125I-labeled fasciculin 2:: A new tool for quantitation of acetylcholinesterase densities at synaptic sites by EM-autoradiography

Radio-iodinated fasciculin 2 (Fas2), a polypeptide anticholinesterase toxin from Mamba venom, was used as a new probe for localizing and quantifying acetylcholinesterase (AChE) at mouse neuromuscular junctions (NMJs) by quantitative electron microscope autoradiography. We demonstrate that I-125-Fas2 binds very specifically to the NMJs of mouse sternomastoid muscles, with very little binding to other regions in the muscles. Junctional AChE-size densities obtained from the autoradiograms were similar to those previously obtained for the same muscles using H-3-DFP. The use of I-125-Fas2 with EM-autoradiography is simpler and provides higher resolution and sensitivity, as well as considerably lower non-specific binding than previously attainable with H-3-DFP. The advantages and limitations of this procedure are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.