Jiri Popelar

Auditory function in presbycusis: peripheral vs. central changes

The hearing abilities of a group of 30 elderly (67-93 yr of age) subjects were compared with those of a group of 30 young (19-27 yr of age) normal hearing volunteers with the aim of characterizing the changes in the peripheral and central parts of the auditory system. In elderly subjects the pure-tone thresholds were typically represented by a gradually sloping curve with a significantly greater decline in men than in women at frequencies of 3 and 4 kHz. In spite of pure tone threshold elevation in the elderly, the difference limen for intensity at 1 and 3 kHz were not significantly smaller than in the young subjects. The incidence and levels of spontaneous, transient and distortion product otoacoustic emissions were low, which would suggest the involvement of outer hair cell pathology. Also, contralateral suppression was less marked in elderly than in young subjects. Speech audiometry in the elderly revealed serious difficulties in understanding speech. Deteriorated temporal resolution, as demonstrated by increased gap detection thresholds, correlated significantly with increased speech recognition thresholds. The results support the view that presbycusis represents a combination of deteriorated function of the auditory periphery with deteriorated function of the central auditory system.

Responses to species-specific vocalizations in the auditory cortex of awake and anesthetized guinea pigs

Species-specific vocalizations represent an important acoustical signal that must be decoded in the auditory system of the listener. We were interested in examining to what extent anesthesia may change the process of signal decoding in neurons of the auditory cortex in the guinea pig. With this aim, the multiple-unit activity, either spontaneous or acoustically evoked, was recorded in the auditory cortex of guinea pigs, at first in the awake state and then after the injection of anesthetics (33 mg/kg ketamine with 6.6 mg/kg xylazine). Acoustical stimuli, presented in free-field conditions, consisted of four typical guinea pig calls (purr, chutter, chirp and whistle), a time-reversed version of the whistle and a broad-band noise burst. The administration of anesthesia typically resulted in a decrease in the level of spontaneous activity and in changes in the strength of the neuronal response to acoustical stimuli. The effect of anesthesia was mostly, but not exclusively, suppressive. Diversity in the effects of anesthesia led in some recordings to an enhanced response to one call accompanied by a suppressed response to another call. The temporal pattern of the response to vocalizations was changed in some cases under anesthesia, which may indicate a change in the synaptic input of the recorded neurons. In summary, our results suggest that anesthesia must be considered as an important factor when investigating the processing of complex sounds such as species-specific vocalizations in the auditory cortex.

Age-related changes in cochlear and brainstem auditory functions in Fischer 344 rats

Auditory function in Fischer 344 (F344) and Long Evans (LE) rats was monitored during their lifespan by evaluating hair cell loss, middle-ear compliance and the recording of otoacoustic emissions and auditory brainstem responses. The results revealed a faster deterioration of hearing function in F344 rats compared with LE rats, resulting in larger hearing threshold shifts, a decrease in the latency and amplitude of click-evoked auditory brainstem responses, diminution of the distortion product otoacoustic emissions and a decrease in middle-ear compliance. However, hair cell loss, observed only at the most basal and apical parts of the organ of Corti, was comparable in older individuals of both rat strains. The results suggest involvement of cochlear (stria vascularis) and extracochlear (middle-ear) pathological changes during ageing. Thus, F344 rats represent a complex mix of conductive hearing loss (with low-frequency threshold shift, declining parameters of the middle-ear admittance and asymmetric otoacoustic emissions) and sensorineural hearing loss (with a decrease in the amplitudes of auditory brainstem response and a high-frequency threshold shift).

Aging cochleas in the F344 rat: Morphological and functional changes

The Fischer 344 rat strain has been frequently used as an animal model of rapid aging. The present study was aimed at evaluating the incidence of apoptotic cells in the inner ear of 20-24-month-old F344 rats and to correlate it with cochlear function using otoacoustic emissions. Staining with cresyl violet and the enzymatic labeling (terminal deoxynucleotidyl transferase, TdT) of fragmented DNA revealed large numbers of apoptotic cells in the marginal and basal layers of the stria vascularis and in adjacent cells of the spiral ligament. The amplitudes of distortion products otoacoustic emissions (DPOAEs), which reflect functional state of the outer hair cells, were significantly reduced or totally absent in these animals. In contrast to old F344 rats, no marked DPOAE amplitude reduction and smaller numbers of apoptotic cells were found in young 4-month-old F344 rats or in aged 24-28-month-old Long Evans rats. The accumulation of apoptotic cells, mainly in the basal layer of the stria vascularis and in adjacent cells of the spiral ligament, leads to a detachment of the stria vascularis from the spiral ligament and results in the impairment of outer hair cell function. This specific type of strial deterioration suggests that aged F344 rats can serve as an animal model of strial presbycusis.

Comparison of noise-induced changes of auditory brainstem and middle latency response amplitudes in rats

Auditory brainstem responses (ABRs) and middle latency responses (MLRs) were compared after noise exposure to elucidate the specific effects of a loud sound on the central auditory system in rats. Rats were exposed twice for 1 h to broad-band noise (BBN) of 118 dB SPL (first exposure) and 122 dB SPL (second exposure) with an interval between the exposures of three weeks. The first noise exposure produced threshold shifts (TSs) amounting to 5-45 dB, and the second exposure resulted in 40-70 dB TSs. The slope of MLR amplitude-intensity functions (AIFs) increased significantly in correlation with the TS, resembling loudness recruitment. However, maximal MLR amplitudes measured at 8 kHz increased after the first and second noise exposures to almost equal values in individual animals regardless of the TS. In addition, maximum MLR amplitude enhancement was dependent on pre-exposure MLR voltage, probably reflecting the level of metabolic activity or neurotransmitter processes in individual animals. In contrast to MLR amplitudes, ABR amplitudes were suppressed after noise exposure without changing the slope of ABR AIFs. The MLR changes reflect the specific effects of noise exposure on the central auditory system.

