Synaptic remodeling in the dorsal motor nucleus of the vagus nerve following vagal-hypoglossal nerve anastomosis in the cat

Horseradish peroxidase (HRP) retrograde tracing techniques and morphometric analyses were performed to investigate synaptic remodeling associated with neuronal and glial changes in the dorsal motor nucleus of the vagus (DMV) of cats after vagal-hypoglossal nerve anastomosis (VHA). At 25 days postoperation (dpo), in the early target-reinnervation stage, there were 50% fewer presynaptic boutons containing round vesicles (R) or round and large dense-cored synaptic vesicles (R+D) contacting HRP-labeled DMV motoneurons. The loss of R boutons was maintained throughout the remaining postoperative intervals up to 500 dpo, whereas R+D boutons were further reduced at 123 dpo but were restored at 315 dpo, so that, by 500 dpo, 71.4% of them had gained access to the DMV motoneurons. Boutons containing pleomorphic synaptic vesicles (P) were completely disconnected from the DMV motoneurons at 25 dpo and did not reappear even in the long-term reinnervation stage. Loss and recovery of presynaptic boutons occurred in parallel with changes in astroglial ensheathment of the DMV motoneurons. It is suggested that synaptic remodeling associated with astroglial ensheathment in the DMV may be influenced by some retrogradely transported factors/ signals derived from the newly acquired target organ, viz. tongue skeletal musculature. Our results further suggest that the observed changes in boutonal configurations may be attributable to modified functions of the DMV motoneurons induced by VHA.