Long‐term synaptic enhancement and short‐term potentiation in rat fascia dentata act through different mechanisms

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Abstract
The component processes contributing to post-activation change in synaptic efficacy in the perforant pathway to the fascia dentata were studied in rats under sodium pentobarbitone anesthesia. With low stimulus strength, which activated only a relatively small number of perforant path fibers, repetitive stimulation led to effects which had very similar characteristics to those observed at neuromuscular synapses under similar conditions. Paired shocks resulted in a short (.apprx. 100 ms) facilitation superimposed on a depression, possibly due to depletion of available transmitter, which recovered more slowly (.apprx. 4 s). Short trains of stimuli at 125-250 Hz led to a longer lasting increase in synaptic strength which decayed to control levels with a double exponential time course. The 2 exponential components behaved like augmentation and potentiation at neuromuscular synapses, with time constants at 33.degree. C of .apprx. 5 and .apprx. 90 s, respectively. High-intensity stimulus trains of identical frequency and duration led to an enhancement of synaptic strength which lasted > 30 min. The paired shock depletion effect was increased in direct proportion to the amount of augmentation and potentiation present following low-intensity stimulus trains. Following high-intensity trains the paired shock depletion effect was increased by the same amount, an recovered with the same time course as following low-intensity stimulus trains, even though there remained a significant enhancement of the synaptic response. Augmentation and potentiation are due to an increase in the probability of transmitter release; long-term enhancement acts through some other, as yet undetermined, mechanism. Following high-intensity stimulation all processes are activated.