, vigabatrin, clonazepam).
- Status epilepticus: Requires emergency management. First-line drugs are intravenous benzodiazepines (diazepam, lorazepam, clonazepam). Phenobarbital and phenytoin are used as alternatives or add-on therapy.
Characteristics of Phenytoin
Phenytoin is a hydantoin derivative. Historically, it is the first modern anticonvulsant introduced into clinical practice in 1938.
Its key pharmacodynamic feature is use-dependent kinetics. The drug binds to sodium channels exclusively in their inactivated state and slows their recovery. The degree of binding is directly proportional to the frequency of channel opening. This means that the more hyperactive a neuron is (within the epileptic focus), the stronger the effect of phenytoin on it. This selective action on pathological discharges allows the drug to suppress seizures without generalized CNS depression or sedative effects.
In addition to its antiepileptic action (prevention of partial and generalized tonic-clonic seizures, termination of status epilepticus), phenytoin possesses antiarrhythmic properties, particularly useful in cardiac glycoside toxicity, and an analgesic effect utilized in the treatment of trigeminal neuralgia.