Spontaneous synaptic activity modulates action potential generation in neocortical pyramidal cells in vivo.
Alain Destexhe and Denis Paré

Society for Neuroscience Abstracts 23: 1127, 1997.

Computational models based on intracellular recordings in vivo were built to characterize the impact of spontaneous synaptic activity on the properties of neocortical pyramidal cells. Several detailed morphologies were used to simulate pyramidal cells. Passive properties were adjusted to match passive responses in the absence of synaptic activity. GABAergic (GABA_A) and glutamatergic (AMPA) synapses were distributed in soma and dendrites according to previous ultrastructural findings. Quantal conductances of AMPA- and GABA_A- mediated synaptic events were estimated from amplitude histograms of miniature postsynaptic potentials (mPSPs). With low rates of spontaneous release at each terminal, the model reproduced the amplitude fluctuations and small change of input resistance (Rin) produced by mPSPs in neocortical pyramidal cells in vivo. To reproduce the larger Rin change associated with synaptic activity of the intact network, we had to assume much higher average frequency of release at each terminal. Uncorrelated release gave rise to the correct Rin change, but incorrect amplitude fluctuations. Correlated release had to be assumed (correlation of 0.4-0.7) to reproduce all features of spontaneous synaptic activity recorded intracellularly in vivo. In these conditions, we found major effects on the firing properties of pyramidal cells. Dendritic Na spikes occurred frequently, but tended to stay local, rarely reaching the soma and rarely backpropagating into distal ends (about 30% of the cases). The intense network activity in vivo tends to subdivide the dendritic tree into localized regions that process synaptic inputs independently, by contrast to the electrical compactness of dendrites seen in vitro. These results therefore suggest that localized dendritic regions may constitute the basic computational unit of pyramidal cells in vivo.

Supported by NSERC, FRSQ and MRC of Canada.