Biomedical subjects

E W Rubel

Publications and source records attributed to E W Rubel.

At least 19 recordsLinked to original sources

PubMed1992

Rapid changes in protein synthesis and cell size in the cochlear nucleus following eighth nerve activity blockade or cochlea ablation.

Destruction of the cochlea causes secondary changes in the central auditory pathway through transynaptic regulation. These changes appear to be mediated by an activity-dependent process and can be detected in the avian auditory system as early as 30 minutes after deafferentation. We compared the early changes in cochlear nucleus neurons following deafferentation by cochlea ablation with those seen following activity deprivation by perilymphatic tetrodotoxin (TTX) exposure. Protein synthesis and size of large spherical cells in the anteroventral cochlear nucleus (AVCN) of 14-day-old gerbils were measured during the first 48 hours after the manipulations. Both cochlea ablation and TTX produced a reliable decrease in protein synthesis by AVCN neurons (30-40%) by 1 hour. The magnitude of change in tritiated leucine incorporation was similar at all survival times, in both experimental groups. In contrast to the rapid changes in protein synthesis, the decrease in cell size was first evident 18 hours after TTX exposure and 48 hours after cochlea ablation. There was no significant change in protein synthesis or cell size in control groups at any of the survival times. These findings are consistent with changes in the avian auditory system in response to deafferentation and TTX exposure. Cochlea ablation and TTX exposure induced similar transneuronal changes, supporting the hypotheses that activity of auditory afferents in young mammals plays a regulatory role in the metabolism and morphology of their target neurons in the central auditory pathway, and that early changes following destruction of the peripheral receptor are due to reduction of activity-dependent interactions of presynaptic and postsynaptic cells.

Analysis of Variance↗

PubMed1992

Rapid growth of astrocytic processes in N. magnocellularis following cochlea removal.

Removal of the cochlea or pharmacological blockade of eighth nerve activity in young postnatal chickens results in rapid transneuronal cell death and atrophy in neurons of n. magnocellularis. The present experiments were designed to examine the influence of afferent input on astrocyte structure in n. magnocellularis. Young chickens were subjected to unilateral cochlea removal. At times ranging from 5 minutes to 72 hours later, the brainstems were histologically processed with a polyclonal antibody against glial fibrillary acidic protein (GFAP). A second group of chick brainstems was impregnated by a Golgi method 6 hours after unilateral cochlea removal and impregnated three-dimensional reconstructions were made of glial cells in n. magnocellularis (NM). Analyses of GFAP positive processes in NM revealed an observable increase in the number of astrocytic processes at the borders of the nucleus within 30 minutes of cochlea removal and a twofold increase in GFAP + glial processes by 6 hours. A secondary increase in the number and density of GFAP + processes occurred between 24 and 72 hours following cochlea removal, during the period of axonal degeneration, and transneuronal cell atrophy and death. Analyses of astrocytes impregnated by the Golgi method revealed that individual glial cells had increased their total process length and the number of processes by approximately twofold by 6 hours after cochlea removal. These results suggest that the structure of astrocytes is rapidly and dramatically influenced by the level of excitatory activity in a neuronal system. Furthermore, the similarity of results obtained with GFAP immunohistochemistry and three-dimensional reconstruction of astrocytes provides evidence that the short-term changes observed following cochlea removal represent the actual growth of glial processes. We speculate that modulations in glial processes as a function of afferent activity may act to influence synaptic efficacy.

Afferent Pathways↗

PubMed1992

Ultrastructure of hyaline, border, and vacuole cells in chick inner ear.

