Different calcium-binding proteins identify subpopulations of vestibular ganglion neurons in the rat

Calretinin Immunoreactivity in the VIIIth Nerve and Inner Ear Endorgans of Ranid Frogs

Calcium-binding proteins are essential for buffering intracellular calcium concentrations, which are critical for regulating cellular processes involved in neuronal computations. One such calcium-binding protein, calretinin, is present in many neurons of the central nervous system as well as those which innervate cranial sensory organs, although often with differential distributions in adjacent cellular elements. Here, we determined the presence and distribution of calretinin-immunoreactivity in the peripheral vestibular and auditory system of ranid frogs. Calretinin-immunoreactivity was observed in ganglion cells innervating the basilar and amphibian papilla, and in a subpopulation of ganglion cells innervating the saccular epithelium. In contrast, none of the ganglion cells innervating the lagena, the utricle, or the three semicircular canals were calretinin-immunopositive, suggesting that this calcium-binding protein is a marker for auditory but not vestibular afferent fibers in the frog. The absence of calretinin in vestibular ganglion cells corresponds with the lack of type I hair cells in anamniote vertebrates, many of which in amniotes are contacted by the neurites of large, calyx-forming calretinin-immunopositive ganglion cells. In the sensory epithelia of all endorgans, the majority of hair cells were strongly calretinin-immunopositive. Weakly calretinin-immunopositive hair cells were distributed in the intermediate region of the semicircular canal cristae, the central part of the saccular macula, the utricular, and lagenar striola and the medial part of the amphibian papilla. The differential presence of calretinin in the frog vestibular and auditory sensory periphery might reflect a biochemical feature related to firing patterns and frequency bandwidths of self-motion versus acoustic stimulus encoding, respectively.

Characterization of Adult Vestibular Organs in 11 CreER Mouse Lines

Utricles are vestibular sense organs that encode linear head movements. They are composed of a sensory epithelium with type I and type II hair cells and supporting cells, sitting atop connective tissue, through which vestibular nerves project. We characterized utricular Cre expression in 11 murine CreER lines using the ROSA26^tdTomato reporter line and tamoxifen induction at 6 weeks of age. This characterization included Calbindin2^CreERT2, Fgfr3-iCreER^T2, GFAP-A-CreER™, GFAP-B-CreER™, GLAST-CreER^T2, Id2^CreERT2, Otoferlin^CreERT2, Parvalbumin^CreERT2, Prox1^CreERT2, Sox2^CreERT2, and Sox9-CreER^T2. Otoferlin^CreERT2 mice had inducible Cre activity specific to hair cells. GLAST-CreER^T2, Id2^CreERT2, and Sox9-CreER^T2 had inducible Cre activity specific to supporting cells. Sox2^CreERT2 had inducible Cre activity in supporting cells and most type II hair cells. Parvalbumin^CreERT2 mice had small numbers of labeled vestibular nerve afferents. Calbindin2^CreERT2 mice had labeling of most type II hair cells and some type I hair cells and supporting cells. Only rare (or no) tdTomato-positive cells were detected in utricles of Fgfr3-iCreER^T2, GFAP-A-CreER™, GFAP-B-CreER™, and Prox1^CreERT2 mice. No Cre leakiness (tdTomato expression in the absence of tamoxifen) was observed in Otoferlin^CreERT2 mice. A small degree of leakiness was seen in GLAST-CreER^T2, Id2^CreERT2, Sox2^CreERT2, and Sox9-CreER^T2 lines. Calbindin2^CreERT2 mice had similar tdTomato expression with or without tamoxifen, indicating lack of inducible control under the conditions tested. In conclusion, 5 lines-GLAST-CreER^T2, Id2^CreERT2, Otoferlin^CreERT2, Sox2^CreERT2, and Sox9-CreER^T2-showed cell-selective, inducible Cre activity with little leakiness, providing new genetic tools for researchers studying the vestibular periphery.

Species Differences in the Organization of the Ventral Cochlear Nucleus

The mammalian cochlear nuclei (CN) consist of two major subdivisions, the dorsal (DCN) and ventral (VCN) nuclei. We previously reported differences in the structural and neurochemical organization of the human DCN from that in several other species. Here we extend this analysis to the VCN, considering both the organization of subdivisions and the types and distributions of neurons. Classically, the VCN in mammals is composed of two subdivisions, the anteroventral (VCA) and posteroventral cochlear nuclei (VCP). Anatomical and electrophysiological data in several species have defined distinct neuronal types with different distributions in the VCA and VCP. We asked if VCN subdivisions and anatomically defined neuronal types might be distinguished by patterns of protein expression in humans. We also asked if the neurochemical characteristics of the VCN are the same in humans as in other mammalian species, analyzing data from chimpanzees, macaque monkeys, cats, rats and chinchillas. We examined Nissl- and immunostained sections, using antibodies that had labeled neurons in other brainstem nuclei in humans. Nissl-stained sections supported the presence of both VCP and VCA in humans and chimpanzees. However, patterns of protein expression did not differentiate classes of neurons in humans; neurons of different soma shapes and dendritic configurations all expressed the same proteins. The patterns of immunostaining in macaque monkey, cat, rat, and chinchilla were different from those in humans and chimpanzees and from each other. The results may correlate with species differences in auditory function and plasticity. Anat Rec, 301:862-886, 2018. © 2017 Wiley Periodicals, Inc.

