L-glutamate as an Excitatory Transmitter of Motor Giant Neurons in the Crayfish Procambarus Clarkii

Motor giant neurons (MoGs) of the crayfish are identifiable motor neurons that innervate abdominal fast flexor muscles. The role of L-glutamate as a neurotransmitter at the neuromuscular junction between MoG and the abdominal fast flexor muscles was investigated using electrophysiological and pharmacological techniques. Local application of L-glutamate to a fast flexor muscle induced a depolarization of the muscle fibers. This glutamatergic depolarization was dose-dependent and was associated with a conductance increase. Bath application of L-glutamate caused a reversible reduction in the amplitude of the excitatory junctional potentials (EJPs) of the fast flexor muscles evoked by MoG stimulation. Twenty micromole L-glutamate was sufficient to reduce the amplitude of MoG-evoked EJP. The effect of depression was dose-dependent and MoG stimulation elicited almost no response in flexor muscle fibers under 1 mM L-glutamate application. Joro spider toxin, an L-glutamate antagonist, reduced the amplitude of EJPs elicited by MoG stimulation at a concentration of 0.1 µM. The response of the flexor muscle to MoG stimulation did not recover completely after washing. When 5 µM Joro spider toxin was applied, the EJP evoked by MoG was completely blocked and no recovery was observed after washing. These results strongly suggest that L-glutamate is the neurotransmitter at the neuromuscular junction between MoG and the abdominal fast flexor muscle in the crayfish.