Acoustic chiasm. III: Nature, distribution, and sources of afferents to the lateral superior olive in the cat

Cholinergic modulation in the vertebrate auditory pathway

Acetylcholine (ACh) is a prevalent neurotransmitter throughout the nervous system. In the brain, ACh is widely regarded as a potent neuromodulator. In neurons, ACh signals are conferred through a variety of receptors that influence a broad range of neurophysiological phenomena such as transmitter release or membrane excitability. In sensory circuitry, ACh modifies neural responses to stimuli and coordinates the activity of neurons across multiple levels of processing. These factors enable individual neurons or entire circuits to rapidly adapt to the dynamics of complex sensory stimuli, underscoring an essential role for ACh in sensory processing. In the auditory system, histological evidence shows that acetylcholine receptors (AChRs) are expressed at virtually every level of the ascending auditory pathway. Despite its apparent ubiquity in auditory circuitry, investigation of the roles of this cholinergic network has been mainly focused on the inner ear or forebrain structures, while less attention has been directed at regions between the cochlear nuclei and midbrain. In this review, we highlight what is known about cholinergic function throughout the auditory system from the ear to the cortex, but with a particular emphasis on brainstem and midbrain auditory centers. We will focus on receptor expression, mechanisms of modulation, and the functional implications of ACh for sound processing, with the broad goal of providing an overview of a newly emerging view of impactful cholinergic modulation throughout the auditory pathway.

An inhibitory glycinergic projection from the cochlear nucleus to the lateral superior olive

Auditory brainstem neurons in the lateral superior olive (LSO) receive excitatory input from the ipsilateral cochlear nucleus (CN) and inhibitory transmission from the contralateral CN via the medial nucleus of the trapezoid body (MNTB). This circuit enables sound localization using interaural level differences. Early studies have observed an additional inhibitory input originating from the ipsilateral side. However, many of its details, such as its origin, remained elusive. Employing electrical and optical stimulation of afferents in acute mouse brainstem slices and anatomical tracing, we here describe a glycinergic projection to LSO principal neurons that originates from the ipsilateral CN. This inhibitory synaptic input likely mediates inhibitory sidebands of LSO neurons in response to acoustic stimulation.

Electrical signaling in cochlear efferents is driven by an intrinsic neuronal oscillator

Efferent neurons are believed to play essential roles in maintaining auditory function. The lateral olivocochlear (LOC) neurons-which project from the brainstem to the inner ear, where they release multiple transmitters including peptides, catecholamines, and acetylcholine-are the most numerous yet least understood elements of efferent control of the cochlea. Using in vitro calcium imaging and patch-clamp recordings, we found that LOC neurons in juvenile and young adult mice exhibited extremely slow waves of activity (∼0.1 Hz). These seconds-long bursts of Na⁺ spikes were driven by an intrinsic oscillator dependent on L-type Ca²⁺ channels and were not observed in prehearing mice, suggesting an age-dependent mechanism underlying the intrinsic oscillator. Using optogenetic approaches, we identified both ascending (T-stellate cells of the cochlear nucleus) and descending (auditory cortex) sources of synaptic excitation, as well as the synaptic receptors used for such excitation. Additionally, we identified potent inhibition originating in the glycinergic medial nucleus of trapezoid body (MNTB). Conductance-clamp experiments revealed an unusual mechanism of electrical signaling in LOC neurons, in which synaptic excitation and inhibition served to switch on and off the intrinsically generated spike burst mechanism, allowing for prolonged periods of activity or silence controlled by brief synaptic events. Protracted bursts of action potentials may be essential for effective exocytosis of the diverse transmitters released by LOC fibers in the cochlea.

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.

Roles for Coincidence Detection in Coding Amplitude-Modulated Sounds

Many sensory neurons encode temporal information by detecting coincident arrivals of synaptic inputs. In the mammalian auditory brainstem, binaural neurons of the medial superior olive (MSO) are known to act as coincidence detectors, whereas in the lateral superior olive (LSO) roles of coincidence detection have remained unclear. LSO neurons receive excitatory and inhibitory inputs driven by ipsilateral and contralateral acoustic stimuli, respectively, and vary their output spike rates according to interaural level differences. In addition, LSO neurons are also sensitive to binaural phase differences of low-frequency tones and envelopes of amplitude-modulated (AM) sounds. Previous physiological recordings in vivo found considerable variations in monaural AM-tuning across neurons. To investigate the underlying mechanisms of the observed temporal tuning properties of LSO and their sources of variability, we used a simple coincidence counting model and examined how specific parameters of coincidence detection affect monaural and binaural AM coding. Spike rates and phase-locking of evoked excitatory and spontaneous inhibitory inputs had only minor effects on LSO output to monaural AM inputs. In contrast, the coincidence threshold of the model neuron affected both the overall spike rates and the half-peak positions of the AM-tuning curve, whereas the width of the coincidence window merely influenced the output spike rates. The duration of the refractory period affected only the low-frequency portion of the monaural AM-tuning curve. Unlike monaural AM coding, temporal factors, such as the coincidence window and the effective duration of inhibition, played a major role in determining the trough positions of simulated binaural phase-response curves. In addition, empirically-observed level-dependence of binaural phase-coding was reproduced in the framework of our minimalistic coincidence counting model. These modeling results suggest that coincidence detection of excitatory and inhibitory synaptic inputs is essential for LSO neurons to encode both monaural and binaural AM sounds.

Detecting coincident arrivals of synaptic inputs is a shared fundamental property of many sensory neurons. Such 'coincidence detection' usually refers to the detection of synchronized excitatory inputs only. Experimental evidence, however, indicated that some auditory neurons are also sensitive to the relative timing of excitatory and inhibitory synaptic inputs. This type of sensitivity is suggested to be important for coding temporal information of amplitude-modulated sounds, such as speech and other naturalistic sounds. In this study, we used a minimal model of coincidence detection to identify the key elements for temporal information processing. Our series of simulations demonstrated that (1) the threshold and time window for coincidence detection were the major factors for determining the response properties to excitatory inputs, and that (2) timed interactions between excitatory and inhibitory synaptic inputs are responsible for determining the temporal tuning properties of the neuron. These results suggest that coincidence detection is an essential function of neurons that detect the 'anti-coincidence' of excitatory and inhibitory inputs to encode temporal information of sounds.

Differential morphology of the superior olivary complex of Meriones unguiculatus and Monodelphis domestica revealed by calcium-binding proteins

In mammals, the superior olivary complex (SOC) of the brainstem is composed of nuclei that integrate afferent auditory originating from both ears. Here, the expression of different calcium-binding proteins in subnuclei of the SOC was studied in distantly related mammals, the Mongolian gerbil (Meriones unguiculatus) and the gray short-tailed opossum (Monodelphis domestica) to get a better understanding of the basal nuclear organization of the SOC. Combined immunofluorescence labeling of the calcium-binding proteins (CaBPs) parvalbumin, calbindin-D28k, and calretinin as well as pan-neuronal markers displayed characteristic distribution patterns highlighting details of neuronal architecture of SOC nuclei. Parvalbumin was found in almost all neurons of SOC nuclei in both species, while calbindin and calretinin were restricted to specific cell types and axonal terminal fields. In both species, calbindin displayed a ubiquitous and mostly selective distribution in neurons of the medial nucleus of trapezoid body (MNTB) including their terminal axonal fields in different SOC targets. In Meriones, calretinin and calbindin showed non-overlapping expression patterns in neuron somata and terminal fields throughout the SOC. In Monodelphis, co-expression of calbindin and calretinin was observed in the MNTB, and hence both CaBPs were also co-localized in terminal fields within the adjacent SOC nuclei. The distribution patterns of CaBPs in both species are discussed with respect to the intrinsic neuronal SOC circuits as part of the auditory brainstem system that underlie the binaural integrative processing of acoustic signals as the basis for localization and discrimination of auditory objects.

Structural Changes and Lack of HCN1 Channels in the Binaural Auditory Brainstem of the Naked Mole-Rat (Heterocephalus glaber)

Naked mole-rats (Heterocephalus glaber) live in large eu-social, underground colonies in narrow burrows and are exposed to a large repertoire of communication signals but negligible binaural sound localization cues, such as interaural time and intensity differences. We therefore asked whether monaural and binaural auditory brainstem nuclei in the naked mole-rat are differentially adjusted to this acoustic environment. Using antibody stainings against excitatory and inhibitory presynaptic structures, namely the vesicular glutamate transporter VGluT1 and the glycine transporter GlyT2 we identified all major auditory brainstem nuclei except the superior paraolivary nucleus in these animals. Naked mole-rats possess a well structured medial superior olive, with a similar synaptic arrangement to interaural-time-difference encoding animals. The neighboring lateral superior olive, which analyzes interaural intensity differences, is large and elongated, whereas the medial nucleus of the trapezoid body, which provides the contralateral inhibitory input to these binaural nuclei, is reduced in size. In contrast, the cochlear nucleus, the nuclei of the lateral lemniscus and the inferior colliculus are not considerably different when compared to other rodent species. Most interestingly, binaural auditory brainstem nuclei lack the membrane-bound hyperpolarization-activated channel HCN1, a voltage-gated ion channel that greatly contributes to the fast integration times in binaural nuclei of the superior olivary complex in other species. This suggests substantially lengthened membrane time constants and thus prolonged temporal integration of inputs in binaural auditory brainstem neurons and might be linked to the severely degenerated sound localization abilities in these animals.

