An attempt was made to determine electrophysiologically and immunocytochemically the neurotransmitter at neuromuscular junctions (NMJs) in cricket abdominal muscles. One mM bath-applied Lglutamate reduced the amplitudes of large and small excitatory junctional potentials (I- and s-EJPs) reversibly by about 75% of the control on the average. It also produced a slow, transient depolarization of about 10 mV. Joro spider toxin, which is an antagonist of L-glutamate, depressed the amplitudes of I- and s-EJPs almost completely and irreversibly at 3.5 × 10−6 M. By using the antibody to glutamate, glutamate-immunoreactive processes whose configuration resembled that of NMJs revealed by nickel staining were obtained. The present results strongly suggest that the neurotransmitter at cricket NMJs is L-glutamate.
