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

Hearing loss-related altered neuronal activity in the inferior colliculus

Hearing loss is well known to cause plastic changes in the central auditory system and pathological changes such as tinnitus and hyperacusis. Impairment of inner ear functions is the main cause of hearing loss. In aged individuals, not only inner ear dysfunction but also senescence of the central nervous system is the cause of malfunction of the auditory system. In most cases of hearing loss, the activity of the auditory nerve is reduced, but that of the successive auditory centers is increased in a compensatory way. It has been reported that activity changes occur in the inferior colliculus (IC), a critical nexus of the auditory pathway. The IC integrates the inputs from the brainstem and drives the higher auditory centers. Since abnormal activity in the IC is likely to affect auditory perception, it is crucial to elucidate the neuronal mechanism to induce the activity changes of IC neurons with hearing loss. This review outlines recent findings on hearing-loss-induced plastic changes in the IC and brainstem auditory neuronal circuits and discusses what neuronal mechanisms underlie hearing-loss-induced changes in the activity of IC neurons. Considering the different causes of hearing loss, we discuss age-related hearing loss separately from other forms of hearing loss (non-age-related hearing loss). In general, the main plastic change of IC neurons caused by both age-related and non-age-related hearing loss is increased central gain. However, plastic changes in the IC caused by age-related hearing loss seem to be more complex than those caused by non-age-related hearing loss.

Age-Related Changes in Interaural-Level-Difference-Based Across-Frequency Binaural Interference

Low-frequency interaural time differences and high-frequency interaural level differences (ILDs) are used to localize sounds in the horizontal plane. Older listeners appear to be worse at horizontal-plane sound localization to compared younger listeners, but little is understood about age-related changes to across-frequency binaural processing. This study investigated if the frequency dependence of across-frequency ILD processing is altered for older compared to younger listeners, which was done by using an across-frequency binaural interference task (when the interaural difference sensitivity for a target sound is decreased by a spectrally remote interfering sound with zero interaural differences). It was hypothesized that as listeners experience advancing age and age-related high-frequency hearing loss (i.e., presbycusis), they will demonstrate worse binaural performance and experience more across-channel binaural interference (because of age-related temporal processing deficits), and will increasingly be affected by interferers at lower frequencies (because of age-related hearing loss) when compared to younger listeners. There were 11 older (>65 yrs) and 20 younger (<30 yrs) listeners with normal to near-normal audiometric thresholds up to 2 kHz. They were tested using a left-right ILD lateralization discrimination task. Single-tone ILD discrimination thresholds and across-frequency binaural interference were measured at 0.5, 1, 2, 4, and 8 kHz. ILD thresholds and interference were about twice as large for older compared to younger listeners. Interferers ≤1 kHz produced 2-3 times as much across-frequency binaural interference for older compared to younger listeners. Hearing thresholds were significant predictors of single-tone ILD thresholds; in addition, both target and interferer hearing thresholds were significant predictors of binaural interference. The results suggest a reweighting of binaural information that occurs with advancing age and age-related high-frequency hearing loss. This evidence of plasticity may help explain some of the age-related changes in spatial-hearing abilities.

Robustness of neuronal tuning to binaural sound localization cues against age-related loss of inhibitory synaptic inputs

