Regulation of Synaptic Vesicle Recycling by Calcium and Serotonin (original) (raw)
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Journal of Neurobiology, 2004
We analyzed the contribution of calcium (Ca 2؉ )-induced Ca 2؉ release to somatic secretion in serotonergic Retzius neurons of the leech. Somatic secretion was studied by the incorporation of fluorescent dye FM1-43 upon electrical stimulation with trains of 10 impulses and by electron microscopy. Quantification of secretion with FM1-43 was made in cultured neurons to improve optical resolution. Stimulation in the presence of FM1-43 produced a frequency-dependent number of fluorescent spots. While a 1-Hz train produced 19.5 ؎ 5.0 spots/soma, a 10-Hz train produced 146.7 ؎ 20.2 spots/soma. Incubation with caffeine (10 mM) to induce Ca 2؉ release from intracellular stores without electrical stimulation and external Ca 2؉ , produced 168 ؎ 21.7 spots/soma. This staining was reduced by 49% if neurons were preincubated with the Ca 2؉ -ATPase inhibitor thapsigargin (200 nM). Moreover, in neurons stim-ulated at 10 Hz in the presence of ryanodine (100 M) to block Ca 2؉ -induced Ca 2؉ release, FM1-43 staining was reduced by 42%. In electron micrographs of neurons at rest or stimulated at 1 Hz in the ganglion, endoplasmic reticulum lay between clusters of dense core vesicles and the plasma membrane. In contrast, in neurons stimulated at 20 Hz, the vesicle clusters were apposed to the plasma membrane and flanked by the endoplasmic reticulum. These results suggest that Ca 2؉ -induced Ca 2؉ release produces vesicle mobilization and fusion in the soma of Retzius neurons, and supports the idea that neuronal somatic secretion shares common mechanisms with secretion by excitable endocrine cells.
Frontiers in Cellular Neuroscience, 2014
The soma of many neurons releases large amounts of transmitter molecules through an exocytosis process that continues for hundreds of seconds after the end of the triggering stimulus. Transmitters released in this way modulate the activity of neurons, glia and blood vessels over vast volumes of the nervous system. Here we studied how somatic exocytosis is maintained for such long periods in the absence of electrical stimulation and transmembrane Ca 2+ entry. Somatic exocytosis of serotonin from dense core vesicles could be triggered by a train of 10 action potentials at 20 Hz in Retzius neurons of the leech. However, the same number of action potentials produced at 1 Hz failed to evoke any exocytosis. The 20-Hz train evoked exocytosis through a sequence of intracellular Ca 2+ transients, with each transient having a different origin, timing and intracellular distribution. Upon electrical stimulation, transmembrane Ca 2+ entry through L-type channels activated Ca 2+ -induced Ca 2+ release. A resulting fast Ca 2+ transient evoked an early exocytosis of serotonin from sparse vesicles resting close to the plasma membrane. This Ca 2+ transient also triggered the transport of distant clusters of vesicles toward the plasma membrane. Upon exocytosis, the released serotonin activated autoreceptors coupled to phospholipase C, which in turn produced an intracellular Ca 2+ increase in the submembrane shell. This localized Ca 2+ increase evoked new exocytosis as the vesicles in the clusters arrived gradually at the plasma membrane. In this way, the extracellular serotonin elevated the intracellular Ca 2+ and this Ca 2+ evoked more exocytosis. The resulting positive feedback loop maintained exocytosis for the following hundreds of seconds until the last vesicles in the clusters fused. Since somatic exocytosis displays similar kinetics in neurons releasing different types of transmitters, the data presented here contributes to understand the cellular basis of paracrine neurotransmission.
Journal of Neuroscience, 2003
Short-term homosynaptic depression and heterosynaptic facilitation of transmitter release from mechanoreceptor sensory neurons of Aplysia are involved in habituation and sensitization, respectively, of defensive withdrawal reflexes. We investigated whether synaptic transmission is regulated in these forms of plasticity by means of changes in the size of the pool of transmitter available for immediate release [the readily releasable pool (RRP)] or in the efficacy of release from an unchanging pool. Using sensorimotor synapses formed in cell culture, we estimated the number of transmitter quanta in the RRP from the asynchronous release of neurotransmitter caused by application of a hypertonic bathing solution. Our experiments indicate that the transmitter released by action potentials and by hypertonic solution comes from the same pool. The RRP was reduced after homosynaptic depression of the EPSP by low-frequency stimulation and increased after facilitation of the EPSP by application of the endog-enous facilitatory transmitter serotonin (5-HT) after homosynaptic depression. However, although the fractional changes in the RRP and in the EPSP were similar for both synaptic depression and facilitation when depression was induced by repeated hypertonic stimulation, the changes in the EPSP were significantly greater than the changes in the RRP when depression was induced by repeated electrical stimulation. These observations indicate that homosynaptic depression and restoration of depressed transmission by 5-HT are caused by changes in both the amount of transmitter available for immediate release and in processes involved in the coupling of the action potential to transmitter release.
