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|Title:||SYNAPTIC SCALING IN THE DEVELOPING ZEBRAFISH SPINAL CORD|
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|Authors/Affiliations:||1 Laura Knogler*; 1 Pierre Drapeau; |
1 Université de Montréal, QC, Canada
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|Content:||Objectives: Based on in vitro studies, it has been proposed that homeostatic plasticity is an important means by which neurons can regulate their own excitability relative to network activity, but there is little understanding of these processes in vivo. Synaptic scaling, a uniform increase or decrease in the size of miniature excitatory post-synaptic currents (mEPSCs), is one proposed means of homeostasis which could compensate for abnormal levels of activity during development. Our goal is to investigate the effects of chronically blocking or exciting synaptic activity in vivo in the developing zebrafish embryo at the level of its well-characterized spinal cord activity. |
Materials and methods: Wild-type zebrafish embryos were injected 16-17 hours post-fertilization (hpf) with either 5mM TTX to block voltage-gated sodium channels, 25mM AP-5 to block or 1mM NMDA to activate excitatory NMDAR activity, 5mM CNQX to block excitatory AMPAR activity, or vehicle as a control. Embryos were raised and whole-cell patch clamp recordings of mEPSCs from spinal neurons were made in Mg2+ free solution at 4dpf. Recordings were done in the presence of 1uM TTX and 1uM strychnine to isolate glutamatergic mEPSCs. Results: None of the treatments resulted in morphological abnormalities. Blocking activity with TTX resulted in immotile embryos, whereas AP-5, CNQX, and NMDA-injected fish, like controls, could swim. Blocking activity with TTX and CNQX resulted in a significant and multiplicative increase in glutamatergic mEPSC size, affecting each synapse proportionally by a synaptic scaling factor of approximately 1.4. AP-5 and NMDA treated fish showed trends towards larger glutamatergic mEPSCs but differences were not significant. No significant changes were seen following any treatment in glutamatergic mEPSC frequency, kinetics, or AMPA:NMDA ratio.
Conclusions: These results support a role for synaptic scaling in vivo as a means of maintaining homeostasis during development. Multiplicative scaling up of glutamatergic mEPSCs without a change in their frequency suggests a strengthening of existing synapses rather than addition of new ones, but this will require a morphological analysis. If correct, this observation suggests that blocking synaptic transmission results in an enhancement of postsynaptic activity. Of particular interest is our observation of swimming activity following chronic blockade of synaptic activity as this provides new evidence that synaptic scaling may contribute to the stabilization of behaviorally relevant neural networks during embryonic development. This work was supported by the CIHR and NSERC.
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