Functional organization of ascending and descending connections of the cochlear nucleus of horseshoe bats

An Anatomical and Physiological Basis for Flexible Coincidence Detection in the Auditory System

Animals navigate the auditory world by recognizing complex sounds, from the rustle of a predator to the call of a potential mate. This ability depends in part on the octopus cells of the auditory brainstem, which respond to multiple frequencies that change over time, as occurs in natural stimuli. Unlike the average neuron, which integrates inputs over time on the order of tens of milliseconds, octopus cells must detect momentary coincidence of excitatory inputs from the cochlea during an ongoing sound on both the millisecond and submillisecond time scale. Here, we show that octopus cells receive inhibitory inputs on their dendrites that enhance opportunities for coincidence detection in the cell body, thereby allowing for responses both to rapid onsets at the beginning of a sound and to frequency modulations during the sound. This mechanism is crucial for the fundamental process of integrating the synchronized frequencies of natural auditory signals over time.

Medial superior olive in the rat: Anatomy, sources of input and axonal projections

Although rats and mice are among the preferred animal models for investigating many characteristics of auditory function, they are rarely used to study an essential aspect of binaural hearing: the ability of animals to localize the sources of low-frequency sounds by detecting the interaural time difference (ITD), that is the difference in the time at which the sound arrives at each ear. In mammals, ITDs are mostly encoded in the medial superior olive (MSO), one of the main nuclei of the superior olivary complex (SOC). Because of their small heads and high frequency hearing range, rats and mice are often considered unable to use ITDs for sound localization. Moreover, their MSO is frequently viewed as too small or insignificant compared to that of mammals that use ITDs to localize sounds, including cats and gerbils. However, recent research has demonstrated remarkable similarities between most morphological and physiological features of mouse MSO neurons and those of MSO neurons of mammals that use ITDs. In this context, we have analyzed the structure and neural afferent and efferent connections of the rat MSO, which had never been studied by injecting neuroanatomical tracers into the nucleus. The rat MSO spans the SOC longitudinally. It is relatively small caudally, but grows rostrally into a well-developed column of stacked bipolar neurons. By placing small, precise injections of the bidirectional tracer biotinylated dextran amine (BDA) into the MSO, we show that this nucleus is innervated mainly by the most ventral and rostral spherical bushy cells of the anteroventral cochlear nucleus of both sides, and by the most ventrolateral principal neurons of the ipsilateral medial nucleus of the trapezoid body. The same experiments reveal that the MSO densely innervates the most dorsolateral region of the central nucleus of the inferior colliculus, the central region of the dorsal nucleus of the lateral lemniscus, and the most lateral region of the intermediate nucleus of the lateral lemniscus of its own side. Therefore, the MSO is selectively innervated by, and sends projections to, neurons that process low-frequency sounds. The structural and hodological features of the rat MSO are notably similar to those of the MSO of cats and gerbils. While these similarities raise the question of what functions other than ITD coding the MSO performs, they also suggest that the rat MSO is an appropriate model for future MSO-centered research.

Structural arrangement of auditory brainstem nuclei in the bats Phyllostomus discolor and Carollia perspicillata

The structure of the mammalian auditory brainstem is evolutionarily highly plastic, and distinct nuclei arrange in a species-dependent manner. Such anatomical variability is present in the superior olivary complex (SOC) and the nuclei of the lateral lemniscus (LL). Due to the structure-function relationship in the auditory brainstem, the identification of individual nuclei supports the understanding of sound processing. Here, we comparatively describe the nucleus arrangement and the expression of functional markers in the auditory brainstem of the two bat species Phyllostomus discolor and Carollia perspicillata. Using immunofluorescent labeling, we describe the arrangement and identity of the SOC and LL nuclei based on the expression of synaptic markers (vesicular glutamate transporter 1 and glycine transporter 2), calcium-binding proteins, as well as the voltage-gated ion channel subunits Kv1.1 and HCN1. The distribution of excitatory and inhibitory synaptic labeling appears similar between both species and matches with that of other mammals. The detection of calcium-binding proteins indicates species-dependent differences and deviations from other mammals. Kv1.1 and HCN1 show largely the same expression pattern in both species, which diverges from other mammals, indicating functional adaptations in the cellular physiology of bat neurons.

Cyto- and myeloarchitectural brain atlas of the pale spear-nosed bat (Phyllostomus discolor) in CT Aided Stereotaxic Coordinates

The pale spear-nosed bat Phyllostomus discolor, a microchiropteran bat, is well established as an animal model for research on the auditory system, echolocation and social communication of species-specific vocalizations. We have created a brain atlas of Phyllostomus discolor that provides high-quality histological material for identification of brain structures in reliable stereotaxic coordinates to strengthen neurobiological studies of this key species. The new atlas combines high-resolution images of frontal sections alternately stained for cell bodies (Nissl) and myelinated fibers (Gallyas) at 49 rostrocaudal levels, at intervals of 350 µm. To facilitate comparisons with other species, brain structures were named according to the widely accepted Paxinos nomenclature and previous neuroanatomical studies of other bat species. Outlines of auditory cortical fields, as defined in earlier studies, were mapped onto atlas sections and onto the brain surface, together with the architectonic subdivisions of the neocortex. X-ray computerized tomography (CT) of the bat's head was used to establish the relationship between coordinates of brain structures and the skull. We used profile lines and the occipital crest as skull landmarks to line up skull and brain in standard atlas coordinates. An easily reproducible protocol allows sectioning of experimental brains in the standard frontal plane of the atlas. An electronic version of the atlas plates and supplementary material is available from https://doi.org/10.12751/g-node.8bbcxy.

Octopus Cells in the Posteroventral Cochlear Nucleus Provide the Main Excitatory Input to the Superior Paraolivary Nucleus

Auditory streaming enables perception and interpretation of complex acoustic environments that contain competing sound sources. At early stages of central processing, sounds are segregated into separate streams representing attributes that later merge into acoustic objects. Streaming of temporal cues is critical for perceiving vocal communication, such as human speech, but our understanding of circuits that underlie this process is lacking, particularly at subcortical levels. The superior paraolivary nucleus (SPON), a prominent group of inhibitory neurons in the mammalian brainstem, has been implicated in processing temporal information needed for the segmentation of ongoing complex sounds into discrete events. The SPON requires temporally precise and robust excitatory input(s) to convey information about the steep rise in sound amplitude that marks the onset of voiced sound elements. Unfortunately, the sources of excitation to the SPON and the impact of these inputs on the behavior of SPON neurons have yet to be resolved. Using anatomical tract tracing and immunohistochemistry, we identified octopus cells in the contralateral cochlear nucleus (CN) as the primary source of excitatory input to the SPON. Cluster analysis of miniature excitatory events also indicated that the majority of SPON neurons receive one type of excitatory input. Precise octopus cell-driven onset spiking coupled with transient offset spiking make SPON responses well-suited to signal transitions in sound energy contained in vocalizations. Targets of octopus cell projections, including the SPON, are strongly implicated in the processing of temporal sound features, which suggests a common pathway that conveys information critical for perception of complex natural sounds.

Heterogeneity of Intrinsic and Synaptic Properties of Neurons in the Ventral and Dorsal Parts of the Ventral Nucleus of the Lateral Lemniscus

The ventral nucleus of the lateral lemniscus (VNLL) provides a major inhibitory projection to the inferior colliculus (IC). Neurons in the VNLL respond with various firing patterns and different temporal precision to acoustic stimulation. The present study investigates the underlying intrinsic and synaptic properties of various cell types in different regions of the VNLL, using in vitro electrophysiological recordings from acute brain slices of mice and immunohistochemistry. We show that the biophysical membrane properties and excitatory input characteristics differed between dorsal and ventral VNLL neurons. Neurons in the ventral VNLL displayed an onset-type firing pattern and little hyperpolarization-activated current (I_h). Stimulation of lemniscal inputs evoked a large all-or-none excitatory response similar to Calyx of Held synapses in neurons in the lateral part of the ventral VNLL. Neurons that were located within the fiber tract of the lateral lemniscus, received several and weak excitatory input fibers. In the dorsal VNLL onset-type and sustained firing neurons were intermingled. These neurons showed large I_h and were strongly immunopositive for the hyperpolarization-activated cyclic nucleotide-gated channel 1 (HCN1) subunit. Both neuron types received several excitatory inputs that were weaker and slower compared to ventrolateral VNLL neurons. Using a mouse model that expresses channelrhodopsin under the promotor of the vesicular GABA transporter (VGAT) suggests that dorsal and ventral neurons were inhibitory since they were all depolarized by light stimulation. The diverse membrane and input properties in dorsal and ventral VNLL neurons suggest differential roles of these neurons for sound processing.

