The use of patient-report measures and intervention strategies for children and adolescents with chronic tinnitus: a scoping review

BACKGROUND

Chronic tinnitus during childhood/adolescence can be associated with impaired quality of life. Guidelines for managing paediatric tinnitus recommend assessment and interventions are often based upon the experiences and opinions of guideline committee members.

OBJECTIVE

To examine patient response tools used for the assessment and management of childhood tinnitus and how interventions had been evaluated.

DESIGN

A structured scoping review (i) identifying and critically appraising patient response measures (PRMs) assessing tinnitus in children/adolescents, and (ii) critically appraising evidence supporting reported interventions. Original papers written in English, involving paediatric participants ≤19 years, reporting (i) application of established PRMs to assess the experience of chronic tinnitus or (ii) application and evaluation of tinnitus interventions were included.

STUDY SAMPLE

Papers written in English, identifying, or assessing the experience of chronic tinnitus (>3 months) as a primary complaint during childhood/adolescence in participants ≤19 years of age using a PRM and studies evaluating the application of non-pharmaceutical interventions for tinnitus in children/adolescents.

RESULTS

Six studies involving the assessment of tinnitus during childhood/adolescence using a PRM were identified and evaluated. Three established (previously named, described, and published) PRMs were applied of which none were developed specifically for children/adolescents. Three behavioural tinnitus interventions and three combination intervention strategies (coupling of psychological intervention with sound enrichment) had been applied to and evaluated within paediatric populations.

CONCLUSIONS

Although clinicians are seeing children/adolescents with tinnitus, they are evaluating and managing children's distress without appropriate PRMs, and little evidence exists to support clinical interventions.

A repeated measures cognitive affective bias test in rats: comparison with forced swim test

RATIONALE

There is an urgent need to identify behaviours in animals that can provide insight into the aetiology and potential treatment of depression in humans.

OBJECTIVES

This study aimed to validate a repeated measures cognitive affective bias (CAB) test in a rat model of chronic stress and compare CAB with forced swim test (FST) measures.

METHODS

Male and female Sprague Dawley rats were trained to associate large and small rewards with scent, spatial, and tactile cues, and their response to an ambiguous tactile stimulus tested. Rats underwent weekly CAB testing for 4 weeks with no intervention, or for 2 weeks of chronic restraint stress (CRS), followed by 2 weeks of fluoxetine, vehicle, or no treatment. CRS rats also underwent the FST at selected timepoints.

RESULTS

In control rats, CAB was positive and remained stable over the 4-week period. In CRS-fluoxetine and CRS-vehicle groups, CAB was initially positive, became negative during chronic restraint stress, and returned to positive by 2 weeks after treatment. However, in the CRS-no treatment group, CAB was variable at the outset and unstable over time. Behaviour in the FST was not affected by treatment, and there was no correlation between CAB and FST outcomes.

CONCLUSIONS

Instability in the CRS-no treatment group precluded interpretation of the impact of fluoxetine on CAB post-CRS. Our results suggest that behaviour in the FST does not reflect or alter affective state and support the use of CAB tests as part of the behavioural testing repertoire for preclinical animal models of affective disorders.

Acoustic trauma increases inhibitory effects of amygdala electrical stimulation on thalamic neurons in a rat model

Acoustic trauma (AT) induced hearing loss elicits plasticity throughout the central auditory pathway, including at the level of the medial geniculate nucleus (MGN). Hearing loss also results in altered neuronal responses in the amygdala, which is involved in sensory gating at the level of the MGN. However, whether these altered responses in the amygdala affect sensory gating at the level of the MGN requires further evaluation. The current study aimed to investigate the effects of AT-induced hearing loss on the functional connectivity between the amygdala and the MGN. Male Sprague-Dawley rats were exposed to either sham (n = 5; no sound) or AT (n = 6; 16 kHz, 1 h, 124 dB SPL) under full anaesthesia. Auditory brainstem response (ABR) recordings were made to determine hearing thresholds. Two weeks post-exposure, extracellular recordings were used to assess the effect of electrical stimulation of the amygdala on tone-evoked (sham n = 22; AT n = 30) and spontaneous (sham n = 21; AT n = 29) activity of single neurons in the MGN. AT caused a large temporary and small permanent ABR threshold shift. Electrical stimulation of the amygdala induced differential effects (excitatory, inhibitory, or no effect) on both tone-evoked and spontaneous activity. In tone-evoked activity, electrical stimulation at 300 µA, maximum current, caused a significantly larger reduction in firing rate in AT animals compared to sham, due to an increase in the magnitude of inhibitory effects. In spontaneous activity, there was also a significantly larger magnitude of inhibitory effects following AT. The findings confirm that activation of the amygdala results in changes in MGN neuronal activity, and suggest the functional connectivity between the amygdala and the MGN is significantly altered following AT and subsequent hearing loss.

Ear and hearing outcomes in Aboriginal infants living in an urban Australian area: the Djaalinj Waakinj birth cohort study

OBJECTIVE

Describe the ear and hearing outcomes in Aboriginal infants in an Australian urban area.

DESIGN

Aboriginal infants enrolled in the Djaalinj Waakinj prospective cohort study had ear health screenings at ages 2-4, 6-8 and 12-18 months and audiological assessment at ∼12 months of age. Sociodemographic, environmental characteristics, otoscopy, otoacoustic emissions, tympanometry and visual reinforcement audiometry data were collected.

STUDY SAMPLE

125 infants were enrolled in the study; 67 completed audiological assessment, 62, 54, and 58 of whom attended ear screenings at 2-4, 6-8 and 12-18 months.

RESULTS

Of the children that attended the audiological assessment, 36.5%, 50% and 64.3% of infants had otitis media (OM) at 2-4, 6-8 and 12-18 months. Using a 10 dB correction factor, 44.8% of infants had hearing loss (HL) (≥ 25 dB HL) at ∼ 12 months of age. More males (X2=5.4 (1df, p = 0.02)) and infants with OM at audiological assessment (X2=5.8 (1df, p = 0.02)) had HL. More infants that used a pacifier at 12-18 months of age had HL (X2=4.7 (1df, p = 0.03)).

CONCLUSION

Aboriginal infants in an urban area have high rates of HL and OM, which requires early surveillance and timely treatment to reduce the medical and developmental impacts of OM and HL.

Auditory Event-Related Potentials in Older Adults with Subjective Memory Complaints

BACKGROUND

Auditory event-related potentials (AERPs) have been suggested as possible biomarkers for the early diagnosis of Alzheimer's disease (AD). However, no study has investigated AERP measures in individuals with subjective memory complaints (SMCs), who have been suggested to be at a pre-clinical stage of AD.

OBJECTIVE

This study investigated whether AERPs in older adults with SMC can be used to objectively identify those at high risk of developing AD.

