Morphological relationships of peptidergic and noradrenergic nerve terminals to olivocochlear neurones in the rat

The nuclei of the lateral lemniscus: unexpected players in the descending auditory pathway

Introduction

In the mammalian auditory pathway, the nuclei of the lateral lemniscus (NLL) are thought to be exclusively involved in the bottom-up transmission of auditory information. However, our repeated observation of numerous NLL neurons labeled after injection of retrograde tracers into the superior olivary complex (SOC) led us to systematically investigate with retrograde tracers the descending projections from the NLL to the SOC of the rat.

Methods

We performed large injections of FluoroGold into the SOC to determine NLL contributions to descending projections, and focal injections of biotinylated dextran amine (BDA) to pinpoint the specific nuclei of the SOC innervated by each NLL.

Results

The SOC is innervated by thousands of neurons distributed across four nuclei or regions associated with the lateral lemniscus: the ipsilateral ventral and intermediate nuclei of the lateral lemniscus (VNLL and INLL); the medial paralemniscal region (PL) of both sides; and the ipsilateral semilunar nucleus (SLN), a previously unrecognized nucleus that wraps around the INLL dorsally, medially, and caudally and consists of small, flat neurons. In some experiments, at least 30% of neurons in the VNLL and INLL were retrogradely labeled. All nuclei of the SOC, except the medial and lateral superior olives, are innervated by abundant lemniscal neurons, and each SOC nucleus receives a unique combination of lemniscal inputs. The primary target of the projections from the VNLL is the ventral nucleus of the trapezoid body (VNTB), followed by the superior paraolivary nucleus (SPON), and the medial nucleus of the trapezoid body (MNTB). The INLL selectively innervates the VNTB. The PL innervates dorsal periolivary regions bilaterally. The SLN preferentially innervates the MNTB and may provide the first identified non-calyceal excitatory input to MNTB neurons.

Discussion

Our novel findings have strong implications for understanding acoustic information processing in the initial stages of the auditory pathway. Based on the proportion of lemniscal neurons involved in all the projections described, the NLL should be considered major players in the descending auditory pathway.

Sympathetic Nervous System Regulation of Auditory Function

BACKGROUND

The auditory system processes how we hear and understand sounds within the environment. It comprises both peripheral and central structures. Sympathetic nervous system projections are present throughout the auditory system. The function of sympathetic fibers in the cochlea has not been studied extensively due to the limited number of direct projections in the auditory system. Nevertheless, research on adrenergic and noradrenergic regulation of the cochlea and central auditory system is growing. With the rapid development of neuroscience, auditory central regulation is an extant topic of focus in research on hearing.

SUMMARY

As such, understanding sympathetic nervous system regulation of auditory function is a growing topic of interest. Herein, we review the distribution and putative physiological and pathological roles of sympathetic nervous system projections in hearing. Key Messages: In the peripheral auditory system, the sympathetic nervous system regulates cochlear blood flow, modulates cochlear efferent fibers, affects hair cells, and influences the habenula region. In central auditory pathways, norepinephrine is essential for plasticity in the auditory cortex and affects auditory cortex activity. In pathological states, the sympathetic nervous system is associated with many hearing disorders. The mechanisms and pathways of sympathetic nervous system modulation of auditory function is still valuable for us to research and discuss.

Distinct forms of synaptic plasticity during ascending vs descending control of medial olivocochlear efferent neurons

Activity in each brain region is shaped by the convergence of ascending and descending axonal pathways, and the balance and characteristics of these determine the neural output. The medial olivocochlear (MOC) efferent system is part of a reflex arc that critically controls auditory sensitivity. Multiple central pathways contact MOC neurons, raising the question of how a reflex arc could be engaged by diverse inputs. We examined functional properties of synapses onto brainstem MOC neurons from ascending (ventral cochlear nucleus, VCN) and descending (inferior colliculus, IC) sources in mice using an optogenetic approach. We found that these pathways exhibited opposing forms of short-term plasticity, with the VCN input showing depression and the IC input showing marked facilitation. By using a conductance-clamp approach, we found that combinations of facilitating and depressing inputs enabled firing of MOC neurons over a surprisingly wide dynamic range, suggesting an essential role for descending signaling to a brainstem nucleus.