Different tonotopic regions of the lateral superior olive receive a similar combination of afferent inputs

The mammalian lateral superior olive (LSO) codes disparities in the intensity of the sound that reaches the two ears by integrating ipsilateral excitation and contralateral inhibition, but it remains unclear what particular neuron types convey acoustic information to the nucleus. It is also uncertain whether the known conspicuous morphofunctional differences and gradients along the tonotopic axis of the LSO relate to qualitative and/or quantitative regional differences in its afferents. To clarify these issues, we made small, single injections of the neuroanatomical tracer biotinylated dextran amine (BDA) into different tonotopic regions of the LSO of albino rats and analyzed the neurons labeled retrogradely in brainstem auditory nuclei. We demonstrate that the LSO is innervated tonotopically by four brainstem neuron types: spherical bushy cells and planar multipolar neurons of the ipsilateral ventral cochlear nucleus, principal neurons of the ipsilateral medial nucleus of the trapezoid body, and small multipolar neurons of the contralateral ventral nucleus of the trapezoid body. Unexpectedly, the proportion of labeled neurons of each type was virtually identical in all cases, thus indicating that all tonotopic regions of the LSO receive a similar combination of inputs. Even more surprisingly, our data also suggest that the representation of frequencies in the LSO differs from that of the nuclei that innervate it: compared to the latter nuclei, the LSO seems to possess a relatively larger portion of its volume devoted to processing frequencies in the lower-middle part of the spectrum, and a relative smaller portion devoted to higher frequencies. J. Comp. Neurol. 524:2230-2250, 2016. © 2015 Wiley Periodicals, Inc.

Developmental expression of inhibitory synaptic long-term potentiation in the lateral superior olive

Principal neurons of the lateral superior olivary nucleus (LSO) respond selectively to interaural level differences (ILD). To perform this computation, LSO neurons integrate excitatory synaptic drive from the ipsilateral ear with inhibitory synaptic drive from the contralateral ear via the medial nucleus of the trapezoid body (MNTB). Previous research demonstrated that inhibitory terminals from the MNTB to the LSO are eliminated during development. Furthermore, MNTB synapses display an activity- and age-dependent long-term depression (iLTD) that may contribute to inhibitory synapse elimination. However, inhibitory synapses that are stabilized become stronger. Here, we asked whether MNTB synapses displayed activity-dependent strengthening. Whole-cell recordings were obtained from LSO neurons in a gerbil brain slice before and after hearing onset. The inhibitory MNTB afferents were stimulated at a low rate, similar to spontaneous discharge rates observed in vivo. The MNTB-evoked inhibitory responses were strengthened by 40–300% when synaptic activity was coupled with postsynaptic membrane depolarization, exogenous glutamate application, or activation of ipsilateral excitatory synaptic inputs. This inhibitory long-term potentiation (iLTP) was associated with increased spontaneous inhibitory postsynaptic current (IPSC) amplitude and frequency. One hour after iLTP induction, IPSCs could not be de-potentiated by the MNTB stimulation pattern that induces iLTD in control slices. iLTP could only be induced after hearing onset (>P12), and was blocked in the presence of a GABA_B receptor antagonist. Together, these results suggest a developmental period during which the induction of iLTP depends on the conjoint activation of GABA_B receptors and postsynaptic depolarization. We propose that iLTP may support stabilization of un-pruned MNTB connections and contribute to the emergence of ILD processing in the mature LSO.

Sensitivity of cochlear nucleus neurons to spatio-temporal changes in auditory nerve activity

The spatio-temporal pattern of auditory nerve (AN) activity, representing the relative timing of spikes across the tonotopic axis, contains cues to perceptual features of sounds such as pitch, loudness, timbre, and spatial location. These spatio-temporal cues may be extracted by neurons in the cochlear nucleus (CN) that are sensitive to relative timing of inputs from AN fibers innervating different cochlear regions. One possible mechanism for this extraction is "cross-frequency" coincidence detection (CD), in which a central neuron converts the degree of coincidence across the tonotopic axis into a rate code by preferentially firing when its AN inputs discharge in synchrony. We used Huffman stimuli (Carney LH. J Neurophysiol 64: 437-456, 1990), which have a flat power spectrum but differ in their phase spectra, to systematically manipulate relative timing of spikes across tonotopically neighboring AN fibers without changing overall firing rates. We compared responses of CN units to Huffman stimuli with responses of model CD cells operating on spatio-temporal patterns of AN activity derived from measured responses of AN fibers with the principle of cochlear scaling invariance. We used the maximum likelihood method to determine the CD model cell parameters most likely to produce the measured CN unit responses, and thereby could distinguish units behaving like cross-frequency CD cells from those consistent with same-frequency CD (in which all inputs would originate from the same tonotopic location). We find that certain CN unit types, especially those associated with globular bushy cells, have responses consistent with cross-frequency CD cells. A possible functional role of a cross-frequency CD mechanism in these CN units is to increase the dynamic range of binaural neurons that process cues for sound localization.

Discharge patterns in the lateral superior olive of decerebrate cats

Anatomical and pharmacological studies have shown that the lateral superior olive (LSO) receives inputs from a number of sources and that LSO cells can alter the balance of their own excitatory and inhibitory drive. It is thus likely that the ongoing sound-evoked responses of LSO cells reflect a complex interplay of excitatory and inhibitory events, which may be affected by anesthesia. The goal of this study was to characterize the temporal discharge patterns of single units in the LSO of unanesthetized, decerebrate cats in response to long-duration ipsilateral best-frequency tone bursts. A decision tree is presented to partition LSO units on the basis of poststimulus time histogram shape, adaptation of instantaneous firing rate as a function of time, and sustained discharge rate. The results suggest that LSO discharge patterns form a continuum with four archetypes: sustained choppers that show two or more peaks of activity at stimulus onset and little adaptation of rate throughout the response, transient choppers that undergo a decrease in rate that eventually stabilizes with time, primary-like units that display an initial peak of activity followed by a monotonic decline in rate to a steady-state value, and onset-sustained units that exhibit an initial peak of activity at stimulus onset followed by a low sustained activity. Compared with the chopper units, the nonchopper units tend to show longer first-spike latencies, lower peak firing rates, and more irregular sustained discharge patterns. Modeling studies show that the full range of LSO response types can be obtained from an underlying sustained chopper by varying the strength and latency of a sound-driven ipsilateral inhibition relative to that of excitation. Together, these results suggest that inhibition plays a major role in shaping the temporal discharge patterns of units in unanesthetized preparations.

Influence of inhibitory synaptic kinetics on the interaural time difference sensitivity in a linear model of binaural coincidence detection

Temporal correlations between the sound waves arriving at the two ears are used to extract the azimuthal position of sound sources. Nerve cells in the mammalian medial superior olive (MSO) that extract these binaural correlations are sensitive to interaural time differences (ITDs) in the range of about 10 micros. These neurons receive inputs from the two ears via four pathways, two excitatory and two inhibitory ones. In this paper, a simple linear model is fitted to the frequency dependence of ITD sensitivity of MSO neurons, which is quantified by the two parameters, characteristic phase and characteristic delay. The fit parameters are the relative delays and the relative strengths of the two inhibitory pathways and thus specify the underlying ITD-detecting circuit assuming a non-Jeffress-like situation, i.e., no excitatory delay lines but phase-locked inhibition. The fitting procedure finds the parameters of these inhibitory pathways such that they account for a desired frequency dependence of ITD sensitivity. It is found that positive characteristic delays require a finite amount of ipsilateral inhibition that arrives at roughly the same time as ipsilateral excitation as well as contralateral inhibition that lags contralateral excitation so much that it effectively leads excitation of the next cycle.