Decreased temporal precision of neuronal signaling as a candidate mechanism of auditory processing disorder

The sense of hearing is the fastest of our senses and provides the first all-or-none action potential in the auditory nerve in less than four milliseconds. Short stimulus evoked latencies and their minimal variability are hallmarks of auditory processing from spiral ganglia to cortex. Here, we review how even small changes in first spike latencies (FSL) and their variability (jitter) impact auditory temporal processing. We discuss a number of mouse models with degraded FSL/jitter whose mutations occur exclusively in the central auditory system and therefore might serve as candidates to investigate the cellular mechanisms underlying auditory processing disorders (APD).

Large somatic synapses on neurons in the ventral lateral lemniscus work in pairs

In the auditory system, large somatic synapses convey strong excitation that supports temporally precise information transfer. The information transfer of such synapses has predominantly been investigated in the endbulbs of Held in the anterior ventral cochlear nucleus and the calyx of Held in the medial nucleus of the trapezoid body. These large synapses either work as relays or integrate over a small number of inputs to excite the postsynaptic neuron beyond action potential (AP) threshold. In the monaural system, another large somatic synapse targets neurons in the ventral nucleus of the lateral lemniscus (VNLL). Here, we comparatively analyze the mechanisms of synaptic information transfer in endbulbs in the VNLL and the calyx of Held in juvenile Mongolian gerbils. We find that endbulbs in the VNLL are functionally surface-scaled versions of the calyx of Held with respect to vesicle availability, release efficacy, and synaptic peak currents. This functional scaling is achieved by different calcium current kinetics that compensate for the smaller AP in VNLL endbulbs. However, the average postsynaptic current in the VNLL fails to elicit APs in its target neurons, even though equal current suffices to generate APs in neurons postsynaptic to the calyx of Held. In the VNLL, a postsynaptic A-type outward current reduces excitability and prevents AP generation upon a single presynaptic input. Instead, coincidence detection of inputs from two converging endbulbs is ideal to reliably trigger APs. Thus, even large endbulbs do not guarantee one-to-one AP transfer. Instead, information flow appears regulated by circuit requirements.

Substrates of auditory frequency integration in a nucleus of the lateral lemniscus

In the intermediate nucleus of the lateral lemniscus (INLL), some neurons display a form of spectral integration in which excitatory responses to sounds at their best frequency are inhibited by sounds within a frequency band at least one octave lower. Previous work showed that this response property depends on low-frequency-tuned glycinergic input. To identify all sources of inputs to these INLL neurons, and in particular the low-frequency glycinergic input, we combined retrograde tracing with immunohistochemistry for the neurotransmitter glycine. We deposited a retrograde tracer at recording sites displaying either high best frequencies (>75 kHz) in conjunction with combination-sensitive inhibition, or at sites displaying low best frequencies (23-30 kHz). Most retrogradely labeled cells were located in the ipsilateral medial nucleus of the trapezoid body (MNTB) and contralateral anteroventral cochlear nucleus. Consistent labeling, but in fewer numbers, was observed in the ipsilateral lateral nucleus of the trapezoid body (LNTB), contralateral posteroventral cochlear nucleus, and a few other brainstem nuclei. When tracer deposits were combined with glycine immunohistochemistry, most double-labeled cells were observed in the ipsilateral MNTB (84%), with fewer in LNTB (13%). After tracer deposits at combination-sensitive recording sites, a striking result was that MNTB labeling occurred in both medial and lateral regions. This labeling appeared to overlap the MNTB labeling that resulted from tracer deposits in low-frequency recording sites of INLL. These findings suggest that MNTB is the most likely source of low-frequency glycinergic input to INLL neurons with high best frequencies and combination-sensitive inhibition. This work establishes an anatomical basis for frequency integration in the auditory brainstem.

Going native: voltage-gated potassium channels controlling neuronal excitability

In this review we take a physiological perspective on the role of voltage-gated potassium channels in an identified neuron in the auditory brainstem. The large number of KCN genes for potassium channel subunits and the heterogeneity of the subunit combination into K(+) channels make identification of native conductances especially difficult. We provide a general pharmacological and biophysical profile to help identify the common voltage-gated K(+) channel families in a neuron. Then we consider the physiological role of each of these conductances from the perspective of the principal neuron in the medial nucleus of the trapezoid body (MNTB). The MNTB is an inverting relay, converting excitation generated by sound from one cochlea into inhibition of brainstem nuclei on the opposite side of the brain; this information is crucial for binaural comparisons and sound localization. The important features of MNTB action potential (AP) firing are inferred from its inhibitory projections to four key target nuclei involved in sound localization (which is the foundation of auditory scene analysis in higher brain centres). These are: the medial superior olive (MSO), the lateral superior olive (LSO), the superior paraolivary nucleus (SPN) and the nuclei of the lateral lemniscus (NLL). The Kv families represented in the MNTB each have a distinct role: Kv1 raises AP firing threshold; Kv2 influences AP repolarization and hyperpolarizes the inter-AP membrane potential during high frequency firing; and Kv3 accelerates AP repolarization. These actions are considered in terms of fidelity of transmission, AP duration, firing rates and temporal jitter. An emerging theme is activity-dependent phosphorylation of Kv channel activity and suggests that intracellular signalling has a dynamic role in refining neuronal excitability and homeostasis.

Inhibitory projections from the ventral nucleus of the lateral lemniscus and superior paraolivary nucleus create directional selectivity of frequency modulations in the inferior colliculus: a comparison of bats with other mammals

This review considers four auditory brainstem nuclear groups and shows how studies of both bats and other mammals have provided insights into their response properties and the impact of their convergence in the inferior colliculus (IC). The four groups are octopus cells in the cochlear nucleus, their connections with the ventral nucleus of the lateral lemniscus (VNLL) and the superior paraolivary nucleus (SPON), and the connections of the VNLL and SPON with the IC. The theme is that the response properties of neurons in the SPON and VNLL map closely onto the synaptic response features of a unique subpopulation of cells in the IC of bats whose inputs are dominated by inhibition. We propose that the convergence of VNLL and SPON inputs generates the tuning of these IC cells, their unique temporal responses to tones, and their directional selectivities for frequency modulated (FM) sweeps. Other IC neurons form directional properties in other ways, showing that selective response properties are formed in multiple ways. In the final section we discuss why multiple formations of common response properties could amplify differences in population activity patterns evoked by signals that have similar spectrotemporal features.

Functional role of GABAergic and glycinergic inhibition in the intermediate nucleus of the lateral lemniscus of the big brown bat

The intermediate nucleus of the lateral lemniscus (INLL) is a major input to the inferior colliculus (IC), the auditory midbrain center where multiple pathways converge to create neurons selective for specific temporal features of sound. However, little is known about how INLL processes auditory information or how it contributes to integrative processes at the IC. INLL receives excitatory projections from the cochlear nucleus and inhibitory projections from the medial nucleus of the trapezoid body (MNTB), so it must perform some form of integration. To address the question of what role inhibitory synaptic inputs play in the INLL of the big brown bat (Eptesicus fuscus), we recorded sound-evoked responses of single neurons and iontophoretically applied bicuculline to block GABA(A) receptors or strychnine to block glycine receptors. Neither bicuculline nor strychnine had a consistent effect on response latency or frequency response areas. Bicuculline increased spike counts and response durations in most units, suggesting that GABAergic input suppressed the late part of the response and provided some gain control. Strychnine reduced the responses of some units with sustained discharge patterns to one or a few spikes at stimulus onset, but increased others. INLL is the only part of the auditory system where reduced responsiveness has been seen in vivo while blocking glycine. However, in vitro studies in the MNTB suggest that glycine can be facilitatory, possibly through presynaptic action. These results show that GABA consistently reduces spike counts and response durations, whereas glycine is suppressive in some INLL neurons but facilitatory in others.

