Serotonin projection patterns to the cochlear nucleus

Regulation of Noise-Induced Loss of Serotonin Transporters with Resveratrol in a Rat Model Using 4-[18F]-ADAM/Small-Animal Positron Emission Tomography

Serotonin (5-HT) plays a crucial role in modulating the afferent fiber discharge rate in the inferior colliculus, auditory cortex, and other nuclei of the ascending auditory system. Resveratrol, a natural polyphenol phytoalexin, can inhibit serotonin transporters (SERT) to increase synaptic 5-HT levels. In this study, we investigated the effects of resveratrol on noise-induced damage in the serotonergic system. Male Sprague-Dawley rats were anaesthetized and exposed to an 8-kHz tone at 116 dB for 3.5 h. Resveratrol (30 mg/kg, intraperitoneal injection [IP]) and citalopram (20 mg/kg, IP), a specific SERT inhibitor used as a positive control, were administered once a day for four consecutive days, with the first treatment occurring 2 days before noise exposure. Auditory brainstem response testing and positron emission tomography (PET) with N,N-dimethyl-2-(2-amino-4-[¹⁸F]fluorophenylthio)benzylamine (4-[¹⁸F]-ADAM, a specific radioligand for SERT) were used to evaluate functionality of the auditory system and integrity of the serotonergic system, respectively, before and after noise exposure. Finally, immunohistochemistry was performed 1 day after the last PET scan. Our results indicate that noise-induced serotonergic fiber loss occurred in multiple brain regions including the midbrain, thalamus, hypothalamus, striatum, auditory cortex, and frontal cortex. This noise-induced damage to the serotonergic system was ameliorated in response to treatment with resveratrol and citalopram. However, noise exposure increased the hearing threshold in the rats regardless of drug treatment status. We conclude that resveratrol has protective effects against noise-induced loss of SERT.

Serotonergic Modulation of Sensory Representation in a Central Multisensory Circuit Is Pathway Specific

Many studies have explored how neuromodulators affect synaptic function, yet little is known about how they modify computations at the microcircuit level. In the dorsal cochlear nucleus (DCN), a region that integrates auditory and multisensory inputs from two distinct pathways, serotonin (5-HT) enhances excitability of principal cells, predicting a generalized reduction in sensory thresholds. Surprisingly, we found that when looked at from the circuit level, 5-HT enhances signaling only from the multisensory input, while decreasing input from auditory fibers. This effect is only partially explained by an action on auditory nerve terminals. Rather, 5-HT biases processing for one input pathway by simultaneously enhancing excitability in the principal cell and in a pathway-specific feed-forward inhibitory interneuron. Thus, by acting on multiple targets, 5-HT orchestrates a fundamental shift in representation of convergent auditory and multisensory pathways, enhancing the potency of non-auditory signals in a classical auditory pathway.

Serotonin modulates response properties of neurons in the dorsal cochlear nucleus of the mouse

The neurochemical serotonin (5-hydroxytryptamine, 5-HT) is involved in a variety of behavioral functions including arousal, reward, and attention, and has a role in several complex disorders of the brain. In the auditory system, 5-HT fibers innervate a number of subcortical nuclei, yet the modulatory role of 5-HT in nearly all of these areas remains poorly understood. In this study, we examined spiking activity of neurons in the dorsal cochlear nucleus (DCN) following iontophoretic application of 5-HT. The DCN is an early site in the auditory pathway that receives dense 5-HT fiber input from the raphe nuclei and has been implicated in the generation of auditory disorders marked by neuronal hyperexcitability. Recordings from the DCN in awake mice demonstrated that iontophoretic application of 5-HT had heterogeneous effects on spiking rate, spike timing, and evoked spiking threshold. We found that 56% of neurons exhibited increases in spiking rate during 5-HT delivery, while 22% had decreases in rate and the remaining neurons had no change. These changes were similar for spontaneous and evoked spiking and were typically accompanied by changes in spike timing. Spiking increases were associated with lower first spike latencies and jitter, while decreases in spiking generally had opposing effects on spike timing. Cases in which 5-HT application resulted in increased spiking also exhibited lower thresholds compared to the control condition, while cases of decreased spiking had no threshold change. We also found that the 5-HT₂ receptor subtype likely has a role in mediating increased excitability. Our results demonstrate that 5-HT can modulate activity in the DCN of awake animals and that it primarily acts to increase neuronal excitability, in contrast to other auditory regions where it largely has a suppressive role. Modulation of DCN function by 5-HT has implications for auditory processing in both normal hearing and disordered states.