Neuronal subtype identity in the rat auditory brainstem as defined by molecular profile and axonal projection

The nuclei of the auditory brainstem harbor a diversity of neuronal cell types and are interconnected by excitatory as well as inhibitory ascending, descending, and commissural pathways. Classically, neurons have been characterized by size and shape of their cell body and by the geometry of their dendrites. Our study is based on the use of axonal tracers in combination with immunocytochemistry to identify and distinguish neuronal subtypes by their molecular signature in dorsal and ventral cochlear nucleus, lateral superior olive, medial superior olive, medial nucleus of the trapezoid body, and inferior colliculus of the adult rat. The presumed neurotransmitters glutamate, glycine, and GABA were used alongside the calcium-binding proteins parvalbumin, calretinin, and calbindin-D28k as molecular markers. Our data provide distinct extensions to previous characterizations of neuronal subtypes and reveal regularities and differences across auditory brainstem nuclei that are discussed for their functional implications.

Characterization of the rhesus monkey superior olivary complex by calcium binding proteins and synaptophysin

This study was performed in order to characterize the main nuclei of the rhesus monkey superior olivary complex by means of antibodies against the calcium binding proteins parvalbumin, calbindin and calretinin and the synaptic vesicle protein synaptophysin. These markers revealed the neuronal morphology and organization of nuclei located within the rhesus monkey superior olivary complex. The architectural details included the distribution of axonal terminals on neurons. The medial superior olivary nucleus was present as a column of neurons. No clear segregation of calretinin-positive terminals was noticed on the medial and lateral dendritic fields of these neurons. The lateral superior olivary nucleus was characterized by a distinct nuclear shape. Calretinin-, parvalbumin- or calbindin-positive terminals contacted somata and dendrites. The medial nucleus of trapezoid body could be clearly differentiated as a distinct region in the rhesus monkey superior olivary complex. Somata of that nucleus showed calbindin- and parvalbumin-labelling whereas somatic calyces of Held were reavealed by calretinin and synaptophysin labelling. The results are discussed with respect to the processing of acoustic information in primate species and their ability to hear high and low frequencies, which is reflected by anatomical correlates.

Interaural phase and level difference sensitivity in low-frequency neurons in the lateral superior olive

The lateral superior olive (LSO) is believed to encode differences in sound level at the two ears, a cue for azimuthal sound location. Most high-frequency-sensitive LSO neurons are binaural, receiving inputs from both ears. An inhibitory input from the contralateral ear, via the medial nucleus of the trapezoid body (MNTB), and excitatory input from the ipsilateral ear enable level differences to be encoded. However, the classical descriptions of low-frequency-sensitive neurons report primarily monaural cells with no contralateral inhibition. Anatomical and physiological evidence, however, shows that low-frequency LSO neurons receive low-frequency inhibitory input from ipsilateral MNTB, which in turn receives excitatory input from the contralateral cochlear nucleus and low-frequency excitatory input from the ipsilateral cochlear nucleus. Therefore, these neurons would be expected to be binaural with contralateral inhibition. Here, we re-examined binaural interaction in low-frequency (less than approximately 3 kHz) LSO neurons and phase locking in the MNTB. Phase locking to low-frequency tones in MNTB and ipsilaterally driven LSO neurons with frequency sensitivities <1.2 kHz was enhanced relative to the auditory nerve. Moreover, most low-frequency LSO neurons exhibited contralateral inhibition: ipsilaterally driven responses were suppressed by raising the level of the contralateral stimulus; most neurons were sensitive to interaural time delays in pure tone and noise stimuli such that inhibition was nearly maximal when the stimuli were presented to the ears in-phase. The data demonstrate that low-frequency LSO neurons of cat are not monaural and can exhibit contralateral inhibition like their high-frequency counterparts.

Intercollicular commissural projections modulate neuronal responses in the inferior colliculus

The right and left inferior colliculi (ICs) in the auditory midbrain are connected to one another by a bundle of fibres, the commissure of the IC. Previous studies show that this commissural projection connects corresponding frequency regions in the two sides and originates mainly from excitatory neurons, although some studies suggest a smaller number of GABAergic inhibitory neurons may also project via the commissure. Although the commissure of the IC is a major pathway connecting the most important nuclei of the auditory tectum, little is known about its functional significance. To investigate its role in auditory processing in the rat, we recorded sound-evoked responses of single neurons in one IC while injecting kynurenic acid into a corresponding region of the opposite IC. This procedure enabled us to block reversibly excitation of commissural projections to the recorded IC. The changes in the neural responses when input from the opposite IC was blocked are consistent with the commissural projection exerting both an excitatory and an inhibitory influence. The inhibition could be accounted for by monosynaptic or disynaptic connections. The responses to both monaural and binaural stimulation were affected, and the effects were proportionately greater at near-threshold sound levels. The results suggest that one function of the commissure of the IC may be to modulate the response gain of IC neurons to acoustic stimulation.

Inhibitory synapses in the developing auditory system are glutamatergic

Activity-dependent synapse refinement is crucial for the formation of precise excitatory and inhibitory neuronal circuits. Whereas the mechanisms that guide refinement of excitatory circuits are becoming increasingly clear, the mechanisms guiding inhibitory circuits have remained obscure. In the lateral superior olive (LSO), a nucleus in the mammalian sound localization system that receives inhibitory input from the medial nucleus of the trapezoid body (MNTB), specific elimination and strengthening of synapses that are both GABAergic and glycinergic (GABA/glycinergic synapses) is essential for the formation of a precise tonotopic map. We provide evidence that immature GABA/glycinergic synapses in the rat LSO also release the excitatory neurotransmitter glutamate, which activates postsynaptic NMDA receptors (NMDARs). Immunohistochemical studies demonstrate synaptic colocalization of the vesicular glutamate transporter 3 with the vesicular GABA transporter, indicating that GABA, glycine and glutamate are released from single MNTB terminals. Glutamatergic transmission at MNTB-LSO synapses is most prominent during the period of synapse elimination. Synapse-specific activation of NMDARs by glutamate release at GABAergic and glycinergic synapses could be important in activity-dependent refinement of inhibitory circuits.

Alterations in calretinin immunostaining in the ferret superior olivary complex after cochlear ablation

In this study, we used image analysis to assess changes in calretinin immunoreactivity in the lateral (LSO) and medial (MSO) superior olivary nuclei in ferrets 2 months after unilateral cochlear ablations at 30-40 days of age, soon after hearing onset. These two nuclei are the first significant sites of binaural convergence in the ascending auditory system, and both receive direct projections from the deafferented cochlear nucleus. Cochlear ablation results in a decrease in the overall level of calretinin immunostaining within the LSO ipsilaterally compared with the contralateral side and with control animals and within the MSO bilaterally compared with control ferrets. In addition, the level of calretinin immunostaining ipsilaterally within neurons in the LSO was significantly less in cochlear ablated than control animals. In contrast, there was no effect of cochlear ablation on the level of calretinin immunostaining within neurons either in the contralateral LSO or in the MSO. These results are consistent with a downregulation in calretinin within the neuropil of MSO bilaterally and LSO ipsilaterally, as well as a downregulation in calretinin within somata in the ipsilateral LSO as a result of unilateral cochlear ablation soon after hearing onset. Thus, cochlear-driven activity appears to affect calcium binding protein levels in both neuropil and neurons within the superior olivary complex.

Protein kinase A and calcium/calmodulin-dependent protein kinase II regulate glycine and GABA release in auditory brain stem nuclei

We reported previously that unilateral cochlear ablation (UCA) in young adult guinea pigs induced protein kinase C (PKC)-dependent plastic changes in the electrically evoked release of exogenous [14C]glycine ([14C]Gly) or [14C]-gamma-aminobutyric acid ([14C]GABA) in several brain stem auditory nuclei. The present study assessed whether such changes depended on protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII). In the major subdivisions of the cochlear nucleus (CN) and the main nuclei of the superior olivary complex (SOC) dissected from intact animals, dibutyryl-cyclic adenosine monophosphate (DBcAMP) (0.2 mM), a PKA activator, elevated release by 1.6-2.3-fold. The PKA inhibitor, H-89 (2 microM), did not alter the release but blocked the stimulatory effects of DBcAMP. These findings suggested that PKA could positively regulate glycinergic and GABAergic release. After UCA, PKA regulation declined and failed in the ventral CN but persisted in the SOC nuclei. After 145 postablation days, H-89 reversed elevations of [14C]GABA release in the medial nucleus of the trapezoid body (MNTB). A CaMKII inhibitor, KN-93, reversed depressions of [14C]Gly release in the DCN. Thus, the postablation plasticities in these nuclei probably depended on PKA or CaMKII. Both H-89 and KN-93 depressed [14C]Gly release in the lateral superior olive (LSO) and ipsilateral medial superior olive (MSO), suggesting that either kinase was used by endogenous mechanisms in these nuclei to upregulate glycinergic release. In contrast, KN-93 elevated [14C]GABA release in the contralateral MNTB, suggesting a downregulatory action of CaMKII, an action opposite to that of PKA.

