Catecholamine-independent transient expression of tyrosine hydroxylase in primary auditory neurons is coincident with the onset of hearing in the rat cochlea

Adult Neurogenesis of the Medial Geniculate Body: In Vitro and Molecular Genetic Analyses Reflect the Neural Stem Cell Capacity of the Rat Auditory Thalamus over Time

Neural stem cells (NSCs) have been recently identified in the neonatal rat medial geniculate body (MGB). NSCs are characterized by three cardinal features: mitotic self-renewal, formation of progenitors, and differentiation into all neuroectodermal cell lineages. NSCs and the molecular factors affecting them are particularly interesting, as they present a potential target for treating neurologically based hearing disorders. It is unclear whether an NSC niche exists in the rat MGB up to the adult stage and which neurogenic factors are essential during maturation. The rat MGB was examined on postnatal days 8, 12, and 16, and at the adult stadium. The cardinal features of NSCs were detected in MGB cells of all age groups examined by neurosphere, passage, and differentiation assays. In addition, real-time quantitative polymerase chain reaction arrays were used to compare the mRNA levels of 84 genes relevant to NSCs and neurogenesis. In summary, cells of the MGB display the cardinal features of NSCs up to the adult stage with a decreasing NSC potential over time. Neurogenic factors with high importance for MGB neurogenesis were identified on the mRNA level. These findings should contribute to a better understanding of MGB neurogenesis and its regenerative capacity.

mRNA Abundance of Neurogenic Factors Correlates with Hearing Capacity in Auditory Brainstem Nuclei of the Rat

Neural stem cells (NSCs) have previously been described up to the adult stage in the rat cochlear nucleus (CN). A decreasing neurogenic potential was observed with critical changes around hearing onset. A better understanding of molecular factors affecting NSCs and neurogenesis is of interest as they represent potential targets to treat the cause of neurologically based hearing disorders. The role of genes affecting NSC development and neurogenesis in CN over time on hearing capacity has remained unclear. This study investigated the mRNA abundance of genes influencing NSCs and neurogenesis in rats' CN over time. The CN of rats on postnatal days 6, 12, and 24 were examined. Real-time quantitative polymerase chain reaction arrays were used to compare mRNA levels of 84 genes relevant to NSCs and neurogenesis. Age- and hearing-specific patterns of changes in mRNA abundance of neurogenically relevant genes were detected in the rat CN. Additionally, crucial neurogenic factors with significant and relevant influence on neurogenesis were identified. The results of this work should contribute to a better understanding of the molecular mechanisms underlying the neurogenesis of the auditory pathway.

Diverse identities and sites of action of cochlear neurotransmitters

Accurate encoding of acoustic stimuli requires temporally precise responses to sound integrated with cellular mechanisms that encode the complexity of stimuli over varying timescales and orders of magnitude of intensity. Sound in mammals is initially encoded in the cochlea, the peripheral hearing organ, which contains functionally specialized cells (including hair cells, afferent and efferent neurons, and a multitude of supporting cells) to allow faithful acoustic perception. To accomplish the demanding physiological requirements of hearing, the cochlea has developed synaptic arrangements that operate over different timescales, with varied strengths, and with the ability to adjust function in dynamic hearing conditions. Multiple neurotransmitters interact to support the precision and complexity of hearing. Here, we review the location of release, action, and function of neurotransmitters in the mammalian cochlea with an emphasis on recent work describing the complexity of signaling.

