Cerebellopontine angle cisternal infusion of NGF, BDNF and NT-3: effects on cochlear neurons disconnected from central target, cochlear nucleus. An in vivo quantitative study

Rebuilding lost hearing using cell transplantation

OBJECTIVE

The peripheral auditory nervous system (cochlea and auditory nerve) has a complex anatomy, and it has traditionally been thought that once the sensorineural structures are damaged, restoration of hearing is impossible. In the past decade, however, the potential to restore lost hearing has been intensively investigated using molecular and cell biological techniques, and we can now part with such a pessimistic view. In this review, we examine an important field in hearing restoration research: cell transplantation.

METHODS

Most efforts in this field have been directed to the replacement of hair cells by transplantation to the cochlea. Here, we focus on transplantation to the auditory nerve, from the side of the cerebellopontine angle rather than the cochlea.

RESULTS

Delivery of cells to the cochlea is potentially damaging, and nerve cells transplanted distally to the Schwann-glial transitional zone (cochlear side) may become inhibited when they reach the transitional zone. The auditory nerve is probably the most suitable route for cell transplantation.

CONCLUSION

The auditory nerve occupies an important position not only in neurosurgery but also in various diseases in other disciplines, and several lines of recent evidence indicate that it is a key target for hearing restoration. It is familiar to most neurosurgeons, and the recent advances in the molecular and cell biology of inner-ear development are of direct importance to neurorestorative medicine. In this article, we review the anatomy, development, and molecular biology of the auditory nerve and cochlea, with emphasis on the advances in cell transplantation.

Magnetic Resonance Appearance of the Inner Ear After Hearing-Preservation Surgery

OBJECTIVE

To determine whether the appearance of the inner ear on T2-weighted follow-up magnetic resonance imaging correlates with hearing status after hearing-preservation surgery for vestibular schwannoma.

STUDY DESIGN

Retrospective chart review.

SETTING

Tertiary referral medical center.

PATIENTS

The study includes patients undergoing hearing-preservation surgery for vestibular schwannoma from 1998 to 2003.

INTERVENTION

Diagnostic evaluation with magnetic resonance imaging and audiometric evaluation.

MAIN OUTCOME MEASURES

Hearing results as reported in charts was correlated with appearance of membranous labyrinth on T2-weighted magnetic resonance images obtained at least 1 year after surgery.

RESULTS

Twenty-nine patients were identified, 16 of whom satisfied the inclusion criteria. All 16 of the patients underwent middle fossa removal of vestibular schwannoma. Serviceable hearing according to American Academy of Otolaryngology-Head and Neck Surgery criteria was preserved in eight patients (50%). Of the eight patients without serviceable hearing, six had the cochlear nerve sacrificed at the time of surgery. All patients with serviceable hearing had normal appearing cochleovestibular signal on T2-weighted images, whereas six of eight patients (75%) with no hearing or poor hearing had abnormal low signal in the inner ear, suggesting inner ear ossification. The positive predictive value of a normal labyrinth for preserved hearing was 90%, whereas the negative predictive value of an abnormal labyrinth for no hearing was 100%. All but one patient who had the cochlear nerve sacrificed showed abnormal morphology of the labyrinth on T2-weighted magnetic resonance imaging.

CONCLUSION

We describe the T2-weighted magnetic resonance findings after hearing-preservation surgery for acoustic tumor removal. Loss of inner ear signal on T2-weighted images correlates with loss of hearing postoperatively, whereas preserved inner ear signal correlates with hearing preservation after middle fossa surgery for vestibular schwannoma removal.

Macrophage colony stimulating factor (M-CSF) protects spiral ganglion neurons following auditory nerve injury: morphological and functional evidence

Because hearing disturbance due to auditory nerve dysfunction imposes a formidable burden on human beings, intense efforts have been expended in experimental and clinical studies to discover ways to restore normal hearing. However, the great majority of these investigations have focused on the peripheral process side of bipolar auditory neurons, and very few trials have focused on ways to halt degenerative processes in auditory neurons from the central process side (in the cerebellopontine angle). In the present study, we investigated whether administration of macrophage colony-stimulating factor (M-CSF) could protect auditory neurons in a rat model of nerve injury. The electrophysiological and morphological results of our study indicated that M-CSF could ameliorate both anterograde (Wallerian) and retrograde degeneration in both the CNS and PNS portions of the auditory nerve. We attribute the success of M-CSF therapy to the reported functional dichotomy (having the potential to cause both neuroprotective and neurotoxic effects) of microglia and macrophages. Whether the activities of microglia/macrophages are neuroprotective or neurotoxic may depend upon the nature of the stimulus that activates the cells. In the present study, the neuroprotective effects of M-CSF that were observed could have been due to M-CSF we administered and to M-CSF released from endothelial cells, resident cells of the CNS parenchyma, or infiltrating macrophages. Another possibility is that M-CSF ameliorated apoptotic auditory neuronal death, although this hypothesis remains to be proved in future studies.

