Expression of Kv3.1 and Kv4.2 genes in developing cerebellar granule cells

The expression of voltage-gated potassium channels plays an important role in the acquisition of membrane excitability in neurons. We examined the exp ression pattern of genes in developing cerebellar granule neurons in vivo a nd in vitro, In situ hybridization of Kv3.1 mRNA demonstrated that the gene was expressed at high levels in the external granule layer (EGL) as well a s in the internal granule layer (IGL) at all postnatal stages (P) examined (from P3 to P10). In contrast, Kv4.2 mRNA was detected in the premigratory zone (PMZ) of the EGL, but not in the proliferative zone (PLZ), in addition to the IGL. This indicates that Kv4.2 gene expression initiates in the pos tmitotic migrating neurons. We also examined the expression of the channel genes in microexplant culture systems. Kv3.1 polypeptide was detected in pa rallel fibers of granule cells at 2 days in vitro, and the expression conti nued in later stages. The signal of Kv4.2 protein was very low at 2 days in vitro; however, the number of positive cells and the intensity of the sign als were increased at 6 days in vitro. These in vitro observations matched those in vivo and our previous electrophysiological studies in which we dem onstrated that delayed-rectifier-type current was predominant in the immatu re granule cells followed by the later appearance of A-type current. The pa tterns of K+ channel expression suggest that sequential expression of these channel genes primarily determines the membrane excitability.