Box 4

SHORT-TERM PLASTICITY: PPF AND PTP

There are two types of short-term plasticity exhibited at hippocampal Schaffer collateral synapses and elsewhere that are activity-dependent just as is LTP. These are paired-pulse facilitation (PPF) and post-tetanic potentiation (PTP). Paired-pulse facilitation is a form of short-term synaptic plasticity that is commonly held to be due to residual calcium augmenting neurotrans-mitter release presynaptically. When two single stimulus pulses are applied with interpulse intervals ranging from 20 to 300 msec, the second EPSP produced is larger than the first (see Panel A). This effect is referred to as PPF. The role of this type of synaptic plasticity in the behaving animal is unknown at this time; however, it clearly is a robust form of temporal integration of synaptic transmission and could be used in information processing behaviorally. The second form of short-term plasticity, PTP, is a large enhancement of synaptic efficacy observed after brief periods of high-frequency synaptic activity. For example, in experiments where LTP is induced with one or two 1-second, 100-Hz tetani, a large and transient increase in synaptic efficacy is produced immediately after high-frequency tetanus (see, for example, Figure 6 and Panel B). This is post-tetanic potentiation. The mechanisms for PTP are unknown, but both PTP and PPF are NMDA receptor-independent phenomena.

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