In Vitro Wedge Slice Preparation for Mimicking In Vivo Neuronal Circuit Connectivity

In vitro slice electrophysiology techniques measure single-cell activity with precise electrical and temporal resolution. Brain slices must be relatively thin to properly visualize and access neurons for patch-clamping or imaging, and in vitro examination of brain circuitry is limited to only what is physically present in the acute slice. To maintain the benefits of in vitro slice experimentation while preserving a larger portion of presynaptic nuclei, we developed a novel slice preparation. This "wedge slice" was designed for patch-clamp electrophysiology recordings to characterize the diverse monaural, sound-driven inputs to medial olivocochlear (MOC) neurons in the brainstem. These neurons receive their primary afferent excitatory and inhibitory inputs from neurons activated by stimuli in the contralateral ear and corresponding cochlear nucleus (CN). An asymmetrical brain slice was designed which is thickest in the rostro-caudal domain at the lateral edge of one hemisphere and then thins towards the lateral edge of the opposite hemisphere. This slice contains, on the thick side, the auditory nerve root conveying information about auditory stimuli to the brain, the intrinsic CN circuitry, and both the disynaptic excitatory and trisynaptic inhibitory afferent pathways that converge on contralateral MOC neurons. Recording is performed from MOC neurons on the thin side of the slice, where they are visualized using DIC optics for typical patch-clamp experiments. Direct stimulation of the auditory nerve is performed as it enters the auditory brainstem, allowing for intrinsic CN circuit activity and synaptic plasticity to occur at synapses upstream of MOC neurons. With this technique, one can mimic in vivo circuit activation as closely as possible within the slice. This wedge slice preparation is applicable to other brain circuits where circuit analyses would benefit from preservation of upstream connectivity and long-range inputs, in combination with the technical advantages of in vitro slice physiology.

Synaptic Inhibition of Medial Olivocochlear Efferent Neurons by Neurons of the Medial Nucleus of the Trapezoid Body

Medial olivocochlear (MOC) efferent neurons in the brainstem comprise the final stage of descending control of the mammalian peripheral auditory system through axon projections to the cochlea. MOC activity adjusts cochlear gain and frequency tuning, and protects the ear from acoustic trauma. The neuronal pathways that activate and modulate the MOC somata in the brainstem to drive these cochlear effects are poorly understood. Evidence suggests that MOC neurons are primarily excited by sound stimuli in a three-neuron activation loop from the auditory nerve via an intermediate neuron in the cochlear nucleus. Anatomical studies suggest that MOC neurons receive diverse synaptic inputs, but the functional effect of additional synaptic influences on MOC neuron responses is unknown. Here we use patch-clamp electrophysiological recordings from identified MOC neurons in brainstem slices from mice of either sex to demonstrate that in addition to excitatory glutamatergic synapses, MOC neurons receive inhibitory GABAergic and glycinergic synaptic inputs. These synapses are activated by electrical stimulation of axons near the medial nucleus of the trapezoid body (MNTB). Focal glutamate uncaging confirms MNTB neurons as a source of inhibitory synapses onto MOC neurons. MNTB neurons inhibit MOC action potentials, but this effect depresses with repeat activation. This work identifies a new pathway of connectivity between brainstem auditory neurons and indicates that MOC neurons are both excited and inhibited by sound stimuli received at the same ear. The pathway depression suggests that the effect of MNTB inhibition of MOC neurons diminishes over the course of a sustained sound.SIGNIFICANCE STATEMENT Medial olivocochlear (MOC) neurons are the final stage of descending control of the mammalian auditory system and exert influence on cochlear mechanics to modulate perception of acoustic stimuli. The brainstem pathways that drive MOC function are poorly understood. Here we show for the first time that MOC neurons are inhibited by neurons of the MNTB, which may suppress the effects of MOC activity on the cochlea.

