Medicine – Mechanisms of Action of Antiseizure Drugs

Antiseizure drugs work by reducing excessive neuronal excitability and preventing abnormal repetitive firing in the brain. They do this mainly by blocking excitatory ion channels, enhancing inhibitory neurotransmission through GABA, or reducing excitatory neurotransmission through glutamate.

1. Sodium Channel Blockade

Voltage-gated Na⁺ channels are essential for generation and propagation of action potentials.

During seizures, neurons fire repeatedly at high frequency.

Blocking sodium channels stabilises the neuronal membrane and keeps the channels in an inactive state for longer.

Therefore:

Na⁺ channel blockade → reduced repetitive neuronal firing → reduced seizure activity.

2. Drugs Acting on Sodium Channels

Important examples include:

Phenytoin.

Carbamazepine.

Lamotrigine.

Valproate also has some sodium-channel effects as part of its broader mechanism.

3. Calcium Channel Blockade

Calcium channels are involved in neuronal excitability and neurotransmitter release.

A particularly important type in epilepsy is the:

T-type calcium channel in the thalamus.

These channels contribute to the rhythmic thalamocortical activity responsible for absence seizures.

4. Thalamic T-Type Calcium Channels

Inhibition of thalamic T-type Ca²⁺ channels reduces abnormal oscillatory activity.

Therefore:

T-type Ca²⁺ channel inhibition → reduced thalamocortical rhythmic firing → reduced absence seizures.

The classic drug is:

Ethosuximide.

5. GABA

Gamma-aminobutyric acid, GABA, is the major inhibitory neurotransmitter in the central nervous system.

Its overall function is to reduce neuronal excitability.

Therefore:

↑ GABA activity → ↑ inhibition → ↓ seizures.

Several antiseizure drugs work by increasing GABA-mediated inhibition.

6. GABA-A Receptors

The GABA-A receptor is a ligand-gated chloride channel.

When GABA activates this receptor, chloride conductance increases, making the neuron less excitable.

This produces neuronal inhibition.

7. Benzodiazepines

Benzodiazepines enhance the action of GABA at GABA-A receptors.

They are more accurately described as positive allosteric modulators rather than direct receptor agonists.

They increase the:

Frequency of chloride-channel opening.

This rapidly suppresses neuronal activity.

8. Barbiturates

Barbiturates also enhance GABA-A receptor activity.

They characteristically increase the:

Duration of chloride-channel opening.

An important antiseizure example is:

Phenobarbital.

A useful memory aid is:

Benzodiazepines = frequency.

Barbiturates = duration.

9. GABA Synthesis

GABA is produced from:

Glutamate.

The enzyme responsible is:

Glutamic acid decarboxylase, GAD.

Therefore:

Glutamate → GAD → GABA.

Increasing GABA synthesis increases inhibitory neurotransmission.

10. Important Clarification About GAD

The original note states that activation of glutamic acid decarboxylase increases GABA.

Conceptually this is correct.

However, most antiseizure drugs do not work primarily by directly stimulating GAD.

Rather, they increase GABA through several different mechanisms.

11. GABA Breakdown

GABA is metabolised by:

GABA transaminase.

If this enzyme is inhibited, less GABA is broken down.

Therefore:

GABA transaminase inhibition → ↑ GABA concentration → ↑ neuronal inhibition.

12. Vigabatrin

The classic drug acting through this mechanism is:

Vigabatrin.

It irreversibly inhibits GABA transaminase.

Therefore:

Vigabatrin → reduced GABA breakdown → increased GABA.

13. GABA Reuptake

Another way to increase GABA is to prevent its reuptake from the synaptic cleft.

The classic drug is:

Tiagabine.

It inhibits GABA transporters.

Therefore:

Reduced GABA reuptake → more GABA remains in the synapse → increased inhibition.

14. Valproate and GABA

Valproate increases GABA activity through several mechanisms.

It also affects sodium channels and other neuronal processes.

Therefore, valproate is considered a broad-spectrum antiseizure drug with multiple mechanisms.

15. Glutamate

Glutamate is the major excitatory neurotransmitter in the central nervous system.

It promotes neuronal depolarisation and firing.

Excessive glutamate activity can contribute to seizure generation and spread.

16. Reducing Glutamate Activity

Reducing glutamate transmission decreases neuronal excitation.

Therefore:

↓ glutamate activity → ↓ neuroexcitability → ↓ seizures.

This can be achieved by either:

Reducing glutamate release.

or

Blocking glutamate receptors.

17. Lamotrigine and Glutamate

Lamotrigine reduces glutamate release partly through sodium-channel blockade.

Therefore:

Lamotrigine → Na⁺ channel blockade + reduced glutamate release → reduced excitation.

18. AMPA Receptor Blockade

Glutamate acts on several receptor types, including the:

AMPA receptor.

Blocking AMPA receptors reduces excitatory neurotransmission.

A classic example is:

Perampanel.

Therefore:

Perampanel → AMPA receptor antagonism → reduced glutamate-mediated excitation.

19. Overall Balance of Excitation and Inhibition

Seizures can be thought of as resulting from excessive excitation relative to inhibition.

Antiseizure drugs restore balance by:

Reducing excitatory firing.

and/or

Increasing inhibitory activity.

The four main mechanisms in your original notes fit into this framework.

20. Sodium Channel Mechanism – Note Form

Target: voltage-gated Na⁺ channels.

Effect: stabilises neuronal membranes.

Result: prevents repetitive high-frequency firing.

Examples: phenytoin, carbamazepine, lamotrigine.

21. Calcium Channel Mechanism – Note Form

Target: thalamic T-type Ca²⁺ channels.

Effect: suppresses thalamocortical oscillations.

Main seizure type: absence seizures.

Classic drug: ethosuximide.

22. GABA Mechanism – Note Form

GABA: major inhibitory neurotransmitter.

↑ GABA → ↓ neuronal excitability.

Benzodiazepines: enhance GABA-A receptor activity and increase frequency of chloride-channel opening.

Barbiturates: enhance GABA-A receptor activity and increase duration of chloride-channel opening.

Vigabatrin: inhibits GABA transaminase.

Tiagabine: inhibits GABA reuptake.

23. GABA Synthesis – Note Form

Precursor: glutamate.

Enzyme: glutamic acid decarboxylase.

Reaction:

Glutamate → GABA.

Increasing GABA synthesis increases inhibitory neurotransmission.

24. GABA Breakdown – Note Form

Enzyme: GABA transaminase.

If inhibited: GABA accumulates.

Result: increased neuronal inhibition.

Drug example: vigabatrin.

25. Glutamate Mechanism – Note Form

Glutamate: major excitatory neurotransmitter.

Inhibition: reduces neuronal excitation.

Lamotrigine: reduces glutamate release.

Perampanel: blocks AMPA glutamate receptors.

Key Clinical Pattern

The easiest framework is:

Na⁺ channels → stop repetitive firing.

Ca²⁺ channels → suppress thalamic absence-seizure rhythms.

GABA → increase inhibition.

Glutamate → reduce excitation.

And the classic drug associations are:

Phenytoin / Carbamazepine / Lamotrigine → Na⁺ blockade.

Ethosuximide → T-type Ca²⁺ blockade.

Benzodiazepines / Barbiturates → enhance GABA-A.

Vigabatrin → inhibits GABA transaminase.

Lamotrigine / Perampanel → reduce glutamate-mediated excitation.