Noradrenergic and serotonergic projections to the superior olive: potential for modulation of olivocochlear neurons

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.

Cochlear protection against noise exposure requires serotonin type 3A receptor via the medial olivocochlear system

The cochlear efferent feedback system plays important roles in auditory processing, including regulation of the dynamic range of hearing, and provides protection against acoustic trauma. These functions are performed through medial olivocochlear (MOC) neurons. However, the underlying cellular and molecular mechanisms are not fully understood. The serotonin type 3A (5-HT3A) receptor is widely expressed throughout the nervous system, which suggests important roles in various neural functions. However, involvement of the 5-HT3A receptor in the MOC system remains unclear. We used mice in this study and found that the 5-HT3A receptor was expressed in MOC neurons that innervated outer hair cells in the cochlea and was involved in the activation of MOC neurons by noise exposure. 5-HT3A receptor knockout impaired MOC functions, potentiated noise-induced hearing loss, and increased loss of ribbon synapses following noise exposure. Furthermore, 5-HT3 receptor agonist treatment alleviated the noise-induced hearing loss and loss of ribbon synapses, which enhanced cochlear protection provided by the MOC system. Our findings demonstrate that the 5-HT3A receptor plays fundamental roles in the MOC system and critically contributes to protection from noise-induced hearing impairment.

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.

Chemical Effects of Kainic Acid Injection into the Rat Superior Olivary Region

Kainic acid injections have been used to destroy neuron somata in particular regions without damaging fiber tracts. We injected a solution of kainic acid into the region of the rat superior olivary complex in an effort to destroy its cholinergic projections to the cochlea and cochlear nucleus, which derive especially from the lateral superior olivary nucleus and ventral nucleus of the trapezoid body. In the lateral superior olivary nucleus, there were relatively small but fairly consistent decreases of choline acetyltransferase (ChAT) activity, larger decreases of acetylcholinesterase (AChE) activity, and consistent decreases of malate dehydrogenase activity, as a marker for oxidative metabolism. Other superior olivary regions were less affected by the kainic acid injections, but most showed overall significant decreases of AChE activity. Our results suggest that the cholinergic neurons giving rise to the centrifugal pathways to the cochlea and cochlear nucleus are more resistant to the effects of kainic acid than are those that receive major ascending input from the cochlear nucleus and project to higher levels of the auditory system. Comparison with published anatomical studies suggests that this resistance to the effects of kainic acid is related to relatively little glutamatergic input to the somata and proximal dendrites of these neurons. We also found a consistent approximately 16 % decrease of ChAT activity in the injected-side facial nerve root, which is most easily explained as a small effect of kainic acid on the facial nerve fibers passing through the injection site.

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.

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.

Noradrenergic refinement of glutamatergic neuronal circuits in the lateral superior olivary nucleus before hearing onset

Neuronal circuit plasticity during development is fundamental for precise network formation. Pioneering studies of the developmental visual cortex indicated that noradrenaline (NA) is crucial for ocular dominance plasticity during the critical period in the visual cortex. Recent research demonstrated tonotopic map formation by NA during the critical period in the auditory system, indicating that NA also contributes to synaptic plasticity in this system. The lateral superior olive (LSO) in the auditory system receives glutamatergic input from the ventral cochlear nucleus (VCN) and undergoes circuit remodeling during postnatal development. LSO is innervated by noradrenergic afferents and is therefore a suitable model to study the function of NA in refinement of neuronal circuits. Chemical lesions of the noradrenergic system and chronic inhibition of α2-adrenoceptors in vivo during postnatal development in mice disrupted functional elimination and strengthening of VCN-LSO afferents. This was potentially mediated by activation of presynaptic α2-adrenoceptors and inhibition of glutamate release because NA presynaptically suppressed excitatory postsynaptic current (EPSC) through α2-adrenoceptors during the first two postnatal weeks in an in vitro study. Furthermore, NA and α2-adrenoceptor agonist induced long-term suppression of EPSCs and decreased glutamate release. These results suggest that NA has a critical role in synaptic refinement of the VCN-LSO glutamatergic pathway through failure of synaptic transmission. Because of the ubiquitous distribution of NA afferents and the extensive expression of α2-adrenoceptors throughout the immature brain, this phenomenon might be widespread in the developing central nervous system.