Morphofunctional alterations in the olivocochlear efferent system of the genetic audiogenic seizure-prone hamster GASH:Sal

The genetic audiogenic seizure hamster (GASH:Sal) is a model of a form of reflex epilepsy that is manifested as generalized tonic-clonic seizures induced by external acoustic stimulation. The morphofunctional alterations in the auditory system of the GASH:Sal that may contribute to seizure susceptibility have not been thoroughly determined. In this study, we analyzed the olivocochlear efferent system of the GASH:Sal from the organ of Corti, including outer and inner hair cells, to the olivocochlear neurons, including shell, lateral, and medial olivocochlear (LOC and MOC) neurons that innervate the cochlear receptor. To achieve this, we carried out a multi-technical approach that combined auditory hearing screenings, scanning electron microscopy, morphometric analysis of labeled LOC and MOC neurons after unilateral Fluoro-Gold injections into the cochlea, and 3D reconstruction of the lateral superior olive (LSO). Our results showed that the GASH:Sal exhibited higher auditory brain response (ABR) thresholds than their controls, as well as absence of distortion-product of otoacoustic emissions (DPOAEs) in a wide range of frequencies. The ABR and DPOAE results also showed differences between the left and right ears, indicating asymmetrical hearing alterations in the GASH:Sal. These alterations in the peripheral auditory activity correlated with morphological alterations. At the cochlear level, the scanning electron microscopy analysis showed marked distortions of the stereocilia from basal to apical cochlear turns in the GASH:Sal, which were not observed in the control hamsters. At the brainstem level, MOC, LOC, and shell neurons had reduced soma areas compared with control animals. This LOC neuron shrinkage contributed to reduction in the LSO volume of the GASH:Sal as shown in the 3D reconstruction analysis. Our study demonstrated that the morphofunctional alterations of the olivocochlear efferent system are innate components of the GASH:Sal, which might contribute to their susceptibility to audiogenic seizures. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Connectivity and ultrastructure of dopaminergic innervation of the inner ear and auditory efferent system of a vocal fish

Dopamine (DA) is a conserved modulator of vertebrate neural circuitry, yet our knowledge of its role in peripheral auditory processing is limited to mammals. The present study combines immunohistochemistry, neural tract tracing, and electron microscopy to investigate the origin and synaptic characteristics of DA fibers innervating the inner ear and the hindbrain auditory efferent nucleus in the plainfin midshipman, a vocal fish that relies upon the detection of mate calls for reproductive success. We identify a DA cell group in the diencephalon as a common source for innervation of both the hindbrain auditory efferent nucleus and saccule, the main hearing endorgan of the inner ear. We show that DA terminals in the saccule contain vesicles but transmitter release appears paracrine in nature, due to the apparent lack of synaptic contacts. In contrast, in the hindbrain, DA terminals form traditional synaptic contacts with auditory efferent neuronal cell bodies and dendrites, as well as unlabeled axon terminals, which, in turn, form inhibitory-like synapses on auditory efferent somata. Our results suggest a distinct functional role for brain-derived DA in the direct and indirect modulation of the peripheral auditory system of a vocal nonmammalian vertebrate.

