Development and Application of Automated Vocal Fold Tracking Software in a Rat Surgical Model

OBJECTIVE

The rat is a widely used model for studying vocal fold (VF) function after recurrent laryngeal nerve injury, but common techniques for evaluating rat VF motion remain subjective and imprecise. To address this, we developed a software package, called RatVocalTracker1.0 (RVT1.0), to quantify VF motion and tested it on rats with iatrogenic unilateral vocal fold paralysis (VFP).

METHODS

A deep neural network was trained to identify the positions of the VFs and arytenoid cartilages (ACs) in transoral laryngoscope videos of the rat glottis. Software was developed to estimate glottic midline, VF displacement, VF velocity, and AC angle. The software was applied to laryngoscope videos of adult rats before and after right recurrent and superior laryngeal nerve transection (N = 15; 6M, 9F). All software calculated metrics were compared before and after injury and validated against manually calculated metrics.

RESULTS

RVT1.0 accurately tracked and quantified VF displacement, VF velocity, and AC angle. Significant differences were found before and after surgery for all RVT1.0 calculated metrics. There was strong agreement between programmatically and manually calculated measures. Automated analysis was also more efficient than nearly all manual methods.

CONCLUSION

This approach provides fast, accurate assessment of VF motion in rats with minimal labor and allows for quantitative comparison of lateral differences in movement. Through this novel analysis method, we can differentiate healthy movement from unilateral VFP. RVT1.0 is open-source and will be a valuable tool for researchers using the rat model for laryngology research.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:340-346, 2024.

An optimized method for high-quality RNA extraction from distinctive intrinsic laryngeal muscles in the rat model

Challenges related to high-quality RNA extraction from post-mortem tissue have limited RNA-sequencing (RNA-seq) application in certain skeletal muscle groups, including the intrinsic laryngeal muscles (ILMs). The present study identified critical factors contributing to substandard RNA extraction from the ILMs and established a suitable method that permitted high-throughput analysis. Here, standard techniques for tissue processing were adapted, and an effective means to control confounding effects during specimen preparation was determined. The experimental procedure consistently provided sufficient intact total RNA (N = 68) and RIN ranging between 7.0 and 8.6, which was unprecedented using standard RNA purification protocols. This study confirmed the reproducibility of the workflow through repeated trials at different postnatal time points and across the distinctive ILMs. High-throughput diagnostics from 90 RNA samples indicated no sequencing alignment scores below 70%, validating the extraction strategy. Significant differences between the standard and experimental conditions suggest circumvented challenges and broad applicability to other skeletal muscles. This investigation remains ongoing given the prospect of therapeutic insights to voice, swallowing, and airway disorders. The present methodology supports pioneering global transcriptome investigations in the larynx previously unfounded in literature.

Expression of trophic factors receptors during reinnervation after recurrent laryngeal nerve injury

OBJECTIVE

An injury of the recurrent laryngeal nerve (RLN) triggers axonal regeneration but results in a poor functional recovery. Netrin-1 and glial cell-derived neurotrophic factor (GDNF) expression are up-regulated in laryngeal muscles during RLN regeneration, but the role of their receptors produced in the nucleus ambiguus is unknown. The aim of this work was to determine the timing of the production of Netrin-1 and GDNF receptors during RLN regeneration and correlate this with the previously identified timing of up-regulation of their trophic factors in the laryngeal muscles.

STUDY DESIGN

Laboratory experiment with rat model.

METHODS

The right RLN was transected and dextran amine tracer applied. At 7, 14, and 21 days postinjury (DPI), brainstems were removed and harvested. Immunostaining was performed for Netrin-1 (deleted in colorectal carcinoma [DCC], UNC5A) and GDNF receptors (rearranged during transfection [Ret], glycosylphosphatidylinositol-linked cell surface receptors [GFRα1, GFRα2, GFRα3]). The timing and type of receptor production relative to injury as well as their position in the nucleus ambiguus was analyzed.

