The Journal of Neurophysiology Vol. 86 No. 6 December 2001, pp. 2951-2956
Copyright ©2001 by the American Physiological Society

Limits to the Development of Fast Neuromuscular Transmission in Zebrafish

 ¹Centre for Research in Neuroscience, Montreal General Hospital Research Institute, Department of Neurology and Neurosurgery, and Department of Biology, McGill University, Montreal, Quebec H3G 1A4, Canada;  ²Institut des Neurosciences, Centre National de la Recherche Scientifique-Unité Mixte de Recherche 7624, Université Pierre et Marie Curie, 75252 Paris, France; and  ³Department of Cell Biology and Anatomy, University of Miami School of Medicine, Miami, Florida 33136

Drapeau, Pierre, Robert R. Buss, Declan W. Ali, Pascal Legendre, and Richard L. Rotundo. Limits to the Development of Fast Neuromuscular Transmission in Zebrafish. J. Neurophysiol. 86: 2951-2956, 2001. Zebrafish embryos have small and slow miniature end-plate currents (mEPCs), whereas only a few days later larval mEPCs are an order of magnitude larger and faster, being among the fastest of all neuromuscular synapses. To identify the bases for these changes we compared, in embryos and larvae, the properties and distributions of acetylcholine (ACh) receptors (AChRs) and acetylcholinesterase (AChE) as well as the ultrastructure of the developing neuromuscular junctions (NMJs). To mimic synaptic release, patches of muscle membrane were exposed briefly (for 1 ms) to a saturating concentration (10 mM) of ACh. The AChR deactivation kinetics were twice as slow in embryos compared with larvae. In both embryos and larvae, AChRs demonstrated open channel block by millimolar ACh, and this was detected during mEPCs, indicating that a high concentration of ACh is released at immature and mature NMJs. AChR and AChE distributions were compared using the selective fluorescently conjugated labels -bungarotoxin and fasciculin 2, respectively. In larvae, punctate AChR clusters were detected whereas junctional AChE staining was less intense than that found at adult NMJs. Transmission electron microscopy revealed immature nerve endings in embryos that were closely juxtaposed to the surrounding muscle cells, whereas mature larval NMJs had a wider synaptic cleft with a conspicuous basal lamina over a limited region of synaptic contact. Our results indicate that ACh is released at high concentrations at immature NMJs, but its clearance is prolonged and the AChRs are dispersed, resulting in a slow mEPC time course until a mature cleft appears with densely packed faster AChRs and abundant AChE.