Age-related changes in the response properties of cartwheel cells in rat dorsal cochlear nucleus

Abstract

The fusiform cell and deep layers of the dorsal cochlear nucleus (DCN) show neurotransmitter and functional age-related changes suggestive of a downregulation of inhibitory efficacy onto DCN output neurons. Inhibitory circuits implicated in these changes include vertical and D-multipolar cells. Cartwheel cells comprise a large additional population of DCN inhibitory neurons. Cartwheel cells receive excitatory inputs from granule cell parallel fibers and provide a source of glycinergic inhibitory input onto apical dendrites of DCN fusiform cells. The present study compared the response properties from young and aged units meeting cartwheel-cell criteria in anesthetized rats. Single unit recordings from aged cartwheel cells revealed significantly higher thresholds, increased spontaneous activity and significantly altered rate-level functions characterized by hyperexcitability at higher intensities. Aged cartwheel cells showed a significant reduction in off-set suppression. Collectively, these findings suggest a loss of tonic and perhaps response inhibition onto aged DCN cartwheel neurons. These changes likely reflect a compensatory downregulation of synaptic inhibition in response to a loss of excitatory drive from auditory and non-auditory excitatory inputs via granule cells. The impact of increased excitability of cartwheel cells on DCN output neurons is likely to be complex, influenced by loss of glycinergic release and/or subunit receptor changes which would only partially off-set age-related loss of inhibition onto the somata and basal dendrites of fusiform cells.