Multiple dosing strategies with acetyl L-carnitine (ALCAR) fail to alter age-related hearing loss in the Fischer 344/NHsd rat

BACKGROUND: The Fischer 344/NHsd rat undergoes age-related, progressive, high-frequency hearing loss beginning at age 12 months. The loss has been linked to defects/death in the outer hair cells related to oxidative stress originating in the mitochondria. Acetyl L-carnitine (ALCAR) is known to enhance mitochondrial bioenergetics and membrane efficiency. Therefore, ALCAR was targeted as a possible pharmacologic intervention to prevent, or even restore, hearing loss from aging. METHODS: Three different paradigms were used to deliver ALCAR to aging Fischer 344/NHsd rats. Rats in each condition had their hearing evaluated by auditory brainstem responses before, during, and after treatment. First, 24-month-old rats were given ALCAR (100 mg/kg dissolved 25 mg/ml in saline) by IP injection daily for one month. Second, 18-month-old rats were given ALCAR (100 mg/kg) by oral gavage for 90 days. Third, 15-month-old rats were given ALCAR (100 mg/kg) by oral gavage for 90 days. Control rats in each condition received saline by i.p. injection or gavage. RESULTS: Hearing thresholds of the three sets of ALCAR-treated animals were never significantly different from their matched controls before, during, or after the treatments at any of the five test stimuli (5, 10, 20, and 40 kHz tone bursts and a click). CONCLUSION: The current study does not provide evidence that age-related hearing loss in the Fischer 344/NHsd rat can be altered with systemic administration of ALCAR.