Fractal behavior of spontaneous neurotransmitter release: from single-synapse to whole-cell recordings

Spontaneous release of neurotransmitter vesicles at brain chemical synapses has been deeply investigated in the last decades at several levels. First and second order statistics have been widely adopted as a tool for assessing, inter-alia, dependence of spontaneous release on the concentration of ionic species in the intra- or extra-cellular environment. Furthermore, several studies demonstrated that spontaneous release exhibits fractal, and generally non purely random, behavior. Most experimental work on this topic exploits population whole-cell patch-clamp recordings in order to acquire post-synaptic currents elicited by neurotransmitter release into the synaptic cleft. Since several synapses merge on the dendritic arbor of a single neuronal cell, whole-cell recordings of miniature excitatory post-synaptic currents (mEPSCs) implies the temporal superimposition of releasing events from all active synapses on the arbor. This limitation can be overcome by exploiting the loose-patch clamp technique on single synapses, thus acquiring spontaneous release events from individual synapses. Here, we present results obtained by applying well-established methods for the quantification of fractal behavior in the series of mEPSCs acquired through the use of both whole-cell and single-synapse loose-patch recording techniques on hippocampal neurons and synapses. Our long-term aim is to get a better understanding of the release process and of the mechanisms of neuronal integration when the information is coming from several simultaneously active synaptic sites.