Sound localization relies on minute differences in the timing and intensity of sound arriving at both ears. Neurons of the lateral superior olive (LSO) in the brainstem process these interaural disparities by precisely detecting excitatory and inhibitory synaptic inputs. Aging generally induces selective loss of inhibitory synaptic transmission along the entire auditory pathways, including the reduction of inhibitory afferents to LSO. Electrophysiological recordings in animals, however, reported only minor functional changes in aged LSO. The perplexing discrepancy between anatomical and physiological observations suggests a role for activity-dependent plasticity that would help neurons retain their binaural tuning function despite loss of inhibitory inputs. To explore this hypothesis, we use a computational model of LSO to investigate mechanisms underlying the observed functional robustness against age-related loss of inhibitory inputs. The LSO model is an integrate-and-fire type enhanced with a small amount of low-voltage activated potassium conductance and driven with (in)homogeneous Poissonian inputs. Without synaptic input loss, model spike rates varied smoothly with interaural time and level differences, replicating empirical tuning properties of LSO. By reducing the number of inhibitory afferents to mimic age-related loss of inhibition, overall spike rates increased, which negatively impacted binaural tuning performance, measured as modulation depth and neuronal discriminability. To simulate a recovery process compensating for the loss of inhibitory fibers, the strength of remaining inhibitory inputs was increased. By this modification, effects of inhibition loss on binaural tuning were considerably weakened, leading to an improvement of functional performance. These neuron-level observations were further confirmed by population modeling, in which binaural tuning properties of multiple LSO neurons were varied according to empirical measurements. These results demonstrate the plausibility that homeostatic plasticity could effectively counteract known age-dependent loss of inhibitory fibers in LSO and suggest that behavioral degradation of sound localization might originate from changes occurring more centrally.

Sex difference in the efferent inner hair cell synapses of the aging murine cochlea

Efferent innervation of the inner hair cells changes over time. At an early age in mice, inner hair cells receive efferent feedback, which helps fine-tune tonotopic maps in the brainstem. In adulthood, inner hair cell efferent innervation wanes but increases again in older animals. It is not clear, however, whether age-related inner hair cell efferents increase along the entire range of the cochlear frequencies, or if this increase is restricted to a particular frequency-region, and whether this phenomenon occurs in both sexes. Age-related hearing loss, presbycusis, affects men and women differently. In mice, this difference is also strain specific. In aging black six mice, the auditory brainstem response thresholds increase in females earlier than in males. Here, we study age-related increase of the inner hair cell efferent innervation throughout the cochlea before hearing onset, in one month old and in ten months old and older male and female black six mice. We collected confocal images of immunostained inner hair cell efferents and quantified the labeled terminals in the entire cochlea using a machine learning algorithm. The overall number of the inner hair cell efferents in both sexes did not change significantly between age-groups. The distribution of the inner hair cell efferent innervation did not differ across frequencies in the cochlea. However, in females, inner hair cells received on average up to four times more efferent innervation than in males per each of the frequency regions tested. Sex differences were also found in the oldest age-group tested (≥ 10 months) where on average inner hair cells received six times more efferents in females than in males of matching age. Our findings emphasize the importance of including both sexes in sensorineural hearing loss research.

Physiological models of the lateral superior olive

In computational biology, modeling is a fundamental tool for formulating, analyzing and predicting complex phenomena. Most neuron models, however, are designed to reproduce certain small sets of empirical data. Hence their outcome is usually not compatible or comparable with other models or datasets, making it unclear how widely applicable such models are. In this study, we investigate these aspects of modeling, namely credibility and generalizability, with a specific focus on auditory neurons involved in the localization of sound sources. The primary cues for binaural sound localization are comprised of interaural time and level differences (ITD/ILD), which are the timing and intensity differences of the sound waves arriving at the two ears. The lateral superior olive (LSO) in the auditory brainstem is one of the locations where such acoustic information is first computed. An LSO neuron receives temporally structured excitatory and inhibitory synaptic inputs that are driven by ipsi- and contralateral sound stimuli, respectively, and changes its spike rate according to binaural acoustic differences. Here we examine seven contemporary models of LSO neurons with different levels of biophysical complexity, from predominantly functional ones (‘shot-noise’ models) to those with more detailed physiological components (variations of integrate-and-fire and Hodgkin-Huxley-type). These models, calibrated to reproduce known monaural and binaural characteristics of LSO, generate largely similar results to each other in simulating ITD and ILD coding. Our comparisons of physiological detail, computational efficiency, predictive performances, and further expandability of the models demonstrate (1) that the simplistic, functional LSO models are suitable for applications where low computational costs and mathematical transparency are needed, (2) that more complex models with detailed membrane potential dynamics are necessary for simulation studies where sub-neuronal nonlinear processes play important roles, and (3) that, for general purposes, intermediate models might be a reasonable compromise between simplicity and biological plausibility.

