Input-output relationships of the dorsal nucleus of the lateral lemniscus: Possible substrate for the processing of dynamic spatial cues

GABAB receptor-mediated modulation in the developing dorsal nucleus of the lateral lemniscus

The dorsal nucleus of the lateral lemniscus (DNLL) is a GABAergic, reciprocally connected auditory brainstem structure that continues to develop postnatally in rodents. One key feature of the DNLL is the generation of a strong, prolonged, ionotropic, GABAA receptor-mediated inhibition. Possible GABAB receptor-mediated signalling is unexplored in the DNLL. Here, we used Mongolian gerbils of either sex to describe GABAB receptor-mediated modulation of postsynaptic potassium currents and synaptic inputs in postnatal (P) animals of days 10/11 and 23-28. Throughout development, we observed the presence of a Baclofen-activated GABAB receptor-enhanced potassium outward conductance that is capable of suppressing action potential generation. In P10/11, old gerbils GABAB receptor activation enhances glutamatergic and suppresses ionotropic GABAergic synaptic transmission. During development, this differential modulation becomes less distinct, because in P22-28, old animals Baclofen-activated GABAB receptors rather enhance ionotropic GABAergic synaptic transmission, whereas glutamatergic transmission is both enhanced and suppressed. Blocking GABAB receptors causes an increase in ionotropic GABAergic transmission in P10/11 old gerbils that was independent on stimulation frequency but depended on the type of short-term plasticity. Together with the lack of Baclofen-induced changes in the synaptic paired-pulse ratio of either input type, we suggest that GABAB receptor-mediated modulation is predominantly postsynaptic and activates different signalling cascades. Thus, we argue that in DNLL neurons, the GABAB receptor is a post-synaptically located signalling hub that alters signalling cascades during development for distinct targets.

A Temporal Filter for Binaural Hearing Is Dynamically Adjusted by Sound Pressure Level

In natural environments our auditory system is exposed to multiple and diverse signals of fluctuating amplitudes. Therefore, to detect, localize, and single out individual sounds the auditory system has to process and filter spectral and temporal information from both ears. It is known that the overall sound pressure level affects sensory signal transduction and therefore the temporal response pattern of auditory neurons. We hypothesize that the mammalian binaural system utilizes a dynamic mechanism to adjust the temporal filters in neuronal circuits to different overall sound pressure levels. Previous studies proposed an inhibitory mechanism generated by the reciprocally coupled dorsal nuclei of the lateral lemniscus (DNLL) as a temporal neuronal-network filter that suppresses rapid binaural fluctuations. Here we investigated the consequence of different sound levels on this filter during binaural processing. Our in vivo and in vitro electrophysiology in Mongolian gerbils shows that the integration of ascending excitation and contralateral inhibition defines the temporal properties of this inhibitory filter. The time course of this filter depends on the synaptic drive, which is modulated by the overall sound pressure level and N-methyl-D-aspartate receptor (NMDAR) signaling. In psychophysical experiments we tested the temporal perception of humans and show that detection and localization of two subsequent tones changes with the sound pressure level consistent with our physiological results. Together our data support the hypothesis that mammals dynamically adjust their time window for sound detection and localization within the binaural system in a sound level dependent manner.

Electrogenic N-methyl-D-aspartate receptor signaling enhances binaural responses in the adult brainstem

In sensory systems, the neuronal representation of external stimuli is enhanced along the sensory pathway. In the auditory system, strong enhancement of binaural information takes place between the brainstem and the midbrain; however, the underlying cellular mechanisms are unknown. Here we investigated the transformation of binaural information in the dorsal nucleus of the lateral lemniscus (DNLL), a nucleus that connects the binaural nuclei in the brainstem and the inferior colliculus in the midbrain. We used in vitro and in vivo electrophysiology in adult Mongolian gerbils to show that N-methyl-D-aspartate receptor (NMDARs) play a critical role in neuronal encoding of stimulus properties in the DNLL. While NMDARs increase firing rates, the timing and the accuracy of the neuronal responses remain unchanged. NMDAR-mediated excitation increases the information about the acoustic stimulus. Taken together, our results show that NMDARs in the DNLL enhance the auditory information content in adult mammal brainstem.

Evolution of mammalian sound localization circuits: A developmental perspective

Localization of sound sources is a central aspect of auditory processing. A unique feature of mammals is the smooth, tonotopically organized extension of the hearing range to high frequencies (HF) above 10kHz, which likely induced positive selection for novel mechanisms of sound localization. How this change in the auditory periphery is accompanied by changes in the central auditory system is unresolved. I will argue that the major VGlut2(+) excitatory projection neurons of sound localization circuits (dorsal cochlear nucleus (DCN), lateral and medial superior olive (LSO and MSO)) represent serial homologs with modifications, thus being paramorphs. This assumption is based on common embryonic origin from an Atoh1(+)/Wnt1(+) cell lineage in the rhombic lip of r5, same cell birth, a fusiform cell morphology, shared genetic components such as Lhx2 and Lhx9 transcription factors, and similar projection patterns. Such a parsimonious evolutionary mechanism likely accelerated the emergence of neurons for sound localization in all three dimensions. Genetic analyses indicate that auditory nuclei in fish, birds, and mammals receive contributions from the same progenitor lineages. Anatomical and physiological differences and the independent evolution of tympanic ears in vertebrate groups, however, argue for convergent evolution of sound localization circuits in tetrapods (amphibians, reptiles, birds, and mammals). These disparate findings are discussed in the context of the genetic architecture of the developing hindbrain, which facilitates convergent evolution. Yet, it will be critical to decipher the gene regulatory networks underlying development of auditory neurons across vertebrates to explore the possibility of homologous neuronal populations.