The sense organ for hearing in birds, the basilar papilla, is capable of replacing lost or damaged hair cells and supporting cells through regeneration. Potential candidates for precursor-cell populations include cells within the auditory receptor epithelium and nonsensory cells inferior to the sensory epithelium. Ultrastructural characteristics of hyaline cells, border cells, and vacuole cells, nonsensory cells which border or lie inferior to the receptor epithelium proper, were studied with transmission electron microscopy. Data were obtained from normal neonatal and adult chickens. Several rows of epithelial cells separate hyaline cells from inferiorly located organ supporting cells and hair cells. Ultrastructural characteristics and location of these epithelial cells differentiate them from organ supporting cells and hyaline cells; consequently, we have termed them "border cells." Synaptic specializations are observed between neural elements and border cells, and gap junctions are found between adjacent border cells, between border cells and neighboring organ supporting cells, and between juxtaposed border and hyaline cells. Hyaline cells, in contrast to border cells, are highly specialized. Dense bundles of filaments are present in hyaline cells from the basal one-half of the papilla, and an unusual structure, a rough tubular aggregate, is present in hyaline-cell cytoplasm. Pre- and postsynaptic specializations are observed between neural elements and hyaline cells, and gap-junctional complexes link neighboring hyaline cells. Vacuole cells lie inferior to the hyaline cells and rest on the inferior fibrocartilaginous plate. They are unspecialized morphologically. Their only remarkable morphological feature is the abundance of spherical vacuoles within their cytoplasmic matrix.

Animals↗

PubMed1992

Normal and abnormal pathfinding of facial nerve fibers in the chick embryo.

Development of the facial nerve was studied in normal chicken embryos and after surgical disruption of ingrowing sensory facial nerve fibers at 38-72 h of incubation. Disruption of facial nerve fibers by otocyst removal often induced a rostral deviation of the facial nerve and ganglion to the level of the trigeminal ganglion. Cell bodies of the geniculate ganglion trailed their deviating neurites and occupied an abnormal rostral position adjacent to the trigeminal ganglion. Deviating facial nerve fibers were labeled with the carbocyanine fluorescent tracer DiI in fixed tissue. Labeled fibers penetrated the cranium adjacent to the trigeminal ganglion, but they did not follow the trigeminal nerve fibers into the brain stem. Rather, after entering the cranium, they projected caudally to their usual site of entrance and proceeded towards their normal targets. This rostral deviation of the facial nerve was observed only after surgery at 48-72 h of incubation, but not in cases with early otocyst removal (38-48 h). A rostral deviation of the facial nerve was seen in cases with partial otocyst removal when the vestibular nerve was absent. The facial nerve followed its normal course when the vestibular nerve persisted. We conclude that disruption of the developing facial pathway altered the routes of navigating axons, but did not prevent pathfinding and innervation of the normal targets. Pathfinding abilities may not be restricted to pioneering axons of the facial nerve; later-developing facial nerve fibers also appeared to have positional information. Our findings are consistent with the hypothesis that navigating axons may respond to multiple guidance cues during development. These cues appear to differ as a function of position of the navigating axon.

Acetylcholinesterase↗

PubMed1992

Hair cell regeneration in the avian vestibular epithelium.

Research conducted in the past 4 years has shown that the avian vestibular system retains the capacity to generate hair cells postnatally. In the present paper we review information on postnatal proliferation and differentiation of hair cells in the avian vestibular system. In addition, we present preliminary accounts of recent experiments regarding regeneration of vestibular hair cells following aminoglycoside toxicity. The overall consensus is that the avian vestibular system is able to regenerate hair cells, both on an ongoing basis and after damage.

Animals↗

PubMed1992

Ongoing production of sensory cells in the vestibular epithelium of the chick.

Recent studies have shown that the vestibular and auditory systems of some species of birds have the capacity to generate sensory hair cells postnatally. We used a traditional technique, 3H-thymidine autoradiography, and a newer method, bromodeoxyuridine immunocytochemistry, to determine whether ongoing proliferation of hair cells occurs in the intact chick vestibular epithelium. A ten-day course of 3H-thymidine, bromodeoxyuridine, or both was administered to twelve-day-old chicks. Both autoradiographic and immunocytochemical labeling demonstrated ongoing production of supporting cells and Type II hair cells in all chick vestibular organs. No evidence for production of Type I hair cells was seen in this investigation. New sensory cells were distributed throughout the epithelium; there was no peripheral growth zone analogous to that found in other vertebrates. Labeled Type II hair cells were frequently seen immediately above labeled supporting cells. This observation suggests that supporting cells are precursors for new hair cells. The ongoing, postnatal regeneration of vestibular epithelial cells also suggests that this epithelium may retain the potential for repair after trauma or ototoxic damage.

