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Neurobiology

Long-distance long-term depression

Nature volume 388, pages 427–428 (1997)Cite this article

Those who study learning and memory generally agree that many forms of these processes are due to long-lasting, use-dependent changes in the strength of synapses in the brain. Depending on the pattern of synaptic activation, synapses can show either a long-term potentiation (LTP) or a long-term depression (LTD). Both forms of plasticity are spatially restricted — that is, they seem to be limited only to those synapses in which the plasticity has been induced. This synapse-specificity has been used in neural-network models that store information, and it suggests that the signals responsible for the changes in synaptic strength are highly localized.

The idea that LTD2,3,4,5,6 and LTP7,8,9 may not be strictly synapse-specific is not entirely new. So why is the paper by Fitzsimonds et al.1 so provocative? First, it challenges the classic view — as originally put forward by Cajal — that the neuron is ‘dynamically polarized’; that is, the information received by the dendrites is passed forward through the axon to the synapses on the next cell. The newly discovered backward-propagation of LTD indicates that there is also an intracellular, retrograde, axon-terminal-to-dendritic flow of information (on a much slower timescale). In this way, the input of the neuron is ‘told’ the state of the output of the neuron. So dendrites not only integrate synaptic potentials, but they also integrate retrograde signals from various axonal outputs of the cell. This has important implications for models of learning that are based on back-propagation algorithms10.

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Figure 1: The spread of LTD (long-term depression) in a neural network in culture, as proposed by Fitzsimonds et al1.

References

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Authors and Affiliations

  1. the Departments of Cellular and Molecular Pharmacology, and Physiology, University of California, San Francisco, 94143, California, USA

    Roger A. Nicoll & Robert C. Malenka

  2. the Departments of Psychiatry and Physiology, University of California, San Francisco, 94143, California, USA

    Robert C. Malenka

Authors
  1. Roger A. Nicoll
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  2. Robert C. Malenka
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Nicoll, R., Malenka, R. Long-distance long-term depression. Nature 388, 427–428 (1997). https://doi.org/10.1038/41222

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