Previous results show that mice expressing α4-containing nicotinic receptors with a Leu9’Ala (L9´A) mutation have increased (~20-fold) sensitivity in nicotine-induced seizures compared to their wild type (WT) littermates (Fonck et al, SFN, 2003). To further characterize the L9´A mutation, nicotine and/or acetylcholine (ACh) responses were measured in: a) voltage-clamped Xenopus oocytes expressing mouse α4L9´Aβ2 and α4β2 receptors, (b) thalamic primary cell cultures obtained from heterozygous (het) and homozygous (hom) L9´A mouse embryos, and c) synaptosomes made from cortex and thalamus of het and hom L9´A mice. In oocytes, the EC50s of the nicotine and ACh concentration-response relationships showed a 30-fold decrease in L9´A compared to WT receptors. At saturating concentrations of nicotine, the maximal mutant and WT nicotine responses were similar. Fura 2-loaded cell cultures derived from L9´A embryos were ~15-25 times more sensitive to nicotine and ACh-induced increases in intracellular Ca2+ compared t...