Transient expression of A-type K⁺ channel α subunits Kv4.2 and Kv4.3 in rat spinal neurons during development

A-type K⁺ currents (I_As) have been detected from the ventral horn neurons in rat spinal cord during embryonic day (E) 14 to postnatal day (P) 8 but not in adulthood. It is not known which types of neurons and which A-type K⁺ channel α subunits express the I_As and what the possible function might be. Here, we examined the expression of two A-type K⁺ channel α subunits, Kv4.2 and Kv4.3, in rat spinal cord at various developmental stages by immunohistochemistry. We found a transient expression of Kv4.2 in somatic motoneurons during E13.5-P8 with a peak around E17.5, which coincides temporally with the natural selection of motoneurons. Transient expression of Kv4.2 and Kv4.3 was also observed in the intermediate gray (IG) interneurons. During E19.5-P14, some IG interneurons express Kv4.2, some express Kv4.3 and a subset co-express Kv4.2 and Kv4.3. Peak expression of Kv4.2 and Kv4.3 in the IG interneurons was detected around P1, which coincides temporally with the developmental selection of IG interneurons. In contrast to the I_A-expressing subunits Kv4.2 and Kv4.3, a delayed-rectifier K⁺ channel α subunit Kv1.6 is persistently expressed in somatic motoneurons and IG interneurons. Together, these data support the hypothesis that expression of I_As may protect I _A-expressing somatic motoneurons, and possibly also IG interneurons, from naturally occurring cell death during developmental selection.