Auditory Brainstem Implants and Optical Stimulation
Efferent Auditory Brainstem Circuits
Our laboratory has focused on auditory brainstem circuits that control the middle ear muscle reflexes, one of two major descending systems that provide feedback to the auditory periphery. Collaborating with M. Christian Brown, Ph.D., we are characterizing the anatomy and physiology of the middle ear muscle reflexes in a rat model using retrograde labeling studies as well as lesioning experiments of the cochlear nucleus. We also perform transneuronal tracing of these auditory brainstem pathways using pseudorabies virus (PRV), a powerful neurotropic viral tracer. Our group is using PRV to identify synaptically linked neurons in the CNS that are involved in both the medial olivocochlear and middle ear muscle reflex pathways.
Auditory Brainstem Implants
My clinical research interests in pediatric and adult cochlear implants have extended to work in the central auditory system as principal investigator and director of the Helene and Grant Wilson Auditory Brainstem Implant (ABI) Program, a multidisciplinary research and clinical effort with collaborators at Mass. Eye and Ear and Massachusetts General Hospital. Our goals are to 1) provide ABIs to patients who are deaf and are not candidates for cochlear implants due to injured or absent auditory nerves (patients with Neurofibromatosis Type 2, cochlear ossification / labyrinthitis ossificans, severe cochlear hypoplasia, or traumatic bilateral auditory nerve injury and 2) conduct basic and clinical research on how to improve the performance of ABIs.
Optical Stimulation of the Auditory System
Our recent work in the ABI lab has included the first efforts to optically stimulate neurons in the central auditory system using optogenetics. In contrast to electricity, light offers a theoretical advantage as it can be focused and may allow for the selective activation of hundreds of independent acoustic channels.
Light can depolarize unmodified neurons through thermal and physical means, as has been shown by Richter and colleagues using the delivery of radiant infrared energy to the cochlea (Izzo, Richter et al. 2006; Izzo, Walsh et al. 2008). An alternative approach is the use of optogenetics and light-sensitive proteins that, when delivered through viral vector-mediated gene therapy, can make specific cells activatable or silenceable by multiple colors of low-level visible light (Hirase, et al. 2002; Wells, Kao et al. 2005; Boyden, et al. 2005; Han, et al. 2007; Wells, Thomsen et al. 2007; Chow, et al. 2010).
For our central auditory system research, we employ channelrhodopsin-2 (ChR2), which is a light-gated single-component microbial transmembrane ion channel originally identified in the Chlamydomonas reinhardtii. ChR2 opens to allow ions into the interior of the cell upon stimulation by blue light (Boyden, et al. 2005).
Our lab has teamed with Dr. Edward Boyden and colleagues at the Massachusetts Institute of Technology to demonstrate that adeno-associated viral vector mediated delivery of ChR2 is possible in auditory neurons of the cochlear nucleus (CN) (Fig. 1; Acker et al., 2011).
The goals of our lab are to demonstrate that the mouse cochlear nucleus or central nucleus of the inferior colliculus can be 1) photosensitized with ChR2 using an adeno-associated viral (AAV) vector and 2) activated with blue light to generate auditory responses in vivo.
These experiments may provide the basis for an auditory implant based on light.