METHODS

AERPs were measured in older adults. Presence of SMC was determined using the Memory Assessment Clinics Questionnaire (MAC-Q). Hearing thresholds using pure-tone audiometry, neuropsychological data, levels of amyloid-β burden and Apolipoprotein E (APOE)ɛ genotype were also obtained A classic two-tone discrimination (oddball) paradigm was used to elicit AERPs (i.e., P50, N100, P200, N200, and P300).

RESULTS

Sixty-two individuals (14 male, mean age 71.9±5.2 years) participated in this study, of which, 43 (11 male, mean age 72.4±5.5 years) were SMC and 19 (3 male, mean age 70.8±4.3 years) were non-SMC (controls). P50 latency was weakly but significantly correlated with MAC-Q scores. In addition, P50 latencies were significantly longer in Aβ+ individuals compared to Aβ- individuals.

CONCLUSION

Results suggest that P50 latencies may be a useful tool to identify individuals at higher risk (i.e., participants with high Aβ burden) of developing measurable cognitive decline. Further longitudinal and cross-sectional studies in a larger cohort on SMC individuals are warranted to determine if AERP measures could be of significance for the detection of pre-clinical AD.

Long-Term Effects of Repetitive Transcranial Magnetic Stimulation on Tinnitus in a Guinea Pig Model

The auditory phantom sensation of tinnitus is associated with neural hyperactivity. Modulating this hyperactivity using repetitive transcranial magnetic stimulation (rTMS) has shown beneficial effects in human studies. Previously, we investigated rTMS in a tinnitus animal model and showed that rTMS over prefrontal cortex (PFC) attenuated tinnitus soon after treatment, likely via indirect effects on auditory pathways. Here, we explored the duration of these beneficial effects. Acoustic trauma was used to induce hearing loss and tinnitus in guinea pigs. Once tinnitus developed, high-frequency (20 Hz), high-intensity rTMS was applied over PFC for two weeks (weekdays only; 10 min/day). Behavioral signs of tinnitus were monitored for 6 weeks after treatment ended. Tinnitus developed in 77% of animals between 13 and 60 days post-trauma. rTMS treatment significantly reduced the signs of tinnitus at 1 week on a group level, but individual responses varied greatly at week 2 until week 6. Three (33%) of the animals showed the attenuation of tinnitus for the full 6 weeks, 45% for 1–4 weeks and 22% were non-responders. This study provides further support for the efficacy of high-frequency repetitive stimulation over the PFC as a therapeutic tool for tinnitus, but also highlights individual variation observed in human studies.

Comparison of Auditory Steady-State Responses With Conventional Audiometry in Older Adults

Behavioral measures, such as pure-tone audiometry (PTA), are commonly used to determine hearing thresholds, however, PTA does not always provide reliable hearing information in difficult to test individuals. Therefore, objective measures of hearing sensitivity that require little-to-no active participation from an individual are needed to facilitate the detection and treatment of hearing loss in difficult to test people. Investigation of the reliability of the auditory steady-state response (ASSR) for measuring hearing thresholds in older adults is limited. This study aimed to investigate if ASSR can be a reliable, objective measure of frequency specific hearing thresholds in older adults. Hearing thresholds were tested at 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz in 50 participants aged between 60 and 85 years old, using automated PTA and ASSR. Hearing thresholds obtained from PTA and ASSR were found to be significantly correlated (p < .001) in a cohort consisting of participants with normal hearing or mild hearing loss. ASSR thresholds were significantly higher as compared to PTA thresholds, but for the majority of cases the difference remained within the clinically acceptable range (15 dB). This study provides some evidence to suggest that ASSR can be a valuable tool for estimating objective frequency-specific hearing thresholds in older adults and indicate that ASSR could be useful in creating hearing treatment plans for older adults who are unable to complete behavioral PTA. Further research on older adults is required to improve the methodological features of ASSR to increase consistency and reliability, as well as minimize some of the limitations associated with this technique.

Hearing Loss Increases Inhibitory Effects of Prefrontal Cortex Stimulation on Sound Evoked Activity in Medial Geniculate Nucleus

Sensory gating is the process whereby irrelevant sensory stimuli are inhibited on their way to higher cortical areas, allowing for focus on salient information. Sensory gating circuitry includes the thalamus as well as several cortical regions including the prefrontal cortex (PFC). Defective sensory gating has been implicated in a range of neurological disorders, including tinnitus, a phantom auditory perception strongly associated with cochlear trauma. Recently, we have shown in rats that functional connectivity between PFC and auditory thalamus, i.e., the medial geniculate nucleus (MGN), changes following cochlear trauma, showing an increased inhibitory effect from PFC activation on the spontaneous firing rate of MGN neurons. In this study, we further investigated this phenomenon using a guinea pig model, in order to demonstrate the validity of our finding beyond a single species and extend data to include data on sound evoked responses. Effects of PFC electrical stimulation on spontaneous and sound-evoked activity of single neurons in MGN were recorded in anaesthetised guinea pigs with normal hearing or hearing loss 2 weeks after acoustic trauma. No effect, inhibition and excitation were observed following PFC stimulation. The proportions of these effects were not different in animals with normal hearing and hearing loss but the magnitude of effect was. Indeed, hearing loss significantly increased the magnitude of inhibition for sound evoked responses, but not for spontaneous activity. The findings support previous observations that PFC can modulate MGN activity and that functional changes occur within this pathway after cochlear trauma. These data suggest hearing loss can alter sensory gating which may be a contributing factor toward tinnitus development.

Reflections on How Tinnitus Impacts the Lives of Children and Adolescents

Objectives The aim of this study was to generate a conceptual framework describing which aspects of children and adolescents' lives are affected by chronic tinnitus. Design Views and experiences of 32 participants from two participant groups informed this study: (a) a tinnitus group, consisting of adults who had experienced tinnitus during childhood and/or adolescence and primary carers of children/adolescents with tinnitus, and (b) a clinicians' group, consisting of clinicians who provided care for children/adolescents with tinnitus. Participants produced statements describing aspects of children/adolescents' lives that may be affected by chronic tinnitus. Key concepts were identified through the processes of sorting the statements and rating them for degree of associated impact. Result Participants identified 118 unique aspects of the lives of children/adolescents who may be affected by chronic tinnitus. These were clustered into four concepts: (a) emotional well-being, (b) academic performances, (c) social/relationa, and (d) auditory/cognitive processing. At a group level, participants rated the impact of tinnitus as above a slight degree but below a moderate degree of impact. However, individual participant's ratings indicated a range of perceived impact for each statement. Conclusions The experience of chronic tinnitus during childhood and adolescence extends beyond the mere perception of sound. The perception of tinnitus may impact a child's emotional well-being, academic performances, social/relational, and auditory/cognitive processing. The impact of tinnitus in one aspect of a child's life may influence other aspects of their life. While at a group level, participants regarded the impact of tinnitus as "somewhat more than mild" to "less than moderate"; individual participant's ratings indicate that the impact from chronic tinnitus may be highly individual and highlighted the importance of individual assessment and management. Clinically, tinnitus management during childhood and adolescence may be improved if clinicians consider the impact and manifestation of tinnitus within each child's daily life and tailor tinnitus education and management strategies accordingly.

