Trends in Neurosciences
Discrete synaptic states define a major mechanism of synapse plasticity
Section snippets
Synaptic states: a mechanism of dictating synaptic strength
A key role of synaptic plasticity is to allow the synapse to operate over a large dynamic range. Two possible models could explain the behavior of synapses over this range. In the first, synapses undergo changes in efficacy by adjusting their strength along a continuum, such that the properties of strengthening or weakening occur in a graded fashion with fixed underlying mechanisms (i.e. the ‘continuum model’). In the second, synapses might exist in different discrete states that represent and
Why have discrete plastic states?
Synaptic plasticity that occurs in a state-dependent manner increases the information-carrying capacity of a synapse, in that the potentiation or depression of a synapse has an historical aspect that is absent from a simple continuum model. In a continuum model, information is coded solely in the current strength of the synapse, whereas a state model adds to the coded information the history of the synapse, because the ability of a synapse to undergo, and mechanisms for undergoing, further
Future questions: what molecular changes could define synaptic states?
Clearly, synapses can undergo plastic changes by switching between different states, but what cellular mechanisms underlie these states? Currently, each state is defined physiologically by AMPA receptor retrieval from the membrane, and/or the triggering of this retrieval. It is important to appreciate that AMPA receptor regulation is probably not the sole property defining a given state. Many other known presynaptic or postsynaptic processes could play a role in the definition of plastic states
Concluding remarks
That synapses exist in several discrete plasticity states represents a new paradigm for understanding the mechanistic underpinnings of synaptic plasticity, and perhaps also the roles of such plasticity in higher brain functions. Much work remains to be done to define and understand the mechanisms and roles these states play. Emerging data are beginning to elucidate how synaptic plasticity states could arise. Although these states will probably not be specified by a single simple mechanism, it
Acknowledgements
We would like to acknowledge Craig Garner for his insightful comments, and also members of the Madison Laboratory for reading the manuscript and providing their input.
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