Potentiation of spontaneous acetylcholine release from motor nerve terminals by glutamate in Xenopus tadpoles

Abstract

Extracellular application of glutamate (100 μM) increased the spontaneous secretion of acetylcholine, as well as the amplitude and decay time of miniature endplate potentials at developing neuromuscular synapses in Xenopus tadpoles. Kainate, quisqualate and N-methyl- d-aspartate (100 μM each) increased miniature endplate potential frequency by 26-, 13- and four-fold, respectively. The rank order of efficacy at 100 μM was kainate > quisqualate > N-methyl- d-aspartate > glutamate. The effect of kainate on miniature endplate potential frequency was inhibited by 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (20 μM), but not by (±)-2-amino-5-phosphonovalerate (20 μM). Treatment with the voltage-dependent Ca 2+ channel blockers verapamil (10 μM), Cd 2+ (100 μM) or ω-conotoxin (1 μM) inhibited the potentiating action of kainate on miniature endplate potential frequency. On the other hand, 1 S,3 R-1-aminocyclopentane-1,3-dicarboxylate (300 μM), a glutamate metabotropic receptor agonist, inhibited the spontaneous acetylcholine release, which was antagonized by the application of 2-amino-3-phosphonopropionate (500 μM). The potentiating effect of glutamate receptor agonists on the miniature endplate potential frequency declined or disappeared in older Xenopus tadpoles. Quisqualate (100 μM) and N-methyl- d-aspartate (100 μM) but not kainate (30 μM) increased the amplitude and decay time of miniature endplate potential, whereas 1 S,3 R-1-aminocyclopentane-1,3-dicarboxylate (300 μM) only increased the decay time of miniature endplate potentials. These results suggest that there are kainate/quisqualate and N-methyl- d-aspartate receptors existing in the motor nerve terminals of younger Xenopus tadpoles and the activation of these receptors potentiates spontaneous acetylcholine release through increasing Ca 2+ influx. Our data suggest that the presynaptic glutamate receptors on cholinergic terminals may be involved in feedback regulation of acetylcholine secretion at earlier embryonic stages.