Analysis of projections from the cochlear nucleus to the lateral paragigantocellular reticular nucleus in the rat

Characterization of three cholinergic inputs to the cochlear nucleus

Acetylcholine modulates responses throughout the auditory system, including at the earliest brain level, the cochlear nucleus (CN). Previous studies have shown multiple sources of cholinergic input to the CN but information about their relative contributions and the distribution of inputs from each source is lacking. Here, we used staining for cholinergic axons and boutons, retrograde tract tracing, and acetylcholine-selective anterograde tracing to characterize three sources of acetylcholine input to the CN in mice. Staining for cholinergic axons showed heavy cholinergic inputs to granule cell areas and the dorsal CN with lighter input to the ventral CN. Retrograde tract tracing revealed that cholinergic cells from the superior olivary complex, pontomesencephalic tegmentum, and lateral paragigantocellular nucleus send projections to the CN. When we selectively labeled cholinergic axons from each source to the CN, we found surprising similarities in their terminal distributions, with patterns that were overlapping rather than complementary. Each source heavily targeted granule cell areas and the dorsal CN (especially the deep dorsal CN) and sent light input into the ventral CN. Our results demonstrate convergence of cholinergic inputs from multiple sources in most regions of the CN and raise the possibility of convergence onto single CN cells. Linking sources of acetylcholine and their patterns of activity to modulation of specific cell types in the CN will be an important next step in understanding cholinergic modulation of early auditory processing.

Multiple Sources of Cholinergic Input to the Superior Olivary Complex

The superior olivary complex (SOC) is a major computation center in the brainstem auditory system. Despite previous reports of high expression levels of cholinergic receptors in the SOC, few studies have addressed the functional role of acetylcholine in the region. The source of the cholinergic innervation is unknown for all but one of the nuclei of the SOC, limiting our understanding of cholinergic modulation. The medial nucleus of the trapezoid body, a key inhibitory link in monaural and binaural circuits, receives cholinergic input from other SOC nuclei and also from the pontomesencephalic tegmentum. Here, we investigate whether these same regions are sources of cholinergic input to other SOC nuclei. We also investigate whether individual cholinergic cells can send collateral projections bilaterally (i.e., into both SOCs), as has been shown at other levels of the subcortical auditory system. We injected retrograde tract tracers into the SOC in gerbils, then identified retrogradely-labeled cells that were also immunolabeled for choline acetyltransferase, a marker for cholinergic cells. We found that both the SOC and the pontomesencephalic tegmentum (PMT) send cholinergic projections into the SOC, and these projections appear to innervate all major SOC nuclei. We also observed a small cholinergic projection into the SOC from the lateral paragigantocellular nucleus of the reticular formation. These various sources likely serve different functions; e.g., the PMT has been associated with things such as arousal and sensory gating whereas the SOC may provide feedback more closely tuned to specific auditory stimuli. Further, individual cholinergic neurons in each of these regions can send branching projections into both SOCs. Such projections present an opportunity for cholinergic modulation to be coordinated across the auditory brainstem.

Cholinergic boutons are closely associated with excitatory cells and four subtypes of inhibitory cells in the inferior colliculus

Acetylcholine (ACh) is a neuromodulator that has been implicated in multiple roles across the brain, including the central auditory system, where it sets neuronal excitability and gain and affects plasticity. In the cerebral cortex, subtypes of GABAergic interneurons are modulated by ACh in a subtype-specific manner. Subtypes of GABAergic neurons have also begun to be described in the inferior colliculus (IC), a midbrain hub of the auditory system. Here, we used male and female mice (Mus musculus) that express fluorescent protein in cholinergic cells, axons, and boutons to look at the association between ACh and four subtypes of GABAergic IC cells that differ in their associations with extracellular markers, their soma sizes, and their distribution within the IC. We found that most IC cells, including excitatory and inhibitory cells, have cholinergic boutons closely associated with their somas and proximal dendrites. We also found that similar proportions of each of four subtypes of GABAergic cells are closely associated with cholinergic boutons. Whether the different types of GABAergic cells in the IC are differentially regulated remains unclear, as the response of cells to ACh is dependent on which types of ACh receptors are present. Additionally, this study confirms the presence of these four subtypes of GABAergic cells in the mouse IC, as they had previously been identified only in guinea pigs. These results suggest that cholinergic projections to the IC modulate auditory processing via direct effects on a multitude of inhibitory circuits.

