Trends in Neurosciences
Structure–stability–function relationships of dendritic spines
Section snippets
Spine structure and stability
Many ultrastructural investigations have described morphological changes in neurons that accompany neuronal activity. In particular, increases in the number [4] or volume [5] of dendritic spines or changes in spine shape [6] have been observed. It is not possible, however, to study the stability and function of such plasticity by electron microscopy. Recent progress in two-photon excitation imaging has, thus, been largely responsible for the demonstration that the structure of central synapses
Glutamate application with a femtosecond-pulse laser
Investigation of the function of individual spines requires the systematic application of glutamate to identifiable spines along a dendrite. Such studies have been impossible with methodologies that rely only on microelectrodes because brain tissue is too compact to allow the arbitrary movement of an electrode with a resolution at the micrometer level (Fig. 1c). The light-induced release of glutamate from caged compounds was, therefore, applied as a solution to this problem. The spatial
Spine structure and function
Matsuzaki et al. attempted to measure glutamate sensitivity at the surface of dendrites of CA1 pyramidal neurons in fresh slice preparations obtained from the rat hippocampus [22]. The two-photon uncaging of MNI-glutamate was induced at 1064 points in a cubic region encompassing a small portion of a dendrite (Fig. 1f). Sampling points were pseudo-randomized to minimize receptor desensitization. The glutamate-induced currents were recorded at the soma, and their peak amplitudes were displayed by
Memory density
Given that glutamate-mediated synaptic transmission depends on a diffusible molecule, the length of glutamate action is a crucial determinant of the practical density of synaptic connections. The length of glutamate action is, in turn, dependent on the gating properties of postsynaptic AMPA receptors. If the activation of AMPA receptors is slow, then glutamate released from a presynaptic terminal will reach neighboring synapses before the corresponding postsynaptic receptors are activated. Such
Implications of structure–stability–function relationships of spines
In memory devices, memory storage and readout occur in two different operational modes. New memory thus cannot be stored in the readout mode. By contrast, storage and readout of memory occur by inseparable processes in the brain. This feature has been a major theme for the operation of realistic neuronal networks and it has been ascribed mainly to the highly distributed nature of memory in neuronal networks and in the superposition of memories, distributed over many synapses or ‘weights’, as in
Molecular basis of spine structure–stability–function relationships
The trafficking and turnover of AMPA receptors are rapid 24, 37, 38, 39, 40 and have been proposed to account for the changes in synaptic strength during LTP [41] and LTD [42]. By contrast, given that AMPA-receptor expression is dynamically regulated in spines, the maintenance of memory must depend on stable factors that regulate AMPA-receptor expression; otherwise, the strength of synaptic connections could not be maintained in the long term. Such factors might include spine shape and the
Recapitulation
Recent progress in biophysical techniques and molecular biology has provided insight into the structure–function relationships of dendritic spines in the cerebral cortex, as well as support for the century-old hypothesis that spine structure is the basis for memory in the brain. The structure–stability–function relationships of spines have further suggested that small and large spines play distinct roles in learning and memory, enabling rapid acquisition of new memory and robust readout,
Acknowledgements
We thank Y. Hata for critical reading of the manuscript. Our work was supported by Grants-in-Aid from the Japanese Ministry of Education, Culture, Sports, Science and Technology and from the Japan Society for the Promotion of Science, and by a research grant from the Human Frontier Science Program Organization.
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