A patient with absence epilepsy is started on ethosuximide. The anti-absence mechanism of ethosuximide involves blockade of which specific ion channel, and in which neuronal tissue is this effect most relevant?

• A Persistent sodium (Na+) current in cortical pyramidal cells
• B T-type (low-voltage-activated) calcium channels in thalamocortical relay neurons ✓
• C N-type calcium channels in cerebellar Purkinje cells
• D NMDA receptor-mediated calcium entry in hippocampal dentate gyrus neurons

Explanation

Ethosuximide selectively blocks T-type (low-voltage-activated) calcium channels, which are responsible for the rhythmic burst firing of thalamocortical relay neurons that generates the 3-Hz spike-and-wave discharges characteristic of absence epilepsy. These T-type channels in the thalamus are critical for producing the oscillatory synchrony between thalamus and cortex seen in absence seizures. Sodium channel blockade is the mechanism of carbamazepine and phenytoin (effective in focal/tonic-clonic, not absence). N-type channels and NMDA receptors are not relevant to ethosuximide's mechanism.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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