Program of the 2004 East Coast Worm Meeting - Caenorhabditis ...

23. Ryanodine Receptors Regulate Neurotransmitter Release at the C. elegans
Neuromuscular Junction
Qiang Liu 1 , Michael Nonet 2 , Lawrence Salkoff 2 , Zhao-Wen Wang 1
1Department of Neuroscience, University of Connecticut Health Center, Farmington, CT 06030
2Department of Anatomy and Neurobiology, Washington University School of Medicine, St.
Louis, MO 63110
Traditionally, calcium influx through voltage-gated calcium channels was thought as the sole
source of calcium for synaptic release. Emerging evidence suggests that calcium release from
intracellular stores may also play a role. We investigated a potential role of presynaptic ryanodine
receptors (RYRs), which are calcium release channels in the endoplasmic reticulum membrane,
in spontaneous and evoked synaptic exocytosis using C. elegans as a model system. The
existence of a single RYR gene (ryr-1) and the availability of ryr-1 mutants in C. elegans make
the analysis convenient. The potential role of RYRs was investigated by analyzing miniature and
evoked postsynaptic currents (mPSCs and ePSCs) at the neuromuscular junction. In ryr-1
loss-of-function mutants, both the frequency and mean amplitude of mPSCs were significantly
reduced compared with the wild-type. These changes appeared due to presynaptic RYR-1 defect
since the sensitivity of body-wall muscle cells to exogenously applied neurotransmitters was
similar to that of the wild-type. Acute pharmacological blockade of RYRs produced similar
changes in mPSCs as the ryr-1 mutations, suggesting that a secondary developmental defect
was not involved. In wild-type preparations, elimination of extracellular calcium reduced mPSC
frequency by ~70%. By contrast, similar treatment essentially abolished mPSCs in the ryr-1
mutant. Thus, calcium influx and RYR-mediated calcium release are likely the exclusive sources
of calcium for spontaneous synaptic release. The ryr-1 mutant also showed a significant reduction
in ePSC amplitude. However, the quantal content (ePSC current integral divided by the mean
mPSC current integral) did not change, suggesting that RYRs regulate ePSCs by controlling
quantal size but not quantal number. Quantal number appeared to be solely determined by
depolarization-mediated calcium influx. Given their large effects on mPSCs and ePSCs, RYRs
may play an important role in synaptic plasticity.