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Geometry of Calcium Signaling

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Science  03 Jun 2005:
Vol. 308, Issue 5727, pp. 1381
DOI: 10.1126/science.308.5727.1381b

Changes in intracellular calcium concentration ([Ca2+]i) that occur after Ca2+ influx through N-methyl-D-aspartate-type glutamate receptors (NMDARs) play a key role in long-term plastic changes in postsynaptic function that are thought to underlie learning and memory. For most excitatory synapses in the central nervous system, the postsynaptic partners are dendritic spines: small protrusions on the dendritic shaft that have the effect of localizing changes in [Ca2+]i to individual synapses (as opposed to the entire dendrite).

Noguchi et al. used two-photon photolysis of caged glutamate and two-photon Ca2+ imaging to release transmitter onto single spines of rat hippocampal neurons and to assess quantitatively the influence of spine structure on [Ca2+]i. NMDAR-dependent current increased with spine head volume. On the other hand, NMDAR-mediated increases in the [Ca2+]i at the spine head were larger in small mushroom-shaped spines, whereas increases in dendritic shaft [Ca2+]i at the base of the spine were greater for large stubby spines. These differences were dictated by the geometry of the spine neck. The stubby spine morphology favored a rapid diffusion (an energetically downhill process) of Ca2+ from the spine head through the neck into the dendritic shaft, whereas in small spines, the lower conductance of the thin necks means that clearance of calcium from the head relies in part on the energetically uphill and slower process of calcium extrusion. The authors conclude that these differences in Ca2+ handling enable the preferential induction of long-term potentiation, which depends on changes in [Ca2+]i, in smaller spines. — EMA

Neuron 46, 609 (2005).

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