The ventral nucleus of the lateral lemniscus of the gerbil (Meriones unguiculatus): organization of connections with the cochlear nucleus and the inferior colliculus

The spatial organization of projections from the ventral cochlear nucleus (VCN) to the ventral nucleus of the lateral lemniscus (VNLL) and from the VNLL to the central nucleus of the inferior colliculus (CNIC) was investigated by using neuroanatomical tracing methods in the gerbil. In order to label cells in the VNLL that project to the CNIC, focal injections of biotinylated dextran amine (BDA) were made into different CNIC regions. Retrogradely labeled cells were distributed throughout the dorsal-to-ventral axis of the VNLL in all cases. In contrast, the distribution of labeled cells across the lateral-to-medial dimension of the VNLL was related to the location of the injection site along the dorsolateral to ventromedial (frequency) axis of the CNIC. Cells projecting to dorsolateral (low-frequency) regions of the CNIC were located peripherally in the VNLL, mainly laterally and caudally, whereas those projecting to ventromedial (high-frequency) regions of the CNIC tended to be clustered centrally. Projections to the VNLL were labeled anterogradely following injections of BDA in the VCN. The distribution of terminal fields in the VNLL closely paralleled the topographic arrangement of cells projecting to the CNIC; projections from ventrolateral (low-frequency) areas of the VCN terminated mainly along the lateral and caudal borders of the VNLL, whereas projections from dorsomedial (high-frequency) areas terminated in more central regions. The results demonstrate a topographic organization of the major afferent and efferent connections of the gerbil VNLL.

The serotonin releaser fenfluramine alters the auditory responses of inferior colliculus neurons

Local direct application of the neuromodulator serotonin strongly influences auditory response properties of neurons in the inferior colliculus (IC), but endogenous stores of serotonin may be released in a distinct spatial or temporal pattern. To explore this issue, the serotonin releaser fenfluramine was iontophoretically applied to extracellularly recorded neurons in the IC of the Mexican free-tailed bat (Tadarida brasiliensis). Fenfluramine mimicked the effects of serotonin on spike count and first spike latency in most neurons, and its effects could be blocked by co-application of serotonin receptor antagonists, consistent with fenfluramine-evoked serotonin release. Responses to fenfluramine did not vary during single applications or across multiple applications, suggesting that fenfluramine did not deplete serotonin stores. A predicted gradient in the effects of fenfluramine with serotonin fiber density was not observed, but neurons with fenfluramine-evoked increases in latency occurred at relatively greater recording depths compared to other neurons with similar characteristic frequencies. These findings support the conclusion that there may be spatial differences in the effects of exogenous and endogenous sources of serotonin, but that other factors such as the identities and locations of serotonin receptors are also likely to play a role in determining the dynamics of serotonergic effects.

Responses of Neurons in the Rat's Ventral Nucleus of the Lateral Lemniscus to Amplitude-Modulated Tones

Recordings were made from single neurons in the rat's ventral nucleus of the lateral lemniscus (VNLL) to determine responses to amplitude-modulated (AM) tones. The neurons were first characterized on the basis of their response to tone bursts presented to the contralateral ear and a distinction was made between those with transient onset responses and those with sustained responses. Sinusoidal AM tones were then presented to the contralateral ear with a carrier that matched the neuron's characteristic frequency (CF). Modulation transfer functions were generated on the basis of firing rate (MTFFR) and vector strength (MTFVS). Ninety-two percent of onset neurons that responded continuously to AM tones had band-pass MTFFRs with best modulation frequencies from 10 to 300 Hz. Fifty-four percent of sustained neurons had band-pass MTFFRs with best modulation frequencies from 10 to 500 Hz; other neurons had band-suppressed, all-pass, low-pass, or high-pass functions. Most neurons showed either band-pass or low-pass MTFVS. Responses were well synchronized to the modulation cycle with maximum vector strengths ranging from 0.37 to 0.98 for sustained neurons and 0.78 to 0.99 for onset neurons. The upper frequency limit for response synchrony was higher than that reported for inferior colliculus, but lower than that seen in more peripheral structures. Results suggest that VNLL neurons, especially those with onset responses to tone bursts, are sensitive to temporal features of sounds and narrowly tuned to different modulation rates. However, there was no evidence of a topographic relation between dorsoventral position along the length of VNLL and best modulation frequency as determined by either firing rate or vector strength.

Structural and functional classes of multipolar cells in the ventral cochlear nucleus

Multipolar cells in the ventral cochlear nucleus (VCN) are a structurally and functionally diverse group of projection neurons. Understanding their role in the ascending pathway involves partitioning multipolar cells into distinct populations and determining where in the brain each sends its coded messages. In this study, we used retrograde labeling techniques in rats to identify multipolar neurons that project their axons to the ipsilateral dorsal cochlear nucleus (DCN), the contralateral CN, or both structures. Three rats received injections of biotinylated dextran amine in the ipsilateral DCN and diamidino yellow in the contralateral CN. Several radiate multipolar neurons (defined by their axonal projections to the ipsilateral DCN and their dendrites that traverse VCN isofrequency sheets) were double-labeled but over 70% were not. This result suggests two distinct populations: (1) radiate-commissural (RC) multipolar cells that project to the ipsilateral DCN and the contralateral CN, and (2) radiate multipolar cells that project exclusively (in this context) to the ipsilateral DCN. In a different group of animals, we retrogradely labeled multipolar neurons that project their axons to the contralateral CN and measured the size of their cell bodies. The mean size of this population (266 +/- 156 microm2) was significantly smaller than those of RC-multipolar cells (418 +/- 140 microm2). We conclude that the CN commissural pathway is composed of at least two components: (1) RC multipolar cells and (2) commissural multipolar cells that are small- and medium-sized neurons that project exclusively (in this context) to the contralateral CN. These results identify separate structural groups of multipolar cells that may correspond to physiological unit types described in the literature. They also provide protocols for isolating and studying different populations of multipolar cells to determine the neural mechanisms that govern their responses to sound.

Synaptic transmission mediated by ionotropic glutamate, glycine and GABA receptors in the rat’s ventral nucleus of the lateral lemniscus

The synaptic pharmacology of the ventral nucleus of the lateral lemniscus (VNLL) was investigated in brain slices obtained from rats of 14-37 days old using intracellular recording techniques. Excitatory and inhibitory synaptic potentials (EPSPs and IPSPs) were elicited by electrical stimulation of the lemniscal pathway and recorded from neurons with five types of intrinsic firing patterns (onset, pause, adapting, regular and bursting types). Synaptic receptors that mediated the EPSPs and IPSPs were identified using AMPA, NMDA, GABA(A) and glycine receptor antagonists. The early/short EPSPs were mediated by AMPA receptors. The late/long EPSPs, encountered only in neurons of younger animals, were mediated by NMDA receptors. The IPSPs in most neurons were mediated by glycine receptors. In some neurons the IPSPs were mediated by GABA(A) receptors or both glycine and GABA(A) receptors. The temporal dynamics of fast AMPA EPSPs and glycinergic IPSPs were very similar. AMPA EPSPs and glycinergic (and/or GABAergic) IPSPs could be encountered in a single neuron. The results suggest that the VNLL not only relays incoming signals rapidly from the lower brainstem to the inferior colliculus, but also integrates excitatory and inhibitory inputs to modify and process auditory information.