The precedence effect in sound localization

In ordinary listening environments, acoustic signals reaching the ears directly from real sound sources are followed after a few milliseconds by early reflections arriving from nearby surfaces. Early reflections are spectrotemporally similar to their source signals but commonly carry spatial acoustic cues unrelated to the source location. Humans and many other animals, including nonmammalian and even invertebrate animals, are nonetheless able to effectively localize sound sources in such environments, even in the absence of disambiguating visual cues. Robust source localization despite concurrent or nearly concurrent spurious spatial acoustic information is commonly attributed to an assortment of perceptual phenomena collectively termed "the precedence effect," characterizing the perceptual dominance of spatial information carried by the first-arriving signal. Here, we highlight recent progress and changes in the understanding of the precedence effect and related phenomena.

Late postnatal development of intrinsic and synaptic properties promotes fast and precise signaling in the dorsal nucleus of the lateral lemniscus

The dorsal nucleus of the lateral lemniscus (DNLL) is an auditory brain stem structure that generates a long-lasting GABAergic output, which is important for binaural processing. Despite its importance in binaural processing, little is known about the cellular physiology and the synaptic input kinetics of DNLL neurons. To assess the relevant physiological parameters of DNLL neurons, their late postnatal developmental profile was analyzed in acute brain slices of 9- to 26-day-old Mongolian gerbils. The observed developmental changes in passive membrane and action potential (AP) properties all point toward an improvement of fast and precise signal integration in these neurons. Accordingly, synaptic glutamatergic and GABAergic current kinetics accelerate with age. The changes in intrinsic and synaptic properties contribute nearly equally to reduce the latency and jitter in AP generation and thus enhance the temporal precision of DNLL neurons. Furthermore, the size of the synaptic NMDA current is developmentally downregulated. Despite this developmental reduction, DNLL neurons display an NMDA-dependent postsynaptic amplification of AP generation, known to support high firing rates, throughout this developmental period. Taken together, our findings indicate that during late postnatal development DNLL neurons are optimized for high firing rates with high temporal precision.

Laminar inputs from dorsal cochlear nucleus and ventral cochlear nucleus to the central nucleus of the inferior colliculus: Two patterns of convergence

The central nucleus of the inferior colliculus is a laminated structure composed of oriented dendrites and similarly oriented afferent fibers that provide a substrate for tonotopic organization. Although inputs from many sources converge in the inferior colliculus, how axons from these sources contribute to the laminar pattern has remained unclear. Here, we investigated the axons from the cochlear nuclei that terminate in the central nucleus of the cat and rat. After characterization of the best frequency of the neurons at the injection sites in the cochlear nucleus, the neurons were labeled with dextran in order to visualize their axons and synaptic boutons in the central nucleus. Quantitative methods were used to determine the size and distribution of the boutons within the laminar organization. Two components in the laminae were identified: (1) a narrow axonal lamina that included the largest fibers and largest boutons; (2) a wide axonal lamina, surrounding the narrow lamina, composed of thin fibers and only small boutons. The wide lamina was approximately 30-40% wider than the narrow lamina, and it often extended more than 100 microm beyond the larger boutons on each side. The presence of both thick and thin fibers within the acoustic striae following these injections suggests that large and small fibers/boutons within these bands may originate from different neuronal types in the dorsal and ventral cochlear nucleus. We conclude that the narrow laminae that contain large fibers and boutons originate from larger cell types in the cochlear nucleus. In contrast, the wide lamina composed exclusively of small boutons may represent an input from other, perhaps smaller neurons in the cochlear nucleus. Thus, two types of inferior colliculus laminar structures may originate from the cochlear nucleus, and the small boutons in the wide laminae may contribute a functionally distinct input to the neurons of the inferior colliculus.

Neural Correlates of the Precedence Effect in the Inferior Colliculus of Behaving Cats

Several auditory spatial illusions, collectively called the precedence effect (PE), occur when transient sounds are presented from two different spatial locations but separated in time by an interstimulus delay (ISD). For ISDs in the range of localization dominance (<10 ms), a single fused sound is typically located near the leading source location only, as if the location of the lagging source were suppressed. For longer ISDs, both the leading and lagging sources can be heard and localized, and the shortest ISD where this occurs is called the echo threshold. Previous physiological studies of the extracellular responses of single neurons in the inferior colliculus (IC) of anesthetized cats and unanesthetized rabbits with sounds known to elicit the PE have shown correlates of these phenomena though there were differences in the physiologically measured echo thresholds. Here we recorded in the IC of awake, behaving cats using stimuli that we have shown to evoke behavioral responses that are consistent with the precedence effect. For small ISDs, responses to the lag were reduced or eliminated consistent with psychophysical data showing that sound localization is based on the leading source. At longer ISDs, the responses to the lagging source recovered at ISDs comparable to psychophysically measured echo thresholds. Thus it appears that anesthesia, and not species differences, accounts for the discrepancies in the earlier studies.

