Nerve cell injury induced by unconjugated bilirubin (UCB) has been implicated in 
brain damage during severe neonatal hyperbilirubinemia, although the molecular 
mechanisms underlying UCB neurotoxicity are still not clarified. It has been 
suggested recently that there is an association between hyperbilirubinemia and 
long-term neurologic dysfunctions. We incubated immature neurons with UCB to 
evaluate the short- and long-term effects of UCB on apoptotic death and on 
neuritic outgrowth and ramification. We also evaluated whether mature neurons, 
exposed previously to UCB in an early stage of differentiation, are more 
sensitive to apoptosis or to neuritic breakdown when treated with inflammatory 
agents, such as lipopolysaccharide and tumor necrosis factor-alpha. Results show 
that exposure of immature neurons to UCB increased apoptosis and provoked a 
reduction of both neurite extension and number of nodes. These injurious effects 
observed in immature cells treated with UCB were increasingly perpetuated along 
cell differentiation, as compared to neurons incubated in the absence of UCB. In 
addition, neurons that were exposed to UCB when immature showed an increased 
susceptibility to death by apoptosis, as well as an additional decrease in 
neurite outgrowth when incubated with an inflammatory agent afterward. This work 
shows, for the first time, that UCB induces neurite changes consistent with 
neurodevelopment abnormalities. Furthermore, pre-exposure to UCB followed by an 
inflammatory stimulus leads to an enhanced susceptibility to long-term 
apoptosis, as well as a greater neuritic breakdown. These data support the 
association between neonatal hyperbilirubinemia and the later development of 
mental illness, such as schizophrenia.