Excitatory Repetitive Transcranial Magnetic Stimulation Over Prefrontal Cortex in a Guinea Pig Model Ameliorates Tinnitus

Tinnitus, a phantom auditory perception that can seriously affect quality of life, is generally triggered by cochlear trauma and associated with aberrant activity throughout the auditory pathways, often referred to as hyperactivity. Studies suggest that non-auditory structures, such as prefrontal cortex (PFC), may be involved in tinnitus generation, by affecting sensory gating in auditory thalamus, allowing hyperactivity to reach the cortex and lead to perception. Indeed, human studies have shown that repetitive transcranial magnetic stimulation (rTMS) of PFC can alleviate tinnitus. The current study investigated whether this therapeutic effect is achieved through inhibition of thalamic hyperactivity, comparing effects of two common clinical rTMS protocols with sham treatment, in a guinea pig tinnitus model. Animals underwent acoustic trauma and once tinnitus developed were treated with either intermittent theta burst stimulation (iTBS), 20 Hz rTMS, or sham rTMS (10 days, 10 min/day; weekdays only). Tinnitus was reassessed and extracellular recordings of spontaneous tonic and burst firing rates in auditory thalamus made. To verify effects in PFC, densities of neurons positive for calcium-binding proteins, calbindin and parvalbumin, were investigated using immunohistochemistry. Both rTMS protocols significantly reduced tinnitus compared to sham. However, spontaneous tonic firing decreased following 20 Hz stimulation and increased following iTBS in auditory thalamus. Burst rate was significantly different between 20 Hz and iTBS stimulation, and burst duration was increased only after 20 Hz treatment. Density of calbindin, but not parvalbumin positive neurons, was significantly increased in the most dorsal region of PFC indicating that rTMS directly affected PFC. Our results support the involvement of PFC in tinnitus modulation, and the therapeutic benefit of rTMS on PFC in treating tinnitus, but indicate this is not achieved solely by suppression of thalamic hyperactivity.

Reducing the Impact of Tinnitus on Children and Adolescents' Lives: A Mixed-Methods Concept Mapping Study

OBJECTIVES

To generate a conceptual framework describing what is done to reduce the impact of chronic tinnitus on the lives of children and adolescents.

DESIGN

Views and experiences of 32 adults from two participant groups informed this concept mapping study: (i) a tinnitus group (adults who experienced tinnitus during childhood/adolescence, and primary carers of children/adolescents with tinnitus) and (ii) a clinicians' group (clinicians who provided care for children/adolescents with tinnitus). Participants produced statements describing what is done to reduce the impact of chronic tinnitus on the lives of children and adolescents who experience it. Through grouping and rating processes, they identified key concepts and inferred their associated benefit.

RESULTS

The participants generated 102 unique statements across four concepts: (1) Education, Support, and Counselling; (2) Support from Parents and Teachers; (3) Clinical Assessments and Management; and (4) Self-Management Techniques. Many statements highlighted the need for child-friendly and patient-centred care. Adults with personal experience of childhood tinnitus tended to perceive many of the statements as more beneficial than did the clinician group.

CONCLUSIONS

Although many children will develop management strategies to assist them with their tinnitus, both the adults who experienced tinnitus as children and their parents valued strategies involving clinical care, knowledge, and expertise. Participants from the tinnitus group perceived a greater degree of benefit associated with strategies from all four clusters than the clinicians' group. However, both groups perceived the greatest degree of benefit as being associated with activities and strategies within the Education, Support, and Counselling and the Clinical Assessments and Management clusters. Both groups identified that recognising the occurrence of tinnitus for children and adolescents, acknowledging the potential for associated distress, and initiating clinical care provide the nexus of effective management. Addressing the concerns and needs of parents was also perceived as valuable; hence, approaching the management of tinnitus during childhood and adolescence from a family-centred care framework is recommended.

Spontaneous firing patterns in the medial geniculate nucleus in a guinea pig model of tinnitus

The mechanism of tinnitus, the perception of sound in the absence of acoustic stimulation, remains as yet unknown. It has been proposed that tinnitus is caused by altered spontaneous activity in the auditory pathway following cochlear damage in combination with inadequate gating at the level of the auditory thalamus, the medial geniculate nucleus (MGN). To investigate this further we made electrophysiological recordings in MGN of guinea pigs (n = 9) with and without tinnitus after acoustic trauma (continuous loud tone at 10 kHz, 124 dB SPL for 2 h). Parameters of interest were spontaneous tonic and burst firing. After acoustic trauma, 5 out of 9 guinea pigs developed signs of tinnitus as determined by the gap prepulse inhibition of acoustic startle. Spontaneous firing rates were significantly increased in the tinnitus animals as compared to the non-tinnitus animals and this change was specific to pure-tone responsive MGN neurons. However, burst firing parameters, including number of bursts per minute, burst duration, number of spikes in each burst, and percentage of spikes occurring in a burst, were not different between tinnitus and non-tinnitus animals. In addition, our data showed a strong dependence of spontaneous firing rates with heart rate, which implies that monitoring physiological status in animals is pertinent to obtaining reliable data when recording at higher levels of the auditory pathway. Our results suggest that increases in the tonic spontaneous fining rate of pure-tone responsive MGN neurons but not changes in burst firing parameters, are a robust neural signature of tinnitus in anaesthetised animals.

Changes in Prefrontal Cortex-Thalamic Circuitry after Acoustic Trauma

In the adult auditory system, loss of input resulting from peripheral deafferentation is well known to lead to plasticity in the central nervous system, manifested as reorganization of cortical maps and altered activity throughout the central auditory pathways. The auditory system also has strong afferent and efferent connections with cortico-limbic circuitry including the prefrontal cortex and the question arises whether this circuitry is also affected by loss of peripheral input. Recent studies in our laboratory showed that PFC activation can modulate activity of the auditory thalamus or medial geniculate nucleus (MGN) in normal hearing rats. In addition, we have shown in rats that cochlear trauma resulted in altered spontaneous burst firing in MGN. However, whether the PFC influence on MGN is changed after cochlear trauma is unknown. We investigated the effects of electrical stimulation of PFC on single neuron activity in the MGN in anaesthetized Wistar rats 2 weeks after acoustic trauma or sham surgery. Electrical stimulation of PFC showed a variety of effects in MGN neurons both in sham and acoustic trauma groups but inhibitory responses were significantly larger in the acoustic trauma animals. These results suggest an alteration in functional connectivity between PFC and MGN after cochlear trauma. This change may be a compensatory mechanism increasing sensory gating after the development of altered spontaneous activity in MGN, to prevent altered activity reaching the cortex and conscious perception.

