Common strength and localization of spontaneous and evoked synaptic vesicle release sites
© Loy et al.; licensee BioMed Central Ltd. 2014
Received: 8 July 2013
Accepted: 29 March 2014
Published: 2 April 2014
Different pools and functions have recently been attributed to spontaneous and evoked vesicle release. Despite the well-established function of evoked release, the neuronal information transmission, the origin as well as the function of spontaneously fusing synaptic vesicles have remained elusive. Recently spontaneous release was found to e.g. regulate postsynaptic protein synthesis or has been linked to depressive disorder. Nevertheless the strength and cellular localization of this release form was neglected so far, which are both essential parameters in neuronal information processing.
Here we show that the complete recycling pool can be turned over by spontaneous trafficking and that spontaneous fusion rates critically depend on the neuronal localization of the releasing synapse. Thereby, the distribution equals that of evoked release so that both findings demonstrate a uniform regulation of these fusion modes.
In contrast to recent works, our results strengthen the assumption that identical vesicles are used for evoked and spontaneous release and extended the knowledge about spontaneous fusion with respect to its amount and cellular localization. Therefore our data supported the hypothesis of a regulatory role of spontaneous release in neuronal outgrowth and plasticity as neurites secrete neurotransmitters to initiate process outgrowth of a possible postsynaptic neuron to form a new synaptic connection.
Central neurons display two different forms of vesicle release: stimulation dependent, i.e. evoked release and stimulation independent spontaneous release, which occurs at resting membrane potential. Spontaneous neurotransmitter release is thought to play a crucial role in synaptic plasticity, memory and learning  as well as in pathophysiology . Furthermore spontaneous neurotransmitter release was found to regulate postsynaptic dendritic protein synthesis [3, 4]. The origin of spontaneous release remains controversial, with some evidence arguing for the recycling pool [5, 6] of vesicles and some for the reserve pool . Here we seek to analyze the amount of spontaneous release regarding its turnover kinetics and cellular location. While a pool can be defined for each form of release, our results indicate a common regulation of both vesicle populations and thus a common origin of both pools from the recycling pool of vesicles.
Conclusion and discussion
We provide new evidence that spontaneous vesicle turnover can reach the level of the recycling pool of vesicles in a time and calcium dependent manner (Figure 1D, Additional file 1: Figure S1C). Considering these results together with the fact that the amount of recycling pool vesicles correlates robustly with the spontaneous release at each bouton (Figure 1E), suggest that spontaneous vesicles originate from the recycling pool of vesicles rather than from the reserve pool. This underlines previous findings [5, 6, 19], but is contrary to studies that pointed at the reserve pool as origin of spontaneous vesicles . These previous studies used a sequential labeling paradigm instead of a simultaneous labeling that we used in our study. We cannot exclude with our experimental approach that, after total recycling pool turnover, additional vesicles recycle spontaneously from within the reserve pool of vesicles, due to a lack of discrimination without sequential labeling after saturation. However such vesicles would account for a minority of spontaneously fusing vesicles due to the lack of correlation with the reserve pool (Figure 2F). Besides we found that the soma has absolutely the larger synapses with the larger recycling pool and the higher spontaneous release, but if spontaneous and evoked turnover is normalized on the size of the synapse, the relative release is higher at the processes. Recent publications found a distance from soma dependency of synapse size and evoked release at the processes [20, 21]. In accordance with evoked release , spontaneous release declined along the processes with increasing distance to the soma (Additional file 1: Figure S5A), but remained constant, if spontaneous release was normalized on synapse size (Additional file 1: Figure S5B). Our functional measurements indicated, that both forms of release exhibit the same relationship regarding distance from the soma with smaller, but more effective synapses at the process . These results therefore point to a common developmental origin of these release modes with the vesicle populations stemming both from the recycling pool of vesicles. We also found differences between soma and processes regarding synapse size, relative and absolute release and confirmed, that smaller synapses release more efficiently. In conclusion we found a multitude of commonalities of spontaneous release and evoked release, e.g. correlation and identical size with recycling pool, vesicles were immediately re-releasable, same cellular localization with respect to release characteristics which together suggests the recycling pool as the common source of spontaneous and evoked released vesicles. Nevertheless a definition of a spontaneous vesicle pool is valid as vesicles differ with respect to their neuronal function and more future research is needed to substantiate the origin of spontaneous vesicles and to further address the function of these vesicles.
We thank D. Gilbert and O. Friedrich (Institute of Medical Biotechnology, Friedrich-Alexander-University Erlangen-Nuremberg, Paul-Gordan-Strasse 3, Erlangen, Germany) for providing their imaging setup.
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