Modulatory effects of amino acids on neuromuscular transmission on the crayfish fast flexor muscle

Abstract

The modulatory effects of amino acids on neuromuscular transmission from an identified giant motor neuron (MoG) and abdominal fast flexor muscles of the crayfish were exam- ined using electrophysiological and pharmacological techniques. The distribution of amino acids in the cell body and axon of a single MoG was revealed using high-performance liquid chromatogra- phy with electrochemical detection. Eight different amino acids—aspartate, glutamate, glutamine, arginine, glycine, taurine, alanaine and γ-aminobutyric acid—were simultaneously detected in ei- ther the cell body or the axon of MoG. Aspartate, glutamate, alanine and arginine were present at relatively high concentration. Local pressure ejection of glutamate caused a depolarization of the abdominal fast flexor muscle fibers. On the other hand, aspartate, alanine and arginine had no clear effects on the same muscle fibers. Aspartate and arginine, however, had modulatory effects on neuromuscular transmission. Alanine had no significant effect on the neuromuscular transmission. Aspartate at a concentration of 200 μM decreased the amplitude of EJPs in the fast flexor muscle mediated by stimulation of both the MoG and non-giant fast flexor motor neurons. Arginine at a concentration of 200 μM reduced the EJP amplitude of the muscle fibers in response to MoG stimulation but enhanced the EJP amplitude of the same muscle fibers by stimulation of non-giant fast flexor motor neurons. Although rather high concentration (1 mM) were required, aspartate increased and arginine de- creased the depolarization of the muscle fibers induced by local ejection of glutamate. The oppo- site effect of arginine on the fast flexor muscles in response to the stimulation of different motor neurons suggested its modulatory role in the different effects of these motor neurons (depression and facilitation) in the fast flexor muscles. J. Exp. Zool. 283:531n540, 1999. © 1999 Wiley-Liss, Inc.