RESULTS

Netrin-1 UNC5A receptors were minimal in the nucleus ambiguus during RLN regeneration. DCC, the receptor that plays an attract role, was immunopositive from 7 to 21 DPI. All GDNF receptors, except GFRα2, were clearly positive from 7 to 14 DPI. No differences of production were observed according to the position of the motor neurons in the nucleus ambiguus.

CONCLUSION

An injury of the RLN leads to a higher production of Netrin-1 DCC and GDNF receptors in the nucleus ambiguus. The timing of receptor production is similar to up-regulation of their trophic factors in the laryngeal muscles.

LEVEL OF EVIDENCE

NA. Laryngoscope, 129:2537-2542, 2019.

Automated Quantification of Vocal Fold Motion in a Recurrent Laryngeal Nerve Injury Mouse Model

OBJECTIVES/HYPOTHESIS

The goal of this study was to objectively examine vocal fold (VF) motion dynamics after iatrogenic recurrent laryngeal nerve (RLN) injury in a mouse surgical model. Furthermore, we sought to identify a method of inducing injury with a consistent recovery pattern from which we can begin to evaluate spontaneous recovery and test therapeutic interventions.

STUDY DESIGN

Animal model.

METHODS

The right RLN in C57BL/6J mice was crushed for 30 seconds using an aneurysm clip with 1.3-N closing force. Transoral laryngoscopy enabled visualization of VF movement prior to surgery, immediately post-crush, and at two endpoints: 3 days (n = 5) and 2 weeks (n = 5). VF motion was quantified with our custom motion-analysis software. At each endpoint, RLN samples were collected for transmission electron microscopy for correlation with VF motion dynamics.

RESULTS

Our VF tracking software permitted automated quantification of several measures of VF dynamics, such as range and frequency of motion. By 2 weeks post-injury, the frequency of VF movement on the right (injured) side equaled the left, yet range of motion only partially recovered. These objective outcome measures enabled detection of VF dysfunction that persisted at 2 weeks post-crush. Transmission electron microscopy images revealed RLN degeneration 3 days post-crush and partial regeneration at 2 weeks, consistent with functional results obtained with automated VF tracking.

CONCLUSIONS

Our motion-analysis software provides novel objective, quantitative, and repeatable metrics to detect and describe subtle VF dysfunction in mice that corresponds with underlying RLN degeneration and recovery. Adaptation of our tracking software for use with human patients is underway.

LEVEL OF EVIDENCE

NA Laryngoscope, 129:E247-E254, 2019.

Comparison of inhaled versus intravenous anesthesia for laryngoscopy and laryngeal electromyography in a rat model

Background

Propofol and remifentanil intravenous combination is one popular form of total intravenous anesthesia (TIVA) in mainstream clinical practice, but it has rarely been applied to a rat model for laryngoscopy and laryngeal electromyography (LEMG). Our objective was to establish a safe and reproducible general anesthetic protocol for laryngoscopy and endoscopic LEMG in a rat model. Our hypothesis is that TIVA allows a minimally morbid, and feasible laryngoscopy and LEMG.

Methods

Sprague Dawley rats were subjected to either inhalational anesthesia (IA) (isoflurane) or TIVA (propofol and remifentanil) and underwent laryngoscopy and LEMG. The primary outcome was a complete minimally interrupted rigid laryngoscopy and obtaining reproducible motor unit potentials from the posterior cricoarytenoid muscles. The secondary outcome was morbidity and mortality.

Results

Seventeen out of twenty-two rats underwent both TIVA and IA. Only two underwent IA only. All nineteen rats that underwent IA had a successful experiment. Seventeen rats underwent TIVA, however, only nine completed a successful experiment due to difficulty achieving a surgical plane, and respiratory events. Upon comparing the success of the two anaesthetic regimens, IA was superior to TIVA (P = 0.0008). There was no statistical difference between the amplitudes (p = 0.1985) or motor units burst duration (p = 0.82605) of both methods. Three mortalities were encountered, one of which was due to lidocaine toxicity and two were during anesthetic induction. Respiratory related morbidity was encountered in two rats, all seen with TIVA.

