Relationship between the in vivo and in vitro activity of some naturally occurring glutamate analogues on the somatic neuromuscular junction of Lucilia sericata

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Abstract
A preparation of Lucilia sericata flight motor system was arranged so that ganglionic and neuromuscular function could be monitored while experimental compounds were injected into the intact insect. Injections of l‐glutamate, the putative excitatory transmitter at the insect neuromuscular junction, caused a reversible paralysis of the flight muscles. A number of structural analogues of l‐glutamic acid, found in various seed plants, were injected and the results compared. The salts of several of these compounds were as active or more active in causing the paralysis than glutamate itself. Two of the most toxic compounds, salts of 4‐methylene glutamic acid and quisqualic acid were further tested in vitro by iontophoretically applying them directly to exposed insect neuromuscular junctions. Both compounds showed glutamate‐agonistic activity when applied directly to the neuromuscular junction but were less active than glutamate. This difference between in vivo and in vitro effects is caused by removal mechanisms which protect the muscle membranes from the effects of glutamate. These mechanisms do not so readily remove or inactivate 4‐methylene glutamate or quisqualate. Consequently, for a given dose, the concentration of the analogues at the neuromuscular junction remains longer above the critical level which causes paralysis.