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Hippocampal bidirectional plasticity arising from transitions in a
synaptic ensemble
Daniel H. O'Connor, Gayle M. Wittenberg, and Samuel S.-H. Wang
Princeton University
In bidirectional synaptic plasticity, change in synaptic strength has
been observed to occur in graded amounts of potentiation or
depression. However, measurements are almost always made from
ensembles of synapses; this is true even in recordings from connected
pairs of neurons. Such measurements therefore represent the sum of
many single-synapse potentiation or depression events occurring at
once. This point of view suggests that it may be possible to dissect
bidirectional plasticity into separate processes of potentiation and
depression/depotentiation. We tested this at the Schaffer
collateral-CA1 synapse by driving synaptic ensembles into conditions
from which only potentiation or depression/depotentiation are
possible. When LTP is saturated with theta-burst stimulation or its
induction blocked with the kinase inhibitor K252a, only long-term
depression (LTD) or depotentiation can be induced. Conversely, when
LTD is saturated or its induction blocked with the phosphatase
inhibitor okadaic acid, only LTP can be induced. From a fully
potentiated state, or in the presence of K252a, stimulus frequencies
of 1 Hz or higher lead to LTD/depotentiation. From a fully depressed
state, or after treatment with okadaic acid, stimulus frequencies of 5
Hz or higher lead to LTP. To account for our data we present a model
of bidirectional synaptic plasticity in which synapses begin from a
mixture of low-strength, high-strength, and recently-potentiated
states.