Attempts at localizing calcium in relation to synaptic transmission: an X-ray microanalytical study.

1. In view of the importance of calcium in triggering the transmitter secretion during nerve stimulation or depolarization, the localization of calcium binding sites was studied in stimulated nerve endings that were fixed in calcium-containing s-collidine buffered paraformaldehyde solution. 2. In cholinergic synapses, such as the superior cervical ganglion of cat, the myoneural junction of rat diaphragm and the electric organ of Torpedo marmorata, a stimulation-dependent accumulation of calcium was found in the mitochondria of presynaptic nerve endings. 3. In synaptosomes prepared from rat cerebral cortex, the activity-dependent accumulation of calcium in mitochondria of pinched off nerve endings was also observed. The mitochondrial accumulation of calcium in synaptosomes was dependent on ATP, temperature and could be inhibited by quercetin. 4. In stimulated synapses, only the mitochondria seemed to accumulate calcium in appreciable amounts, whereas other intra-terminal structures, such as the smooth endoplasmic reticulum and synaptic vesicles were devoid of calcium.