Spike sequences and mean firing rate in rat neocortical neurons in vitro

Properties of neuronal discharges and their interrelationship with mean evoked firing rate (MF) were investigated in detail in the rat somatosensory cortex in vitro. Firing was evoked by 1-min depolarizing current injection (0.1-1.0 nA) into pyramidal neurons (n=93) of layers II/III and V. Spike sequence patterns (SSPs) with consecutive increasing or decreasing interspike intervals were analyzed. SSP rate (change of pattern, 2-50 min(-1)) and the deviations of afterhyperpolarizing potentials (up to 1.5 mV) associated with the SSPs were significantly correlated with MF. The number of transitions in spike frequency was dependent on the MF and showed a biphasic interrelation with a reversible point at 5-8 Hz. All neurons analysed were subdivided into two groups: with depolarizing potential after single spike and without such afterdepolarizing potential. It was found that despite significant differences in electrophysiological properties of the cells from these groups, similar dependency in patterns of their discharges upon MF was observed. A possible role of spike sequence patterns in neocortical rhythmical processes is discussed.