β-Arrestin-Dependent Dopaminergic Regulation of Calcium Channel Activity in the Axon Initial Segment

G-protein-coupled receptors (GPCRs) initiate a variety of signaling cascades, depending on effector coupling. β-arrestins, which were initially characterized by their ability to "arrest" GPCR signaling by uncoupling receptor and G protein, have recently emerged as important signaling effectors for GPCRs. β-arrestins engage signaling pathways that are distinct from those mediated by G protein. As such, arrestin-dependent signaling can play a unique role in regulating cell function, but whether neuromodulatory GPCRs utilize β-arrestin-dependent signaling to regulate neuronal excitability remains unclear. Here, we find that D3 dopamine receptors (D3R) regulate axon initial segment (AIS) excitability through β-arrestin-dependent signaling, modifying CaV3 voltage dependence to suppress high-frequency action potential generation. This non-canonical D3R signaling thereby gates AIS excitability via pathways distinct from classical GPCR signaling pathways.

Serotonergic regulation of excitability of principal cells of the dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) is one of the first stations within the central auditory pathway where the basic computations underlying sound localization are initiated and heightened activity in the DCN may underlie central tinnitus. The neurotransmitter serotonin (5-hydroxytryptamine; 5-HT), is associated with many distinct behavioral or cognitive states, and serotonergic fibers are concentrated in the DCN. However, it remains unclear what is the function of this dense input. Using a combination of in vitro electrophysiology and optogenetics in mouse brain slices, we found that 5-HT directly enhances the excitability of fusiform principal cells via activation of two distinct 5-HT receptor subfamilies, 5-HT2A/2CR (5-HT2A/2C receptor) and 5-HT7R (5-HT7 receptor). This excitatory effect results from an augmentation of hyperpolarization-activated cyclic nucleotide-gated channels (Ih or HCN channels). The serotonergic regulation of excitability is G-protein-dependent and involves cAMP and Src kinase signaling pathways. Moreover, optogenetic activation of serotonergic axon terminals increased excitability of fusiform cells. Our findings reveal that 5-HT exerts a potent influence on fusiform cells by altering their intrinsic properties, which may enhance the sensitivity of the DCN to sensory input.

Tinnitus: Maladaptive auditory-somatosensory plasticity

Tinnitus, the phantom perception of sound, is physiologically characterized by an increase in spontaneous neural activity in the central auditory system. However, as tinnitus is often associated with hearing impairment, it is unclear how a decrease of afferent drive can result in central hyperactivity. In this review, we first assess methods for tinnitus induction and objective measures of the tinnitus percept in animal models. From animal studies, we discuss evidence that tinnitus originates in the cochlear nucleus (CN), and hypothesize mechanisms whereby hyperactivity may develop in the CN after peripheral auditory nerve damage. We elaborate how this process is likely mediated by plasticity of auditory-somatosensory integration in the CN: the circuitry in normal circumstances maintains a balance of auditory and somatosensory activities, and loss of auditory inputs alters the balance of auditory somatosensory integration in a stimulus timing dependent manner, which propels the circuit towards hyperactivity. Understanding the mechanisms underlying tinnitus generation is essential for its prevention and treatment. This article is part of a Special Issue entitled <Tinnitus>.

