Neuromuscular synaptic transmission in Limulus polyphemus--I. Actions of aspartate, glutamate and the natural transmitter

Aspartate and glutamate were examined as excitatory transmitter candidates for the tibia flexor muscle of the chelicerate arthropod, Limulus polyphemus. Bath application of aspartate or glutamate caused dose-dependent depolarizations of Limulus muscle fibers and contractions of the whole muscle. Glutamate was about 10 times more potent than aspartate. Aspartate and glutamate depolarizations were associated with a conductance increase in muscle fibers, although aspartate depolarizations were dependent on external sodium, while glutamate depolarizations persisted in the absence of sodium. Although the Limulus excitatory postsynaptic potential (epsp) was associated with a conductance increase the ionic basis of the epsp could not be determined. If, however, the Limulus epsp, like other arthropod epsps, is sodium-dependent then the sodium-dependence of the aspartate depolarization is consistent with the action of the natural excitatory transmitter. The sodium-independence of glutamate action, however, is not consistent with generally accepted models of arthropod neuromuscular transmitter action. The rank order of potency for amino acid agonists indicates that the Limulus neuromuscular junction is pharmacologically very similar to other arthropod junctions which are well-accepted to be glutamatergic. Pentobarbital reversibly attenuated the amplitudes of the epsp and aspartate and glutamate depolarizations, and it was found to be the only useful antagonist in Limulus.