Jared Daiber
A Study of Synaptic Connections in the Organ of Corti
Mentor: Dr. Jianxin Bao, Department of Otolaryngology, Washington University School of Medicine

Sound perception from simple tones through highly complex components of speech and music is mediated through the Organ of Corti in our ear, which is comprised about half million interconnected cells. Sound frequency is decoded in the cochlea: high frequency waves activate hair cells in the Organ of Corti at the base of the cochlea, and low frequency waves activate hair cells at the apex of the cochlea. Sound waves move the hair bundles in hair cells, which in turn activate spiral ganglion neurons by their synaptic connections. Spiral ganglion neurons transmit this sound information into the brain. Damage of synaptic connections between hair cells and spiral ganglion neurons due to noise or aging could lead to hearing loss. Therefore, it is important to establish an effective way to study these synaptic connections.

With the advance of modern genetic technology, it is now possible to label specific neuronal connections with green fluorescent protein (GFP). We have recently found a specific expression of GFP in spiral ganglion neurons in one transgenic mouse line, in which the expression of GFP is under the control of a neuronal specific thy1 promoter. Using the multi-photon microscope, we have observed GFP-labeled synaptic terminals under hair cells from spiral ganglion neurons. 3-D images have been reconstructed with computer software to quantify synaptic terminals from spiral ganglion neurons at the apex, middle, and base of the cochlea. We have found that, on average, there are about 175 terminals per 40 mm section at the apex, 76 at the middle, and 74 at the base. There is a statistical difference between the amount of terminal at the apex compared to the middle and base. In addition, we have also examined the size of these synaptic terminals. On average, the size of synaptic terminals at the apex is 11.012 micrometers; 10.36 micrometers in the middle, and 10.09 micrometers at the base. We have also examined the number and size of synaptic terminals at the same locations after noise-exposure (110dB, 15 minutes). On average, there are about 117 terminals per 40 mm section at the apex, 68 at the middle, and 64 at the base. The average size of synaptic terminals at the apex is 8.30 micrometers; 9.257 micrometers in the middle, and 12.1 micrometers at the base. Therefore, compared to the normal Organ of Corti, there are fewer synaptic terminals in the Organ of Corti after the noise exposure, which could be a basis for noise-induced hearing loss.

In summary, we have found a transgenic mouse line expressing GFP specifically in spiral ganglion neurons. In this transgenic line, the synaptic terminal of spiral ganglion neurons can be easily identified and quantified. We have found that there are more synaptic terminals at the apex of the cochlea than the base of the cochlea. After noise exposure, the number of these synaptic terminals is decreased. Therefore, one cellular mechanism for noise-induced hearing loss may be the damage of the synaptic connections between hair cells and spiral ganglion neurons.