Either nitric oxide or nerve growth factor is required for dorsal root ganglion neurons to survive during embryonic and neonatal development

During early embryonic (E12) development almost all dorsal root ganglion (DRG) neurons express the neuronal isoform of nitric oxide synthase (nNOS). At this stage, the axons of these neurons are rudimentary and have not made contact with peripheral tissue targets. As their axons establish contact with peripheral targets such as the skin, the number of neurons expressing nNOS decrease that correspond to increased immunoreactivity for nerve growth factor (NGF) in the skin, and its high affinity receptor, tyrosine kinase A (trkA) in both skin and DRG neurons. During late postnatal development, very few DRG neurons express nNOS; however, axotomy or NGF deprivation of cultured DRG neurons induce nNOS and NOS blockade causes neuronal death. In contrast, NGF-deprived embryonic and neonatal DRG neurons die by apoptosis, while NOS blockade has no effect. Overall, these observations suggest that NGF and nitric oxide (NO) interact during embryonic and postnatal development to facilitate neuronal selection and survival. The roles of NO, NGF and its receptor trkA in DRG neurons during different stages of development are discussed.