Protein kinase A and calcium/calmodulin-dependent protein kinase II regulate D-[3H]aspartate release in auditory brain stem nuclei

We noted previously that after unilateral cochlear ablation (UCA) in young adult guinea pigs, plastic changes in glutamatergic transmitter release in several brain stem auditory nuclei depended on protein kinase C. In this study, we assessed whether such changes depended on protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII). The electrically-evoked release of D-[3H]aspartate (D-[3H]Asp) was quantified in vitro as an index of glutamatergic transmitter release in the major subdivisions of the cochlear nucleus (CN) and the main nuclei of the superior olivary complex (SOC). In tissues from intact animals, dibutyryl-cyclic adenosine monophosphate (DBcAMP), a PKA activator, elevated D-[3H]Asp release by 1.9-3.7-fold. The PKA inhibitor, H-89 (2 microM), did not alter the evoked release but blocked the stimulatory effects of DBcAMP. These findings suggested that PKA could positively regulate glutamatergic transmitter release. Seven days after the ablation of one cochlea and its cochlear nerve, the stimulatory effect of DBcAMP remained evident. After 145 postablation days, H-89 blocked the plastic elevations of D-[3H]Asp release in the ipsilateral CN and lateral (LSO) and medial (MSO) superior olive. A CaMKII inhibitor, KN-93, produced similar blocks, suggesting that the postablation plasticities in these nuclei depended on PKA or CaMKII. Both H-89 and KN-93 elevated release in the medial nucleus of the trapezoid body (MNTB) and the contralateral MSO, suggesting that either kinase could be used by endogenous mechanisms in these nuclei to downregulate glutamatergic release.

Parallel auditory pathways: projection patterns of the different neuronal populations in the dorsal and ventral cochlear nuclei

The cochlear nuclear complex gives rise to widespread projections to nuclei throughout the brainstem. The projections arise from separate, well-defined populations of cells. None of the cell populations in the cochlear nucleus projects to all brainstem targets, and none of the targets receives inputs from all cell types. The projections of nine distinguishable cell types in the cochlear nucleus-seven in the ventral cochlear nucleus and two in the dorsal cochlear nucleus-are described in this review. Globular bushy cells and two types of spherical bushy cells project to nuclei in the superior olivary complex that play roles in sound localization based on binaural cues. Octopus cells convey precisely timed information to nuclei in the superior olivary complex and lateral lemniscus that, in turn, send inhibitory input to the inferior colliculus. Cochlear root neurons send widespread projections to areas of the reticular formation involved in startle reflexes and autonomic functions. Type I multipolar cells may encode complex features of natural stimuli and send excitatory projections directly to the inferior colliculus. Type II multipolar cells send inhibitory projections to the contralateral cochlear nuclei. Fusiform cells in the dorsal cochlear nucleus appear to be important for the localization of sounds based on spectral cues and send direct excitatory projections to the inferior colliculus. Giant cells in the dorsal cochlear nucleus also project directly to the inferior colliculus; some of them may convey inhibitory inputs to the contralateral cochlear nucleus as well.

The lateral superior olive: a functional role in sound source localization

Sound location in azimuth is signaled by differences in the times of arrival (interaural time difference, ITDs) and the amplitudes (interaural level differences, ILDs) of the stimuli at the ears. Psychophysical studies have shown that low- and high-frequency sounds are localized based on ITDs and ILDs, respectively, suggesting that dual mechanisms mediate localization. The anatomical and physiological bases for this "duplex theory" of localization are found in the medial (MSO) and lateral (LSO) superior olives, two of the most peripheral sites in the ascending auditory pathway receiving inputs from both ears. The MSO and LSO are believed to be responsible for the initial encoding of ITDs and ILDs, respectively. Here the author focuses on ILDs as a cue to location and the role of the LSO in encoding ILDs. Evidence from disparate fields of study supports the hypothesis that the LSO is the initial ILD processor in the mammalian auditory system.

Characterization of the human superior olivary complex by calcium binding proteins and neurofilament H (SMI-32)

This study provides a morphologic characterization of the human superior olivary complex as revealed by immunohistochemistry by using antibodies against the calcium binding proteins parvalbumin, calbindin, calretinin, and the nonphosphorylated neurofilament H SMI-32. By combining these markers, it was possible to establish the neuronal architecture and details of the morphologic organization (including axonal terminals) of the different nuclei. The medial superior olivary nucleus is formed by a sheet of parallel-oriented cells. A clear segregation of axon terminals was noticed on the medially and laterally oriented dendrites of the mostly bipolar neurons. The lateral superior olivary nucleus lacked a distinct nuclear shape but was formed by several patches of rather irregularly arranged neurons. Calretinin or parvalbumin immunoreactive afferent terminals were observed which contacted somata or dendrites of these neurons. The immunolabeling also revealed the boundaries of the dorsal periolivary nucleus and morphologic detail of its neurons. A coherent nuclear structure that could be addressed as the medial nucleus of the trapezoid body was not identified by any single one or by combinations of the markers used. The data were also used to establish a three-dimensional-reconstruction of the three major subnuclei of the superior olivary complex. The results are discussed with respect to the possible role of the superior olivary complex in the processing of spatial acoustic information in the azimuthal plane.

Glycinergic and GABAergic calcium responses in the developing lateral superior olive

The lateral superior olive (LSO), a binaural nucleus involved in sound localization, receives tonotopically organized inhibitory inputs from the medial nucleus of the trapezoid body (MNTB). During development, the tonotopic organization of this glycinergic/GABAergic MNTB-LSO pathway is established by activity-dependent axonal reorganization. However, the underlying mechanisms by which this reorganization takes place have remained largely unknown. As cytosolic calcium is one of the most important second messengers responsible for inducing synaptic plasticity and reorganization, we examined whether and how activity in the MNTB-LSO pathway changes the intracellular calcium concentration ([Ca2+]i) in developing LSO neurons. By applying calcium imaging techniques to Fura-2-labelled slices from neonatal rats and mice, we found that glycine and GABA (gamma-aminobutyric acid) affect [Ca2+]i in LSO neurons in an age-dependent manner; during the first postnatal week, the period at which glycine and GABA are depolarizing in the LSO, glycine and GABA always increased [Ca2+]i. However, in 2-week-old animals, the time around hearing onset when glycine and GABA are hyperpolarizing, glycine and GABA slightly decreased [Ca2+]i. Calcium responses could also be elicited by stimulation of afferent fibres from the MNTB, and these synaptic responses were mediated by glycine and GABA(A) receptors. Furthermore, GABA, which is a neurotransmitter only in the immature MNTB-LSO pathway, played a major role in generating MNTB-elicited Ca2+ responses. The direct link of glycinergic/GABAergic synaptic activity to intracellular calcium signalling during the period of inhibitory synaptic plasticity could be one of the mechanisms by which tonotopic MNTB-LSO connections become established.

The coding of spatial location by single units in the lateral superior olive of the cat. II. The determinants of spatial receptive fields in azimuth

The lateral superior olive (LSO) is one of the most peripheral nuclei in the auditory pathway to receive inputs from both ears, and its cells are sensitive to interaural level disparities (ILDs) when stimulated by sounds presented over earphones. It has, accordingly, long been hypothesized that the functional role of the LSO is to encode a correlate of ILDs, one of the acoustical cues to the spatial location of sound. In the companion paper, we used the virtual space (VS) technique to present over earphones stimuli containing all the acoustical cues to the location of broadband stimuli and measured the spatial receptive fields (SRFs) in azimuth of single LSO cells. The shapes of the SRFs were generally consistent with the ILD sensitivity of the cells (Tollin and Yin, 2002), but because the only variable under our control was azimuth, and not ILD directly, the precise cues responsible for the SRFs could not be unambiguously determined. Here, we test more directly the hypothesis that ILDs are the primary determinants of the SRFs in azimuth of LSO cells by digitally manipulating the head-related transfer functions used to create the VS stimuli by independently varying (or holding constant) in azimuth each of the primary localization cues in isolation while holding constant (or varying) the others. Our results support the classical view of the LSO that the form of the SRFs of the cells in azimuth is determined primarily by the ILDs in a small band of frequencies around the characteristic frequencies of the cells.

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.

