Inner-ear delivery of AAV2-retro robustly labels lateral olivocochlear efferents but results in off-target transfection in the contralateral cochlea and the brain

Gene delivery via adeno-associated viruses (AAVs) is a valuable tool for understanding the organization and function of auditory neuronal circuitry. In this study, we explored the use of AAVs injected into the inner ear to transfect the lateral olivocochlear (LOC) pathway, a poorly understood ipsilateral component of the auditory efferent feedback system. AAV serotypes exhibit a wide range of properties, including cell type selectivity and the capacity for retrograde transport, but none of the AAV serotypes used for inner ear delivery have been reported to successfully transfect the LOC pathway. Here, we show that unilateral inner ear delivery of AAV2-retro robustly labeled the LOC pathway. However, the ability of AAV2-retro to diffuse via the cochlear aqueduct from the perilymph into the cerebrospinal fluid resulted in off-target transfection in the brain and the labeling of LOC projections in the contralateral cochlea. The extent of contralateral transfection of LOC neurons depended on the age of the animal and the amount of virus delivered to the inner ear. These findings highlight the importance of considering the dosage and properties of AAV serotype when interpreting experimental results.