Auditory electrophysiological assessments of Alzheimer's disease and preclinical stages: protocol for a systematic review and meta-analysis

INTRODUCTION

Investigating auditory functions in populations at risk of developing Alzheimer's disease (AD) using auditory neurophysiological measurements can potentially identify a crucial and sensitive diagnostic window of opportunity in preclinical AD. Auditory electrophysiological assessments have gained interest as possible tools for early diagnosis of AD. This paper outlines the protocol that will be used to systematically review the published literature currently available on auditory electrophysiological assessments that have been used to assess the auditory functions of adults over the age of 60 years diagnosed with AD or its preclinical stages.

METHODS AND ANALYSIS

All full-length peer-reviewed publications of original data that use auditory electrophysiological assessments in AD and its preclinical stages (subjective cognitive decline (SCD) and mild cognitive impairment (MCI)) will be considered in this review. The search will be performed on major electronic databases (Ovid MEDLINE, Ovid Embase, PsycINFO, PubMed, Scopus and CINAHL Plus) using keywords alone or in combination with Medical Subject Headings divided into two domains; (i) auditory tests and (ii) AD. The database search will be conducted on the 7^th of May 2019. Data analysis will be completed and reported in the full review. A random effects meta-analysis will also be conducted using the Comprehensive Meta-Analysis software, V.3. This review will describe which auditory electrophysiological tests have been found to be useful in assessing the auditory function in cognitively impaired adults (MCI and AD) or adults with serious complaints about their cognition (SCD). This review will also identify and describe which auditory electrophysiological test demonstrates the most sensitivity in differentiating people at different stages of cognitive decline.

ETHICS AND DISSEMINATION

This systematic review focusses on analysing already available literature. Therefore, there will be no requirement for ethical approval. The systematic review findings will be disseminated through peer-reviewed publication as well as relevant media platforms, for example, conferences.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO CRD42019133553.

Non-chewing diets and cerumen impaction in the external ear canal in a residential aged care population

OBJECTIVE

To examine whether diets involving reduced chewing affect the degree of cerumen impaction in older people in residential care.

METHODS

A total of 51 people (53% male) over 65 years (80.5 ± 9.3) were recruited from two residential aged care facilities in Perth, Australia. Participants were classified as "chewers" (on unrestricted or soft food diets), or "non-chewers" (on pureed or nil by mouth diets), and completed a survey about predisposing factors for cerumen impaction. Otoscopy and tympanometry were used to assess cerumen impaction and ear canal occlusion. Participants with excess cerumen were referred for wax removal.

RESULTS

No significant difference in cerumen accumulation was found between groups. However, 57% of participants showed excess cerumen requiring removal.

CONCLUSION

Diets involving reduced chewing are not associated with increased cerumen in older people in residential care. The prevalence of cerumen impaction is high in this population, and improved cerumen screening and management is needed in residential aged care.

Hearing aid review appointment: clients' reasons for attendance and non-attendance

Objective: To investigate hearing aid owners' decisions to attend or not to attend an annual hearing aid review (HAR) appointment. To investigate the possible factors associated with appointment attendance, including age, gender, transportation, travel time, and hearing aid outcomes.Design: A prospective cohort study. Potential participants were notified of their annual HAR appointment in the usual process employed by their clinic. Two months later, potential participants were identified as those who had attended and those who had not attended an appointment.Study sample: One hundred and twenty adult hearing aid users ranging in age from 26 to 100 (M = 74, SD = 11) years recruited from a single hearing clinic in Perth, Western Australia.Results: Factors found to be significantly associated with attendance at an annual HAR appointment included hearing aid funding source (government subsidised), participants valuing the importance and benefit of the appointment, and superior hearing aid outcomes.Conclusions: Within a controlled practice setting, appointment attendance is influenced by some factors modifiable by the clinician, including providing better education about the process and purpose of the HAR appointment. The value of the HAR appointment was emphasised by the positive association between better hearing aid outcomes HAR appointment attendance.

Cholinergic responses of acoustically-characterized cochlear nucleus neurons: An in vivo iontophoretic study in Guinea pig

The responses of guinea pig cochlear nucleus neurons to in vivo iontophoretic application of various neurotransmitter agonists were recorded with extracellular multi-barrelled electrodes. Where possible, neurons were physiologically identified using strict criteria. Emphasis was placed on the action of cholinergic agonists in relation to the possible action of olivocochlear collateral innervation. Excitatory responses (increase in action potential firing) to glutamate were confirmed in a number of neuronal response types. Application of acetylcholine (ACh) or the broad spectrum cholinergic agonist carbachol produced reliable excitatory responses in about 47% of neurons (n = 29 out of 61 neurons). The remaining neurons were unresponsive to cholinergic agonists and no inhibitory responses were observed. Cholinergic responses were more common in dorsal cochlear nucleus (DCN) (73% of 30 neurons tested) than in ventral cochlear nucleus (VCN) (23% of 31 neurons). Of the total neuron sample in which cholinergic responses were investigated, 41 neurons were able to be categorized according to established acoustic response features. Excitatory responses to cholinergic agonists were seen in "Pauser-buildup" (Pb) and "Transient chopper" (Ct) response types. Primary-like neurons (PL and Pn) as well as "Onset chopper" (Oc) neurons (n = 6) were unresponsive to either ACh or carbachol. Oc neurons also did not show any effect on their acoustic responses. Robust cholinergic responses were also seen in several VCN and DCN neurons that were either unresponsive to sound, or had acoustic response properties that did not fit standard classification. The results suggest a relatively more robust cholinergic innervation of DCN compared to VCN. The excitatory cholinergic responses of some Ct neurons and the lack of effect on Oc neurons are consistent with previous results in mouse brain slice studies, but are in conflict with reports of medial olivocochlear collateral excitatory responses in onset-type neurons in vivo. The results also indicate that a number of neurons of unknown identity may also receive cholinergic input.

Persistent hair cell malfunction contributes to hidden hearing loss

Noise exposures that result in fully reversible changes in cochlear neural threshold can cause a reduced neural output at supra-threshold sound intensity. This so-called "hidden hearing loss" has been shown to be associated with selective degeneration of high threshold afferent nerve fiber-inner hair cell (IHC) synapses. However, the electrophysiological function of the IHCs themselves in hidden hearing loss has not been directly investigated. We have made round window (RW) measurements of cochlear action potentials (CAP) and summating potentials (SP) after two levels of a 10 kHz acoustic trauma. The more intense acoustic trauma lead to notch-like permanent threshold changes and both CAP and SP showed reductions in supra-threshold amplitudes at frequencies with altered thresholds as well as from fully recovered regions. However, the interpretation of the results in normal threshold regions was complicated by the likelihood of reduced contributions from adjacent regions with elevated thresholds. The milder trauma showed full recovery of all neural thresholds, but there was a persistent depression of the amplitudes of both CAP and SP in response to supra-threshold sounds. The effect on SP amplitude in particular shows that occult damage to hair cell transduction mechanisms can contribute to hidden hearing loss. Such damage could potentially affect the supra-threshold output properties of surviving primary afferent neurons.