Animals↗

PubMed1992

Aberrant projection induced by otocyst removal maintains normal tonotopic organization in the chick cochlear nucleus.

Nucleus magnocellularis (NM), a second-order nucleus in the chick auditory system, is topographically and tonotopically organized. The basilar papilla (cochlea) projects onto the ipsilateral NM via the auditory nerve. The anteromedial region of NM is innervated by the proximal end of the basilar papilla and responds to high-frequency sounds; more posterolateral regions receive input from more distal locations along the papilla and respond to progressively lower frequencies. NM projects exclusively to the third-order neurons of nucleus laminaris (NL). Otocyst removal prevents the formation of the ipsilateral cochlea and cochlear nerve and results in the development of an aberrant functional projection from the contralateral NM to the "deafferented" NM on the operated side of the brain (Jackson and Parks, 1988). In the present experiment, the otocyst was removed unilaterally and the tonotopic organization of the deafferented NM was physiologically mapped at 17-18 d of embryonic age (E17-E18). Quantitative analyses revealed that the frequency organization of the deafferented NM is almost identical to that in normal embryos. Progressively higher characteristic frequencies were recorded at successively more anterior and more medial locations in the nucleus, and the orientation of the tonotopic axis was indistinguishable from normal. Furthermore, the correlation between characteristic frequency and anatomical location is comparable in the deafferented (r = 0.91) and normal (r = 0.87) NM. The only noticeable discrepancy is that characteristic frequencies in NM on both sides of the brain of operated embryos are higher than the frequencies observed previously at comparable regions of the nucleus in unoperated controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Rapid and reversible astrocytic reaction to afferent activity blockade in chick cochlear nucleus.

We describe here rapid proliferation of astrocytic processes immunoreactive for glial fibrillary acidic protein (GFAP) in the chick cochlear nucleus following blockade of action potentials in the afferent nerve. Unilateral eighth nerve activity blockade was achieved through intralabyrinthine injection of TTX. Within 1 hr of activity blockade, a 56% increase in area density of GFAP-immunoreactive processes was found in the ipsilateral cochlear nucleus as compared to the contralateral side of the brain. This increase reached 152% by 3 hr. When eighth nerve activity was allowed to recover and animals were studied 1 week after TTX injection, no difference was found in GFAP immunoreactivity between the ipsilateral and contralateral cochlear nuclei. This is the first report of a glial reaction to documented neuronal inactivity in the absence of neuropathology. These results indicate that neuronal activity may regulate the structure of astrocytic processes.

Animals↗

PubMed1992

A circuit for coding interaural time differences in the chick brainstem.