Tinnitus and anxiety disorders: A review

BACKGROUND

The most common form of tinnitus is a subjective, auditory, and distressing phantom phenomenon. Comorbidity with depression is high but other important psychiatric disorders such as anxiety disorders have received less attention. The current paper reviews the literature on the associations between tinnitus and anxiety disorders and the underlying pathophysiology, and discusses the clinical implications.

METHODOLOGY

PubMed and Web of Science were searched for all articles published up until October 2014 using combinations of the following search strings "Tinnitus", "Anxiety disorder", "Panic Disorder", "Generalized Anxiety Disorder", "Post traumatic stress disorder", "PTSD" "Social Phobia", "Phobia Disorder", "Obsessive Compulsive Disorder", "Agoraphobia".

RESULTS

A total of 117 relevant papers were included. A 45% lifetime prevalence of anxiety disorders is reported in tinnitus populations, while an important overlap in associated (sub)cortical brain areas and cortico-subcortical networks involved in attention, distress, and memory functions is suggested. A disturbed hypothalamic-pituitary-adrenal axis function can be found in tinnitus and in anxiety disorders but, in comorbidity, the direction of the dysfunction is unclear.

CONCLUSION

Comorbidity is high and screening for and treatment of anxiety disorders is recommended in moderate to severe tinnitus, as, given the overlap in the structural and functional brain circuitries involved, theoretically, their management could improve (subjective) levels of tinnitus although further empirical research on this topic is required.

A mu-delta opioid receptor brain atlas reveals neuronal co-occurrence in subcortical networks

Opioid receptors are G protein-coupled receptors (GPCRs) that modulate brain function at all levels of neural integration, including autonomic, sensory, emotional and cognitive processing. Mu (MOR) and delta (DOR) opioid receptors functionally interact in vivo, but whether interactions occur at circuitry, cellular or molecular levels remains unsolved. To challenge the hypothesis of MOR/DOR heteromerization in the brain, we generated redMOR/greenDOR double knock-in mice and report dual receptor mapping throughout the nervous system. Data are organized as an interactive database offering an opioid receptor atlas with concomitant MOR/DOR visualization at subcellular resolution, accessible online. We also provide co-immunoprecipitation-based evidence for receptor heteromerization in these mice. In the forebrain, MOR and DOR are mainly detected in separate neurons, suggesting system-level interactions in high-order processing. In contrast, neuronal co-localization is detected in subcortical networks essential for survival involved in eating and sexual behaviors or perception and response to aversive stimuli. In addition, potential MOR/DOR intracellular interactions within the nociceptive pathway offer novel therapeutic perspectives.

[Is the middle ear the first filter of frequency selectivity?]

INTRODUCTION AND OBJECTIVES

The cochlea has traditionally been considered as the first frequency selection filter in the auditory pathway due to the contraction of its external ciliated cells. Yet, much evidence has emerged from work carried out during experiments with animals, some of which is anatomical (connections between the auditory pathway and motor nuclei of the middle ear muscles) and other physiological, which indicates that the middle ear might be the first filter through which specific sounds from noisy environments may initially be isolated.

METHODS

In cooperation with the Department of Mechanical Engineering of the Technical School of Industrial Engineering at the University of Valladolid (UVa) we have developed and refined a new admittance meter capable of evaluating changes in impedance that occur in the human middle ear depending on frequency. Using this device we have measured variation in impedance in 7 otologically healthy volunteers submitted to a varied range of sound environments.

RESULTS

We have found that hearing impedance is not constant but rather that the attention offered by the examined subjects when following a conversation in a noisy environment leads to variations in hearing impedance at high frequencies.

CONCLUSIONS

In the light of these findings we feel that the middle ear does not play a merely passive role in hearing but rather that the contraction of the endotympanic muscles makes possible variations in impedance such that the resonance frequency of the ear shifts towards higher frequencies, thus enhancing sound discrimination in noisy environments.

