Clin Exp Otorhinolaryngol.  2019 Feb;12(1):33-39. 10.21053/ceo.2018.00542.

Losartan Prevents Maladaptive Auditory-Somatosensory Plasticity After Hearing Loss via Transforming Growth Factor-β Signaling Suppression

• Mun SK
• Han KH
• Baek JT
• Ahn SW
• Cho HS
• Chang MY

• KMID: 2437489
• DOI: http://doi.org/10.21053/ceo.2018.00542

Abstract

OBJECTIVES
Hearing loss disrupts the balance of auditory-somatosensory inputs in the cochlear nucleus (CN) of the brainstem, which has been suggested to be a mechanism of tinnitus. This disruption results from maladaptive auditory-somatosensory plasticity, which is a form of axonal sprouting. Axonal sprouting is promoted by transforming growth factor (TGF)-β signaling, which can be inhibited by losartan. We investigated whether losartan prevents maladaptive auditory-somatosensory plasticity after hearing loss.
METHODS
The study consisted of two stages: determining the time course of auditory-somatosensory plasticity following hearing loss and preventing auditory-somatosensory plasticity using losartan. In the first stage, rats were randomly divided into two groups: a control group that underwent a sham operation and a deaf group that underwent cochlea ablation on the left side. CNs were harvested 1 and 2 weeks after surgery. In the second stage, rats were randomly divided into either a saline group that underwent cochlear ablation on the left side and received normal saline or a losartan group that underwent cochlear ablation on the left side and received losartan. CNs were harvested 2 weeks after surgery. Hearing was estimated with auditory brainstem responses (ABRs). Western blotting was performed for vesicular glutamate transporter 1 (VGLUT1), reflecting auditory input; vesicular glutamate transporter 2 (VGLUT2), reflecting somatosensory input; growth-associated protein 43 (GAP-43), reflecting axonal sprouting; and p-Smad2/3.
RESULTS
Baseline ABR thresholds before surgery ranged from 20 to 35 dB sound pressure level. After cochlear ablation, ABR thresholds were higher than 80 dB. In the first experiment, VGLUT2/VGLUT1 ratios did not differ significantly between the control and deaf groups 1 week after surgery. At 2 weeks after surgery, the deaf group had a significantly higher VGLUT2/VGLUT1 ratio compared to the control group. In the second experiment, the losartan group had a significantly lower VGLUT2/VGLUT1 ratio along with significantly lower p-Smad3 and GAP-43 levels compared to the saline group.
CONCLUSION
Losartan might prevent axonal sprouting after hearing loss by blocking TGF-β signaling thereby preventing maladaptive auditory-somatosensory plasticity.

Keyword

Hearing Loss; Auditory-Somatosensory Plasticity; Tinnitus; Losartan

MeSH Terms

Animals
Axons
Blotting, Western
Brain Stem
Cochlea
Cochlear Nucleus
Evoked Potentials, Auditory, Brain Stem
GAP-43 Protein
Hearing Loss*
Hearing*
Losartan*
Plastics*
Rats
Tinnitus
Transforming Growth Factors
Vesicular Glutamate Transport Protein 1
Vesicular Glutamate Transport Protein 2
GAP-43 Protein
Losartan
Plastics
Transforming Growth Factors
Vesicular Glutamate Transport Protein 1
Vesicular Glutamate Transport Protein 2

Figure

• 

  Fig. 1. Auditory brainstem response thresholds for the control and deaf groups before (A) and after (B) surgery. Error bars indicate standard deviation. SPL, sound pressure level.

• 

  Fig. 2. Increase in the VGLUT2/VGLUT1 ratio after hearing loss. VGLUT1 and VGLUT2 expression levels in the cochlear nucleus were examined with a Western blot assay 1 and 2 weeks after surgery. (A) Representative Western blots. (B) Quantitative analysis of VGLUT1 and VGLUT2. At 2 weeks after surgery, the deaf group had a significantly higher VGLUT2/VGLUT1 ratio compared to the control group. Values are presented as mean±standard error. VGLUT1, vesicular glutamate transporter 1; VGLUT2, vesicular glutamate transporter 2. *Statistically significant differences between groups, P<0.05.

• 

  Fig. 3. Prevention of the increase in the VGLUT2/VGLUT1 ratio using losartan. VGLUT1 and VGLUT2 expression levels in the cochlear nucleus were examined with a Western blot assay 2 weeks after surgery. (A) Representative Western blots. (B) Quantitative analysis of VGLUT1 and VGLUT2. The losartan group had a significantly lower VGLUT2/VGLUT1 ratio compared to the deaf group. Values are presented as mean±standard error. VGLUT1, vesicular glutamate transporter 1; VGLUT2, vesicular glutamate transporter 2. *Statistically significant differences between groups, P<0.05.

• 

  Fig. 4. Changes in p-Smad2 and 3 signaling, and GAP-43 following losartan administration. p-Smad2 and 3, and GAP-43 expression levels in the cochlear nucleus were examined with a Western blot assay 2 weeks after surgery. (A) Representative Western blots. (B) Quantitative analysis of p-Smad2 and 3, and GAP-43. The losartan group had significantly lower p-Smad3 and GAP-43 levels compared to the deaf group. Values are presented as mean±standard error. GAP-43, growth-associated protein 43. *Statistically significant differences between groups, P<0.05.

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