Permeability of the blood-labyrinth barrier in mice following acoustic injury

Abstract

Problem: The blood-brain barrier is a well-described structure that excludes the transport of molecules from the blood stream into the brain. The cochlea has been demonstrated to maintain a similar barrier with the intravascular space, termed the blood-labyrinth barrier (BLB). Its presence has been established with the use of radio-labeled proteins and ions, showing that a gradient exists between the serum and the perilymph. However, an anatomic study of the permeability gradients in different regions of the cochlea has not been performed. The purpose of this study is, therefore (1) to assess the presence of a BLB in the cochlea using Evans blue (EB), and (2) to investigate the effects of acoustic injury on the integrity of such a barrier. Methods: Normal control and noise-exposed mice were intravascularly injected with EB and after 20 minutes, brain and cochlea were harvested. EB staining was viewed under fluorescence microscopy. Results: In the cochlea, EB fluorescence demonstrated areas of high permeability including the bone marrow and blood vessels within the stria vascularis. There were also areas of moderate permeability such as the intra-strial compartment of the stria vascularis. Finally there were areas of relative impermeability found in the cochlear duct, spiral ligament, spiral limbus, scala vestibuli, and scala tympani. In this experiment we demonstrated that acoustic injury can change the permeability characteristics of the cochlea and causes leakiness of the BLB in the spiral ligament. Conclusion: The EB dye test is a technique that can be applied to assess the integrity of the BLB. Within the inner ear, there are specific regions that demonstrate high permeability, moderate permeability, and relative impermeability to EB. Significance: Acoustic trauma causes changes in the intrinsic properties of the BLB and results in leakiness of the BLB in the spiral ligament, precisely in the location where inflammatory cells migrate in response to acoustic injury. Support: This study was funded by the American Academy of Otolaryngology-Head and Neck Surgery resident research grant award.