A structural study of the squid synapse after intraaxonal injection of calcium

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Abstract
The giant synapse of the squid, [Loligo vulgaris], was examined by EM after ionophoretic injection of Ca2+ into the pre- or postsynaptic axon. There seem to be differences in the Ca buffering mechanims in pre- and postsynaptic axons. After injection of Ca2+ into the postsynaptic axon, mitochondria were heavily loaded with granular inclusions, but mitochondria of injected presynaptic terminals did not contain inclusions. In the postsynaptic axon, besides inclusions in mitochondria, dense deposits were found in axoplasmic vesicles and cisternae that appeared locally at the site of injection. Injection of Ca2+ into the presynaptic terminal produced non-membrane bound dense deposits associated with the filamentous ground structure of the axoplasm. Some Ca may be bound to the presynaptic membrane which appears dense after injection. In both axons the alterations produced by injected Ca2+ were confined mainly to the area of injection. After Ca2+ injection into the presynaptic nerve terminal, synaptic vesicles disappeared and a large number of coated vesicles appeared, and membrane invaginations developed involving the presynaptic membrane, postsynaptic processes and glial cells. After injection of large quantities of Ca2+ into the postsynaptic axon, electron-dense precipitates were seen in the presynaptic terminal indicating retrograde transfer of material from post-to presynaptic axons.