Third-order auditory neurons in the avian nucleus laminaris (NL) are the first to receive binaural input. In the chick, NL consists of a monolayer of neurons with polarized dendritic arbors oriented dorsally and ventrally. Afferents from second-order neurons in the ipsilateral nucleus magnocellularis (NM) innervate the dorsal dendrites of NL neurons, distributing processes of approximately equal length to NL neurons along an isofrequency band (roughly caudomedial to rostrolateral). Afferents from the contralateral NM innervate the ventral dendrites of NL neurons, distributing collateral branches sequentially as they proceed from caudomedial to rostrolateral along the isofrequency band of neurons. This innervation pattern could be the basis of a "delay line" circuit, as postulated in models of neural networks mediating sound localization. We examined this circuit by analyzing evoked field potentials using a brain slice preparation containing both NL and NM. The results were consistent with the previous anatomical findings. When the ipsilateral auditory nerve or ipsilateral NM was stimulated, there was no consistent variation in the latency of postsynaptic field potentials across the medial-to-lateral extent of NL. In contrast, when the contralateral NM or NM axons in the crossed dorsal cochlear tract were stimulated, a linear increase in the latency of postsynaptic potentials was observed from medial to lateral positions in NL. When stimulation amplitudes for both the ipsilateral and contralateral inputs were adjusted so as to produce little or no postsynaptic field potential, simultaneous bilateral stimulation evoked a pronounced response. Thus, NL neurons can act as "coincidence detectors." The amplitude of the postsynaptic response was dependent on the relative timing of stimulation of the two inputs. The optimal time difference changed systematically across the medial-to-lateral extent of NL. This system of delay lines and coincidence detectors could provide a mechanism for converting interaural time differences into a "place map" within NL.

Animals↗

PubMed1991

Afferent influences on brainstem auditory nuclei of the chick: nucleus magnocellularis neuronal activity following cochlea removal.

Elimination of presynaptic elements often results in marked changes, such as atrophy and death, in postsynaptic neurons in the central nervous system. These transneuronal changes are particularly rapid and profound in young animals. In order to understand the cellular events underlying transneuronal regulation it is necessary to explore changes in the local environment of neurons following manipulations of their afferents. In previous investigations we have documented a variety of rapid and marked cellular changes in neurons of the cochlear nucleus of neonatal chicks (n. magnocellularis) following cochlea removal. In adult chickens, however, these transneuronal changes are either absent or minor. The goals of the studies presented here were to examine changes in the electrical activity of nucleus magnocellularis cells and their afferents following removal of the cochlea and to determine if these changes were similar in adult and neonatal animals. Two measures of electrical activity were used; multiunit recording with microelectrodes and incorporation of radiolabeled 2-deoxyglucose (2-DG). Microelectrode recordings revealed high levels of spontaneous activity in n. magnocellularis and n. laminaris, the binaural target of n. magnocellularis neurons. Neither puncturing of the tympanic membrane nor removal of the columella causes significant changes in spontaneous activity, although the latter results in a profound hearing loss (40-50 dB). Removal of the cochlea, on the other hand, results in immediate cessation of all extracellular electrical activity in the ipsilateral n. magnocellularis. Recordings from the same location for up to 6 h failed to reveal any return of spontaneous activity. When the electrode tip was placed in n. laminaris, unilateral cochlea removal had no discernible effect on extracellularly recorded spontaneous activity, probably due to the high levels of excitatory input from the intact ear. Bilateral cochlea removal, however, completely eliminated activity in n. laminaris. 2-DG studies conducted 1 h to 8 days following unilateral cochlea removal revealed marked decreases in 2-DG incorporation in the ipsilateral n. magnocellularis and bilaterally in the n. laminaris target of the ablated cochlea. No compensatory return of 2-DG incorporation was observed for up to 8 days. Comparisons of adult and neonatal chicks failed to reveal significant differences in the effects of cochlea removal on multiunit activity or 2-DG incorporation, suggesting that age differences in transneuronal regulation are due to intrinsic biochemical differences in young and adult neurons rather than differences in the proportion of synaptic input that has been abolished.

Acoustic Stimulation↗

PubMed1991

Hair cell regeneration in the avian inner ear.

The postembryonic production of hair cells in fish and reptiles has been known for several decades. Until recently it was assumed that this capacity was absent in the more highly specialized inner ears of birds and mammals. Recent research has shown, however, that birds have the capacity to rebuild a damaged inner ear. Summarized here are studies conducted in our laboratory which address the following questions: (1) Which are the precursors of the regenerated hair cells? (2) Are the new hair cells functional? (3) What are the ultrastructural properties of regenerated hair cells? and (4) Can the level of proliferation be regulated? Both the auditory and the vestibular systems of the avian inner ear were studied. Our results provide some answers to these questions. The implications of the results are discussed.

