VESTIBULAR NERVE REGENERATION IN THE BULLFROG, RANA-CATESBEIANA - PERIPHERAL DENDRITES

Three experiments were conducted on healthy adult bullfrogs (Rana cate sbeiana) for the purpose of investigating three characteristics of cen trifugal vestibular afferent regeneration after complete transection o f the anterior division of the vestibular nerve (AVN). In experiment 1 total fiber count and axon diameter measurements were obtained from t he anterior canal nerve at three different time periods and compared w ith normal. The normal group (n = 3) demonstrated a total fiber count of 1001 +/- 76 (SEM). The early time period (1 to 2 weeks, n = 3) did not completely regenerate as demonstrated by a total fiber count of 28 2 +/- 23. The intermediate (4 to 6 weeks, n = 3) and late (8 to 16 wee ks, n = 3) groups exhibited total fiber counts of 907 +/- 29 and 946 /- 50, respectively, which were not different from normal (Mann-Whitne y U, p > 0.2). Evaluation of fiber diameter distribution of the interm ediate and late regenerated nerves revealed a reduction in axon diamet er caliber compared with normal (analysis of variance, p < 0.0001). Th us transection of the AVN results in regeneration of ail afferents tha t exhibit a reduction in axon diameter. In experiment 2 fibers innerva ting the anterior canal crista (ACC) were prelabeled before nerve tran section. After the labeling procedure the AVN (n = 3) was sectioned at a location that resulted in denervation of three vestibular receptors : the ACC, horizontal canal cristae (HCC), and utricular macula. After 4 weeks of regeneration the ACC fibers that were prelabeled were obse rved innervating all three denervated vestibular receptors. This resul t demonstrated that reinnervation of the peripheral vestibular end org ans after AVN transection is a nonspecific process. In experiment 3, 1 67 regenerated canal afferents were evaluated for functional recovery 16 weeks after transection. Both spontaneous and rotation-induced disc harge characteristics were obtained and compared with those obtained f rom a sample of 254 normal afferents in a previous study (Hoffman LF. Factors affecting the response dynamics of canalicular primary afferen t neurons in the bullfrog. St. Petersburg (FL): Association for Resear ch in Otolaryngology; 1989). The mean spontaneous discharge coefficien t of variation (CV) +/- standard deviation was 0.60 +/- 0.32 and 0.49 +/- 0.33 for ACC and HCC regenerated afferents, respectively, which di d not differ from the normal means of 0.63 +/- 0.33 and 0.54 +/- 0.36 (Mann-Whitney, p > 0.2). Response gains and phases obtained during 0.0 5 Hz sinusoid rotations at 15 degrees/second maximum horizontal table velocity also demonstrated normal discharge characteristics. The mean phases were -28.2 +/- 25.2 degrees and -55.9 +/- 21.5 degrees for rege nerated ACC and HCC afferents, respectively, which were not different from the normal means of -33.77 +/- 24.31 degrees and -58.0 +/- 23.3 d egrees (Mann-Whitney U). Furthermore, regenerated afferents exhibited a positive association between phase and CV, which was also true for n ormal afferents (correlation analysis, p 0.001). Although the mean gai ns for regenerated ACC and HCC (7.13 +/- 5.5 and 3.3 +/- 2.4 spikes.se c(-1)/degrees.sec(-2), respectively) afferents were reduced from norma l ACC and HCC (14.8 +/- 12.52 and 7.76 +/- 6.58 spikes.sec(-1)/degrees .sec(-2), respectively) afferents (Mann-Whitney U, p > 0.0001), a posi tive association between gain and CV was also demonstrated by regenera ted afferents, as was the case for normal afferents (correlation analy sis, p < 0.001). Thus the overall response discharges of regenerated a fferents were comparable with normal afferents. Normally, large fibers innervate central regions of the receptor, and smaller fibers innerva te the peripheral regions. However, the data from experiments 1 and 2 demonstrate that vestibular nerve regeneration results in a dissociati on between the normal topographic organization of fiber size and regio nal innervation of the receptor epithelium. Nonetheless, both spontane ous and stimulus-induced discharge characteristics remain unaltered in the regenerated condition. Because the presynaptic epithelial feature s also remained unaltered after nerve transection, these data demonstr ate the importance presynaptic factors in governing afferent discharge characteristics.