Proceedings of the National Academy of Sciences, 1990
The molecular events that control synaptic vesicle availability in chemical synapticjunctions have not been fully clarified. Among the protein molecules specifically located in presynaptic terminals, synapsin I and calcium/calmodulindependent protein kinase H (CaM kinase II) have been shown to modulate evoked transmitter release in the squid giant synapse. In the present study, analysis of synaptic noise in this chemical junction was used to determine whether these proteins also play a role in the control of spontaneous and enhanced spontaneous transmitter release. Injections of dephosphorylated synapsin I into the presynaptic terminal reduced the rate of spontaneous and enhanced quantal release, whereas injection of phosphorylated synapsin I did not modify such release. By contrast CaM kinase II injection increased enhanced miniature release without affecting spontaneous miniature frequency. These results support the view that dephosphorylated synapsin I "cages" synaptic vesicles while CaM kinase II, by phosphorylating synapsin I, "decages" these organelles and increases their availability for release without affecting the release mechanism itself.
Serotonin release from the neuronal cell body and its long-lasting effects on the nervous system
Philosophical Transactions of the Royal Society B: Biological Sciences, 2015
Serotonin, a modulator of multiple functions in the nervous system, is released predominantly extrasynaptically from neuronal cell bodies, axons and dendrites. This paper describes how serotonin is released from cell bodies of Retzius neurons in the central nervous system (CNS) of the leech, and how it affects neighbouring glia and neurons. The large Retzius neurons contain serotonin packed in electrodense vesicles. Electrical stimulation with 10 impulses at 1 Hz fails to evoke exocytosis from the cell body, but the same number of impulses at 20 Hz promotes exocytosis via a multistep process. Calcium entry into the neuron triggers calcium-induced calcium release, which activates the transport of vesicle clusters to the plasma membrane. Exocytosis occurs there for several minutes. Serotonin that has been released activates autoreceptors that induce an inositol trisphosphate-dependent calcium increase, which produces further exocytosis. This positive feedback loop subsides when the la...
Neuroscience, 1998
Diterbutyl-1,4-benzohydroquinone, a specific blocker of Ca 2+-ATPase pumps, increased acetylcholine release from an identified synapse of Aplysia, as well as from Torpedo and mouse caudate nucleus synaptosomes. Because 2,5-diterbutyl-1,4-benzohydroquinone does not change the presynaptic Ca 2+ influx, the enhancement of acetylcholine release could be due to an accumulation of Ca 2+ in the terminal. This possibility was further checked by studying the effects of 2,5-diterbutyl-1,4benzohydroquinone on twin pulse facilitation, classically attributed to residual Ca 2+. While preventing the fast sequestration of Ca 2+ by presynaptic organelles, 2,5-diterbutyl-1,4-benzohydroquinone magnified both twin pulse facilitation observed under low extracellular Ca 2+ concentration and twin pulse dysfacilitation observed under high extracellular Ca 2+ concentration. Thus, it is concluded that 2,5-diterbutyl-1,4-benzohydroquinone, by preventing Ca 2+ buffering near transmitter release sites, modulates acetylcholine release. As 2,5-diterbutyl-1,4-benzohydroquinone was also shown to decrease by 50% the uptake of 45 Ca 2+ by isolated synaptic vesicles, we propose that synaptic vesicles can control the presynaptic Ca 2+ concentration triggering the release of neurotransmitter. 1998 IBRO. Published by Elsevier Science Ltd.
Synaptic and Extrasynaptic Secretion of Serotonin
Cellular and Molecular Neurobiology, 2005
1. Serotonin is a major modulator of behavior in vertebrates and invertebrates and deficiencies in the serotonergic system account for several behavioral disorders in humans.
European Journal of Neuroscience, 2008
Neuromodulation is central to all nervous system function, although the precise mechanisms by which neurotransmitters affect synaptic efficacy between central neurons remain to be fully elucidated. In this study, we examined the neuromodulatory action of serotonin [5-hydroxytryptamine (5-HT)] at central synapses between identified neurons from the pond snail Lymnaea stagnalis. Using whole-cell voltage-clamp and sharp electrode recording, we show that 5-HT strongly depresses synaptic strength between cultured, cholinergic neuron visceral dorsal 4 (VD4 ) presynaptic) and its serotonergic target left pedal dorsal 1 (LPeD1 ) postsynaptic). This inhibition was accompanied by a reduction in synaptic depression, but had no effect on postsynaptic input resistance, indicating a presynaptic origin. In addition, serotonin inhibited the presynaptic calcium current (I Ca ) on a similar time course as the change in synaptic transmission. Introduction of a non-condensable GDP analog, GDP-b-S, through the presynaptic pipette inhibited the serotonin-mediated effect on I Ca. Similar results were obtained with a membrane-impermeable inactive cAMP analog, 8OH-cAMP. Furthermore, stimulation of the serotonergic postsynaptic cell also inhibited presynaptic currents, indicating the presence of a negative feedback loop between LPeD1 and VD4. Taken together, this study provides direct evidence for a negative feedback mechanism, whereby the activity of a presynaptic respiratory central pattern-generating neuron is regulated by its postsynaptic target cell. We demonstrate that either serotonin or LPeD1 activity-induced depression of presynaptic transmitter release from VD4 involves voltage-gated calcium channels and is mediated through a G-protein-coupled and cAMP-mediated system.