Computational models help our understanding of complex biological systems, by identifying their key elements and revealing their operational principles. Close comparisons between model predictions and empirical observations ensure our confidence in a model as a building block for further applications. Most current neuronal models, however, are constructed to replicate only a small specific set of experimental data. Thus, it is usually unclear how these models can be generalized to different datasets and how they compare with each other. In this paper, seven neuronal models are examined that are designed to reproduce known physiological characteristics of auditory neurons involved in the detection of sound source location. Despite their different levels of complexity, the models generate largely similar results when their parameters are tuned with common criteria. Comparisons show that simple models are computationally more efficient and theoretically transparent, and therefore suitable for rigorous mathematical analyses and engineering applications including real-time simulations. In contrast, complex models are necessary for investigating the relationship between underlying biophysical processes and sub- and suprathreshold spiking properties, although they have a large number of unconstrained, unverified parameters. Having identified their advantages and drawbacks, these auditory neuron models may readily be used for future studies and applications.

Age-dependent changes in the lateral superior olive of the gerbil (Meriones unguiculatus)

Data from humans and animal models provide evidence for an age-dependent impairment in the ability to localize sound. The lateral superior olive (LSO) in the ascending auditory pathway is one important center involved in processing of binaural auditory stimuli. To identify potential age-dependent changes we characterized the LSO in young (< 15 months) and old (> or =3 years) gerbils with a special emphasis on the expression of GABA- and glycine-like immuno-reactivity. The dimensions of the LSO, as well as the number and density of glycine- and GABA-immuno-reactive neurons, were not significantly different between young and old gerbils. The size of glycine- and GABA-immuno-reactive neurons was significantly reduced in the high-frequency (medial) limb of the LSO. Over all, age-dependent changes in the LSO of the gerbil were small.

Hyperpolarization-activated (Ih) conductances affect brainstem auditory neuron excitability

Blockade of the hyperpolarization-activated cyclic-nucleotide-gated mixed-cationic conductance (I(h)) by ZD7288 markedly reduces excitability of neurons in the superior olivary complex (SOC), in vivo. Following pressure ejection application of 100 microM ZD7288, extracellular recorded single unit responses of 47/47 SOC neurons to monaural or binaural pure tone best frequency (BF) stimuli (30 dB above threshold) decreased by 49.7+/-19%, and background activity decreased by 56.3+/-18.1%. Pressure ejection of the vehicle did not affect excitability. The dose- and time-dependence of ZD7288 (10-100 microM) effects are consistent with specific blockade of I(h) currents. SOC neuron responses to pressure-ejected glutamate were also decreased following application of 100 microM ZD7288 by 76.7+/-28.0%, which suggests a predominant direct effect of ZD7288 on auditory cell excitability. The considerable variability in the magnitude of ZD7288 effects between cells was only partially accounted for by greater effects on neurons with BFs greater than 16 kHz. Therefore, I(h) channels significantly contribute to auditory brainstem neuron excitability, affecting their response level to acoustic stimuli. The variability in the ZD7288 reduction in excitability and its variation with the BF of units could be an indication of regulation and plasticity in neuronal encoding of sounds.