Distribution of 5-HT1F Receptors in Monkey Vestibular and Trigeminal Ganglion Cells

Background

Evidence of serotonergic involvement in vestibular pathway contributions to migraine and balance disorders is compelling. Serotonergic 5-HT_1B and 5-HT_1D receptors are expressed extensively in inner ear ganglia of monkeys and rats. The serotonergic 5-HT_1F receptor is also a target of triptans. This study describes its distribution in vestibular and trigeminal ganglia of monkeys.

Methods

Using primary polyclonal antibodies raised against oligopeptides specific for the human 5-HT_1F receptor, neuronal somatic area and intensity of immunoreactive vestibular and trigeminal ganglia were quantified.

Results and Discussion

Virtually all vestibular and considerable trigeminal ganglia showed positive 5-HT_1F receptor immunoreactivity. Inferior and superior vestibular ganglia staining appeared confined to distinct cell regions, varying considerably among cells of different sizes: more intense in small, punctate in some medium and regionally polarized in some large cells. Analyses of average somatic vestibular neuronal immunoreactive intensity identified mainly medium sized cells with high standard deviation of intensity corresponding to punctately stained cells. Less variability occurred in somatic intensity staining and cellular distribution among 5-HT_1F receptor immunopositive trigeminal ganglia. Most exhibited similar punctate staining patterns, higher mean somatic immunoreactive intensity and larger neuronal somatic size proportions per size distribution subpopulation compared to vestibular ganglia size distribution populations. Centrally directed vestibular ganglion neuronal processes, cochlear inner hair cells, vestibular hair cells and blood vessels in vestibular maculae and cristae were immunoreactive. The 5-HT_1F receptor expression in vestibular ganglia shows complex variable staining intensity patterns associated with cell size of immunopositive neurons, not seen in immunopositive trigeminal ganglia and not previously evident with 5-HT_1B and 5-HT_1D receptor subtype immunoreactivity in vestibular ganglia. These data motivate exploration of 5-HT₁ receptor oligomerization and ligand functional selectivity in differential serotonergic involvement in co-morbidity of migraine and balance disorders. Similar findings in cochlear inner hair cell afferents are applicable to migraine-related tinnitus or hypercusis (phonophobia).

Implementation of linear sensory signaling via multiple coordinated mechanisms at central vestibular nerve synapses

Signal transfer in neural circuits is dynamically modified by the recent history of neuronal activity. Short-term plasticity endows synapses with nonlinear transmission properties, yet synapses in sensory and motor circuits are capable of signaling linearly over a wide range of presynaptic firing rates. How do such synapses achieve rate-invariant transmission despite history-dependent nonlinearities? Here, ultrastructural, biophysical, and computational analyses demonstrate that concerted molecular, anatomical, and physiological refinements are required for central vestibular nerve synapses to linearly transmit rate-coded sensory signals. Vestibular synapses operate in a physiological regime of steady-state depression imposed by tonic firing. Rate-invariant transmission relies on brief presynaptic action potentials that delimit calcium influx, large pools of rapidly mobilized vesicles, multiple low-probability release sites, robust postsynaptic receptor sensitivity, and efficient transmitter clearance. Broadband linear synaptic filtering of head motion signals is thus achieved by coordinately tuned synaptic machinery that maintains physiological operation within inherent cell biological limitations.

Vestibular nuclei characterized by calcium-binding protein immunoreactivity and tract tracing in Gekko gecko

Immunohistochemical techniques were used to describe the distribution of the calcium binding proteins calretinin, calbindin and parvalbumin as well as synaptic vesicle protein 2 in the vestibular nuclei of the Tokay gecko (Gekko gecko). In addition, tract tracing was used to investigate connections between the vestibular nerves and brainstem nuclei. Seven vestibular nuclei were recognized: the nuclei cerebellaris lateralis (Cerl), vestibularis dorsolateralis (Vedl), ventrolateralis (Vevl), ventromedialis (Vevm), tangentialis (Vetg), ovalis (VeO) and descendens (Veds). Vestibular fibers entered the brainstem with the ascending branch projecting to Vedl and Cerl, the lateral descending branch to Veds, and the medial descending branch to ipsilateral Vevl. Cerl lay most rostral, in the cerebellar peduncle. Vedl, located rostrally, was ventral to the cerebellar peduncle, and consisted of loosely arranged multipolar and monopolar cells. Vevl was found at the level of the vestibular nerve root and contained conspicuously large cells and medium-sized cells. Veds is a large nucleus, the most rostral portion of which is situated lateral and ventral to Vevl, and occupies much of the dorsal brainstem extending caudally through the medulla. VeO is a spherically shaped cell group lateral to the auditory nucleus magnocellularis and dorsal to the caudal part of Vevl. Vevm and Vetg were small in the present study. Except for VeO, all other vestibular nuclei appear directly comparable to counterparts in other reptiles and birds based on their location, cytoarchitecture, and connections, indicating these are conserved features of the vestibular system.

