Acquired pharmacoresistance in temporal lobe epilepsy is driven by Na_v1.6-mediated subicular hyperexcitability

Pharmacoresistance to anti-seizure medications (ASMs) remains a major unmet challenge in temporal lobe epilepsy (TLE), and occurs diversely, as classified to primary or acquired manner. The pathophysiological underpinnings of acquired pharmacoresistance remain elusive. Here, using a hippocampal kindling mouse model, we established that prolonged lamotrigine (LTG) treatment-either during or after kindling-induces broad-spectrum resistance to multiple ASMs, effectively recapitulating clinical patterns of acquired pharmacoresistance. Multimodal interrogation revealed hyperexcitability of subicular pyramidal neurons as a critical factor in pharmacoresistance, characterized by elevated c-Fos expression specifically within the subiculum, as well as hyperexcitability of subicular glutamatergic pyramidal neurons. This hyperexcitability phenotype stemmed from Na_v1.6 upregulation, driving both enhanced persistent sodium current (I_NaP) and a pro-excitatory shift in voltage-dependent activation kinetics of voltage-gated sodium channel (VGSC). Crucially, pharmacological activation of subicular Na_v1.6 sufficed to induce acquired pharmacoresistance in pharmaco-responsive mice. Conversely, subiculum-specific Na_v1.6 knockdown in pyramidal neurons (but not GABAergic neurons) prevented or reversed pharmacoresistance, while analogous genetic manipulation in the CA1 had no such impact. Chemogenetic inhibition of subicular pyramidal neurons (mimicking ASM effects) restored drug responsiveness, directly implicating compensatory increases in Na_v1.6 offsetting ASM's inhibitory function on subicular excitability in acquired pharmacoresistance. These findings collectively identify Na_v1.6 upregulation in subicular pyramidal neurons as a critical driver of acquired pharmacoresistance in TLE, highlighting a novel therapeutic target for refractory epilepsy.