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Published on: March 16, 2009
Ca²+ buffering at a drosophila larval synaptic terminal.
1Department of Biological Sciences, University at Albany, Suny, Albany, New York 12222, USA.
Synapse (New York, N.Y.)
|January 11, 2011
Summary
The Ca²+-binding ratio (κ(S)) was measured in Drosophila motor terminals, revealing that endogenous buffers bind nearly 99% of Ca²+. This quantifies calcium dynamics crucial for synaptic transmission.
Area of Science:
- Neuroscience
- Cellular Physiology
- Biophysics
Background:
- Quantitative analysis of calcium dynamics (Ca²+) is essential for understanding synaptic function.
- The Ca²+-binding ratio (κ(S)), a key parameter, has not been previously measured in invertebrate synaptic terminals.
- Accurate κ(S) values are critical for interpreting Ca²+ transients and their impact on neurotransmission.
Purpose of the Study:
- To measure the Ca²+-binding ratio (κ(S)) at Drosophila Ib motor terminals.
- To determine the rate constant for Ca²+ removal and Ca²+ influx under physiological conditions.
- To investigate the influence of external Ca²+ and Mg²+ concentrations on Ca²+ influx.
Main Methods:
- Measured κ(S) in Drosophila larvae Ib motor terminals using varying concentrations of the Ca²+ indicator Oregon Green 488 BAPTA-1 (OGB-1).
- Employed a linear single-compartment model to calculate κ(S) based on the effect of [OGB-1] on the decay time constant (τ(decay)) of intracellular free Ca²+ concentration ([Ca²+](i)).
- Analyzed Ca²+ transients following single action potentials (APs) and AP trains, and examined the impact of altered extracellular Ca²+ and Mg²+ levels.
Main Results:
- A κ(S) of 77 was determined, indicating that approximately 99% of Ca²+ entering the terminal is rapidly bound by endogenous buffers.
- Extrapolation to zero OGB-1 concentration yielded a τ(decay) of 46 ms and a Ca²+-removal rate constant of 1641 s⁻¹ for single APs.
- A single AP increased [Ca²+](i) by 196 nM and total intracellular Ca²+ ([Ca²+](free + bound)) by 15.3 μM (at 1.0 mM external Ca²+); AP trains (10 Hz) increased [Ca²+](i) by 185 nM.
Conclusions:
- The high κ(S) value underscores the significant role of endogenous fast Ca²+ buffers in buffering synaptic Ca²+ transients.
- The calculated Ca²+-removal rate constant (827 s⁻¹) for AP trains likely reflects plasma membrane Ca²+-ATPase activity.
- This study provides crucial quantitative data on Ca²+ dynamics in invertebrate synapses, essential for future modeling and research.
