The cisternal organelle as a Ca2+-storing compartment associated with GABAergic synapses in the axon initial segment of hippocampal pyramidal neurones
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The axon initial segment of cortical principal neurones contains an organelle consisting of two to four stacks of flat, membrane-delineated cisternae alternating with electron-dense, fibrillar material. These cisternal organelles are situated predominantly close to the synaptic junctions of GABAergic axo-axonic cell terminals. To examine the possibility that the cisternal organelle is involved in Ca2+ sequestration, we tested for the presence of Ca2+-ATPase in the cisternal organelles of pyramidal cell axons in the CA1 and CA3 regions of the hippocampus. Electron microscopic immunocyto-chemistry using antibodies to muscle sarcoplasmic reticulum ATPase revealed immunoreactivity associated with cisternal organelle membranes. The localisation of Ca2+-ATPase in cisternal organelles was also confirmed by enzyme cytochemistry, which produced reaction product in the lumen of the cisternae. These experiments provide evidence for the presence of a Ca2+ pump in the cisternal organelle membrane, which may play a role in the sequestration and release of Ca2+. Cisternal organelles are very closely aligned to the axolemma and the outermost cisternal membrane is connected to the plasma membrane by periodic electron-dense bridges as detected in electron micrographs. It is suggested that the interface acts as a voltage sensor, releasing Ca2+ from cisternal organelles upon depolarisation of the axon initial segment, in a manner similar to the sarcoplasmic reticulum of skeletal muscle. The increase in intra-axonal Ca2+ may regulate the GABAA receptors associated with the axo-axonic cell synapses, and could affect the excitability of pyramidal cells.
Key wordsCalcium Calcium-activated ATP-ase GABA Immunocytochemistry Synapse
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