Ion channels in mammalian vestibular afferents may set regularity of firing

Rodent vestibular afferent neurons offer several advantages as a model system for investigating the significance and origins of regularity in neuronal firing interval. Their regularity has a bimodal distribution that defines regular and irregular afferent classes. Factors likely to be involved in setting firing regularity include the morphology and physiology of the afferents' contacts with hair cells, which may influence the averaging of synaptic noise and the afferents' intrinsic electrical properties. In vitro patch clamp studies on the cell bodies of primary vestibular afferents reveal a rich diversity of ion channels, with indications of at least two neuronal populations. Here we suggest that firing patterns of isolated vestibular ganglion somata reflect intrinsic ion channel properties, which in vivo combine with hair cell synaptic drive to produce regular and irregular firing.

Sox2 and JAGGED1 expression in normal and drug-damaged adult mouse inner ear

Inner ear hair cells detect environmental signals associated with hearing, balance, and body orientation. In humans and other mammals, significant hair cell loss leads to irreversible hearing and balance deficits, whereas hair cell loss in nonmammalian vertebrates is repaired by the spontaneous generation of replacement hair cells. Research in mammalian hair cell regeneration is hampered by the lack of in vivo damage models for the adult mouse inner ear and the paucity of cell-type-specific markers for non-sensory cells within the sensory receptor epithelia. The present study delineates a protocol to drug damage the adult mouse auditory epithelium (organ of Corti) in situ and uses this protocol to investigate Sox2 and Jagged1 expression in damaged inner ear sensory epithelia. In other tissues, the transcription factor Sox2 and a ligand member of the Notch signaling pathway, Jagged1, are involved in regenerative processes. Both are involved in early inner ear development and are expressed in developing support cells, but little is known about their expressions in the adult. We describe a nonsurgical technique for inducing hair cell damage in adult mouse organ of Corti by a single high-dose injection of the aminoglycoside kanamycin followed by a single injection of the loop diuretic furosemide. This drug combination causes the rapid death of outer hair cells throughout the cochlea. Using immunocytochemical techniques, Sox2 is shown to be expressed specifically in support cells in normal adult mouse inner ear and is not affected by drug damage. Sox2 is absent from auditory hair cells, but is expressed in a subset of vestibular hair cells. Double-labeling experiments with Sox2 and calbindin suggest Sox2-positive hair cells are Type II. Jagged1 is also expressed in support cells in the adult ear and is not affected by drug damage. Sox2 and Jagged1 may be involved in the maintenance of support cells in adult mouse inner ear.

The anatomy of the vestibular nuclei

The vestibular portion of the eighth cranial nerve informs the brain about the linear and angular movements of the head in space and the position of the head with respect to gravity. The termination sites of these eighth nerve afferents define the territory of the vestibular nuclei in the brainstem. (There is also a subset of afferents that project directly to the cerebellum.) This chapter reviews the anatomical organization of the vestibular nuclei, and the anatomy of the pathways from the nuclei to various target areas in the brain. The cytoarchitectonics of the vestibular brainstem are discussed, since these features have been used to distinguish the individual nuclei. The neurochemical phenotype of vestibular neurons and pathways are also summarized because the chemical anatomy of the system contributes to its signal-processing capabilities. Similarly, the morphologic features of short-axon local circuit neurons and long-axon cells with extrinsic projections are described in detail, since these structural attributes of the neurons are critical to their functional potential. Finally, the composition and hodology of the afferent and efferent pathways of the vestibular nuclei are discussed. In sum, this chapter reviews the morphology, chemoanatomy, connectivity, and synaptology of the vestibular nuclei.