Sodium salicylate suppresses GABAergic inhibitory activity in neurons of rodent dorsal raphe nucleus

Sodium salicylate (NaSal), a tinnitus inducing agent, can activate serotonergic (5-HTergic) neurons in the dorsal raphe nucleus (DRN) and can increase serotonin (5-HT) level in the inferior colliculus and the auditory cortex in rodents. To explore the underlying neural mechanisms, we first examined effects of NaSal on neuronal intrinsic properties and the inhibitory synaptic transmissions in DRN slices of rats by using whole-cell patch-clamp technique. We found that NaSal hyperpolarized the resting membrane potential, decreased the input resistance, and suppressed spontaneous and current-evoked firing in GABAergic neurons, but not in 5-HTergic neurons. In addition, NaSal reduced GABAergic spontaneous and miniature inhibitory postsynaptic currents in 5-HTergic neurons. We next examined whether the observed depression of GABAergic activity would cause an increase in the excitability of 5-HTergic neurons using optogenetic technique in DRN slices of the transgenic mouse with channelrhodopsin-2 expressed in GABAergic neurons. When the GABAergic inhibition was enhanced by optical stimulation to GABAergic neurons in mouse DRN, NaSal significantly depolarized the resting membrane potential, increased the input resistance and increased current-evoked firing of 5-HTergic neurons. However, NaSal would fail to increase the excitability of 5-HTergic neurons when the GABAergic synaptic transmission was blocked by picrotoxin, a GABA receptor antagonist. Our results indicate that NaSal suppresses the GABAergic activities to raise the excitability of local 5-HTergic neural circuits in the DRN, which may contribute to the elevated 5-HT level by NaSal in the brain.

Catecholaminergic connectivity to the inner ear, central auditory, and vocal motor circuitry in the plainfin midshipman fish porichthys notatus

Although the neuroanatomical distribution of catecholaminergic (CA) neurons has been well documented across all vertebrate classes, few studies have examined CA connectivity to physiologically and anatomically identified neural circuitry that controls behavior. The goal of this study was to characterize CA distribution in the brain and inner ear of the plainfin midshipman fish (Porichthys notatus) with particular emphasis on their relationship with anatomically labeled circuitry that both produces and encodes social acoustic signals in this species. Neurobiotin labeling of the main auditory end organ, the saccule, combined with tyrosine hydroxylase immunofluorescence (TH-ir) revealed a strong CA innervation of both the peripheral and central auditory system. Diencephalic TH-ir neurons in the periventricular posterior tuberculum, known to be dopaminergic, send ascending projections to the ventral telencephalon and prominent descending projections to vocal-acoustic integration sites, notably the hindbrain octavolateralis efferent nucleus, as well as onto the base of hair cells in the saccule via nerve VIII. Neurobiotin backfills of the vocal nerve in combination with TH-ir revealed CA terminals on all components of the vocal pattern generator, which appears to largely originate from local TH-ir neurons but may include input from diencephalic projections as well. This study provides strong neuroanatomical evidence that catecholamines are important modulators of both auditory and vocal circuitry and acoustic-driven social behavior in midshipman fish. This demonstration of TH-ir terminals in the main end organ of hearing in a nonmammalian vertebrate suggests a conserved and important anatomical and functional role for dopamine in normal audition.

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.

Pathophysiology and treatment of tinnitus: an elusive disease

Tinnitus is a perception of sound in proximity to the head with the absence of an external source. It is estimated to occur in 15-20% of the world's population, with 1-3% of cases severely affecting quality of life. Severe tinnitus is frequently associated with depression, anxiety and insomnia. Tinnitus has been associated with a variety of risk factors, including prolonged noise exposure, head and neck injury and infection. The most recent pathophysiologic theory of tinnitus suggests that the central nervous system is the source or "generator" of this condition. However, treatment modalities are still aimed at lessening the awareness of tinnitus and its impact on quality of life rather than attaining a definitive cure. Currently, no drug is available that has demonstrated replicable, long-term reduction of tinnitus impact in excess of placebo effects. However, the market value of such an agent is estimated to be $1.1 billion, with a potential for increase with an aging and longer-living population. This review assesses the current developments in the pathophysiology and treatment for tinnitus, which remains a chronic and debilitating condition.