The effects of repetitive transcranial magnetic stimulation in an animal model of tinnitus

Tinnitus (phantom auditory perception associated with hearing loss) can seriously affect wellbeing. Its neural substrate is unknown however it has been linked with abnormal activity in auditory pathways. Though no cure currently exists, repetitive transcranial magnetic stimulation (rTMS) has been shown to reduce tinnitus in some patients, possibly via induction of cortical plasticity involving brain derived neurotrophic factor (BDNF). We examined whether low intensity rTMS (LI-rTMS) alleviates signs of tinnitus in a guinea pig model and whether this involves changes in BDNF expression and hyperactivity in inferior colliculus. Acoustic trauma was used to evoke hearing loss, central hyperactivity and tinnitus. When animals developed tinnitus, treatment commenced (10 sessions of 10 minutes 1 Hz LI-rTMS or sham over auditory cortex over 14 days). After treatment ceased animals were tested for tinnitus, underwent single-neuron recordings in inferior colliculus to assess hyperactivity and samples from cortex and inferior colliculus were taken for BDNF ELISA. Analysis revealed a significant reduction of tinnitus after LI-rTMS compared to sham, without a statistical significant effect on BDNF levels or hyperactivity. This suggests that LI-rTMS alleviates behavioural signs of tinnitus by a mechanism independent of inferior colliculus hyperactivity and BDNF levels and opens novel therapeutic avenues for tinnitus treatment.

Effects of pulsatile electrical stimulation of the round window on central hyperactivity after cochlear trauma in guinea pig

Partial hearing loss induced by acoustic trauma has been shown in animal models to result in an increased spontaneous firing rate in central auditory structures. This so-called hyperactivity has been suggested to be involved in the generation of tinnitus, a phantom auditory sensation. Although there is no universal cure for tinnitus, electrical stimulation of the cochlea, as achieved by a cochlear implant, can result in significant reduction of the tinnitus percept. However, the mechanism by which this tinnitus suppression occurs is as yet unknown and furthermore cochlear implantation may not be an optimal treatment option for tinnitus sufferers who are not profoundly deaf. A better understanding of the mechanism of tinnitus suppression by electrical stimulation of the cochlea, may lead to the development of more specialised devices for those for whom a cochlear implant is not appropriate. This study aimed to investigate the effects of electrical stimulation in the form of brief biphasic shocks delivered to the round window of the cochlea on the spontaneous firing rates of hyperactive inferior colliculus neurons following acoustic trauma in guinea pigs. Effects during the stimulation itself included both inhibition and excitation but spontaneous firing was suppressed for up to hundreds of ms after the cessation of the shock train in all sampled hyperactive neurons. Pharmacological block of olivocochlear efferent action on outer hair cells did not eliminate the prolonged suppression observed in inferior colliculus neurons, and it is therefore likely that activation of the afferent pathways is responsible for the central effects observed.

Effects of chronic furosemide on central neural hyperactivity and cochlear thresholds after cochlear trauma in Guinea pig

Increased neuronal spontaneous firing rates have been observed throughout the central auditory system after trauma to the cochlea and this hyperactivity is believed to be associated with the phantom perception of tinnitus. Previously, we have shown in an animal model of hearing loss, that an acute injection with furosemide can significantly decrease hyperactivity after cochlear trauma and eliminate behavioral evidence of tinnitus of early onset. However, furosemide also has the potential to affect cochlear thresholds. In this paper, we measured the effects of a chronic (daily injections for 7 days) furosemide treatment on the spontaneous firing rate of inferior colliculus neurons and on cochlear thresholds in order to establish whether a beneficial effect on hyperactivity can be obtained without causing additional hearing loss. Guinea pigs were exposed to a 10–kHz, 124 dB, 2 h acoustic trauma, and after 5 days of recovery, were given daily i.p. injections of 80 mg/kg furosemide or an equivalent amount of saline. The activity of single IC neurons was recorded 24 h following the last injection. The furosemide treatment had no effect on cochlear thresholds compared to saline injections but did result in significant reductions in spontaneous firing rates recorded in inferior colliculus. These results that suggest a long-term beneficial effect of furosemide on hyperactivity after cochlear trauma may be achievable without detrimental effects on hearing, which is important when considering therapeutic potential.

Effects of furosemide on cochlear neural activity, central hyperactivity and behavioural tinnitus after cochlear trauma in guinea pig

Cochlear trauma causes increased spontaneous activity (hyperactivity) to develop in central auditory structures, and this has been suggested as a neural substrate for tinnitus. Using a guinea pig model we have previously demonstrated that for some time after cochlear trauma, central hyperactivity is dependent on peripheral afferent drive and only later becomes generated intrinsically within central structures. Furosemide, a loop diuretic, reduces spontaneous firing of auditory afferents. We investigated in our guinea pig model the efficacy of furosemide in reducing 1) spontaneous firing of auditory afferents, using the spectrum of neural noise (SNN) from round window recording, 2) hyperactivity in inferior colliculus, using extracellular single neuron recordings and 3) tinnitus at early time-points after cochlear trauma. Tinnitus was assessed using gap prepulse inhibition of acoustic startle (GPIAS). Intraperitoneal furosemide, but not saline, caused a marked decrease in both SNN and central hyperactivity. Intracochlear perfusion with furosemide similarly reversed central hyperactivity. In animals in which GPIAS measurements suggested the presence of tinnitus (reduced GPIAS), this could be reversed with an intraperitoneal injection with furosemide but not saline. The results are consistent with furosemide reducing central hyperactivity and behavioural signs of tinnitus by acting peripherally to decrease spontaneous firing of auditory afferents. The data support the notion that hyperactivity may be involved in the generation of tinnitus and further suggest that there may be a therapeutic window after cochlear trauma using drug treatments that target peripheral spontaneous activity.

Modulation of gene expression in guinea pig paraflocculus after induction of hearing loss

Hearing loss often results in plastic changes in the central auditory pathways, which may be involved in the generation of tinnitus, a phantom auditory sensation. However, although animal studies have consistently shown increased neural activity in auditory structures after hearing loss, tinnitus does not always develop. It has therefore been suggested that non-auditory structures perform a gating or regulatory role that determines whether the increased activity in auditory structures leads to conscious perception. Recent evidence points to the paraflocculus of the cerebellum as having such a role. Therefore, we investigated the early effects of hearing loss on gene expression in guinea pig paraflocculus. Gene expression was investigated after two weeks recovery from either acoustic or mechanical cochlear trauma. The genes investigated in our study were associated with inhibitory neurotransmission (GABA-A receptor subunit alpha 1; glutamate decarboxylase 1), excitatory neurotransmission (glutamate receptor NMDA subunit 1), and regulation of transmitter release (member of RAB family of small GTPase). Our results show increased mRNA levels of glutamate decarboxylase 1 in ipsilateral paraflocculus with no difference between the different methods of cochlear trauma. Early modulation of gene expression in the paraflocculus suggests that an early effect of hearing loss may affect the influence of this structure on auditory processing.

