Mechanisms That Initiate Spontaneous Network Activity in the Developing Chick Spinal Cord

Abstract

Many developing networks exhibit a transient period of spontaneous activity that is believed to be important developmentally. Here we investigate the initiation of spontaneous episodes of rhythmic activity in the embryonic chick spinal cord. These episodes recur regularly and are separated by quiescent intervals of many minutes. We examined the role of motoneurons and their intraspinal synaptic targets (R-interneurons) in the initiation of these episodes. During the latter part of the inter-episode interval, we recorded spontaneous, transient ventral root depolarizations that were accompanied by small, spatially diffuse fluorescent signals from interneurons retrogradely labeled with a calcium-sensitive dye. A transient often could be resolved at episode onset and was accompanied by an intense pre-episode (approximately 500 ms) motoneuronal discharge (particularly in adductor and sartorius) but not by interneuronal discharge monitored from the ventrolateral funiculus (VLF). An important role for this pre-episode motoneuron discharge was suggested by the finding that electrical stimulation of motor axons, sufficient to activate R-interneurons, could trigger episodes prematurely. This effect was mediated through activation of R-interneurons because it was prevented by pharmacological blockade of either the cholinergic motoneuronal inputs to R-interneurons or the GABAergic outputs from R-interneurons to other interneurons. Whole-cell recording from R-interneurons and imaging of calcium dye-labeled interneurons established that R-interneuron cell bodies were located dorsomedial to the lateral motor column (R-interneuron region). This region became active before other labeled interneurons when an episode was triggered by motor axon stimulation. At the beginning of a spontaneous episode, whole-cell recordings revealed that R-interneurons fired a high-frequency burst of spikes and optical recordings demonstrated that the R-interneuron region became active before other labeled interneurons. In the presence of cholinergic blockade, however, episode initiation slowed and the inter-episode interval lengthened. In addition, optical activity recorded from the R-interneuron region no longer led that of other labeled interneurons. Instead the initial activity occurred bilaterally in the region medial to the motor column and encompassing the central canal. These findings are consistent with the hypothesis that transient depolarizations and firing in motoneurons, originating from random fluctuations of interneuronal synaptic activity, activate R-interneurons, which then trigger the recruitment of the rest of the spinal interneuronal network. This unusual function for R-interneurons is likely to arise because the output of these interneurons is functionally excitatory during development.

GABA A transmission is a critical step in the process of triggering homeostatic increases in quantal amplitude

“…White Leghorn chicken embryos (Hy-Line Hatcheries) were dissected as described in ref. 48. Briefly, spinal cords from stage 36 (E10) embryos were isolated with intact muscle nerves.…”

Fast noninvasive activation and inhibition of neural and network activity by vertebrate rhodopsin and green algae channelrhodopsin

“…Our next goal was to show that these light-sensitive proteins could be used to control circuit behavior in whole animal preparations. Early embryonic chick spinal cords exhibit rhythmic episodes of spontaneous bursting activity, which are generated by recurrent excitatory connections between motoneurons and GABAergic and glycinergic interneurons, all of which are excitatory at this stage of development (23)(24)(25). Recently, it has been shown that the normal pattern and frequency of this early spontaneous activity is required for appropriate motor axon pathfinding in the chick (14) and for the development of cord circuits that enable appropriate flexorextensor and right-left phasing during locomotor-like activity in the mouse (26).…”

“…This finding was not entirely unexpected because regions of cord not electroporated with RO4 would still be able to depolarize and contribute to the excitation required to elicit a bursting episode (see ref. 23) for details of episode generation). Surprisingly, however, a 2-s pulse of light actually elicited a premature episode (4I, 2) 1 min after a spontaneous episode (Fig.…”

The onset of hyperreflexia in the rat following complete spinal cord transection

“…13,19 Studies of the developing chick cord showed that spinal networks experience transient increases in synaptic activity, presumably arising from the coordinated firing of clusters of neurons, and that these events can trigger synchronized network activity. 19 The ensemble activity of these networks allows the chick to synchronize motor activity (co-contractions) to break out of the shell, and then changes in organization allow the chick to manifest bipedal (alternating) locomotion while maintaining the ability to co-contract. 17 Gap junctions are required for synchronizing the oscillatory responses of neurons and for clustering of coherent rhythmic activity.…”