Ascending efferent projections of the superior olivary complex

The superior olivary complex conveys information about binaural time and intensity to higher centers in the auditory pathway. This information is sent primarily to the subdivisions of the inferior colliculus and to the nuclei of the lateral lemniscus. Olivary projections are the predominant afferents to the central nucleus of the inferior colliculus. Electron microscopic observations of axonal endings in the central nucleus suggest that the ipsilateral medial superior olive and contralateral lateral superior olive make excitatory synapses. In contrast, the axons from the ipsilateral lateral superior olive to the central nucleus contain glycine and have a morphology consistent with inhibitory synapses. Little is known about the transmitter types used by olivary projections to the nuclei of the lateral lemniscus, but they are presumed to be similar to the collicular projections. Olivary ascending efferents are tonotopically organized and terminate in laminae in the inferior colliculus. They combine with other laminar afferents and postsynaptic neurons to create fibro-dendritic laminae in the colliculus. The key to the functional organization of the olivary efferents is the possible segregation of excitatory olivary efferents from each other in "synaptic domains" located on the laminae. This segregation may be the major determinant of response properties in the colliculus. Olivary efferents may converge with other non-olivary afferents on the same postsynaptic neurons in the colliculus. Inhibitory efferents from the lateral superior olive are essential in shaping the response properties of neurons in the colliculus. Olivary efferents to the nuclei of the lateral lemniscus are also key components of ascending pathways that inhibit neurons in the midbrain.

Afferent projections of the superior olivary complex

Based on current literature, the afferents of the superior olivary complex (SOC) are described including those from the cochlear nucleus, inferior colliculus, thalamus, and auditory cortex. Intrinsic SOC afferents and non-auditory afferents from the serotoninergic and noradrenergic systems are also described. New data are provided that show a differential distribution of serotoninergic afferents within the SOC: serotoninergic fibers were relatively sparse in the lateral and medial superior olives and the medial nucleus of the trapezoid body and were most numerous in periolivary regions. There are variations in the density of serotoninergic fibers within periolivary regions themselves. New data is also provided on auditory and non-auditory afferents to SOC neurons, which have known targets. These include: cochlear nucleus afferents to periolivary (lateral nucleus of the trapezoid body, LNTB) cells that project to the inferior colliculus; cortical afferents to periolivary (ventral nucleus of the trapezoid body, VNTB) cells that project to the cochlear nucleus; and serotoninergic and noradrenergic afferents to periolivary (LNTB and VNTB) cells that project to the cochlear nucleus. The relationships between other types of afferents and SOC neurons with known projections are also described as functional circuits. The circuits include those that are part of the ascending auditory system (to the inferior and superior colliculi, lateral lemniscus, and medial geniculate nucleus), the descending auditory system (to the cochlea and cochlear nucleus), and the middle ear reflex circuits.

Effects of stimulus frequency and intensity on c-fos mRNA expression in the adult rat auditory brainstem

Induction of the cellular fos gene (c-fos) is one of the earliest transcriptional changes observed following neuronal excitation. Although not an activity marker in the strict electrophysiological sense, many neurons in the central nervous system increase their c-fos expression after periods of sustained stimulation at physiological levels of intensity. In the present study, induction of c-fos mRNA expression was examined in the auditory brainstem after 1 hour of continuous free-field acoustic stimulation. Sprague-Dawley rats were exposed to pure tones of 2, 8, 16, or 32 kHz or half-octave noise bands centered on 2, 8, or 32 kHz at 80-120 dB SPL. Stimulation-induced c-fos mRNA expression was evident at all levels of the auditory brainstem, and this expression was intensity dependent. In some brain areas, induced expression manifested a clear tonotopic organization, i.e., in dorsal, posteroventral, and anteroventral cochlear nuclei, and in the medial nucleus of the trapezoid body. The inferior colliculus exhibited multiple tonotopic representations. The dorsal nucleus of the lateral lemniscus had a crude tonotopy. Although expression was present, tonotopy was not evident in periolivary nuclei or in the ventral or intermediate nuclei of the lateral lemniscus. Free-field diotic stimulation did not induce c-fos mRNA expression in the medial or lateral superior olivary nuclei. Expression was induced in the lateral superior olive by dichotic stimulation (after a unilateral cochlear ablation), and that expression was tonotopically organized. The results suggest that stimulation-induced c-fos mRNA expression can be an effective way of mapping neuronal activity in the central auditory system under both normal and pathological conditions.

The vesicular GABA transporter, VGAT, localizes to synaptic vesicles in sets of glycinergic as well as GABAergic neurons

A transporter thought to mediate accumulation of GABA into synaptic vesicles has recently been cloned (McIntire et al., 1997). This vesicular GABA transporter (VGAT), the first vesicular amino acid transporter to be molecularly identified, differs in structure from previously cloned vesicular neurotransmitter transporters and defines a novel gene family. Here we use antibodies specific for N- and C-terminal epitopes of VGAT to localize the protein in the rat CNS. VGAT is highly concentrated in the nerve endings of GABAergic neurons in the brain and spinal cord but also in glycinergic nerve endings. In contrast, hippocampal mossy fiber boutons, which although glutamatergic are known to contain GABA, lack VGAT immunoreactivity. Post-embedding immunogold quantification shows that the protein specifically associates with synaptic vesicles. Triple labeling for VGAT, GABA, and glycine in the lateral oliva superior revealed a higher expression of VGAT in nerve endings rich in GABA, with or without glycine, than in others rich in glycine only. Although the great majority of nerve terminals containing GABA or glycine are immunopositive for VGAT, subpopulations of nerve endings rich in GABA or glycine appear to lack the protein. Additional vesicular transporters or alternative modes of release may therefore contribute to the inhibitory neurotransmission mediated by these two amino acids.

Glutamate receptors underlying excitatory synaptic transmission in the rat's lateral superior olive studied in vitro

Glutamate receptors underlying synaptic excitation in the rat's lateral superior olive were studied by whole-cell patch clamp recordings in a brain slice preparation. Recordings from two morphological types of cells, bipolar and multipolar, identified by intracellular labeling with biocytin, showed that there were no obvious differences in responses mediated or modulated by ionotropic and metabotropic receptors between these two types of neurons. The excitatory postsynaptic potentials (EPSPs) elicited by ipsilateral stimulation of the trapezoid body consisted of two components. An earlier component, which had faster rise time constant and decay time constant, was mediated by non-NMDA receptors. A later component, which had slower rise time and decay time constants, was mediated by NMDA receptors. Suprathreshold responses (action potentials), which arose from the early component, were always abolished by the non-NMDA antagonist, CNQX, but not by the NMDA antagonist, APV. These results suggest that both non-NMDA and NMDA receptors are present in LSO neurons, and that fast excitatory transmission in LSO is primarily mediated by non-NMDA receptors. The metabotropic glutamate receptor agonists, t-ACPD and L-AP4, reduced the size of EPSPs evoked by stimulation of the ipsilateral trapezoid body in LSO neurons; the reductive action of t-ACPD was reversed by the antagonist, MCPG, indicating that metabotropic glutamate receptors, probably group II and III subtypes, can modulate excitatory synaptic transmission in LSO.

Afferent regulation of glycine receptor distribution in the gerbil LSO

Synaptic activity plays an important role in many aspects ofneuronal development, particularly the expression of proteins. In this study, the influence of inhibitory and excitatory afferents on the development of glycine receptor density in the lateral superior olive (LSO) of Mongolian gerbils was investigated. Afferent activity was manipulated by removing one or both cochleas at postnatal day 7, prior to the onset of sound-evoked responses. Due to the anatomy of the LSO, these manipulations result in either excitatory denervation, inhibitory denervation, or both. The density of glycine receptors in the LSO was determined at 21 days postnatal. Glycine receptors were either labeled with tritiated strychnine (3H-SN) or with an antibody directed against gephyrin, a protein closely associated with the receptor complex. Antibody binding was used to quantify the differential glycine receptor density between the medial limb (high frequency area) and the lateral limb (low frequency area) of the LSO. 3H-SN was used to quantify the amount of glycine receptors in each part of the LSO in control and experimental animals. In addition, changes in neuron density and neuron cross-sectional area were quantified following cochlear ablations. In control animals, the amount of glycine receptors is about 2- to 3-fold higher in the high-frequency than in the low-frequency region. In bilaterally ablated animals, the same density of glycine receptors was measured in the high- and low-frequency region. Unilateral ablations had no significant effect on glycine receptor distribution, either ipsi- or contralateral to the ablation. The neuron cross-sectional area decreased about 30% in the ipsilateral LSO of unilaterally ablated animals and in bilaterally ablated animals. However, alterations of soma density and cross-sectional area were similar in the high- and low-frequency projection region. These results suggest that the distribution of glycine receptors is only changed when excitatory and inhibitory afferents have been denervated.

