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

1. 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. 2. Rt constructions 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, 3. In P2 animals, both the tuft and soma had a high input, resistance (> 500 M Omega) and the tuft was electrotonically close to the some. In contrast, the apical tuft and soma of P56 neurons had a low input resistance (< 50 M Omega) and they were electrotonically isolated From each other. 4. Depolarizing current pulses injected into the tuft of P2 cells generated mostly Na+-dependent regenerative dendritic potentials of short duration (similar to 15 ms) while in the tuft of P56 animals, complex regenerative potentials were generated which had a longer duration (similar to 55 ms) and were Na+ and Ca2+ dependent. In young and juvenile animals (P14-28) dendritic regenerative potentials could br restricted tu the apical dendritic tuft whereas in adult animals (> P42), the: complex regenerative potentials frequently occurred simultaneously wit ll somatic action potentials. 5. 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 thr: dendritic tuft from the soma. This change is concomitant with that 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.