Activity- and Target-Dependent Regulation of Large-Conductance Ca -Activated K Channels in Developing Chick Lumbar Motoneurons

The functional expression of large-conductance (BK-type) Ca -activated K (KCa ) channels was examined in developing chick lumbar motoneurons (LMNs) between embryonic day 6 (E6) and E13 using patch-clamp recording techniques. The macroscopic KCa 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 KCa channels could not be detected in inside-out patches from E8 LMNs but were readily detected at E11. The density of voltageactivated Ca currents did not change between E8 and E11. Surgical ablation of target tissues at E5 caused a significant reduction in average KCa 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 KCa density measured at E11. Electrical activity also contributed to developmental regulation of LMN KCa density. A significant reduction in E11 KCa 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 KCa expression. Conversely, tetrodotoxin caused a decrease in KCa 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.