TIME-COURSE OF APOPTOSIS IN THE OLFACTORY EPITHELIUM OF RAINBOW-TROUTEXPOSED TO A LOW COPPER LEVEL

Copper at low doses is known to specifically induce olfactory neuron d eath in fish olfactory epithelium. Using light and electron transmissi on microscopy, we have investigated the features and the time-course o f receptor cell death in rainbow trout exposed for 15 days to 20 mu g Cu2+/l. Ultrastructural observations demonstrate that degenerating cel ls, which included both mature and immature neurons, exhibited morphol ogical changes characteristic of a cell death by apoptosis, Quantitati ve analysis shows that the number of apoptotic cells increased signifi cantly 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 ol factory 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 proc ess to balance the receptor cell death, despite continued copper expos ure, 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.