Modulation of gene expression in guinea pig paraflocculus after induction of hearing loss

Hearing loss often results in plastic changes in the central auditory pathways, which may be involved in the generation of tinnitus, a phantom auditory sensation. However, although animal studies have consistently shown increased neural activity in auditory structures after hearing loss, tinnitus does not always develop. It has therefore been suggested that non-auditory structures perform a gating or regulatory role that determines whether the increased activity in auditory structures leads to conscious perception. Recent evidence points to the paraflocculus of the cerebellum as having such a role. Therefore, we investigated the early effects of hearing loss on gene expression in guinea pig paraflocculus. Gene expression was investigated after two weeks recovery from either acoustic or mechanical cochlear trauma. The genes investigated in our study were associated with inhibitory neurotransmission (GABA-A receptor subunit alpha 1; glutamate decarboxylase 1), excitatory neurotransmission (glutamate receptor NMDA subunit 1), and regulation of transmitter release (member of RAB family of small GTPase). Our results show increased mRNA levels of glutamate decarboxylase 1 in ipsilateral paraflocculus with no difference between the different methods of cochlear trauma. Early modulation of gene expression in the paraflocculus suggests that an early effect of hearing loss may affect the influence of this structure on auditory processing.

Development of hyperactivity after acoustic trauma in the guinea pig inferior colliculus

The time of onset of hyperactivity (increased spontaneous firing rates) was investigated by single neuron recording in the inferior colliculus (IC) of guinea pigs subjected to unilateral acoustic trauma (exposure to a loud 10 kHz tone). Hyperactivity was present by 12 h post acoustic trauma whereas data obtained within approximately 4 h of the cessation of acoustic trauma found no evidence of hyperactivity. These data suggest that hyperactivity in the IC begins at some time between 4 and 12 h post trauma and is a relatively rapid plastic event beginning within hours rather than days post cochlear trauma. This is consistent with results reported in the cat auditory cortex (Norena and Eggermont, 2003). Hyperactivity did not show any further systematic increase between 12 h and up to 2 weeks post acoustic trauma. At recovery times of 12 and 24 h hyperactivity was widespread across most regions of the IC but at longer recovery times, it became progressively more restricted to ventral regions corresponding to the regions of the cochlea where there was persistent damage.

Relationship between auditory thresholds, central spontaneous activity, and hair cell loss after acoustic trauma

Acoustic trauma caused by exposure to a very loud sound increases spontaneous activity in central auditory structures such as the inferior colliculus. This hyperactivity has been suggested as a neural substrate for tinnitus, a phantom hearing sensation. In previous studies we have described a tentative link between the frequency region of hearing impairment and the corresponding tonotopic regions in the inferior colliculus showing hyperactivity. In this study we further investigated the relationship between cochlear compound action potential threshold loss, cochlear outer and inner hair cell loss, and central hyperactivity in inferior colliculus of guinea pigs. Two weeks after a 10-kHz pure tone acoustic trauma, a tight relationship was demonstrated between the frequency region of compound action potential threshold loss and frequency regions in the inferior colliculus showing hyperactivity. Extending the duration of the acoustic trauma from 1 to 2 hours did not result in significant increases in final cochlear threshold loss, but did result in a further increase of spontaneous firing rates in the inferior colliculus. Interestingly, hair cell loss was not present in the frequency regions where elevated cochlear thresholds and central hyperactivity were measured, suggesting that subtle changes in hair cell or primary afferent neural function are sufficient for central hyperactivity to be triggered and maintained.

Tonotopic changes in GABA receptor expression in guinea pig inferior colliculus after partial unilateral hearing loss

Immunohistochemistry was used to investigate the topographic distribution of the alpha1 subunit of the GABA receptor (GABRA1) in guinea pig inferior colliculus after treatments that caused a unilateral loss of peripheral neural sensitivity in the high-frequency regions of the cochlea. Both forms of treatment (direct mechanical lesion of the cochlea and acoustic overstimulation) resulted in a significant decrease in GABRA1 labeling in regions of the contralateral inferior colliculus in which high-frequency sound stimuli are represented. This localized region of reduced inhibitory receptor expression corresponds to the region in which hyperactivity of inferior colliculus neurons has been shown to develop after such treatments. The results strengthen the notion of a causal link between reduced GABRA1 expression and neural hyperactivity in central auditory nuclei and provide a possible mechanism for the development of phantom auditory sensations, or tinnitus.

Electrically evoked responses in onset chopper neurons in guinea pig cochlear nucleus

Extracellular recordings were obtained from single cochlear nucleus neurons in guinea pigs anesthetized with Nembutal and Hypnorm. Neurons were classified by their spontaneous firing rates and responses to acoustic stimuli. In addition, electrical shocks were applied to the midline at the level of the IVth ventricle and spike responses were recorded. Spikes were evoked by shocks only in neurons that were classified as onset choppers (O(c)). The shock-evoked spikes could be extinguished by acoustically evoked action potentials in the same neurons. In roughly 30% of the sample of O(c) neurons, quantitative aspects of the timing of this extinction were not compatible with the shock-evoked spike being antidromically conducted from O(c) output axons. Together with the presence of temporal jitter at high shock rates, the data suggest the possibility that at least some of the shock-evoked spikes may be generated by excitatory synaptic input to the O(c) neurons, most likely from the collaterals of the medial olivocochlear system (MOCS), whose axons pass close to the floor of the IVth ventricle. This excitatory synaptic input may operate to modulate the activity of O(c) neurons in addition to MOCS actions in the auditory periphery.

Gentamicin abolishes all cochlear effects of electrical stimulation of the inferior colliculus

Electrical stimulation of the inferior colliculus (IC) has been shown to result in suppression of cochlear output, due to activation of the medial olivocochlear system. This auditory efferent system originates in the brainstem and terminates on the outer hair cells in the cochlea. Recently, excitatory effects of IC stimulation have also been reported, both on cochlear gross potentials and on primary auditory afferents. It has been hypothesized that this excitation is due to co-activation of the lateral olivocochlear system, which synapses on the primary auditory afferent fibres contacting the inner hair cells. If stimulation of the IC leads to the activation of both the medial and lateral olivocochlear system, resulting in a mixture of inhibitory and excitatory effects in the cochlea, then removal of the inhibitory effects, by blocking the medial system, should lead to more pronounced excitatory effects out in the periphery. To investigate this hypothesis, we recorded the effect of IC stimulation on cochlear gross potentials as well as on single auditory primary afferents in guinea pigs following block of the medial olivocochlear system with gentamicin. We found that administration of gentamicin, whether intraperitoneally or by intracochlear perfusion, blocked all effects of IC stimulation, whether inhibitory or excitatory. These data strongly suggest that all effects observed after IC stimulation, both inhibitory as well as excitatory, are due to the activation of the medial olivocochlear system.

