Nitric oxide signaling in developing trigeminal neurons

Aims: This study aimed to investigate the roles of neuronal (nNOS) and endothelial (eNOS) nitric oxide synthase in the nerve growth factor (NGF) response during the development of the trigeminal ganglion (TG). In addition the roles of these nitric oxide synthases following NGF withdrawal were ascertained. Finally the effects of administration of high doses of exogenous reactive nitrogen species were considered.Study Design and Methodology: Primary cultures of trigeminal sensory neurons were pharmacologically treated with reagents known to specifically modify the activity of NOS isoforms or were treated with reagents that release exogenous reactive nitrogen species in vitro. Cultures were maintained for 24 hours after treatment and the number of surviving neurons determined. In vivo determination of NOS isoform expression was carried out using immunohistochemistry.Results: eNOS and nNOS are widely expressed during TG neurodevelopment as ascertained immunohistochemically. Pharmacological inhibition of these enzymes reduces the in vitro NGF survival response of E16 neurons, but only eNOS inhibition reduces E19 TG neuron survival. Both developmental stages are vulnerable to the neurotoxic effects of high levels of exogenous nitric oxide when grown with NGF but susceptibility is higher at E19.Conclusion: During the period of naturally occurring neuronal death in the trigeminal ganglion, eNOS and nNOS are required for the survival of a proportion of trigeminal sensory neurons but when this period of cell death has ended there is a switch, and only eNOS is required for the NGF survival response. Furthermore, as these neurons mature beyond the period of naturally occurring cell death, their susceptibility to damage by elevated concentrations of reactive nitrogen species increases. This implies that there may be important mechanisms found within developing neurons that confer protection from nitrosative stress that warrant further investigation.