PEP-19 immunoreactivity in the cochlear nucleus and superior olive of the cat

We applied antiserum to PEP-19, a presumptive calcium-binding polypeptide, to the auditory brainstem of cats to determine whether this antiserum would selectively reveal cochlear nucleus neurons and their projections. We report that the entire populations of ventral cochlear nucleus bushy and multipolar cells, but not octopus cells, express this peptide in their somata and dendrites. Presumed axons of spherical bushy cells located dorsally and thicker globular bushy cell fibers located ventrally in the trapezoid body are immunostained, as are thin fibers presumed to represent the axons of multipolar cells. Large calyceal endings in the medial nucleus of the trapezoid body are densely immunoreactive as are smaller punctate profiles that outline immunonegative neuronal profiles in the medial and lateral superior olives. These features of immunolabeling indicate that PEP-19 is expressed in all neuronal compartments. Within the entire superior olivary complex, relatively few neurons are immunolabeled, and the vast majority of these are found in the periolivary nuclei. There are many more immunostained neurons in lateral than in medial periolivary cell groups, but their combined numbers are dwarfed by the numbers of immunolabeled cells in the ventral cochlear nucleus. The borders of the principal nuclei and some of the periolivary cell groups are well defined by the distribution of PEP-19-immunoreactive fibers and puncta. Since ventral cochlear nucleus bushy cells comprise the predominant input to principal nuclei of the superior olive, and the entire bushy cell population is immunolabeled by PEP-19 antiserum, the numbers and distribution of their inputs can be quantified. In this study we report that immunoreactive puncta apposed to the cell bodies and proximal dendrites of neurons in the medial superior olive occur at a density of 20/100 microns2. Moreover, we demonstrate by pre-embedding immunoelectron microscopy that the PEP-19-immunoreactive punctate profiles observed in the medial superior olive by light microscopy represent presynaptic terminal boutons that contain round synaptic vesicles and form asymmetric synaptic junctions, features traditionally associated with excitatory synapses. Thus, this antiserum represents a useful tool for investigating the distribution of ventral cochlear nucleus fibers and synaptic terminals within their target nuclei in the superior olive.

Envelope coding in the lateral superior olive. III. Comparison with afferent pathways

Binaural cues for spatial localization of complex high-frequency sounds are interaural level and time differences (ILDs and ITDs). We previously showed that cells in the lateral superior olive (LSO) are sensitive to ITDs in the envelope of sinusoidally amplitude-modulated (AM) signals up to a modulation frequency of only approximately 800 Hz. To understand the limitations in this ITD-sensitivity, we here compare responses to monaural modulation in LSO and its input pathways, derived from cochlear nucleus and medial nucleus of the trapezoid body. These pathways have marked functional and morphological specializations, suggestive of adaptations for timing. Afferent cell populations were identified on the basis of electrophysiological signatures, and for each population, average firing rate and synchronization to AM tones were compared with auditory-nerve fibers and LSO cells. Except for an increase in modulation gain in some subpopulations, synchronization of LSO afferents was very similar to that in auditory nerve fibers in its dependency on sound pressure level (SPL), modulation depth, and modulation frequency. Distributions of cutoff frequencies of modulation transfer functions were largely coextensive with the distribution in auditory nerve. Group delays, measured from the phase of the response modulation as a function of modulation frequency, showed an orderly dependence on characteristic frequency and cell type and little dependence on SPL. Similar responses were obtained to a modulated broadband carrier. Compared with their afferents, LSO cells synchronized to monaurally modulated stimuli with a higher gain but often over a narrower range of modulation frequencies. Considering the scatter in afferent and LSO cell populations, ipsi- and contralateral responses were well matched in cutoff frequency and magnitude of delays. In contrast to their afferents, LSO cells show a decrease in average firing rate at high modulation frequencies. We conclude that the restricted modulation frequency range over which LSO cells show ITD-sensitivity does not result from loss of envelope information along the afferent pathway but is due to convergence or postsynaptic effects at the level of the LSO. The faithful transmission of envelope phase-locking in LSO afferents is consistent with their physiological and morphological adaptations, but these adaptations are not commensurate with the rather small effects of physiological ITDs reported previously, especially when compared with effects of ILDs. We suggest that these adaptations have evolved to allow a comparison of instantaneous amplitude fluctuations at the two ears rather than to extract interaural timing information per se.

Sources of GABAergic input to the inferior colliculus of the rat

We have studied the GABAergic projections to the inferior colliculus (IC) of the rat by combining the retrograde transport of horseradish peroxidase (HRP) and immunohistochemistry for gamma-amino butyric acid (GABA). Medium-sized (0.06-0.14 microliter) HRP injections were made in the ventral part of the central nucleus (CNIC), in the dorsal part of the CNIC, in the dorsal cortex (DCIC), and in the external cortex (ECIC) of the IC. Single HRP-labeled and double (HRP-GABA)-labeled neurons were systematically counted in all brainstem auditory nuclei. Our results revealed that the IC receives GABAergic afferent connections from ipsi- and contralateral brainstem auditory nuclei. Most of the contralateral GABAergic input originates in the IC and the dorsal nucleus of the lateral lemniscus (DNLL). The dorsal region of the IC (DCIC and dorsal part of the CNIC) receives connections mostly from its homonimous contralateral region, and the ventral region from the contralateral DNLL. The commissural GABAergic projections originate in a morphologically heterogeneous neuronal population that includes small to medium-sized round and fusiform neurons as well as large and giant neurons. Quantitatively, the ipsilateral ventral nucleus of the lateral lemniscus is the most important source of GABAergic input to the CNIC. In the superior olivary complex, a smaller number of neurons, which lie mainly in the periolivary nuclei, display double labeling. In the contralateral cochlear nuclei, only a few of the retrogradely labeled neurons were GABA immunoreactive. These findings give us more information about the role of GABA in the auditory system, indicating that inhibitory inputs from different ipsi- and contralateral, mono- and binaural auditory brainstem centers converge in the IC.

Neurotransmitter and neuromodulator systems of the rat inferior colliculus and auditory brainstem studied by in situ hybridization

This study was concerned with the distribution of a variety of putative neuromodulator and neurotransmitter systems in auditory regions of the rat brainstem using in situ hybridization histochemistry. Serial brain sections were screened for the presence of mRNAs for (i) precursors of the neuroactive substances cholecystokinin, somatostatin, proenkephalin and substance P (preprotachykinin), (ii) glutamic acid decarboxylase, the key synthesizing enzyme for GABA, or (iii) subunits alpha 1, alpha 2 and alpha 3 of the GABAA receptor. Detectable message for all of these probes was found in at least one auditory brainstem area. There were clear differences in the distribution of the various mRNAs in subregions of the inferior colliculus, superior olivary complex, lateral lemniscus and cochlear nucleus. Cells expressing mRNA for glutamic acid decarboxylase were most prominent in the inferior colliculus, but were also present in all lower auditory brainstem nuclei, except the medial superior olivary nucleus and medial nucleus of trapezoid body. The mRNA for GABAA alpha 1 receptor subunits was detectable in all auditory regions investigated, although at different levels of expression. GABAA alpha 2 and alpha 3 mRNA signals were seen in inferior colliculus, lateral lemniscus and in almost all superior olivary complex regions, but in fewer cells and at lower levels than the GABAA alpha 1 subtype. Moderate to high levels of preprocholecystokinin mRNA expression were seen in all subregions of the inferior colliculus. In other auditory brainstem areas, preprocholecystokinin mRNA levels were either low or absent. With regard to mRNAs for the neuroactive peptides somatostatin, preprotachykinin and preproenkephalin, all were expressed in the inferior colliculus but there were differences in their cellular distribution. For example, there were almost no preprotachykinin mRNA expressing cells in the central nucleus of inferior colliculus and levels of somatostatin mRNA were especially high in the dorsal cortex and in layer 3 of the external cortex of inferior colliculus. There were also differences in the pattern of expression of these mRNAs in the various brainstem auditory nuclei; there was no preprotachykinin mRNA in any part of the superior olivary complex, only somatostatin mRNA was found in the ventral cochlear nucleus, and expression of preproenkephalin mRNA was pronounced in the ventral nucleus of the trapezoid body and the rostral periolivary zone. The data are considered in light of the connectivity and functional organization of the auditory brainstem.

Expression of glycine receptor subunits in the cochlear nucleus and superior olivary complex using non-radioactive in-situ hybridization

The distribution of glycine receptor (GlyR) subunit mRNAs was examined in the cochlear nucleus (CN) and superior olivary complex of 5-6-week-old and 8-10-week-old rats using a non-radioactive in-situ hybridization method. In the younger rats, GlyR alpha 1-, alpha 2-, and alpha 3- and beta-subunits were observed in all major ventral cochlear nucleus (VCN) and superior olivary complex (SOC) neurons, while only alpha 1-, alpha 3- and beta-subunits were observed in dorsal cochlear nucleus (DCN) neurons. In 8-10-week-old rats, GlyR alpha 1-, alpha 3- and beta-subunits were observed in all major CN and SOC neurons, while mRNA for GlyR alpha 2-subunit was not observed. These results indicate that GlyR is being expressed all major CN and SOC neurons, with alpha 1-, alpha 3- and beta-subunit components of the mature receptor and the alpha 2-subunit, a component of the immature GlyR, which is not down-regulated until after 6 weeks of age in most CN and SOC neurons.

