The hypoglossal-facial anastomosis as model of neuronal plasticity in the rat.

Hypoglossal-facial cross anastomosis (HFA) causes regeneration with change of function, as the axotomized hypoglossal motoneurons sprout into the facial plexus and reinnervate the mimic musculature. Following HFA, hypoglossal-hypoglossal single anastomosis (HHA) and resection of 8-10 mm peripheral hypoglossal nerve in 190 female adult Wistar rats, we compared the axon reactions in the hypoglossal nucleus during 1) regeneration with change of function, 2) regeneration with restoration of original function and 3) degeneration of the nucleus. Following postoperative survival times of 1-16 weeks we estimated the volume of the hypoglossal nucleus and counted the number of hypoglossal neurons with the physical disector on both sides of the brainstem. Additional sections of the same animals were reacted with anti-synaptophysin, anti-GFAP and the isolectin Griffonia simplicifolia I-B4 (GSA I-B4) as cytochemical markers for presynaptic boutons, activated astroglia and microglia. After HHA and HFA all hypoglossal neurons survive and the volume of the hypoglossal nucleus remains constant. Resection of the hypoglossal nerve leads to the loss of one third of the hypoglossal neurons and of one third of the volume of the hypoglossal nucleus within 16 weeks post operation. Hypoglossal-facial anastomosis and hypoglossal-hypoglossal anastomosis differ in postoperative swelling of the hypoglossal nucleus, microglia and astroglia activation and the duration of synaptic stripping. All differences are limited to the acute growth phase during regeneration. It is concluded that hypoglossal-facial anastomosis provides more stimulation and facilitates faster recovery of the hypoglossal nucleus than does hypoglossal-hypoglossal anastomosis.