Impact of spontaneous network activity and miniature synaptic potentials on membrane properties of cortical pyramidal neurons: an in vivo intracellular study.
Hélène Gaudreau, Eric Lang, Alain Destexhe and Denis Paré

Society for Neuroscience Abstracts 22: 790, 1996.

In vivo intracellular recordings were obtained from area 5-7 cortical pyramidal neurons with pipettes containing KCl. TTX was applied locally to the recording area to block all network activity as shown by the lack of evoked responses to local stimuli and loss of Na spikes. Under these conditions, cells had Rin of 40-60 MOhm, whereas in the absence of TTX the Rin ranged between 20-30 MOhm. Cells recorded with pipettes containing KCl and QX314, had intermediate Rin values of 30-40 MOhm (no TTX present). TTX increased the time constant approx. 2-fold. Thus, spontaneous network activity significantly influences the membrane properties of pyramidal neurons. After TTX application, we observed frequent (5-20/sec), low-amplitude (0.5-2 mV) depolarizing events, which were interpreted as miniature synaptic potentials (minis) independent of Na spikes. The amplitude of the minis was correlated with the Rin of the cell. suggesting additional unobserved events occurring at more distant sites. Larger potentials were also seen, and appeared to result from summating minis. Presumably, minis were generated at sites electrotonically close to the soma where most synapses are GABAergic. Consistent with the idea that minis were reversed GABA_A IPSPs, membrane depolarization reduced their amplitude. Experiments are currently underway to identify the involved transmitter. The minis produced transient drops in Rin, suggesting that ongoing action potential-independent transmitter release is a major determinant of the membrane properties of pyramidal neurons in intact networks.

Support: MRC, NINDS and FRSQ.