Aminergic projections to cochlear nucleus via descending auditory pathways

A parahippocampal-sensory Bayesian vicious circle generates pain or tinnitus: a source-localized EEG study

Pain and tinnitus share common pathophysiological mechanisms, clinical features, and treatment approaches. A source-localized resting-state EEG study was conducted in 150 participants: 50 healthy controls, 50 pain, and 50 tinnitus patients. Resting-state activity as well as functional and effective connectivity was computed in source space. Pain and tinnitus were characterized by increased theta activity in the pregenual anterior cingulate cortex, extending to the lateral prefrontal cortex and medial anterior temporal lobe. Gamma-band activity was increased in both auditory and somatosensory cortex, irrespective of the pathology, and extended to the dorsal anterior cingulate cortex and parahippocampus. Functional and effective connectivity were largely similar in pain and tinnitus, except for a parahippocampal-sensory loop that distinguished pain from tinnitus. In tinnitus, the effective connectivity between parahippocampus and auditory cortex is bidirectional, whereas the effective connectivity between parahippocampus and somatosensory cortex is unidirectional. In pain, the parahippocampal-somatosensory cortex is bidirectional, but parahippocampal auditory cortex unidirectional. These modality-specific loops exhibited theta-gamma nesting. Applying a Bayesian brain model of brain functioning, these findings suggest that the phenomenological difference between auditory and somatosensory phantom percepts result from a vicious circle of belief updating in the context of missing sensory information. This finding may further our understanding of multisensory integration and speaks to a universal treatment for pain and tinnitus-by selectively disrupting parahippocampal-somatosensory and parahippocampal-auditory theta-gamma activity and connectivity.

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.

Dopamine in Auditory Nuclei and Lemniscal Projections is Poised to Influence Acoustic Integration in the Inferior Colliculus

Dopamine (DA) modulates the activity of nuclei within the ascending and descending auditory pathway. Previous studies have identified neurons and fibers in the inferior colliculus (IC) which are positively labeled for tyrosine hydroxylase (TH), a key enzyme in the synthesis of dopamine. However, the origins of the tyrosine hydroxylase positive projections to the inferior colliculus have not been fully explored. The lateral lemniscus (LL) provides a robust inhibitory projection to the inferior colliculus and plays a role in the temporal processing of sound. In the present study, immunoreactivity for tyrosine hydroxylase was examined in animals with and without 6-hydroxydopamine (6-OHDA) lesions. Lesioning, with 6-OHDA placed in the inferior colliculus, led to a significant reduction in tyrosine hydroxylase immuno-positive labeling in the lateral lemniscus and inferior colliculus. Immunolabeling for dopamine beta-hydroxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT), enzymes responsible for the synthesis of norepinephrine (NE) and epinephrine (E), respectively, were evaluated. Very little immunoreactivity for DBH and no immunoreactivity for PNMT was found within the cell bodies of the dorsal, intermediate, or ventral nucleus of the lateral lemniscus. The results indicate that catecholaminergic neurons of the lateral lemniscus are likely dopaminergic and not noradrenergic or adrenergic. Next, high-pressure liquid chromatography (HPLC) analysis was used to confirm that dopamine is present in the inferior colliculus and nuclei that send projections to the inferior colliculus, including the cochlear nucleus (CN), superior olivary complex (SOC), lateral lemniscus, and auditory cortex (AC). Finally, fluorogold, a retrograde tracer, was injected into the inferior colliculus of adult rats. Each subdivision of the lateral lemniscus contained fluorogold within the somata, with the dorsal nucleus of the lateral lemniscus showing the most robust projections to the inferior colliculus. Fluorogold-tyrosine hydroxylase colocalization within the lateral lemniscus was assessed. The dorsal and intermediate nuclei neurons exhibiting similar degrees of colocalization, while neurons of the ventral nucleus had significantly fewer colocalized fluorogold-tyrosine hydroxylase labeled neurons. These results suggest that several auditory nuclei that project to the inferior colliculus contain dopamine, dopaminergic neurons in the lateral lemniscus project to the inferior colliculus and that dopaminergic neurotransmission is poised to play a pivotal role in the function of the inferior colliculus.

