Reduced spike frequency adaptation in Purkinje cells of the vestibulocerebellum

Firing regularity has long been an issue of firing dynamics in the vestibular circuitry. Spike frequency adaption (SFA) is ubiquitous in neuronal activity and can modulate neural coding, which may disrupt the regularity or accuracy of firing. We previously observed different characteristics of intrinsic excitability in Purkinje cells (PCs) of lobule X (vestibulocerebellum) compared to lobules III-V (spinocerebellum). However, systematic comparison of the extent of SFA in PCs of different lobules has not yet been made. In this study we examined the degree of SFA and compared the firing regularity by measuring interspike interval (ISI). During the course of low-frequency spike trains, PCs in lobules III-V showed gradual lengthening of ISI due to SFA. In contrast, ISI showed little change during the propagation of spikes in lobule X PCs. In high-frequency firing, PCs in lobules III-V exhibited gradual SFA, whereas lobule X neurons showed dramatic increase in ISI during the first four spikes and then stayed unchanged. The coefficient of variation of ISI of lobule X PCs was significantly lower in lobules III-V PCs during low-frequency firing. The comparison of duration of action potential showed no significant difference between lobules III-V and lobule X PCs during SFA even in low-frequency firing. The lack of SFA in lobule X PCs, as a part of vestibulocerebellum, might be involved in a consistent and regular coordination of vestibular function by the cerebellar cortex in response to low vestibular stimulation. However, the difference of SFA between lobules may be explained by other mechanisms than those which have been reported to be responsible for the SFA formation.