Altered Gap Junction Network Topography in Mouse Models for Human Hereditary Deafness.

Anisotropic gap junctional coupling is a distinct feature of astrocytes in many brain regions. In the lateral superior olive (LSO), astrocytic networks are anisotropic and oriented orthogonally to the tonotopic axis. In CaV1.3 knock-out (KO) and otoferlin KO mice, where auditory brainstem nuclei are deprived from spontaneous cochlea-driven neuronal activity, neuronal circuitry is disturbed. So far it was unknown if this disturbance is also accompanied by an impaired topography of LSO astrocyte networks. To answer this question, we immunohistochemically analyzed the expression of astrocytic connexin (Cx) 43 and Cx30 in auditory brainstem nuclei. Furthermore, we loaded LSO astrocytes with the gap junction-permeable tracer neurobiotin and assessed the network shape and orientation. We found a strong elevation of Cx30 immunoreactivity in the LSO of CaV1.3 KO mice, while Cx43 levels were only slightly increased. In otoferlin KO mice, LSO showed a slight increase in Cx43 as well, whereas Cx30 levels were unchanged. The total number of tracer-coupled cells was unaltered and most networks were anisotropic in both KO strains. In contrast to the WTs, however, LSO networks were predominantly oriented parallel to the tonotopic axis and not orthogonal to it. Taken together, our data demonstrate that spontaneous cochlea-driven neuronal activity is not required per se for the formation of anisotropic LSO astrocyte networks. However, neuronal activity is required to establish the proper orientation of networks. Proper formation of LSO astrocyte networks thus necessitates neuronal input from the periphery, indicating a critical role of neuron-glia interaction during early postnatal development in the auditory brainstem.

Activity-dependent formation of a vesicular inhibitory amino acid transporter gradient in the superior olivary complex of NMRI mice.

BACKGROUND: In the mammalian superior olivary complex (SOC), synaptic inhibition contributes to the processing of binaural sound cues important for sound localization. Previous analyses demonstrated a tonotopic gradient for postsynaptic proteins mediating inhibitory neurotransmission in the lateral superior olive (LSO), a major nucleus of the SOC. To probe, whether a presynaptic molecular gradient exists as well, we investigated immunoreactivity against the vesicular inhibitory amino acid transporter (VIAAT) in the mouse auditory brainstem. RESULTS: Immunoreactivity against VIAAT revealed a gradient in the LSO and the superior paraolivary nucleus (SPN) of NMRI mice, with high expression in the lateral, low frequency processing limb and low expression in the medial, high frequency processing limb of both nuclei. This orientation is opposite to the previously reported gradient of glycine receptors in the LSO. Other nuclei of the SOC showed a uniform distribution of VIAAT-immunoreactivity. No gradient was observed for the glycine transporter GlyT2 and the neuronal protein NeuN. Formation of the VIAAT gradient was developmentally regulated and occurred around hearing-onset between postnatal days 8 and 16. Congenital deaf Claudin14 -/- mice bred on an NMRI background showed a uniform VIAAT-immunoreactivity in the LSO, whereas cochlear ablation in NMRI mice after hearing-onset did not affect the gradient. Additional analysis of C57Bl6/J, 129/SvJ and CBA/J mice revealed a strain-specific formation of the gradient. CONCLUSIONS: Our results identify an activity-regulated gradient of VIAAT in the SOC of NRMI mice. Its absence in other mouse strains adds a novel layer of strain-specific features in the auditory system, i.e. tonotopic organization of molecular gradients. This calls for caution when comparing data from different mouse strains frequently used in studies involving transgenic animals. The presence of strain-specific differences offers the possibility of genetic mapping to identify molecular factors involved in activity-dependent developmental processes in the auditory system. This would provide an important step forward concerning improved auditory rehabilitation in cases of congenital deafness.

Mechanism of auditory hypersensitivity in human autism using autism model rats.

BACKGROUND: Auditory hypersensitivity is one of the major complications in autism spectrum disorder. The aim of this study was to investigate whether the auditory brain center is affected in autism model rats. METHODS: Autism model rats were prepared by prenatal exposure to thalidomide on embryonic day 9 and 10 in pregnant rats. The superior olivary complex (SOC), a complex of auditory nuclei, was immunostained with anti-calbindin d28k antibody at postnatal day 50. RESULTS: In autism model rats, SOC immunoreactivity was markedly decreased. Strength of immunostaining of SOC auditory fibers was also weak in autism model rats. Surprisingly, the size of the medial nucleus of trapezoid body, a nucleus exerting inhibitory function in SOC, was significantly decreased in autism model rats. CONCLUSIONS: Auditory hypersensitivity may be, in part, due to impairment of inhibitory processing by the auditory brain center.