Axonal projections from the lateral and medial superior olive to the inferior colliculus of the cat: a study using electron microscopic autoradiography

The superior olivary complex is the first site in the central auditory system where binaural interactions occur. The output of these nuclei is direct to the central nucleus of the inferior colliculus, where binaural inputs synapse with monaural afferents such as those from the cochlear nuclei. Despite the importance of the olivary pathways for binaural information processing, little is known about their synaptic organization in the colliculus. The present study investigates the structure of the projections from the lateral and medial superior olivary nuclei to the inferior colliculus at the electron microscopic level. Stereotaxic placement and electrophysiological responses to binaural sounds were used to locate the superior olive. Anterograde axonal transport of 3H-leucine was combined with light and electron microscopic autoradiography to reveal the location and morphology of the olivary axonal endings. The results show that the superior olivary complex contributes different patterns of synaptic input to the central nucleus of the inferior colliculus. Each projection from the superior olivary complex to the colliculus differs in the number and combinations of endings. Axonal endings from the ipsilateral medial superior olive were exclusively the round (R) type that contain round synaptic vesicles and make asymmetrical synaptic junctions. This morphology is usually associated with excitatory synapses and neurotransmitters such as glutamate. Endings from medial superior olive terminate densely in the central nucleus. The projection from the contralateral lateral superior olive also terminates primarily as R endings. This projection also includes small numbers of pleomorphic (PL) endings that contain pleomorphic synaptic vesicles and usually make symmetrical synaptic junctions. The PL morphology is associated with inhibitory synapses and transmitters such as gamma-aminobutyric acid and glycine. All endings from the contralateral lateral superior olive terminate much less densely than endings from the medial olive. In contrast, the projection from the ipsilateral lateral superior olive contributes both R and PL endings in roughly equal proportions. These ipsilateral afferents are heterogeneous in density and can terminate in lower or higher concentrations than endings from the contralateral side. These data show that the superior olive is a major contributor to the synaptic organization of the central nucleus of the inferior colliculus. The ipsilateral projections of the medial and lateral superior olive may produce higher concentrations of R endings than other inputs to the central nucleus. Such endings may participate in excitatory synapses. The highest concentrations of PL endings come from the ipsilateral lateral superior olive.(ABSTRACT TRUNCATED AT 400 WORDS)

Binaural response characteristics in isofrequency sheets of the gerbil inferior colliculus

The spatial distribution of neurons with different binaural response properties was studied in two isofrequency areas of the inferior colliculus (IC) of the gerbil (Meriones unguiculatus). Single units were recorded in dorsomedial-to-ventrolaterally oriented electrode penetrations at different rostrocaudal positions, within a low (1-2 kHz; N = 108) and a high (23-27 kHz; N = 90) 'iso-frequency' sheet. Response types were not equally represented within the two layers. Within low-frequency regions, 41% of the units were excited following sound stimulation of either ear (EE), 21% received an excitatory input from one and an inhibitory input from the other ear (EI), and 12% were monaural (E0), while within high-frequency regions the distribution was 20% EE, 47% EI and 18% E0. However, the spatial arrangement of EE, EI and E0, within the dorsomedial-to-ventrolateral extent of the isofrequency sheets, was on the whole comparable within the two regions: EE-units were accumulated dorsomedially and EI-units ventrolaterally in the respective isofrequency sheets. E0-units were distributed equally within the low-frequency sheet but occurred more frequently in the middle part of the high-frequency lamina. About equal proportions of the units (low frequencies 9%; high frequencies 7%) had complex binaural response characteristics and were characterized as E/IE. In the rostrocaudal dimension of the nucleus, orderly arrangement of different unit types was less obvious. The results show that there is a tendency for grouping of different binaural unit types within the inferior colliculus. However, these unit types are not strictly segregated. Furthermore, it was shown that, despite the differences found in low- and high-frequency sites, there is a common scheme of 'binaural representation' across frequencies within the IC. Provided that EE-, EI-, and monaural response characteristics originate from separate sources, the data point to a convergence of binaural brainstem afferents within the IC.

Development of ventral cochlear nucleus projections to the superior olivary complex in gerbil

The postnatal development of the projection from the ventral cochlear nucleus to the principal nuclei in the superior olivary complex in gerbil (Meriones unguiculatus) was studied in an age-graded series of pups ranging from 0 to 18 days old. Small crystals of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) were inserted into the ventral cochlear nucleus of aldehyde-fixed brains, and the labeled projections were examined with epifluorescence microscopy. Selected sections were photooxidized in a solution of diaminobenzidine and subsequently processed for electron microscopy to examine the development of labeled synapses in the target nuclei. Horseradish peroxidase was injected into the ventral cochlear nucleus of adult gerbils to assess the form and persistence of projections observed in the neonatal animals. In addition, electrophysiological responses to acoustic stimuli of single units in the adult auditory brainstem were analyzed to confirm the functionality of the novel projection from the ventral cochlear nucleus to the contralateral lateral superior olive. By the day of birth (P0), developing axons from the ventral cochlear nucleus have already established highly ordered pathways to the three primary nuclei of the superior olivary complex: the ipsilateral lateral superior olive, the contralateral medial nucleus of the trapezoid body, and at the lateral and medial dendrites of the ipsilateral and contralateral medial superior olive, respectively. Developing axons from the ventral cochlear nucleus that innervated the contralateral medial nucleus of the trapezoid body lacked the terminal morphology characteristic of the calyx of Held, but began to adopt a more characteristic form on P5. The mature calyx appeared around P14-16. Exuberant developmental projections to topographically inappropriate areas of the superior olivary complex were not observed at the postnatal ages studied. In addition to the projections of the ventral cochlear nucleus to the superior olivary complex described in other species, we observed the development and maintenance of a major direct projection from the ventral cochlear nucleus to the contralateral lateral superior olive. On P0, ventral cochlear nucleus axons decussate in the dorsal trapezoid body, form a plexus at the dorsal edge of the contralateral medial superior olive, and enter the ventrolateral limb of the contralateral lateral superior olive. Over the next 2 weeks, fascicles of fibers form on the hilar and ventral aspects of the ventrolateral limb. Fibers arising from these fascicles form converging, but nonoverlapping, arborizations within the ventrolateral limb at right angles to the curvature of the nucleus. The medial region was devoid of labeled axons.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of ectopic projections from the ventral cochlear nucleus to the superior olivary complex induced by neonatal ablation of the contralateral cochlea

The ability of an animal to localize a sound in space requires the precise innervation of the superior olivary complex by the ventral cochlear nuclei on each side of the lower brainstem. This precise pattern of innervation could require an immutable recognition of appropriate targets by afferent processes arising from these nuclei. This possibility was investigated by destroying one cochlea of gerbil pups (Meriones unguiculatus) on the second postnatal day and assessing the projections from the ventral cochlear nucleus (VCN) on the unablated side to the superior olivary complex during the subsequent 2 weeks and after the animals had reached maturity. A crystal of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was inserted into VCN on the unablated side in animals ranging in age from 3 to 14 days. To assess the permanence of any altered pattern of innervation, horseradish peroxidase was injected into VCN on the unablated side in adult, neonatally ablated animals. Finally, electrophysiological responses to acoustic stimuli delivered to the ear on the unablated side were recorded in the superior olivary complex of adult animals to assess whether altered innervation patterns were functional. Normative data were derived from our accompanying study of the development of VCN projections to the superior olivary complex in normal gerbils (Kil et al., this issue). Whereas VCN normally projects to the lateral aspect of the ipsilateral medial superior olive and to the medial aspect of the contralateral medial superior olive in control animals, in experimental animals VCN on the unablated side projects to both sides of these nuclei. Whereas in the gerbil, VCN normally projects only to the hilar area and to the ventrolateral limb of the contralateral lateral superior olive, in experimental animals VCN on the unablated side projects throughout this nucleus. This induced projection is specific in that the efferents to each limb of the contralateral nucleus are linked to the normal projection to the homotopic region of the ipsilateral nucleus. Whereas VCN innervates the contralateral medial nucleus of the trapezoid body in control animals, in experimental animals VCN on the unablated side provides calyces of Held in the ipsilateral nucleus as well. The induced projections to these three major subnuclei of the superior olivary complex first appear within 24 hours of the cochlear ablation and continue to develop over at least the subsequent 11 days. Thus, prior to the day when the cochlea becomes functional, VCN has established specific ectopic projections to loci normally innervated by VCN on the ablated side.(ABSTRACT TRUNCATED AT 400 WORDS)