Morphological characterization of bushy cells and their inputs in the laboratory mouse (Mus musculus) anteroventral cochlear nucleus

Spherical and globular bushy cells of the AVCN receive huge auditory nerve endings specialized for high fidelity neural transmission in response to acoustic events. Recent studies in mice and other rodent species suggest that the distinction between bushy cell subtypes is not always straightforward. We conducted a systematic investigation of mouse bushy cells along the rostral-caudal axis in an effort to understand the morphological variation that gives rise to reported response properties in mice. We combined quantitative light and electron microscopy to investigate variations in cell morphology, immunostaining, and the distribution of primary and non-primary synaptic inputs along the rostral-caudal axis. Overall, large regional differences in bushy cell characteristics were not found; however, rostral bushy cells received a different complement of axosomatic input compared to caudal bushy cells. The percentage of primary auditory nerve terminals was larger in caudal AVCN, whereas non-primary excitatory and inhibitory inputs were more common in rostral AVCN. Other ultrastructural characteristics of primary auditory nerve inputs were similar across the rostral and caudal AVCN. Cross sectional area, postsynaptic density length and curvature, and mitochondrial volume fraction were similar for axosomatic auditory nerve terminals, although rostral auditory nerve terminals contained a greater concentration of synaptic vesicles near the postsynaptic densities. These data demonstrate regional differences in synaptic organization of inputs to mouse bushy cells rather than the morphological characteristic of the cells themselves.

Context-dependent modulation of auditory processing by serotonin

Context-dependent plasticity in auditory processing is achieved in part by physiological mechanisms that link behavioral state to neural responses to sound. The neuromodulator serotonin has many characteristics suitable for such a role. Serotonergic neurons are extrinsic to the auditory system but send projections to most auditory regions. These projections release serotonin during particular behavioral contexts. Heightened levels of behavioral arousal and specific extrinsic events, including stressful or social events, increase serotonin availability in the auditory system. Although the release of serotonin is likely to be relatively diffuse, highly specific effects of serotonin on auditory neural circuitry are achieved through the localization of serotonergic projections, and through a large array of receptor types that are expressed by specific subsets of auditory neurons. Through this array, serotonin enacts plasticity in auditory processing in multiple ways. Serotonin changes the responses of auditory neurons to input through the alteration of intrinsic and synaptic properties, and alters both short- and long-term forms of plasticity. The infrastructure of the serotonergic system itself is also plastic, responding to age and cochlear trauma. These diverse findings support a view of serotonin as a widespread mechanism for behaviorally relevant plasticity in the regulation of auditory processing. This view also accommodates models of how the same regulatory mechanism can have pathological consequences for auditory processing.

The multiple functions of T stellate/multipolar/chopper cells in the ventral cochlear nucleus

Acoustic information is brought to the brain by auditory nerve fibers, all of which terminate in the cochlear nuclei, and is passed up the auditory pathway through the principal cells of the cochlear nuclei. A population of neurons variously known as T stellate, type I multipolar, planar multipolar, or chopper cells forms one of the major ascending auditory pathways through the brainstem. T Stellate cells are sharply tuned; as a population they encode the spectrum of sounds. In these neurons, phasic excitation from the auditory nerve is made more tonic by feedforward excitation, coactivation of inhibitory with excitatory inputs, relatively large excitatory currents through NMDA receptors, and relatively little synaptic depression. The mechanisms that make firing tonic also obscure the fine structure of sounds that is represented in the excitatory inputs from the auditory nerve and account for the characteristic chopping response patterns with which T stellate cells respond to tones. In contrast with other principal cells of the ventral cochlear nucleus (VCN), T stellate cells lack a low-voltage-activated potassium conductance and are therefore sensitive to small, steady, neuromodulating currents. The presence of cholinergic, serotonergic and noradrenergic receptors allows the excitability of these cells to be modulated by medial olivocochlear efferent neurons and by neuronal circuits associated with arousal. T Stellate cells deliver acoustic information to the ipsilateral dorsal cochlear nucleus (DCN), ventral nucleus of the trapezoid body (VNTB), periolivary regions around the lateral superior olivary nucleus (LSO), and to the contralateral ventral lemniscal nuclei (VNLL) and inferior colliculus (IC). It is likely that T stellate cells participate in feedback loops through both medial and lateral olivocochlear efferent neurons and they may be a source of ipsilateral excitation of the LSO.

