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 neur ons that innervate abdominal fast flexor muscles. The role of L-glutam ate as a neurotransmitter at the neuromuscular junction between MoG an d the abdominal fast flexor muscles was investigated using electrophys iological and pharmacological techniques. Local application of L-gluta mate to a fast flexor muscle induced a depolarization of the muscle fi bers. This glutamatergic depolarization was dose-dependent and was ass ociated with a conductance increase. Bath application of L-glutamate c aused a reversible reduction in the amplitude of the excitatory juncti onal potentials (EJPs) of the fast flexor muscles evoked by MoG stimul ation. Twenty micromole L-glutamate was sufficient to reduce the ampli tude of MoG-evoked EJP. The effect of depression was dose-dependent an d 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 mu M. The response of the flexor muscle to M oG stimulation hid not recover completely after washing. When 5 mu M J oro spider toxin was applied, the EJP-evoked by MoG was completely blo cked and no recovery was observed after washing. These results strongl y suggest that L-glutamate is the neurotransmitter at the neuromuscula r junction between MoG and the abdominal fast flexor muscle in the cra yfish.