Cytoarchitectural and functional abnormalities of the inferior colliculus in sudden unexplained perinatal death.

The inferior colliculus is a mesencephalic structure endowed with serotonergic fibers that plays an important role in the processing of acoustic information. The implication of the neuromodulator serotonin also in the aetiology of sudden unexplained fetal and infant death syndromes and the demonstration in these pathologies of developmental alterations of the superior olivary complex (SOC), a group of pontine nuclei likewise involved in hearing, prompted us to investigate whether the inferior colliculus may somehow contribute to the pathogenetic mechanism of unexplained perinatal death. Therefore, we performed in a wide set of fetuses and infants, aged from 33 gestational weeks to 7 postnatal months and died of both known and unknown cause, an in-depth anatomopathological analysis of the brainstem, particularly of the midbrain. Peculiar neuroanatomical and functional abnormalities of the inferior colliculus, such as hypoplasia/structural disarrangement and immunonegativity or poor positivity of serotonin, were exclusively found in sudden death victims, and not in controls. In addition, these alterations were frequently related to dysgenesis of connected structures, precisely the raphé nuclei and the superior olivary complex, and to nicotine absorption in pregnancy. We propose, on the basis of these results, the involvement of the inferior colliculus in more important functions than those related to hearing, as breathing and, more extensively, all the vital activities, and then in pathological conditions underlying a sudden death in vulnerable periods of the autonomic nervous system development, particularly associated to harmful risk factors as cigarette smoking.

Organization of the human superior olivary complex in 15q duplication syndromes and autism spectrum disorders.

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by a number of behavioral and social features. Although the etiology of most cases of ASD is idiopathic, a significant number of cases can be attributed to genetic causes, such as chromosome 15q duplications [dup(15q)]. Recent neuropathological investigations have provided evidence for distinct patterns of heterotopias and dysplasias in ASD and subjects with both ASD and dup(15q). Individuals with ASD characteristically have hearing difficulties and we have previously demonstrated significant and consistent hypoplasia in a number of auditory brainstem nuclei in subjects with ASD. Herein, we compare results from a morphometric investigation of auditory brainstem nuclei in subjects with ASD, dup(15q) and controls. Our observations in subjects with ASD support our previous reports. However, in subjects with dup(15q), we find significantly fewer neurons and in many nuclei, neurons were significantly smaller than in ASD subjects. Finally, we find a notably higher incidence of ectopic neurons in dup(15q). These results suggest that in the brainstem, these neuropathological conditions may evolve from some of the same developmental errors but are distinguished on microscopic features.

Dexamethasone's effect in the retrocochlear auditory centers of a noise-induced hearing loss mouse model.

OBJECTIVE: Examine prophylactic effects of dexamethasone (Dex) in retrocochlear auditory centers in a noise-induced hearing loss (NIHL) mouse model. STUDY DESIGN: Prospective animal study. SETTING: Academic research center. SUBJECTS AND METHODS: Thirty-two mice were divided into control, untreated, saline (2 and 10 µL), and Dex (2 and 10 µL) groups. Dex was applied intratympanically (IT) prior to 110 to 120 dB noise over 6 hours. Auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE) were performed at 1 day, 1 week, 1 month, and 2 months. Retrocochlear neuronal cells were labeled with FluoroGold and counted. Hair cells of the organ of Corti were labeled with fluorescein isothiocyanate-conjugated phalloidin and counted. RESULTS: Auditory brainstem response thresholds of untreated NIHL, 2 and 10 µL IT saline, and 2 and 10 µL IT Dex were 21.7 ± 2.9 dB, 20 ± 0 dB, 20 ± 5 dB, 18.3 ± 2.9 dB, and 18.3 ± 2.9 dB, respectively. At 1-day post NIHL, all groups demonstrated profound hearing loss. At 2 weeks, 2 and 10 µL Dex thresholds improved to 47.5 ± 3.5 dB and 48.8 ± 18.9 dB, respectively, whereas the untreated and saline groups remained unchanged. Mean cell counts in the cochlear nucleus (CN), superior olivary complex (SOC), and lateral lemniscus (LL) of control mice were 1483 ± 190, 2807 ± 67, and 112 ± 20, respectively. After acoustic trauma, the untreated, saline, and 2 µL Dex groups yielded decreased neuronal counts in the SOC. In contrast, the 10 µL Dex group had 1883 ± 186 (CN), 2774 ± 182 (SOC), and 166 ± 18 (LL). There was sporadic hair cell loss for all traumatized groups. CONCLUSION: Our NIHL mouse model demonstrated dose-dependent Dex pretreatment otoprotection against NIHL with preservation of retrocochlear auditory neurons.