Selective gentamicin uptake by cytochemical subpopulations of guinea-pig geniculate ganglion cells

Cytochemical subpopulations of geniculate ganglion (GG) cells were identified in guinea-pigs using immunohistochemistry and selective gentamicin accumulation. Two subpopulations of GG cells were evident based upon their location and immunoreactivity for peptide 19 (PEP 19), for plasma membrane Ca2+-ATPase (PMCA-ATPase), and for neurofilament proteins. Cells within the posterior part of GG were positive for PEP 19 and PMCA-ATPase, but not for 68 kD or 160 kD neurofilament proteins. Cells within the anterior part showed complementary staining properties. Cells within these populations showed differences in accumulation of gentamicin, depending upon the administration route. Cells within the posterior part showed avid accumulation of gentamicin when animals received the drug systemically. When the drug was administered directly into the middle ear, cells within the anterior part showed avid gentamicin accumulation. Immunostaining for gentamicin in both cell populations was much more extreme and remained so for longer post-administration times when compared with spiral ganglion and vestibular ganglion cells. The results suggest that cells in the anterior part of GG have little exposure to gentamicin in the serum and that perhaps they innervate the middle ear mucosa or they absorb the drug through their axons within the middle ear. In contrast, cells in the posterior part of GG have greater access to systemically administered gentamicin either directly or via their axon terminals.

Comparative morphology of rodent vestibular periphery. I. Saccular and utricular maculae

Calyx afferents, a group of morphologically and physiologically distinct afferent fibers innervating the striolar region of vestibular sensory epithelia, are selectively labeled by antibodies to the calcium-binding protein calretinin. In this study, the population of calretinin-stained calyx afferents was used to delineate and quantify the striolar region in six rodent species: mouse, rat, gerbil, guinea pig, chinchilla, and tree squirrel. Morphometric studies and hair cell and calyx afferent counts were done. Numbers of hair cells, area, length, and width of the sensory epithelium increase from mouse to tree squirrel. In the mouse and rat, calretinin is found in 5-9% of all type I hair cells, 20-40% of striolar type II hair cells, and 70-80% of extrastriolar type II hair cells. Numbers of calyx afferents increase from mouse to squirrel, with more complex calyx afferents in larger species. About 10% of calyx afferents are branched. Based on our counts of total numbers of calyx afferents in chinchilla maculae and in comparison to fiber counts in the literature, the proportion of calyx afferents is greater than previously described, constituting nearly 20% of the total. Because morphometric measures increase with body weight, we obtained additional data on vestibular end organ surface areas from the literature and used this to construct a power law function describing this relationship. The function holds for species with body weights less than approximately 4 kg. Greater than 4 kg, the surface area of the sensory epithelia remains constant even with increasing body weight.

Calcium-binding proteins map the postnatal development of rat vestibular nuclei and their vestibular and cerebellar projections

We investigated whether three calcium-binding proteins, calretinin, parvalbumin, and calbindin, could identify specific aspects of the postnatal development of the rat lateral (LVN) and medial (MVN) vestibular nuclei and their vestibular and cerebellar connections. Calretinin levels in the vestibular nuclei, increased significantly between birth and postnatal day (P) 45. In situ hybridization and immunocytochemical staining showed that calretinin-immunoreactive neurons were mostly located in the parvocellular MVN at birth and that somatic and dendritic growth occurred between birth and P14. During the first week, parvalbumin-immunoreactive fibers and endings were confined to specific areas, i.e., the ventral LVN and magnocellular MVN, and identified exclusively the maturation of the vestibular afferents. Calbindin was located within the dorsal LVN and the parvocellular MVN and identified the first arrival of the corticocerebellar afferents. From the second week, in addition to labeling vestibular afferents in their specific target areas, parvalbumin was also found colocalized with calbindin in mature Purkinje cell afferents. Thus, the specific spatiotemporal distribution of parvalbumin and calbindin could correspond to two successive phases of synaptic remodeling involving integration of the vestibular sensory messages and their cerebellar control. On the basis of the sequence of distribution patterns of these proteins during the development of the vestibular nuclei, calretinin is an effective marker for neuronal development of the parvocellular MVN, parvalbumin is a specific marker identifying maturation of the vestibular afferents and endings, and calbindin is a marker of the first appearance and development of Purkinje cell afferents.

Molecular probes of the vestibular nerve. I. Peripheral termination patterns of calretinin, calbindin and peripherin containing fibers

Vestibular afferents have different physiological properties that can be at least partially correlated with the morphology that the peripheral ending makes with type I and type II hair cells. If the location of the ending in the sensory epithelium is included, the correlations are further improved. It is also known that vestibular afferents can be immunohistochemically stained for a variety of different substances. We have concentrated on three of these markers, calretinin, calbindin and peripherin, because the sources of afferents to the vestibular nuclear complex that contain these substances are restricted, in two cases to the primary afferents. We demonstrate that calretinin is found only in the calyx-only afferents that are located at the apex of the cristae ampullaris and along the striola of the maculae. The area containing stained calyces is equal to or slightly smaller than the central zone of the cristae as defined by the Goldberg group [J. Neurophysiol. 60 (1988) 167]. Calbindin is also found in calyces at the apex of the cristae and along the striola of the otoliths. Examination of adjacent sections of all endorgans indicates that calbindin staining overlaps with calretinin, but is always several hair cells wider on each side. Peripherin also stains fibers in the neuroepithelium. The greatest density of staining is in the peripheral zone of the cristae, i.e. at the base and toward the planum semilunatum. We suggest that these substances are useful markers for specific sets of vestibular afferents.

