Inositol trisphosphate and cyclic adenosine diphosphate-ribose increase quantal transmitter release at frog motor nerve terminals: possible involvement of smooth endoplasmic reticulum

The release of chemical transmitter from nerve terminals is critically dependent on a transient increase in intracellular Ca²⁺.6., 25. The increase in Ca²⁺ may be due to influx of Ca²⁺ from the extracellular fluid¹⁵ or release of Ca²⁺ from intracellular stores such as mitochondria.1., 8., 18. Whether Ca²⁺ utilized in transmitter release is liberated from organelles other than mitochondria is uncertain. Smooth endoplasmic reticulum is known to release Ca²⁺, e.g., on activation by inositol trisphosphate or cyclic adenosine diphosphate-ribose,² so the possibility exists that Ca²⁺ from this source may be involved in the events leading to exocytosis. We examined this hypothesis by testing whether inositol trisphosphate and cyclic adenosine diphosphate-ribose modified transmitter release. We used liposomes to deliver these agents into the cytoplasmic compartment and binomial analysis to determine their effects on the quantal components of transmitter release. Administration of inositol trisphosphate (10⁻⁴ M) caused a rapid, 25% increase in the number of quanta released. This was due to an increase in the number of functional release sites, as the other quantal parameters were unaffected. The effect was reversed with 40 min of wash. Virtually identical results were obtained with cyclic adenosine diphosphate-ribose (10⁻⁴ M). Inositol trisphosphate caused a 10% increase in quantal size, whereas cyclic adenosine diphosphate-ribose had no effect. The results suggest that quantal transmitter release can be increased by Ca²⁺ released from smooth endoplasmic reticulum upon stimulation by inositol trisphosphate or cyclic adenosine diphosphate-ribose. This may involve priming of synaptic vesicles at the release sites or mobilization of vesicles to the active zone. Inositol trisphosphate may have an additional action to increase the content of transmitter within the vesicles. These findings raise the possibility of a role of endogenous inositol phosphate and smooth endoplasmic reticulum in the regulation of cytoplasmic Ca²⁺ and transmitter release.