Altered peripheral and brainstem auditory function in aged rats

The role of the Ventral Nucleus of the Trapezoid Body in the auditory prepulse inhibition of the acoustic startle reflex

Cholinergic signaling is essential to mediate the auditory prepulse inhibition (PPI), an operational measure of sensorimotor gating, that refers to the reduction of the acoustic startle reflex (ASR) when a low-intensity, non-startling acoustic stimulus (the prepulse) is presented just before the onset of the acoustic startle stimulus. The cochlear root neurons (CRNs) are the first cells of the ASR circuit to receive cholinergic inputs from non-olivocochlear neurons of the ventral nucleus of the trapezoid body (VNTB) and subsequently decrease their neuronal activity in response to auditory prepulses. Yet, the contribution of the VNTB-CRNs pathway to the mediation of PPI has not been fully elucidated. In this study, we used the immunotoxin anti-choline acetyltransferase (ChAT)-saporin as well as electrolytic lesions of the medial olivocochlear bundle to selectively eliminate cholinergic VNTB neurons, and then assessed the ASR and PPI paradigms. Retrograde track-tracing experiments were conducted to precisely determine the site of lesioning VNTB neurons projecting to the CRNs. Additionally, the effects of VNTB lesions and the integrity of the auditory pathway were evaluated via auditory brain responses tests, ChAT- and FOS-immunohistochemistry. Consequently, we established three experimental groups: 1) intact control rats (non-lesioned), 2) rats with bilateral lesions of the olivocochlear bundle (OCB-lesioned), and 3) rats with bilateral immunolesions affecting both the olivocochlear bundle and the VNTB (OCB/VNTB-lesioned). All experimental groups underwent ASR and PPI tests at several interstimulus intervals before the lesion and 7, 14, and 21 days after it. Our results show that the ASR amplitude remained unaffected both before and after the lesion across all experimental groups, suggesting that the VNTB does not contribute to the ASR. The%PPI increased across the time points of evaluation in the control and OCB-lesioned groups but not in the OCB/VNTB-lesioned group. At the ISI of 50 ms, the OCB-lesioned group exhibited a significant increase in%PPI (p < 0.01), which did not occur in the OCB/VNTB-lesioned group. Therefore, the ablation of cholinergic non-olivocochlear neurons in the OCB/VNTB-lesioned group suggests that these neurons contribute to the mediation of auditory PPI at the 50 ms ISI through their cholinergic projections to CRNs. Our study strongly reinforces the notion that auditory PPI encompasses a complex mechanism of top-down cholinergic modulation, effectively attenuating the ASR across different interstimulus intervals within multiple pathways.

Objective Detection of Tinnitus Based on Electrophysiology

Tinnitus, a common disease in the clinic, is associated with persistent pain and high costs to society. Several aspects of tinnitus, such as the pathophysiology mechanism, effective treatment, objective detection, etc., have not been elucidated. Any change in the auditory pathway can lead to tinnitus. At present, there is no clear and unified mechanism to explain tinnitus, and the hypotheses regarding its mechanism include auditory plasticity theory, cortical reorganization theory, dorsal cochlear nucleus hypothesis, etc. Current theories on the mechanism of tinnitus mainly focus on the abnormal activity of the central nervous system. Unfortunately, there is currently a lack of objective diagnostic methods for tinnitus. Developing a method that can detect tinnitus objectively is crucial, only in this way can we identify whether the patient really suffers from tinnitus in the case of cognitive impairment or medical disputes and the therapeutic effect of tinnitus. Electrophysiological investigations have prompted the development of an objective detection of tinnitus by potentials recorded in the auditory pathway. However, there is no objective indicator with sufficient sensitivity and specificity to diagnose tinnitus at present. Based on recent findings of studies with various methods, possible electrophysiological approaches to detect the presence of tinnitus have been summarized. We analyze the change of neural activity throughout the auditory pathway in tinnitus subjects and in patients with tinnitus of varying severity to find available parameters in these methods, which is helpful to further explore the feasibility of using electrophysiological methods for the objective detection of tinnitus.

Sound-Evoked Responses in the Vestibulo-Ocular Reflex Pathways of Rats

Vestibular evoked myogenic potentials (VEMP) have been used to assess otolith function in clinics worldwide. However, there are accumulating evidence suggesting that the clinically used sound stimuli activate not only the otolith afferents, but also the canal afferents, indicating canal contributions to the VEMPs. To better understand the neural mechanisms underlying the VEMPs and develop discriminative VEMP protocols, we further examined sound-evoked responses of the vestibular nucleus neurons and the abducens neurons, which have the interneurons and motoneurons of the vestibulo-ocular reflex (VOR) pathways. Air-conducted clicks (50–80 dB SL re ABR threshold, 0.1 ms duration) or tone bursts (60–80 dB SL, 125–4,000 Hz, 8 ms plateau, 1 ms rise/fall) were delivered to the ears of Sprague-Dawley or Long-Evans rats. Among 425 vestibular nucleus neurons recorded in anesthetized rats and 18 abducens neurons recorded in awake rats, sound activated 35.9% of the vestibular neurons that increased discharge rates for ipsilateral head rotation (Type I neuron), 15.7% of the vestibular neurons that increased discharge rates for contralateral head rotation (Type II neuron), 57.2% of the vestibular neurons that did not change discharge rates during head rotation (non-canal neuron), and 38.9% of the abducens neurons. Sound sensitive vestibular nucleus neurons and abducens neurons exhibited characteristic tuning curves that reflected convergence of canal and otolith inputs in the VOR pathways. Tone bursts also evoked well-defined eye movements that increased with tone intensity and duration and exhibited peak frequency of ∼1,500 Hz. For the left eye, tone bursts evoked upward/rightward eye movements for ipsilateral stimulation, and downward/leftward eye movements for contralateral stimulation. These results demonstrate that sound stimulation results in activation of the canal and otolith VOR pathways that can be measured by eye tracking devices to develop discriminative tests of vestibular function in animal models and in humans.

Longitudinal auditory pathophysiology following mild blast-induced trauma

Blast-induced hearing difficulties affect thousands of veterans and civilians. The long-term impact of even a mild blast exposure on the central auditory system is hypothesized to contribute to lasting behavioral complaints associated with mild blast traumatic brain injury (bTBI). Although recovery from mild blast has been studied separately over brief or long time windows, few, if any, studies have investigated recovery longitudinally over short-term and longer-term (months) time windows. Specifically, many peripheral measures of auditory function either recover or exhibit subclinical deficits, masking deficits in processing complex, real-world stimuli that may recover differently. Thus, examining the acute time course and pattern of neurophysiological impairment using appropriate stimuli is critical to better understanding and intervening in bTBI-induced auditory system impairments. Here, we compared auditory brainstem response, middle-latency auditory-evoked potentials, and envelope following responses. Stimuli were clicks, tone pips, amplitude-modulated tones in quiet and in noise, and speech-like stimuli (iterated rippled noise pitch contours) in adult male rats subjected to mild blast and sham exposure over the course of 2 mo. We found that blast animals demonstrated drastic threshold increases and auditory transmission deficits immediately after blast exposure, followed by substantial recovery during the window of 7-14 days postblast, although with some deficits remaining even after 2 mo. Challenging conditions and speech-like stimuli can better elucidate mild bTBI-induced auditory deficit during this period. Our results suggest multiphasic recovery and therefore potentially different time windows for treatment, and deficits can be best observed using a small battery of sound stimuli.NEW & NOTEWORTHY Few studies on blast-induced hearing deficits go beyond simple sounds and sparsely track postexposure. Therefore, the recovery arc for potential therapies and real-world listening is poorly understood. Evidence suggested multiple recovery phases over 2 mo postexposure. Hearing thresholds largely recovered within 14 days and partially explained recovery. However, midlatency responses, responses to amplitude modulation in noise, and speech-like pitch sweeps exhibited extended changes, implying persistent central auditory deficits and the importance of subclinical threshold shifts.

