Abstract
Parallel fiber (PF) synapses show pronounced and lasting facilitation during bursts of high-frequency activity. They typically connect to their target neurons via a single active zone (AZ), harboring few release sites (~2–8) with moderate initial vesicular release probability (~0.2–0.4). In light of these biophysical characteristics, it seems surprising that PF synapses can sustain facilitation during high-frequency periods of tens of action potentials (APs). Recent findings suggest an increase in the number of occupied release sites due to ultra-rapid (~180 s−1), Ca2+ dependent recruitment of synaptic vesicles (SVs) from replenishment sites as major presynaptic mechanism of this lasting facilitation. On the molecular level, Synaptotagmin 7 or Munc13s have been suggested to be involved in mediating facilitation at PF synapses. The recruitment of SVs from replenishment sites appears to be reversible on a slower time-scale, thereby, explaining that PF synapses rapidly depress and ultimately become silent during low-frequency activity. Hence, PF synapses show high-frequency facilitation (HFF) but low-frequency depression (LFD). This behavior is explained by regulation of the number of occupied release sites at the AZ by AP frequency.
Highlights
Parallel fiber (PF) synapses are major sites for conveying sensory information to the cerebellar cortical output neurons, the Purkinje cells (PCs), and to interneurons
In electron microscopy ∼8 docked vesicles were found in PF terminals (Xu-Friedman et al, 2001). These results indicate that single PF active zone (AZ) harbor more than one release site, consistent with multi-vesicular release (Crowley et al, 2007)
During high-frequency trains, N, which is equal to Nocc in this study, increased via rapid recruitment from the reluctant pool while release sites became progressively depleted during low-frequency stimulation
Summary
Parallel fiber (PF) synapses are major sites for conveying sensory information to the cerebellar cortical output neurons, the Purkinje cells (PCs), and to interneurons. Occupied release sites are continuously emptied by the fusion processes, which, without further mechanisms, would result in synaptic depression due to progressive depletion of the pool of release-ready SVs. How effectively the information transfer can be maintained during an AP train depends on the speed with which Nocc can be restored or newly recruited and on their pv, which may increase.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
