SLOW AXONAL REGROWTH BUT EXTREME HYPERINNERVATION OF TARGET MUSCLE AFTER SUTURE OF THE FACIAL-NERVE IN AGED RATS

Unilateral transection and suture of the facial nerve was performed in 60 old rats (70 months of age). The time course of mimetic reinnervat ion was studied by counting all retrogradely labeled motoneurons in th e facial nucleus after injection of HRP into the whiskerpad muscles fu r 14-112 days post operation. The comparison between these neuron coun ts and data for young rats yielded four conclusions. First, the qualit ative equivalent of the phenomenon ''misdirected reinnervation'' in ag ed mis was the same as in young adults: HRP-labeled motoneurons were s cattered throughout the facial nucleus lacking myotopic organization f rom 18 until 112 days post operation. Second, no age-related loss of m otoneurons was detected. Third, the axonal regrowth was delayed in age d rats. Fourth, the postoperative hyperinnervation (the projection of more motoneurons into a muscle than under normal conditions, i.e., the quantitative aspect of misdirected reinnervation) was more than two t imes higher than in young rats. These data may provide reasonable expl anations for the poor functional recovery after reconstructive surgery on the facial were in old patients. (C) 1998 Elsevier Science Inc.