Apoptosis of auditory neurons following central process injury

Although apoptotic changes in auditory neurons induced by injury to peripheral processes (dendrites) have been intensively studied, apoptotic changes in auditory neurons induced by injury to central processes (axons of spiral ganglion cells, SGCs) have not been reported previously, probably due to lack of an experimental model. The present study reports for the first time the appearance, extent, and time course of SGC apoptosis following injury to the central processes. Apoptosis was studied in a rat model that consisted of compression of the auditory nerve in the cerebellopontine (CP) angle cistern with intraoperative recordings of auditory nerve compound action potentials (CAPs) to ensure highly reproducible results. Rats were killed between day 0 and day 14 after compression and apoptosis of SGCs was evaluated quantitatively as well as qualitatively by terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate nick-end labeling (TUNEL) staining, anti-activated caspase-3 immunostaining, Hoechst 33342 staining, and electron microscopy. The average number of TUNEL-positive apoptotic SGCs in each cochlear turn increased from day 1 to day 5 and then decreased gradually to an undetectable level on day 14 after compression. The average proportion of apoptotic SGCs identified in any cochlear turn on any day was always lower than 10%. The results of our present study should be useful in determining the therapeutic time window for rescuing auditory neurons undergoing apoptosis due to injury during surgery in the CP angle.

Effect of topically applied basic fibroblast growth factor on injured cochlear nerve

OBJECTIVE

Trauma-induced hearing loss after cerebellopontine angle manipulation has been regarded as having a hopeless natural course once it occurs. To challenge such a pessimistic view, we investigated whether pharmacological interventions with basic fibroblast growth factor (bFGF) could ameliorate trauma-induced cochlear nerve degeneration.

METHODS

The cerebellopontine angle portion of the cochlear nerve of rats was quantitatively compressed, and bFGF was topically administered for 2 weeks with a bFGF-soaked absorbable sponge and an osmotic minipump. The animals were killed 2 weeks after the compression procedure. The effect of bFGF in ameliorating cochlear neuronal death was evaluated from the residual number of spiral ganglion cells.

RESULTS

Cerebellopontine angle cisternal application of bFGF ameliorated cochlear nerve degeneration after the compression. Immunocytochemical studies of FGF receptors indicated that topically administered bFGF was internalized by a receptor-mediated mechanism through FGF receptor-1 and/or FGF receptor-2.

CONCLUSION

This report demonstrated that therapeutic application of bFGF was feasible to ameliorate trauma-induced cochlear nerve degeneration. Recent technological advances for deafened ears, such as cochlear implants and auditory brainstem implants, in combination with neurotrophic and/or growth factor therapeutic intervention, would be of great potential benefit for patients with hearing loss.

Axonal injury in auditory nerve observed in reversible latency changes of brainstem auditory evoked potentials (BAEP) during cerebellopontine angle manipulations in rats

Intraoperative monitoring of brainstem auditory evoked potentials (BAEP) has been widely utilized to reduce the incidence of postoperative hearing disturbance due to cerebellopontine angle manipulations. The prolongation of wave V of BAEP is usually used as a criterion to warn the surgeons to modify their surgical maneuvers. However, it is not known whether all neuropathological changes are avoided if BAEP latency intraoperatively returns to the baseline level or some neuropathological changes 'silently' occur even if BAEP normalizes. The aim of this study was to experimentally clarify this point that would be important for the long-term prognosis of patients' hearing. The cerebellopontine angle portion of the auditory nerve was quantitatively compressed in the rats and reversible prolongation of BAEP latency was reproduced just as it occurs during surgery in humans. Twenty-four hours after the compression, the auditory nerve was removed for beta-APP immunostaining to investigate the degree of axonal injury. The results of the present study disclosed that axonal injury occurred even in the cases where the intraoperative normalization of prolonged wave IV (equivalent to wave V in humans) latency had been obtained. Therefore, the interpretation of BAEP changes based only on the prolongation of the latency of BAEP was not enough to prevent the auditory nerve from developing morphological changes. Changes in the amplitude of wave V of BAEP appears to be more sensitive than its latency change as an intraoperative indicator for axonal injury in the auditory nerve.

Temporal pattern of cochlear nerve degeneration following compression injury: a quantitative experimental observation

OBJECT

It has been empirically recognized that the cochlear nerve is highly vulnerable to traumatic stress resulting from surgical procedures; therefore, careful manipulation of the cochlear nerve is mandatory in preventing trauma-induced hearing loss during cerebellopontine angle (CPA) surgery. There is, however, no precise knowledge about the temporal pattern of cochlear nerve degeneration following trauma. This study was performed to determine the temporal pattern of injury that occurs after cochlear nerve trauma, knowledge of which is indispensable not only to neurosurgeons but also to all those who manage lesions involving the cochlear nerve.

METHODS

Right suboccipital craniectomies were performed in groups of rats with the aid of a surgical microscope, and the seventh and eighth cranial nerve trunks were identified at the internal auditory meatus. The cochlear nerve was quantifiably compressed while compound action potentials of the cochlear nerve were monitored and recorded. Following injury, one group of rats was killed for histological examination at the end of each week for 4 weeks. Data from this study disclosed that the degeneration of the compressed cochlear nerve progressed in a relatively rapid manner and was complete within 1 week after the insult. The main pathophysiological mechanisms responsible for cochlear neuronal death in this experimental setting appeared to be necrosis, and an apoptotic mechanism seemed to play a subsidiary role.

CONCLUSIONS

Accurate knowledge about the temporal profile of trauma-induced cochlear nerve degeneration is closely linked with the problem of the therapeutic time window. The results of the present study indicated that any measures to ameliorate cochlear nerve degeneration following trauma should be started as early as possible (within 1 week) after an injury.