Nucleus paragigantocellularis afferents in male and female rats: organization, gonadal steroid receptor expression, and activation during sexual behavior

The supraspinal regulation of genital reflexes is poorly understood. The brainstem nucleus paragigantocellularis (nPGi) of rats is a well-established source of tonic inhibition of genital reflexes. However, the organization, gonadal steroid receptor expression, and activity of nPGi afferents during sex have not been fully characterized in male and female rats. To delineate the anatomical and physiological organization of nPGi afferents, the retrograde tracer Fluoro-Gold (FG) was injected into the nPGi of sexually experienced male and female rats. Animals engaged in sexual behavior 1 hour before sacrifice. Cells containing FG, estrogen receptor-alpha (ER(alpha)), androgen receptor (AR), and the immediate-early gene product Fos were identified immunocytochemically. Retrograde labeling from the nPGi was prominent in the bed nucleus of the stria terminalis, paraventricular nucleus (PVN), posterior hypothalamus, precommissural nucleus, deep mesencephalic nucleus, and periaqueductal gray (PAG) of both sexes. Sex differences were observed in the caudal medial preoptic area (MPO), with significantly more FG+ cells observed in males, and in the PAG and inferior colliculus, where significantly more FG+ cells were observed in females. The majority of regions that contained FG+ cells also contained ER(alpha) or AR, indicating sensitivity to gonadal steroids. The proportions of FG+ cells that co-localized with sex-induced Fos was high in the PVN of both sexes and high in the MPO of males but low in the PAG of both sexes despite the large number of PAG-nPGi output neurons and Fos+ cells in both sexes. The characterization of these afferents will lead to a further understanding of the neural regulation of genital reflexes.

The whisking rhythm generator: a novel mammalian network for the generation of movement

Using the rat vibrissa system, we provide evidence for a novel mechanism for the generation of movement. Like other central pattern generators (CPGs) that underlie many movements, the rhythm generator for whisking can operate without cortical inputs or sensory feedback. However, unlike conventional mammalian CPGs, vibrissa motoneurons (vMNs) actively participate in the rhythmogenesis by converting tonic serotonergic inputs into the patterned motor output responsible for movement of the vibrissae. We find that, in vitro, a serotonin receptor agonist, alpha-Me-5HT, facilitates a persistent inward current (PIC) and evokes rhythmic firing in vMNs. Within each motoneuron, increasing the concentration of alpha-Me-5HT significantly increases the both the magnitude of the PIC and the motoneuron's firing rate. Riluzole, which selectively suppresses the Na(+) component of PICs at low concentrations, causes a reduction in both of these phenomena. The magnitude of this reduction is directly correlated with the concentration of riluzole. The joint effects of riluzole on PIC magnitude and firing rate in vMNs suggest that the two are causally related. In vivo we find that the tonic activity of putative serotonergic premotoneurons is positively correlated with the frequency of whisking evoked by cortical stimulation. Taken together, these results support the hypothesized novel mammalian mechanism for movement generation in the vibrissa motor system where vMNs actively participate in the rhythmogenesis in response to tonic drive from serotonergic premotoneurons.

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.

Mice with the deleted neurofilament of low-molecular-weight (Nefl) gene: 1. Effects on regional brain metabolism

Neuronal intermediate filaments consist of the NFL subunit linked with NFM and NFH, and their alterations have been proposed as a pathogenesic cause in motor neuron diseases. Depletion of the Nefl gene in mice mimicks the reduced NFL mRNA levels seen in amyotrophic lateral sclerosis and causes perikaryal accumulation of neurofilament proteins and axonal hypotrophy in motoneurons. NFL -/- mice were evaluated for regional brain metabolism by means of quantitative histochemical estimation of cytochrome oxidase (COx) activity. The NFL null mice displayed enzymatic activity alterations in numerous hindbrain regions, mainly the cerebellum, connected regions of the brainstem (red nucleus, vestibular nuclei, and reticular formation), and cranial nerve nuclei. All of the affected regions presented elevated COx activity, except for the Purkinje cells of the cerebellum and the magnocellular red nucleus, where enzymatic activity was lower. NFL-disrupted mice displayed functional alterations in brainstem sensorimotor regions affected in amyotrophic lateral sclerosis.

Regional brainstem expression of Fos associated with sexual behavior in male rats

This study utilized Fos expression to map the distribution of activated cells in brainstem areas following masculine sexual behavior. Males displaying both appetitive and consumatory sexual behaviors (Cop) were compared to animals prevented from copulation (NC) and to socially isolated (SI) animals. Following copulation, Fos was preferentially augmented in the caudal ventral medulla (CVM), a region mediating descending inhibition of penile reflexes, and which may be regulated by a forebrain circuit that includes the medial preoptic area (MPOA). Copulation-induced Fos was observed in the medial divisions of both the dorsal cochlear nucleus (DC) and trapezoid bodies (Tz), areas which are part of a circuit processing auditory information. In addition, the medullary linear nucleus (Li) displayed comparable amounts of Fos in Cop and NC as compared to the SI animals. Other regions of the pontomedullary reticular system, which may mediate sleep and arousal, did not exhibit Fos expression associated with consumatory sexual behavior. We suggest that Fos is associated with the inhibition of sexual behavior following ejaculation in the CVM, and that auditory information arising from the DC and Tz is combined with copulation-related sensory information in the subparafasicular nucleus and projected to the hypothalamus. In addition, equal amounts of Fos expression observed in the Li in both the Cop and NC animals suggests that this region is involved in sexual arousal. Overall, the data suggest that processing by brainstem nuclei directly contributes to the regulation of mating behavior in male rats.

