Fast changes in the functional status of release sites during short-term plasticity:: involvement of a frequency-dependent bypass of Rac at Aplysia synapses

Synaptic transmission can be described as a stochastic quantal process defined by three main parameters: N, the number of functional release sites; P, the release probability; and Q, the quantum of response. Many changes in synaptic strength that are observed during expression of short term plasticity rely on modifications in P. Regulation of N has been also suggested. We have investigated at identified cholinergic inhibitory Aplysia synapses the cellular mechanism of post-tetanic potentiation (PTP) expressed under control conditions or after N has been depressed by applying lethal toxin (LT) from Clostridium sordellii or tetanus toxin (TeNT). The analysis of the Ca2+ dependency, paired-pulse ratio and variance to mean amplitude relationship of the postsynaptic responses elicited at distinct extracellular [Ca2+]/[Mg2+] elicited during control post-tetanic potentiation (PTPcont) indicated that PTPcont is mainly driven by an increase in release probability, P. The PTP expressed at TeNT-treated synapses (PTPTeNT) was found to be similar to PTPcont, but scaled to the extent of reduction in N produced by TeNT. Despite LT inducing a decrease in N as TeNT does, the PTP expressed at LT-treated synapses (PTPLT) was characterized by exceptionally large amplitude and bi-exponential time course, as compared to PTPcont or the PTPTeNT. Analysis of the Ca2+ dependency of PTPLT, paired-pulse ratio and fluctuations in amplitude of the postsynaptic responses elicited during PTPLT or the variance to mean amplitude relationship of time-locked postsynaptic responses in a series of subsequent PTPLT indicated that an N-driven change is involved in the early phase (1 s time scale) of PTPLT, while at a later stage PTPLT is composed of both N and P increases. Our results suggest that fast switching on of the functional status of the release sites occurs also during the early events of PTPcont. The early N-driven phase of PTPLT is likely to be a functional recovery of the release sites silenced by Rac inactivation. This effect did not appear to result from reversion of LT inhibitory action but from bypassing the step regulated by Rac. Altogether the data suggest that Rac and the intracellular pathway which allows the bypassing of Rac are key players in new forms of short-term plasticity that rely on fast, activity-dependent changes in the functional status of the release sites.