Gap junctions mediate electrical coupling between neurons and promote neuronal circuit formation. Recently we found that excitatory neurons form dense and precise gap junction networks in the developing neocortex. In layer 5 of the mouse visual cortex, subcerebral projection neurons (SCPNs) and callosal projection neurons (CPNs) selectively coupled with neurons of the same class, at frequencies ~50% in the early second postnatal week. Neurons of each subnetwork showed synchronous firing. Because extensive synapse formation occurs during the same developmental period, coordinated activities mediated by the electrical coupling may facilitate synapse formation in a cell-type-specific manner.
To test the hypothesis, we conducted electrophysiological analysis in mouse brain slices at postnatal day 10-11. At this stage, SCPN-SCPN pairs and CPN-CPN pairs had electrical coupling with similar probability (~20%). In contrast, chemical synapses were much more frequently found in SCPN-SCPN pairs. Moreover, electrically coupled SCPN-SCPN pairs had chemical synapses with a high probability. These results suggest the possibility that electrical coupling of these two cell types has different functions in synapse formation. Whether gap junctions regulate the synapse formation is now under investigation using dominant-negative connexin mutants. Possible contribution of the gap junction subnetworks in the formation of cell-type-specific synaptic circuits will be discussed.