Fast spiking neocortical interneurons can produce very high frequency firing rates. Their electrophysiological properties include a high amplitude, narrow spike and a large fast AHP.58–60 Fast AHP is known to assist rapid recovery of Na+ channel inactivation,61 allowing for a high persistent rate of firing. Interestingly, our first sub-population of BD neurons, the LR neurons, displayed a similar phenotypic behavior to fast spiking interneurons—a high amplitude, narrow spike, with a large fast AHP—that explains their hyperexcitability and fast spiking abilities. The other sub-population of BD neurons, the NR neurons, were very different from both the first population (LR) and from the control neurons. They had a smaller spike amplitude and a wider spike, and they exhibited an almost 50% reduction in sodium currents along with a more depolarized threshold. The large differences between the two subgroups of BD neurons suggest two distinct subdisorders, with very different intrinsic properties, where the common cellular phenotype is hyperexcitability. Their shared electrophysiological feature, a large fast AHP, suggests a possible expedited recovery of sodium channel inactivation, which allows BD neurons to sustain spiking activity.