Conclusions

TIVA is not an ideal anesthetic regimen for laryngeal endoscopy and LEMG in rat models. Contrary to our hypothesis, IA did not affect the quality of the LEMG and allowed a seamless rigid endoscopy.

Electronic supplementary material

The online version of this article (10.1186/s40463-018-0312-9) contains supplementary material, which is available to authorized users.

Characteristics of Experimental Recurrent Laryngeal Nerve Surgical Injury in Dogs

Objectives

We characterized various recurrent laryngeal nerve (RLN) injuries in dogs.

Methods

Sixteen dogs were classified as having complete injuries (transection) or incomplete injuries (ligation, half-section, and crush). The characteristics of nerve injuries were evaluated by endoscopic examination, laryngeal electromyography (LEMG), and histopathologic examination at 0 to 12 months after the injury.

Results

After the RLN injury, the average muscle fiber diameter and the average muscle bundle diameter of the affected muscles were decreased, and the average number of muscular cell nuclei per square inch increased. Fibrillation potentials were found 1 to 3 months after injury, and reinnervation potentials appeared 3 to 6 months after incomplete injury. For nerve transection and ligation, there was no reaction with LEMG instantly after injury involving vocal fold fixation. Vocal fold motion did not improve in members of the complete injury group, whereas all of the vocal folds in the members of the nerve ligation subgroup had limited activity in the later period. Various forms of vocal fold mobility were observed after injury in the half-section subgroup. Animals in the crush subgroup had normal EMG signals combined with abnormal LEMG signals with lower amplitudes after injury. Vocal fold fixation was not observed in this subgroup.

Conclusions

We found the causes of nerve injury, in order of decreasing severity, to be transection, ligation, half-section, and crush.

Electrical stimulation and testosterone enhance recovery from recurrent laryngeal nerve crush

OBJECTIVE

This study investigated the effects of a combinatorial treatment, consisting of a brief period of nerve electrical stimulation (ES) and systemic supraphysiologic testosterone, on functional recovery following a crush of the recurrent laryngeal nerve (RLN).

STUDY DESIGN

Prospective, controlled animal study.

METHODS

After a crush of the left RLN, adult male Sprague-Dawley rats were divided into four treatment groups: 1) no treatment, 2) ES, 3) testosterone propionate (TP), and 4) ES + TP. Each group was subdivided into 1, 2, 3, or 4 weeks post-operative survival time points. Groups had an n of 4- 9. Recovery of vocal fold mobility (VFM) was assessed.

RESULTS

Brief ES of the proximal nerve alone or in combination with TP accelerated the initiation of functional recovery. TP administration by itself also produced increased VFM scores compared to controls, but there were no statistical differences between the ES-treated and TP-treated animals. Treatment with brief ES alone was sufficient to decrease the time required to recover complete VFM. Animals with complete VFM were seen in treatment groups as early as 1 week following injury; in the untreated group, this was not observed until at least 3 weeks post-injury, translating into a 66% decrease in time to complete recovery.

CONCLUSIONS

Brief ES, alone or in combination with TP, promise to be effective therapeutic interventions for promoting regeneration following RLN injury.

Predictors for Permanent Medialization Laryngoplasty in Unilateral Vocal Fold Paralysis

OBJECTIVE

Early differentiation of patients with unilateral vocal fold paralysis (VFP) who recover from those who do not and consequently require permanent medialization laryngoplasty (ML) remains a challenge. The goal of this study is to identify factors that predict the need for ML.

STUDY DESIGN

Case series with chart review.

SETTING

Academic center.