Molecular probes of the vestibular nerve. II. Characterization of neurons in Scarpa's ganglion to determine separate populations within the nerve

An unambiguous delineation of the exact numbers and/or proportions of calyx-only, dimorph, and bouton-only vestibular afferents is needed to continue studies concerning vestibular integration in the nervous system. Here, we take advantage of immunocytochemical properties of three groups of vestibular afferents. We utilize calretinin to delineate the calyx-only population, and peripherin to stain the bouton-only afferents. An additional subgroup of afferents that stain with calbindin, but not calretinin is also introduced. The size of the cells that stain with these markers was determined. Cells that are calbindin-positive overlap the sizes of Nissl-stained somata. Cells that stain with peripherin or calretinin are non-overlapping with calretinin cells being the largest and peripherin-positive cells the smallest. Twenty percent of the ganglion cells were peripherin positive, another 20% stained with calretinin antibodies, 30% stained with calbindin, and all cells in Scarpa's ganglion stained with parvalbumin. Most of the calretinin-positive cells also stained with calbindin. One-third of the calbindin-positive population stained only with calbindin. These studies indicate that the calyx- and bouton-only populations of vestibular afferents in gerbil comprise at least 40% of the nerve. In addition, at least 10% of the nerve also stains with calbindin and neither calretinin nor peripherin. Based on indirect evidence, we hypothesize that these are a subpopulation of dimorph afferents. This study has provided an anatomical instrument (in addition to intracellular physiological methods) to study separate populations of vestibular afferents.

Afferent diversity and the organization of central vestibular pathways

This review considers whether the vestibular system includes separate populations of sensory axons innervating individual organs and giving rise to distinct central pathways. There is a variability in the discharge properties of afferents supplying each organ. Discharge regularity provides a marker for this diversity since fibers which differ in this way also differ in many other properties. Postspike recovery of excitability determines the discharge regularity of an afferent and its sensitivity to depolarizing inputs. Sensitivity is small in regularly discharging afferents and large in irregularly discharging afferents. The enhanced sensitivity of irregular fibers explains their larger responses to sensory inputs, to efferent activation, and to externally applied galvanic currents, but not their distinctive response dynamics. Morphophysiological studies show that regular and irregular afferents innervate overlapping regions of the vestibular nuclei. Intracellular recordings of EPSPs reveal that some secondary vestibular neurons receive a restricted input from regular or irregular afferents, but that most such neurons receive a mixed input from both kinds of afferents. Anodal currents delivered to the labyrinth can result in a selective and reversible silencing of irregular afferents. Such a functional ablation can provide estimates of the relative contributions of regular and irregular inputs to a central neuron's discharge. From such estimates it is concluded that secondary neurons need not resemble their afferent inputs in discharge regularity or response dynamics. Several suggestions are made as to the potentially distinctive contributions made by regular and irregular afferents: (1) Reflecting their response dynamics, regular and irregular afferents could compensate for differences in the dynamic loads of various reflexes or of individual reflexes in different parts of their frequency range; (2) The gating of irregular inputs to secondary VOR neurons could modify the operation of reflexes under varying behavioral circumstances; (3) Two-dimensional sensitivity can arise from the convergence onto secondary neurons of otolith inputs differing in their directional properties and response dynamics; (4) Calyx afferents have relatively low gains when compared with irregular dimorphic afferents. This could serve to expand the stimulus range over which the response of calyx afferents remains linear, while at the same time preserving the other features peculiar to irregular afferents. Among those features are phasic response dynamics and large responses to efferent activation; (5) Because of the convergence of several afferents onto each secondary neuron, information transmission to the latter depends on the gain of individual afferents, but not on their discharge regularity.

Unbiased number of vestibular ganglion neurons in the mouse

Vestibular sensory information from the labyrinth and otolith organs is conducted to the central nervous system exclusively via primary vestibular afferents (PVA) originating from neurons located in the vestibular ganglion (VG). In the present study, the total number of VG neurons was determined in two different wild-type mouse strains using the principles of unbiased stereological counting methods by means of the physical disector. 3316 (+/-225 SD) neurons were present in the VG of the B6CBA-strain and 3551 (+/-239 SD) in C57BL/6J-mice. Since no statistical difference was detected between the two strains, the pooled mean number was 3433 (+/-232 SD) neurons. This is the first unbiased estimate of VG neurons aimed at providing a numerical basis for comparative studies and for the impact of experimental, pharmacological and pathological conditions as well as ageing on the survival and maintenance of VG neurons.