Neurobiological specializations in echolocating bats

Although the bat's nervous system follows the general mammalian plan in both its structure and function, it has undergone a number of modifications associated with flight and echolocation. The most obvious neuroanatomical specializations are seen in the cochleas of certain species of bats and in the lower brainstem auditory pathways of all microchiroptera. This article is a review of peripheral and central auditory neuroanatomical specializations in echolocating bats. Findings show that although the structural features of the central nervous system of echolocating microchiropteran bats are basically the same as those of more generalized mammals, certain pathways, mainly those having to do with accurate processing of temporal information and auditory control of motor activity, are hypertrophied and/or organized somewhat differently from those same pathways in nonecholocating species. Through the resulting changes in strengths and timing of synaptic inputs to neurons in these pathways, bats have optimized the mechanisms for analysis of complex sound patterns to derive accurate information about objects in their environment and direct behavior toward those objects.

Axonal pathways to the lateral superior olive labeled with biotinylated dextran amine injections in the dorsal cochlear nucleus of rats

The lateral superior olive (LSO) contains cells that are sensitive to intensity differences between the two ears, a feature used by the brain to localize sounds in space. This report describes a source of input to the LSO that complements bushy cell projections from the ventral cochlear nucleus (VCN). Injections of biotinylated dextran amine (BDA) into the dorsal cochlear nucleus (DCN) of the rat label axons and swellings in several brainstem structures, including the ipsilateral LSO. Labeling in the ipsilateral LSO was confined to a thin band that extended throughout the length of the structure such that it resembled an LSO isofrequency lamina. The source of this labeled pathway was not obvious, because DCN neurons do not project to the LSO, and VCN bushy cells were not filled by these injections. Filled neurons in several brainstem structures emerged as possible sources. Three observations suggest that most of the axonal labeling in the LSO derives from a single source. First, the number of labeled VCN planar multipolar cells and the amount of labeling in the LSO were consistent and robust across animals. In contrast, the number of labeled cells in most other structures was small and highly variable. Second, the locations of planar cells and filled axons in the LSO were related topographically to the position of the DCN injection site. Third, labeled terminal arborizations in the LSO arose from collaterals of axons in the trapezoid body (output tract of planar cells). We infer that planar multipolar cells, in addition to bushy cells, are a source of ascending input from the cochlear nucleus to the LSO.

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.

Aminergic projections to cochlear nucleus via descending auditory pathways

The cochlear nucleus (CN) receives descending input from a variety of auditory nuclei. Descending inputs from the superior olive in particular have been well described, especially those of olivocochlear neurons, which terminate ultimately in the cochlea. It has been demonstrated that olivocochlear neurons receive serotonergic and noradrenergic inputs and thus form a route by which the aminergic system may modulate cochlear mechanisms. Since olivocochlear neurons send collaterals into the CN, it is possible that they also from a route by which the aminergic systems modulate CN processes. The goal of the current study was to determine if neurons in the superior olive that projected to the CN received serotonergic or noradrenergic inputs. The retrograde tracer WGAapoHRP-Au was injected into the CN of cats. The brainstems were silver-enhanced to visualize the tracer and then immunohistochemically processed with antibodies raised against serotonin or dopamine-beta-hydroxylase (DBH) to label serotonergic or noradrenergic fibers, respectively. The sections were viewed with high power light microscopy to determine if the retrogradely labeled neurons were contacted by serotonin- or DBH-immunoreactive varicosities. Retrogradely labeled cells were observed in auditory brainstem nuclei known to project to the CN including the superior olivary complex and inferior colliculus bilaterally and the opposite CN. In these regions, retrogradely labeled neurons were closely associated with serotonin- and/or DBH-immunoreactive varicosities. Assuming a synaptic relationship between the projection neurons and varicosities, these results indicate that the serotonergic and noradrenergic systems innervate the descending pathways to the CN. Since the serotonergic and noradrenergic systems modulate their targets based on level of arousal, these results support the theory that descending systems are involved in selective attention.

Processing of interaural temporal disparities in the medial division of the ventral nucleus of the lateral lemniscus

The medial division of the ventral nucleus of the lateral lemniscus (VNLLm) contains a specialized population of neurons that is sensitive to interaural temporal disparities (ITDs), a potent cue for sound localization along the azimuth. Unlike many ITD-sensitive neurons elsewhere in the auditory system, neurons in the VNLLm respond only at the onset of tones. An onset response may be significant for behavior because, under echoic conditions, tones require sharp onsets for accurate localization. In contrast, noise can generally be localized even with gradual onsets, presumably because transients occur at random intervals in noise. We recorded responses of neurons in the VNLLm to tones and noise in unanesthetized rabbits. We found that although tones elicited a transient response, noise elicited a sustained response as if it was a sequence of transients. The responses to tones indicate that these neurons represent a secondary stage in the processing of ITDs. The onset response to tones was only weakly synchronized to the phase of the tone, indicating that neurons in the VNLLm inherit their sensitivity to ITDs from their inputs. The latencies were short (~8 ms), implying that the ITD sensitivity is derived from ascending inputs. Most neurons in the VNLLm discharged maximally at the same ITD at all frequencies, a characteristic shared with neurons of the medial superior olive. However, the latency of neurons in the VNLLm to interaurally delayed stimuli is linked strongly to the timing of the contralateral stimulus. This suggests that these neurons receive a suprathreshold, contralateral input that is modulated by a subthreshold input conveying information about ITDs. Other stations in the auditory pathway contain a subset of neurons that respond transiently to tones and are sensitive to ITDs. These neurons may represent a novel pathway that assists in localizing sounds in the presence of reflections.

Cellular and subcellular distribution of AMPA-type glutamate receptor subunits and metabotropic glutamate receptor 1alpha in the cochlear nucleus of the horseshoe bat (Rhinolophus rouxi)

Ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) selective glutamate receptors (GluRs) are the main mediators of fast excitatory neurotransmission and composed of a variable combination of four different subunits (GluR1-4). The metabotropic glutamate receptor 1alpha (mGluR1alpha) is involved in plastic synaptic events. Since horseshoe bats strongly depend on temporal cues for acoustic imaging by echolocation and exhibit prominent species specific specializations of the cochlear nucleus (CN), the subunit distribution of AMPA selective GluRs and the distribution of mGluR1alpha was studied at the light and electron microscopic level with preembedding immunocytochemistry. Immunoreactivity to GluR1 was low throughout the CN. All types of projection neurons of the ventral CN expressed distinct GluR2/3 and GluR4 immunoreactivity with GluR4-labeling especially prominent in multipolar and octopus cell-like neurons of the posteroventral CN. The AMPA and metabotropic receptor inventory of the laminated ventral subdivision of the dorsal CN (DCNv) agreed with that reported in other mammals, whereas the specialized dorsal non-laminated subdivision of DCN (DCNd) lacked the prominent labeling for GluR2/3 and mGluR1alpha that characterizes cartwheel cells of DCNv. Distinct GluR2/3 and GluR4 immunoreactivity combined with low expression of mGluR1alpha immunoreactivity was characteristic for fusiform cells of DCNv and DCNd. Tuberculoventral cells of both the deep DCNv and the DCNd exhibited light to moderate GluR2/3 and GluR4 immunoreactivity. The staining patterns in DCNd thus indicate a loss of cerebellar-like microcircuits and a conservation of frequency specific circuitry of the deep and fusiform cell layers of the mammalian DCN.

Morphology and physiology of neurons in the ventral nucleus of the lateral lemniscus in rat brain slices

The ventral nucleus of the lateral lemniscus (VNLL) is a prominent neuronal group that lies within the auditory pathway connecting the auditory lower brainstem and midbrain. Previous physiologic studies showed that VNLL neurons respond mainly to contralaterally presented sounds and display various firing patterns. To understand better the role that VNLL neurons play in transmitting and processing of auditory information, we examined the morphology of VNLL neurons and their cellular physiology in young rat brain slices. We made whole-cell patch-clamp recordings and labeled cells intracellularly with neurobiotin to investigate the relation between morphologic neuronal types, intrinsic membrane properties, and postsynaptic responses. VNLL neurons fell into two distinct morphologic groups, i.e., bushy cells and stellate cells, based on their dendritic patterns. Stellate cells were grouped further into stellate I, II, and elongate cells according to soma shape, dendritic branches, and orientation. Bushy cells showed an onset firing pattern and a nonlinear current-voltage relationship. All three subtypes of stellate cells had a linear current-voltage relationship, but exhibited different firing patterns. Stellate I cells showed regular and onset-pause firing patterns, whereas stellate II cells showed adapting and elongate cells showed burst firing patterns. Bushy cells and stellate cells responded to stimulation of the lateral lemniscus with excitatory and/or inhibitory synaptic potentials. These results suggest that the VNLL is a heterogeneous neuronal group and that it contains many channels for processing different kinds of auditory information. Neuronal morphology and intrinsic membrane properties contribute to the behavior of individual neurons.