SUBJECTS AND METHODS

A total of 507 records of patients with VFP were analyzed, of which 252 met criteria and were stratified according to whether or not they obtained ML. Demographic information and clinical features were analyzed to determine predictors of ML. A nomogram was generated according to the significance and utility of these parameters.

RESULTS

Of 252 patients, 86 underwent ML, and 166 did not. No differences in age or sex were observed between the ML and non-ML patients (P = .27 and P = .35, respectively). The most common cause of VFP was iatrogenic injury (62.79%, ML; 49.40%, non-ML). ML correlated with VFP secondary to neoplastic disease (odds ratio [OR], 2.14; 95% confidence interval [95% CI], 1.01-4.53) and iatrogenic injury (OR 1.73; 95% CI 1.01-2.94). ML had an inverse correlation with idiopathic VFP (OR, 0.40; 95% CI, 0.20-0.79). Patients in the ML group were more likely to have left-sided VFP, to have a history of aspiration, and to present ≥90 days from onset and less likely to have had temporary injection augmentation.

CONCLUSION

Clinical features may be used to predict the likelihood of a patient obtaining ML. Nomograms may be useful to counsel patients who would benefit from early definitive surgery.

Neurotrophin expression and laryngeal muscle pathophysiology following recurrent laryngeal nerve transection

Laryngeal palsy often occurs as a result of recurrent laryngeal or vagal nerve injury during oncological surgery of the head and neck, affecting quality of life and increasing economic burden. Reinnervation following recurrent laryngeal nerve (RLN) injury is difficult despite development of techniques, such as neural anastomosis, nerve grafting and creation of a laryngeal muscle pedicle. In the present study, due to the limited availability of human nerve tissue for research, a rat model was used to investigate neurotrophin expression and laryngeal muscle pathophysiology in RLN injury. Twenty-five male Sprague-Dawley rats underwent right RLN transection with the excision of a 5-mm segment. Vocal fold movements, vocalization, histology and immunostaining were evaluated at different time-points (3, 6, 10 and 16 weeks). Although vocalization was restored, movement of the vocal fold failed to return to normal levels following RLN injury. The expression of brain‑derived neurotrophic factor and glial cell line-derived neurotrophic factor differed in the thyroarytenoid (TA) and posterior cricoarytenoid muscles. The number of axons did not increase to baseline levels over time. Furthermore, normal muscle function was unlikely with spontaneous reinnervation. During regeneration following RLN injury, differences in the expression levels of neurotrophic factors may have resulted in preferential reinnervation of the TA muscles. Data from the present study indicated that neurotrophic factors may be applied for restoring the function of the laryngeal nerve following recurrent injury.

Factors associated with the improvement of vocal fold movement: an analysis of LEMG and laryngeal CT parameters

The aim of this study is to elucidate the relationship of laryngeal electromyography (LEMG) and computed tomographic (CT) parameters to improve the prognosis of recurrent laryngeal nerve injury. 22 patients clinically suspected of having recurrent laryngeal nerve injury were examined with LEMG and CT studies. Bilateral thyroarytenoid (TA) muscles were examined and findings were interpreted by a single blind technique. Laryngeal CT image analysis of the ventricle dilation symmetry determined TA muscle atrophy. Finally, a follow-up laryngoscopic examination determined improvement of vocal fold movement. Ventricle dilation symmetry and the dichotomized TA muscle atrophy parameter significantly relate to the improvement of vocal fold movement (χ(2)=4.029, P=0.039, and χ(2)=3.912, P=0.048, respectively). When the severity of vocal fold impairment was classified as severe TA muscle atrophy or none/discrete MUAP recruitment, it was found to significantly relate with the improvement of vocal fold movement (χ(2)=6.712, P=.010). From this study, image analysis of the ventricle dilation symmetry to determine the severity of TA muscle atrophy shows promise for the improved prognosis of vocal fold immobility.