Membrane properties of principal neurons of the lateral superior olive

In the lateral superior olive (LSO) the firing rate of principal neurons is a linear function of inter-aural sound intensity difference (IID). The linearity and regularity of the "chopper response" of these neurons have been interpreted as a result of an integration of excitatory ipsilateral and inhibitory contralateral inputs by passive soma-dendritic cable properties. To account for temporal properties of this output, we searched for active time- and voltage-dependent nonlinearities in whole cell recordings from a slice preparation of the rat LSO. We found nonlinear current-voltage relations that varied with the membrane holding potential. Repetitive regular firing, supported by voltage oscillations, was evoked by current pulses injected from holding potentials near rest, but the response was reduced to an onset spike of fixed short latency when the pulse was injected from de- or hyperpolarized holding potentials. The onset spike was triggered by a depolarizing transient potential that was supported by T-type Ca(2+)-, subthreshold Na(+)-, and hyperpolarization-activated (I(H)) conductances sensitive, respectively, to blockade with Ni2+, tetrodotoxin (TTX), and Cs+. In the hyperpolarized voltage range, the I(H), was largely masked by an inwardly rectifying K+ conductance (I(KIR)) sensitive to blockade with 200 microM Ba2+. In the depolarized range, a variety of K+ conductances, including A-currents sensitive to blockade with 4-aminopyridine (4-AP) and additional tetraethylammonium (TEA)-sensitive currents, terminated the transient potential and firing of action potentials, supporting a strong spike-rate adaptation. The "chopper response," a hallmark of LSO principal neuron firing, may depend on the voltage- and time-dependent nonlinearities. These active membrane properties endow the LSO principal neurons with an adaptability that may maintain a stable code for sound direction under changing conditions, for example after partial cochlear hearing loss.

Short-term adaptation of excitation and inhibition shapes binaural processing

The temporal dependence of neuronal responses in the superior olivary complex (SOC) and central nucleus of the inferior colliculus (ICC) were examined using modified forward masking paradigms. Masking and probe tones were at the unit's best frequency and at the same intensity (20-30 dB above threshold). Short-term adaptation was observed in 85 of 113 SOC and in 32 of 50 ICC neurons, and resulted in an average decrease of probe responses (1 or 2 ms after the masker) of 56.3% in SOC neurons and 83.1% in ICC neurons. Recovery from adaptation followed exponential trends, with mean time constants of 106.1 ms and 226.9 ms for SOC and ICC neurons, respectively. Adaptation of inhibition was observed in the lateral superior olive, and may also affect many of the neurons studied. Other ICC neurons (n = 7) exhibited facilitation of probe tone responses, while 6 ICC neurons exhibited more complex temporal changes in responsiveness following a masker.

Excitatory and inhibitory response adaptation in the superior olive complex affects binaural acoustic processing

Short-term adaptation was examined in single unit recordings from 113 superior olive neurons of anaesthetized 3- to 6-month-old Long-Evans rats. Responses to an equal intensity BF probe tone presented 1 ms after an 'adapting' BF tone were adapted by 56.3 +/- 2.6% (mean +/- S.E.) compared to responses at a 512 ms delay. The rapid decrease in discharge rate during adapting tones often approximated exponential time courses with time constants of less than 20 ms. The recovery from adaptation was exponential with time constants of 106 +/- 20.0 ms. The magnitude of adaptation and time course of recovery following monaural stimulation of binaurally excited (EE) neurons were not significantly different in both input pathways. Additionally, in 60% of EE neurons, an 'adapting' tone presented to one ear reduced subsequent responses to probe tones presented to the opposite ear. Binaural stimulation resulted in equal or greater adaptation of responses than monaural stimulation of either ear. The recovery of binaural excitatory responses generally followed a time course between recovery functions for ipsilateral and contralateral monaural stimuli. Lateral Superior Olive (LSO) neurons encode sound source location through the interaction of ipsilateral excitation and contralateral inhibition (IE). Ipsilaterally driven excitatory responses in LSO neurons exhibited the greatest magnitude of adaptation (68.5 +/- 21.1%). Adaptation of inhibition was observed in over half of IE neurons. Responses of LSO neurons to binaural BF probe stimuli were greatest immediately after a 200 ms BF 'inhibitory adapting' stimulus to the contralateral ear, and decreased with greater interstimulus delays. Responses to binaural stimulation were constant after prior binaural adaptation, when the magnitude and recovery of adaptation to monaural stimuli were similar for excitation and inhibition (8/25 IE cells). The functional significance and possible sites of adaptation processes are discussed.