Animals↗

PubMed1991

Lack of correspondence between mRNA expression for a putative cell death molecule (SGP-2) and neuronal cell death in the central nervous system.

Neuronal death during nervous system development, a widely observed phenomenon, occurs through unknown mechanisms. Recent evidence suggests an active, destructive process requiring new gene expression. Sulfated glycoprotein-2 (SGP-2), a secretory product of testicular Sertoli cells has been shown to up-regulate in several nonneural tissues undergoing programmed cell death and in several types of neuronal degeneration. In order to determine if this message up-regulates in neurons undergoing developmentally determined cell death, we have studied the expression of SGP-2 mRNA in the developing and adult rat central nervous system (CNS) with in situ hybridization. We also report on the expression of this message in nonneural tissues from several regions of the developing embryo. The developing and adult rat central nervous system as well as widely varied tissues in the rat embryo express SGP-2 mRNA in a pattern that does not correlate with regions undergoing developmental cell death. In the nervous system, SGP-2 mRNA is expressed in neuronal populations including motor neurons, cortical neurons, and hypothalamic neurons at ages when the period of developmental cell death has passed. In a nonneural tissue (palatal shelve epithelium) for which a developmental cell death period has been described, SGP-2 mRNA was not present in the region where cell death occurs. We conclude that SGP-2 mRNA expression cannot be correlated with programmed cell death in neural or nonneural tissues. The results of this study as well as recently reported SGP-2 homologies indicate a possible role for this protein in secretion and lipid transport.

Aging↗

PubMed1991

Development of otoacoustic emissions in gerbil: evidence for micromechanical changes underlying development of the place code.

The development of the acoustic distortion product (ADP) 2f1-f2 was studied in gerbils, beginning 12 days after birth (P12). ADPs were measured as a function of stimulus frequency region (1.0 to 13.0 kHz) and level (10 to 80 dB SPL). There was an orderly progression in the appearance and maturation of the emissions, with responses to high-frequency stimuli (f2 = 13.0 kHz) appearing first, at P13-14. Responses to mid and high frequencies (f2 = 3.9 to 13.0 kHz) matured earlier than responses to lower frequencies. Responses to low-frequency stimuli (f2 = 1.3 KHz) did not appear until P18-19 and were not mature until after one month of age. The first emissions to develop in a given frequency region had elevated thresholds, were reduced in amplitude, and displayed monotonic input-output functions. As the auditory system matured, emission growth functions became non-monotonic displaying saturation, but initially retained a reduced dynamic range. Data from the developing gerbil suggest that initially its cochlear mechanics are passive and that active elements associated with normal outer hair cell function mature first in the basal turn and last near the apex. Furthermore, the development of active nonlinear elements underlying ADP generation is consistent with the development of frequency selectivity and developmental shifts in the place code which have been demonstrated in the gerbil.

Acoustic Stimulation↗

PubMed1991

Cochlear nucleus cell size is regulated by auditory nerve electrical activity.

Accumulating evidence suggests that sensorineural hearing loss in animals is rapidly followed by degenerative changes in central auditory neurons. For example, cochlear removal in birds and mammals results in a reduction in central auditory neuron cell size within 48 hours. A similar decrease in cell size after pharmacologic blockade of auditory nerve electrical activity with tetrodotoxin has been reported. In the present study, we evaluate the reversibility of central auditory changes after a profound sensorineural hearing loss caused by blockade of auditory nerve actions potentials. Tetrodotoxin, which blocks voltage-sensitive sodium channels, was embedded in a slow-release vehicle and placed next to the round window membrane of gerbils. Tetrodotoxin diffused into perilymph and unilaterally blocked electrical activity in auditory nerve axons. Electrical activity blockade was confirmed with recordings of auditory brainstem response. Animals were killed immediately after 24 hours of electrical blockade or 7 days after a transient 24- or 48-hour blockade. Large spherical cells of the anteroventral cochlear nucleus ipsilateral to manipulation were measured and compared to large spherical cells on the opposite, unmanipulated side of the brain. Animals killed immediately after a 24-hour blockade of electrical activity showed a mean decrease of 16% in cell size ipsilateral to the blockade (p less than 0.05). In animals allowed to recover for 7 days after blockade for 24 or 48 hours, cell size returned to previous levels. There was no longer a consistent difference in cell size between the two sides of the brain in these animals (p greater than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

