RABBIT FACIAL-NERVE REGENERATION IN NGF-CONTAINING SILASTIC TUBES

Previous reports suggest that exogenous nerve growth factor (NGF) enha nced nerve regeneration in rabbit facial nerves.1 Rabbit facial nerve regeneration in 10-mm Silastic(R) tubes prefilled with NGF was compare d to cytochrome C (Cyt. C), bridging an 8-mm nerve gap. Three weeks fo llowing implantation, NGF-treated regenerates exhibited a more mature fascicular organization and more extensive neovascularization than cyt ochrome-C-treated controls. Morphometric analysis at the midtube of 3- and 5-week regenerates revealed no significant difference in the mean number of myelinated or unmyelinated axons between NGF- and cytochrom e-C-treated implants. However, when the number of myelinated fibers in 5-week regenerates were compared to their respective preoperative con trols, NGF-treated regenerates had recovered a significantly greater p ercentage of myelinated axons than cytochrome-C-treated implants (46% vs. 18%, respectively). In addition, NGF-containing chambers reinnerva ted a higher percentage of myelinated axons in the distal transected n eural stumps (49% vs. 34%). Behavioral and electrophysiologic studies demonstrated spontaneous and induced activities in the target muscles when approximately one third of the myelinated axons were recovered in the midchamber (1280 axons). Horseradish peroxidase (HRP) studies dem onstrated retrograde axonal transport to the midchamber and proximal t ransected neural stump. PC12 bioassay demonstrated persistent NGF acti vity in the intrachamber fluids at 3 (5:1 dilution) and 5 (2:1 dilutio n) weeks of entubation. Electrophysiologic tests demonstrated a slow c onduction velocity of a propagated electrical impulse (43.5 m/s-1 vs. 67 m/s-1) and shallow wide compound action potential. In wider defects (15-mm chambers) and longer entubation periods (7 weeks), no regenera tion or NGF activity was seen. Therefore, exogenous NGF provides an ea rly but limited neurotrophic effect on the regeneration of the rabbit buccal division of the facial nerve and a limited behavioral and physi ological improvement in the target muscles.