We proceeded to characterize the most excitable neurons from each group (see Materials and Methods for definition of the most excitable neurons). Generally, the more excitable neurons, which we termed ‘hyper’ neurons, had larger sodium currents that opened at a less depolarized potential (Figure 5b) compared with the ‘hypo’ neurons. The fast potassium currents were also 10–15% higher in the control and LR ‘hyper’ groups, compared with the matching ‘hypo’ neurons (Figure 5c), and generally in the more excitable neurons the fast potassium channels opened in a less depolarized potential. The sodium-to-potassium ratio was larger for the ‘hyper’ neurons of all three groups (Figure 5e), indicating that this ratio of the sodium currents at −20 mV (approximately the potential where the sodium channels opened) to the potassium currents at 20 mV (a potential where many of the potassium channels have opened, close to the peak of the action potential) is an important feature of cell excitability. Interestingly, only BD neurons were hyperexcitable when the sodium-to-potassium ratio was close to 0 (Figure 5e), indicating that another cell-intrinsic property had a role.
