Maturation of layer 5 neocortical pyramidal neurons: amplifying salient layer 1 and layer 4 inputs by Ca2+ action potentials in adult rat tuft dendrites

Changes in the arborization and electrical excitability of the apical dendritic tufts of pyramidal cells of cortical layer 5 were examined during the first 2 months (postnatal days (P)2-56) of postnatal development in rats. Reconstructions of biocytin-filled neurons showed that the apical dendritic trunk was continually growing, becoming longer and thicker and that the distance between the tuft and soma increased more than 5-fold. In P2 animals, both the tuft and soma had a high input resistance (> 500 MOmega) and the tuft was electrotonically close to the soma. In contrast, the apical tuft and soma of P56 neurons had a low input resistance ( P42), the complex regenerative potentials frequently occurred simultaneously with somatic action potentials. The main developmental change in layer 5 pyramidal neurons, as assayed with square pulse current injections and synaptic stimulations, is the progressive electrotonic isolation of the dendritic tuft from the soma. This change is concomitant with the appearance of complex, mostly Na+- and Ca2+-dependent, regenerative dendritic potentials initiated partly in the tuft and partly in the axon. The coupling of the dendritic tuft and axonal initiation zones for regenerative potentials by active dendritic Na+ and Ca2+ conductances enables mature layer 5 pyramidal neurons to detect selectively the salient distal synaptic inputs and coincident synaptic inputs arriving at different cortical layers.