The central acoustic tract and audio-vocal coupling in the horseshoe bat, Rhinolophus rouxi

Doppler shift compensation (DSC) behaviour in horseshoe bats is a remarkable example of sensorimotor feedback that stabilizes the echo frequency at the bat's optimum hearing range regardless of motion-induced frequency shifts in the echoes. Searching for a related neural interface, the nucleus of the central acoustic tract (NCAT) was investigated in the echolocating horseshoe bat, Rhinolophus rouxi, using various neurophysiological and tracer methods. The NCAT receives bilateral auditory input from the cochlear nuclei and sends projections to regions outside the classical acoustic pathway like the pretectal area or the superior colliculus. The binaural input is excitatory from the contralateral and inhibitory from the ipsilateral ear to 53% of the units, and auditory responses were biased to frontal and contralateral directions. The best frequencies of NCAT neurons match a narrow range above the main frequency component of the bat's species-specific echolocation call (62% of the units), and the neurons exhibit extremely sharp tuning (Q10dB up to 632). DSC is degraded by unilateral electrical or pharmacological microstimulation of the NCAT, and heavily impaired by unilateral lesion of the region. Altogether, the efferents of the NCAT to prevocal areas, the tuning of its neurons to the DSC-relevant echo frequency range, and the possibility to affect DSC by manipulation of the NCAT, support the assumption that the nucleus plays an important role in audio-vocal control in the horseshoe bat.

Structure and function of the bat superior olivary complex

The superior olivary complex (SOC) is a mammalian auditory brainstem structure that contains several nuclei. Some of them are part of the ascending system projecting to higher auditory centers, others belong to the descending system projecting to the cochlear nuclei or the cochlea itself. The main nuclei of the ascending system, the lateral and medial superior olive (LSO, MSO), as well as the lateral and medial nuclei of the trapezoid body (LNTB, MNTB), have been traditionally associated with sound localization. Here we review the results of recent studies on the main SOC nuclei in echolocating bats. These studies suggest that some SOC structures and functions are highly conserved across mammals (e.g., the LSO, which is associated with interaural intensity difference processing), while others are phylogenetically highly variable in both form and function (e.g., the MSO, traditionally associated with interaural time difference processing). For the MSO, these variations indicate that we should broaden our view regarding what functions the MSO might participate in, since its function in echolocation seems to lie in the context of pattern recognition rather than sound localization. Furthermore, across bat species, variations in the form and physiology of the MSO can be linked to specific behavioral adaptations associated with different echolocation strategies. Finally, the comparative approach, including auditory specialists such as bats, helps us to reach a more comprehensive view of the functional anatomy of auditory structures that are still poorly understood, like the nucleus of the central acoustic tract (NCAT).

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.

Detection of synchrony in the activity of auditory nerve fibers by octopus cells of the mammalian cochlear nucleus

The anatomical and biophysical specializations of octopus cells allow them to detect the coincident firing of groups of auditory nerve fibers and to convey the precise timing of that coincidence to their targets. Octopus cells occupy a sharply defined region of the most caudal and dorsal part of the mammalian ventral cochlear nucleus. The dendrites of octopus cells cross the bundle of auditory nerve fibers just proximal to where the fibers leave the ventral and enter the dorsal cochlear nucleus, each octopus cell spanning about one-third of the tonotopic array. Octopus cells are excited by auditory nerve fibers through the activation of rapid, calcium-permeable, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors. Synaptic responses are shaped by the unusual biophysical characteristics of octopus cells. Octopus cells have very low input resistances (about 7 M Omega), and short time constants (about 200 microsec) as a consequence of the activation at rest of a hyperpolarization-activated mixed-cation conductance and a low-threshold, depolarization-activated potassium conductance. The low input resistance causes rapid synaptic currents to generate rapid and small synaptic potentials. Summation of small synaptic potentials from many fibers is required to bring an octopus cell to threshold. Not only does the low input resistance make individual excitatory postsynaptic potentials brief so that they must be generated within 1 msec to sum but also the voltage-sensitive conductances of octopus cells prevent firing if the activation of auditory nerve inputs is not sufficiently synchronous and depolarization is not sufficiently rapid. In vivo in cats, octopus cells can fire rapidly and respond with exceptionally well-timed action potentials to periodic, broadband sounds such as clicks. Thus both the anatomical specializations and the biophysical specializations make octopus cells detectors of the coincident firing of their auditory nerve fiber inputs.

The evolution of temporal processing in the medial superior olive, an auditory brainstem structure

A basic concept in neuroscience is to correlate specific functions with specific neuronal structures. By discussing a specific example, an alternative concept is proposed: structures may be linked to rules of processing and these rules may serve different functions in different species or at different stages of evolution. The medial superior olive (MSO), a mammalian auditory brainstem structure, has been thought to solely process interaural time differences (ITD), the main cue for localizing low frequency sounds. Recent findings, however, indicate that this is not its only function since mammals that do not hear low frequencies and do not use ITDs for sound localization also possess a MSO. Recordings from the bat MSO indicate that it processes temporal cues in the milli- and submillisecond range, based on monaural or binaural inputs. In bats, and most likely in other small mammals, this temporal processing is related to pattern recognition and echo suppression rather than sound localization. However, the underlying mechanism, coincidence detection of several inputs, creates an epiphenomenal ITD sensitivity that is of no use for small mammals like bats or ancestral mammals. Such an epiphenomenal ITD sensitivity would have been a pre-adaptation which, when mammals grew larger during evolution and when localization of low frequency sounds became a question of survival, suddenly gained relevance. This way the MSO became involved in a new function without changing its basic rules of processing.

Discharge patterns of neurons in the ventral nucleus of the lateral lemniscus of the unanesthetized rabbit

The ventral nucleus of the lateral lemniscus (VNLL) is a major auditory nucleus that sends a large projection to the inferior colliculus. Despite its prominence, the responses of neurons in the VNLL have not been extensively studied. Previous studies in nonecholocating species have used anesthesia, which is known to affect discharge patterns. In addition, there is disagreement about the proportion of neurons that are sensitive to binaural stimulation. This report examines the responses of neurons in the VNLL of the unanesthetized rabbit to monaural and binaural stimuli. Most neurons responded to contralateral tone bursts at their best frequency and had either sustained or phasic discharge patterns. A few neurons were only inhibited. Most sustained neurons were classified as short-latency sustained (SL-sustained), but a few were of long latency. Some SL-sustained neurons exhibited multiple peaks in their discharge pattern, i.e., they had a "chopper" discharge pattern, whereas other SL-sustained neurons did not exhibit this pattern. In ordinary chopper neurons, the multiple peaks corresponded to the evenly spaced action potentials of a regular discharge. In unusual chopper neurons, the action potential associated with a particular peak could fail to occur during any one presentation of the stimulus. Unusual chopper neurons had a relatively irregular discharge. Phasic neurons were of two types: onset and transient. Onset neurons typically responded with a single action potential at the onset of the tone, whereas transient neurons produced a burst of action potentials. Transient neurons were relatively rare. About half the neurons also were influenced by ipsilateral stimulation. Most binaurally influenced neurons were either sensitive to interaural temporal disparities (ITDs) or excited by contralateral stimulation and inhibited by ipsilateral stimulation. Neurons sensitive to ITDs were mostly of the onset type and were embedded in the fiber tract medial to the main part of the nucleus. Neurons inhibited by ipsilateral stimulation could be of the sustained or onset type. The sustained neurons were located on the periphery of the main nucleus as well as in the fiber tract. Most of the monaural neurons were in the main, high-density part of VNLL. The present results demonstrate that the VNLL contains neurons with a heterogeneous set of responses, and that many of the neurons are binaural.

