Visualization of alpha 9 acetylcholine receptor expression in hair cells of transgenic mice containing a modified bacterial artificial chromosome

The alpha 9 acetytcholine receptor (alpha 9 AChR) is specifically expressed in hair cells of the inner ear and is believed to be involved in synaptic transmission between efferent nerves and hair cells. Using a recently devel oped method, we modified a bacterial artificial chromosome containing the m ouse alpha 9 AChR gene with a reporter gene encoding green fluorescent prot ein (GFP) to generate transgenic mice. GFP expression in transgenic mice re capitulated the known temporal and spatial expression of alpha 9 AChR. Howe ver, we observed previously unidentified dynamic changes in alpha 9 AChR ex pression in cochlear and vestibular sensory epithelia during neonatal devel opment. In the cochlea, inner hair cells persistently expressed high revels of alpha 9 AChR in both the apical and middle turns, whereas both outer an d inner hair cells displayed dynamic changes of alpha 9 AChR expression in the basal turn. In the utricle, we observed high levels of alpha 9 AChR exp ression in the striolar region during early neonatal development and high l evels of alpha 9 AChR in the extrastriolar region in adult mice. Further, s imultaneous visualization of efferent innervation and alpha 9 AChR expressi on showed that dynamic expression of alpha 9 AChR in developing hair cells was independent of efferent contacts. We propose that alpha 9 AChR expressi on in developing auditory and vestibular sensory epithelia correlates with maturation of hair cells and is hair-cell autonomous.