Tyrosine Hydroxylase Expression in Type II Cochlear Afferents in Mice

Acoustic information propagates from the ear to the brain via spiral ganglion neurons that innervate hair cells in the cochlea. These afferents include unmyelinated type II fibers that constitute 5 % of the total, the majority being myelinated type I neurons. Lack of specific genetic markers of type II afferents in the cochlea has been a roadblock in studying their functional role. Unexpectedly, type II afferents were visualized by reporter proteins induced by tyrosine hydroxylase (TH)-driven Cre recombinase. The present study was designed to determine whether TH-driven Cre recombinase (TH-2A-CreER) provides a selective and reliable tool for identification and genetic manipulation of type II rather than type I cochlear afferents. The "TH-2A-CreER neurons" radiated from the spiral lamina, crossed the tunnel of Corti, turned towards the base of the cochlea, and traveled beneath the rows of outer hair cells. Neither the processes nor the somata of TH-2A-CreER neurons were labeled by antibodies that specifically labeled type I afferents and medial efferents. TH-2A-CreER-positive processes partially co-labeled with antibodies to peripherin, a known marker of type II afferents. Individual TH-2A-CreER neurons gave off short branches contacting 7-25 outer hair cells (OHCs). Only a fraction of TH-2A-CreER boutons were associated with CtBP2-immunopositive ribbons. These results show that TH-2A-CreER provides a selective marker for type II versus type I afferents and can be used to describe the morphology and arborization pattern of type II cochlear afferents in the mouse cochlea.

Dopamine in the auditory brainstem and midbrain: co-localization with amino acid neurotransmitters and gene expression following cochlear trauma

Dopamine (DA) modulates the effects of amino acid neurotransmitters (AANs), including GABA and glutamate, in motor, visual, olfactory, and reward systems (Hnasko et al., 2010; Stuber et al., 2010; Hnasko and Edwards, 2012). The results suggest that DA may play a similar modulatory role in the auditory pathways. Previous studies have shown that deafness results in decreased GABA release, changes in excitatory neurotransmitter levels, and increased spontaneous neuronal activity within brainstem regions related to auditory function. Modulation of the expression and localization of tyrosine hydroxylase (TH; the rate limiting enzyme in the production of DA) in the IC following cochlear trauma has been previously reported (Tong et al., 2005). In the current study the possibility of co-localization of TH with AANs was examined. Changes in the gene expression of TH were compared with changes in the gene expression of markers for AANs in the cochlear nucleus (CN) and inferior colliculus (IC) to determine whether those deafness related changes occur concurrently. The results indicate that bilateral cochlear ablation significantly reduced TH gene expression in the CN after 2 months while in the IC the reduction in TH was observed at both 3 days and 2 months following ablation. Furthermore, in the CN, glycine transporter 2 (GLYT2) and the GABA transporter (GABAtp) were also significantly reduced only after 2 months. However, in the IC, DA receptor 1 (DRDA1), vesicular glutamate transporters 2 and 3 (VGLUT2, VGLUT3), GABAtp and GAD67 were reduced in expression both at the 3 days and 2 months time points. A close relationship between the distribution of TH and several of the AANs was determined in both the CN and the IC. In addition, GLYT2 and VGLUT3 each co-localized with TH within IC somata and dendrites. Therefore, the results of the current study suggest that DA is spatially well positioned to influence the effects of AANs on auditory neurons.

Transient Appearance of Tyrosine Hydroxylase Immunoreactive Cells in the Midline Epithelial Seam of the Human Fetal Secondary Palate

Objective

Transient immunoreactivity for tyrosine hydroxylase, which mediates the conversion of the amino acid L-tyrosine to dihydroxyphenylalanine, in the midline epithelial seam between the bilateral palatal shelves was investigated in human fetuses.

Materials and Methods

Horizontal or frontal paraffin sections of two human fetuses at 9 and 15 weeks of gestation were used to examine the distribution of tyrosine hydroxylase–immunoreactive cells in regions of the entire head other than the brain. Immunohistochemical staining for S100 protein, calretinin, cytokeratin 14, and vimentin was examined using adjacent or near sections.

Results

Tyrosine hydroxylase–immunoreactive cells were large and densely distributed in the midline epithelial seam at the site of palatal fusion in fetuses at 9 weeks but not in fetuses at 15 weeks, in which the midline epithelial seam had already disappeared. No expression of S100 protein, calretinin, or vimentin was detected, but the midline epithelial seam was positive for cytokeratin 14. Tyrosine hydroxylase immunoreactivity was not detected in epithelia during the process of palatal fusion in mice from E 14.0 to 15.0.

