Extensive dye coupling between rat neocortical neurons during the period of circuit formation

A low molecular weight intracellular tracer, Neurobiotin, was injected into single neurons in living slices of rat neocortex made at postnatal days 5-18. Between days 5 and 12, 66% of single-neuron injections labeled clusters of up to 80 neurons surrounding the injected cell. Coupling between neurons occurred primarily through dendrites. Injections done in the presence of halothane, a gap junction blocker, abolished the spread of tracer to surrounding neurons, implying that gap junctions mediate coupling. Injections done after day 16 resulted in little or no dye coupling. We conclude that transient local coupling via gap junctions in developing cortex may provide a pathway for communicating intercellular signals, including subthreshold electrical activity, and thereby enable temporal coordination of local neuronal ensembles during circuit formation.