Tetanic stimulation in Ca²⁺-free Ringer's solution induced Ca²⁺- and Mg²⁺-independent short term enhancement of EPPs

Abstract

We have reported that tetanic stimulation (100Hz, 50s) enhanced miniature end-plate potential (MEPP) frequency in Ca²⁺-free Ringer's solution (0.2mM EGTA, 5mM Mg²⁺) at frog neuromuscular junctions. Furthermore, this enhancement was induced by the accumulation of [Mg²⁺]_i in presynapse during tetanus through Ca²⁺ channels. We examined whether this presynaptic activation affected end-plate potentials(EPPs) using conventional electrophysiological techniques and imaging techniques. The nerve-muscle preparation was circulated with Ca²⁺-free Ringer's solution. EPPs were evoked by 0.25Hz stimulation with puffing 0.9mM Ca²⁺ Ringer's solution to the observed synapse locally. Puff was stopped 3 minutes before tetanus, therefore the extracellar condition was Ca²⁺-free during tetanus. After tetanus, EPP amplitude increased to 5 times and decayed exponentially with a time constant of about 140s. EGTA loaded into the terminal had no effect on this plasticity. Imaging techniques indicated that [Ca²⁺]_i did not increased but [Mg²⁺]_i increased in presynaptic terminals during tetanus. When the extracellar concentration of Mg²⁺ increased from 2 mM to 10 mM, [Mg²⁺]_i peak increased, but this plasticity did not change. Therefore, we concluded that this plasticity depend on neither [Ca²⁺]_i nor [Mg²⁺]_i. Casein kinase 2(CK2) inhibitor, 5,6-dichlorobenzimidazole riboside, reduced this plasticity to 60%. CK2 must be partially responsible for this plasticity. [Jpn J Physiol 54 Suppl:S159 (2004)]