Pharmacology - ANTIEPILEPTIC DRUGS (MADE EASY)

Antiepileptic Drugs: Understanding Pharmacology

Overview of Seizures

• The primary use of antiepileptic drugs (AEDs) is to prevent or control epilepsy, characterized by sudden bursts of uncontrolled electrical activity in the brain.

• Seizures are classified into two major groups:

• Focal seizures: Affect a portion of the brain, typically one hemisphere, with or without awareness impairment.

• Generalized seizures: Affect both sides of the brain simultaneously and almost always result in loss of consciousness.

Mechanisms Behind Seizures

• Seizures can be viewed as an imbalance between inhibitory and excitatory processes in the brain, leading to either too little inhibition or excessive excitation.

• Neurons communicate through action potentials, which are initiated by voltage-gated sodium channels opening and allowing sodium ions to enter the cell.

• Glutamate release occurs when calcium ions enter neurons; it binds to AMPA and NMDA receptors on postsynaptic neurons, facilitating further depolarization.

Role of Inhibitory Neurotransmitters

• Excessive glutamate can lead to hyperexcitability; however, inhibitory neurons release GABA that counteracts this effect by making the inside of neurons more negative.

• GABA-A receptors allow chloride ions to enter upon binding with GABA, limiting neuronal response to stimulation.

• After dissociation from GABA-A receptors, GABA is removed from the synaptic cleft via reuptake through GAT-1 and degraded by GABA-T.

Pharmacology of Antiepileptic Drugs

• The main goal of AED therapy is to lower neuronal excitability or enhance inhibition.

• Some AEDs block voltage-gated sodium channels (e.g., Carbamazepine, Lamotrigine), reducing sodium influx into neurons.

Additional Mechanisms and Drug Classes

• Many AEDs target multiple mechanisms; for instance:

• Lamotrigine & Topiramate: Block both sodium and calcium channels.

• Topiramate: Also inhibits excitatory neurotransmission via AMPA receptor blockade.

Unique Mechanisms

• Gabapentin and Pregabalin bind specifically to alpha-2-delta subunits on high-voltage calcium channels, modulating glutamate release for their antiepileptic effects.

• Levetiracetam binds SV2A proteins in vesicles containing glutamate, impairing its release and decreasing neuronal excitability.

Other Notable AED Actions

• Felbamate blocks NMDA receptors inhibiting excitatory neurotransmission.

• Benzodiazepines and Barbiturates enhance GABA-A receptor function by prolonging channel openings for increased chloride ion influx.

Antiepileptic Drugs and Their Side Effects

Mechanisms of Action and Drug Examples

• Vigabatrin: This drug irreversibly inhibits GABA-aminotransferase, the enzyme responsible for breaking down GABA. This action increases GABA concentrations in the brain, enhancing receptor binding on postsynaptic neurons.

Common Side Effects of Antiepileptic Drugs

• General Side Effects: Sedation and dizziness are common side effects associated with all antiepileptic drugs.

• Specific Drug Reactions:

• Carbamazepine & Oxcarbazepine: Can lead to hyponatremia (low sodium levels).

• Vigabatrin: Associated with visual field loss.

• Lamotrigine & Phenytoin: May cause double vision.

Additional Drug-Specific Side Effects

• Phenytoin: Known to cause gingival hyperplasia (gum overgrowth) and hirsutism (excessive hair growth).

• Cognitive Issues: Topiramate and Zonisamide can result in cognitive problems.

• Weight Changes:

• Topiramate is linked to weight loss.

• Valproic acid is more frequently associated with weight gain, though Gabapentin and Pregabalin can also contribute to this issue.