Ageing and multisensory integration: A review of the evidence, and a computational perspective

The processing of multisensory signals is crucial for effective interaction with the environment, but our ability to perform this vital function changes as we age. In the first part of this review, we summarise existing research into the effects of healthy ageing on multisensory integration. We note that age differences vary substantially with the paradigms and stimuli used: older adults often receive at least as much benefit (to both accuracy and response times) as younger controls from congruent multisensory stimuli, but are also consistently more negatively impacted by the presence of intersensory conflict. In the second part, we outline a normative Bayesian framework that provides a principled and computationally informed perspective on the key ingredients involved in multisensory perception, and how these are affected by ageing. Applying this framework to the existing literature, we conclude that changes to sensory reliability, prior expectations (together with attentional control), and decisional strategies all contribute to the age differences observed. However, we find no compelling evidence of any age-related changes to the basic inference mechanisms involved in multisensory perception.

Vestibular short-latency evoked potential abolished by low-frequency noise exposure in rats

The vestibular system plays a critical role in detection of head movements and is essential for normal postural control. Because of their anatomical proximity to the cochlea, the otolith organs are selectively exposed to sound pressure and are at risk for noise overstimulation. Clinical reports suggest a link between noise exposure and balance problems, but the structural and physiological basis for this linkage is not well understood. The goal of this study was to determine the effects of low-frequency noise (LFN) on the otolith organs by correlating changes in vestibular short-latency evoked potentials (VsEPs) with changes in saccular afferent endings following noise exposure. LFN exposure transiently abolished the VsEP and reduced the number of stained calyces within the sacculus. Although some recovery of the VsEP waveform could be observed within 3 days after noise, at 3 wk recovery was only partial in most animals, consistent with a reduced number of afferents with calyceal endings. These data show that a single intense noise exposure is capable of causing a vestibular deficit that appears to mirror the synaptic deficit associated with hidden hearing loss after noise-induced cochlear injury. NEW & NOTEWORTHY This is the first study to explore the effects of low-frequency high-intensity noise on vestibular short-latency evoked potential (VsEP) responses, which shows a linkage between attenuated noise-induced VsEPs and pathological changes to otolith organ afferents. This finding suggests a potential limitation of the VsEP for evaluation of vestibular dysfunction, since the VsEP measurement may assess the activity of a specific class rather than all afferents.

Hearing assessment during deep brain stimulation of the central nucleus of the inferior colliculus and dentate cerebellar nucleus in rat

BACKGROUND

Recently it has been shown in animal studies that deep brain stimulation (DBS) of auditory structures was able to reduce tinnitus-like behavior. However, the question arises whether hearing might be impaired when interfering in auditory-related network loops with DBS.

METHODS

The auditory brainstem response (ABR) was measured in rats during high frequency stimulation (HFS) and low frequency stimulation (LFS) in the central nucleus of the inferior colliculus (CIC, n = 5) or dentate cerebellar nucleus (DCBN, n = 5). Besides hearing thresholds using ABR, relative measures of latency and amplitude can be extracted from the ABR. In this study ABR thresholds, interpeak latencies (I-III, III-V, I-V) and V/I amplitude ratio were measured during off-stimulation state and during LFS and HFS.

RESULTS

In both the CIC and the CNBN groups, no significant differences were observed for all outcome measures.

DISCUSSION

DBS in both the CIC and the CNBN did not have adverse effects on hearing measurements. These findings suggest that DBS does not hamper physiological processing in the auditory circuitry.

The impact of aging, hearing loss, and body weight on mouse hippocampal redox state, measured in brain slices using fluorescence imaging

The relationships between oxidative stress in the hippocampus and other aging-related changes such as hearing loss, cortical thinning, or changes in body weight are not yet known. We measured the redox ratio in a number of neural structures in brain slices taken from young and aged mice. Hearing thresholds, body weight, and cortical thickness were also measured. We found striking aging-related increases in the redox ratio that were isolated to the stratum pyramidale, while such changes were not observed in thalamus or cortex. These changes were driven primarily by changes in flavin adenine dinucleotide, not nicotinamide adenine dinucleotide hydride. Multiple regression analysis suggested that neither hearing threshold nor cortical thickness independently contributed to this change in hippocampal redox ratio. However, body weight did independently contribute to predicted changes in hippocampal redox ratio. These data suggest that aging-related changes in hippocampal redox ratio are not a general reflection of overall brain oxidative state but are highly localized, while still being related to at least one marker of late aging, weight loss at the end of life.

Effects of high intensity noise on the vestibular system in rats

Some individuals with noise-induced hearing loss (NIHL) also report balance problems. These accompanying vestibular complaints are not well understood. The present study used a rat model to examine the effects of noise exposure on the vestibular system. Rats were exposed to continuous broadband white noise (0-24 kHz) at an intensity of 116 dB sound pressure level (SPL) via insert ear phones in one ear for three hours under isoflurane anesthesia. Seven days after the exposure, a significant increase in ABR threshold (43.3 ± 1.9 dB) was observed in the noise-exposed ears, indicating hearing loss. Effects of noise exposure on vestibular function were assessed by three approaches. First, fluorescein-conjugated phalloidin staining was used to assess vestibular stereocilia following noise exposure. This analysis revealed substantial sensory stereocilia bundle loss in the saccular and utricular maculae as well as in the anterior and horizontal semicircular canal cristae, but not in the posterior semicircular canal cristae. Second, single unit recording of vestibular afferent activity was performed under pentobarbital anesthesia. A total of 548 afferents were recorded from 10 noise-treated rats and 12 control rats. Noise exposure produced a moderate reduction in baseline firing rates of regular otolith afferents and anterior semicircular canal afferents. Also a moderate change was noted in the gain and phase of the horizontal and anterior semicircular canal afferent's response to sinusoidal head rotation (1 and 2 Hz, 45°/s peak velocity). Third, noise exposure did not result in significant changes in gain or phase of the horizontal rotational and translational vestibulo-ocular reflex (VOR). These results suggest that noise exposure not only causes hearing loss, but also causes substantial damage in the peripheral vestibular system in the absence of immediate clinically measurable vestibular signs. These peripheral deficits, however, may lead to vestibular disorders over time.

Hearing impairment in the P23H-1 retinal degeneration rat model

The transgenic P23H line 1 (P23H-1) rat expresses a variant of rhodopsin with a mutation that leads to loss of visual function. This rat strain is an experimental model usually employed to study photoreceptor degeneration. Although the mutated protein should not interfere with other sensory functions, observing severe loss of auditory reflexes in response to natural sounds led us to study auditory brain response (ABR) recording. Animals were separated into different hearing levels following the response to natural stimuli (hand clapping and kissing sounds). Of all the analyzed animals, 25.9% presented auditory loss before 50 days of age (P50) and 45% were totally deaf by P200. ABR recordings showed that all the rats had a higher hearing threshold than the control Sprague-Dawley (SD) rats, which was also higher than any other rat strains. The integrity of the central and peripheral auditory pathway was analyzed by histology and immunocytochemistry. In the cochlear nucleus (CN), statistical differences were found between SD and P23H-1 rats in VGluT1 distribution, but none were found when labeling all the CN synapses with anti-Syntaxin. This finding suggests anatomical and/or molecular abnormalities in the auditory downstream pathway. The inner ear of the hypoacusic P23H-1 rats showed several anatomical defects, including loss and disruption of hair cells and spiral ganglion neurons. All these results can explain, at least in part, how hearing impairment can occur in a high percentage of P23H-1 rats. P23H-1 rats may be considered an experimental model with visual and auditory dysfunctions in future research.

