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Review
. 2012 Jul 1;4(7):a005702.
doi: 10.1101/cshperspect.a005702.

Short-term presynaptic plasticity

Wade G Regehr  1
Affiliations
Review

Short-term presynaptic plasticity

Wade G Regehr. Cold Spring Harb Perspect Biol. .

Abstract

Different types of synapses are specialized to interpret spike trains in their own way by virtue of the complement of short-term synaptic plasticity mechanisms they possess. Numerous types of short-term, use-dependent synaptic plasticity regulate neurotransmitter release. Short-term depression is prominent after a single conditioning stimulus and recovers in seconds. Sustained presynaptic activation can result in more profound depression that recovers more slowly. An enhancement of release known as facilitation is prominent after single conditioning stimuli and lasts for hundreds of milliseconds. Finally, tetanic activation can enhance synaptic strength for tens of seconds to minutes through processes known as augmentation and posttetantic potentiation. Progress in clarifying the properties, mechanisms, and functional roles of these forms of short-term plasticity is reviewed here.

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Figures

Figure 1.
Figure 1.
Forms of short-term, use-dependent plasticity. Simulated experiments show the properties of various forms of short-term plasticity. (A) Short-lived depression is observed at some synapses when the presynaptic axon is stimulated twice with a time between stimuli of Δt. (B) At some synapses low-frequency stimulation results in stable synaptic response, but sustained high-frequency stimulation results in a depression that persists for tens of seconds even when low-frequency stimulation is resumed. (C) Paired-pulse facilitation that lasts for hundreds of milliseconds is observed at some synapses. (D) Augmentation or posttetanic potentiation of synaptic responses lasting tens of seconds or minutes after tetanic stimulation is observed at some synapses.
Figure 2.
Figure 2.
Factors relevant to depression. (A) Different pools of vesicles in a presynaptic bouton. For simplicity the pools are depicted as being clustered together and at different distances from the active zone. It is clear that at least in some cases the pools intermingle, indicating that spatial location within the bouton is insufficient to account for the different properties of synaptic vesicles. (B) The dependence of paired-pulse plasticity on initial release probability. (C) Schematic showing mechanisms of synaptic depression. Abbreviations: PF, parallel fiber; CF, climbing fiber.
Figure 3.
Figure 3.
Proposed mechanisms of facilitation. (A) The residual calcium hypothesis based on a single type of low-affinity calcium sensor is shown for two types of calcium signals. When the residual calcium signal is a significant fraction of the local calcium signal, significant facilitation occurs (red traces), but when the residual calcium signal is much smaller than the calcium signal near the calcium channel, this mechanism results in very little enhancement (blue traces). (B) Another type of residual calcium model is shown that is based on two types of calcium sensors, a fast, low-affinity sensor and a slow, high-affinity sensor. The residual calcium signal can activate the high-affinity receptor to produce facilitation. (C) A comparison of the calcium signals and resulting EPSCs for a synapse in which a presynaptic bouton contains either a high concentration (blue) or a low concentration (red) of a rapid calcium buffer illustrates another mechanism of facilitation. (D) A slow calcium buffer binds presynaptic calcium slowly and by doing so accelerates the decay of presynaptic calcium, which can in turn affect facilitation. (E) Schematic illustrating mechanisms of facilitation.
Figure 4.
Figure 4.
Proposed mechanisms of augmentation and PTP. (A) The Cares evoked by tetanic stimulation. (Inset) The buildup of Cares. After the stimulus Cares decays with a fast and slow component. Two mechanisms have been shown to account for the slow component of Cares decay. (B) One possibility is that calcium loads the mitochondria during tetanic stimulation, and then afterwards calcium leaks out of the mitochondria and thereby leads to a sustained elevation of calcium in the presynaptic bouton. (C) Another possibility is that the interplay of Na/Ca exchanger and the Ca-ATPase leads to sustained calcium increases. During and immediately after tetanic stimulation, calcium is rapidly extruded through both the Na/Ca exchanger and the Ca-ATPase. At some point the Na/Ca exchanger has removed sufficient calcium, and there is a sufficient buildup of sodium in the terminal that it reaches its reversal potential, opposes the Ca-ATPase, and leads to a very slow Cares decay. (D) Schematic showing mechanisms of augmentation and PTP.
Figure 5.
Figure 5.
Examples of functional roles of short-term synaptic plasticity. (A) If a sensory stimulus activates a neuron (red), as a result of depression the synaptic current in the postsynaptic cell (blue) is only prominent at the onset of stimulus, and as a result the postsynaptic cell only fires at the onset of stimulation. (B) A schematic illustrating the effect of a depressing synapse on the response of postsynaptic cell evoked by changing steady-state firing from 1 Hz to 5 Hz to 25 Hz. Because of synaptic depression the synaptic current decreases to a steady-state value during sustained activation, such that the extent of depression offsets the change in firing frequency. As a result the only change in charge transfer occurs when the firing rate is changed and the magnitude of the response is proportional to the percentage change in the firing rate of the presynaptic cell. (C) Direction selectivity can be mediated by a simple circuit of three cells, provided two of the cells are sensory neurons that target a common postsynaptic cell, and the synapses have different synaptic plasticity. In this case the red synapse depresses and the blue synapse facilitates. A sensory stimulus moving from left to right results in a net synaptic current (black) that is the sum of the two synaptic inputs. The potential change does not reach threshold (dashed line) in this case, but if the stimulus moves from right to left, a larger synaptic response is observed and the potential of the postsynaptic cell reaches threshold.
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