[Quantitative determination of specific reinnervation after suture of the facial nerve in the rat].

INTRODUCTION

In recent studies we identified two morphological phenomena in the facial nucleus of the brainstem that play an important role in the recovery of facial movement: hyperinnervation and misdirected reinnervation. While the hyperinnervation could easily be quantified by cell counting, the extent of misdirected reinnervation could not be estimated accurately. In the present study we developed a method for accurate quantification of this misdirected reinnervation.

MATERIAL AND METHODS

In 6 rats we injected the fluorescent tracer FluoroGold into the whiskerpad. After 4 days the facial nerve was transected and a facial-facial anastomosis (FFA) performed. Eight weeks later the fluorescent tracer Fast Blue was injected in the same site of the whiskerpad, as done before with the FluoroGold. Following sacrifice, the brainstems of the animals were removed and labeled motoneurons in the facial nucleus counted.

RESULTS

Three different types of labeled motoneurons could be identified: (a) white fluorescent motoneurons (labeled preoperatively only with fluoroGold, (b) blue fluorescent motoneurons (labeled only postoperatively with Fast Blue) and (c) green-grey fluorescent motoneurons double-labeled pre- and postoperatively. The double-labeled motoneurons were seen to project to the same sites in the whiskerpad pre- and postoperatively, demonstrating no misdirected reinnervation. In our experiment we counted 478 +/- 45 green-grey labeled neurons from a total number of 1446 +/- 131 postoperatively labeled cells (double-labeled and single FB-labeled). These findings show that 33% of the regenerating motoneurons were correctly redirected after FFA in our animal model and 67% were misdirected.