Input-output functions of vestibular afferent responses to air-conducted clicks in rats

Sound-evoked vestibular myogenic potentials recorded from the sternocleidomastoid muscles (the cervical vestibular-evoked myogenic potential or cVEMP) and the extraocular muscles (the ocular VEMP or oVEMP) have proven useful in clinical assessment of vestibular function. VEMPs are commonly interpreted as a test of saccular function, based on neurophysiological evidence showing activation of saccular afferents by intense acoustic click stimuli. However, recent neurophysiological studies suggest that the clicks used in clinical VEMP tests activate vestibular end organs other than the saccule. To provide the neural basis for interpreting clinical VEMP testing results, the present study examined the extent to which air-conducted clicks differentially activate the various vestibular end organs at several intensities and durations in Sprague-Dawley rats. Single unit recordings were made from 562 vestibular afferents that innervated the otoliths [inferior branch otolith (IO) and superior branch otolith (SO)], the anterior canal (AC), the horizontal canal (HC), and the posterior canal (PC). Clicks higher than 60 dB SL (re-auditory brainstem response threshold) activated both semicircular canal and otolith organ afferents. Clicks at or below 60 dB SL, however, activated only otolith organ afferents. Longer duration clicks evoked larger responses in AC, HC, and SO afferents, but not in IO afferents. Intra-axonal recording and labeling confirmed that sound sensitive vestibular afferents innervated the horizontal and anterior canal cristae as well as the saccular and utricular maculae. Interestingly, all sound sensitive afferents are calyx-bearing fibers. These results demonstrate stimulus-dependent acoustic activation of both semicircular canals and otolith organs, and suggest that sound activation of vestibular end organs other than the saccule should not be ruled out when designing and interpreting clinical VEMP tests.

Age-related changes in the distribution of Kv1.1 and Kv3.1 in rat cochlear nuclei

OBJECTIVES

To identify age-related changes in voltage-gated K(+) (Kv) channels that contribute to temporal processing in neurons of the central auditory system, we investigated the distribution of Kv1.1 and Kv3.1 in the auditory brainstem of adult and aged rats.

METHODS

Immunohistochemistry was performed in accordance with the free-floating method described earlier.

RESULTS

Among the auditory nuclei, only the posterior ventral cochlear nucleus (PVCN) showed age-related changes. Kv1.1 immunoreactivity was increased in the octopus cell bodies, while the staining intensity was significantly decreased in the neuropil. Image analysis demonstrated the specific increase in Kv1.1 immunoreactivity in aged cochlear nucleus neurons although the mean density of the entire selection was significantly decreased. In contrast, the number of Kv1.1-immunoreactive neurons was not significantly different between control and aged groups. The immunoreactivity for Kv3.1 was decreased in the octopus cells and neuropil of aged PVCN, which was confirmed by image analysis. The number of Kv3.1-positive cells was also significantly decreased in aged PVCN.

DISCUSSION

This study may provide useful data to compare age-related changes in Kv1.1 and Kv3.1 with known physiological properties of auditory neurons.

Dopamine from cirrhotic liver contributes to the impaired learning and memory ability of hippocampus in minimal hepatic encephalopathy

Background Defective learning/memory ability is a feature of MHE. However, the exact pathophysiological mechanisms leading to the impairment of learning/memory ability in MHE remain not clearly understood. Methods MHE rat modeling by intraperitoneal injection of TAA was successfully established using a Morris water maze, BAEP, and EEG tests. COMT inhibitor, a protein involved in the accumulation of dopamine (DA), was found to be up-regulated in cirrhotic livers in MHE by 2-DE/MS. Results The levels of DA in cirrhotic livers, serums and hippocampuses in the MHE group were more significantly increased than in the control group. In the hippocampuses of MHE rats, NMDA-induced formation of cGMP was reduced by 40 % as determined by in vivo brain microdialysis. Activation of sGC by NO was reduced by 38 %. The expression of NMDAR1, CaM, nNOS and sGC in the hippocampus in the MHE group were more significantly decreased than in controls. Chronic exposure of cultured hippocampus neurons to DA (50 μM) reduced by 53 % the NMDA-induced formation of cGMP. Activation of sGC by NO in these neurons was reduced by 44 %. Down-regulated NMDAR1, CaM, nNOS and sGC were also detected in neurons treated with dopamine, in contrast with the controls. Conclusions This study suggests that when the glutamate-NO-cGMP pathway in the hippocampus is inhibited by the elevation of DA from cirrhotic livers, this in turn may lead to the impairment of learning and memory ability of MHE.

Normal variations in the morphology of auditory brainstem response (ABR) waveforms: a study in Wistar rats

Auditory brainstem evoked responses (ABR) have been used for decades to assess auditory function. Surprisingly, despite the fact that rats are one of the most widely used experimental models in hearing, there have been no studies that have characterized in detail the normal morphological variations that occur in ABR waves. Therefore, the goal of this study was to characterize the patterns of ABR waves in rats to establish baseline criteria that could be used to identify abnormalities. Rats were stimulated with pure tone sounds at different frequencies and ABR waves were classified based on morphology. The most definitive finding was that, unlike what is observed in human ABRs, wave II of the rat ABR was the most prominent. Additionally, wave III was the smallest and, in many cases, was not apparent at low frequencies. Wave III was frequently involved in the formation of complexes, often appearing as a small wave or adjoining primarily wave IV. Complexes were common at low and medium frequencies and rare at high frequencies. These results indicate that knowledge of the different wave patterns in normal rats is fundamental to understanding how the wave morphology changes in pathological conditions that could lead to hearing impairment.

Click-evoked responses in vestibular afferents in rats

Sound activates not only the cochlea but also the vestibular end organs. Research on this phenomenon led to the discovery of the sound-evoked vestibular myogenic potentials recorded from the sternocleidomastoid muscles (cervical VEMP, or cVEMP). Since the cVEMP offers simplicity and the ability to stimulate each labyrinth separately, its values as a test of human vestibular function are widely recognized. Currently, the cVEMP is interpreted as a test of saccule function based on the assumption that clicks primarily activate the saccule. However, sound activation of vestibular end organs other than the saccule has been reported. To provide the neural basis for interpreting clinical VEMP testing, we employed the broadband clicks used in clinical VEMP testing to examine the sound-evoked responses in a large sample of vestibular afferents in Sprague-Dawley rats. Recordings were made from 924 vestibular afferents from 106 rats: 255 from the anterior canal (AC), 202 from the horizontal canal (HC), 177 from the posterior canal (PC), 207 from the superior vestibular nerve otolith (SO), and 83 from the inferior nerve otolith (IO). Sound sensitivity of each afferent was quantified by computing the cumulative probability of evoking a spike (CPE). We found that clicks activated irregular afferents (normalized coefficient of variation of interspike intervals >0.2) from both the otoliths (81%) and the canals (43%). The order of end organ sound sensitivity was SO = IO > AC > HC > PC. Since the sternocleidomastoid motoneurons receive inputs from both the otoliths and the canals, these results provide evidence of a possible contribution from both of them to the click-evoked cVEMP.

BDNF mRNA expression and protein localization are changed in age-related hearing loss

A decline in neuronal plasticity during the adult life span has been proposed to be associated with a reduced level of the effectors of plasticity responses (e.g., BDNF). Alteration of plasticity is also correlated with age-related hearing loss (presbycusis), but to date no detailed studies of BDNF expression have been performed in the young or aging mature cochlea. We have used rat and gerbil animal models for presbycusis, which displayed hearing loss in the final third of the animals' natural life span. We demonstrate for the first time a co-localization of BDNF protein, transcripts III and IV in cochlear neurons with a declining distribution towards low-frequency processing cochlear turns. BDNF protein was also found within the neuronal projections of the cochlea. A significant reduction of BDNF transcripts in high-frequency processing cochlear neurons was observed during aging, though this did not coincide with a major reduction of BDNF protein. In contrast, BDNF protein in peripheral and central projections was drastically reduced. Our results suggest that reduced BDNF protein levels in auditory nerves over age may be a crucial factor in the altered brainstem plasticity observed during presbycusis.