Cellular localization of voltage-gated calcium channels and synaptic vesicle-associated proteins in the guinea pig cochlea

The cellular localization of voltage-gated calcium channels (VGCCs) and synaptic vesicle-associated proteins, SV2, synapsin I, and vesicle-associated membrane protein (VAMP) (synaptobrevin), was investigated in the guinea pig cochlea using immunocytochemistry and confocal laser scanning microscopy. Reactivity, in guinea pig, of antibodies to the alpha1 subunits of L-type, alpha1C [Cav1.2] and alpha 1D [Cav1.3]; P/Q-type, alpha1A [Cav2.1]; and R-type, a1E [Cav2.3] high voltage-activated calcium channels, was determined by Western blotting and immunolabeling of cerebellum. In the cochlea the sensory inner hair cells of the organ of Corti displayed strong intracellular staining, predominantly localized to their basolateral poles, with an antibody directed against the alpha1C subunit. Some alpha1C labeling was also observed in the inner pillar cells, in cell bodies of afferent neurons in the spiral ganglion, and in the inferior region of the spiral ligament. The supporting pillar cells were strongly immunoreactive throughout for alpha1D, but no alpha1D labeling of the inner hair cells was seen. The alpha1A subunit showed a cytoplasmic distribution in all three rows of outer hair cells. alpha1E labeling localized to the outer hair cells, predominantly in the subcuticular plate region, and also to nerve fiber bundles beneath these hair cells. Strong immunoreactivity was consistently seen with antibodies directed against SV2 and synapsin I in neuronal structures surrounding the basolateral surfaces of both the inner and outer hair cells but was absent from the sensory cells themselves. VAMP labeling was found throughout the cytoplasm of the inner hair cells and in neuronal structures beneath the hair cells. These results reveal a differential distribution of VGCC-types in the sensory and nonsensory elements of the guinea pig cochlea, with the inner hair cells expressing alpha1C L-type channels and VAMP but not synapsin I or SV2.

Catecholaminergic innervation of guinea pig superior olivary complex

In mammals, olivocochlear neurons in the superior olivary complex project to the cochlea, providing input to outer hair cells and auditory afferents contacting inner hair cells. In the rat it has been demonstrated that olivocochlear neurons receive noradrenergic input, arising from the locus coeruleus and it has been demonstrated in this species using in vitro brain slices that noradrenaline exerts a direct, mostly excitatory effect on an olivocochlear subpopulation. The guinea pig is a more commonly used animal in auditory physiology than the rat and anatomical data on noradrenaline in the auditory brainstem in this species are lacking. Because it has been shown that a compact locus coeruleus is not present in the guinea pig, subtle species differences might be expected. Therefore, using immunohistochemical and tracing techniques we have investigated in the guinea pig (1) the noradrenergic and dopaminergic innervation of the superior olivary complex, (2) the anatomical relationship between noradrenergic fibres and olivocochlear neurons and (3) the origin of the noradrenergic input to this brainstem region. The results show that the guinea pig superior olivary complex receives moderately dense noradrenergic innervation and no dopaminergic innervation. In addition, noradrenergic fibres and varicosities were observed in close contact with both somata and dendrites of olivocochlear neurons, strongly suggestive of synaptic contacts. Finally the results show that a significant component of the noradrenergic innervation of the guinea pig superior olivary complex arises in the locus subcoeruleus, which is a structure likely to be the homologue of the locus coeruleus in rats and other species.

Noradrenergic modulation of brainstem nuclei alters cochlear neural output

The peripheral auditory sense organ, the cochlea, receives innervation from lateral and medial olivocochlear neurons in the brainstem. These neurons are able to modulate cochlear neural output. Anatomical studies have shown that one of the neurotransmitters which is present in varicosities surrounding the olivocochlear neurons in the brainstem is noradrenaline and previous work on brainstem slices has demonstrated a generally excitatory effect of noradrenaline on medial olivocochlear neurons. In order to assess in vivo the function of the noradrenergic inputs to olivocochlear neurons, we injected noradrenaline in the brainstem of anaesthetised guinea pigs and recorded ipsilateral cochlear electrical activity. Injections of noradrenaline close to the lateral olivocochlear neurons evoked increases in the sound-driven neural activity from the cochlea, measured as compound action potential (CAP) amplitude, as well as in the spontaneous activity, measured as amplitude of the 900 Hz peak of the spectrum of the neural noise in the cochlear fluids. In contrast, noradrenaline in the vicinity of the medial olivocochlear neurons evoked inhibitory effects on both the CAP amplitude and 900 Hz peak. These results indicate most likely an excitatory action of noradrenaline on both the lateral and medial olivocochlear neurons in the brainstem, and show that such noradrenergic inputs can modulate cochlear function.

Diverse responses of single auditory afferent fibres to electrical stimulation of the inferior colliculus in guinea-pig

Medial olivocochlear (MOC) neurons in the auditory brainstem project to the cochlea and inhibit cochlear neural output by their action on the cochlear outer hair cells. The function of the lateral olivocochlear (LOC) neurons, projecting to the auditory primary afferents is still under debate. Recent studies have suggested that the olivocochlear system can have frequency-specific, spatially restricted effects within the cochlea. It has been shown that the inferior colliculus (IC) projects to the MOC neurons in a tonotopic manner and that electrical stimulation of the IC can activate the MOC system, suppressing cochlear gross potentials. In addition, it has been shown that stimulation of the IC may be able to activate the LOC neurons. We investigated the effect of IC stimulation on single units in the cochlea of guinea-pigs and searched for evidence of spatially restricted effects of the MOC system and effects of the LOC system. We found a variety of effects on single units. About 40% of units were unchanged whereas others (53%) showed inhibitory effects, reflected in a rightward shift of their rate-level function, sometimes accompanied by a suppression of the spontaneous rate. About 18% of the inhibited neurons showed an increased spontaneous rate. In 5% of the units we observed an excitatory effect of IC stimulation, resulting in a leftward shift of the rate-level functions. We also found that the effect could vary greatly between units of the same and adjacent frequencies within a single animal. These results imply an involvement of another regulatory system besides the MOC system, possibly the LOC system, which acts directly on the primary afferents. These data also demonstrate that the olivocochlear system is capable of eliciting highly localized effects on different frequency regions in the cochlea.

