The influence of prenatal gamma irradiation on the ageing of the cochlea.

Pregnant CBA/Ca mice were subjected to whole-body irradiation on the 12th, 13th or 16th day of gestation with 0.5, 1 or 2 Gy, respectively. Auditory brainstem response (ABR) thresholds in the offspring at one month of age were significantly elevated. The irradiated mice were raised to an age of between 25 and 36 months. Before the animals were killed, the ABR thresholds were again determined and compared with those in age-matched controls. The most pronounced ABR threshold loss at a high age occurred in those animals irradiated on gestation day 12. There was a significant (up to p less than 0.001) potentiation of the age-related loss of ABR threshold in the old irradiated mice. In the scanning electron microscope, both inner and outer hair cells were missing in large numbers in exposed animals, irrespective of irradiation dose. Remaining hair cells and also the pillar cells showed signs of degeneration. In particular, damage observed in the mid-cochlear region was significantly more pronounced in the old irradiated mice than in the old control mice. It is concluded 1) that elevation of ABR thresholds in ageing irradiated animals is due to damage to cells and nerve endings in the peripheral receptor organ, and 2) that prenatal irradiation significantly potentiates age-related hearing loss. Exposure to gamma rays seems to function as a sensitizer causing premature ageing.