Figure 3.

A large diversity in excitability is observed in NR hippocampal neurons due to a physiological instability. Electrophysiological measurements (A-H) compared to the computational simulation (I-J). A. Measuring the diversity in excitability within each experiment (see Supplementary Methods) reveals that, in NR DG neurons, there is a significantly larger diversity in excitability between different neurons during an experiment as compared to control DG neurons. B. Measuring the diversity in excitability in each experiment (see Supplementary Methods) reveals that, in NR CA3 neurons, there is a significantly larger diversity in excitability between different neurons during an experiment as compared to control CA3 neurons. C. Dividing the NR DG neurons that we recorded in experiment into hypoexcitable neurons vs. all the rest, there is a very large and significant change in the sodium currents of the hypoexcitable neurons vs. all the rest. D. Dividing the NR CA3 neurons that we recorded into hypoexcitable neurons vs. all the rest, there is a very large and significant change in the sodium currents of the hypoexcitable neurons vs. all the rest. E-H. Examples of different experimental days recordings from a control (physiologically stable) DG culture and from a DG NR (physiologically unstable) culture. On each line, recordings during one experiment are plotted. The evoked potentials are presented in the main graph, and in the inset in the left corner, the normalized sodium currents are presented at −20 mV and −10 mV. E. An example of a control culture in which neurons are not hyperexcitable or hypoexcitable. The total evoked potentials do not change much from neuron to neuron during the experiment. The mean sodium currents vary from around 15 pA/pF to 33 pA/pF. F. Similarly, an example with recordings from several NR neurons during the same experiment is presented. Neurons tend to be hyperexcitable or hypoexcitable (“mixed physiological state”), where a main effector is the sodium current; the hypoexcitable neurons have sodium currents that are close to 0. G. Although most of the NR networks tend to be in a mixed state of hyperexcitable and hypoexcitable cells, in approximately 15% of the experiments we found the entire culture was hypoexcitable, with neurons producing very little action potentials (and the sodium currents were very low throughout the experiment). Although most of the NR networks tend to be in a mixed state of hyperexcitable and hypoexcitable cells, in approximately 12% of the experiments we found the entire culture was hyperexcitable, with most neurons producing many action potentials (above the neurotypical-mean plus a half of a standard deviation). I. Applying changes to the sodium conductance in the numerical simulation of the DG neurons (as these vary throughout the different neurons in the experimental settings) causes mild changes in excitability of the control DG neuron (blue) vs. drastic changes in excitability in the BD NR DG neuron (green). J. Applying changes to the sodium conductance in the numerical simulation of the CA3 neurons (as these vary throughout the different neurons in the experimental settings) causes mild changes in excitability of the control CA3 neuron (blue) vs. drastic changes in excitability in the BD NR CA3 neuron (green). We conclude that low sodium currents are the main driver of the large number of hypoexcitable neurons that we observe in NR cultures. Note that NR neurons have a significantly reduced sodium current ((3)Fig. 2). In the experiment, like in the simulation, NR neurons with the average (decreased) sodium currents compared to control neurons are on average hyperexcitable, but those neurons with sodium currents that are below the average (already very reduced) sodium current become hypoexcitable. We termed this phenomenon a “physiological instability,” since the NR neurons are easily shifted between a hyperexcitable and a hypoexcitable state. Asterisks represent statistical significance by the following code: * p value<0.05, ***p value<0.001, **** p value<0.0001. Error bars represent standard error.