Activity- and target-dependent regulation of large-conductance Ca2+-activated K+ channels in developing chick lumbar motoneurons

The functional expression of large-conductance (BK-type) Ca2+-activated K+ (K-Ca) channels was examined in developing chick lumbar motoneurons (LMNs) between embryonic day 6 (E6) and E13 using patch-clamp recording techniques. The macroscopic K-Ca current of E13 LMNs is inhibited by iberiotoxin and resistant to apamin. The average macroscopic KCa density was low before E8 and increased 3.3-fold by E11, with an additional 1.8-fold increase occurring by E13. BK-type K-Ca channels could not be detected in inside-out patches from E8 LMNs but were readily detected at E11. The density of voltage-activated Ca2+ currents did not change between E8 and E11. Surgical ablation of target tissues at E5 caused a significant reduction in average K-Ca density in LMNs measured at E11. Conversely, chronic in ovo administration of D-tubocurarine, which causes an increase in motoneuron branching on the surface of the muscle target tissue, evoked a 1.8-fold increase in average LMN K-Ca density measured at E11. Electrical activity also contributed to developmental regulation of LMN K-Ca density. A significant reduction in E11 K-Ca density was found after chronic in ovo treatment with the neuronal nicotinic antagonist mecamylamine or the GABA receptor agonist muscimol, agents that reduce activation of LMNs in ovo. Moreover, 3 d exposure to depolarizing concentrations of external K+ to LMNs cultured at E8 caused an increase in K-Ca expression. Conversely, tetrodotoxin caused a decrease in K-Ca expression in cultured E8 LMNs developing for 3 d in the presence of neurotrophic factors that promote neuronal survival in the absence of target tissues.