Impact of cochlear ablation on calbindin and synaptophysin in the gerbil medial nucleus of the trapezoid body before hearing onset

Spontaneous bursting activity is already generated in the cochlea before hearing onset and represents an important condition of the functional and anatomical organization of auditory brainstem nuclei. In the present study, cochlea ablation induced changes were characterized in auditory brainstem nuclei indirectly innervated by auditory nerve fibers before hearing onset. In Meriones unguiculatus immunohistochemical labeling of calbindin-D28k (CB) and synaptophysin (SYN) were performed. The influence of cochlea-ablation on CB or SYN was analyzed by considering their differential immunoreaction during development. During the normal postnatal development, CB was first detected in somata of the medial nucleus of the trapezoid body (MNTB) at postnatal day (P)4. The immunoreaction increased gradually in parallel to the appearance of CB-immunoreactive terminal fields in distinct superior olivary complex (SOC) nuclei. Cochlear removal at P5 or P9 in animals with 24 and 48 h survival times resulted in an increase in somatic CB-labeling in the lesioned MNTB including terminal fields compared to the non-lesioned MNTB. SYN-immunolabeling was first detected at P0 and began to strongly encircle the MNTB neurons at P4. A further progression was observed with age. Cochlear ablation resulted in a significant reduction of SYN-labeled MNTB areas of P5-cochlea-ablated gerbils after 48 h post-lesion. In P9 cochlea-ablated gerbils, a redistribution of SYN-positive terminals was seen after 24 and 48 h. Taken together, the destruction of cochlea differentially influences CB- and SYN-labeling in the MNTB, which should be considered in association with different critical periods before hearing onset.

The ion channels and synapses responsible for the physiological diversity of mammalian lower brainstem auditory neurons

The auditory part of the brainstem is composed of several nuclei specialized in the computation of the different spectral and temporal features of the sound before it reaches the higher auditory regions. There are a high diversity of neuronal types in these nuclei, many with remarkable electrophysiological and synaptic properties unique to these structures. This diversity reflects specializations necessary to process the different auditory signals in order to extract precisely the acoustic information necessary for the auditory perception by the animal. Low threshold Kv1 channels and HCN channels are expressed in neurons that use timing clues for auditory processing, like bushy and octopus cells, in order to restrict action potential firing and reduce input resistance and membrane time constant. Kv3 channels allow principal neurons of the MNTB and pyramidal DCN neurons to fire fast trains of action potentials. Calcium channels on cartwheel DCN neurons produce complex spikes characteristic of these neurons. Calyceal synapses compensate the low input resistance of bushy and principal neurons of the MNTB by releasing hundreds of glutamate vesicles resulting in large EPSCs acting in fast ionotropic glutamate receptors, in order to reduce temporal summation of synaptic potentials, allowing more precise correspondence of pre- and post-synaptic potentials, and phase-locking. Pre-synaptic calyceal sodium channels have fast recovery from inactivation allowing extremely fast trains of action potential firing, and persistent sodium channels produce spontaneous activity of fusiform neurons at rest, which expands the dynamic range of these neurons. The unique combinations of different ion channels, ionotropic receptors and synaptic structures create a unique functional diversity of neurons extremely adapted to their complex functions in the auditory processing.

Age-Related Changes in Ang II Receptor Localization and Expression in the Developing Auditory Pathway

We studied Ang II receptor localization in different nuclei of the auditory system, by means of binding autoradiography, during brain development. The inferior colliculus (IC), a large midbrain structure which serves as an obligatory synaptic station in both the ascending and descending auditory pathways, exhibited high Ang II AT₂ binding at all ages (P0, P8, P15, P30), being maximal at P15. These observations were confirmed by in situ hybridization and immunofluorescence at P15, demonstrating that AT₂ receptor mRNA localized at the same area recognized by AT₂ antibodies and anti β III-tubulin suggesting the neuronal nature of the reactive cells. Ang II AT₁ receptors were absent at early developmental ages (P0) in all nuclei of the auditory system and a low level was observed in the IC at the age P8. AT₂ receptors were present at ventral cochlear nucleus and superior olivary complex, being higher at P15 and P8, respectively. We also explored the effect of prenatal administration of Ang II or PD123319 (AT₂ antagonist) on binding of Ang II receptors at P0, P8, P15. Both treatments increased significantly the level of AT₂ receptors at P0 and P8 in the IC. Although total binding in the whole IC from P15 animals showed no difference between treatments, the central nucleus of the IC exhibited higher binding. Our results supports a correlation between the timing of the higher expression of Ang II AT₂ receptors in different nuclei, the onset of audition and the establishment of neuronal circuits of the auditory pathway.