Identification of inputs to olivocochlear neurons using transneuronal labeling with pseudorabies virus (PRV)

Olivocochlear (OC) neurons respond to sound and provide descending input that controls processing in the cochlea. The identities of neurons in the pathways providing inputs to OC neurons are incompletely understood. To explore these pathways, the retrograde transneuronal tracer pseudorabies virus (Bartha strain, expressing green fluorescent protein) was used to label OC neurons and their inputs in guinea pigs. Labeling of OC neurons began 1 day after injection into the cochlea. On day 2 (and for longer survival times), transneuronal labeling spread to the cochlear nucleus, inferior colliculus, and other brainstem areas. There was a correlation between the numbers of these transneuronally labeled neurons and the number of labeled medial (M) OC neurons, suggesting that the spread of labeling proceeds mainly via synapses on MOC neurons. In the cochlear nucleus, the transneuronally labeled neurons were multipolar cells including the subtype known as planar cells. In the central nucleus of the inferior colliculus, transneuronally labeled neurons were of two principal types: neurons with disc-shaped dendritic fields and neurons with dendrites in a stellate pattern. Transneuronal labeling was also observed in pyramidal cells in the auditory cortex and in centers not typically associated with the auditory pathway such as the pontine reticular formation, subcoerulean nucleus, and the pontine dorsal raphe. These data provide information on the identity of neurons providing input to OC neurons, which are located in auditory as well as non-auditory centers.

Ultrastructure of spines and associated terminals on brainstem neurons controlling auditory input

Spines are unique cellular appendages that isolate synaptic input to neurons and play a role in synaptic plasticity. Using the electron microscope, we studied spines and their associated synaptic terminals on three groups of brainstem neurons: tensor tympani motoneurons, stapedius motoneurons, and medial olivocochlear neurons, all of which exert reflexive control of processes in the auditory periphery. These spines are generally simple in shape; they are infrequent and found on the somata as well as the dendrites. Spines do not differ in volume among the three groups of neurons. In all cases, the spines are associated with a synaptic terminal that engulfs the spine rather than abuts its head. The positions of the synapses are variable, and some are found at a distance from the spine, suggesting that the isolation of synaptic input is of diminished importance for these spines. Each group of neurons receives three common types of synaptic terminals. The type of terminal associated with spines of the motoneurons contains pleomorphic vesicles, whereas the type associated with spines of olivocochlear neurons contains large round vesicles. Thus, spine-associated terminals in the motoneurons appear to be associated with inhibitory processes but in olivocochlear neurons they are associated with excitatory processes.

Planar multipolar cells in the cochlear nucleus project to medial olivocochlear neurons in mouse

Medial olivocochlear (MOC) neurons originate in the superior olivary complex and project to the cochlea, where they act to reduce the effects of noise masking and protect the cochlea from damage. MOC neurons respond to sound via a reflex pathway; however, in this pathway the cochlear nucleus cell type that provides input to MOC neurons is not known. We investigated whether multipolar cells of the ventral cochlear nucleus have projections to MOC neurons by labeling them with injections into the dorsal cochlear nucleus. The projections of one type of labeled multipolar cell, planar neurons, were traced into the ventral nucleus of the trapezoid body, where they were observed terminating on MOC neurons (labeled in some cases by a second cochlear injection of FluoroGold). These terminations formed what appear to be excitatory synapses, i.e., containing small, round vesicles and prominent postsynaptic densities. These data suggest that cochlear nucleus planar multipolar neurons drive the MOC neuron's response to sound.

Dendrodendritic connections between the cochlear efferent neurons in guinea pig

The outer hair cells of organ of Corti are innervated by the efferent neurons of medial olivocochlear neurons (MOC) of the brainstem which modify the cochlear auditory processing and sensitivity. Most of the MOC neurons are excited by a dominant ear and only a small portion of them is excited by both ears resulting in a binaural facilitation. The functional role of the feedback system between the organ of Corti and the cochlear efferent neurons is the protection of the ear from acoustic injury. The rapid impulse propagation in the bilateral olivocochlear system is suggestive of an electrotonic interaction between the bilateral olivocochlear neurons. The morphological background of the MOC pathway is not yet completely characterized. Therefore, we have labeled the bilateral cochlear nerves with different neuronal tracers in guinea pigs. In the anesthetized animals the cochlear nerves were exposed in the basal part of the modiolus and labeled simultaneously with different retrograde fluorescent tracers. By using confocal laser scanning microscope we could detect close appositions between the dendrites of the neurons of bilateral MOC. The distance between the neighboring profiles suggested close membrane appositions without interposing glial elements. These connections might serve as one of the underlying mechanisms of the binaural facilitation mediated by the olivocochlear system.