Descending auditory pathways: projections from the inferior colliculus contact superior olivary cells that project bilaterally to the cochlear nuclei

Multiple retrograde and anterograde tracers were used to characterize a pathway that extends from the inferior colliculus to both the left and right cochlear nuclei via a synaptic relay in the superior olivary complex. Different fluorescent tracers were injected into the left and right cochlear nuclei to identify cells in the superior olivary complex that project bilaterally. Double-labeled cells were present in almost all periolivary nuclei; the majority were located in the ventral nucleus of the trapezoid body and the anteroventral periolivary nucleus. Because these two nuclei are targets of descending projections from the inferior colliculus, triple-labeling experiments were performed to determine whether collicular axons contact the periolivary cells that project to the cochlear nuclei. The results demonstrate that descending axons from the inferior colliculus contact periolivary cells that project to the cochlear nuclei, including periolivary cells that project bilaterally. This pathway could provide an opportunity for higher levels of the auditory system to influence activity bilaterally in the cochlear nuclei and thus to modulate the initial processing of acoustic information by the brain.

Physiological properties of neurons in the ventral nucleus of the lateral lemniscus of the rat: intrinsic membrane properties and synaptic responses

The physiological properties including current-voltage relationships, firing patterns, and synaptic responses of the neurons in the ventral nucleus of the lateral lemniscus (VNLL) were studied in brain slices taken through the young rat's (17-37 days old) auditory brain stem. Intracellular recordings were made from VNLL neurons, and synaptic potentials were elicited by electrical stimulation of the lateral lemniscus ventral to the VNLL. Current-voltage relations and firing patterns were tested by recording the electrical potentials produced by intracellular injection of positive and negative currents. There were two types of VNLL neurons (type I and II) that exhibited different current-voltage relationships. In response to negative current, both type I and II neurons produced a graded hyperpolarization. Type I neurons responded to positive current with a graded depolarization and multiple action potentials the number of which was related to the strength of the current injected. The current-voltage relations of type I neurons were nearly linear. Type II neurons responded to positive current with a limited depolarization and only one or a few action potentials. The current-voltage relations of type II neurons were nonlinear near the resting potential. The membrane properties of the type II VNLL neurons may play an important role for processing information about time of onset of a sound. Type I neurons showed three different firing patterns, i.e., regular, onset-pause and adaptation, in response to small positive current. The onset-pause and adaptation patterns could become sustained when a large current was injected. The regular, onset-pause, and adaptation patterns in type I neurons and the onset pattern in type II neurons resemble "chopper," "pauser, " "primary-like," and "on" responses, respectively, as defined in in vivo VNLL studies. The results suggest that different responses to acoustic stimulation could be attributed to intrinsic membrane properties of VNLL neurons. Many VNLL neurons responded to stimulation of the lateral lemniscus with excitatory or inhibitory responses or both. Excitatory and inhibitory responses showed interaction, and the output of the synaptic integration depended on the relative strength of excitatory and inhibitory responses. Neurons with an onset-pause firing pattern were more likely to receive mixed excitatory and inhibitory inputs from the lower auditory brain stem.

THE ROLE OF TIMING IN THE BRAIN STEM AUDITORY NUCLEI OF VERTEBRATES

Vertebrate animals gain biologically important information from environmental sounds. Localization of sound sources enables animals to detect and respond appropriately to danger, and it allows predators to detect and localize prey. In many species, rapidly fluctuating sounds are also the basis of communication between conspecifics. This information is not provided directly by the output of the ear but requires processing of the temporal pattern of firing in the tonotopic array of auditory nerve fibers. The auditory nerve feeds information through several parallel ascending pathways. Anatomical and electrophysiological specializations for conveying precise timing, including calyceal synaptic terminals and matching axonal conduction times, are evident in several of the major ascending auditory pathways through the ventral cochlear nucleus and its nonmammalian homologues. One pathway that is shared by all higher vertebrates makes an ongoing comparison of interaural phase for the localization of sound in the azimuth. Another pathway is specifically associated with higher frequency hearing in mammals and is thought to make use of interaural intensity differences for localizing high-frequency sounds. Balancing excitation from one ear with inhibition from the other in rapidly fluctuating signals requires that the timing of these synaptic inputs be matched and constant for widely varying sound stimuli in this pathway. The monaural nuclei of the lateral lemniscus, whose roles are not understood (although they are ubiquitous in higher vertebrates), receive input from multiple pathways that encode timing with precision, some through calyceal endings.

Processing of sinusoidally amplitude modulated signals in the nuclei of the lateral lemniscus of the big brown bat, Eptesicus fuscus

Changes in amplitude are a characteristic feature of most natural sounds, including the biosonar signals used by bats for echolocation. Previous evidence suggests that the nuclei of the lateral lemniscus play an important role in processing timing information that is essential for target range determination in echolocation. Neurons that respond to unmodulated tones with a sustained discharge are found in the dorsal nucleus (DNLL), intermediate nucleus (INLL) and multipolar cell division of the ventral nucleus (VNLLm). These neurons provide a graded response over a broad dynamic range of intensities, and would be expected to provide information about the amplitude envelope of a modulated signal. Neurons that respond only at the onset of a tone make up a small proportion of cells in DNLL, INLL and VNLLm, but are the only type found in the columnar division of the ventral nucleus (VNLLc). Onset neurons in VNLLc maintain a constant latency across a wide range of stimulus frequencies and intensities, thus providing a precise marker for when a sound begins. To determine how these different functional classes of cells respond to amplitude changes, we presented sinusoidally amplitude modulated (SAM) signals monaurally to awake, restrained bats and recorded the responses of single neurons extracellularly. There were clear differences in the ability of neurons in the different cell groups to respond to SAM. In the VNLLm, INLL and DNLL, 90% of neurons responded to SAM with a synchronous discharge. Neurons in the VNLLc responded poorly or not at all to SAM signals. This finding was unexpected given the precise onset responses of VNLLc neurons to unmodulated tones and their ability to respond synchronously to sinusoidally frequency modulated (SFM) signals. Among neurons that responded synchronously to SAM, synchronization as a function of modulation rate described either a bandpass or a lowpass function, with the majority of bandpass functions in neurons that responded to unmodulated tones with a sustained discharge. The maximal modulation rates that elicited synchronous responses were similar for the different cell groups, ranging from 320 Hz in VNLLm to 230 Hz in DNLL. The range of best modulation rates was greater for SAM than for SFM; this was also true of the range of maximal modulation rates at which synchronous discharge occurred. There was little correlation between a neuron's best modulation rate or maximal modulation rate for SAM signals and those for SFM signals, suggesting that responsiveness to amplitude and frequency modulations depends on different neural processing mechanisms.

Sensitivity to interaural time differences in the medial superior olive of a small mammal, the Mexican free-tailed bat

Neurons in the medial superior olive (MSO) are thought to encode interaural time differences (ITDs), the main binaural cues used for localizing low-frequency sounds in the horizontal plane. The underlying mechanism is supposed to rely on a coincidence of excitatory inputs from the two ears that are phase-locked to either the stimulus frequency or the stimulus envelope. Extracellular recordings from MSO neurons in several mammals conform with this theory. However, there are two aspects that remain puzzling. The first concerns the role of the MSO in small mammals that have relatively poor low-frequency hearing and whose heads generate only very small ITDs. The second puzzling aspect of the scenario concerns the role of the prominent binaural inhibitory inputs to MSO neurons. We examined these two unresolved issues by recording from MSO cells in the Mexican free-tailed bat. Using sinusoidally amplitude-modulated tones, we found that the ITD sensitivities of many MSO cells in the bat were remarkably similar to those reported for larger mammals. Our data also indicate an important role for inhibition in sharpening ITD sensitivity and increasing the dynamic range of ITD functions. A simple model of ITD coding based on the timing of multiple inputs is proposed. Additionally, our data suggest that ITD coding is a by-product of a neuronal circuit that processes the temporal structure of sounds. Because of the free-tailed bat's small head size, ITD coding is most likely not the major function of the MSO in this small mammal and probably other small mammals.

