Exposure to the microgravity conditions of spaceflight alleviates the load normally imposed by the Earth's gravitational field upon the inner ear utricular epithelia. Previous ultrastructural investigations showed that spaceflight induced an increase in synapse density within hair cells of the rat utricle. However, the utricle exhibits broad physiologic heterogeneity across different epithelial regions, and it is unknown whether capabilities for synaptic plasticity generalize to hair cells across its topography. To achieve systematic and broader sampling of the epithelium than previously conducted we used immunohistochemistry and volumetric image analyses to quantify synapse distributions across representative utricular regions in specimens from mice exposed to spaceflight (a 15-day mission of the space shuttle Discovery). These measures were compared to similarly-sampled Earth-bound controls. Following paraformaldehyde fixation and microdissection, immunohistochemistry was performed on intact specimens to label presynaptic ribbons (anti-CtBP2) and postsynaptic receptor complexes (anti-Shank1A). Synapses were identified as closely-apposed pre- and postsynaptic puncta. Epithelia from horizontal semicircular canal cristae served as 'within specimen' controls, while utricles and cristae from Earth-bound cohorts served as experimental controls. We found that synapse densities decreased in the medial extrastriolae of Microgravity specimens compared to experimental Controls, while they were unchanged in the striolae and horizontal cristae from the two conditions. These data demonstrate that structural plasticity was topographically localized to the utricular region that encodes very low frequency and static changes in linear acceleration, and illuminates the remarkable capabilities of utricular hair cells for synaptic plasticity in adapting to novel gravitational environments.
- synaptic ribbon
- resampling linear mixed effects
- Copyright © 2016, Journal of Neurophysiology