Serotonin in the inferior colliculus fluctuates with behavioral state and environmental stimuli

Neuromodulation by serotonin (5-HT) could link behavioral state and environmental events with sensory processing. Within the auditory system, the presence of 5-HT alters the activity of neurons in the inferior colliculus (IC), but the conditions that influence 5-HT neurotransmission in this region of the brain are unknown. We used in vivo voltammetry to measure extracellular 5-HT in the IC of behaving mice to address this issue. Extracellular 5-HT increased with the recovery from anesthesia, suggesting that the neuromodulation of auditory processing is correlated with the level of behavioral arousal. Awake mice were further exposed to auditory (broadband noise), visual (light) or olfactory (2,5-dihydro-2,4,5-trimethylthiazoline, TMT) stimuli, presented with food or confined in a small arena. Only the auditory stimulus or restricted movement increased the concentration of extracellular 5-HT in the IC. Changes occurred within minutes of stimulus onset, with the auditory stimulus increasing extracellular 5-HT by an average of 5% and restricted movement increasing it by an average of 14%. These findings suggest that the neuromodulation of auditory processing by 5-HT is a dynamic process that is dependent on internal state and behavioral conditions.

Mice lacking adrenergic signaling have normal cochlear responses and normal resistance to acoustic injury but enhanced susceptibility to middle-ear infection

The vasculature and neurons of the inner ear receive adrenergic innervation from the cervical sympathetic chain, and adrenergic receptors may be expressed by cells of the organ of Corti and stria vascularis, despite a lack of direct sympathetic innervation. To assess the functional role of adrenergic signaling in the auditory periphery, we studied mice with targeted deletion of the gene for dopamine beta-hydroxylase (DBH), which catalyzes the conversion of dopamine to noradrenaline; thus, these mutant mice have no measurable adrenaline or noradrenaline. Dbh (-/-) mice were more susceptible to spontaneous middle-ear infection than their control littermates, consistent with a role for sympathetics in systemic and/or local immune response. At 6-8 weeks of age, cochlear thresholds and suprathreshold responses assessed by auditory brainstem responses and distortion product otoacoustic emissions, as well as light-microscopic morphology, were indistinguishable from controls, if ears with conductive hearing loss were eliminated. Dbh (-/-) mice were no more susceptible to acoustic injury than controls, despite prior reports that sympathectomy reduces noise damage. Dbh (-/-) mice showed enhancement of shock-evoked olivocochlear suppression of cochlear responses, which may arise from the loss of adrenergic inputs to olivocochlear neurons in the brainstem. However, adrenergic modulation of olivocochlear efferents does not mediate the protective effect of contralateral cochlear destruction on ipsilateral response to acoustic overexposure.

Chronic low-level lead exposure affects the monoaminergic system in the mouse superior olivary complex

Low-level lead (Pb) exposure is associated with behavioral and cognitive dysfunction, but it is not clear how Pb produces these behavioral changes. Pb has been shown to alter auditory temporal processing in both humans and animals. Auditory temporal processing occurs in the superior olivary complex (SOC) in the brainstem, where it is an important component in sound detection in noisy environments and in selective auditory attention. The SOC receives a serotonergic innervation from the dorsal raphe, and serotonin has been implicated in auditory temporal processing within the brainstem and inferior colliculus. Because Pb exposure modulates auditory temporal processing, the serotonergic system is a potential target for Pb. The current study was undertaken to determine whether developmental Pb exposure preferentially changes the serotonergic system within the SOC. Pb-treated mice were exposed to no Pb, very low Pb (0.01 mM), or low Pb (0.1 mM) throughout gestation and through 21 days postnatally. Brainstem sections from control and Pb-exposed mice were immunostained for the vesicular monoamine transporter 2 (VMAT2), serotonin (5-HT), and dopamine-beta-hydroxylase (DbetaH; a marker for norepinephrine) in order to elucidate the effect of Pb on monoaminergic input into the SOC. Sections were also immunolabeled with antibodies to vesicular glutamate transporter 1 (VGLUT1), vesicular gamma-aminobutyric acid (GABA) transporter (VGAT), and vesicular acetylcholine transporter (VAChT) to determine whether Pb exposure alters the glutaminergic, GABAergic, or cholinergic systems. Pb exposure caused a significant decrease in VMAT2, 5-HT, and DbetaH expression, whereas VGLUT1, VGAT, and VAChT showed no change. These results provide evidence that Pb exposure during development alters normal monoaminergic expression in the auditory brainstem.

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.

