Visual deprivation increases accumulation of dense core vesicles in developing optic tectal synapses in Xenopus laevis

Abstract

Despite considerable progress in understanding the molecular components of synapses in the central nervous system, the ultrastructural rearrangements underlying synaptic development remain unclear. We used serial section transmission electron microscopy and three-dimensional reconstructions of the optic tectal neuropil of Xenopus laevis tadpoles to detect and quantify changes in synaptic ultrastructure over a 1-week period from stages 39 and 47, during which time the visual system of Xenopus tadpoles becomes functional. Synapse density, presynaptic maturation index, and number of synapses per axon bouton increase, whereas the number of DCVs per bouton decreases, between stages 39 and 47. The width of the synaptic cleft decreased and the diameter of postsynaptic profiles increased between stages 39 and 47 and then remained relatively unchanged after stage 47. We found no significant difference in synapse maturation between GABAergic and non-GABAergic synapses. To test the effect of visual experience on synaptogenesis, animals were deprived of visual experience for 3 days from stage 42 to 47. Visual deprivation decreased synapse maturation and the number of connections per bouton. Furthermore, visual deprivation increased the number of DCVs per bouton by more than twofold. The visual-deprivation-induced decrease in synaptic connections is specific to asymmetric nonGABAergic synapses; however, both symmetric GABAergic and asymmetric synapses show comparable increases in the number DCVs with visual deprivation. In both the control and the visually deprived animals, the number of DCVs per bouton is highly variable and does not correlate with either synapse maturation or the number of connected partners per bouton. These data suggest that synaptogenesis and DCV accumulation are regulated by visual experience and further suggest a complex spatial and temporal relation between DCV accumulation and synapse formation. Keywordsdense core vesicles; multiple synapse boutons; optic tectum; ultrastructure; 3D reconstruction; visual deprivation; synaptogenesis Synaptogenesis requires the coordinated assembly of the pre-and postsynaptic elements, each of which is composed of a dense complex of proteins, lipids, and organelles. Despite considerable effort to describe the protein components of pre-and postsynaptic specializations and the time course of their assembly (Jin and Garner, 2008;McAllister, 2007), the cellular mechanisms underlying synaptogenesis are still not clear. Ultrastructural studies have demonstrated that key events in synaptic differentiation are the accumulation of synaptic vesicles in presynaptic terminals (Blue and Parnavelas, 1983 1980), the enlargement of the presynaptic active zone (Blue and Parnavelas, 1983;Dyson and Jones, 1980), and the accumulation of polyribosome (Steward and Falk, 1986). Other parameters such as the length of the postsynaptic density, the width of the synaptic cleft, and the diameter of the presynaptic terminals remain relatively unchanged (Vaughn, 1989). In addition, dense ...

In Vivo Time-Lapse Imaging and Serial Section Electron Microscopy Reveal Developmental Synaptic Rearrangements

“…In addition, we often observed that axonal boutons contacting extending dendrites, which had at most a 4h lifetime, contained dense core vesicles (Fig. 5M), consistent with the idea that they are involved in synaptogenesis (Li and Cline, 2010). As observed in the neuron imaged at daily intervals (Fig 2A), extending dendrites formed synapses with MSBs.…”

“…We previously reported that the proportion of the presynaptic terminal area that is occupied by clustered synaptic vesicles increased during development when synapses mature, and termed this metric the maturation index (Li and Cline, 2010). Here, we mapped the maturation index of synapses on stable, extended and retracted branches (Fig.…”

Insulin receptor signaling in the development of neuronal structure and function

“…From the presynaptic point of view, a prominent ultrastructural feature of synaptic maturation is the increase in the number of synaptic vesicles per terminal [21-23], which likely contributes to the increase in probability of transmitter release in mature synapses [17]. Transmission at immature glutamatergic synapses is mainly mediated by NMDA receptors, which shift their kinetics by replacing NMDA receptor subunit 2B-containing receptors with NMDA receptor subunit 2A-containing receptors [24,25].…”

“…To test whether synaptic contacts onto dnIR-expressing tectal neurons were changed in dnIR-expressing neurons, we used electron microscopy to estimate synapse density on tectal neurons. This methodology gives both definite identification of synaptic contacts onto transfected neurons and ultrastructural information about both pre- and post-synaptic profiles [23]. We estimated synapse density by measuring the number of green fluorescent protein (GFP)-labeled synapses normalized to the total area of GFP-labeled dendritic profiles and found that dnIR-expressing dendrites had less than half of the synapse density of GFP-labeled neuron controls, although no changes in other ultrastructural features or synapse maturation were observed (Figures 6A-C) [84].…”

Illuminating the Multifaceted Roles of Neurotransmission in Shaping Neuronal Circuitry

“…These alterations include changes to connectivity (e.g. number of synapses) as well as modifications at the level of individual synapses (Aizenman and Cline, 2007; Li and Cline, 2010; Zhao et al, 2013). Hippocampal, neocortical and spinal neurons in culture exhibit ‘synaptic scaling’ where globally blocking activity leads to an increase in the amplitudes of miniature EPSCs and ionotropic glutamate receptor expression at excitatory synapses.…”