Lamotrigine (LTG) is a popular modern antiepileptic drug (AED); however, its mechanism of action has yet to be fully understood, as it is known to modulate many members of several ion channel families. In heterologous systems, LTG inhibits Cav2.3 (R-type) calcium currents, which contribute to kainic-acid (KA)–induced epilepsy in vivo. To gain insight into the role of R-type currents in LTG drug action in vivo, we compared the effects of LTG to two other AEDs in Cav2.3-deficient mice and controls on KA-induced seizures.
Behavioral seizure rating and quantitative electrocorticography were performed after injection of 20 mg/kg (and 30 mg/kg) KA. One hour before KA injection, mice were pretreated with 30 mg/kg LTG, 50 mg/kg topiramate (TPM), or 30 mg/kg lacosamide (LSM).
Ablation of Cav2.3 reduced total seizure scores by 28.6% (p = 0.0012), and pretreatment with LTG reduced seizure activity of control mice by 23.2% (p = 0.02). In Cav2.3-deficient mice, LTG pretreatment increased seizure activity by 22.1% (p = 0.018) and increased the percentage of degenerated CA1 pyramidal neurons (p = 0.02). All three AEDs reduced seizure activity in control mice; however, only the non–calcium channel modulating AED, LSM, had an anticonvulsive effect in Cav2.3-deficient mice. Furthermore, LTG altered electrocorticographic parameters differently in the two genotypes: decreasing relative power of ictal spikes in control mice but increasing relative power of high frequency fast ripple discharges during seizures in Cav2.3-deficient mice.
These findings provided the first in vivo evidence for an essential role for Cav2.3 in LTG pharmacology and shed light on a paradoxical effect of LTG in their absence. Furthermore, LTG appears to promote ictal activity in Cav2.3-deficient mice by increasing high frequency components of seizures, resulting in increased neurotoxicity in the CA1. This paradoxical mechanism, possibly reflecting rebound hyperexcitation of pyramidal CA1 neurons after increased inhibition, may be key in understanding LTG-induced seizure aggravation observed in clinical practice.