A bushy cell network in the rat ventral cochlear nucleus

Geometry of the dendritic tree and synaptic organization of afferent inputs are essential factors in determining how synaptic input is integrated by neurons. This information remains elusive for one of the first brainstem neurons involved in processing of the primary auditory signal from the ear, the bushy cells (BCs) of the ventral cochlear nucleus (VCN). Here, we labeled the BC dendritic trees with retrograde tracing techniques to analyze their geometry and synaptic organization after immunofluorescence for excitatory and inhibitory synaptic markers, electron microscopy, morphometry, double tract-tracing methods, and 3D reconstructions. Our study revealed that BC dendrites provide space for a large number of compartmentalized excitatory and inhibitory synaptic interactions. The dendritic inputs on BCs are of cochlear and noncochlear origin, and their proportion and distribution are dependent on the branching pattern and orientation of the dendritic tree in the VCN. Three-dimensional reconstructions showed that BC dendrites branch and cluster with those of other BCs in the core of the VCN. Within the cluster, incoming synaptic inputs establish divergent multiple-contact synapses (dyads and triads) between BCs. Furthermore, neuron-neuron connections including puncta adherentia, sarcoplasmic junctions, and gap junctions are common between BCs, which suggests that these neurons are electrically coupled. Overall, our study demonstrates the existence of a BC network in the rat VCN. This network may establish the neuroanatomical basis for acoustic information processing by individual BCs as well as for enhanced synchronization of the output signal of the VCN.

Two distinct types of inhibition mediated by cartwheel cells in the dorsal cochlear nucleus

Individual neurons have been shown to exhibit target cell-specific synaptic function in several brain areas. The time course of the postsynaptic conductances (PSCs) strongly influences the dynamics of local neural networks. Cartwheel cells (CWCs) are the most numerous inhibitory interneurons in the dorsal cochlear nucleus (DCN). They are excited by parallel fiber synapses, which carry polysensory information, and in turn inhibit other CWCs and the main projection neurons of the DCN, pyramidal cells (PCs). CWCs have been implicated in "context-dependent" inhibition, producing either depolarizing (other CWCs) or hyperpolarizing (PCs) post synaptic potentials. In the present study, we used paired whole cell recordings to examine target-dependent inhibition from CWCs in neonatal rat DCN slices. We found that CWC inhibitory postsynaptic potentials (IPSPs) onto PCs are large (1.3 mV) and brief (half-width = 11.8 ms), whereas CWC IPSPs onto other CWCs are small (0.2 mV) and slow (half-width = 36.8 ms). Evoked IPSPs between CWCs exhibit paired-pulse facilitation, while CWC IPSPs onto PCs exhibit paired-pulse depression. Perforated-patch recordings showed that spontaneous IPSPs in CWCs are hyperpolarizing at rest with a mean estimated reversal potential of -67 mV. Spontaneous IPSCs were smaller and lasted longer in CWCs than in PCs, suggesting that the kinetics of the receptors are different in the two cell types. These results reveal that CWCs play a dual role in the DCN. The CWC-CWC network interactions are slow and sensitive to the average rate of CWC firing, whereas the CWC-PC network is fast and sensitive to transient changes in CWC firing.

Activation of serotonin 3 receptors changes in vivo auditory responses in the mouse inferior colliculus

Metabotropic serotonin receptors such as 5-HT1A and 5-HT1B receptors shape the level, selectivity, and timing of auditory responses in the inferior colliculus (IC). Less is known about the effects of ionotropic 5-HT3 receptors, which are cation channels that depolarize neurons. In the present study, the influence of the 5-HT3 receptor on auditory responses in vivo was explored by locally iontophoresing a 5-HT3 receptor agonist and antagonists onto single neurons recorded extracellularly in mice. Three main findings emerge from these experiments. First, activation of the 5-HT3 receptor can either facilitate or suppress auditory responses, but response suppressions are not consistent with 5-HT3 effects on presynaptic GABAergic neurons. Both response facilitations and suppressions are less pronounced in neurons with high precision in response latency, suggesting functional differences in the role of receptor activation for different classes of neuron. Finally, the effects of 5-HT3 activation vary across repetition rate within a subset of single neurons, suggesting that the influence of receptor activation sometimes varies with the level of activity. These findings contribute to the view of the 5-HT3 receptor as an important component of the serotonergic infrastructure in the IC, with effects that are complex and neuron-selective.