Conclusions

These findings indicate that tyrosine hydroxylase–immunoreactive cells in the midline epithelial seams are nonneural epithelial cells and suggest that the tyrosine hydroxylase is a novel factor involved in normal palatal formation, especially the fate of the midline epithelial seam in humans.

Partial requirement of endothelin receptor B in spiral ganglion neurons for postnatal development of hearing

Impairments of endothelin receptor B (Ednrb/EDNRB) cause the development of Waardenburg-Shah syndrome with congenital hearing loss, hypopigmentation, and megacolon disease in mice and humans. Hearing loss in Waardenburg-Shah syndrome has been thought to be caused by an Ednrb-mediated congenital defect of melanocytes in the stria vascularis (SV) of inner ears. Here we show that Ednrb expressed in spiral ganglion neurons (SGNs) in inner ears is required for postnatal development of hearing in mice. Ednrb protein was expressed in SGNs from WT mice on postnatal day 19 (P19), whereas it was undetectable in SGNs from WT mice on P3. Correspondingly, Ednrb homozygously deleted mice (Ednrb(-/-) mice) with congenital hearing loss showed degeneration of SGNs on P19 but not on P3. The congenital hearing loss involving neurodegeneration of SGNs as well as megacolon disease in Ednrb(-/-) mice were markedly improved by introducing an Ednrb transgene under control of the dopamine β-hydroxylase promoter (Ednrb(-/-);DBH-Ednrb mice) on P19. Neither defects of melanocytes nor hypopigmentation in the SV and skin in Ednrb(-/-) mice was rescued in the Ednrb(-/-);DBH-Ednrb mice. Thus, the results of this study indicate a novel role of Ednrb expressed in SGNs distinct from that in melanocytes in the SV contributing partially to postnatal hearing development.

Neurofilament localization and phosphorylation in the developing inner ear of the rat

Detailed understanding of neurofilament protein distribution in the inner ear can shed light on regulatory mechanisms involved in neuronal development of this tissue. We assessed the spatio-temporal changes in the distribution of neurofilaments in the developing rat inner ear between embryonic day 12 and 30 days after birth, using antibodies against phosphorylated as well as non-phosphorylated light (NFL), medium (NFM) and heavy (NFH) neurofilament subunits. Our results show that during development, the onset of neurofilament expression in the rat inner ear is on embryonic day 12, earlier than previously shown. We demonstrate that neurofilament subunits of different molecular weight emerge in a developmental stage-dependent order. In addition, we determined that neurofilaments of the vestibular nerve mature earlier than neurofilaments of the cochlear nerve. Cochlear neurofilament maturation progresses in a gradient from base to apex, and from inner to outer hair cells. The sequential pattern of neurofilament expression we describe may help understand the consequences of certain mutations, and contribute to develop therapeutic strategies.

Impairment of SLC17A8 encoding vesicular glutamate transporter-3, VGLUT3, underlies nonsyndromic deafness DFNA25 and inner hair cell dysfunction in null mice

Autosomal-dominant sensorineural hearing loss is genetically heterogeneous, with a phenotype closely resembling presbycusis, the most common sensory defect associated with aging in humans. We have identified SLC17A8, which encodes the vesicular glutamate transporter-3 (VGLUT3), as the gene responsible for DFNA25, an autosomal-dominant form of progressive, high-frequency nonsyndromic deafness. In two unrelated families, a heterozygous missense mutation, c.632C-->T (p.A211V), was found to segregate with DFNA25 deafness and was not present in 267 controls. Linkage-disequilibrium analysis suggested that the families have a distant common ancestor. The A211 residue is conserved in VGLUT3 across species and in all human VGLUT subtypes (VGLUT1-3), suggesting an important functional role. In the cochlea, VGLUT3 accumulates glutamate in the synaptic vesicles of the sensory inner hair cells (IHCs) before releasing it onto receptors of auditory-nerve terminals. Null mice with a targeted deletion of Slc17a8 exon 2 lacked auditory-nerve responses to acoustic stimuli, although auditory brainstem responses could be elicited by electrical stimuli, and robust otoacoustic emissions were recorded. Ca(2+)-triggered synaptic-vesicle turnover was normal in IHCs of Slc17a8 null mice when probed by membrane capacitance measurements at 2 weeks of age. Later, the number of afferent synapses, spiral ganglion neurons, and lateral efferent endings below sensory IHCs declined. Ribbon synapses remaining by 3 months of age had a normal ultrastructural appearance. We conclude that deafness in Slc17a8-deficient mice is due to a specific defect of vesicular glutamate uptake and release and that VGLUT3 is essential for auditory coding at the IHC synapse.

