Amino acid concentrations in the hamster central auditory system and long‐term effects of intense tone exposure

Abstract

Exposure to intense sounds often leads to loss of hearing of environmental sounds and hearing of a monotonous tonal sound not actually present, a condition known as tinnitus. Chronic physiological effects of exposure to intense tones have been reported for animals and should be accompanied by chemical changes present at long times after the intense sound exposure. By using a microdissection mapping procedure combined with high-performance liquid chromatography (HPLC), we have measured concentrations of nine amino acids, including those used as neurotransmitters, in the cochlear nucleus, inferior colliculus, medial geniculate, and auditory cortex of hamsters 5 months after exposure to an intense tone, compared with control hamsters of the same age. No very large differences in amino acid concentrations were found between exposed and control hamsters. However, increases of glutamate and γ-aminobutyrate (GABA) in some parts of the inferior colliculus of exposed hamsters were statistically significant. The most consistent differences between exposed and control hamsters were higher aspartate and lower taurine concentrations in virtually all regions of exposed hamsters, which reached statistical significance in many cases. Although these amino acids are not considered likely neurotransmitters, they indirectly have roles in excitatory and inhibitory neurotransmission, respectively. Thus, there is evidence for small, widespread, long-term increases in excitatory transmission and decreases in inhibitory transmission after a level of acoustic trauma previously shown to produce hearing loss and tinnitus.

Brain Metabolic Changes in Rats following Acoustic Trauma

“…These metabolites are predominantly involved in amino acid metabolism, the urea cycle as well as oxidation-reduction reactions. The long-term effects of acoustic trauma on amino acid levels in auditory brain regions have been investigated in a previous study, which reported a significant increase in glutamate, GABA and aspartate and a decrease in taurine (Godfrey et al, 2012). This is in a general agreement with the present results except that our study extended the finding into some non-auditory areas, such as the CB and VNC, which may suggest a more widespread effect of acoustic trauma in disturbing the balance between excitatory and inhibitory amino acids.…”

Effects of cochlear ablation on amino acid levels in the rat cochlear nucleus and superior olive

“…The cochlear damage that results from exposure to intense sound leads to degeneration of relatively small numbers of auditory nerve fibers (Morest et al, 1998; Kim et al, 2004) compared to cochlear ablation. Likewise, the effects of intense sound on amino acid levels in the cochlear nucleus, based on our studies in hamsters (Godfrey et al, 2008b, 2012), are much less than those of cochlear ablation. Also, the changes measured are often in opposite directions.…”

“…Also, the changes measured are often in opposite directions. Thus, the decreased glutamate and aspartate levels in cochlear nucleus regions after cochlear ablation contrast with a tendency toward increased glutamate and aspartate levels after intense tone exposure (Godfrey et al, 2012). The trend for levels of the other amino acids to increase in cochlear nucleus regions after cochlear ablation was not found after intense tone exposure.…”

“…In the auditory system, damage to the cochlea part of the inner ear can result in hearing loss and distorted hearing, such as tinnitus ( Eggermont, 2012). Chemical changes associated with these deficits have been studied at many levels of the rodent auditory system (Fisher and Davies, 1976; Wenthold and Gulley, 1977; Wenthold, 1978, 1979; Ryan et al, 1992; Hackney et al, 1996; Abbott et al, 1999; Milbrandt et al, 2000; Potashner et al, 1985, 1997, 2000; Suneja et al, 1998a,b, 2000; Michler and Illing, 2003; Zhang et al, 2003a,b; Jin et al, 2005, 2006; Jin and Godfrey, 2006; Godfrey et al, 2008a,b, 2012, 2013; Wang et al, 2009, 2011; Dong et al, 2010; Hildebrandt et al, 2011; Kraus et al, 2011; Fredrich et al, 2013), but the most immediate effects occur at its earliest levels, where the auditory nerve carries information about sounds from the cochlea to the cochlear nucleus (Harrison and Warr, 1962; Harrison and Irving, 1965, 1966; Merchán et al, 1985). The superior olivary complex, which receives extensive input from the cochlear nucleus, is also connected to the cochlea via a centrifugal pathway, the olivocochlear bundle (Warr, 1992).…”

“…Previous studies in rodents have indicated important functions for the amino acids glutamate, glycine, and γ-aminobutyrate (GABA) as major neurotransmitters in the cochlear nucleus (Fisher and Davies, 1976; Wenthold and Gulley, 1977; Wenthold, 1978, 1979; Hackney et al, 1996; Potashner et al, 1985, 1997, 2000; Suneja et al, 1998a,b, 2000; Godfrey et al, 2008a,b, 2012; Wang et al, 2009; Dong et al, 2010). Glutamate is strongly supported as a neurotransmitter of auditory nerve fibers, whereas glycine and GABA, as well as glutamate, are associated with cochlear nucleus interneurons and non-auditory nerve innervations (Godfrey et al, 2000, 2008b).…”