Brainstem auditory-evoked potential habituation and intensity-dependence related to serotonin metabolism in migraine: a longitudinal study

OBJECTIVE

Reduced habituation and increased intensity-dependence of cortical auditory-evoked potentials have been reported in migraine, but it is not known if brainstem mechanisms are chiefly or partly responsible for this hypersensitivity, if brainstem excitability or habituation changes across the migraine cycle, or how excitability relates to symptoms and serotonin metabolism.

METHODS

Brainstem auditory-evoked potentials (BAEPs) to 40, 55, and 70dB binaural rarefaction clicks were recorded in four blocks of 750 stimuli in a blinded longitudinal study in 41 migraine patients. Serotonin was measured in a blood sample from the cubital vein. The test day was classified as baseline, attack, pre-attack or post-attack.

RESULTS

Pre-attack BAEP changes were not found. Wave I, V and interpeak III-V latency increased after the attack. III-V latency correlated with headache history duration and usual headache attack duration. Habituation in wave IV-V dispersion to 40dB was found in controls but not in migraine (p=0.04). Serotonin correlated with BAEP amplitude in controls. Low serotonin correlated with more autonomic symptoms. BAEP intensity-dependence was normal in migraine.

CONCLUSIONS

BAEP latencies, but not amplitude, increase temporarily after a migraine attack. Abnormal habituation of brainstem wave IV-V dispersion in migraine may suggest increased excitation in colliculus inferior at low sound intensities, but no relation to the migraine cycle was found for wave IV-V amplitude, dispersion or habituation. The correlation between BAEP amplitude and serotonin was deranged in migraine patients, but reappeared temporarily within 72h after an attack.

SIGNIFICANCE

No evidence for pre-attack brainstem auditory sensitization was found in migraine. Intensity-dependence of AEP in migraine is probably not a passive reflection of brainstem dysfunction. BAEP changes seem to reflect a slight impact of migraine on serotonergic brainstem pathways.

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.

Dense serotonergic innervation of principal nuclei of the superior olivary complex in mouse

To evaluate species differences in the serotonergic innervation of the superior olivary complex, serotonergic fibers and varicosities were labeled with immunohistochemistry in mouse. Many immunoreactive fibers and varicosities were observed in two of the three principal nuclei, in addition to some periolivary nuclei. This pattern of staining differs greatly from that observed in other mammals in which periolivary, but not principal nuclei are richly innervated by serotonin (5-HT). These results indicate a functional relationship between the 5-HT system and both the ascending and descending auditory systems in the mouse.

The emotional ear in stress

Stress of some kind is encountered everyday and release of stress hormones is essential for adaptation to change. Stress can be physical (pain, noise exposure, etc.), psychological (apprehension to impending events, acoustic conditioning, etc.) or due to homeostatic disturbance (hunger, blood pressure, inner ear pressure, etc.). Persistent elevated levels of stress hormones can lead to disease states. The aim of the present review is to bring together data describing morphological or functional evidence for hormones of stress within the inner ear. The present review describes possible multiple interactions between the sympathetic and the complex feed-back neuroendocrine systems which interact with the immune system and so could contribute to various inner ear dysfunctions such as tinnitus, vertigo, hearing losses. Since there is a rapidly expanding list of genes specifically expressed within the inner ear this clearly allows for possible genomic and non-genomic local action of steroid hormones. Since stress can be encountered at any time throughout the life-time, the effects might be manifested starting from in-utero. These are avenues of research which remain relatively unexplored which merit further consideration. Progress in this domain could lead towards integration of stress concept into the overall clinical management of various inner ear pathologies.

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.

Effects of salicylate on serotoninergic activities in rat inferior colliculus and auditory cortex

In vivo microdialysis offers a unique approach to monitor biochemical events related to brain function and metabolism, and has been used extensively in many systems to measure the release of endogenous transmitters and other neuroactive substances during normal and pathological conditions. The characterization of neurotransmitters' changes induced by salicylate in the inferior colliculus (IC) and the auditory cortex (AC) may provide insight into the action of salicylate on the auditory system and, through this, provide a better understanding of neurological mechanism of salicylate-induced tinnitus. In the present study, the effect of salicylate on 5-HT system in IC and AC has been monitored by microdialysis in salicylate-induced tinnitus animal models. Glucose and lactate levels in IC and AC were significantly increased after application of salicylate (350 mg/kg, i.p.), indicating a salicylate-related increase in regional neuronal activity. The 5-HT level increased to a maximum of 268+/-27% basal level in IC 2 h after application and of 277+/-24% basal level in AC around 3 h after application. These data suggest that the increases of 5-HT levels in IC and AC may be involved in the tinnitus generation.

