Mechanisms that initiate spontaneous network activity in the developing chick spinal cord

Wenner, Peter andMichael J. O'Donovan.Mechanisms That Initiate Spontaneous Network Activity in theDeveloping Chick Spinal Cord. J. Neurophysiol. 86: 1481-1498, 2001. Many developing networks exhibit a transient period ofspontaneous activity that is believed to be important developmentally. Here we investigate the initiation of spontaneous episodes of rhythmicactivity in the embryonic chick spinal cord. These episodes recurregularly and are separated by quiescent intervals of many minutes. Weexamined the role of motoneurons and their intraspinal synaptic targets(R-interneurons) in the initiation of these episodes. During the latterpart of the inter-episode interval, we recorded spontaneous,transient ventral root depolarizations that were accompanied by small,spatially diffuse fluorescent signals from interneurons retrogradelylabeled with a calcium-sensitive dye. A transient often could beresolved at episode onset and was accompanied by an intense pre-episode(~500 ms) motoneuronal discharge (particularly in adductor andsartorius) but not by interneuronal discharge monitored from theventrolateral funiculus (VLF). An important role for this pre-episodemotoneuron discharge was suggested by the finding that electricalstimulation of motor axons, sufficient to activate R-interneurons,could trigger episodes prematurely. This effect was mediated throughactivation of R-interneurons because it was prevented bypharmacological blockade of either the cholinergic motoneuronal inputsto R-interneurons or the GABAergic outputs from R-interneurons to otherinterneurons. Whole-cell recording from R-interneurons and imaging ofcalcium dye-labeled interneurons established that R-interneuron cellbodies were located dorsomedial to the lateral motor column(R-interneuron region). This region became active before other labeledinterneurons when an episode was triggered by motor axonstimulation. At the beginning of a spontaneous episode, whole-cellrecordings revealed that R-interneurons fired a high-frequency burstof spikes and optical recordings demonstrated that the R-interneuronregion became active before other labeled interneurons. In the presenceof cholinergic blockade, however, episode initiation slowed and theinter-episode interval lengthened. In addition, optical activityrecorded from the R-interneuron region no longer led that of otherlabeled interneurons. Instead the initial activity occurred bilaterallyin the region medial to the motor column and encompassing thecentral canal. These findings are consistent with the hypothesis thattransient depolarizations and firing in motoneurons, originating fromrandom fluctuations of interneuronal synaptic activity, activateR-interneurons, which then trigger the recruitment of the rest of thespinal interneuronal network. This unusual function for R-interneuronsis likely to arise because the output of these interneurons isfunctionally excitatory during development.