Agonists, antagonists and modulators of excitatory amino acid receptors in the guinea‐pig myenteric plexus

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Abstract
1 The receptors for glutamic acid (L-Glu) present in the guinea-pig myenteric plexus-ileal longitudinal muscle preparation have been studied by measuring the muscle contraction induced by numerous putative endogenous agonists acting at these receptors. Furthermore, the actions of different concentrations of antagonists, glycine, Mg2+ and Ca2+ on the ileal contractions induced by l-G1u have been evaluated. 2 The EC50 values of the most common putative endogenous agonists of these receptors were: L-Glu 1.9 × 10−5 m; L-aspartate 8 × 10−5 m; quinolinate 5 × 10−4M; L-homocysteate 1.4 × 10−4M; the dipeptide aspartyl-glutamate 8 × 10−5m, while N-acetyl-aspartyl-glutamate was inactive. Among the molecules used to classify excitatory amino acid receptors, N-methyl-D-aspartate (NMDA) was the most potent (EC50 5 × 10−4m). Kainic and quisqualic acids were almost completely inactive. 3 The responses to L-Glu were competitively antagonized by 2-amino-5-phosphonovaleric acid. They were, also, prevented by hyoscine (10−7m) and by tetrodotoxin (3 × 10−7 m), suggesting that the L-Glu-induced ileal contraction was in some way dependent upon an action on the myenteric cholinergic neurones. Kynurenic acid was a non-competitive antagonist, γ-D-glutamyl-taurine (10−4m) and aminophosphonobutyric acid (10−4m) did not modify the L-Glu-induced contractions. 4 Glycine (10−5 m) significantly potentiated the effects of glutamate especially when the ionic composition of the superfusion medium contained concentrations of Ca2+ in the range of 0.6–1.2 mM. Strychnine 3 × 10−5 m did not modify the actions of glycine. 5 The data presented here confirm the presence of NMDA receptors in the guinea-pig myenteric plexus, and show that these receptors, similar to those present in primary neuronal cultures may be modulated by glycine.