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.

Pathways involved in somatosensory electrical modulation of dorsal cochlear nucleus activity

Our recent study has shown that somatosensory electrical stimulation may be useful to modulate sound-induced hyperactivity in the dorsal cochlear nucleus (DCN), a neural correlate of certain forms of tinnitus. Somatosensory electrical stimulation induced both suppressive and excitatory effects on neural activity in the DCN of both control and tone-exposed animals. However, it is unclear what neural pathways underlie the somatosensory electrical stimulation-induced effects on DCN activity. To address this issue, we conducted c-fos immunocytochemistry using hamsters and mapped neural activation in both auditory and non-auditory structures following transcutaneous electrical stimulation of the basal part of the pinna. We also conducted tracing experiments to investigate the anatomical relations between the DCN and structures that showed a significant increase in the number of Fos-positive neurons as a result of electrical stimulation. Electrical stimulation of the pinna induced significant increases in the number of Fos-positive neurons in the DCN, spinal trigeminal nucleus (Sp5), dorsal raphe nucleus (DR) and locus coeruleus (LC). Results of tracing experiments indicate that the DCN received inputs from the Sp5, DR and LC. The above results suggest that modulation of DCN activity through somatosensory electrical stimulation may involve both direct pathways via the Sp5 and indirect pathways via the DR and LC. Therefore, relieving tinnitus through somatosensory electrical stimulation may require manipulations of both auditory and non-auditory functions.

Regional variations of cytochrome oxidase activity in the central auditory system of Relnrl-Orl (reeler) mutant mice

Despite preserved cell differentiation, the Reln(rl-Orl) phenotype comprises laminar abnormalities of cell position in auditory cortex and dorsal cochlear nucleus. The metabolic consequences of the cell ectopias were determined by estimating cytochrome oxidase (CO) activity, a marker of neuronal activity. CO activity increased in the granular cell layer of dorsal cochlear nucleus, trapezoid body nucleus, intermediate lateral lemniscus, central and external inferior colliculus, and pyramidal cell layer of primary auditory cortex. On the contrary, CO activity decreased in the superficial molecular layer of dorsal cochlear nucleus as well as in the medioventral periolivary nucleus. These metabolic variations are discussed in terms of their possible relation to morphologic anomalies observed in the mutant.

Serotonin 2B receptor: Upregulated with age and hearing loss in mouse auditory system

Serotonin (5-HT) is a monoamine neurotransmitter. Serotonin may modulate afferent fiber discharges in the cochlea, inferior colliculus (IC) and auditory cortex. Specific functions of serotonin are exerted upon its interaction with specific receptors; one of those receptors is the serotonin 2B receptor. The aim of this study was to investigate the differences in gene expression of serotonin 2B receptors with age in cochlea and IC, and the possible correlation between gene expression and functional hearing measurements in CBA/CaJ mice. Immunohistochemical examinations of protein expression of IC in mice of different age groups were also performed. Gene expression results showed that serotonin 2B receptor gene was upregulated with age in both cochlea and IC. A significant correlation between gene expression and functional hearing results was established. Immunohistochemical protein expression studies of IC showed more serotonin 2B receptor cells in old mice relative to young adult mice, particularly in the external nucleus. We conclude that serotonin 2B receptors may play a role in the pathogenesis of age-related hearing loss.

