Presynaptic Activation of Silent Synapses and Growth of New Synapses Contribute to Intermediate and Long-Term Facilitation in Aplysia

Howard Hughes Medical Institute, Columbia University, New York, NY 10032, USA.
Neuron (Impact Factor: 15.05). 10/2003; 40(1):151-65. DOI: 10.1016/S0896-6273(03)00595-6
Source: PubMed


The time course and functional significance of the structural changes associated with long-term facilitation of Aplysia sensory to motor neuron synaptic connections in culture were examined by time-lapse confocal imaging of individual sensory neuron varicosities labeled with three different fluorescent markers: the whole-cell marker Alexa-594 and two presynaptic marker proteins-synaptophysin-eGFP to monitor changes in synaptic vesicle distribution and synapto-PHluorin to monitor active transmitter release sites. Repeated pulses of serotonin induce two temporally, morphologically, and molecularly distinct presynaptic changes: (1) a rapid activation of silent presynaptic terminals by filling of preexisting empty varicosities with synaptic vesicles, which parallels intermediate-term facilitation, is completed within 3-6 hr and requires translation but not transcription and (2) a slower generation of new functional varicosities which occurs between 12-18 hr and requires transcription and translation. Enrichment of empty varicosities with synaptophysin accounts for 32% of the newly activated synapses at 24 hr, whereas newly formed varicosities account for 68%.

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    • "This schema places all presynaptic terminals at equal distance from the cell body, hence implying equal probabilities for the distribution of subcellular organelles, proteins and mRNA (Kim et al., 2003; Bailey and Kandel, 2008). Whereas significant differences in the properties of the individual terminal boutons are considered in relation to long-term plasticity of the neurons, the underlying mechanisms are not discussed (Kim et al., 2003). Intuitively if all terminal boutons are equally distributed from the cell body, no innate presynaptic differences in their subcellular structure and physiology would be expected. "
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    ABSTRACT: Much of what we know about the mechanisms underlying Homosynaptic Depression (HSD) and heterosynaptic facilitation is based on intracellular recordings of integrated postsynaptic potentials (PSPs). This methodological approach views the presynaptic apparatus as a single compartment rather than taking a more realistic representation reflecting the fact that it is made up of tens to hundreds of individual and independent Presynaptic Release Boutons (PRBs). Using cultured Aplysia sensorimotor synapses, we reexamined HSD and its dishabituation by imaging the release properties of individual PRBs. We find that the PRB population is heterogeneous and can be clustered into three groups: ~25% of the PRBs consistently release neurotransmitter throughout the entire habituation paradigm (35 stimuli, 0.05 Hz) and have a relatively high quantal content, 36% of the PRBs display intermittent failures only after the tenth stimulation, and 39% are low quantal-content PRBs that exhibit intermittent release failures from the onset of the habituation paradigm. 5HT-induced synaptic dishabituation by a single 5HT application was generated by the enhanced recovery of the quantal content of the habituated PRBs and did not involve the recruitment of new release boutons. The characterization of the PRB population as heterogeneous in terms of its temporal pattern of release-probability and quantal content provides new insights into the mechanisms underlying HSD and its dishabituation.
    Frontiers in Cellular Neuroscience 09/2013; 7:165. DOI:10.3389/fncel.2013.00165 · 4.29 Impact Factor
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    • "We assessed the long-term structural changes by comparing the images of each sensory neuron 12 hr and 48 hr after ApNT(+) and/or RFP overexpression . Sensory neuron varicosities were identified as elongated spheres (R3 mm in diameter) in apposition to the initial segment and major neurites of L7 motor neurons (Kim et al., 2003). Quantitative analysis followed a blind procedure. "
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    ABSTRACT: Neurotrophins control the development and adult plasticity of the vertebrate nervous system. Failure to identify invertebrate neurotrophin orthologs, however, has precluded studies in invertebrate models, limiting our understanding of fundamental aspects of neurotrophin biology and function. We identified a neurotrophin (ApNT) and Trk receptor (ApTrk) in the mollusk Aplysia and found that they play a central role in learning-related synaptic plasticity. Blocking ApTrk signaling impairs long-term facilitation, whereas augmenting ApNT expression enhances it and induces the growth of new synaptic varicosities at the monosynaptic connection between sensory and motor neurons of the gill-withdrawal reflex. Unlike vertebrate neurotrophins, ApNT has multiple coding exons and exerts distinct synaptic effects through differentially processed and secreted splice isoforms. Our findings demonstrate the existence of bona fide neurotrophin signaling in invertebrates and reveal a posttranscriptional mechanism that regulates neurotrophin processing and the release of proneurotrophins and mature neurotrophins that differentially modulate synaptic plasticity.
    Cell Reports 04/2013; 3(4). DOI:10.1016/j.celrep.2013.03.008 · 8.36 Impact Factor
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    • "In our model, this increase in probability and speed of neurotransmitter release may reflect a positive modulation of release machinery induced by epileptiform activity, perhaps through transient protein modification such as phosphorylation or increased Ca2+ concentration in the presynaptic terminals [72]. Alternatively, we cannot exclude the rapid activation of other active zones that were previously silent [73]. An enhancement in amplitude of evoked postsynaptic currents and a shortening of their rise times have also been described in basolateral amygdala neurons of rats which undergo to epileptic discharges induced by electrical kindling [74]. "
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    ABSTRACT: Epileptic activity is generally induced in experimental models by local application of epileptogenic drugs, including pentylenetetrazol (PTZ), widely used on both vertebrate and invertebrate neurons. Despite the high prevalence of this neurological disorder and the extensive research on it, the cellular and molecular mechanisms underlying epileptogenesis still remain unclear. In this work, we examined PTZ-induced neuronal changes in Helix monosynaptic circuits formed in vitro, as a simpler experimental model to investigate the effects of epileptiform activity on both basal release and post-tetanic potentiation (PTP), a form of short-term plasticity. We observed a significant enhancement of basal synaptic strength, with kinetics resembling those of previously described use-dependent forms of plasticity, determined by changes in estimated quantal parameters, such as the readily releasable pool and the release probability. Moreover, these neurons exhibited a strong reduction in PTP expression and in its decay time constant, suggesting an impairment in the dynamic reorganization of synaptic vesicle pools following prolonged stimulation of synaptic transmission. In order to explain this imbalance, we determined whether epileptic activity is related to the phosphorylation level of synapsin, which is known to modulate synaptic plasticity. Using western blot and immunocytochemical staining we found a PTZ-dependent increase in synapsin phosphorylation at both PKA/CaMKI/IV and MAPK/Erk sites, both of which are important for modulating synaptic plasticity. Taken together, our findings suggest that prolonged epileptiform activity leads to an increase in the synapsin phosphorylation status, thereby contributing to an alteration of synaptic strength in both basal condition and tetanus-induced potentiation.
    PLoS ONE 02/2013; 8(2):e56968. DOI:10.1371/journal.pone.0056968 · 3.23 Impact Factor
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