Can antioxidants help you hear better?.

The effects of antioxidants in the senescent auditory cortex

Age-related hearing loss is among the most common problems affecting the elderly population. One of its more debilitating forms is a difficulty in discriminating speech, even in a quiet environment. One possible cause may be an age-related decline in processing speed throughout the central nervous system (CNS). For the senescent auditory system, a deterioration in processing the rapidly changing consonant–vowel formant transitions (which are comprised of rapid changes in frequency and amplitude over time), may be an important source of diminished speech discrimination in the elderly. Unfortunately, relatively few studies have explored the effects of aging on processing these more dynamic aspects of speech.

One stimulus that lends itself well to investigate the effects of aging on temporal processing speed in the auditory system is the frequency modulated (FM) sweep. FM sweeps, which are characterized by changes in frequency over time, share features in common with formant transitions. Recently, we found that cells in the primary auditory cortex (AI) of aged rats respond more vigorously to slower as compared to faster FM sweeps. These results suggest the presence of an age-related decline in the rate of change of frequency that can be processed by the auditory cortex.

In recent years a great deal of research has been devoted to characterizing age-related changes in the CNS in an effort to develop techniques that can halt or even reverse these changes. One such area has focused on attempting to reduce the amount of oxidative stress (OS) that accompanies aging. Researchers have found that by increasing the amount of antioxidants in the organism's system some of the debilitating effects of aging can be reversed. For example, we have showed that an antioxidant-enriched diet impedes the onset of age-related neuronal signal transduction and cognitive behavioural deficits in rats. We also found that aged rats placed on a diet supplemented with blueberries for 2 months showed less of an age-related decline than control animals. In fact, these animals showed an improvement in motor and cognitive behaviours when compared to control animals. In addition, in an in vitro model, antioxidants from blueberries were shown to ameliorate the experimentally induced formation of reactive oxygen species (ROS) in red blood cells.

Recently, researchers have found a negative correlation between ROS and presbycusis. Using an in vitro model, researchers were able to demonstrate that antioxidants from blueberries improved the experience-induced formation of ROS. In the auditory system, studies have demonstrated that OS plays a deleterious role in noise exposure and ototoxicity.

While the effects of antioxidants appear to be encouraging little is known about their potential effects on sensory processing, particularly in the auditory system. Because ROS causes pathological changes in the morphology of neurons, such detrimental modifications could compromise normal cell functioning and potentially act to disrupt synaptic activity. This in turn could affect temporal processing speed within the auditory system. Thus, the goal of the present study was to determine if the introduction of antioxidants into the diet of aged rats would affect auditory cortical temporal processing speed. Specifically, we asked if an antioxidant-enriched diet containing blueberry phytochemicals would reverse the effects of aging on temporal processing speed of speech-related acoustic signals.

We investigated whether a 2-month dietary supplementation of antioxidants, in the form of blueberry phytochemicals, could reverse or retard the age-related decline in temporal processing speed observed in the aged rat. To this end, extracellular single unit responses to frequency modulated (FM) sweeps were recorded in the primary auditory cortex (AI) of aged rats that had been placed on either a blueberry-supplemented or control diet 2 months prior to the physiological recordings. Results showed that most cells recorded from the blueberry-fed rats responded most vigorously to fast FM sweeps, similar to that observed in young rats. In contrast, the majority of cells recorded from the control rats showed a preference for slow FM sweep rates. These results suggest that age-related changes in temporal processing speed in A1 may be reversed by dietary supplementation of blueberry phytochemicals.

In summary, it seems that free radical damage may be imparted on the cortex of aged rats. In particular, this type of age-related change in the cortex could have profound effects on neurotransmission and, consequently, on central nervous system processing of sensory information such as auditory stimuli. Such damage may have been reversed by the 2-month supplementation of dietary antioxidants in the form of blueberry phytochemicals. However, it is unclear if the same effects could have been attained had classical antioxidants, such as alpha-tocopherol or ascorbic acid been administered; to our knowledge such studies have not been performed. Furthermore, the exact mechanism by which flavonoids exert their antioxidant activity particularly in the auditory cortex is not clear and is the subject of future research.