SUMMARY Background 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. Results 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 anti-correlated 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 VeLD interneurons, and resulted in more neurons exhibiting uncoordinated activity patterns at later time points. Conclusions 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.

We demonstrate that single interneurons can toggle the output neurons of the cerebellar cortex (the Purkinje cells) between their two states. The firing of Purkinje cells has previously been shown to alternate between an "up" state in which the cell fires spontaneous action potentials and a silent "down" state. We show here that small hyperpolarizing currents in Purkinje cells can bidirectionally toggle Purkinje cells between down and up states and that blockade of the hyperpolarization-activated cation channels (H channels) with the specific antagonist ZD7288 (10 μM) blocks the transitions from down to up states. Likewise, hyperpolarizing inhibitory postsnyaptic potentials (IPSPs) produced by small bursts of action potentials (10 action potentials at 50 Hz) in molecularlayer interneurons induce these bidirectional transitions in Purkinje cells. Furthermore, single interneurons in paired interneuron → Purkinje cell recordings, produce bidirectional switches between the two states of Purkinje cells. The ability of molecular-layer interneurons to toggle Purkinje cells occurs when Purkinje cells are recorded under whole-cell patch-clamp conditions as well as when action potentials are recorded in an extracellular loose cell-attached configuration. The mode switch demonstrated here indicates that a single presynaptic interneuron can have opposite effects on the output of a given Purkinje cell, which introduces a unique type of synaptic interaction that may play an important role in cerebellar signaling.A s the sole output of the cerebellar cortex, Purkinje cells are ultimately responsible for relaying the computation performed by the entire network. Even though membrane properties and synaptic currents have been extensively studied in the cerebellar circuit, it still remains unclear how Purkinje cells process information coming from the rest of the cerebellar cortex before passing it on to the deep cerebellar nuclei. Purkinje cells are known to intrinsically generate action potentials at high rates and also to have an intrinsic membrane bistability (1-4). The cell switches between hyperpolarized potentials during which it is silent and depolarized potentials during which it fires action potentials (1, 2, 5-11). The frequency of changes between the two states in awake versus anesthetized animals in vivo has been disputed, but it is agreed that the bistability exists in a fraction of Purkinje cells in both anesthetized and unanesthetized animals (5, 6). This unique characteristic of the output of Purkinje cells potentially plays a significant role in passing the computation of the cerebellar cortex onto the cerebellar nuclei. Therefore to understand how Purkinje cells process this information it is essential to understand how the cells within the cerebellar cortex influence this bistable output of Purkinje cells.The climbing fibers, which provide glutamatergic inputs to Purkinje cells from outside the cerebellar cortex, have been shown to produce complex spikes in Purkinje cells (1). In experiments performed in vivo a...