Intravenous mesenchymal stem cell therapy after recurrent laryngeal nerve injury: a preliminary study

OBJECTIVES/HYPOTHESIS

Intravenous administration of mesenchymal stem cells (MSCs) has been recently shown to enhance functional recovery after stroke and spinal cord injury. The therapeutic properties of MSCs are attributed to their secretion of a variety of potent antiinflammatory and neurotrophic factors. We hypothesize that intravenous administration of MSCs after recurrent laryngeal nerve (RLN) injury in the rat may enhance functional recovery.

STUDY DESIGN

Animal Research.

METHODS

Twelve 250-gram Sprague-Dawley rats underwent a controlled crush injury to the left RLN. After confirming postoperative vocal fold immobility, each rat was intravenously infused with either green fluorescent protein-expressing MSCs or control media in a randomized and blinded fashion. Videolaryngoscopy was performed weekly. The laryngoscopy video recordings were reviewed and rated by a fellowship-trained laryngologist who remained blinded to the intervention using a 0 to 3 scale.

RESULTS

At 1 week postinjury, the MSC-infused group showed a trend for higher average functional recovery scores compared to the control group (2.2 vs 1.3), but it did not reach statistical significance (P value of 0.06). By 2 weeks, however, both groups exhibited complete return of function.

CONCLUSIONS

These pilot data indicate that with complete nerve transection by crush injury of the RLN in rat, there is complete recovery of vocal fold mobility at 2 weeks. At 1 week postinjury, animals receiving intravenous infusion of MSCs showed a trend for greater functional recovery, suggesting a potential beneficial effect of MSCs; however, this did not reach statistical significance. Therefore, no definite conclusions can be drawn from these data and further study is required.

LEVEL OF EVIDENCE

N/A.

Spontaneous laryngeal reinnervation following chronic recurrent laryngeal nerve injury

OBJECTIVES/HYPOTHESIS

To enhance understanding of spontaneous laryngeal muscle reinnervation following severe recurrent laryngeal nerve injury by testing the hypotheses that 1) nerve fibers responsible for thyroarytenoid muscle reinnervation can originate from multiple sources and 2) superior laryngeal nerve is a source of reinnervation.

STUDY DESIGN

Prospective, controlled, animal model.

METHODS

A combination of retrograde neuronal labeling techniques, immunohistochemistry, electromyography, and sequential observations of vocal fold mobility were employed in rat model of chronic recurrent laryngeal nerve injury. The current study details an initial set of experiments in sham surgical and denervated group animals and a subsequent set of experiments in a denervated group.

RESULTS

At 3 months after recurrent laryngeal nerve resection, retrograde brainstem neuronal labeling identified cells in the characteristic superior laryngeal nerve cell body location as well as cells in a novel caudal location. Regrowth of neuron fibers across the site of previous recurrent laryngeal nerve resection was seen in 87% of examined animals in the denervated group. Electromyographic data support innervation by both the superior and recurrent laryngeal nerves following chronic recurrent laryngeal nerve injury.

CONCLUSIONS

Following chronic recurrent laryngeal nerve injury in the rat, laryngeal innervation is demonstrated through the superior laryngeal nerve from cells both within and outside of the normal cluster of cells that supply the superior laryngeal nerve. The recurrent laryngeal nerve regenerates across a surgically created gap, but functional significance of regenerated nerve fibers is unclear.

Reorganization of laryngeal motoneurons after crush injury in the recurrent laryngeal nerve of the rat

