Chronology-based architecture of descending circuits that underlie the development of locomotor repertoire after birth.

The emergence of new and increasingly sophisticated behaviors after birth is accompanied by dramatic increase of newly established synaptic connections in the nervous system. Little is known, however, of how nascent connections are organized to support such new behaviors alongside existing ones. To understand this, in the larval zebrafish we examined the development of spinal pathways from hindbrain V2a neurons and the role of these pathways in the development of locomotion. We found that new projections are continually layered laterally to existing neuropil, and give rise to distinct pathways that function in parallel to existing pathways. Across these chronologically layered pathways, the connectivity patterns and biophysical properties vary systematically to support a behavioral repertoire with a wide range of kinematics and dynamics. Such layering of new parallel circuits equipped with systematically changing properties may be central to the postnatal diversification and increasing sophistication of an animal's behavioral repertoire.

Brain-wide circuit interrogation at the cellular level guided by online analysis of neuronal function.

Whole-brain imaging allows for comprehensive functional mapping of distributed neural pathways, but neuronal perturbation experiments are usually limited to targeting predefined regions or genetically identifiable cell types. To complement whole-brain measures of activity with brain-wide manipulations for testing causal interactions, we introduce a system that uses measured activity patterns to guide optical perturbations of any subset of neurons in the same fictively behaving larval zebrafish. First, a light-sheet microscope collects whole-brain data that are rapidly analyzed by a distributed computing system to generate functional brain maps. On the basis of these maps, the experimenter can then optically ablate neurons and image activity changes across the brain. We applied this method to characterize contributions of behaviorally tuned populations to the optomotor response. We extended the system to optogenetically stimulate arbitrary subsets of neurons during whole-brain imaging. These open-source methods enable delineating the contributions of neurons to brain-wide circuit dynamics and behavior in individual animals.

Mutual inhibition of lateral inhibition: a network motif for an elementary computation in the brain.

A series of classical studies in non-human primates has revealed the neuronal activity patterns underlying decision-making. However, the circuit mechanisms for such patterns remain largely unknown. Recent detailed circuit analyses in simpler neural systems have started to reveal the connectivity patterns underlying analogous processes. Here we review a few of these systems that share a particular connectivity pattern, namely mutual inhibition of lateral inhibition. Close examination of these systems suggests that this recurring connectivity pattern ('network motif') is a building block to enforce particular dynamics, which can be used not only for simple behavioral choice but also for more complex choices and other brain functions. Thus, a network motif provides an elementary computation that is not specific to a particular brain function and serves as an elementary building block in the brain.

Motoneurons regulate the central pattern generator during drug-induced locomotor-like activity in the neonatal mouse.

Motoneurons are traditionally viewed as the output of the spinal cord that do not influence locomotor rhythmogenesis. We assessed the role of motoneuron firing during ongoing locomotor-like activity in neonatal mice expressing archaerhopsin-3 (Arch), halorhodopsin (eNpHR), or channelrhodopsin-2 (ChR2) in Choline acetyltransferase neurons (ChAT+) or Arch in LIM-homeodomain transcription factor Isl1+ neurons. Illumination of the lumbar cord in mice expressing eNpHR or Arch in ChAT+ or Isl1+ neurons, depressed motoneuron discharge, transiently decreased the frequency, and perturbed the phasing of the locomotor-like rhythm. When the light was turned off motoneuron firing and locomotor frequency both transiently increased. These effects were not due to cholinergic neurotransmission, persisted during partial blockade of gap junctions and were mediated, in part, by AMPAergic transmission. In spinal cords expressing ChR2, illumination increased motoneuron discharge and transiently accelerated the rhythm. We conclude that motoneurons provide feedback to the central pattern generator (CPG) during drug-induced locomotor-like activity.

Interactions between Dorsal and Ventral Root Stimulation on the Generation of Locomotor-Like Activity in the Neonatal Mouse Spinal Cord.

