Adenosine‐induced apoptosis in chick embryonic sympathetic neurons: a new physiological role for adenosine.

1. A newly found action of adenosine in neurons, which may have an important physiological function in the growth and development of the sympathetic nervous system, is described. Adenosine (1‐100 microM) inhibited neurite outgrowth within the first 24 h and killed about 80% of sympathetic neurons supported by nerve growth factor over the next 2 days in culture. Neurons supported by excess KCl, forskolin or phorbol 12,13‐dibutyrate were equally susceptible to the toxic actions of adenosine. Inosine, guanosine or hypoxanthine (all 100‐300 microM) were without effect on neuronal growth and survival. 2. Specific agonists of adenosine A1 and A2 receptors were not neurotoxic, and toxic effects of adenosine were not antagonized by aminophylline. These results rule out involvement of adenosine receptors and the adenylyl cyclase‐cAMP signalling system in neurotoxic actions of adenosine. 3. Adenosine toxicity was prevented by inhibitors of the adenosine membrane transporter, suggesting an intracellular site of action of adenosine. 4. Inhibitors of adenosine deaminase dramatically facilitated the toxic action so that physiologically relevant concentrations of adenosine were neurotoxic. 5. Adenosine kinase activity of sympathetic neurons was dose‐dependently inhibited by 5'‐iodotubercidin (3‐100 nM). 5'‐Iodotubercidin (100 nM) completely protected neurons against toxicity of adenosine plus adenosine deaminase inhibitors. These results provide convincing evidence that phosphorylation of the nucleoside is an essential requirement for initiation of adenosine toxicity. 6. Sympathetic neurons were successfully rescued from the lethal effects of adenosine deaminase inhibitor plus adenosine by uridine or 2‐deoxycytidine, but not by nicotinamide or 2‐deoxyguanosine, suggesting that depletion of pyrimidine nucleotides by phosphorylated adenosine compounds and consequent inhibition of DNA synthesis produces neuronal death. 7. DNA fragmentation, assessed by the fluorescent dye bisbenzimide and by the TUNEL (terminal deoxynucleotidyl transferase‐mediated nick end labelling) method, indicated that neuronal death induced by adenosine was apoptotic. 8. We conclude that adenosine deaminase and adenosine kinase play an important role in the metabolism of intracellular concentrations of adenosine and thereby regulate the growth and development of sympathetic neurons. Our study highlights, for the first time, the importance of adenosine as a mediator of programmed cell death of neurons supported by nerve growth factor.