AXOTOMY-INDUCED APOPTOTIC CELL-DEATH OF NEONATAL RAT FACIAL MOTONEURONS - TIME-COURSE ANALYSIS AND RELATION TO NADPH-DIAPHORASE ACTIVITY

Rapid and massive death of motoneurons occurs following axotomy in neo natal mammals. This likely results from the neurons being deprived of access to target-derived trophic factors, as their death can be preven ted by application of a variety of neurotrophic factors to the proxima l end of the cut nerve. Since trophic factor-deprived embryonic chick motoneurons undergo apoptosis in vitro, we have investigated whether a xotomized neonatal rat facial motoneurons undergo apoptotic cell death in vivo. Following facial nerve transection during the first postnata l day, the dying motoneurons show characteristic morphological changes of apoptosis and undergo DNA fragmentation, as detected by an in situ end labeling technique. An initial sharp burst of DNA fragmentation, between 12 and 24 h postaxotomy, accompanies a steep decline in neuron al numbers, indicating that neuronal cell death rapidly follows endonu clease cleavage of DNA, However, the interval between axotomy and onse t of DNA fragmentation varies widely. By 4 days postnatum only 38% of the lesioned motoneurons have survived an initial rapid phase of neuro nal loss, whereas 11% survive to 10 days postnatum at least. NADPH-dia phorase/nitric oxide synthase activity has been implicated as having a causal role in the death of lesioned motoneurons. We have found that there is a sustained increase in the intensity of NADPH-diaphorase his tochemical staining in axotomized neonatal facial motoneurons, but tha t this is first detectable well after the onset of DNA fragmentation a nd cell death. This suggests that nitric oxide, or its metabolites, do es not initiate cell death in this model. (C) 1996 Academic Press, Inc .