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Probing for calcium at presynaptic nerve terminals

C F McGraw, A V Somlyo, M P Blaustein

Federation Proceedings

|August 1, 1980

Summary

The smooth endoplasmic reticulum (SER) in nerve terminals sequesters calcium, acting as a non-mitochondrial site for calcium buffering. This finding is crucial for understanding neuronal activity and calcium regulation.

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Area of Science:

Neuroscience
Cell Biology
Biochemistry

Background:

Presynaptic nerve terminals (synaptosomes) rely on efficient calcium regulation for neurotransmission.
Mitochondria are known calcium buffers, but non-mitochondrial sites are also implicated.
The role of smooth endoplasmic reticulum (SER) in neuronal calcium handling requires further elucidation.

Purpose of the Study:

To identify the non-mitochondrial ATP-dependent calcium sequestration site within presynaptic nerve terminals.
To investigate the role of smooth endoplasmic reticulum (SER) in intraterminal calcium buffering.

Main Methods:

Morphological techniques including three-dimensional reconstructions of serial sections.
Incubation of saponin-treated synaptosomes with calcium, ATP, and oxalate.
Electron probe microanalyses to detect calcium deposits.
Experiments using mitochondrial poisons, A23187, EGTA, and omission of oxalate.

Main Results:

Electron-dense deposits, indicative of calcium oxalate, were found in mitochondria, SER cisterns, and vesicular profiles.
Calcium sequestration in SER persisted even when mitochondria were inhibited, indicating a non-mitochondrial site.
Calcium sequestration was abolished when oxalate was omitted or when ionophores/chelators were present.

Conclusions:

The nerve terminal SER is capable of sequestering calcium.
The SER plays a significant role in intraterminal calcium buffering during neuronal activity.
These findings support the SER as a key non-mitochondrial calcium storage site in neurons.

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