AMPA receptors in cultured vestibular ganglion neurons: detection and activation

The presence and the activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptors were investigated in mouse cultured vestibular ganglion neurons using immunocytochemistry and measurement of intracellular calcium concentration ([Ca2+]i) by spectrofluorimetry. Cultures of dissociated vestibular ganglia from 18 gestation day mouse embryos were grown in vitro for 3-4 days. Immunocytochemical labelling of AMPA receptor subunits GluR2/R3 and GluR4 was detected in neuron cell bodies and proximal neurites and more lightly in glial cells. There was no clear selective subcellular localization of the different subunits. For the GluR1 subunit a signal was observed only in some neurons and neurites and was weak. Vestibular ganglion neurons responded to fast application of 1 mM glutamate and 10 mM aspartate through unknown receptors by a transient increase in [Ca2+]i. The mean amplitude of this rapid increase was about nine times the resting level and recovery was complete within 30-45 s after the application. If separated by an interval of at least 10 min, consecutive applications produced similar calcium responses. AMPA (1 mM) application induced the same type of responses. Five minutes prior to the AMPA exposure, the application of a specific AMPA antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX, 1.5 mM), in the external medium inhibited the response to AMPA. Chelation of external calcium by EGTA (1.5 mM) abolished the responses to drug applications, indicating that an influx of external calcium is involved in the [Ca2+]i increase. These observations suggest that heteromeric AMPA receptors are expressed in vestibular ganglion neurons in culture and play a functional role in their glutamate-induced depolarization. Experiments are in progress using specific AMPA and NMDA antagonists to characterize the participation of the two types of ionotropic glutamate receptors in the glutamate/aspartate-induced intracellular calcium response.

Immunolocalization of peptide 19 and other calcium-binding proteins in the guinea pig cochlea

Calcium ions are known to play critical roles in a variety of cochlear functions. The distributions of a number of calcium binding proteins that regulate calcium ion levels within the cochlea have previously been described. In this report we extend and refine previous reports of the distribution of immunostaining for calmodulin, calbindin, and calretinin and show for the first time the distribution for peptide 19. There were longitudinal and radial gradients of immunostaining for peptide 19 within outer hair cells that appeared to match previously described gradients of efferent innervation of these cells. Gradients of immunostaining for calbindin within outer hair cells were in the opposite directions, which suggests that levels of this protein are correlated with afferent innervation density and perhaps the abundance of subsurface cisternae. No gradients were seen in the distributions of cells stained for calmodulin and calretinin, which included sensory cells and supporting cells respectively. All ganglion cells were stained for calmodulin but the other proteins appeared to be present in limited ganglion cell subpopulations. In addition to staining of sensorineural elements, antisera to all compounds but peptide 19 showed immunostaining of cells within the lateral wall and the spiral limbus. The results suggest that the proteins under study are involved in a wide variety of calcium-regulated functions within the cochlea. Knowledge of the unique distribution of each of the compounds should facilitate further studies of their roles in cochlear function.

Pattern of selected calcium-binding proteins in the vestibular nuclear complex of two rodent species

The distribution of immunoreactivity to calbindin, calretinin, and parvalbumin in the vestibular nuclear complex and the adjacent nucleus prepositus hypoglossi was studied in rats and gerbils. The distribution of stained fibers was the same for both rodent species. All three calcium-binding proteins were present in vestibular afferents. However, none of the three proteins were present in all afferent fibers. Many fibers were labeled in the vestibular nerve and in fascicles of the descending vestibular nucleus, as well as ascending fibers in the superior vestibular nucleus and fibers directed to the medial vestibular nucleus. Labeled terminals were present in the medial vestibular nucleus, especially along the ventricular border, in the neuropil of the superior vestibular nucleus, and scattered in the descending and ventral portions of the lateral vestibular nucleus. Calbindin- and parvalbumin-positive terminals, but not calretinin-positive terminals, were present in the neuropil of the dorsal lateral vestibular nucleus, especially surrounding the large neuronal somas. Some of these terminals are presumably from cerebellar Purkinje cells, which were also labeled by both antibodies. The pattern of parvalbumin immunoreactivity was slightly different from that of calbindin, indicating that parvalbumin might be contained in additional fibers. Some neurons in the vestibular nuclear complex were labeled with antibodies to calretinin, but few cells were stained with either calbindin or parvalbumin antibodies. The largest group of calretinin-positive cells was a cluster of small- to medium-sized neurons located in a densely stained mesh of dendrites and terminals in the medial vestibular nucleus, adjacent to the ventricular border.

