Time-course of apoptosis in the olfactory epithelium of rainbow trout exposed to a low copper level

Copper at low doses is known to specifically induce olfactory neuron death in fish olfactory epithelium. Using light and electron transmission microscopy, we have investigated the features and the time-course of receptor cell death in rainbow trout exposed for 15 days to 20 mug Cu(2+)/l. Ultrastructural observations demonstrate that degenerating cells, which included both mature and immature neurons, exhibited morphological changes characteristic of a cell death by apoptosis. Quantitative analysis shows that the number of apoptotic cells increased significantly already after 1 day of exposure, reaching a peak at day 5. From this timepoint of exposure, no more mature neuron was noted in the olfactory epithelium. Following a significant decrease in the number of apoptotic cells at day 10, a second wave of neuron death was noted at day 15. These findings argue for the occurrence of a neurogenesis process to balance the receptor cell death, despite continued copper exposure, and for a higher vulnerability to the metal of olfactory neurons presenting more advanced stages of cell differentiation. The molecular mechanisms by which copper may induce olfactory neuron apoptosis are discussed.