Motoneurons innervating laryngeal muscles are located in the nucleus ambiguus (Amb), but there is no general agreement on the somatotopic representation and even less is known on how an injury in the recurrent laryngeal nerve (RLN) affects this pattern. This study analyzes the normal somatotopy of those motoneurons and describes its changes over time after a crush injury to the RLN. In the control group (control group 1, n = 9 rats), the posterior cricoarytenoid (PCA) and thyroarytenoid (TA) muscles were injected with cholera toxin-B. In the experimental groups the left RLN of each animal was crushed with a fine tip forceps and, after several survival periods (1, 2, 4, 8, 12 weeks; minimum six rats per time), the PCA and TA muscles were injected as described above. After each surgery, the motility of the vocal folds was evaluated. Additional control experiments were performed; the second control experiment (control group 2, n = 6 rats) was performed labeling the TA and PCA immediately prior to the section of the superior laryngeal nerve (SLN), in order to eliminate the possibility of accidental labeling of the cricothyroid (CT) muscle by spread from the injection site. The third control group (control group 3, n = 5 rats) was included to determine if there is some sprouting from the SLN into the territories of the RLN after a crush of this last nerve. One week after the crush injury of the RLN, the PCA and TA muscles were injected immediately before the section of the SLN. The results show that a single population of neurons represents each muscle with the PCA in the most rostral position followed caudalwards by the TA. One week post-RLN injury, both the somatotopy and the number of labeled motoneurons changed, where the labeled neurons were distributed randomly; in addition, an area of topographical overlap of the two populations was observed and vocal fold mobility was lost. In the rest of the survival periods, the overlapping area is larger, but the movement of the vocal folds tends to recover. After 12 weeks of survival, the disorganization within the Amb is the largest, but the number of motoneurons is similar to control, and all animals recovered the movement of the left vocal fold. Our additional controls indicate that no tracer spread to the CT muscle occurred, and that many of the labeled motoneurons from the PCA after 1 week post-RLN injury correspond to motoneurons whose axons travel in the SLN. Therefore, it seems that after RLN injury there is a collateral sprouting and collateral innervation. Although the somatotopic organization of the Amb is lost after a crush injury of the RLN and does not recover in the times studied here, the movement of the vocal folds as well as the number of neurons that supply the TA and the PCA muscles recovered within 8 weeks, indicating that the central nervous system of the rat has a great capacity of plasticity.

Recurrent Laryngeal Nerve Regeneration through a Silicone Tube Produces Reinnervation without Vocal Fold Mobility in Rats

Objectives:

We established an animal model of recurrent laryngeal nerve reinnervation with persistent vocal fold immobility following recurrent laryngeal nerve injury.

Methods:

In 36 rats, the left recurrent laryngeal nerve was transected and the stumps were abutted in a silicone tube with a 1-mm interspace, facilitating regeneration. The mobility of the vocal folds was examined endoscopically 5, 10, and 15 weeks later. Electromyography of the thyroarytenoid muscle was performed. Reinnervation was assessed by means of a quantitative immunohistologic evaluation with anti-neurofilament antibody in the nerve both proximal and distal to the silicone tube. The atrophy of the thyroarytenoid muscle was assessed histologically.

Results:

We observed that all animals had a fixed left vocal fold throughout the study. The average neurofilament expression in the nerve both distal and proximal to the silicone tube, the muscle area, and the amplitude of the compound muscle action potential recorded from the thyroarytenoid muscle on the treated side increased significantly (p < 0.05) over time, demonstrating regeneration through the silicone tube.

Conclusions:

Recurrent laryngeal nerve regeneration through a silicone tube produced reinnervation without vocal fold mobility in rats. The efficacy of new laryngeal reinnervation treatments can be assessed with this model.

Short-term laryngeal electromyography and histopathological findings after primary reconstruction of the inferior laryngeal nerve in rabbits: prospective study

INTRODUCTION

The recurrent laryngeal nerve can be injured during surgery. This study investigated recurrent laryngeal nerve reinnervation.

OBJECTIVE

To study the short-term effects of primary anastomosis of the recurrent laryngeal nerve, by laryngeal electromyography and histopathological analysis, in a rabbit model.

METHOD

Twenty Zealand rabbits underwent either right recurrent laryngeal nerve (1) transection with excision of 1 cm or (2) transection and end-to-end primary anastomosis. Vocal fold movements, laryngeal electromyography results and histological changes were recorded.

