Trophic effect of cholera toxin B subunit in cultured cerebellar granule neurons: Modulation of intracellular calcium by GM1 ganglioside

Survival of cerebellar granule cells (CGC) in culture was significantly improved in the presence of cholera toxin B subunit (Ctx B), a ligand which binds to GM1 with specificity and high affinity. This trophic effect was linked to elevation of intracellular calcium ([Ca ²⁺](i)), and was additive to that of high K ⁺. Survival was optimized when Ctx B was present for several days during the early culture period. ⁴⁵Ca ²⁺ and cell survival studies indicated the mechanism to involve enhanced influx of Ca ²⁺ through L-type voltage-sensitive channels, since the trophic effect was blocked by antagonists specific for that channel type. Inhibitors of N-methyl-D- aspartate receptor/channels were without effect. During the early stage of culture Ctx B, together with 25 mM K ⁺, caused [Ca ²⁺](i) to rise to 0.2- 0.7 μM in a higher proportion of cells than 25 mM K ⁺ alone. A significant change in the nature of GM1 modulation of Ca ²⁺ flux occurred after 7 days in culture, at which time Ctx B ceased to elevate and instead reduced [Ca ²⁺](i) below the level attained with 25 mM K ⁺. GM1 thus appears to serve as intrinsic inhibitor of one or more L-type Ca ²⁺ channels during the first 7 days in vitro, and then as intrinsic activator of (possibly other) L- type channels after that period. This is the first demonstration of a modulatory role for GM1 ganglioside affecting Ca ²⁺ homeostasis in cultured neurons of the CNS.