Heterogeneous projections of the cat posteroventral cochlear nucleus

The anterograde tracer Phaseolus vulgaris-leucoagglutinin was used to identify the projections of the posteroventral cochlear nucleus in cats. After labeling predominately cells of the core and multipolar regions, varicose fibers were observed in a variety of auditory nuclei. Ipsilaterally, most varicose fibers were located in periolivary regions situated lateral to the medial superior olive of the superior olivary complex. Contralaterally, the majority of labeled fibers were located in the ventral nucleus of the trapezoid body and the ventral nucleus of the lateral lemniscus. Labeled varicose fibers were also observed in regions not commonly identified as receiving input from the posteroventral cochlear nucleus. These regions included bilaterally the principal nuclei of the superior olivary complex, some periolivary regions, and the sagulum, as well as the ipsilateral intermediate and dorsal nucleus of the lateral lemniscus, inferior colliculus, and lateral pontine nucleus. Both similarities and differences were observed in the projections of the core and multipolar regions. With the exception of calyceal-type endings in the contralateral ventral nucleus of the lateral lemniscus, the varicose fibers in all regions, including the contralateral medial nucleus of the trapezoid body, were beaded, en passant type terminal varicosities.

Ventral nucleus of the lateral lemniscus in guinea pigs: cytoarchitecture and inputs from the cochlear nucleus

Cytoarchitectonic criteria were used to distinguish three subdivisions of the ventral nucleus of the lateral lemniscus in guinea pigs. Axonal tracing techniques were used to examine the projections from the cochlear nucleus to each subdivision. Based on the cell types they contain and their patterns of input, we distinguished ventral, dorsal, and anterior subdivisions of the ventral nucleus of the lateral lemniscus. All three subdivisions receive bilateral inputs from the cochlear nucleus, with contralateral inputs greatly outnumbering ipsilateral inputs. However, the relative density of the inputs varies: the ventral subdivision receives the densest projection, whereas the anterior subdivision receives the sparsest projection. Further differences are apparent in the morphology of the afferent axons. Following an injection of Phaseolus vulgaris-leucoagglutinin into the ventral cochlear nucleus, most of the axons on the contralateral side and all of the axons on the ipsilateral side are thin. Thick axons are present only in the ventral subdivision contralateral to the injection site. The evidence from both anterograde and retrograde tracing studies suggests that the thick axons originate from octopus cells, whereas the thin axons arise from multipolar cells and spherical bushy cells. The differences in constituent cell types and in patterns of inputs suggest that each of the three subdivisions of the ventral nucleus of the lateral lemniscus makes a distinct contribution to the analysis of acoustic signals.

Medial superior olive in the free-tailed bat: response to pure tones and amplitude-modulated tones

In mammals with good low-frequency hearing and a moderate to large interear distance, neurons in the medial superior olive (MSO) are sensitive to interaural time differences (ITDs). Most small mammals, however, do not hear low frequencies and do not experience significant ITDs, suggesting that their MSOs participate in functions other than ITD coding. In one bat species, the mustached bat, the MSO is a functionally monaural nucleus, acting as a low-pass filter for the rate of sinusoidally amplitude-modulated (SAM) stimuli. We investigated whether the more typical binaural MSO of the MExican free-tailed bat also acts as an SAM filter. We recorded from 60 MSO neurons with their best frequencies covering the entire audiogram of this bat. The majority revealed bilateral excitation and indirect evidence for inhibition (EI/EI; 55%). The remaining neurons exhibited reduced inputs, mostly lacking ipsilateral inputs (28% I/EI; 12% O/EI; 5% EI/O). Most neurons (64%) responded with a phasic discharge to pure tones; the remaining neurons exhibited an additional sustained component. For stimulation with pure tones, two thirds of the cells exhibited monotonic rate-level functions for ipsilateral, contralateral, or binaural stimulation. In contrast, nearly all neurons exhibited nonmonotonic rate-level functions when tested with SAM stimuli. Eighty-eight percent of the neurons responded with a phase-locked discharge to SAM stimuli at low modulation rates and exhibited low-pass filter characteristics in the modulation transfer function (MTF) for ipsilateral, contralateral, and binaural stimulation. The MTF for ipsilateral stimulation usually did not match that for contralateral stimulation. Introducing interaural intensity differences (IIDs) changed the MTF in unpredictable ways. We also found that responses to SAMs depended on the carrier frequency. In some neurons we measured the time course of the ipsilaterally and contralaterally evoked inhibition by presenting brief frequency-modulated sweeps at different ITDs. The duration and timing of inhibition could be related to the SAM cutoff for binaural stimulation. We conclude that the response of the MSO in the free-tailed bat is created by a complex interaction of inhibition and excitation. The different time constants of inputs create a low-pass filter for SAM stimuli. However, the MSO output is an integrated response to the temporal structure of a stimulus as well as its azimuthal position, i.e., IIDs. There are no in vivo results concerning filter characteristics in a "classical" MSO, but our data confirm an earlier speculation about this interdependence based on data accessed from a gerbil brain slice preparation.

Multiple projections from the ventral nucleus of the trapezoid body in the rat

An analysis of the central projections of the ventral nucleus of the trapezoid body (VNTB) in the rat, a region of the superior olivary complex known for its neuronal heterogeneity, was made using two anterograde axonal tracers, [3H]leucine and biotinylated dextran amine (BDA). A mixture of these tracers was injected iontophoretically into the VNTB and the results analyzed by first assessing magnitudes of autoradiographic signal in nuclei receiving projections and then identifying the axons and terminals responsible for this signal in parallel sets of sections processed for BDA. Our analysis showed that in addition to its projections to each cochlea via the olivocochlear bundle, the VNTB has 3 major central sites of axonal terminations: (1) the cochlear nucleus, particularly the molecular layer of the contralateral dorsal cochlear nucleus, (2) the contralateral lateral superior olive, and (3) the ipsilateral inferior colliculus. Other sites receiving projections from the VNTB included the VNTB itself and the nuclei of the lateral lemniscus. Significantly, the relative magnitudes of labeling within the nuclei receiving inputs from the VNTB varied consistently as a function of the dorsoventral location of the injection site, confirming previous work showing that there is a partial segregation within this nucleus of neurons according to their projections. Our data also revealed an orderly topographic pattern of projections to the cochlear nuclei, lateral superior olive and the inferior colliculus which is consistent with the known tonotopic organization both of the VNTB and these projection targets. Methodologically, the co-injection of two tracers was advantageous in that patterns of silver grains in autoradiographs could be used to confirm whether axons and terminals labeled with BDA had originated from labeled somata at the injection site or were the result of uptake of BDA by fibers of passage.

