Novel seizure phenotype and sleep disruptions in knock-in mice with hypersensitive α4*nicotinic receptors

A leucine to alanine substitution (L9'A) was introduced in the M2 region of the mouse alpha 4 neuronal nicotinic acetylcholine receptor (nAChR) subunit. Expressed in Xenopus oocytes, alpha 4(L9'A)beta 2 nAChRs were >= 30-fold more sensitive than wild type (WT) to both ACh and nicotine. We generated knock-in mice with the L9'A mutation and studied their cellular responses, seizure phenotype, and sleep wake cycle. Seizure studies on alpha 4-mutated animals are relevant to epilepsy research because all known mutations linked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) occur in the M2 region of alpha 4 or beta 2 subunits. Thalamic cultures and synaptosomes from L9'A mice were hypersensitive to nicotine-induced ion flux. L9'A mice were similar to 15-fold more sensitive to seizures elicited by nicotine injection than their WT littermates. Seizures in L9'A mice differed qualitatively from those in WT: L9'A seizures started earlier, were prevented by nicotine pretreatment, lacked EEG spike-wave discharges, and consisted of fast repetitive movements. Nicotine-induced seizures in L9'A mice were partial, whereas WT seizures were generalized. When L9'A homozygous mice received a 10 mg/kg nicotine injection, there was temporal and phenomenological separation of mutant and WT-like seizures: an initial seizure similar to 20 s after injection was clonic and showed no EEG changes. A second seizure began 3-4 min after injection, was tonic-clonic, and had EEG spike-wave activity. No spontaneous seizures were detected in L9'A mice during chronic video/EEG recordings, but their sleep-wake cycle was altered. Our findings show that hypersensitive alpha 4* nicotinic receptors in mice mediate changes in the sleep-wake cycle and nicotine-induced seizures resembling ADNFLE.