Delayed rat facial nerve repair leads to accelerated and enhanced muscle reinnervation with reduced collateral axonal sprouting during a definite denervation period using a cross-anastomosis paradigm

To establish the influence of prolonged denervation on the recovery of a motor nerve, the rat facial nerve was transected and denervated for 0 to 224 days. Then, the freshly transected hypoglossal nerve was sutured to the predegenerated facial nerve (hypoglossal-facial nerve anastomosis, HFA). Using this nerve cross-anastomosis paradigm we analyzed the nerve regeneration and muscle reinnervation 7 to 112 days post-suture operation (DPSO). After HRP injection into the whiskerpad 931 ± 27 hypoglossal neurons were labeled at 112 DPSO after immediate HFA. Following 14 to 112 days denervation the number of labeled neurons increased to 138% (14 days delay), 154% (56 days), and 145% (112 days). In contrast, the reinnervation was poorer after 7 days denervation with the number of neurons increasing to 84%, and after long-term denervation of 224 days the number of neurons increased to 81%. The increase in amplitude of evoked electromyography wave after nerve suture correlated with the number of labeled neurons. After immediate HFA each regenerated motoneuron established on average 5.1 myelinated sprouts at 112 DPSO. The number of sprouts remained constant after delayed suture of 14 to 112 days, whereas the slower reinnervation after 7 or 224 days delay was accompanied by a massive sprouting of 9.1 or 8.1, respectively, sprouts per neuron. The muscles showed recovery after any denervation time. The muscle cross-sectional area continuously decreased with longer denervation time. This decrease was only significant after 224 days denervation (67% of the normal value). We conclude that motor nerve reconstruction achieves better functional results after a definite period of denervation when using a nerve cross-anastomosis paradigm. (C) 2000 Academic Press.