Emergence of Patterned Activity in the Developing Zebrafish Spinal Cord

Helen Wills Neuroscience Graduate Program, University of California, Berkeley, Berkeley, CA 94720, USA.
Current biology: CB (Impact Factor: 9.57). 12/2011; 22(2):93-102. DOI: 10.1016/j.cub.2011.12.002
Source: PubMed


Developing neural networks display spontaneous and correlated rhythmic bursts of action potentials that are essential for circuit refinement. In the spinal cord, it is poorly understood how correlated activity is acquired and how its emergence relates to the formation of the spinal central pattern generator (CPG), the circuit that mediates rhythmic behaviors like walking and swimming. It is also unknown whether early, uncorrelated activity is necessary for the formation of the coordinated CPG.
Time-lapse imaging in the intact zebrafish embryo with the genetically encoded calcium indicator GCaMP3 revealed a rapid transition from slow, sporadic activity to fast, ipsilaterally correlated, and contralaterally anticorrelated activity, characteristic of the spinal CPG. Ipsilateral correlations were acquired through the coalescence of local microcircuits. Brief optical manipulation of activity with the light-driven pump halorhodopsin revealed that the transition to correlated activity was associated with a strengthening of ipsilateral connections, likely mediated by gap junctions. Contralateral antagonism increased in strength at the same time. The transition to coordinated activity was disrupted by long-term optical inhibition of sporadic activity in motoneurons and ventral longitudinal descending interneurons and resulted in more neurons exhibiting uncoordinated activity patterns at later time points.
These findings show that the CPG in the zebrafish spinal cord emerges directly from a sporadically active network as functional connectivity strengthens between local and then more distal neurons. These results also reveal that early, sporadic activity in a subset of ventral spinal neurons is required for the integration of maturing neurons into the coordinated CPG network.

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    • "This activity is mostly calcium-mediated and becomes apparent either prior to or during synapse formation. In the embryonic spinal cord, calciumdependent electrical activity has been identified in mouse (Hanson and Landmesser, 2003), rat (Ren and Greer, 2003), chick (Sernagor et al., 1995; Chub and O'Donovan, 1998; O'Donovan et al., 1998), Xenopus laevis (Gu et al., 1994; Gu and Spitzer, 1995; Borodinsky et al., 2004), Xenopus tropicalis (Marek et al., 2010) and zebrafish (Warp et al., 2012; Plazas et al., 2013), arguing for the universal character of this developmental feature. Many studies have investigated the role of this early electrical activity in different aspects of nervous system development. "
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    • "Whilst electrical activity has repeatedly been demonstrated for emerging neuronal circuits [1], [2], [6], [69], [70], the very earliest patterns of activity in the spinal cord have only been reported for zebrafish [70], in which random firing of neurons becomes synchronous over time. The data presented here imply that this phenomenon might occur in the chick spinal cord between HH St. 23 and HH St. 30. "
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