Dendrites of medial olivocochlear neurons in mouse

Stains for acetylcholinesterase (AChE) and retrograde labeling with Fluorogold (FG) were used to study olivocochlear neurons and their dendritic patterns in mice. The two methods gave similar results for location and number of somata. The total number of medial olivocochlear (MOC) neurons in the ventral nucleus of the trapezoid body (VNTB) is about 170 per side. An additional dozen large olivocochlear neurons are located in the dorsal periolivary nucleus (DPO). Dendrites of all of these neurons are long and extend in all directions from the cell bodies, a pattern that contrasts with the sharp frequency tuning of their responses. For VNTB neurons, there were greater numbers of dendrites directed medially than laterally and those directed medially were longer (on average, 25-50% longer). Dendrite extensions were most pronounced for neurons located in the rostral portion of the VNTB. When each dendrite from a single neuron was represented as a vector, and all the vectors summed, the result was also skewed toward the medial direction. DPO neurons, however, had more symmetric dendrites that projected into more dorsal parts of the trapezoid body, suggesting that this small group of olivocochlear neurons has very different physiological properties. Dendrites of both types of neurons were somewhat elongated rostrally, about 20% longer than those directed caudally. These results can be interpreted as extensions of dendrites of olivocochlear neurons toward their synaptic inputs: medially to meet crossing fibers from the cochlear nucleus that are part of the MOC reflex pathway, and rostrally to meet descending inputs from higher centers.

Projections of low spontaneous rate, high threshold auditory nerve fibers to the small cell cap of the cochlear nucleus in cats

The marginal shell of the anteroventral cochlear nucleus houses small cells that are distinct from the overlying microneurons of the granule cell domain and the underlying projection neurons of the magnocellular core. This thin shell of small cells and associated neuropil receives auditory nerve input from only the low (<18 spikes/s) spontaneous rate (SR), high threshold auditory nerve fibers; high SR, low threshold fibers do not project there. It should be noted, that most of these auditory nerve terminations reside in the neuropil and intermix with dendrites that originate outside the shell. Consequently, electron microscopy is necessary to determine the synaptic targets. For this report, the terminations of intracellularly labeled low SR auditory nerve fibers in the small cell of cats cap were mapped through serial sections using a light microscope. The terminals were then examined with an electron microscope and found to form synapses with the somata and dendrites of small cells. Moreover, the small cell dendrites were identifiable by an abundance of microtubules and the presence of polyribosomes that were free or associated with membranous cisterns. These data contribute to the concept of a high threshold feedback circuit to the inner ear, and reveal translational machinery for local control of activity-dependent synaptic modification.

Ultrastructure of synaptic input to medial olivocochlear neurons

Medial olivocochlear (MOC) neurons project from the brain to the cochlea to form the efferent limb of the MOC reflex. To study synaptic inputs to MOC neurons, we retrogradely labeled these neurons using horseradish peroxidase injections into the cochlea. Labeled neurons were identified in the ventral nucleus of the trapezoid body and documented with the light microscope before being studied with serial-section electron microscopy. MOC somata and dendrites were innervated by three different types of synapses, distinguished as either having: 1) large, round synaptic vesicles and forming asymmetric contacts; 2) small, round vesicles plus a few dense core vesicles and forming asymmetric contacts; or 3) pleomorphic vesicles and forming symmetric contacts. The first two types were the most frequent on somata. Acetylcholinesterase-stained material confirmed that the type containing large, round vesicles is most common on dendrites. We kept track of the synaptic terminals in serial sections and compiled them into three-dimensional swellings. Swellings with large, round vesicles formed up to seven synapses per swelling, were largest in size, and sometimes formed complex arrangements engulfing spines of MOC neurons. Swellings with small, round vesicles formed up to four synapses per swelling. The morphology of this type of synapse, and the moderate sizes of the swellings forming it, suggests that it originates from posteroventral cochlear nucleus stellate/multipolar neurons. This input may thus provide the sound-evoked input to MOC neurons that causes their reflexive response to sound.

