Recovery of original nerve supply after hypoglossal–facial anastomosis causes permanent motor hyperinnervation of the whisker‐pad muscles in the rat

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Abstract
Hypoglossal–facial anastomosis (HFA), used in humans for the treatment of facial palsy, was experimentally performed in adult female Wistar rats. The time course of facial reinnervation and the extent of the new motor nerve supply of the vibrissal muscles that develops after HFA were estimated by counting all motoneurons in the brainstem labeled by injection of horseradish peroxidase (HRP) into the whisker pad; muscle innervation by motor endplates was not studied. In untreated animals, HRP injection labels 1,254 ± 54 (mean ± S.D.; n = 6) motoneurons, localized exclusively in the lateral subdivision of the facial nucleus. Immediately following HFA, this number drops to zero. The first HRP-labeled motoneurons appear in the hypoglossal nucleus at 28 days postoperation (dpo) and at 56 dpo their number reaches 1,096 ± 48. Unexpectedly, the facial nerve, whose proximal stump has been left as blind end during surgery, additionally sends axons to the facial periphery. This resprouting is first detected at 42 dpo with HRP-marked neurons throughout the facial nucleus lacking somatotopic organiza-tion. The number of these labeled neurons also rises with time, and at 56 dpo, a total of 1,797 ± 142 facial and hypoglossal motoneurons, that is, 43% more motoneurons than in normal animals, supplies the whisker pad. This hyperinnervation, that is, the projection of more moto-neurons into the target muscle than under normal conditions-further increases to 1,978 ± 92 motoneurons at 224 dpo and may provide a new animal model for studying the competitive relationships between motoneurons in their search for peripheral targets.