Quantitative measurement of afferent layers in the ferret inferior colliculus: DNLL projections to sublayers

In the central nucleus of the inferior colliculus (IC), afferent projections are aligned with dendritic arbors of disk-shaped cells, forming fibrodendritic layers. One feature that may serve as a guide for study of the intrinsic organization of the IC layers is the segregation of certain inputs to bands and patches within the layers of the central nucleus. In this study, we used Phaseolus leucoagglutinin as an anterograde tracer to examine the projections from the dorsal nucleus of the lateral lemniscus to the contralateral IC in adult ferrets. The labeled afferent projections distributed along the IC layers in a series of bands where there were dense endings and interband spaces where there were few if any endings. Branches of individual labeled axons that were reconstructed distributed within a single afferent band. Measurements of both the terminal density distribution and the optical density across the band were similar indicating that afferent bands were approximately 85 microm thick. Quantitative measurements of the labeled afferent bands will enhance comparison with other afferent projections and analysis of afferent development and plasticity.

Spectral determination of responses to species-specific calls in the dorsal nucleus of the lateral lemniscus

This study evaluated how neurons in the dorsal nucleus of the lateral lemniscus (DNLL) in Mexican free-tailed bats respond to both tone bursts and species-specific calls. Up to 20 calls were presented to each neuron, of which 18 were social communication and 2 were echolocation calls. We also measured excitatory response regions (ERRs): the range of tone burst frequencies that evoked discharges at a fixed intensity. Neurons were unselective for one or another call in that each neuron responded to any call so long as the call had energy that encroached on its ERR. Additionally, responses were evoked by the same set of calls, and with similar spike counts, when they were presented normally or reversed. By convolving activity in the ERRs with the spectrogram of each call, we showed that responses to tones accurately predicted discharge patterns evoked by species-specific calls. DNLL cells are remarkably homogeneous in that neurons having similar BFs responded to each of the species-specific calls with similar response profiles. The homogeneity was further illustrated by the ability to accurately predict the response profiles of a particular DNLL cell to species-specific calls from the ERR of another similarly tuned DNLL cell. Thus DNLL neurons tuned to the same or similar frequencies responded to species-specific calls with latencies and temporal discharge patterns that were so similar as to be virtually interchangeable. What this suggests is that DNLL responses evoked by complex sounds can be largely explained by a simple summation of the excitation in each neuron's ERR. Finally, superimposing the spectrograms of each call on the responses evoked by that call revealed that the DNLL population response re-creates both the spectral and the temporal features of each signal.

Roles of inhibition for transforming binaural properties in the brainstem auditory system

This review is concerned with the operation of circuits in the central auditory system, how they transform response features and what functional significance may be attributed to those transformations. We focus on the role that GABAergic inhibition plays in processing interaural intensity disparities (IIDs), the principal cues for localizing high frequencies, and the transformations of IID coding that occur between the superior olivary complex and the inferior colliculus (IC). IIDs are coded by excitatory-inhibitory (EI) cells, so called because they are excited by one ear and inhibited by the other. EI neurons are first created in the lateral superior olive (LSO), but they also dominate the dorsal nucleus of the lateral lemniscus (DNLL) and regions of the IC. The three nuclei are intimately linked through a complex arrangement of excitatory and inhibitory connections. One of these is a crossed excitatory projection from the LSO to both the DNLL and IC. The binaural properties of EI neurons in LSO, DNLL and IC appear strikingly similar, suggesting that the EI properties created in the LSO are simply imposed on the DNLL and IC through the crossed excitatory projections. Recent studies support the idea that EI properties created in lower centers are imposed on some IC cells. However, other studies show that the circuitry linking LSO, DNLL and IC generates a number of response transformations in many IC cells. These transformations include marked changes in EI properties with stimulus duration, the generation of highly focused spatial receptive fields, shifts in sensitivity to IIDs, and the de novo creation of the EI response property. All of these transformations are produced by inhibitory innervation of the IC. An additional emergent property is also imposed on IC cells that receive GABAergic innervation from the DNLL. That property is a change in the binaural features of the IC cell, a change produced by the reception of an earlier sound whose IID is strongly excitatory to the IC cell. We illustrate each of these transformations, propose circuitry that could account for the observed properties and suggest some functional relevance for each. In the final section, we discuss some of the inherent uncertainties associated with attributing functional consequences to response features and then consider whether the transformations found in some mammals are species-specific or are universal features of all mammals.

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.