Catecholaminergic innervation of guinea pig superior olivary complex

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

Noradrenergic modulation of brainstem nuclei alters cochlear neural output

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

Substance P inhibits potassium and calcium currents in inner ear spiral ganglion neurons

Substance P (SP), a member of the tachykinin family of neurotransmitters and neuromodulators, has been identified on spiral ganglion neurons (SGNs) in the inner ear; however, its high affinity receptor, neurokinin-1 (NK1), has not been identified and the physiological effects of SP on SGNs are not well understood. To address these issues, immunolabeling, RT-PCR, Western blots and whole-cell patch-clamp recordings were made from SGNs in P0-P5 mouse cochlear organotypic cultures. The NK1 receptor was detected on SGNs by immunocytochemistry, the protein was detected in cochlear tissues by Western blots, and the mRNA for the NK1 receptor was also found in cochlear tissues of postnatal mice (P2) by RT-PCR. Application of SP (1 to 25 microM) significantly increased the latency of SGN action potentials (APs) (mean increase 7.8 +/- 4 ms; 25 microM of SP), prolonged the duration of the action potential and made the resting potential (RP) more positive (mean 9.0 +/- 7 mV) relative to normal values (-54 +/- 6 mV). SP (1 to 25 microM) also suppressed voltage-activated potassium currents (IK+) and calcium currents (ICa2+). Puffing 25 microM of SP onto SGNs suppressed IK+ by 43 +/- 9% (n = 7) and ICa2+ by 40.6 +/- 5.6% (n = 7); both currents recovered when SP was washed out. A SP antagonist blocked the SP-induced suppression of IK+ and ICa2+. These results indicate that SP acting through NK1 receptors can have direct neuromodulatory effects on SGNs.

Intracochlear injection of pseudorabies virus labels descending auditory and monoaminerg projections to olivocochlear cells in guinea pig

Pseudorabies virus was used to label transneuronally descending auditory projections following intracochlear injections. At different time points after injection, virus-infected cells were detected immunohistochemically in the central nervous system. Initially (25 h), virus was transported retrogradely to olivocochlear cells in the pons. At 32-72 h after injection, labelling occurred in higher order auditory brainstem nuclei as well as in the locus coeruleus and pontine dorsal raphe. At 90-108 h, virus-infected neurons were found bilaterally in the medial geniculate body and in layer V of the auditory cortex. Viral transneuronal labelling in the auditory cortex after intracochlear application confirms the existence of a continuous descending chain of neurons from the auditory cortex to the cochlea, via the medial and lateral olivocochlear systems. The transneuronal labelling of the locus coeruleus and pontine dorsal raphe suggests that noradrenergic and serotonergic inputs may substantially influence the activity of olivocochlear cells, and thus the cochlea.

Ascending and descending projections from the superior olivary complex in guinea pigs: different cells project to the cochlear nucleus and the inferior colliculus

The superior olivary complex is a source of ascending projections to the inferior colliculus and descending projections to the cochlear nucleus. We used multiple-labeling techniques with fluorescent retrograde tracers to determine whether individual superior olivary cells project to the inferior colliculus and the cochlear nucleus. Almost all labeled cells contained one tracer, suggesting that they projected to only one of the injected targets. A small number of cells sent collateral projections to the ipsilateral cochlear nucleus and ipsilateral inferior colliculus. The double-labeled cells constituted fewer than 2% of the cells that projected to the cochlear nucleus or to the inferior colliculus. There was no evidence for cells projecting to both contralateral targets or to one ipsilateral target and one contralateral target. We conclude that the ascending projections to the inferior colliculus and the descending projections to the cochlear nucleus arise almost exclusively from separate populations of cells in the superior olivary complex. Their separate origins suggest that these projections are sending different information to higher and lower centers of the auditory pathways.