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).

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.

Age-related synaptic changes in the anteroventral cochlear nucleus of Fischer-344 rats

Previous studies have demonstrated age-related decreases in the transmitters glycine and glutamate in the cochlear nucleus (CN) of the Fischer-344 (F344) rat, along with declining levels of binding for glycine receptors. The purpose of this study was to evaluate structural correlates to the transmitter and receptor losses that accompany aging in the anteroventral CN (AVCN). Thin sections were obtained from the middle-frequency area of the right AVCNs from five 3-month-, four 19-month-, and five 28-month-old F344 rats. Montages were constructed from electron micrographs taken of several sites in each AVCN section. The presynaptic terminals were classified by vesicle type and postsynaptic target, and their perimeters and synaptic lengths were traced using morphometry software. The calibers of all dendritic profiles were also measured, and cell counts were performed on semi-thin sections. The data were compared among the three age groups using analysis of variance followed by Tukey's Honestly Significant Difference for pairwise comparisons. There were significant age-related decreases in the size of terminals contacting small-caliber (<2 microm) dendrites. Dendrites of this size comprised the largest percentage of dendrites in the AVCN. On these targets, round and pleomorphic-vesicle terminals were reduced in volume by nearly 44% and 24%, respectively, in 28-month olds when compared to the 3-month olds. On the other hand, the densities and numbers of synaptic terminals and dendritic profiles did not differ among age groups, and no neuronal losses were evident in the older animals. Also, there were no detectable changes in synaptic area among groups. The decrease in terminal size may be related to age-associated reductions in neurotransmitter levels previously described in the F344 CN. The observations presented here contrast with those previously described in the inferior colliculus (IC), in which there were significant age-related losses of synaptic terminals and dendrites, but no change in the size of synaptic terminals. The lack of synaptic and dendritic losses suggests that the structural connectivity of the rat AVCN remains relatively intact during aging, which is interesting in light of the synaptic and dendritic changes evident in the IC, a major target of its projections.

Susceptibility of young adult and old rats to noise-induced hearing loss

This study was designed to test whether old rats show signs of presbyacusis and whether they would be either similarly or more or less susceptible to noise-induced hearing loss than young adult rats. Old (24 months) and young adult (3-4 months) Wistar rats were exposed to a broad-band noise of 113 dB SPL for a duration of 1 h (producing temporary threshold shifts) or 3 days (12 h noise/12 h quiet; permanent shifts). Auditory brainstem response (ABR), distortion product otoacoustic emissions (DPOAEs) and transient evoked otoacoustic emissions (TEOAEs) were measured before and after exposure. At the initial recording (before exposure), old rats demonstrated a small mean ABR threshold elevation, a reduction in amplitude of wave I (WI), a shortening of WI latency and a prolongation of the interpeak interval between WI and WIV, as compared to the young rats. The old rats also demonstrated a small DPOAE amplitude reduction and a reduction of peak-to-peak amplitude in the TEOAE response 2 ms after stimulus, but no reduction in TEOAE energy content between 2 and 4 kHz. These are signs of presbyacusis in the old rats. The noise exposures caused elevations in ABR threshold and reductions in DPOAE amplitude and TEOAE energy content that were similar in both the old and young rats. Their recovery from the noise-induced loss was also similar. Thus, the results of this study show that old and young adult rats, at least when considering clinically relevant intensities and durations of noise exposure, are equally susceptible to the effects of the exposure.

Presbyacusis and the auditory brainstem response

Age-related hearing loss (ARHL or presbyacusis) is an increasingly common form of sensorineural hearing loss (SNHL) as a result of changing demographics, and the auditory brainstem response (ABR) is a common experimental and clinical tool in audiology and neurology. Some of the changes that occur in the aging auditory system may significantly influence the interpretation of the ABR in comparison to the ABRs of younger adults. The approach of this review will be to integrate physiological and histopathological data from human and animal studies to provide a better understanding of the array of age-related changes in the ABR and to determine how age-related changes in the auditory system may influence how the ABR should be interpreted in presbyacusis. Data will be described in terms of thresholds, latencies, and amplitudes, as well as more complex auditory functions such as masking and temporal processing. Included in the review of data will be an attempt to differentiate between age-related effects that may strictly be due to threshold elevation from those that may be due to the aging process.

Plastic changes in the central auditory system after hearing loss, restoration of function, and during learning

Traditionally the auditory system was considered a hard-wired sensory system; this view has been challenged in recent years in light of the plasticity of other sensory systems, particularly the visual and somatosensory systems. Practical experience in clinical audiology together with the use of prosthetic devices, such as cochlear implants, contributed significantly to the present view on the plasticity of the central auditory system, which was originally based on data obtained in animal experiments. The loss of auditory receptors, the hair cells, results in profound changes in the structure and function of the central auditory system, typically demonstrated by a reorganization of the projection maps in the auditory cortex. These plastic changes occur not only as a consequence of mechanical lesions of the cochlea or biochemical lesions of the hair cells by ototoxic drugs, but also as a consequence of the loss of hair cells in connection with aging or noise exposure. In light of the aging world population and the increasing amount of noise in the modern world, understanding the plasticity of the central auditory system has its practical consequences and urgency. In most of these situations, a common denominator of central plastic changes is a deterioration of inhibition in the subcortical auditory nuclei and the auditory cortex. In addition to the processes that are elicited by decreased or lost receptor function, the function of nerve cells in the adult central auditory system may dynamically change in the process of learning. A better understanding of the plastic changes in the central auditory system after sensory deafferentation, sensory stimulation, and learning may contribute significantly to improvement in the rehabilitation of damaged or lost auditory function and consequently to improved speech processing and production.

Molecular mechanisms of age-related hearing loss

Age-related hearing loss, known as presbyacusis, is characterized by the progressive deterioration of auditory sensitivity associated with aging and is the most common cause of adult auditory deficiency in the United States. Presbyacusis is defined as a progressive, bilateral, high-frequency hearing loss that is manifested on audiometric assessment by a moderately sloping pure tone audiogram. This condition affects approximately 23% of the population between 65 and 75 years of age and 40% of the population older than 75 years of age. In 1980, it was estimated that 11% of the population was 76 years or older and this number is expected to nearly double by the year 2030 [, Otolaryngol. Head Neck Surg. 100, 262]. When coupled with the fact that the population over 65 years of age is experiencing the most rapid progression of hearing loss, the potential socioeconomic ramifications are staggering. Interestingly, presbyacusis varies in its frequency across differing societies. This discrepancy has been attributed to many factors such as genetics, diet, socioeconomic factors, and environmental variables [, Otolaryngol. Head Neck Surg. 100, 266;. Scand. Audiol. 26 (1997) 133]. The purpose of this discussion is to illuminate the various molecular mechanisms underlying this age-related hearing loss and to offer insights into potential ways to mitigate the effects of aging on hearing impairment.

Regeneration and recovery of the hearing function of the central auditory pathway by transplants of embryonic brain tissue in adult rats

The present study is the first report of successful regeneration and recovery of hearing function of the central auditory pathway after transection in the adult rat. The ventral cochlear tract in the brain stem to pons was transected on one side in adult rats. Tissue from embryos (E14 to E16) was used to cover the lesion site. In 30% of the rats examined, the axons regrew beyond the transected site and regenerated into the denervated side and terminated at the normal targets. The hearing function of rats was elucidated by recording the auditory brain stem response (ABR). Rats with successful regeneration showed nearly normal ABR. In rats receiving simple transection without covering embryonic tissue, there was no regeneration and hearing function did not recover. Thus, the present findings contradict the widely held view that the adult mammalian central auditory system cannot be restored following damage.

