Developmental cholinotoxicants: Nicotine and chlorpyrifos

The stimulation of cholinergic receptors in target cells during a critical developmental period provides signals that influence cell replication and d ifferentiation. Accordingly, environmental agents that promote cholinergic activity evoke neurodevelopmental damage because of the inappropriate timin g or intensity of stimulation. Nicotine evokes mitotic arrest in brain cell s possessing high concentrations of nicotinic cholinergic receptors. In add ition. the cholinergic overstimulation programs the expression of genes tha t evoke apoptosis and delayed cell loss. Effects of cholinesterase inhibito rs exhibit many similarities to those of nicotine. Chlorpyrifos administere d to developing rats in doses that do not evoke signs of overt toxicity dec reased DNA synthesis and caused shortfalls in cell numbers in brain regions enriched in cholinergic innervation. In embryo cultures, chlorpyrifos also evoked apoptosis during neurulation. However, chlorpyrifos also evokes non cholinergic disruption of cell development by interfering with cell signali ng via adenylyl cyclase, leading to widespread disruption that is not limit ed to cholinergic systems. We have tested this hypothesis in vitro with PC1 2 cells, which lack the enzymes necessary to produce chlorpyrifos oxon, the metabolite that inhibits cholinesterase. Chlorpyrifos inhibited DNA synthe sis in undifferentiated PC12 cells, which have relatively few cholinergic r eceptors. Furthermore, chlorpyrifos was more effective than nicotine and it s effects were not blocked by cholinergic antagonists. When cells were allo wed to differentiate in the presence of chlorpyrifos, cell replication was inhibited even more profoundly and cell acquisition was arrested. At higher concentrations, chlorpyrifos also inhibited neuritic outgrowth. Thus, chlo rpyrifos elicits damage by both noncholinergic and cholinergic mechanisms e xtending from early stages of neural cell replication through late stages o f axonogenesis and terminal differentiation. Accordingly the window of deve lopmental vulnerability to chlorpyrifos is likely to extend from the embryo nic period into postnatal life.