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.

Comparative distribution of NK1, NK2, and NK3 receptors in the rat brainstem auditory nuclei

While the distribution of substance P in the auditory system is well illustrated, the localization of its receptors has not yet been documented. The goal of our study was to characterize the distribution of the tachykinin receptors NK1-R, NK2-R and NK3-R in the brainstem auditory nuclei of the adult rat using immunohistochemical techniques. The immunoreactivity of the neurokinin receptors was found to be widely distributed in most neurons of the cochlear nucleus (CN), the lateral superior olive (LSO), the medial nucleus of the trapezoid body (MNTB) and in the inferior colliculus (IC). Immunoreactivity was generally confined to post-synaptic targets (neuronal cell body and proximal or primary dendrites) in all auditory nuclei. However, unlike brainstem nuclei, the IC showed, in addition to neuronal cell body staining, a positive axonal immunolabeling (axons and pre-synaptic terminals) with the anti-NK1-R antibody. This axonal staining, revealing a pre-synaptic expression of NK1-R, is in good agreement with the known presence of substance P in the IC neurons. The absence of axonal staining in the superior olivary complex nuclei which projects afferent to the IC indicated that the NK1-R labeled axons are rather intrinsic IC fibers or descending thalamic projections to the IC. Overall, the wide distribution of the three types of tachykinin receptors observed in the present study argues for an important role of tachykinin neuropeptides in the central auditory system.

Origin of the noradrenergic innervation of the superior olivary complex in the rat

In the rat, the superior olivary complex contains lateral and medial olivocochlear neurones, which respectively innervate two separate targets within the cochlea; the auditory afferents contacting the inner hair cells and the outer hair cells themselves. Previous double label immunohistochemical studies have shown that both lateral and medial olivocochlear neurones are contacted by noradrenergic nerve endings, and electrophysiological studies on in-vitro rat brain slices have demonstrated that noradrenaline exerts a direct, predominantly excitatory effect on medial olivocochlear neurones. In this paper, we have investigated the origin of the noradrenergic input to the superior olivary complex (SOC). A retrograde tracer, Fluorogold, was used to map the inputs to the SOC, and this was combined with immunofluorescent staining for dopamine-beta-hydroxylase (DbetaH) to identify which of the afferent inputs was noradrenergic. These experiments showed small numbers of neurones double-stained for both Fluorogold and DbetaH in the A6 cell group (the locus coeruleus). In the A7 cell group, within and medial to the lateral lemniscus, numerous Fluorogold labelled and DbetaH positive neurones were found, but no neurones were seen that were double-labelled. In none of the other major noradrenergic cell groups were labelled Fluorogold neurones ever detected. To confirm the results obtained by retrograde tracer injections, anterograde tracer injections with biotinylated dextran amine were made in the locus coeruleus. This resulted in labelled fibres within all subdivisions of the superior olivary complex. These experiments indicate that the noradrenergic input to the olivocochlear neurones originates solely from the locus coeruleus. The small numbers of double-labelled neurones found in the locus coeruleus indicate a very divergent non-selective noradrenergic input to the SOC.

Distribution and possible functional roles of some neuroactive peptides in the mammalian superior olivary complex

The mammalian superior olivary complex (SOC) is innervated by neuronal systems that contain a variety of neuroactive peptides. Conversely, neurones of the SOC form peptidergic projections to other targets. In this review, the peptides substance P, calcitonin-gene-related peptide, enkephalins and dynorphins, cholecystokinin and somatostatin are considered. Their distribution in fibres and cell bodies of the SOC are considered, with particular attention to differences between the SOC subdivisions. Evidence for the functional effects of these peptides is also reviewed and some brief speculations are offered about their possible functional role in hearing.