Parallel auditory pathways: projection patterns of the different neuronal populations in the dorsal and ventral cochlear nuclei

The cochlear nuclear complex gives rise to widespread projections to nuclei throughout the brainstem. The projections arise from separate, well-defined populations of cells. None of the cell populations in the cochlear nucleus projects to all brainstem targets, and none of the targets receives inputs from all cell types. The projections of nine distinguishable cell types in the cochlear nucleus-seven in the ventral cochlear nucleus and two in the dorsal cochlear nucleus-are described in this review. Globular bushy cells and two types of spherical bushy cells project to nuclei in the superior olivary complex that play roles in sound localization based on binaural cues. Octopus cells convey precisely timed information to nuclei in the superior olivary complex and lateral lemniscus that, in turn, send inhibitory input to the inferior colliculus. Cochlear root neurons send widespread projections to areas of the reticular formation involved in startle reflexes and autonomic functions. Type I multipolar cells may encode complex features of natural stimuli and send excitatory projections directly to the inferior colliculus. Type II multipolar cells send inhibitory projections to the contralateral cochlear nuclei. Fusiform cells in the dorsal cochlear nucleus appear to be important for the localization of sounds based on spectral cues and send direct excitatory projections to the inferior colliculus. Giant cells in the dorsal cochlear nucleus also project directly to the inferior colliculus; some of them may convey inhibitory inputs to the contralateral cochlear nucleus as well.

Descending spinal projections from the rostral gigantocellular reticular nuclei complex

Electrophysiological and physiological studies have suggested that the ventral medullary gigantocellular reticular nuclei (composed of the gigantocellular ventralis and pars alpha nuclei as well as the adjacent lateral paragigantocellular nucleus; abbreviated Gi-LPGi complex) provide descending control of pelvic floor organs (Mackel [1979] J. Physiol. (Lond.) 294:105-122; Hubscher and Johnson [1996] J. Neurophysiol. 76:2474-2482; Hubscher and Johnson [1999] J. Neurophysiol. 82:1381-1389; Johnson and Hubscher [1998] Neuroreport 9:341-345). Specifically, this complex of paramedian reticular nuclei has been implicated in the inhibition of sexual reflexes. In the present study, an anterograde fluorescent tracer was used to investigate direct descending projections from the Gi-LPGi complex to retrogradely labeled pudendal motoneurons (MN) in the male rat. Our results demonstrated that, although a high density of arborizations from Gi-LPGi fibers appears to be in close apposition to pudendal MNs, this relationship also applies to other MNs throughout the entire spinal cord. The Gi-LPGi also projects to spinal autonomic regions, i.e., both the intermediolateral cell column and the sacral parasympathetic nucleus, as well as to regions of the intermediate gray, which contain interneurons involved in the organization of pelvic floor reflexes. Lastly, throughout the length of the spinal cord, numerous neurons located primarily in laminae VII-X, were retrogradely labeled with Fluoro-Ruby after injections into the Gi-LPGi. The diffuse descending projections and arborizations of this pathway throughout the spinal cord suggest that this brainstem area is involved in the direct, descending control of a variety of spinal activities. These results are in contrast with our observations of the discrete projections of the caudal nucleus raphe obscurus, which target the autonomic and somatic MNs involved specifically in sexual and eliminative functions (Hermann et al. [1998] J. Comp. Neurol. 397:458-474).

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.

Serotoninergic innervation of stapedial and tensor tympani motoneurons

Retrograde tracing and neurotransmitter immunohistochemistry were combined to determine whether serotonin neurons innervated stapedial and tensor tympani motoneurons. With high-power light microscopy, putative axo-somatic and axo-dendritic contacts were observed between serotonin-positive endings and both stapedial and tensor tympani motoneurons, indicating that serotonin neurons terminate on brainstem motoneurons innervating the middle-ear muscles. With this connection, the serotonin system may directly modulate middle-ear muscle activity.

Heterogeneous projections of the cat posteroventral cochlear nucleus

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