Synaptic activity slows vesicular replenishment at excitatory synapses of rat hippocampus

Abstract

Short-term synaptic depression mainly reflects the depletion of the readily releasable pool (RRP) of quanta. Its dynamics, and especially the replenishment rate of the RRP, are still not well characterized in spite of decades of investigation. Main reason is that the vesicular storage and release system is treated as time-independent. If it is time-dependent all parameters thus estimated become problematic. Indeed the reports about how prolonged stimulation affects the dynamics are contradictory. To study this, we used patterned stimulation on the Schaeffer collateral fiber pathway and model-fitting of the excitatory post-synaptic currents (EPSC) recorded from CA1 neurons in rat hippocampal slices. The parameters of a vesicular storage and release model with two pools were estimated by minimizing the squared difference between the ESPC amplitudes and simulated model output. This yields the 'basic' parameters (release coupling, replenishment coupling and RRP size) that underlie the 'derived' and commonly used parameters (fractional release and replenishment rate). The fractional release increases when [Ca ?? ] o is raised, whereas the replenishment rate is [Ca ?? ] o independent. Fractional release rises because release coupling increases, and the RRP becomes less able to contain quanta. During prolonged stimulation, the fractional release remains generally unaltered, whereas the replenishment rate decreases down to *10 % of its initial value with a decay time of *15 s, and this decrease in the replenishment rate significantly contributes to synaptic depression. In conclusion, the fractional release is [Ca ?? ] odependent and stimulation-independent, whereas the replenishment rate is [Ca ?? ] o -independent and stimulation-dependent.

Is replenishment of the readily releasable pool associated with vesicular movement?

“…Steady-state values are represented as a percentage of the replenishment rate at t = 0 s. Each bar or symbol represents the mean and standard error (n C 4). Patterned stimulation consisted of 10 repeating trains of brief high (10 Hz, 5 s)-low (5 Hz, 5 s) frequencies sensitive (Bui and Glavinovic 2013a). These findings suggest that a significant re-formulation of the biophysical picture of the mechanism of evoked release may be needed.…”

“…DL-2-amino-5phosphonovaleric acid (10 lM) was added to reduce NMDA-activated channels (Taniike et al 2008), to ensure no induction of the long-term potentiation, although such an induction was not expected to occur given comparatively low frequency of stimulation (5-10 Hz). The difference in quantal size of miniature excitatory postsynaptic currents prior to and after long patterned stimulation was indeed not statistically significant (Bui and Glavinovic 2013a). Note that it is also simpler to study the short-term depression due to the activation of AMPA receptors, if EPSCs contain only such a contribution, and are not 'contaminated' by an NMDA component.…”

“…The parameter estimates of the first 1-3 windows are sometimes quite inaccurate (data not shown) if an equilibrium assumption is made, not surprisingly since vesicular pools do not have to be necessarily in equilibrium initially. Finally, note that regardless of whether the traditionally used rate-based models or our electrical model are used, the estimated rate values are the same (Bui and Glavinovic 2013a).…”

“…o (Student's t test) parameters of the vesicular storage and release system using a pair of brief high-low frequency stimulation trains. The changes of EPSC amplitudes due to a single pair of such brief trains of stimulation are sufficient to estimate all of the parameters (Bui and Glavinovic 2013a). Following the evaluation at the control conditions (but with DMSO; see ''Methods'') the synapses were exposed to the drugs at least 30-60 min prior to recordings.…”

“…o independent, and the replenishment rate does not increase during prolonged stimulation, which leads to [Ca 2? ] i accumulation, but decreases instead (Bui and Glavinovic 2013a). In this study we further explore this problem and try to answer an even more basic question-Is vesicular replenishment of the RRP associated with significant vesicular movement?…”