Age and diet affect the survival of axotomised motoneurons

Diet restriction can prolong the life of the whole animal. The effects of this procedure on subsets of neurons and their response to injury, however, is less well understood. Here, we have studied the retrograde effects of facial nerve avulsion in ad lib. fed Sprague-Dawley rats aged 1 week, 6 months and 24 months at the time of nerve injury, as well as in 24 month old rats which had been raised on 15g per day standard rat chow from the age of 6 months (diet restricted). All animals were anaesthestised prior to all experimental procedures. Stereological counts revealed that ad lib. fed rats of all ages had similar numbers of facial motoneurons (3000-3,500 per facial nucleus), while 24 month old diet restricted rats had approximately half this number. No obvious difference in vibrissae function was noted between the two groups of 24 month old rats. Following nerve avulsion of ad lib. fed rats, 50-75% of motoneurones were lost by 1 week (1 week old rats) or 1 month (6 & 24 month old rats). In contrast, no loss of motoneurons was found 1 month after nerve avulsion in 24 month old diet restricted rats. Loss of motoneurones was associated with TUNEL staining and increased expression of activated caspase-3 in the ipsilateral facial nuclei of rats operated at 1 week of age and examined 1 week later, but not in the other age groups which were examined 1 month after nerve avulsion. Increased GAP-43 and GFAP expression characterised the injured facial nucleus of all groups. Diet and age therefore affect the timecourse and magnitude of facial motoneuron death following nerve avulsion.