Impact of spontaneous synaptic activity on the resting properties of neocortical pyramidal neurons in vivo.
Denis Paré, Eric Shink, Hélène Gaudreau, Alain Destexhe and Eric Lang

Society for Neuroscience Abstracts 23: 1127, 1997.

The integrative properties of pyramidal neurons might be quite different in vitro versus in vivo considering that much of the connectivity is lost in brain slices. Indeed, each cortical pyramidal cell receives 10 000 inputs, 70% of which originate in other cortical neurons. Considering that pyramidal neurons were reported to fire at 10 Hz in waking animals and since individual synaptic events produce transient increases in membrane conductance, it logically follows that background network activity should have a major impact on the physiological properties of pyramidal neurons. Here, the impact of spontaneous network activity on the passive properties of pyramidal neurons was estimated in vivo and in brain slices using identical recording methods. The amount of synaptic activity was much lower in brain slices as the standard deviation of the intracellular signal was 10-17 times lower in vitro than in vivo. Input resistances measured in vivo during relatively quiescent epochs (“control Rins”) could be reduced by up to 70% during periods of intense spontaneous activity. Further, “control Rins” were increased by 30-70% following tetrodotoxin (TTX) application in vivo, approaching in vitro values. In contrast, TTX produced negligible Rin changes in vitro provided that it was estimated outside the region of inward rectification characterizing these cells in the depolarizing direction. Thus, network activity dramatically reduces the electrical compactness of cortical neurons. This should be taken into account when extrapolating in vitro findings to intact brains.

Supported by: MRC, NSERC and NINDS.