Cholinergic top-down influences on the auditory brainstem

Descending connections are present in many sensory systems and support adaptive information processing. This allows the sensory brain to code a wider range of inputs. A well characterized descending system is the olivo-cochlear cholinergic innervation of the inner ear, which mediates a reduction of the sensitivity of the inner ear upon perception of intense sounds. Because this inhibits the response to background noise, the olivo-cochlear system supports detection of transient sound events. Olivo-cochlear neurons also innervate the cochlear nucleus through axon collaterals. Here, acetylcholine increases the excitability of central neurons without reducing their temporal precision. Thus their target neurons in the superior olivary complex can more effectively process binaural temporal cues. We argue that the central effect of the olivo-cochlear system augments the peripheral effect. In addition, olivo-cochlear cholinergic neurons are under top-down control of cortical inputs, providing further adaptability of information processing on the level of the auditory brainstem.

Cholinergic cells of the pontomesencephalic tegmentum: connections with auditory structures from cochlear nucleus to cortex

Acetylcholine (ACh) is a neuromodulator that is likely to play a role in plasticity as well as other phenomena at many sites in the auditory system. The auditory cortex receives cholinergic innervation from the basal forebrain, whereas the cochlea receives cholinergic innervation from the superior olivary complex. Much of the remainder of the auditory pathways receives innervation from the pedunculopontine and laterodorsal tegmental nuclei, two nuclei referred to collectively as the pontomesencephalic tegmentum (PMT). The PMT provides the major source of ACh to the auditory thalamus and the midbrain, and is a substantial source (in addition to the superior olivary complex) of ACh in the cochlear nucleus. Individual cholinergic cells in the PMT often have axon branches that innervate multiple auditory nuclei, including nuclei on both sides of the brain as well as nuclei at multiple levels of the auditory system. The auditory cortex has direct axonal projections to the PMT cells, including cholinergic cells that project to the inferior colliculus or cochlear nucleus. The divergent projections of PMT cholinergic cells suggest widespread effects on the auditory pathways. These effects are likely to include plasticity as well as novelty detection, sensory gating, reward behavior, arousal and attention. Descending projections from the forebrain, including the auditory cortex, are likely to provide a high level of cognitive input to these cholinergic effects. Dysfunction associated with the cholinergic system may play a role in disorders such as tinnitus and schizophrenia.

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.

Distribution and phenotypes of unipolar brush cells in relation to the granule cell system of the rat cochlear nucleus

In most mammals the cochlear nuclear complex (CN) contains a distributed system of granule cells (GCS), whose parallel fiber axons innervate the dorsal cochlear nucleus (DCN). Like their counterpart in cerebellum, CN granules are innervated by mossy fibers of various origins. The GCS is complemented by unipolar brush (UBCs) and Golgi cells, and by stellate and cartwheel cells of the DCN. This cerebellum-like microcircuit modulates the activity of the DCN's main projection neurons, the pyramidal, giant and tuberculoventral neurons, and is thought to improve auditory performance by integrating acoustic and proprioceptive information. In this paper, we focus on the rat UBCs, a chemically heterogeneous neuronal population, using antibodies to calretinin, metabotropic glutamate receptor 1alpha (mGluR1alpha), epidermal growth factor substrate 8 (Eps8) and the transcription factor T-box gene Tbr2 (Tbr2). Eps8 and Tbr2 labeled most of the CN's UBCs, if not the entire population, while calretinin and mGluR1alpha distinguished two largely separate subsets with overlapping distributions. By double labeling with antibodies to Tbr2 and the alpha6 GABA receptor A (GABAA) subunit, we found that UBCs populate all regions of the GCS and occur at remarkably high densities in the DCN and subpeduncular corner, but rarely in the lamina. Although GCS subregions likely share the same microcircuitry, their dissimilar UBC densities suggest they may be functionally distinct. UBCs and granules are also present in regions previously not included in the GCS, namely the rostrodorsal magnocellular portions of ventral cochlear nucleus, vestibular nerve root, trapezoid body, spinal tract and sensory and principal nuclei of the trigeminal nerve, and cerebellar peduncles. The UBC's dendritic brush receives AMPA- and NMDA-mediated input from an individual mossy fiber, favoring singularity of input, and its axon most likely forms several mossy fiber-like endings that target numerous granule cells and other UBCs, as in the cerebellum. The UBCs therefore, may amplify afferent signals temporally and spatially, synchronizing pools of target neurons.