Selective anterograde tracing of the individual serotonergic and nonserotonergic components of the dorsal raphe nucleus projection to the vestibular nuclei

It is well known that the dorsal raphe nucleus (DRN) sends serotonergic and nonserotonergic projections to target regions in the brain stem and forebrain, including the vestibular nuclei. Although retrograde tracing studies have reported consistently that there are differences in the relative innervation of different target regions by serotonergic and nonserotonergic DRN neurons, the relative termination patterns of these two projections have not been compared using anterograde tracing methods. The object of the present investigation was to trace anterogradely the individual serotonergic and nonserotonergic components of the projection from DRN to the vestibular nuclei in rats. To trace nonserotonergic DRN projections, animals were pretreated with the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), and then, after 7 days, the anterograde tracer biotinylated dextran amine (BDA) was iontophoretically injected into the DRN. In animals treated with 5,7-DHT, nonserotonergic BDA-labeled fibers were found to descend exclusively within the ventricular plexus and to terminate predominantly within the periventricular aspect of the vestibular nuclei. Serotonergic DRN projections were traced by injecting 5,7-DHT directly into DRN, and amino-cupric-silver staining was used to visualize the resulting pattern of terminal degeneration. Eighteen hours after microinjection of 5,7-DHT into the DRN, fine-caliber degenerating serotonergic terminals were found within the region of the medial vestibular nucleus (MVN) that borders the fourth ventricle, and a mixture of fine- and heavier-caliber degenerating serotonergic terminals was located further laterally within the vestibular nuclear complex. These findings indicate that fine-caliber projections from serotonergic and nonserotonergic DRN neurons primarily innervate the periventricular regions of MVN, whereas heavier-caliber projections from serotonergic DRN neurons innervate terminal fields located in more lateral regions of the vestibular nuclei. Thus, serotonergic and nonserotonergic DRN axons target distinct but partially overlapping terminal fields within the vestibular nuclear complex, raising the possibility that these two DRN projection systems are organized in a manner that permits regionally-specialized regulation of processing within the vestibular nuclei.

Anterograde tracing of projections from the dorsal raphe nucleus to the vestibular nuclei

This study used the anterograde transport of biotinylated dextran amine (BDA) to identify the course and terminal distribution of projections from the dorsal raphe nucleus (DRN) to the vestibular nuclei in rats. After iontophoretic injection of BDA into the medial and lateral regions of DRN, anterogradely labeled fibers descend within the medial longitudinal fasciculus and the ventricular fiber plexus to terminate within two discrete regions of the vestibular nuclear complex. One terminal field was located primarily ipsilateral to the injection site and involved rostrodorsal aspects of the vestibular nuclei, including superior vestibular nucleus and rostral portions of the medial vestibular nucleus (MVN) and lateral vestibular nucleus (LVN). The other terminal field involved caudoventral aspects of both ipsilateral and contralateral MVN and LVN and was less heavily innervated. These findings confirm that the vestibular nuclei are targeted by a regionally-selective projection from the DRN. The segregation of DRN terminals into anatomically distinct fields indicates that the DRN-vestibular nucleus projections are organized to selectively modulate processing within specific functional domains of the vestibular nuclear complex. In particular, these terminal fields may be organized to modulate vestibular regions involved in eye movement-related velocity storage, coordination of vestibular and affective responses, and the bilateral coordination of horizontal eye movement reflexes.

The dorsal cochlear nucleus as a participant in the auditory, attentional and emotional components of tinnitus

The dorsal cochlear nucleus (DCN) has been modeled in numerous studies as a possible source of tinnitus-generating signals. This hypothesis was originally developed on the basis of evidence that the DCN becomes hyperactive following exposure to intense noise. Since these early observations, evidence that the DCN is an important contributor to tinnitus has grown considerably. In this paper, the available evidence to date will be summarized. In addition, the DCN hypothesis of tinnitus can now be expanded to include possible involvement in other, non-auditory components of tinnitus. It will be shown by way of literature review that the DCN has direct connections with non-auditory brainstem structures, such as the locus coeruleus, reticular formation and raphe nuclei, that are implicated in the control of attention and emotional responses. The hypothesis will be presented that attentional and emotional disorders, such as anxiety and depression, which are commonly associated with tinnitus, may result from an interplay between these non-auditory brainstem structures and the DCN. Implicit in this hypothesis is that attempts to develop effective anti-tinnitus therapies are likely to benefit from a greater understanding of how the levels of activity in the DCN are influenced by different states of activation of these non-auditory brainstem structures and vice versa.