Neurochemical characterization of tyrosine hydroxylase-immunoreactive interneurons in the developing rat cerebral cortex

Understanding the development of cortical interneuron phenotypic diversity is critical because interneuron dysfunction has been implicated in several neurodevelopmental disorders. Here, tyrosine hydroxylase (TH)-immunoreactive neurons in the developing and adult rat cortex were characterized in light of findings regarding interneuron neurochemistry and development. Cortical TH-immunoreactive neurons were first observed 2 weeks postnatally and peaked in number 3 weeks after birth. At subsequent ages, the number of these cell profiles was gradually reduced, and they were seen less frequently in adults. No DNA fragmentation or active caspase 3 was observed in cortical TH cells at any age examined, eliminating cell death as an explanation for the decrease in cell number. Although cortical TH cells reportedly fail to produce subsequent catecholaminergic enzymes, we found that the majority of these cells at all ages contained phosphorylated TH, suggesting that the enzyme may be active and producing L-DOPA as an end-product. Morphological criteria and colocalization of some TH cells with glutamic acid decarboxylase suggest that these cells are interneurons. Previously, parvalbumin, somatostatin, and calretinin were demonstrated in non-overlapping subsets of interneurons. Cortical TH neurons colocalized with calretinin but not with parvalbumin or somatostatin. These findings suggest that the transitory increase in TH cell number is not due to cell death but possibly due to alterations in the amount of detectable TH present in these cells, and that at least some cortical TH-producing interneurons belong to the calretinin-containing subset of interneurons that originate developmentally in the caudal ganglionic eminence.

FXYD6 is a novel regulator of Na,K-ATPase expressed in the inner ear

The exquisite sensitivity of the cochlea, which mediates the transduction of sound waves into nerve impulses, depends on the endolymph ionic composition and the endocochlear potential. A key protein in the maintenance of the electrochemical composition of the endolymph is the Na,K-ATPase. In this study, we have looked for the presence in the rat inner ear of members of the FXYD protein family, recently identified as tissue-specific modulators of Na,K-ATPase. Only FXYD6 is detected at the protein level. FXYD6 is expressed in various epithelial cells bordering the endolymph space and in the auditory neurons. FXYD6 co-localizes with Na,K-ATPase in the stria vascularis and can be co-immunoprecipitated with Na,K-ATPase. After expression in Xenopus oocytes, FXYD6 associates with Na,K-ATPase alpha1-beta1 and alpha1-beta2 isozymes, which are preferentially expressed in different regions of the inner ear and also with gastric and non-gastric H,K-ATPases. The apparent K(+) and Na(+) affinities of alpha1-beta1 and alpha1-beta2 isozymes are different. Association of FXYD6 with Na,K-ATPase alpha1-beta1 isozymes slightly decreases their apparent K(+) affinity and significantly decreases their apparent Na(+) affinity. On the other hand, association with alpha1-beta2 isozymes increases their apparent K(+) and Na(+) affinity. The effects of FXYD6 on the apparent Na(+) affinity of Na,K-ATPase and the voltage dependence of its K(+) effect are distinct from other FXYD proteins. In conclusion, this study defines the last FXYD protein of unknown function as a modulator of Na,K-ATPase. Among FXYD protein, FXYD6 is unique in its expression in the inner ear, suggesting a role in endolymph composition.