Immunohistochemical localization of adrenergic receptors in the rat organ of corti and spiral ganglion

Alpha(1)-, beta(1)-, and beta(2)-adrenergic receptors (ARs), which mediate responses to adrenergic input, have been immunohistochemically identified within the organ of Corti and spiral ganglion with polyclonal antibodies of established specificity. Alpha(1)-AR was immunolocalized to sites overlapping supranuclear regions of inner hair cells as well as to nerve fibers approaching the base of inner hair cells, most evident in the basal cochlear turn. A similar preponderance across cochlear turns for alpha(1)-AR in afferent cell bodies in the spiral ganglion pointed to type I afferent dendrites as a possible neural source of alpha(1)-AR beneath the inner hair cell. Foci of immunoreactivity for alpha(1)-AR, putatively neural, were found overlapping supranuclear and basal sites of outer hair cells for all turns. Beta(1)- and beta(2)-ARs were immunolocalized to sites overlapping apical and basal poles of the inner and outer hair cells, putatively neural in part, with immunoreactive nerve fibers observed passing through the habenula perforata. Beta(1)- and beta(2)-ARs were also detected in the cell bodies of Deiters' and Hensen's cells. Within the spiral ganglion, beta(1)- and beta(2)-ARs were immunolocalized to afferent cell bodies, with highest expression in the basal cochlear turn, constituting one possible neural source of receptors within the organ of Corti, specifically on type I afferent dendrites. Beta(1)- and beta(2)-ARs in Hensen's and Deiters' cells would couple to Galphas, known to be present specifically in the supporting cells. Overall, adrenergic modulation of neural/supporting cell function within the organ of Corti represents a newly considered mechanism for modifying afferent signaling.

Projections of the lateral reticular nucleus to the cochlear nucleus in rats

The lateral reticular nucleus (LRN) resides in the rostral medulla and caudal pons, is implicated in cardiovascular regulation and cranial nerve reflexes, and gives rise to mossy fibers in the cerebellum. Retrograde tracing data revealed that medium-sized multipolar cells from the magnocellular part of the LRN project to the cochlear nucleus (CN). We sought to characterize the LRN projection to the CN using BDA injections. Anterogradely labeled terminals in the ipsilateral CN appeared as boutons and mossy fibers, and were examined with light and electron microscopy. The terminal field in the CN was restricted to the granule cell domain (GCD), specifically in the superficial layer along the anteroventral CN and in the granule cell lamina. Electron microscopy showed that the smallest LRN boutons formed 1-3 synapses, and as boutons increased in size, they formed correspondingly more synapses. The largest boutons were indistinguishable from the smallest mossy fibers, and the largest mossy fiber exhibited 15 synapses. Synapses were asymmetric with round vesicles and formed against thin dendritic profiles characterized by plentiful microtubules and the presence of fine filopodial extensions that penetrated the ending. These structural features of the postsynaptic target are characteristic of the terminal dendritic claw of granule cells. LRN projections are consistent with known organizational principles of non-auditory inputs to the GCD.