GABAergic terminals in nucleus magnocellularis and laminaris originate from the superior olivary nucleus

The auditory brainstem nuclei, angularis (NA), magnocellularis (NM), and laminaris (NL) of the chicken, Gallus, contain terminals that stain for antibodies against the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA). Some of these terminals originate from cells surrounding nucleus magnocellularis. Results from this study indicate that the majority of the GABAergic terminals found in NA, NM and NL originate from the superior olivary nucleus (SON). Injections of cholera toxin and horseradish peroxidase show that superior olivary nucleus (SON) neurons, which respond to pure tones, project bilaterally to NA, NM, and NL. NA and NL are reciprocally connected with the SON. More NA cells project to the SON than NL cells. While SON neurons project to NM, NM neurons do not project axons back to the SON. The configuration of SON terminals in NA, NM and NL matches the pattern of GABA-immunoreactive puncta seen in these three nuclei: they surround individual NM cells, congregate in the dendritic neuropil of NL, and blanket the NA. The data indicate that NA, NM and NL may be affected by two different inhibitory cell types: local interneurons and SON neurons. Patterns of connectivity described in this report suggest that the activity of NA cells could influence NM and NL cell physiology. Specifically, increases in NA cell activity could augment the effects of GABAergic SON neurons on NM and NL. Hence, binaural perception in the chicken may be more dependent upon changes in intensity cues than previously believed.

Physiological evidence for ipsilateral inhibition in the lateral superior olive: synaptic responses in mouse brain slice

The incidence of ipsilateral inhibition in the lateral superior olive (LSO) was examined in a brain slice preparation of the mouse superior olivary complex. A 400 microns brain slice was taken in the frontal plane and maintained in a warm, oxygenated saline solution. Intracellular recordings were made from the LSO with micropipettes filled with 4 M potassium acetate. Synaptic responses were elicited by electrical stimulation of the trapezoid body in different slices at various locations between the cochlear nucleus and the ipsilateral superior olivary complex (SOC). The results show that ipsilateral stimulation can evoke inhibitory as well as excitatory postsynaptic potentials. The ipsilateral IPSPs have short latencies and are elicited by stimulation of the trapezoid body at any point along its course between cochlear nucleus and LSO. Short-latency IPSPs can also be produced by direct stimulation of the ventral cochlear nucleus itself. Ipsilateral IPSPs are blocked by low concentrations of the glycine antagonist, strychnine. In addition, bath application of sodium pentobarbital in one case eliminated ipsilateral IPSPs without eliminating EPSPs. The results suggest that there is a rapidly conducting, glycinergic pathway from cochlear nucleus through the trapezoid body to the LSO on the same side of the brain. This pathway is probably served by either a direct projection from the ventral cochlear nucleus to the LSO or an indirect one from cochlear nucleus to LSO through the lateral nucleus of the trapezoid body (LNTB).

Response of neurons in the lateral superior olive and medial nucleus of the trapezoid body to repetitive stimulation: intracellular and extracellular recordings from mouse brain slice

The responses of neurons in the lateral superior olive (LSO) and medial nucleus of the trapezoid body (MNTB) to repeated electrical stimulation of the trapezoid body were investigated in a brain slice preparation of the mouse superior olivary complex. Brain slices, 400-500 microns thick, were cut in the frontal plane and were maintained for physiological recording in a bath of warm, oxygenated saline. Both intracellular and extracellular recordings were made with glass micropipettes filled with 4 M potassium acetate. Bipolar stimulating electrodes were placed on the trapezoid body ipsilateral and contralateral to the superior olive. Current levels were set so that an ipsilateral pulse elicited a single action potential in LSO and a contralateral pulse elicited a single action potential in MNTB. Trapezoid body fibers were then stimulated with trains consisting of 10 current pulses. Repeated stimulation at low rates resulted in a number of spikes equal to the number of current pulses. Pulse rate was then increased to determine the limits of response to repeated stimulation. In the MNTB, neurons were capable of following very high rates of stimulation without reduction in response probability. On the average, MNTB neurons responded with a probability of 0.9 at 667 Hz. In contrast, most LSO neurons were incapable of following high rates of stimulation. LSO neurons responded with a probability of 0.9 at 125 Hz. For some LSO neurons, application of strychnine (0.5 microM) to the bath increased the probability of firing at high rates. MNTB neurons were unaffected by strychnine.

Degenerate hearing and sound localization in naked mole rats (Heterocephalus glaber), with an overview of central auditory structures

Behavioral tests of absolute sensitivity and sound localization in African naked mole rats show that, despite their communal social structure and large vocal repertoire, their hearing has degenerated much like that of other subterranean species. First, their ability to detect sound is limited, with their maximum sensitivity being only 35 dB (occurring at 4 kHz). Second, their high-frequency hearing is severely limited, with their hearing range (at 60 dB sound pressure level [SPL]) extending from 65 Hz to only 12.8 kHz. Third, determination of the effect of duration on noise thresholds indicates that, compared with other animals, mole rats require a sound to be present for a much longer duration before reaching asymptotic threshold. Finally, they are unable consistently to localize sounds shorter than 400 ms and cannot accurately localize sounds of longer duration, raising the possibility that they are unable to use binaural locus cues. Thus, it seems that the essentially one-dimensional burrow system of a subterranean habitat produces severe changes in hearing comparable to the changes in vision that result from the absence of light. To explore the relation between vision and sound-localization acuity, retinal ganglion cell densities were determined. The results indicate that naked mole rats have a broad area of best (albeit poor) vision, with maximum acuity estimated at 44 cycles/degree. That mammals with wide fields of best vision have poorer sound-localization acuity than those with narrower fields is consistent with the thesis that a major function of sound localization is to direct the gaze to the source of a sound. However, the fact that subterranean mammals have little use for vision in a lightless environment suggests that they represent an extreme case in this relationship and may explain the fact that, unlike surface-dwelling mammals, they have virtually lost the ability to localize brief sounds. Finally, despite their very limited auditory abilities, the major brainstem auditory nuclei, although relatively small, appear to be present.

Role of the central auditory system in hearing: the new direction

The mammalian central auditory system contains a large number of subcortical auditory nuclei, which were once thought to form a simple relay system, taking signals from the ear to the cortex where all information processing would have occurred. Now it appears that these subcortical nuclei are themselves responsible for the extraction and analysis of the dimensions of sounds. Not only do the nuclei encode dimensions defining the nature of the sound, but also they extract features of sound location. Three major nuclei in the superior olivary complex of mammals extract the horizontal direction of a sound source, and it seems likely that other nuclei in the auditory system encode elevation and distance. This shift in viewpoint away from the attributes of sound to the attributes of sound sources is an important new step in the investigation of the role of the central auditory system in hearing.

Acoustic chiasm V: inhibition and excitation in the ipsilateral and contralateral projections of LSO

When this series of experiments was begun in 1984, the activity of each lateral superior olive (LSO) in the mammalian hindbrain was known to encode the hemifield of acoustic space containing a sound source. However, the almost random bilaterality of its ascending projections seemed to jumble that identification before reaching the midbrain. At the same time, electrophysiological studies of LSO and its efferent target in the inferior colliculus, along with the strictly contralateral deficits in sound localization resulting from unilateral lesions above the level of the superior olives, indicated that hemifield allegiance was largely maintained (though reversed) at the midbrain. Here we present seven lines of biochemical evidence, some combined with prior ablations, supporting the notion that the anatomical segregation of the ipsilateral and contralateral fibers ascending from the LSO is accompanied by a corresponding segregation of their neurotransmitters: most of the ascending ipsilateral projection is probably glycinergic and, hence, inhibitory in effect, while most of the contralateral projection is probably glutamatergic/aspartergic and, hence, excitatory in effect. Taken together, the inhibitory ipsilateral projections and the excitatory contralateral projections serve to amplify functional contralaterality at the higher levels of the auditory system.

NMDA, non-NMDA and glycine receptors mediate binaural interaction in the lateral superior olive: Physiological evidence from mouse brain slice

Binaural interaction was investigated in a 400 microns brain slice taken through the mouse lateral superior olive (LSO). Ipsilateral excitatory and contralateral inhibitory inputs to LSO neurons were examined by recording physiological responses to electrical stimulation of the trapezoid body. Bath application of non-N-methyl-D-aspartate (non-NMDA) antagonists blocked ipsilateral excitation and strychnine blocked contralateral inhibition. N-methyl-D-aspartate (NMDA) had little effect on ipsilateral responses but completely blocked contralateral inhibition. These results suggest that ipsilateral excitation is mediated by non-NMDA receptors and contralateral inhibition by strychnine dependent glycine receptors. NMDA receptors may play a role by modulating contralateral inhibition in LSO.