Article

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

ABSTRACT

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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    • "Red triangles represent transmitter-release sites (active zones). (From Kim et al. 2003; modified, with permission.) "
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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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    • "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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