Origin of hyperactivity in the hamster dorsal cochlear nucleus following intense sound exposure

This study sought to determine whether maintenance of noise-induced dorsal cochlear nucleus (DCN) hyperactivity depends on descending projections. Twenty-two hamsters were exposed under anesthesia to a 10-kHz tone at 125-130 dB SPL for 4 hr, and another 21 unexposed animals served as controls. After approximately 4-6 weeks of recovery, surgical transections were made to isolate the DCN from its adjacent brainstem structures. Spontaneous multiunit activity was recorded from the DCN surface 30-40 min after the surgical manipulations. Spontaneous rates were derived from the recording sites of the DCN along its mediolateral axis for each animal, yielding average spontaneous rates for both control and exposed groups. Histology was performed to assess the degree of sectioning of descending fiber tract connections to the cochlear nucleus, via the acoustic striae route, subpeduncular route, trapezoid body route, and ventral route of the olivocochlear bundle connection. The results showed that complete or nearly complete transections of descending inputs did not affect significantly the magnitude of DCN hyperactivity. However, this manipulation triggered a lateral shift of the peak mean rate, suggesting that descending inputs may play a modulatory role on the profile of DCN hyperactivity. Indeed, exposed animals with transection of only the strial route of entry manifested a level of hyperactivity much higher than that observed in exposed animals in which no sections were performed. This enhancement of DCN hyperactivity was weakened by damage to the subpeduncular or trapezoid routes of input, suggesting that the dorsally located inputs may have an inhibitory effect on DCN hyperactivity.

Postnatal Expression of the Serotonin Transporter in Auditory Brainstem Neurons

To investigate the putative role of serotonin (5-HT) in auditory brainstem development, the expression of the 5-HT transporter (5-HTT) was evaluated in the normal mouse brainstem at 6 different postnatal ages. The brains of C3H/HeJ mice at birth (P0) and P1, P8-P9, P13, P21-P22, P35-P36 and P48-P50 were collected and processed immunohistochemically with an antibody raised against the 5-HTT. 5-HTT immunoreactivity (5-HTT-IR) was first observed in P8 mice and was localized to cell bodies in the ventral cochlear nucleus (VCN) and principal nuclei of the superior olivary complex, including the medial nucleus of the trapezoid body. Labeled neurons were found in similar regions in older mice except at P48-50, where labeled neurons were observed in the VCN only. 5-HTT-IR was especially prominent in VCN neurons at P21 and was observed in all of the brains examined at this age. These results indicate that auditory brainstem neurons of the normal inbred mouse express the 5-HTT postnatally. The presence of 5-HTT-IR in neurons located in the VCN indicates a regional expression of the 5-HTT that is related to the ascending auditory pathway. The timing of 5-HTT expression indicates that 5-HT may modulate developmental processes that rely on cochlear input.

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.

Serotonin in the inferior colliculus

It has been recognized for some time that serotonin fibers originating in raphe nuclei are present in the inferior colliculi of all mammalian species studied. More recently, serotonin has been found to modulate the responses of single inferior colliculus neurons to many types of auditory stimuli, ranging from simple tone bursts to complex species-specific vocalizations. The effects of serotonin are often quite strong, and for some neurons are also highly specific. A dramatic illustration of this is that serotonin can change the selectivity of some neurons for sounds, including species-specific vocalizations. These results are discussed in light of several theories on the function of serotonin in the IC, and of outstanding issues that remain to be addressed.

5-HT(1A) receptor subtype mRNA expression in cochlear nucleus

Previous studies indicate expression of various serotonin receptor subtypes, including the 5-HT(1A) receptor subtype, in rodent cochlear nucleus. Our long-term goal is to identify the types of cochlear nucleus neurons, which are well described in cat, that express 5-HT receptors. In the current study, the reverse transcriptase/polymerase chain reaction and the in situ hybridization method were used to detect the mRNA encoding a portion of the 5-HT(1A) receptor subtype in the cochlear nucleus of the cat.