Two types of neurons in the rat cerebellar nuclei as distinguished by membrane potentials and intracellular fillings

Classically, three classes of neurons in the cerebellar nuclei (CN), define d by different projection targets and content of transmitters, have been di stinguished. However, evidence for different types of neurons based on diff erent intrinsic properties is lacking. The present study reports two types of neurons defined mainly by their intrinsic properties, as determined by w hole-cell patch recordings. The majority of cells (type I, n = 63) showed c yclic burst firing whereas a small subset (type II, n = 7) did not. Burst f iring was used to distinguish the two types of neurons because, as it turne d out, pharmacological interference could not be used to convert the non-bu rsting cells to bursting ones. Some of the membrane potentials exclusively present in type I neurons, such as sodium and calcium plateau potentials, l ow-threshold calcium spikes, and a slow calcium-dependent afterhyperpolariz ation, were found to contribute to the generation of burst firing. Other me mbrane potentials of type I neurons were not obviously related to the gener ation of bursts. These were 1) the lower amplitude and width of the action potential during spontaneous activity, 2) a sequence of afterhyperpolarizat ion-afterdepolarization-afterhyperpolarization following each spike, and 3) the high spontaneous firing rate. In contrast, type II neurons lacked slow plateau potentials and low threshold spikes. Their action potentials showe d higher amplitude and width and were followed by a single deep afterhyperp olarization. Furthermore, they showed a lower firing rate at rest. In both types of neurons, a delayed inward rectification was present. Neurons fille d with neurobiotin revealed that the sizes of the somata and dendritic fiel ds of type I neurons comprised the whole range known from Golgi studies, wh ereas those of the few type II neurons recovered were found to be in the lo west range. In view of their size and scarcity, we propose that type II neu rons may correspond to CN interneurons.