Optical analysis of functional development of the facial motor nucleus in the embryonic rat brainstem

Facial motor neurons of the rat embryo are first generated in rhombomere 4 and then migrate in the caudo‐ventral direction. This migration forms a unique axonal trajectory called the genu, a loop of facial motor axons around the abducens nucleus. It is still unclear when and how this unique structure is functionally established during ontogenesis. Using voltage‐sensitive dye (VSD) recording and the DiI staining method, we identified neural responses evoked by facial nerve (N.VII) stimulation and examined developmental processes of the facial motor nucleus in E12‐E17 rat brainstems. We identified two types of fast spike‐like signals; a long‐duration signal, which corresponded to the action potential in the N.VII soma, and a short‐duration signal, which reflected the action potential in the N.VII axons. The long‐duration signal was detected as early as E13, suggesting that the N.VII motor neuron is already excitable at the beginning of cell migration. The response area of the long‐duration signal extended caudally at E13‐E14, and shifted in a ventral direction at E15. At E16‐E17, the long‐duration signal was concentrated in the caudo‐ventral area, which was comparable to the location of the facial motor nucleus in the adult rat brainstem. These results demonstrate that developmental processes of cell migration and nuclear organization can be visualized and identified functionally with the VSD recording. We discuss the results by comparing functiogenesis and morphogenesis of the N.VII pathway.