Dynamic expression of FXYD6 in the inner ear suggests a role of the protein in endolymph homeostasis and neuronal activity

A key protein in the production and in the maintenance of the endocochlear potential is the Na,K-ATPase. Previously, we have shown that FXYD6 is a modulator of the Na,K-ATPase expressed in the inner ear (Delprat et al. [2007] J Biol Chem 282:7450-7456). To investigate the potential role of FXYD6 in inner ear function, we studied the developmental expression of FXYD6. Reverse transcriptase-polymerase chain reaction analysis demonstrates that FXYD6 is present as two splice variants. Both variants coimmunoprecipitate with Na,K-ATPase after expression in Xenopus oocytes. Immunohistochemistry of the cochlea (from birth to postnatal day 30) shows that FXYD6 is expressed in several epithelial cells important for endolymph homeostasis. Marked similarities were found in the developmental expression patterns of FXYD6 and Na,K-ATPase, suggesting functional cooperation between the two proteins in the generation and maintenance of the endocochlear potential and ion composition of the endolymph.

Pituitary adenylyl cyclase-activating polypeptide (PACAP) and its receptor (PAC1-R) are positioned to modulate afferent signaling in the cochlea

Pituitary adenylyl cyclase-activating polypeptide (PACAP), via its specific receptor pituitary adenylyl cyclase-activating polypeptide receptor 1 (PAC1-R), is known to have roles in neuromodulation and neuroprotection associated with glutamatergic and cholinergic neurotransmission, which, respectively, are believed to form the primary basis for afferent and efferent signaling in the organ of Corti. Previously, we identified transcripts for PACAP preprotein and multiple splice variants of its receptor, PAC1-R, in microdissected cochlear subfractions. In the present work, neural localizations of PACAP and PAC1-R within the organ of Corti and spiral ganglion were examined, defining sites of PACAP action. Immunolocalization of PACAP and PAC1-R in the organ of Corti and spiral ganglion was compared with immunolocalization of choline acetyltransferase (ChAT) and synaptophysin as efferent neuronal markers, and glutamate receptor 2/3 (GluR2/3) and neurofilament 200 as afferent neuronal markers, for each of the three cochlear turns. Brightfield microscopy giving morphological detail for individual immunolocalizations was followed by immunofluorescence detection of co-localizations. PACAP was found to be co-localized with ChAT in nerve fibers of the intraganglionic spiral bundle and beneath the inner and outer hair cells within the organ of Corti. Further, evidence was obtained that PACAP is expressed in type I afferent axons leaving the spiral ganglion en route to the auditory nerve, potentially serving as a neuromodulator in axonal terminals. In contrast to the efferent localization of PACAP within the organ of Corti, PAC1-R immunoreactivity was co-localized with afferent dendritic neuronal marker GluR2/3 in nerve fibers passing beneath and lateral to the inner hair cell and in fibers at supranuclear and basal sites on outer hair cells. Given the known association of PACAP with catecholaminergic neurotransmission in sympathoadrenal function, we also re-examined the issue of whether the organ of Corti receives adrenergic innervation. We now demonstrate the existence of nerve fibers within the organ of Corti which are immunoreactive for the adrenergic marker dopamine beta-hydroxylase (DBH). DBH immunoreactivity was particularly prominent in nerve fibers both at the base and near the cuticular plate of outer hair cells of the apical turn, extending to the non-sensory Hensen's cell region. Evidence was obtained for limited co-localization of DBH with PAC1-R and PACAP. In the process of this investigation, we obtained evidence that efferent and afferent nerve fibers, in addition to adrenergic nerve fibers, are present at supranuclear sites on outer hair cells and distributed within the non-sensory epithelium of the apical cochlear turn for rat, based upon immunoreactivity for the corresponding neuronal markers. Overall, PACAP is hypothesized to act within the organ of Corti as an efferent neuromodulator of afferent signaling via PAC1-R that is present on type I afferent dendrites, in position to afford protection from excitotoxicity. Additionally, PACAP/PAC1-R may modulate secretion of catecholamines from adrenergic terminals within the organ of Corti.

