Readily-releasable pool

Synaptic vesicles are the organelles in axon terminals that store neurotransmitters and release them by exocytosis. There are two types, the large dense-core vesicles, diameter about 90 nm, that contain neuropeptides, and the small synaptic vesicles, diameter about 50nm, that contain non-peptide transmitters. About ten vesicles per synapse are docked to the plasma membrane and ready for release, the readily releasable pool . Many more vesicles per synapse are stored farther away from the plasma membrane, the resting pool . When needed, the latter vesicles may be recruited into the readily releasable pool. Neuronal depolarization and activation of voltage-sensitive Ca2+... 

This technique has permitted the dynamics of the exo-cytic/endocytic cycle to be investigated. At the neuromuscular junction, a readily releasable pool (RRP) and a reserve pool of vesicles coexist, the latter being released... 

FIGURE 2 3-7 Schematic diagram of the synaptic vesicle cycle. Neurotransmitter-filled vesicles held in the reserve pool are trafficked to a readily releasable pool where they are docked, primed and fused with the plasmalemma at the synaptic cleft. Also depicted is the clathrin-mediated endocy-tosis of the fused vesicles, which is followed by their uncoating and recycling via early endosomal fusion and budding of vesicles. This returns the vesicles to the reserve pool. Some of the phosphoproteins which regulate these steps are shown. For a more detailed description of this process and the phosphoproteins involved the reader is directed to the excellent text by Cowen et al. [67]. 

Turner TJ (2004) Nicotine enhancement of dopamine release by a calcium-dependent increase in the size of the readily releasable pool of synaptic vesicles. J Neurosci 24 11328-11336 Unwin N (1995) Acetylcholine receptor channel imaged in the open state. Nature 373 37-43 Unwin N (2003) Structure and action of the nicotinic acetylcholine receptor explored by electron microscopy. FEBS Lett 555 91-95... 

Xu T, Bajjalieh SM (2001) SV2 modulates the size of the readily releasable pool of secretory vesicles. Nat Cell Biol 3 691-698. 

Fig. 5 GPCR regulation of exocytosis downstream of Ca2+-entry. (a) Sequence of steps leading from recruitment to maturation of synaptic vesicles from a reserve pool (RP) to a readily-releasable pool (RRP) displaying slow (asynchronous) and fast (synchronous highly Ca2+-sensitive pool, HCSP synaptotagmin 1 (SYT 1) supported) components, (b) Protein-protein interactions of SNARES (SYX, syntaxin SYB, synaptobrevin and SNAP-2s-7S complex) and major putative regulatory proteins. Phosphoproteins are shown in shaded boxes (phosphorylation sites for PKA and PKC are indicated where known) with phosphorylation-dependent interactions depicted by arrows (increase indicated by filled arrows decrease indicated by open arrows). Circle-end connectors indicate a phosphorylation-independent or as yet unspecified interaction. Potential effects of interactions at various points of the sequence in A are discussed in the text.

Rosenmund C, Stevens CF (1996) Definition of the readily releasable pool of vesicles at hippocampal synapses. Neuron 16 1197-1207... 

Baldelli P, Fassio A, Valtorta F, Benfenati F. 2007. Lack of synapsin I reduces the readily releasable pool of synaptic... 

The ribbon is composed mainly of the sd uctural protein, Ribeye, but also includes a kinesin motor protein, KIF3A, and Rab3-interacdng protein, RIM. Ribbons are attached to the synapdc acdve zone by bassoon, and its structural rela-dve, piccolo. Although the ribbon appears to anchor a readily releasable pool of vesicles, molecular motors do not appear to be involved in vesicle movements near the acdve zone. RIM protein mutadons have been implicated in an autosomal dominant rod-cone dysd ophy (Johnson et al., 2003). 

In close connection with the problem of the readily releasable pool stands the peculiar fact that various stimuli cause a disappearance of the neurosecretory material both by light microscopic and electron microscopic criteria, whereas very little hormone has disappeared (see e. g. Barer c Lederis 1966). Transformation of carrier to a different substance within the granules is one possibility, but a release of hormone to the axoplasm or even the perivascular spaces surrounding the nerve endings would seem possible. 

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