Characterization of neuromuscular transmitters in the horseshoe crab, Limulus polyphemus

Neuromuscular junctions (NMJs) in tibial flexor muscle of first-year horseshoe crabs were tested for immunoreactivity to glutamate, aspartate (known neurotransmitters in other arthropod taxa), and to NO synthase (producer of 2nd messenger NO). Light microscopy of semithin epoxy-embedded sections revealed likely sites of NMJs. Ultrathin (60-90 nm) sections were treated with primary antibodies from rabbit sera, followed by goat-anti-rabbit immunogold. Transmission Electron Microscopy has not shown immunoreactivity to Glu, Asp, or NOS to date. The same methods on this muscle showed Glu- and Asp- immunoreactivity in large dark granules of atypical synapses (L. Lasovsky, MS Thesis, WPI, 1998), therefore our subset of NMJs with small, clear vesicles may contain a different neurotransmitter