Afferent synapses are present in utricular hair cells from otoconia-deficient mice

The head tilt mouse (het/het, abbr. het) is a naturally occurring mutant whose salient phenotypic traits include the complete absence of otoconia in both the utricle and saccule. Cursory histologic evaluation has indicated that the neuroepithelia exhibit a normal appearance. Though evidence exists indicating that utricular function is severely if not completely compromised in these animals, it is not yet known whether afferent synapses exist within utricular hair cells of otoconia-deficient mutants. The absence of synapses would be suggestive of a trophic relationship between stimulus-evoked hair cell activation and the afferent synapse. To address this question, we have conducted an ultrastructural survey of utricular sensory epithelia from confirmed het mice. The specific objective was to determine whether utricular hair cells made synaptic contact with afferent neurons. We found that both type I and II hair cells from utricles of het mice exhibited afferent synapses that were found at numerous sites distributed throughout the utricle. These results indicate that afferent synapses within vestibular hair cells do not critically depend upon stimulus-evoked activity.