Age-related hearing changes and effects of exotoxin on inner ear function in aging rat. A frequency-specific auditory brainstem response study

Frequency-specific auditory brainstem responses to tone bursts (2-31.5 kHz) and tuning curves were recorded in 8 male rats during their entire life. No significant threshold elevation occurred during the 1st year. Mean age at onset of hearing loss was 20 months, with individual variations ranging from 14 up to 23 months. In most animals, threshold elevations were limited to 20 and 31.5 kHz. At age of 18-23 months, 3 rats were treated with Pseudomonas aeruginosa exotoxin A. After instillation, threshold elevation occurred at all frequencies, followed by near-total recovery at low frequencies, but only partial reversibility at high frequencies. It is concluded that aging-related hearing changes in Sprague-Dawley albino rats start at different ages, particularly comprising high frequencies and encroaching on the middle and low areas with increasing age.

Feline immunodeficiency virus envelope protein (FIVgp120) causes electrophysiological alterations in rats

Close to 20% of the patients infected with the AIDS virus develops neurological deficit; eventhough HIV does not invade neurons. Consistently with the neurological deficit, HIV(+) subjects show abnormalities in brainstem auditory and visual evoked potentials (BSAEP and VEP) and in sleep patterns. The HIV-derived glycoprotein 120 has been postulated as a neurotoxic; therefore, it may be playing a crucial role in the generation of BSAEP and VEP, as well as in sleep disturbances. To study the role of the virus-derived proteins on the development of these electrophysiological signals' alterations, we have used the feline immunodeficiency virus (FIV)-derived gp120 and evaluated the changes in these electrophysiological signals. We employed 15 adult male Sprague-Dawley rats (250-350 g), chronically implanted for evoked potential and sleep recordings. Results showed that the i.c.v. administration of FIVgp120 (5 ng/10 microliter) produces changes in the latency of both cortical auditory evoked potentials (CAEPs) and VEPs and a decrease in both REM sleep and SWS. These data support the notion that FIVgp120 is neurotoxic to the central nervous system of cats and rats and that this protein suffices to cause electrophysiological alterations. In addition, it suggests that a similar effect may be occurring in humans as a result of HIVgp120's neurotoxic effects.

Age-related changes in the brainstem auditory evoked potentials of the marmoset

Changes in brain-stem auditory evoked potentials (BAEPs) with age were recorded in common marmosets (Callithrix jacchus) at the age of 1-2, 6-8 and 10-12 years. The auditory function was assessed by thresholds, latencies and amplitudes of BAEPs evoked by use of tone burst stimulations with audible frequencies ranging from 1 to 99 kHz. Prolongation of the latencies of later waves was observed in the animals at the age of 6-8 and 10-12 years at high frequencies, suggesting that aging in marmosets, as reported previously in humans and other animals, may cause earlier hearing loss at high frequency than at low frequency within the hearing range. At 10-12 years of age, the elevations of BAEP thresholds and the declines of BAEP amplitudes in older animals were also observed. As the differences in the parameters are small, it was suggested that only a moderate hearing loss occurred with onset late in life in common marmoset similar to that in CBA/Ca mice. Based on the results obtained in this study, BAEP latencies appear to be more sensitive indicators than BAEP thresholds and amplitudes for the early detection of hearing impairment.

Estrogen treatment and age effects on auditory brainstem responses in the post-breeding Long-Evans rat

The auditory brainstem response (ABR) was recorded from 20-month-old Long Evans hooded female rats to determine if latency reductions occur from estrogen replacement. The ABR in these post-breeding age rats was also examined for reductions in response latencies as a function of adult age. Tone pip stimuli (8 and 40 kHz) were presented at 21, 51, or 81 s(-1). Aging control and ovariectomized animals showed slower response latencies for waves Ib-VI than young adults for 8 and 40 kHz stimulation at 21 s(-1). Increased stimulus rate resulted in longer latencies for all waves at 20 months. In contrast to hormone treatment effects in young adults, ABR latencies in post-breeding age estrogen-treated animals were not reduced, consistent with a general decrease in CNS responsiveness to estrogen steroids associated with age. The results also suggest that sensorineural modifications in the auditory system which prolong ABR latencies can occur early in the aging process of adult female subjects.

Age-related glycine receptor subunit changes in the cochlear nucleus of Fischer-344 rats

Previous studies have shown that levels of binding for the strychnine-sensitive glycine receptor in the cochlear nucleus (CN) of Fischer (F344) rats decrease with age. Given the major role glycine plays in normal CN function, changes in glycine-receptor activity may contribute to central presbycusis. To further evaluate the impact of age on glycine receptors, in situ hybridization was used to assess, in three age groups of F344 rats, changes in levels of gene expression for four of its subunits. When compared with the 3-month-old rats, expression of mRNAs for alpha1 and beta subunits in the anteroventral CN decreased significantly in the 18- and 27-month-old age groups, while mRNA expression for the alpha2 subunit increased. If protein expressions are similar, these subunit changes may alter the function of glycine receptors, thereby affecting binding to its ligands.

Age-related changes in the compound action potentials of the eighth nerve in guinea pigs

The eighth nerve compound action potential (CAP) in 95 guinea pigs was measured using click stimuli to investigate age-related changes in their neural auditory thresholds. The animals were separated into three groups: group A (n = 43, 86 ears; 2-4 months old); group B (n = 29; 58 ears, 13-15 months old); and group C (n = 23; 46 ears, 23-25 months old). With increasing age, a gradual elevation of CAP thresholds was clearly seen among the three groups. The negative peak (N1) latencies of the CAP were prolonged, and the N1 amplitudes of the CAP decreased. There were significant differences in N1 latencies among the three groups and in N1 amplitudes between groups A and B, and between groups A and C. However, the rate of decline of the thresholds as well as the input-output function curves of the CAP varied in some of the oldest animals, suggesting that there were some individual differences in degenerative aging processes of the auditory system.

Midlatency auditory evoked potentials and the startle response in the rat

The P13 midlatency auditory evoked potential in the rat is (i) sleep state dependent, (ii) undergoes rapid habituation and (iii) is blocked by the cholinergic antagonist scopolamine. As such, the P13 potential in the rat shows the same characteristics as the P1 (or P50) potential in the human. These potentials are thought to be mediated, at least in part, by the cholinergic arm of the reticular activating system. Previous studies have linked the reticular activating system with the startle response. The present study was undertaken to explore this relationship by simultaneously recording the P13 potential and the electromyographically recorded startle response using stimuli designed to elicit each response. Simultaneous recordings from the vertex and neck musculature following auditory click stimuli showed that: (i) the mean threshold of the P13 potential was 69.3 +/- 1.9 dB, while that for the startle response was 87.9 +/- 6.4 dB; (ii) the P13 potential was present during waking and paradoxical sleep, but absent during slow-wave sleep, while the startle response was present reliably only during waking; (iii) both responses habituated in response to paired stimuli, but the startle response was more habituated than the P13 potential; and (iv) both responses were facilitated by trains of stimuli in a similar manner. Recordings carried out from the auditory cortex verified that the primary cortical response had properties different from the P13 potential; i.e. it was present during all sleep-wake states, had a lower threshold and did not habituate rapidly. Finally, different patterns of startle responses were detected in the neck muscles. In every case, the P13 potential occurred during the middle, inhibitory phase of the startle response. These results suggest that the P13 potential and the startle response share response features, but the P13 potential appears to be more sensitive to auditory stimulation and to sleep-wake states. The startle response may be modulating descending systems by priming the spinal cord to respond in a "fight vs flight" fashion. On the other hand, the P13 response may be modulating ascending systems by triggering thalamocortical activity and resetting descending systems to allow novel motor strategies.

