Selective Loss of Calretinin-Poor Cochlear Afferent Nerve Fibers in Streptozotocin-Induced Hyperglycemic Mice

Hearing loss is one of the major complications of diabetes mellitus and significantly lowers the quality of life of diabetic patients. In studies using diabetic animal models hearing loss have been frequently associated with damages to cochlear afferent fibers. Recent studies suggested that cochlear afferent neurons are composed of heterogeneous populations and a subgroup of neurons equipped with low level of calretinin might be more vulnerable to various noxious stimuli such as noise and neurotoxins. Here, we tested if cochlear afferent neurons deficient in the Ca2+-buffering protein calretinin are more vulnerable to hyperglycemic insults. Streptozotocin-induced (50 mg/kg, i.p.) hyperglycemic mice (>250 mg/dl) were tested. The expression patterns of calretinin in peripheral processes and the cell bodies of cochlear afferent nerve fibers were examined using immunohistochemistry and confocal microscopy. The proportion of calretinin-poor cochlear afferent fibers was much lower in hyperglycemic mice compared to the normoglycemic control group. (30.0 vs. 55.5% in the peripheral process; 15.7 vs. 24.4 % in spiral ganglion neuron). The results suggest that calretinin-poor cochlear nerve fibers may be selectively lost after the hyperglycemic insults. The finding also supports a calretinin's neuroprotective role against diabetic neuropathy in cochlear afferent neurons.