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 ¹²⁵I-Fas2 binds very specifically to the NMJs of mouse sternomastoid muscles, with very little binding to other regions in the muscles. Junctional AChE-site densities obtained from the autoradiograms were similar to those previously obtained for the same muscles using ³H-DFP. The use of ¹²⁵I- Fas2 with EM-autoradiography is simpler and provides higher resolution and sensitivity, as well as considerably lower non-specific binding than previously attainable with ³H-DFP. The advantages and limitations of this procedure are discussed.