Recent genetic and molecular biological analyses have revealed many forms of inherited channelopathies. Homozygous ataxic mice, tottering (tg) and leaner (tg la ) mice, have mutations in the P/Q-type Ca 2؉ channel ␣ 1A subunit gene. Although their clinical phenotypes, histological changes, and locations of gene mutations are known, it remains unclear what phenotypes the mutant Ca 2؉ channels manifest, or whether the altered channel properties are the primary consequence of the mutations. To

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... ess these questions, we have characterized the electrophysiological properties of Ca 2؉ channels in cerebellar Purkinje cells, where the P-type is the dominant Ca 2؉ channel, dissociated from the normal, tg, and tg la mice, and compared them with the properties of the wild-type and mutant ␣ 1A channels recombinantly expressed with the ␣ 2 and ␤ subunits in baby hamster kidney cells. The most striking feature of Ca 2؉ channel currents of mutant Purkinje cells was a marked reduction in current density, being reduced to ϳ60 and ϳ40% of control in tg and tg la mice, respectively, without changes of cell size. The Ca 2؉ channel currents in the tg Purkinje cells showed a relative increase in non-inactivating component in voltage-dependent inactivation. Besides the same change, those of the tg la mice showed a more distinct change in voltage dependence of activation and inactivation, being shifted in the depolarizing direction by ϳ10 mV, with a broader voltage dependence of inactivation. In the recombinant expression system, the tg channel with a missense mutation (P601L) and one form of the two possible tg la aberrant splicing products, tg la (short) channel, showed a significant reduction in current density, while the other form of the tg la channels, tg la (long), had a current density comparable to the normal control. On the other hand, the shift in voltage dependence of activation and inactivation was observed only for the tg la (long) channel. Comparison of properties of the native and recombinant mutant channels suggests that single tottering mutations are directly responsible for the neuropathic phenotypes of reduction in current density and deviations in gating behavior, which lead to neuronal death and cerebellar atrophy.