Cellular and subcellular localization of AMPA-selective glutamate receptors in the mammalian peripheral vestibular system

The cellular and subcellular distribution of AMPA-selective glutamate receptors in the mammalian peripheral vestibular system was examined using antibodies against peptides corresponding to the C-terminal portions of AMPA receptor subunits: GluR1, GluR2/R3 and GluR4. The light and electron microscopic immunocytochemical studies were carried out on Vibratome sections of rat and guinea pig vestibular sensory epithelial and ganglia. In the epithelium, GluR1 subunit immunoreactivity appeared as accumulations of patches outlining the baso-lateral periphery of the type I sensory cells. The GluR1-immunoreactive microareas were postsynaptically distributed on the membranes of calyceal afferent fibers. GluR2/R3 immunoreactivity was present in the sensory cells. GluR4 was not detected. In the vestibular ganglion, the neurons were densely stained with antibodies to GluR2/R3 and GluR4. The fibroblasts and the Schwann cells were also intensely stained with antibodies to GluR2/R3 and GluR4. In the sensory cells, the AMPA receptors, GluR2/R3, may function as (1) autoreceptors controlling afferent neurotransmitter release or (2) 'postsynaptic' receptors activated by the neurotransmitter release of the afferent calyx. The detection of GluR1 at postsynaptic sites in the afferent fibers provides anatomical evidence for the role of glutamate as a neurotransmitter of sensory cells. In the ganglion neurons, GluR2/R3 and GluR4 may represent reserve intracytoplasmic pools of receptor subunits in transit to the postsynaptic sites. In the Schwann cells, GluR2/R3 and GluR4 may be involved in neuronal-glial signalling at the nodes of Ranvier.

Distribution of calretinin mRNA in the vestibular nuclei of rat and guinea pig and the effects of unilateral labyrinthectomy: a non-radioactive in situ hybridization study

We examined the localization of neurons expressing mRNA for calretinin, a cytosolic EF hand calcium-binding protein, throughout the vestibular nuclei of rat and guinea pig by non-radioactive in situ hybridization, using an alkaline phosphatase labeled oligonucleotide probe. Labeled cells were particularly numerous in the medial vestibular nucleus (mVN) and their distribution was similar in rat and guinea pig, and presented a characteristic rostrocaudal and mediolateral pattern. The effects of hemilabyrinthectomy were assessed at various times post lesion from 10 h to 30 days by comparison of the pattern of labeling in the ipsi- and contra-lateral vestibular nuclei of guinea pig. After up to 48 h no modification in the calretinin mRNA distribution was detected. After 3 to 30 days of survival, there was a decrease (about 30%) of the calretinin expressing neurons in the nucleus on the side of the lesion. The unilateral sensory deprivation seemed to induce a permanent asymmetry in the expression of calretinin which was not abolished after vestibular compensation. These results suggested that the calretinin expression in these neurons depends upon the integrity and activity of sensorineuronal peripheral vestibular influences.

Size-related colocalization of glycine and glutamate immunoreactivity in frog and rat vestibular afferents

Presence and distribution of glutamate, glycine, GABA and beta-alanine in VIIIth nerves of frogs and rats were investigated with postembedding immunocytochemical methods on serial semithin sections. In Scarpa's ganglion of the frog, all cell bodies were glutamate immunoreactive. About 17% of the cells per section were also glycine immunoreactive, but none were GABA or beta-alanine immunoreactive. The mean diameter of glycine-positive cell bodies (26.7 +/- 6.9 microns; N = 130) was significantly (P < 0.0001) larger than that of glycine-negative cell bodies (15.7 +/- 5.4 microns; N = 272). The intensity of glutamate immunostaining decreased with cell diameter, whereas the intensity of glycine immunostaining increased with cell diameter. As a result, the staining intensities for glutamate and glycine in a given cell were negatively correlated. Glycine immunoreactivity was also present in a size-related manner in distal and proximal afferent fibers. The majority of thin fibers (< 4 microns) was glycine negative, whereas most of the thick fibers (> 10 microns) were glycine positive. Glycine-positive fibers were observed in the sensory epithelial of all end organs in the inner ear. The saccular macula and its nerve, however, contained only few glycine immunoreactive structures. In Scarpa's ganglion of the rat, all cells were immunoreactive for glutamate, about 12% for colocalized glycine, and none for GABA or beta-alanine. Glycine-positive cell bodies were significantly (P < 0.0001) larger (32.2 +/- 5.2 microns; N = 82) than glycine-negative cell bodies (25.1 +/- 5.3 microns; N = 274). Cell bodies in the spiral ganglion were only glutamate immunoreactive, whereas staining for glutamate, glycine, and GABA was dense in the ventral cochlear nucleus. These results demonstrate that thicker vestibular afferent fibers represent a particular subpopulation that differs from the majority of thinner afferents due to their glycine immunoreactivity.