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.

Membrane properties of principal neurons of the lateral superior olive

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

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.

Serotonergic innervation of the auditory brainstem of the Mexican free-tailed bat,Tadarida brasiliensis

Anatomical and electrophysiological evidence suggests that serotonin alters the processing of sound in the auditory brainstem of many mammalian species. The Mexican free-tailed bat is a hearing specialist, like other microchiropteran bats. At the same time, many aspects of its auditory brainstem are similar to those in other mammals. This dichotomy raises an interesting question regarding the serotonergic innervation of the bat auditory brainstem: Is the serotonergic input to the auditory brainstem similar in bats and other mammals, or are there specializations in the serotonergic innervation of bats that may be related to their exceptional hearing capabilities? To address this question, we immunocytochemically labeled serotonergic fibers in the brainstem of the Mexican free-tailed bat, Tadarida brasiliensis. We found many similarities in the pattern of serotonergic innervation of the auditory brainstem in Tadarida compared with other mammals, but we also found two striking differences. Similarities to staining patterns in other mammals included a higher density of serotonergic fibers in the dorsal cochlear nucleus and in granule cell regions than in the ventral cochlear nucleus, a high density of fibers in some periolivary nuclei of the superior olive, and a higher density of fibers in peripheral regions of the inferior colliculus compared with its core. The two novel features of serotonergic innervation in Tadarida were a high density of fibers in the fusiform layer of the dorsal cochlear nucleus relative to surrounding layers and a relatively high density of serotonergic fibers in the low-frequency regions of the lateral and medial superior olive.

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.

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.

A review of evidence in support of a role for 5-HT in the perception of tinnitus

Tinnitus is a debilitating condition from which some 0.5-1% of the population of the Western world suffer sufficiently badly as to interfere with their normal working and leisure life. There is no satisfactory treatment at the present time and the uncertainty surrounding the mechanism of its generation makes it difficult to devise an effective cure. After much debate, the consensus of opinion amongst researchers regarding its site of origin is that it is primarily a central nervous system pathology although there certainly exists a class of patients whose tinnitus is peripherally based. In this paper, we provide some speculative ideas on how an initial auditory insult can be translated into central neurological substrates that represent tinnitus. Plastic changes arising from sensory deprivation trigger a change in synaptology and neurotransmission with a consequent change in receptor configuration. From neuroanatomical considerations and analogies with other clinical conditions, we postulate the involvement of serotonin (5-HT) in these plastic changes and consider the evidence available from the use of serotonergic drugs in different conditions. A possible relationship between 5-HT and lidocaine is also discussed.

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

In the rat, the outer hair cells in the cochlea receive direct synaptic input from neurones in the ventral nucleus of the trapezoid body. These so-called medial olivocochlear neurones exert an inhibitory influence on the cochlear neural output. Electrophysiological in vitro studies suggest that the activity of medial olivocochlear neurones may be affected by a variety of neuropeptides as well as noradrenaline, but anatomical confirmation of direct synaptic input is still lacking. We have investigated, at the light microscopical level, the morphological relationships between terminals containing noradrenaline, substance P, cholecystokinin and leu-enkephalin, and medial olivocochlear neurones in the rat. A retrograde tracer was injected into the cochlea to label medial olivocochlear neurones and a double labelling immunocytochemical method was used to visualise the retrograde tracer as well as the neurotransmitters within each brain section. Light microscopical analysis revealed nerve endings containing substance P, cholecystokinin and leu-enkephalin in close apposition to the dendrites of medial olivocochlear neurones, and nerve endings containing dopamine-beta-hydroxylase, a marker for noradrenaline, in close contact with the somata as well as dendrites of medial olivocochlear neurones. Although the technique cannot prove the existence of functional synaptic contacts, the results are broadly consistent with electrophysiological data and suggest a direct input to medial olivocochlear neurones from substance P, cholecystokinin, leu-enkephalin and noradrenaline-containing neural pathways. Differences in the densities and spatial distribution of the various neuropharmacological inputs suggest differences in the relative strengths and possible roles of these diverse inputs to the olivocochlear system.