Dopaminergic projections of the subparafascicular thalamic nucleus to the auditory brainstem

Neuromodulators can alter the response properties of sensory neurons, including those in the auditory system. Dopamine, which plays a major role in reward and movement, has been shown to alter neural responses in the auditory brainstem and midbrain. Recently we identified the subparafascicular thalamic nucleus (SPF), part of the A11 dopaminergic cell group, as the source of dopamine to the inferior colliculus (IC). The superior olivary complex (SOC) is also a likely target of dopaminergic projections from the SPF because it receives projections from the SPF and contains fibers and terminals immunoreactive for tyrosine hydroxylase, the rate limiting enzyme in dopamine synthesis. However, it is unknown if the projections from the SPF to SOC are dopaminergic, and if single neurons in the SPF project to both the IC and SOC. Using anterograde tracing combined with fluorescent immunohistochemistry, we found that the SPF sends dopaminergic projections to the superior paraolivary nucleus and the medial nucleus of the trapezoid body, but not the lateral superior olive. We confirmed these projections using a retrograde tracer. By making dual retrograde deposits in the IC and SOC, we found that individual dopaminergic cells innervate both the IC and SOC. These results suggest dopaminergic innervation, likely released in a context dependent manner, occurs at multiple levels of the auditory pathway.

All the way from the cortex: a review of auditory corticosubcollicular pathways

Enrico Mugnaini has devoted part of his long and fruitful neuroscientific career to investigating the structural similarities between the cerebellar cortex and one of the first relay stations of the mammalian auditory pathway: the dorsal cochlear nucleus. The hypothesis of the cerebellar-like nature of the superficial layers of the dorsal cochlear nucleus received definitive support with the discovery and extensive characterization in his laboratory of unipolar brush cells, a neuron type unique to certain regions of the cerebellar cortex and to the granule cell domains of the cochlear nuclei. Paradoxically, a different line of research carried out in his laboratory revealed that, unlike the mammalian cerebellar cortex, the dorsal cochlear nucleus receives direct projections from the cerebral cortex, a fact that constitutes one of the main differences between the cerebellum and the dorsal cochlear nucleus. In an article published in 1995, Mugnaini's group described in detail the novel direct projections from the rat auditory neocortex to various subcollicular auditory centers, including the nucleus sagulum, the paralemniscal regions, the superior olivary complex, and the cochlear nuclei (Feliciano et al., Auditory Neuroscience 1995; 1:287-308). This review gives Enrico Mugnaini credit for his seminal contribution to the knowledge of auditory corticosubcollicular projections and summarizes how this growing field has evolved in the last 20 years.

Aging effects on the binaural interaction component of the auditory brainstem response in the Mongolian gerbil: Effects of interaural time and level differences

The effect of interaural time difference (ITD) and interaural level difference (ILD) on wave 4 of the binaural and summed monaural auditory brainstem responses (ABRs) as well as on the DN1 component of the binaural interaction component (BIC) of the ABR in young and old Mongolian gerbils (Meriones unguiculatus) was investigated. Measurements were made at a fixed sound pressure level (SPL) and a fixed level above visually detected ABR threshold to compensate for individual hearing threshold differences. In both stimulation modes (fixed SPL and fixed level above visually detected ABR threshold) an effect of ITD on the latency and the amplitude of wave 4 as well as of the BIC was observed. With increasing absolute ITD values BIC latencies were increased and amplitudes were decreased. ILD had a much smaller effect on these measures. Old animals showed a reduced amplitude of the DN1 component. This difference was due to a smaller wave 4 in the summed monaural ABRs of old animals compared to young animals whereas wave 4 in the binaural-evoked ABR showed no age-related difference. In old animals the small amplitude of the DN1 component was correlated with small binaural-evoked wave 1 and wave 3 amplitudes. This suggests that the reduced peripheral input affects central binaural processing which is reflected in the BIC.

