Maturation of Neuromuscular Transmission During Early Development in Zebrafish

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Abstract
Maturation of neuromuscular transmission during early development in zebrafish. We have examined the rapid development of synaptic transmission at the neuromuscular junction (NMJ) in zebrafish embryos and larvae by patch-clamp recording of spontaneous miniature endplate currents (mEPCs) and single acetylcholine receptor (AChR) channels. Embryonic (24–36 h) mEPCs recorded in vivo were small in amplitude (<50 pA). The rate of mEPCs increased in larvae (3.5-fold increase measured by 6 days), and these mEPCs were mostly of larger amplitude (10-fold on average) with (≤5-fold) faster kinetics. Intracellular labeling with Lucifer yellow indicated extensive coupling between muscle cells in both embryos and larvae (≤10 days). Blocking acetylcholinesterase (AChE) with eserine had no effect on mEPC kinetics in embryos at 1 day and only partially slowed (by ∼½) the decay rate in larvae at 6 days. In acutely dissociated muscle cells, we observed the same two types of AChR with conductances of 45 and 60 pS and with similar, brief (<0.5 ms) mean open times in both embryos and larvae. We conclude that AChR properties are set early during development at these early stages; functional maturation of the NMJ is only partly shaped by expression of AChE and may also depend on postsynaptic AChR clustering and presynaptic maturation.