N-methyl-D-aspartate antagonists, glutamate release inhibitors, 4-aminopyridine at neuromuscular transmission

It is thought that glutamate (GLU) and acetylcholine (ACh) are co-released in the neuromuscular junction (NMJ). Consequently, GLU is also a mediator or modulator of neuromuscular transmission (n-m) together with ACh. Therefore we decided to investigate the role of GLU in n-m by using isolated rat phrenic nerve-hemidiaphragm preparations. Since the GLU receptors present at NMJ have been reported to be predominantly N-methyl-D-aspartate (NMDA) subtype, some non-competitive and competitive NMDA receptor blockers, MK801, ketamine, dextromethorphan and CGP 37849, and GLU release inhibitors, clonidine, guanfacine, tizanidine were used at their optimum concentrations in medium after having found them from dose-response curves. The preparations were first stimulated indirectly in the presence of the optimum concentrations of the drugs used and tensions developed were recorded isometrically through a force displacement transducer on a polygraph linked to a computer + Math coprocessor by an analog converter. All drugs at their optimum concentrations suppressed contractions significantly. Prolyl-glycinamide (PLG) or phenyl-succinate, both of which are the inhibitors of GLU production also suppressed the contraction significantly, following depletion of GLU stores by tetanic contraction in nerve endings. 4-Aminopyridine, which has been shown to release GLU augmented the contractions which were also completely abolished by the NMDA receptor antagonists or GLU release inhibitors at their higher concentrations than their optimum ones. The direct stimulation of the muscles elicited statistically insignificant but higher contractions than controls at the optimum concentrations of the antagonists or inhibitors in medium. The results were discussed and it was concluded that blockade of NMDA receptors, the inhibition of GLU released or the suppression of GLU production inhibit the contractions of the rat-isolated hemidiaphragms elicited by indirect electrical stimulation, without altering acetylcholinergic part of the contraction cascade.