RESULTS

Vocal fold analysis showed a paramedian vocal fold in both groups, with perceptible vibratory movements in group two. Electromyography revealed total denervation potentials in group one, but denervation and regeneration signs in group two. Histopathologically, hyperkeratosis and parakeratosis of the vocal fold mucosa were seen in group one, and signs of parakeratosis and hyperplasia in group two.

CONCLUSION

Even under ideal conditions for primary recurrent laryngeal nerve anastomosis, a return to normal muscle function is unlikely. However, such anastomosis prevents muscle atrophy, and should be performed as soon as possible. The degree of nerve recovery is associated with the number, amplitude and myelination level of fibrils returning to the original motor end-plaque.

Modulation of nerve fibers in the rat thyroarytenoid muscle following recurrent laryngeal nerve injury

CONCLUSION

Regeneration of nerve fibers in the thyroarytenoid (TA) muscle occurred actively after damage on the recurrent laryngeal nerve (RLN) compared with the vagus nerve (VN). However, remyelination did not occur after damage on the RLN.

OBJECTIVES

To determine the regeneration process of nerve fibers in the TA muscle following transection and immediate anastomosis of the RLN or VN.

METHODS

Three types of animal model were prepared: an RLN anastomosis model (RLNa), a VN anastomosis model (VNa), and a peroneal nerve anastomosis model (PNa). Animals were sacrificed at five time points following the procedure. The modulation of axons, myelin sheaths, Schwann cells (SCs), nerve terminals (NTs), and acetylcholine receptors (AchRs) in the TA or tibialis anterior muscles was examined by immunohistochemical analysis. The ratios of the expression areas in axons, myelin sheaths, and SCs, and the number of NTs and AchRs in the treated (T) and untreated (U) sides (T/U) were evaluated.

RESULTS

At 18 weeks, the T/U ratios of expression in RLNa, VNa, and PNa were 68.5, 0, and 100.4%, respectively, in axons; 0, 0, and 97.6% in myelin sheaths; 53.7, 0, and 93.6% in SCs; 61.0, 0, and 96.4% in NTs; and 99.4, 67.0, and 101.2% in AchRs.

Quantity and three-dimensional position of the recurrent and superior laryngeal nerve lower motor neurons in a rat model

OBJECTIVES

We sought to elucidate the 3-dimensional position and quantify the lower motor neurons (LMNs) of the recurrent laryngeal nerve (RLN) and the superior laryngeal nerve (SLN) in a rat model. Quantification and mapping of these neurons will enhance the usefulness of the rat model in the study of reinnervation following trauma to these nerves.

METHODS

Female Sprague-Dawley rats underwent microsurgical transection of the RLN, the SLN, or both the RLN and SLN or sham surgery. After transection, either Fluoro-Ruby (FR) or Fluoro-Gold (FG) was applied to the proximal nerve stumps. The brain stems were harvested, sectioned, and examined for fluorolabeling. The LMNs were quantified, and their 3-dimensional position within the nucleus ambiguus was mapped.

RESULTS

Labeling of the RLN was consistent regardless of the labeling agent used. A mean of 243 LMNs was documented for the RLN. The SLN labeling with FR was consistent and showed a mean of 117 LMNs; however, FG proved to be highly variable in labeling the SLN. The SLN LMNs lie rostral and ventral to those of the RLN. In the sham surgical condition, FG was noted to contaminate adjacent tissues--in particular, in the region of the SLN.

CONCLUSIONS

Fluorolabeling is an effective tool to locate and quantify the LMNs of the RLN and SLN. The LMN positions and counts were consistent when FR was used in labeling of either the RLN or the SLN. Fluoro-Gold, however, because of its tendency to contaminate surrounding structures, can only be used to label the RLN. Also, as previously reported, the SLN LMNs lie rostral and ventral to those of the RLN. This information results in further clarification of a rat model of RLN injury that may be used to investigate the effects of neurotrophic factors on RLN reinnervation.