Dopaminergic olivocochlear neurons originate in the high frequency region of the lateral superior olive of guinea pigs

Dopaminergic neurons are known to exist within the lateral superior olive (LSO). The LSO is the nucleus of origin of the lateral olivocochlear neurons, which project to the cochlea and synapse onto the primary afferents contacting the inner hair cells. We investigated whether the dopaminergic neurons in the LSO are part of the lateral olivocochlear neuron population. We combined intracochlear injections of a fluorescent retrograde tracer with immunofluorescent staining of tyrosine hydroxylase (TH). TH was used as a marker for dopaminergic neurons. After the injection with retrograde tracer most of the TH-labelled neurons in the LSO also contained the tracer, which directly demonstrates for the first time that the TH-labelled, dopaminergic neurons in the LSO are lateral olivocochlear neurons. TH-labelled neurons were not equally distributed over the LSO as is observed for the lateral olivocochlear neurons in general. TH-labelled neurons were almost exclusively seen in the medial, high frequency, limb of the LSO. Since the projection of the lateral olivocochlear neurons to the cochlea is known to be tonotopic, we investigated the TH-labelling in the cochlea as well. We found that the staining pattern of TH in the cochlea is in broad agreement with the distribution of TH-labelling in the LSO. Cochlear sections showed dense labelling in the basal and second, high frequency, turns and decreasing intensity of staining in the third turn, while the extreme apical, low frequency, turn was almost devoid of any positive TH-labelling. These observations imply that the dopaminergic neurons of the lateral olivocochlear system may play a role in the selective suppression of the high frequency fibers of the auditory system.

Olivocochlear collaterals evoke excitatory effects in onset neurones of the rat cochlear nucleus

Axons of medial olivocochlear neurones in the superior olivary complex terminate on the outer hair cells of the cochlea and also give off collaterals that terminate in the cochlear nucleus. Previous work in our laboratory, using extracellular recordings in the cochlear nucleus, has indicated that stimulation of the olivocochlear axons may have an excitatory effect on specific cell populations of the cochlear nucleus, such as onset-choppers, in contrast to the peripheral suppressive action of the same axons. We have investigated whether this excitation is produced by action of the olivocochlear collaterals in the cochlear nucleus or whether it is mediated via the peripheral suppression, by measuring intracellular responses in the rat cochlear nucleus to electrical stimulation of the olivocochlear axons in silence. The results demonstrate that single shocks applied to the olivocochlear axons can evoke excitatory postsynaptic potentials in onset neurones. We observed an inhibitory effect in one chopper only. In the same animals in all other neurones investigated (i.e. three primary-like neurones and eight choppers) the same stimulation was without any effect on cell membrane potential. We conclude that the excitatory effects in onset neurones are not caused by suppression in the auditory peripheral organ, but by activation of olivocochlear collaterals in the cochlear nucleus.

Dual action of olivocochlear collaterals in the guinea pig cochlear nucleus

Axons of olivocochlear neurones in the superior olivary complex terminate on hair cells of the cochlea, reducing the sensitivity to sound. These axons also have collateral branches to neurones in the cochlear nucleus, the first processing centre in the brainstem. Anatomical data show that these collaterals terminate mainly in the granule cell area but their precise neuronal targets and the effects they might have are unknown. We have studied the effects of these collaterals in guinea pigs, by electrically stimulating the olivocochlear axons at the floor of the IVth ventricle while recording single neurone responses in the cochlear nucleus. We eliminated the peripheral effects of olivocochlear stimulation either by destruction of the target receptor cells using chronic administration of kanamycin, or by acute perfusion of the cochlea with strychnine, a specific blocker of the postsynaptic receptors. Electrical stimulation of the olivocochlear axons in normal animals caused a variety of effects on cochlear nucleus neurones. In some neurones, there was suppression of spontaneous firing and a reduction in sensitivity to sound, while in others there was an excitatory effect of olivocochlear axon stimulation. When the peripheral olivocochlear action was eliminated, we still found both inhibition and excitation in the cochlear nucleus. These results show that the effects of olivocochlear stimulation on cochlear nucleus responses are not a simple passive reflection of peripheral changes but are a result of complex interactions between peripheral suppression of afferent input and collateral-mediated excitation and possibly also inhibition.

Learning deficits in aged rats related to decrease in extracellular volume and loss of diffusion anisotropy in hippocampus

The extracellular space (ECS) is the microenvironment of the nerve cells and an important communication channel, allowing for long-distance extrasynaptic communication between cells. Changes in ECS size, geometry, and composition have been reported in diverse (patho)physiological states, including aging. In the present study, real-time tetramethylammonium (TMA+) iontophoresis was used to quantify ECS diffusion parameters in different brain regions of adult and behaviorally characterized aged rats. Prior to ECS diffusion measurement, superior and inferior learners were selected from a large group of aged rats, according to their performance in the open-field water maze. The main finding was that the degree of impaired maze performance of old rats correlates, firstly, with decrease in ECS volume, loss of diffusion anisotropy in hippocampus, and degree of astrogliosis, and secondly, with disorganization of the astrocytic processes and reduction of hippocampal ECS matrix molecules. Importantly, no significant differences were found in the density of neurons in any region of the hippocampus or dentate gyrus. The alterations in hippocampal diffusion parameters evident in aged animals with severe learning deficits could account for the learning impairment, due to their effects on extrasynaptic volume transmission and/or on the "cross-talk" between synapses, which has been suggested to be involved in neural processes associated with learning and memory formation.

Inputs from the cochlea and the inferior colliculus converge on olivocochlear neurones

Medial olivocochlear (MOC) neurones, located in the superior olivary complex, can suppress cochlear gain by their action on the cochlear outer hair cells. Inputs from the contralateral cochlea and the inferior colliculus (IC) have been separately shown to increase activity of MOC neurones. In this study we have investigated in guinea-pigs under barbiturate anaesthesia the interactions between these two inputs by combining electrical stimulation of the IC with acoustic stimulation of the contralateral cochlea. Electrical stimulation of the IC resulted in a significant suppression of the amplitude of the compound action potential (CAP) of the auditory nerve to test tones. This suppression was equivalent to an average decrease in sound intensity of 5.7 dB and 3.7 dB for contralateral and ipsilateral stimulation, respectively. Acoustic stimulation of the contralateral cochlea with broadband noise produced no detectable change in the amplitude of the CAP in the test cochlea in all but one animal. However, simultaneous electrical stimulation of the IC and acoustic stimulation of the contralateral cochlea resulted in a reduction in CAP amplitude that was markedly larger than that produced by IC stimulation alone. The suppression with the addition of contralateral noise was equivalent to a mean reduction in sound intensity of 8.7 dB with contralateral and 5.7 dB with ipsilateral IC stimulation. We hypothesise that excitatory input from the contralateral cochlea converges with excitatory input from the IC on the MOC neurones and in this way augments the activity of these neurones, resulting in a larger peripheral effect.