Ultrastructural and immunocytochemical observations on the superior olivary complex of the mustached bat

This study investigates the functional organization of the superior olivary complex of the mustached bat with classical transmission electron microscopy and postembedding immunocytochemistry for gamma-aminobutyric acid (GABA) and glycine antisera in semithin serial sections. The ultrastructure and distribution of terminal types in the lateral superior olive (LSO) and the medial nucleus of the trapezoid body (MNTB) closely resemble that of other mammals; the organization within the medial superior olive (MSO) differs significantly. The differences concern the relative proportion of putatively inhibitory boutons, which appear as symmetrical synapses with flattened vesicles on MSO somata. In the bat, inhibitory boutons comprised 75-100% of perisomatic boutons, a value identical to that observed in the LSO. These terminals most likely arise from the MNTB. In other species, putatively inhibitory terminals form a much smaller proportion of perisomatic boutons in MSO. This difference suggests that in the bat MSO excitatory input to cell somata is considerably reduced and outweighed by inhibitory input. This suggestion is corroborated by immunocytochemical data. Glycine-immunoreactive puncta encrust somata of LSO and MSO cells to a similar degree and in rather homogeneous patterns throughout these nuclei. Putatively GABAergic terminals are located mainly on distal dendrites of MSO and LSO cells. Regional variations in the density of GABA-immunoreactive puncta in LSO suggest that different tonotopic zones are under differential modulatory influence. Both the LSO and MSO of the mustached bat contain significant amounts of putatively inhibitory projection cells. Coexistence of both antigens was commonly observed in subsets of cells. Quantitative analyses of labeling patterns and comparisons with other mammals suggest that the mix of neurotransmitters in projection cells of LSO and MSO is phylogenetically flexible, and thus the details of the functions of ascending pathways are species specific. In contrast to other mammals, the bat MSO forms parallel output pathways with excitatory and inhibitory components. Data are discussed in relation to specialized physiological response features.

Origin of ascending projections to the nuclei of the lateral lemniscus in the big brown bat, Eptesicus fuscus

The nuclei of the lateral lemniscus in the echolocating bat, Eptesicus fuscus, are large and highly differentiated. In each nucleus, different characteristic response properties predominate. To determine whether the dissimilar response properties are due in part to differential ascending input, we examined the retrograde transport from small deposits of horseradish peroxidase (HRP) or HRP conjugated with wheat germ agglutinin (WGA-HRP) in the nuclei of the lateral lemniscus. The intermediate nucleus (INLL) and the two divisions of the ventral nucleus (VNLL) receive almost exclusively monaural input from the anteroventral and posteroventral cochlear nuclei and from the medial nucleus of the trapezoid body. Lesser inputs originate in the lateral nucleus of the trapezoid body and the ventral periolivary area. Although the three monaural nuclei of the lateral lemniscus all receive input from the same set of nuclei, and from the same identified cell types in the cochlear nucleus, there is a difference in the relative proportions of input from these sources. The dorsal nucleus (DNLL) receives input mostly from binaural structures, the lateral and medial superior olives and the contralateral DNLL, with only a minor projection from the cochlear nucleus. The lateral and medial superior olives project bilaterally; the bilateral projection from the medial superior olive is unusual in that it is found in only a few mammalian species. The results show a segregated pattern of binaural projections to DNLL and monaural projections to INLL and VNLL that is consistent with the binaural response properties found in DNLL and the exclusively monaural response properties found in INLL and VNLL. The differences in response properties between monaural nuclei, however, are not due to input from different nuclei or cell types but may be influenced by differing magnitudes of the constituent ascending projections.

Auditory cortex of the rufous horseshoe bat: 1. Physiological response properties to acoustic stimuli and vocalizations and the topographical distribution of neurons

The extent and functional subdivisions of the auditory cortex in the echolocating horseshoe bat, Rhinolophus rouxi, were neurophysiologically investigated and compared to neuroarchitectural boundaries and projection fields from connectional investigations. The primary auditory field shows clear tonotopic organization with best frequencies increasing in the caudorostral direction. The frequencies near the bat's resting frequency are largely over-represented, occupying six to 12 times more neural space per kHz than in the lower frequency range. Adjacent to the rostral high-frequency portion of the primary cortical field, a second tonotopically organized field extends dorsally with decreasing best frequencies. Because of the reversed tonotopic gradient and the consistent responses of the neurons, the field is comparable to the anterior auditory field in other mammals. A third tonotopic trend for medium and low best frequencies is found dorsal to the caudal primary field. This area is considered to correspond to the dorsoposterior field in other mammals. Cortical neurons had different response properties and often preferences for distinct stimulus types. Narrowly tuned neurons (Q10dB > 20) were found in the rostral portion of the primary field, the anterior auditory field and in the posterior dorsal field. Neurons with double-peaked tuning curves were absent in the primary area, but occurred throughout the dorsal fields. Vocalization elicited most effectively neurons in the anterior auditory field. Exclusive response to pure tones was found in neurons of the rostral dorsal field. Neurons preferring sinusoidal frequency modulations were located in the primary field and the anterior and posterior dorsal fields adjacent to the primary area. Linear frequency modulations optimally activated only neurons of the dorsal part of the dorsal field. Noise-selective neurons were found in the dorsal fields bordering the primary area and the extreme caudal edge of the primary field. The data provide a survey of the functional organization of the horseshoe bat's auditory cortex in real coordinates with the support of cytoarchitectural boundaries and connectional data.

Projections to the cochlear nuclei from principal cells in the medial nucleus of the trapezoid body in guinea pigs

Spherical and globular cells in the cochlear nucleus provide input to the cell groups in the superior olivary complex devoted to the analysis of binaural cues. Descending projections from the superior olivary complex appear to inhibit the spherical and globular cells. It is not known which of the numerous cell types in the superior olive provide this descending input, but recent studies have shown that some of the cells are located in the medial nucleus of the trapezoid body (MTB). The present experiments were designed to determine whether the MTB projections arise from principal cells, which are known to play a role in sound localization, and to determine whether their projections terminate on spherical or globular cells. Principal cells in the MTB are characterized by their contacts with synaptic specializations called calyces, which arise from the axons of cells in the contralateral cochlear nucleus. In the first experiment, a fluorescent tracer was injected into one cochlear nucleus to label the calyces anterogradely. A different tracer was injected into the opposite cochlear nucleus to label cells retrogradely in the MTB. In every case, some of the labeled cells were enveloped by a labeled calyx, demonstrating that principal cells do project to the cochlear nucleus. In the second experiment, fluorescent tracers were injected into different parts of the cochlear nucleus. Analysis of the distribution of labeled cells suggested that MTB projections selectively target the globular cell region of the cochlear nucleus. In a third experiment, the axonal arborizations arising from this projection were labeled with biocytin or wheat germ agglutinin conjugated to horseradish peroxidase. Labeled boutons appeared to contact globular cells but not spherical cells. Multipolar cells in the ventral cochlear nucleus and cells in the dorsal cochlear nucleus were also contacted. The results suggest that MTB projections to the cochlear nucleus arise largely from principal cells and contact, at least in part, cells in the cochlear nucleus that give rise to ascending pathways involved in sound localization.

Anatomy and projection patterns of the superior olivary complex in the Mexican free-tailed bat, Tadarida brasiliensis mexicana

The superior olivary complex (SOC) is the first station in the ascending auditory pathway that receives binaural projections. Two of the principal nuclei, the lateral superior olive (LSO) and the medial superior olive (MSO), are major sources of ascending projections to the inferior colliculus. Whereas almost all mammals have an LSO, it has traditionally been thought that only animals that hear low frequencies have an MSO. Recent reports, however, suggest that the medial part of the SOC in bats is highly variable and that at least some bats have a well-developed MSO. Thus, the main goal of this study was to evaluate the cytoarchitecture and connections of the principal superior olivary nuclei of the Mexican free-tailed bat, with specific attention directed at the MSO. Cell and fiber stained material revealed that the LSO and the medial nucleus of the trapezoid body (MNTB) are similar to those described for other mammals. There are two medial nuclei we refer to as dorsomedial periolivary nucleus (DMPO) and MSO. Tracer experiments exhibited that the DMPO receives bilateral projections from the cochlear nucleus, and additional projections from the ipsilateral MNTB. The DMPO sends a strong projection to the ipsilateral inferior colliculus. Positive staining for acetylcholinesterase indicates that the DMPO is a part of the olivocochlear system, as it is in other animals. The MSO in the free-tailed bat meets many of the criteria that traditionally define this nucleus. These include the presence of bipolar and multipolar principal cells, bilateral innervation from the cochlear nucleus, a strong projection from the ipsilateral MNTB, and the absence of cholinergic cells. The major difference from traditional MSO features is that it projects bilaterally to the inferior colliculus. Approximately 30% of its cells provide collateral projections to the colliculi on both sides. Functional implications of the MSO for the free-tailed bat are considered in the Discussion.