Prenatal cocaine exposure accelerates morphological changes and transient expression of tyrosine hydroxylase in the cochlea of developing rats

Prenatal cocaine exposure causes alterations in auditory brainstem response in children and experimental animals and has adverse effects on auditory information processing and language skills in children. These effects may result from lesions in the cochlea since this organ is particularly sensitive to chemical insults during the development. We have thus studied here the effect of prenatal cocaine exposure on the maturation of the rat cochlea using the transient non-catecholaminergic expression of tyrosine hydroxylase in spiral ganglion neurons as an index of cochlear maturation and morphometry to evaluate the maturation of primary auditory neurons and the organ of Corti. We showed that prenatal cocaine exposure accelerated the cochlear maturation. In the basal coil of cochleas from PND8 cocaine-treated pups, the Kölliker's organ had disappeared, the tunnel of Corti was opened, and the stria vascularis no longer contained undifferentiated marginal cells. The maximum expression of tyrosine hydroxylase in type I primary auditory neurons occurred at PND8 instead of PND12 in pair-fed controls. On the other hand, the prenatal cocaine exposure had no effect on the width and height of the organ of Corti, spiral ganglion volume and number and size of primary auditory neurons. In conclusion, our data suggest that prenatal cocaine exposure, though not lethal to primary auditory neurons, accelerates aspects of the cochlear sensorineural maturation. This accelerated cochlear maturation in cocaine-treated rat pups could cause auditory dysfunctions by desynchronizing the development of the whole auditory pathway.

Tyrosine hydroxylase expression in the olfactory/respiratory epithelium in early sheep fetuses (Ovis aries)

Transient expression of tyrosine hydroxylase (TH, the first enzyme in catecholamine synthesis) has been shown in different brain and peripheral structures of various species. TH-immunoreactive neurons have been reported in the nasal region of human and rat fetuses migrating to the forebrain with GnRH neurons during embryogenesis. In the present study, immunohistochemical analysis and in situ hybridization were performed in fetal sheep and in vitro sheep embryo olfactory placode cultures to confirm this population in this species. On embryonic days 33 to 35, TH-immunoreactive cells as well as TH cDNA-hybridized cells were found in the olfactory and respiratory epithelium and were spatially separated from GnRH-immunoreactive neurons. In days 40 to 44 of gestation, TH-immunoreactive neurons were no longer observed in the olfactory epithelium, and TH-immunoreactive fibers were found on the trajectories of the olfactory nerves. At this stage, some TH-immunoreactive fibers were also labeled for GnRH. TH-immunoreactive cells were also found in primary cultures of olfactory placodes of fetal sheep at 10 to 18 days in vitro. Some of them coexpressed GnRH. These results imply that olfactory epithelium is also able to give rise to TH expressing cells in fetal sheep, but this expression is suppressed earlier in ontogenesis than in humans due to some unidentified factors not present in the primary cultures of olfactory placode. The role of TH expression remains unclear as in other previously described examples.

Ontogeny of tyrosine hydroxylase mRNA expression in mid- and forebrain: neuromeric pattern and novel positive regions

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of catecholamines and, thus, critical in determining the catecholaminergic phenotype. In this study, we have examined the expression of TH mRNA by in situ hybridization in the embryonic mouse forebrain and midbrain and have mapped its localization according to the neuromeric pattern. We find that early in embryonic development, 10 to 12 days post coitum (dpc), TH mRNA is expressed in ample continuous regions of the neuroepithelium, extending across several neuromeres. However, from 12.5 dpc onward, the expression becomes restricted to discrete regions, which correspond to the dopaminergic nuclei (A8 to A15). In addition to these nuclei previously described, TH mRNA is also observed in regions that do not express this enzyme according to immunohistochemical studies. This difference in relation to protein expression pattern is consequent with the known posttranscriptional regulation of TH expression. The most representative example of a novel positive region is the conspicuous mRNA expression in both medial and lateral ganglionic eminences. This result agrees with reports describing the capacity of striatal stem cells (that is, located at the lateral ganglionic eminence) to become dopaminergic in vitro. Other regions include the isthmic mantle layer and the early floor plate of the midbrain-caudal forebrain. On the whole, the expression map we have obtained opens new perspectives for evolutionary/comparative studies, as well as for therapeutic approaches looking for potentially dopaminergic cells. Developmental Dynamics 234:709-717, 2005. (c) 2005 Wiley-Liss, Inc.