Physiology of the young adult Fischer 344 rat inferior colliculus: responses to contralateral monaural stimuli

This study was designed to establish the young adult (3 month) Fischer 344 (F344) rat as a model of inferior colliculus (IC) physiology, providing a baseline for analysis of changes in single unit responses as the animals age and for the study of noise induced hearing loss. The response properties of units localized to the central nucleus of the IC (CIC) and those localized to the external cortex of the IC (ECIC) were compared in order to better characterize differences between these two subnuclei in the processing of simple auditory stimuli. In vivo extracellular single unit recordings were made from IC neurons in ketamine/xylazine anesthetized young adult F344 rats. When a unit was electrically isolated, the spontaneous activity level, characteristic frequency (CF) and CF threshold were determined. Rate/intensity functions (RIFs) in response to contralateral CF tones and to contralateral noise bursts were obtained as were tone isointensity functions. The recording site was marked by ejecting horseradish peroxidase (HRP) from an electrode. Locations of recorded units were determined from electrode track marks and HRP marks in serial brain sections. Recordings were made from 320 neurons in the IC; 176 were localized to the CIC and 87 to the ECIC. Thirteen percent of the units in each subdivision were found to be poorly responsive to auditory stimulation (clicks, tones or noise), and spontaneous activity was generally low. Characteristic frequencies representative of the full rat audiogram were found in each subdivision with the mean threshold significantly higher in the ECIC (28.7 dB SPL) than in the CIC (22.3 dB SPL). The mean maximum discharge rate to CF tone bursts was near 24 spikes/s in each subdivision. Dynamic range tended to be higher in the ECIC (28.3 dB) than in the CIC (23.2 dB), reflecting the lower percentage of nonmonotonic units found in the ECIC. Most units responded more robustly with a slower tone presentation rate, displayed lower levels of discharge to noise bursts than to tone bursts, and had differently shaped tone and noise RIFs. Most units were classified as onset responders to CF tone bursts in both subdivisions, with the percentage of onset responders higher in the ECIC (68.9%) than in the CIC (57.8%). First spike latency did not differ significantly between the subdivisions, but tended to be shorter in the CIC. The breadth of the excitatory receptive fields did not differ significantly between subdivisions, although the mean was slightly larger in the ECIC. These results are generally consistent with the results of CIC studies from other species, establishing the F344 rat as a model of CIC physiology. Differences between CIC and ECIC units included a higher percentage of nonmonotonic RIFs and lower percentage of onset temporal response patterns in the CIC than in the ECIC. Some properties which have been previously used as hallmarks for differentiation between CIC and ECIC units, namely broader tuning and longer first spike latencies in the ECIC, did not reach statistical significance in this study. These may reflect species differences and/or the highly variable and largely overlapping sets of responses evident in the large sample size used in this study.

Age-related alterations in the auditory brainstem responses and the compound action potentials in guinea pigs

The auditory brainstem response (ABR) and the eight nerve compound action potential (CAP) were measured using click click stimuli to investigate the age-related alteration in the auditory function in 66 guinea pigs consisting of four age groups. With advancing age, a gradual elevation of the thresholds in both the ABR and CAP was clearly seen, together with the prolonged latencies for waves I, II, III, and IV to clicks at 95 dBpeSPL in the ABR. There were some individual differences in either threshold elevation or latency prolongation of both the ABR and CAP in aged guinea pigs. These findings suggest that the effect of individual differences on degenerative aging processes of the auditory system should be considered in selected aged animals, although a significant elevation of the neural auditory threshold is clearly found with advancing age as a whole.

Distribution of glial fibrillary acidic protein (GFAP) in the cochlear nucleus of adult and aged rats

The age-related change in glial fibrillary acidic protein (GFAP) immunoreactivity was analyzed in young (3 months) and old (24 months) adult rat cochlear nuclei (CN). Quantitative analyses show a significant increase with age, in the number of GFAP positive astrocytes and processes in the old adult when compared with the young adult rat. There was also a differential distribution of GFAP immunoreactivity in the young adult CN where it predominates in the granular cell region, whereas in old rats, the GFAP immunoreactivity distribution was homogeneous in all parts of the nucleus. There was no change in the total number of neurons between these two stages in any part of the nucleus except for the antero-ventral CN, where a decrease in neuronal number was observed in the aged rats. The increase in GFAP immunoreactivity was related to an increase of both GFAP positive astrocyte number and processes. The increase of GFAP positive astrocytes may be due either to an alteration of auditory nerve fibers, changing the trophic interactions with post-synaptic cells, or to intrinsic alterations of CN neurons and local circuits reflecting aging of the CN.

Decreased inhibition to lateral superior olive neurons in young and aged Sprague-Dawley rats

Lateral superior olive (LSO) neurons in young and aged Sprague-Dawley rats have functional properties consistent with a limited contralateral inhibition, which is markedly different from other animals. An unusually low proportion of LSO cells (36/113) exhibited contralateral inhibition (and ipsilateral excitation, IE), while over 25% of LSO units exhibited excitatory responses to contralateral stimuli. Inhibition of most IE LSO neurons was evident only when the contralateral intensity was greater than the ipsilateral intensity, resulting in a marked shift in sensitivity to interaural intensity differences (IID). The firing rate of IE neurons was also affected more by a change in intensity of ipsilateral compared to contralateral stimuli. The shift in the IID sensitivity and the relative decrease in effectiveness of contralaterally driven inhibition in Sprague-Dawley rat LSO neurons could be due to decreased inhibitory inputs from the MNTB principal cells, increased contralateral excitatory effects and/or increased ipsilateral excitatory effects. Age-related decreases in the numbers of MNTB neurons observed anatomically is not reflected in a change in LSO function. The Sprague-Dawley rat may be a useful model for the effect of reduced inhibition in the superior olivary complex on auditory behavior.

Central auditory aging: GABA changes in the inferior colliculus

Age-related hearing loss (presbycusis) is a complex state that reflects pathologic changes along the entire auditory neuraxis. Loss of speech understanding, decreased ability to localize sounds, and a decreased ability to detect and extract signals in noise are characteristic problems encountered by the elderly. Central (neural) presbycusis frequently results in a dramatic loss in speech understanding without a parallel change in pure-tone thresholds. In spite of evidence that suggests these deficits cannot be fully explained by peripheral changes alone, few studies have examined the neurochemical basis of central auditory dysfunction in aging. Age-related alterations in neural circuits involved in the processing of acoustic information could reflect changes in the synthesis, degradation, uptake, release, and receptor sensitivity of neurotransmitters, perhaps secondary to cell loss and/or progressive deafferentation. A series of studies designed to test this hypothesis has examined aging in the central auditory system of the F344 rate. Age-related changes associated with GABA neurotransmitter function in an important auditory midbrain structure, the inferior colliculus, have been investigated. These studies found: (1) decreased numbers of GABA immunoreactive neurons; (2) decreased basal levels (concentrations) of GABA; (3) decreased GABA release; (4) decreased glutamic acid decarboxylase activity; (5) decreased GABAB receptor binding; (6) decreased numbers of presynaptic terminals; and (7) subtle GABAA receptor binding changes. Collectively, these age-related changes suggest altered GABA neurotransmitter function in the IC. Identification of specific neurotransmitter changes in structures important in speech processing could eventually lead to the development of pharmacotherapy for selective types of age-related hearing loss.

