Age-related Changes to Dense Core Vesicles in Excitatory Cells Across the Tonotopic Inferior Colliculus

Summary: PURPOSE Work in our lab demonstrated that dense core vesicles (DCV), which package neuropeptides, neurotrophins, and monoamines, are upregulated with age in the central inferior colliculus (ICc). We sought to determine if excitatory ICc cells upregulate DCVs. METHODS We assessed five young and five old Fischer Brown Norway rats. We used immunogold transmission electron microscopy to characterize the ultrastructure across the tonotopic axis of ICc. Excitatory synapses were identified by asymmetric postsynaptic densities and vesicle shape. DCVs have an electron-dense core surrounded by an outer membrane with diameters between 40-120 nm. A total of 2,228 DCVs were characterized across 96,000 μm2 of ICc. RESULTS We found that in DCVs were equally expressed between boutons/axons and the dendrites of excitatory cells across the high, middle and low frequencies at young age. During aging there were 1) ~45-55% more DCVs in excitatory boutons and axons, 2) ~70% more DCVs in middle and low frequency excitatory dendrites and 3) ~160% more in high frequency excitatory dendrites. Despite an age-related loss of excitatory boutons, the percentage of boutons with more than one dense core vesicle increased. CONCLUSION Our model undergoes age-related hearing loss at low frequencies. Perhaps the substantial increase of DCVs, possibly reflecting an increase of neurotrophins, in the dendrites of higher frequency excitatory cells reflects a more robust cellular maintenance during aging. The increase of DCVs could also reflect an increase of neurotransmission that underlies the hyperexcited stated often characteristic of the aging IC.