Glutamate antagonists at a crayfish neuromuscular junction

Abstract

As Kravitz et al.¹ have pointed out, glutamic acid is at present the only candidate for the excitatory transmitter at crustacean neuromuscular junctions. The evidence is that^1,2 glutamic acid is the most potent excitatory substance present in the crustacean central nervous system (CNS), whole peripheral nerves and isolated excitatory axons^1,3–5; that the glutamic acid content in nerve extracts accounts for the whole of their excitatory activity on crustacean muscles¹, and that the receptors to glutamate are localised at the same junctional spots as the receptors to the natural excitatory transmitter⁶. Also, both glutamic acid and the natural transmitter produce similar changes in the permeability of the postjunctional muscle membrane^7–9 and in spite of a large background leakage, a significant release of glutamic acid can be evoked by stimulation of excitatory nerves¹. There is also a selective uptake of glutamic acid at the neuromuscular junction which could provide an inactivation mechanism¹⁰. There is, however, no information on whether glutamate receptors have the same pharmacological properties as the receptors of the natural excitatory transmitter.