A middle-latency auditory-evoked potential in the rat

Previous studies have established the presence of a middle-latency auditory-evoked potential that is characterized by a) sleep-state dependence, b) low following frequency (i.e., rapid habituation to repetitive stimulation), and c) blockade by the cholinergic antagonist, scopolamine. A vertex-recorded evoked potential having these characteristics was described in humans at a 50-80 ms latency (termed the P1 or.P50 potential) and in the cat at a 20-25 ms latency (termed wave A). These studies were undertaken to determine if a click stimulus-evoked potential having the same characteristics was present in the intact rat. Vertex and auditory cortex recordings in intact rats studied in a sound-attenuating chamber and exposed to free-field click stimuli showed a) the presence of a vertex recorded potential at a 11-15 ms latency, termed P13, and of an auditory cortex recorded potential at a 7-11 ms latency, termed Pa; b) the P13 was present during waking and paradoxical sleep but absent in slow-wave sleep, while Pa was present in all sleep-wake states; c) the P13 habituated markedly at stimulation rates above 1 Hz while Pa did not; and d) the P13 was blocked by low doses of scopolamine while Pa was not. These studies demonstrate the presence of a P1-like potential in the rat at a 13 +/- 2 ms latency.

Age-related changes in brainstem auditory neurotransmitters: measures of GABA and acetylcholine function

This study was designed to determine if there are age-related alterations in the bio-synthetic enzyme glutamic acid decarboxylase (GAD), the degradative enzyme GABA-transaminase (GABA-T), and the uptake system for GABA in the central nucleus of the inferior colliculus (CIC), the cochlear nucleus (CN), and/or nuclei of the lateral lemniscus (NLL) of Fischer-344 rats. For purposes of comparison, the cholinergic neuronal system was studied in parallel in young adult (3-7 months), mature (15-17 months) and aged (24-26 months) rats. In young adults GAD activity was highest in the CIC (219 nmol/mg protein/h; N = 5), intermediate in NLL (82 nmol/mg protein/h), and lowest in CN (34 nmol/mg protein/h). Choline acetyltransferase (ChAT) activity was highest in NLL and CN, and approximately 35-40% lower in CIC. A more uniform pattern was observed with GABA-T activity. Reductions in GAD activity were seen in the CIC of mature (-31%) and aged (-30%) rats that were not graded with age when compared to young adult, P < 0.05 (N = 5). This effect was regionally selective, since the CN did not show any loss of GAD or ChAT activity. The neurotransmitter selectivity of this deficit in CIC is supported by the non-parallel changes in ChAT activity (-22%, aged vs. mature, P < 0.05) that occurred after the changes in GAD activity. In contrast to the loss of GABAergic biosynthetic capacity in aged CIC, high affinity uptake processes (Kd and Vmax) for 14C-GABA and 3H-D-aspartate were not significantly altered (P > 0.05). Similar to the CIC, the NLL showed remarkable age-related deficits, but these deficits were more substantial for the cholinergic system (ChAT activity: -56% aged vs. young adult, P < 0.05; GAD activity: -35% aged vs. mature). None of the areas examined showed a significant loss of GABA-T activity with aging. These data suggest: 1) Age-related loss of GABA-mediated inhibition in the CIC of Fischer-344 rats is not attributable to changes in uptake or degradation of GABA, but may be related loss of biosynthetic capacity (i.e. activity or quantity) of the GAD present; 2) processing centers of the central auditory pathway (i.e. CIC and NLL), but not necessarily primary (i.e. CN) integrative nuclei, demonstrate selective, age-related neurochemical deficits; and 3) age-related neurochemical changes in central auditory structures may contribute substantially to the abnormal perception of signals in noise and loss of speech discrimination observed in neural presbycusis.

GABA in the inferior colliculus plays a critical role in control of audiogenic seizures

Previous studies have implicated a decreased efficacy of GABA as an important defect subserving the audiogenic seizures of the genetically epilepsy-prone rat (GEPR-9). The inferior colliculus (IC) is a critical site for audiogenic seizure (AGS) initiation, and the pontine reticular formation (PRF) is implicated in the propagation of AGS and in other generalized seizure models. The present study observed that microinjection of baclofen, a GABA-B receptor agonist, into IC protects against AGS, and blockade of the breakdown of endogenous GABA by gabaculine, a GABA transaminase inhibitor, increased GABA levels and blocked AGS susceptibility in the GEPR-9. Microinjection of baclofen or gabaculine into the PRF reduced AGS severity, but the doses required were considerably greater and the degree of anticonvulsant effect was less. Uptake of [3H]GABA into GEPR-9 synaptosomes from the IC is significantly increased as compared to normal, which could contribute to the diminished effectiveness of GABA in the GEPR-9. Previous studies indicate that GABA-A receptor agonists block AGS with IC microinjection, and recent data indicate that blockade of GABA uptake in this nucleus significantly reduced AGS severity. These data taken together strongly support the critical importance of the defect in GABA function in the IC in modulating susceptibility to audiogenic seizure initiation in the GEPR-9.

Effect of aging on myo-inositol and phosphoinositide metabolism in the cochlear and vestibular sensory epithelia of the rat

Neurotransmission and transmembrane signaling are among the cellular mechanisms affected in the aging nervous system. In the inner ear, the phosphoinositide second messenger cascade is of particular interest as a target of the aging process. In both the cochlear (CSE) and vestibular sensory epithelia (VSE), the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) to the second messenger inositol 1,4,5-trisphosphate (InsP3) is coupled to muscarinic cholinergic and P2y purinergic receptors and may be linked to calcium homeostasis. The present study compared the turnover of phosphoinositides (PtdInsPs), receptor-mediated release of inositol phosphates (InsPs), and concentrations of endogenous myo-inositol in the CSE and VSE of young (3 months) and aged (24 months) Fischer-344 rats. In the aged rat, there was a significant increase in [3H]inositol incorporation (per mass of protein) into PtdInsPs plus InsPs in both sensory epithelia while the protein content remained unchanged. In contrast, no age-dependent differences were found when pre-labeled [3H]PtdInsPs were 'chased' with non-radiolabeled myo-inositol indicating that the turnover of these lipids was unaffected. The cholinergic receptor agonist carbamylcholine and the P2 purinergic receptor agonist adenosine 5'-O-(3-thiotriphosphate) stimulated the release of [3H]InsPs two- to six-fold in both organs. This agonist-stimulated release of [3H]InsPs (per mass of protein) was significantly higher in aged animals. However, when the same stimulation was expressed as per cent of control values, there was no age-dependent difference.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes in auditory brainstem responses in Fischer 344 rats: effects of rate and intensity

Age-related changes in auditory brainstem responses (ABR) observed in humans may reflect peripheral or centrally-occurring deficits. In clinical studies, high stimulus repetition rates have been used to improve the identification of central auditory pathology. In the present study, interactions between stimulus level and repetition rate were examined in the Fischer 344 rat, an animal demonstrating both peripheral hearing loss and changes in auditory brainstem neurochemistry with age. Monaural threshold and standard ABR morphology were determined in young (3-6 months) and old (20-23 months) rats using clicks at 10/s, with intensity varied from 0-100 dB. The effects of increasing stimulus repetition rate on ABR latency and morphology were evaluated at 60-100 dB using rates of 5, 10, 20, and 40/s. Old animals demonstrated elevated ABR click thresholds, reflected by shifts in the latency-intensity curves. With increased stimulation rates, aged rats exhibited prolonged Wave 4 and 5 latencies, especially at the highest intensities, with degraded waveform morphology. Peak amplitudes were generally reduced in old rats, irrespective of rate or stimulus level. These findings suggest auditory processing is altered in aged animals, while the selective effects of rate increases on Waves 4 and 5 provide supporting evidence for possible involvement of the central auditory generators of these components.

