The cisternal organelle as a Ca2+-storing compartment associated with GABAergic synapses in the axon initial segment of hippocampal pyramidal neurones
- 191 Downloads
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
Unable to display preview. Download preview PDF.
- Buhl EH, Halasy K, Somogyi P (1993) Hippocampal unitary IP-SPs: identified sources and number of release sites. Eur J Neurosci [Suppl 6]:225Google Scholar
- Conradi S (dy1969) Observations on the ultrastructure of the axon hillock and intial axon segment of lumbosacral motoneurones in the cat. Acta Physiol Scand [Supp. 332]: 65–84Google Scholar
- Molnár E, Baude A, Richmond SA, Patel PB, Somogyi P, McIlhinney RAJ (1993) Biochemical and immunocytochemical characterization of antipeptide antibodies to a cloned GluR1 glutamate receptor subunit: cellular and subcellular distribution in the rat forebrain. Neuroscience 53:307–326CrossRefGoogle Scholar
- Nasu F, Inomata K (1990) Ultracytochemical demonstration of Ca2+-ATPase activity in the rat saphenous artery and its innervated nerve terminal. J Electron Microsc (Tokyo) 39:487–491Google Scholar
- Peters A, Palay SL, Webster H (1991) The fine structure of the nervous system, 3rd edn. Oxford University Press, New YorkGoogle Scholar
- Somogyi P, Smith AD, Nunzi MG, Gorio A, Takagi H, Wu J-Y (1983b) Glutamate decarboxylase immunoreactivity in the hippocampus of the cat. Distribution of immunoreactive synaptic terminals with special reference to the axon initial segment of pyramidal neurons. J Neurosci 3:1450–1468CrossRefGoogle Scholar
- Takei K, Stukenbrok H, Metcalf A, Mignery GA, Sudhof TC, Volpe P, De Camilli P (1992) Ca2+ Stores in Purkinje neurons: endoplasmic reticulum subcompartments demonstrated by the heterogenous distribution of the InsP3 receptor, Ca2+- ATPase, and calsequestrin. J Neurosci 12:489–505CrossRefGoogle Scholar
- Whiting P, McKernan RM, Iversen LL (1990) Another mechanism for creating diversity in γ-aminobutyrate type A receptors: RNA splicing directs expression of two forms of γ2 sub-unit, one of which contains a protein kinase C phosphorylation site. Proc Natl Acad Sci USA 87:9966–9970CrossRefGoogle Scholar