We investigated whether dorsal (DR) and ventral root (VR) stimulus trains engage common postsynaptic components to activate the central pattern generator (CPG) for locomotion in the neonatal mouse spinal cord. VR stimulation did not activate the first order interneurons mediating the activation of the locomotor CPG by sacrocaudal afferent stimulation. Simultaneous stimulation of adjacent dorsal or ventral root pairs, subthreshold for evoking locomotor-like activity, did not summate to activate the CPG. This suggests that locomotor-like activity is triggered when a critical class of efferent or afferent axons is stimulated and does not depend on the number of stimulated axons or activated postsynaptic neurons. DR- and VR-evoked episodes exhibited differences in the coupling between VR pairs. In DR-evoked episodes, the coupling between the ipsilateral and contralateral flexor/extensor roots was similar and stronger than the bilateral extensor roots. In VR-evoked episodes, ipsilateral flexor/extensor coupling was stronger than both the contralateral flexor/extensor and the bilateral extensor coupling. For both types of stimulation, the coupling was greatest between the bilateral L1/L2 flexor-dominated roots. This indicates that the recruitment and/or the firing pattern of motoneurons differed in DR and VR-evoked episodes. However, the DR and VR trains do not appear to activate distinct CPGs because trains of DR and VR stimuli at frequencies too low to evoke locomotor-like activity did so when they were interleaved. These results indicate that the excitatory actions of VR stimulation converge onto the CPG through an unknown pathway that is not captured by current models of the locomotor CPG.

Mechanisms of excitation of spinal networks by stimulation of the ventral roots.

It has recently been demonstrated that motoneurons in neonatal rodents release an excitatory amino acid, in addition to acetylcholine, from their central terminals onto Renshaw cells. Although the function of this amino acid release is not understood, it may mediate the excitatory actions of motor axon stimulation on spinal motor networks. Stimulation of motor axons in the ventral roots or muscle nerves can activate the locomotor central pattern generator or entrain bursting in the disinhibited cord. Both of these effects persist in the presence of cholinergic antagonists and are abolished or diminished by ionotropic and metabotropic glutamate antagonists. Calcium imaging in the disinhibited cord shows that a ventral root stimulus evokes ventrolateral activity initially, which subsequently propagates to the rest of the cord. This finding suggests that excitatory interneurons excited by motoneuron recurrent collaterals are located in this region. However, motoneurons do not exhibit short latency excitatory potentials in response to ventral root stimulation indicating that the excitatory effects are mediated polysynaptically. We discuss the significance of these findings.

Excitatory actions of ventral root stimulation during network activity generated by the disinhibited neonatal mouse spinal cord.

To further understand the excitatory effects of motoneurons on spinal network function, we investigated the entrainment of disinhibited rhythms by ventral root (VR) stimulation in the neonatal mouse spinal cord. A brief train of stimuli applied to a VR triggered bursting reliably in 31/32 experiments. The same roots that entrained disinhibited bursting could also produce locomotor-like activity with a similar probability when the network was not disinhibited. The ability of VR stimulation to entrain the rhythm persisted in nicotinic and muscarinic cholinergic antagonists but was blocked by the AMPAR antagonist NBQX. Bath application of the type I mGluR1 receptor antagonist CPCCOEt reduced the ability of both dorsal root and VR stimulation to entrain the disinhibited rhythm and abolished the ability of either type of stimulation to evoke locomotor-like activity. Calcium imaging through the lateral aspect of the cord revealed that VR stimulation and spontaneously occurring bursts were accompanied by a wave of activity that originated ventrally and propagated dorsally. Imaging the cut transverse face of L(5) revealed that the earliest VR-evoked optical activity began ventrolaterally. The optical activity accompanying spontaneous bursts could originate ventrolaterally, ventromedially, or throughout the mediolateral extent of the ventral horn or very occasionally dorsally. Collectively, our data indicate that VR stimulation can entrain disinhibited spinal network activity and trigger locomotor-like activity through a mechanism dependent on activation of both ionotropic and metabotropic glutamate receptors. The effects of entrainment appear to be mediated by a ventrolaterally located network that is also active during spontaneously occurring bursts.