Changes in otoacoustic emissions and high-frequency hearing thresholds in children and adolescents

With the aim of characterizing the loss of high frequency hearing sensitivity in children, hearing thresholds and otoacoustic emissions were measured in a group of 126 normal hearing children and adolescents aged from 6 to 25 years. The subjects were divided into four 5-year age groups. Hearing thresholds over a range of 125 Hz-12.5 kHz were similar in all age groups, the average hearing threshold at 16 kHz was significantly elevated in the oldest age group. The response values of transiently evoked otoacoustic emissions (TEOAEs) significantly declined with age; the decline was negatively correlated with the hearing loss at 16 kHz. Significantly larger TEOAE responses and average distortion-product otoacoustic emission (DPOAE) values at 6.3 kHz were present in the youngest group in comparison with the other three older groups. Spontaneous otoacoustic emissions (SOAEs) were present in 70.8% of the children (in either one or both ears) with the greatest prevalence in the 11-20-year-old subjects. In the 21-25-year-old group, the hearing loss at 16 kHz was significantly smaller in ears with SOAEs than in ears without SOAEs. The results demonstrate that the increase in the high frequency hearing threshold at 16 kHz, which starts at ages over 20 years, is correlated with a decrease in the TEOAE responses at middle frequencies.

Cochlear function in young and adult Fischer 344 rats.

Fischer 344 (F344) rats are often used as an animal model for investigation of the mechanisms underlying age-related hearing loss. The aim of this study was to assess cochlear function in young (1-month-old) and adult (6-month-old) F344 rats using recording of otoacoustic emissions and auditory brainstem responses (ABRs). The results were compared with control groups of Long Evans (LE) rats of the same ages. The results demonstrate a significant increase in the hearing threshold in F344 rats in comparison with LE rats, expressed mainly at low frequencies (1-2 kHz). In F344 rats, transient evoked otoacoustic emissions were not measurable and distortion product otoacoustic emissions could be detected within a frequency range of 2.4-6.3 kHz. Tympanometric measurements did not reveal any differences in middle ear parameters between F344 and LE rats. The amplitudes of click-evoked ABRs were significantly lower in 6-month-old F344 rats than in LE rats, but other parameters of the ABRs were almost identical in both rat strains. The results demonstrate a significant deficit in low-frequency hearing and altered otoacoustic emissions in both young and adult F344 rats, suggesting a defect of the inner ear sensory epithelium at the apical part of the cochlea.

Collagen changes in the cochlea of aged Fischer 344 rats.

Hearing function in the Fischer 344 (F344) albino inbred strain of rats deteriorates with aging faster than in other strains, in spite of the small hair cell loss in old F344 animals [Popelar, J., Groh, D., Pelanova, J., Canlon, B., Syka, J., 2005. Age-related changes in cochlear and brainstem auditory function. Neurobiol. Aging, in press.]. This study was aimed at elucidating the structural changes in the inner ear of this rat strain during aging. Cochlear histopathology was examined in 20-24-month-old F344 rats and compared with that of young F344 rats (4 months) and of old rats of the Long-Evans (LE) strain. Hematoxylin/eosin staining in aged F344 rats showed degenerative changes in the organ of Corti, consisting of a damaged layer of marginal cells, reduced vascularization of the stria vascularis and a distorted tectorial membrane detached from the organ of Corti. Age-related changes in collagen distribution were observed with Massons trichrome staining in the spiral ligament of old F344 rats. The results of immunohistochemical staining for type II collagen revealed a marked decrease in collagen fibers in the area connecting the spiral ligament and stria vascularis and a decrease in area IV fibrocytes in old F344 but not in LE rats. These findings may contribute to an explanation of the substantial hearing loss found in old F344 rats.

Effects of electrical stimulation of the inferior colliculus on 2f1−f2 distortion product otoacoustic emissions in anesthetized guinea pigs

The effects of electrical stimulation of the inferior colliculus (IC) on the activation of olivocochlear nerve fibers were investigated in guinea pigs in which the 2f1-f2 distortion product otoacoustic emissions (DPOAE) were recorded. Animals were anesthetized with ketamine (33 mg/kg) and xylazine (6.6 mg/kg). Bipolar electrical stimulation of the IC by a train of pulses with currents less than the threshold for evoking muscle twitches resulted in a small depression of the DPOAE amplitude by 0.1-2 dB. The maximal effect was observed when the stimulating electrodes were located in the rostro-medial or ventral parts of the IC. The suppression of electrically evoked DPOAE was similar to the DPOAE suppression produced by acoustical stimulation of the contralateral ear by a broad-band noise. Suppression of DPOAE amplitude in response to both acoustical and electrical stimulation was abolished 1-2 h after a single intramuscular injection of gentamicin (210-250 mg/kg). The results indicate that electrical stimulation of the IC can activate the efferent system and produce DPOAE changes by similar mechanisms as does acoustical stimulation of the contralateral ear.