ACTIVITY-DEPENDENT MODULATION OF RIBBON SYNAPSES IN VESTIBULAR HAIR CELLS

The vestibular system comprises five mechanosensory organs containing two types of vestibular hair cells (VHCs), that detect head movements and convey signals to higher brain centers through specialized ribbon synapses. Unlike cochlear inner hair cells, where spherical ribbon precursors mature into active zone-anchored structures, VHCs, especially type-I cells, retain high numbers of free-floating ribbons and ribbon clusters into adulthood. The physiological significance of these floating ribbons remains unclear. Previous studies show a decline of active zone-anchored ribbons in type-I VHCs comparing p15 to 11-month-old mice, suggesting a potential relationship between aging, activity and synaptic functioning, that remains unexplored. In contrast to rather deprived standard-cage housing, so-called “enriched” environments (EE) contain running wheels and mazes for increased physical and cognitive stimulation, which enhance neuronal plasticity and engage multiple sensory and motor systems, including the vestibular, boosting physical exercise. This study investigates whether EE-raising modulates ribbon attributes in VHCs of 15-month-old mice. Using 2D/3D transmission electron microscopy, we analyzed ribbon attachment, abundance and other ultrastructural characteristics in both type-I and type-II utricular VHCs. Compared to standard cage-raised animals, EE-mice displayed more active zone-anchored ribbons and reduced numbers of floating ribbon in smaller clusters. Conversely, absence of otoferlin, a protein essential for quantal synaptic release, may offer insights into synaptic adaptations to reduced cellular activity, which we plan to investigate further. Our findings indicate that “enrichment” preserves hallmarks of a “younger” ribbon organization into old age and contribute to understanding how ultrastructural plasticity may support vestibular function during aging.