Time course of axotomy-induced apoptotic cell death in facial motoneurons of neonatal wild type and bcl-2 transgenic mice

In neonatal animals, axotomy of facial motoneurons induces cell death. Using the TUNEL technique, which labelled apoptotic DNA breaks in situ, the kinetics of motoneuron death were studied. Lesion of the right facial nerve were performed on two-day-old mice. Then, animals were perfused 8, 12, 16, 20, 24, 28, 32, 48, 72 and 120 h after the lesion. Our results provide direct evidence that, following an axotomy, facial motoneurons die through an apoptotic process. We showed that apoptotic neurons can be detected as early as 16 h after the lesion. Facial motoneurons die within 120 h, with a peak observed 28 h after the lesion. The kinetics of appearance of apoptotic cells were correlated with the loss of Cresyl Violet-stained motoneurons. Furthermore, labelled cells were observed in the contralateral side of the lesion, suggesting that spontaneous apoptotic cell death occurs during the postnatal period. The same study was performed on transgenic mice overexpressing the proto-oncogene bcl-2, a gene repressor of cell death. In these mice, no TUNEL-labelled cells were detected on the lesioned and unlesioned sides. In vivo, Bcl-2 may protect motoneurons from apoptotic death following axotomy and during naturally occurring cell death. These results suggest that these two types of cell death may occur via the same mechanism.