Development of calretinin immunoreactivity in the mouse inner ear

Calretinin is a calcium-binding protein of the EF-hand family. It has been previously identified in particular cell types of adult guinea pig, rat, and chinchilla inner ear. Development of calretinin immunoreactivity in the mouse inner ear was investigated from embryonic day 13 (E13) to the adult stage. In the adult mouse vestibule, calretinin immunoreactivity was present in the same structures as described for the rat and guinea pig: the population of afferent fibers forming calyx units and a small number of ganglion neurons. The earliest immunoreactivity was found at E17 in vestibular hair cells (VHCs), then, at E19, in afferent fibers entering the sensory epithelia and in rare ganglion neurons. At postnatal day 4 (P4), a few vestibular nerve fibers and ganglion neurons were reactive. From this stage until P14, immunoreactivity developed in the calyx units and disappeared from VHCs. At P14, immunostaining was adult-like. In the adult mouse cochlea, immunoreactivity was present in the same cell populations as described in the rat: the inner hair cells (IHCs) and most of Corti's ganglion neurons. Calretinin immunoreactivity appeared at E19-P0 in IHCs and ganglion neurons of the basal turn. At P1, outer hair cells (OHCs) of the basal turn were positive. Calretinin immunoreactivity then appeared in IHCs, OHCs, and ganglion neurons of the medial turn, then of the apical turn. At P4, all IHCs and OHCs and most of the ganglion neurons were immunostained. Immunoreactivity gradually disappeared from the OHCs starting at P10 and, at P22, only IHCs and ganglion neurons were positive. The sequences of appearance of calretinin were specific to each cell type of the inner ear and paralleled their respective maturation. Calretinin was transiently expressed in VHCs and OHCs.

Characterization of different neuron populations in mouse statoacoustic ganglion cultures

Statoacoustic ganglion (SAG) cells were grown in primary culture and the appearance of different neuronal phenotypes was investigated. Analysis criteria were shape, size and staining for the immunocytochemical markers: neurofilament proteins (NF-200 kDa), neuron-specific enolase (NSE), calretinin, a calcium-binding protein and substance P, a neurotransmitter. Cultures were prepared from dissociated SAG cells of 13 gestation-day-old mouse embryos. Neurons were identified and counted after 7 days in vitro. At this stage, neurons were organized in small clusters forming an extensive network of neurites grown on a layer of fibroblasts and glia. Most neurons identified by NF or NSE immunoreactivity showed a typical adult-like bipolar profile. The diameters of the neurons were between 5.62 and 17.00 microns and displayed a normal distribution (mean: 10.6 microns). Two distinct subpopulations were identified by the expression of calretinin and substance P. Calretinin-immunoreactive neurons were large and very rare and had a mean diameter of 11.3 microns; the distribution of substance P was more extensive than that of calretinin and identified a population of small neurons with a mean diameter of 8.9 microns. The distributions of these two markers in SAG cultures were consistent with in vivo results. In conclusion, dissociated SAG cell cultures appear to be a suitable model for analyzing the development of the immunocytochemical and functional characteristics of the neurons of this inner ear ganglion.

Cellular distribution of parvalbumin immunoreactivity in the peripheral vestibular system of three rodents

The cellular distribution of parvalbumin immunoreactivity in the vestibular peripheral system of mouse, rat, and guinea pig was investigated by light and electron microscopy. Parvalbumin was found in all neurons of the vestibular ganglia of these species but in the sensory epithelia immunoreactivity was restricted to type I hair cells localized exclusively in the central areas. The very intense staining pattern was similar in the cristae ampullares and utricles of all three species but a faint immunoreaction was also detectable in sensory cells of peripheral areas of rat cristae. The parvalbumin-immunoreactive type I sensory cells are connected by nerve fibres of the calyx unit type which are known selectively to contain calretinin.

Localization of calretinin mRNA in rat and guinea pig inner ear by in situ hybridization using radioactive and non-radioactive probes

The localization of calretinin mRNA was studied in the rat and guinea pig inner ear by in situ hybridization, and compared to the distribution of the protein previously examined by immunocytochemistry. Radioactive and non-radioactive in situ hybridization (ISH) were performed using oligonucleotide probes labelled with 35S or digoxigenin. Radioactive ISH was more sensitive than non-radioactive ISH. In cochlear and vestibular ganglia, calretinin mRNA was localized in subpopulations of neurons with patterns of distribution similar to those shown by immunocytochemistry. By contrast, the observations in the sensory epithelia differed with the two techniques, ISH revealing less positive structures than immunocytochemistry. Rat inner hair cells and guinea pig inner hair cells, Hensen's cells and Deiters cells, which had been described strongly immunoreactive, appeared positive with radioactive but not with non-radioactive ISH. On the other hand, rat vestibular type II hair cells and guinea pig interdental cells of the spiral limbus which were faintly immunoreactive were not positive with both ISH techniques.