Paired-pulse depression of unitary quantal amplitude at single hippocampal synapses

Abstract

At central synapses, quantal size is generally regarded as fluctuating around a fixed mean with little change during short-term synaptic plasticity. We evoked quantal release by brief electric stimulation at single synapses visualized with FM 1-43 dye in hippocampal cultures. The majority of quantal events evoked at single synapses were monovesicular, based on examination of amplitude distribution of ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-receptor-mediated responses. Consistent with previous findings, the quantal size did not change during paired-pulse facilitation (PPF), supporting the notion that the evoked events were monoquantal. However, during paired-pulse depression (PPD), there was a significant decrease in unitary quantal size, which was not due to postsynaptic receptor desensitization. This asymmetry of quantal modulation during PPF and PPD was demonstrated at the same single synapse at different extracellular calcium concentrations. Our results indicate that PPF can be fully accounted for by an increase of release probability, whereas PPD may be caused by decreases in both release probability and quantal size. One possible explanation is that the release of a quantum of neurotransmitter from synaptic vesicles is not invariant but subject to rapid calcium-dependent modulation during short-term synaptic plasticity.S hort-term synaptic plasticity is important for synaptic communication within the brain and is classically assessed with ''pairedpulse stimulation,'' two stimuli in close succession. There are various forms of paired-pulse modulation (PPM), typically attributed to different mechanisms. Paired-pulse facilitation (PPF) is generally explained as an increase of release probability (P r ) during the second stimulus, arising from prior accumulation of residual Ca 2ϩ near active zones or a lingering effect of Ca 2ϩ on a Ca 2ϩ sensor (1, 2). In contrast, paired-pulse depression (PPD) comes in multiple forms (3) and is open to a much wider range of possible explanations: receptor desensitization can be important in some cases (4) but is in general excluded. Instead, PPD is thought to originate presynaptically in most systems, as reflected by decreased transmitter output (5, 6). Reduced presynaptic release is most often attributed to vesicular depletion (7-11), but evidence for additional presynaptic mechanisms independent of vesicular depletion has also been provided (12-16).Deciphering PPM calls for a clear picture of quantal transmission at single synapses, but this is still under debate. Although general agreement has been reached about a lack of receptor saturation at excitatory synapses (17-19), controversy remains as to what generates variability in excitatory postsynaptic current (EPSC) size. One view is that single CNS synapses obey a ''one-vesicle rule,'' whereby presynaptic release is somehow capped at no more than one vesicle per presynaptic spike (8,(20)(21)(22)(23). The unitary evoked EPSC would then constitute a response to exocytosis of a single presynaptic vesicle, the ev...

Quantal Size Is Independent of the Release Probability at Hippocampal Excitatory Synapses

“…It is generally agreed that an increase in the P r occurs during facilitation; however, it is still debated whether the increase in P r is solely responsible for the facilitation or whether postsynaptic modifications also take place. Several studies reported an exclusive presynaptic alteration (Gulyas et al, 1993;Stevens and Wang, 1995;Dobrunz and Stevens, 1997;Hanse and Gustafsson, 2001;Silver et al, 2003;Chen et al, 2004;Lawrence et al, 2004), but evidence has also been published supporting significant postsynaptic modifications, including an increased quantal size after multivesicular release (Oertner et al, 2002;Conti and Lisman, 2003) or removing the polyamine block from certain postsynaptic glutamate receptors (Rozov and Burnashev, 1999). To some extent, the reason for the discrepancy is likely attributable to differences in experimental approaches, such as the preparations, the age of the animals, and the identity of the synapses examined.…”

Roles of the Fast-Releasing and the Slowly Releasing Vesicles in Synaptic Transmission at the Calyx of Held

“…Variance analysis also showed that quantal amplitudes stayed constant during a high-frequency train. It is interesting to note that shortterm synaptic depression accompanies decrease in quantal sizes, which is not attributable to postsynaptic mechanisms at hippocampal synapses (Chen et al, 2004).…”

High Ca2+-phosphate transfection efficiency enables single neuron gene analysis

“…13,16,18 In brief, the hippocampal CA1-CA3 region was dissected out and incubated for 30 min in 0.05% trypsin-EDTA solution (pH 7.2). After enzyme treatment, tissue blocks were gently triturated with a fire-polished Pasteur pipette.…”

“…For example, microisland cultures containing only a single autaptic neuron or a few neurons have been widely used to study molecular and cellular mechanisms of synaptic transmission and synaptic plasticity. [12][13][14][15][16][17][18] With the usually low efficiency of 1-5%, Ca 2+ -phosphate transfection is not suitable for transfecting single autaptic neurons to study gene functions. In order to transfect single autaptic neurons in microisland cultures, the transfection efficiency has to be greatly improved.…”