Tyrosine hydroxylase in rat auditory midbrain: distribution and changes following deafness

Tyrosine hydroxylase (TH), a key enzyme in the catecholaminergic pathway, allows for the differentiation of dopaminergic neurons. We previously showed decreases in TH gene expression in the rat inferior colliculus (IC) 3 and 21 days following deafness. In the present study, we characterized the normal distribution of TH as well as changes following deafness (bilateral cochlear ablation) in the IC and nuclei of the lateral lemniscus. Immunostaining was compared in three groups of rats: normal hearing (n=8), 21 day deaf (n=5) and 90 days following deafening (n=5). Many TH immunoreactive fibers and puncta were identified in the IC and nuclei of the lateral lemniscus of normal hearing animals and labeling was most dense in the external cortex of the IC. We also identified immunolabeling for fibers and puncta for another catecholaminergic enzyme, dopamine beta hydroxylase (DBH), but not phenylethanolamine-N-methyltranferase (PNMT). Neurons immunopositive for TH but not DBH or PNMT were observed in the dorsal cortex and dorsal horn of the central nucleus of the IC and ventral and intermediate lemniscus. In the central nucleus of the IC and dorsal lateral lemniscus many lightly labeled TH neurons were also DBH positive. Although the number of immunopositive cells in the IC and lemniscus declined 3 weeks and 3 months after deafening, the decline was not significant at three weeks in the VNLL nor after three months in the dorsal cortex. Immunolabeling for TH decreased significantly in IC and lemniscus 3 weeks and 3 months following deafening. These results suggest a role for dopaminergic neurons and fibers in deafness-related plasticity.

Distribution of catestatin-like immunoreactivity in the human auditory system

Chromogranin A (CgA) belongs to the family of chromogranin peptides which are contained in large dense-core vesicles. The novel CgA fragment catestatin (bovine CgA(344-364); RSMRLSFRARGYGFRGPGLQL) is a potent inhibitor of catecholamine release by acting as a nicotinic cholinergic antagonist. Catestatin is a recently characterized neuropeptide, consisting of 21 amino acids, which might play an autocrine regulatory role in neuroendocrine secretion through its interaction with different nicotinic acetylcholine receptor subtypes. This study investigates for the first time the distribution of this peptide in the human auditory system using immunohistochemistry. A high density of catestatin-like immunoreactivity (catestatin-LI) is located in the spiral ganglion cells. In the dorsal cochlear nucleus, a high density of catestatin-LI consists of varicose fibers, immunoreactive varicosities and immunoreactive neurons. A moderate density is detected in the ventral cochlear and the medial vestibular nucleus. A low density is found in the inferior colliculus and superior olivary complex. The study indicates that catestatin is distinctly distributed in the auditory system, suggesting a role as a neuromodulatory peptide. Further studies should elucidate a possible interaction with other neurotransmitters in the auditory system.

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.

Pontine sources of norepinephrine in the cat cochlear nucleus

In the current study, the distribution of noradrenergic neurons in the pontine tegmentum that project to the cochlear nucleus was determined with retrograde tract tracing combined with neurotransmitter immunohistochemistry in the cat. Double-labeled neurons were observed in all noradrenergic cell groups, in both the dorsolateral and the ventrolateral tegmentum. Half of the double-labeled cells were located in the locus coeruleus complex. Most of these were situated in its ventral division. Most other double-labeled cells were located in peribrachial regions, especially lateral to the brachium conjunctivum. Relatively few double-labeled cells were observed in both the A4 and the A5 cell groups, 2% and 0.4%, respectively, of the total. Except for neurons in A5, which projected only contralaterally, the projections were bilateral, with an ipsilateral preponderance. The results indicate that neurons located in the ipsilateral dorsolateral tegmentum, namely, in the locus coeruleus complex and the peribrachial region, are the primary source of pontine noradrenergic afferents to the cochlear nucleus of the cat.