Two distinct vesicle loading processes underlying delayed facilitation at excitatory synapses in prefrontal cortex

Synaptic strength is dynamically modulated depending on prior activity called short-term synaptic plasticity (STP). STP is displayed as either or both by facilitation or depression. Facilitation of recurrent excitatory synapses within neocortical networks is proposed to have a critical role in working memory. However, facilitation in the prefrontal cortex (PFC), a key cortical area for working memory, remains unclear. We aimed to investigate the synaptic properties, including STP, at synapses of the rat PFC layer 2/3 network and made a vesicle dynamics model to describe these properties. We found that PFC excitatory synapses exhibited slowly increasing facilitation at low frequency stimulation which was eliminated by knockdown (KD) of synaptotagmin-7 (Syt7), a slow presynaptic calcium sensor. However, upon high frequency stimulation, excitatory synapses initially underwent strong depression, which was converted to facilitation later in a few stimuli. Such delayed facilitation after brief depression can be ascribed to high initial release probability and Syt7-dependent recovery of vesicle pools. The conventional single vesicle pool model was insufficient to explain these stimulation frequency-dependence of STP. Therefore, we propose a model featuring two distinct vesicle pools, each supplied at different rates in a sequence. This model involves a slow replenishment of a reluctant pool and a rapid transition of reluctant vesicles to a fully releasable pool.