Increased inner ear susceptibility to noise injury in mice with streptozotocin-induced diabetes

Abstract

We aimed to investigate the pathophysiology of diabetes-associated hearing impairment in type 1 diabetes using mice with streptozotocin-induced diabetes (C57BL/6J; male). Hearing function was evaluated 1, 3, and 5 months after induction of diabetes (five diabetic and five control animals per time point) using auditory-evoked brain stem responses (ABRs). Mice (four diabetic and four control) were exposed to loud noise (105 dB) 5 months after induction of diabetes. ABRs were measured before and after noise exposure. Cochlear blood flows were measured by laser-Doppler flowmeter. Spiral ganglion cells (SGCs) were counted. Vessel endothelial cells were observed by CD31 immunostaining. Chronologic changes in the ABR threshold shift were not significantly different between the diabetic and control groups. However, vessel walls in the modiolus of the cochleae were significantly thicker in the diabetic group than the control group. Additionally, recovery from noise-induced injury was significantly impaired in diabetic mice. Reduced cochlea blood flows and SGC loss were observed in diabetic mice cochleae after noise exposure. Our data suggest that diabetic cochleae are more susceptible than controls to loud noise exposure, and decreased cochlear blood flow due to sclerosis of the vessels and consequent loss of SGCs are possible mechanisms of hearing impairment in diabetic patients.

FIG. 1.

Time course of changes in body weight (A) and blood glucose level (B) at baseline (n = 15 each for diabetic [DM] and control [Ctrl]) and 1 month (n = 15 each for diabetic and control), 3 months (n = 10 each for diabetic and control), and 5 months (n = 5 each for diabetic and control) after induction of diabetes. Data are means ± SE. **P < 0.01, ***P < 0.001 diabetic vs. control group.

FIG. 2.

Chronologic changes in the ABR threshold shift were not significantly different between the diabetic group and controls throughout the observation period except at 4 kHz at 1 month. The time course of ABR threshold shifts compared with baseline (A–C) at each observation period (1, 3, and 5 months) for the diabetic (DM) group (n = 5 for each period) and control (Ctrl) group (n = 5 for each period). Data are means ± SE. *P < 0.05 diabetic vs. control group.

FIG. 3.

H-E staining (top panel; scale bar = 100 μm) and CD31 immunostaining (bottom panel; scale bar = 20 μm) of the vessel endothelial cells at the modiolus in control (Ctrl) cochlea (A) and diabetic (DM) cochlea (B). The vessel wall thickness of staining for CD31, quantified by a computer-aided image-analysis system, is shown for both diabetic (n = 5) and control (n = 5) mice at 5 months of diabetes as described in research design and methods (C). Data are means ± SE. **P < 0.01 diabetic vs. control group. (A high-quality digital representation of this figure is available in the online issue.)

FIG. 4.

Recovery from noise-induced injury was significantly impaired in diabetic mice at 4 and 8 kHz. The time course of ABR threshold shifts at each frequency in diabetic (DM) (n = 4) and control (Ctrl) (n = 4) groups after noise exposure (4-kHz OBN, 105-dB SPL for 2 h) (A–D). Data are means ± SE. *P < 0.05 diabetic vs. control group.

FIG. 5.

The cochlear blood flow ratio measured by laser-Doppler flowmeter is shown for both diabetic (DM) (n = 4) and control (Ctrl) (n = 4) mice 14 days after noise exposure (4-kHz OBN, 105-dB SPL for 2 h) as described in research design and methods. Data are means ± SE. **P < 0.01 diabetic vs. control group.

FIG. 6.

A and B: Whole cochleae (top panels; scale bar = 100 μm) and SGCs in the apical turn of the cochleae (bottom panels; scale bar = 50 μm) 14 days after noise exposure (4-kHz OBN, 105-dB SPL for 2 h). H-E staining of control [Ctrl] cochlea (A) and diabetic (DM) cochlea (B). Means of the densities of spiral ganglion neurons 14 days after noise exposure are shown in C (n = 4 for each group). Data are means ± SE. *P < 0.05, **P < 0.01 diabetic vs. control group. (A high-quality digital representation of this figure is available in the online issue.)