Age, diet and injury affect the survival of facial motoneurons.

Using the model of facial nerve avulsion, we have compared the effects of injury, age and diet on motoneuronal survival. One to four weeks after nerve avulsion, 50-75% motoneuron loss was quantified in ad libitum-fed rats aged 7 days (neonate), 6 months (adult) and 24 months (aging) at the time of injury. Evidence of apoptosis was found for neonatal rats at 3 days post-injury, but not for neonates examined 7 days or adult or aging rats examined 1 month after injury. Non-operated, ad libitum-fed rats showed no significant loss of facial motoneurons by 24 months. Surprisingly, non-operated rats whose food intake was restricted to 15 g standard rat chow per day from the age of 6 months lost 50% of their motoneurons by 24 months. Facial nerve avulsion of 24-month-old rats raised on this restricted diet did not result in any additional loss of motoneurons one month after injury. These results challenge the common view that aging results in neuronal loss and that dietary restriction is universally beneficial.