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

Journal of Neurophysiology 79: 1450-1460, 1998.

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The frequency of spontaneous synaptic events in vitro is probably lower than in vivo due to the reduced synaptic connectivity present in cortical slices and the lower temperature used during in vitro experiments. Since this reduction in background synaptic activity could modify the integrative properties of cortical neurons, we compared the impact of spontaneous synaptic events on the resting properties of intracellularly-recorded pyramidal neurons in vivo and in vitro by blocking synaptic transmission with tetrodotoxin (TTX).

The amount of synaptic activity was much lower in brain slices (at 34oC) as the standard deviation of the intracellular signal was 10-17 times lower in vitro than in vivo. Rins measured in vivo during relatively quiescent epochs (‘control Rins’) could be reduced by up to 70% during periods of intense spontaneous activity. Further, the ‘control Rins’ were increased by about 30-70% following TTX application in vivo, approaching in vitro values. In contrast, TTX produced negligible Rin changes in vitro (about 4%).

These results indicate that, compared to the in vitro situation, the background synaptic activity present in intact networks dramatically reduces the electrical compactness of cortical neurons and modifies their integrative properties. The impact of the spontaneous synaptic bombardment should be taken into account when extrapolating in vitro findings to the intact brain.