Age-related changes in auditory evoked potentials of gerbils. II. Response latencies

Auditory brainstem responses (ABR) were recorded in young (6-10 month) and aged (36 month) Mongolian gerbils. Data from the young animals served as the baselines for comparison to aged animals which were categorized on the basis of ABR thresholds. Aged gerbils with normal thresholds (re young controls) had wave i and ii latencies of the ABR which were relatively normal at 1-4 kHz and slightly reduced at 8 and 16 kHz. Wave iv latencies in the aged gerbils with normal thresholds were reduced at all frequencies. Aged gerbils with 10-30 dB of hearing loss had wave i, ii, and iv latencies which were prolonged at low sound pressure levels and normal at high stimulus levels. Aged gerbils with 30 dB or greater losses had prolonged wave i, ii, and iv latencies at most levels. Slopes of latency-intensity (L/I) functions were steeper at 1-4 kHz than controls in aged subjects with hearing losses of 10 dB or greater. Slopes of L/I functions for wave iv were normal in aged subjects. The wave i-iv interval was shorter than normal in aged subjects with no hearing loss, normal in aged subjects with 10-30 dB of loss, and prolonged in subjects with greater than 30 dB of loss.

Aging and otitis media as sources of variance in the rat auditory brainstem response

Auditory brainstem response (ABR) latencies were examined in normal female Sprague-Dawley rats at 6 months of age and in 10-12 month old animals which tested positive for Mycoplasma pulmonis, a frequently implicated pathogen in otitis media. Significant differences in the peak latencies of Waves I, II and IIIn were found between the two age groups. A significant age by intensity interaction effect was also identified for Waves II and IIIn. Since the age-related effects observed in Wave I were similarly reflected in Waves II and IIIn, it was concluded that the group differences were due to alterations of the auditory periphery and not to changes in the central auditory system.

Brainstem auditory evoked potentials in experimental autoimmune dementia

Cognitive dysfunction may be induced in rats by immunization with cholinergic neuronal antigens and is associated with degeneration of nuclei in the septum and hippocampus and white matter tracts in the forebrain. Such rats are a putative model of Alzheimer's disease named experimental autoimmune dementia (EAD). The aim of the present study was to investigate brainstem auditory evoked potentials (BAEP) in EAD rats in order to define the extent of white matter tract involvement in this model. Clear reproducible evoked potentials were obtained and the normal range of BAEP in rats was established in adjuvant immunized controls. Measurements of inter peak latencies (IPL) I-IV in 9 EAD rats revealed that they were not significantly prolonged (3.00 +/- 0.22 ms, mean +/- S.E.M.) compared to 9 controls (2.80 +/- 0.08 ms), (P > 0.2, Student's t-test). The lack of significant change in IPL I-IV is compatible with preliminary histological findings and indicates that brainstem structures are generally unaffected in EAD.

Response properties in young and old Fischer-344 rat lateral superior olive neurons: a quantitative approach

Significant age-related functional deficits may result from a selective loss of inhibitory processing in the lateral superior olivary (LSO) nucleus. To test this hypothesis, methods were developed for quantitative comparisons of the excitation evoked by ipsilateral acoustic stimuli and the inhibition evoked by contralateral acoustic stimuli in neurons recorded from this binaural structure. Data were obtained from 103 LSO neurons from 22 young adult (3-6 month) and 70 LSO neurons from 14 old (20-23 month) Fischer-344 (F-344) rats. Age-related increases in the thresholds of auditory brainstem responses as well as in the inhibitory and excitatory responses of LSO neurons were observed. Spike-discharge rates were analyzed using multiple regression analysis for interaural intensity data and by calculating correlation coefficients between excitatory and inhibitory isointensity curves. Most LSO principal cells exhibited ipsilateral excitation and contralateral inhibition of similar strength for similar stimuli. Inhibitory and excitatory response areas of these neurons were similar based on visual inspection and correlation coefficients. No statistically significant age-related changes were observed for (a) rate-level functions generated by ipsilateral or contralateral stimuli; (b) maximal discharge rate; (c) conduction latencies; or (d) measures of binaural function. However, a small percentage of LSO neurons both in young and old rats displayed "unmatched" inhibitory and excitatory response areas. Quantitative methods developed in this study are used to examine age-related changes in binaural function in the inferior colliculus of F-344 rats and in the LSO of the Sprague-Dawley rat. Although no asymmetrical aging changes were observed for the F-344 rat LSO, there appear to be significant differences between the aging auditory system in the F-344 and Sprague-Dawley rats. Age-related changes have been previously described for the F-344 in other brainstem auditory structures.

Multiple brain systems generating the rat auditory evoked potential. II. Dissociation of auditory cortex and non-lemniscal generator systems

This study addressed the issue of multiple parallel auditory processing systems and their relationship to the skull-recorded auditory evoked potentials (AEPs) in the unanesthetized, unrestrained rat. In the preceding paper (Brain Res., 602 (1993) 240-250) it has been shown that auditory cortex activity does not contribute significantly to the vertex maximal AEPs recorded from the dorsal skull of the rat. In the present study, mapping of the AEP skull distribution revealed two sets of components: one set maximal at the dorsal skull vertex, and another set at the lateral skull), but not the early (P7-P11, N15) lateral skull components generated in auditory cortex. Bilateral auditory cortex ablation eliminated the lateral skull maximal AEP components, but not the dorsal skull maximal components. These findings support extensive parallel processing of auditory inputs (reflected by the dorsal AEPs) in the absence of primary auditory cortex. Ablation of primary auditory cortex did result in a modulation of the dorsal skull AEPs, indicative of an interaction between the geniculocortical system and the parallel system which generates the dorsal AEPs.

Multiple brain systems generating the rat auditory evoked potential. I. Characterization of the auditory cortex response

The objectives of this study were to characterize the auditory cortex response in the rat and to examine its contributions to the auditory evoked potentials (AEPs) recorded from the dorsal and lateral skull. This was accomplished by simultaneously recording AEPs from the cortical surface and from skull screw electrodes in anesthetized animals. The initial positive-negative response (P17-N32) was largely restricted to the cortical region corresponding to area 41. More detailed examination of the AEP mapping revealed multiple subcomponents (P9, P14, P17, P19) underlying the initial positivity, with differing topographies. Stimulus-response properties further dissociated the multiple positive subcomponents. Reversible local neurochemical suppression confirmed the auditory cortical origin of these AEPs. The auditory cortex-generated AEPs were refractory to barbiturate anesthesia which eliminated all dorsal skull AEPs, indicating that primary auditory cortical AEPs do not make a significant contribution to the dorsal skull-recorded ('vertex') AEPs. The findings raise issues regarding multiple parallel auditory processing systems and their associated AEPs.

Alteration of brainstem auditory evoked potentials in diethylbenzene and diacetylbenzene-treated rats

Male Sprague-Dawley rats were treated either with 1,2-diethylbenzene (1,2-DEB) or its putative active metabolite, 1,2-diacetylbenzene (1,2-DAB). Experimental rats and appropriate controls were examined electrophysiologically for brainstem auditory evoked potentials (BAEP). Oral administration of 1,2-DEB (75 or 100 mg kg-1 once a day, 4 days a week, for 8 weeks) and intraperitoneal injection of 1,2-DAB (10 or 15 mg kg-1 once a day, 4 days a week, for 8 weeks) produced time- and dose-dependent increases in the peak latencies of all BAEP components as well as in interpeak (I-V) differences, and a decrease in the amplitudes of all the components. The absolute and interpeak latencies recovered partially during an 8-week (1,2-DEB) or a 10-week (1,2-DAB) recovery period, whereas there were long-lasting decreases in peak amplitudes.