Graded and discontinuous EphA-ephrinB expression patterns in the developing auditory brainstem

Eph-ephrin interactions guide topographic mapping and pattern formation in a variety of systems. In contrast to other sensory pathways, their precise role in the assembly of central auditory circuits remains poorly understood. The auditory midbrain, or inferior colliculus (IC) is an intriguing structure for exploring guidance of patterned projections as adjacent subdivisions exhibit distinct organizational features. The central nucleus of the IC (CNIC) and deep aspects of its neighboring lateral cortex (LCIC, Layer 3) are tonotopically-organized and receive layered inputs from primarily downstream auditory sources. While less is known about more superficial aspects of the LCIC, its inputs are multimodal, lack a clear tonotopic order, and appear discontinuous, terminating in modular, patch/matrix-like distributions. Here we utilize X-Gal staining approaches in lacZ mutant mice (ephrin-B2, -B3, and EphA4) to reveal EphA-ephrinB expression patterns in the nascent IC during the period of projection shaping that precedes hearing onset. We also report early postnatal protein expression in the cochlear nuclei, the superior olivary complex, the nuclei of the lateral lemniscus, and relevant midline structures. Continuous ephrin-B2 and EphA4 expression gradients exist along frequency axes of the CNIC and LCIC Layer 3. In contrast, more superficial LCIC localization is not graded, but confined to a series of discrete ephrin-B2 and EphA4-positive Layer 2 modules. While heavily expressed in the midline, much of the auditory brainstem is devoid of ephrin-B3, including the CNIC, LCIC Layer 2 modular fields, the dorsal nucleus of the lateral lemniscus (DNLL), as well as much of the superior olivary complex and cochlear nuclei. Ephrin-B3 LCIC expression appears complementary to that of ephrin-B2 and EphA4, with protein most concentrated in presumptive extramodular zones. Described tonotopic gradients and seemingly complementary modular/extramodular patterns suggest Eph-ephrin guidance in establishing juxtaposed continuous and discrete neural maps in the developing IC prior to experience.

The brainstem efferent acoustic chiasm in pigmented and albino rats

The present study examined whether structural peculiarities in the brain-efferent pathway to the organ of Corti may underlie functional differences in hearing between pigmented and albino individuals of the same mammalian species. Pigmented Brown-Norway rats and albino Wistar rats received unilateral injections of an aqueous solution of the retrograde neuronal tracer Fluorogold (FG) into the scala tympani of the cochlea to identify olivocochlear neurons (OCN) in the brainstem superior olivary complex. After five days, brains were perfusion-fixed and brainstem sections were cut and analyzed with respect to retrogradely labeled neurons. Intrinsic neurons of the lateral system were located exclusively in the ipsilateral lateral superior olive (LSO) in both groups. Shell neurons surrounding the LSO and in periolivary regions, which made up only 5-8% of all OCN, were more often contralaterally located in albino than in pigmented animals. A striking difference was observed in the laterality of neurons of the medial olivocochlear (MOC) system, which provided more than one third of all OCN. These neurons, located in the rostral periolivary region and in the ventral nucleus of the trapezoid body, were observed contralateral to 45% in pigmented and to 68% in albino animals. Our study, the first to compare the origin of the olivocochlear bundle in pigmented and albino rats, provides evidence for differences in the crossing pattern of the olivocochlear pathway. These were found predominantly in the MOC system providing the direct efferent innervation of cochlear outer hair cells. Our findings may account for the alterations in auditory perception observed in albino mammals including man.

Developmental alterations of the auditory brainstem centers--pathogenetic implications in Sudden Infant Death Syndrome

Sudden Infant Death Syndrome (SIDS), despite the success of campaigns to reduce its risks, is the leading cause of infant death in the Western world. Even though the pathogenesis remains unexplained, brainstem abnormalities of the neuronal network that mediates breathing and protective responses to asphyxia, particularly in the arousal phase from sleep, are believed to play a fundamental role. This is the first study to identify, in SIDS, developmental defects of specific brainstem centers involved in hearing pathways, particularly in the cochlear and vestibular nuclei, in the superior olivary complex and in the inferior colliculus, suggesting a possible influence of the acoustic system on respiratory activity. In 49 SIDS cases and 20 controls an in-depth anatomopathological examination of the autonomic nervous system was performed, with the main aim of detecting developmental alterations of brainstem structures controlling both the respiratory and auditory activities. Overall, a significantly higher incidence of cytoarchitectural alterations of both the auditory and respiratory network components were observed in SIDS victims compared with matched controls. Even if there is not sufficient evidence to presume that developmental defects of brainstem auditory structures can affect breathing, our findings, showing that developmental deficit in the control respiratory areas are frequently accompanied by alterations of auditory structures, highlight an additional important element for the understanding the pathogenetic mechanism of SIDS.