Nerve growth factor: increased angiogenesis without improved nerve regeneration.

Nerve growth factor (NGF) and laminin are important factors for neural development and regeneration. We examined the effects of increasing the local concentration and duration of action of NGF and laminin on peripheral nerve regeneration in the adult mouse sciatic nerve. A Silastic (silicone rubber) channel with intraluminal NGF solution was secured between transected nerve ends. The second channel tested, formed from a polysaccharide called chitosan, was prepared with NGF and laminin in the channel walls and provided a sustained release of NGF. At six weeks post-implantation, no improvement in nerve regeneration was identified in those channels prepared with NGF when comparing electromyographic thresholds (microA), maximum potentials (mV), nerve diameter, myelin sheath thickness, myelinated axon counts, or diameter. However, increased angiogenesis was demonstrated within the chitosan and Silastic channels prepared with NGF compared to those channels without NGF. Silastic exhibited minimal inflammation. Chitosan was associated with inflammation in many nerve channels.

Animals↗

PubMed1991

Expression of nerve growth factor (NGF) receptors in the developing inner ear of chick and rat.

The expression of nerve growth factor receptors (NGFRs) was studied in the developing inner ear with in situ hybridization in chick embryos and with immunocytochemistry in rat embryos to determine sites of possible functions of NGF or NGF-like molecules in inner ear development. NGFR expression in the chick otocyst and acoustic ganglion is compared with epithelial differentiation and the onset of afferent innervation as determined with fluorescent carbocyanine tracers. In the inner ear of the chick embryo, NGFR mRNA expression shows an alternating pattern in mesenchymal and epithelial tissues. NGFR mRNA is heavily expressed in the mesenchyme surrounding the otocyst (E2-3), ceases at E3-5, and reappears in a thin layer of mesenchymal cells surrounding the membraneous epithelia (E5-13). In the otocyst epithelium, NGFR mRNA expression develops in one anterior and one posterior focus at E3-4.5. NGFR mRNA is expressed in the primordia of the ampullary cristae (E5-7) and possibly the anlage of the utricle; label transiently concentrates in the planum semilunatum of the cristae ampullares and in superior portions of the semicircular canals at E9, but is not seen in differentiating hair cells. In the acoustic ganglion, NGFR mRNA expression begins at E4; at the same time, the first peripheral acoustic nerve processes penetrate the otic epithelium (E4-4.5). The acoustic ganglia remain weakly NGFR mRNA-labeled in the posthatch animal. In the rat embryo, NGFR immunoreactivity is present in the auditory placode at E9, in the periotic mesenchyme at E9-10, and in the medial half of the otocyst at E10-11. At E12, epithelial NGFR expression becomes restricted anteriorly and posteriorly in a pattern similar to that of the chick otocyst and ceases at E13. NGFR immunoreactivity appears transiently in pillar cells of the cochlea in the third week of gestation. NGFR and NGFR mRNA is expressed after E11 in the acoustic ganglia. While NGFR transcripts are expressed in the cochlear ganglion cell bodies, NGFR protein becomes restricted to neuronal processes by the third week of gestation. The vestibular, but not the cochlear (spiral) ganglia remain NGFR-labeled in the adult rat. Onset of NGFR mRNA expression in the acoustic ganglion during the period of afferent fiber ingrowth into the otocyst epithelium is consistent with the hypothesis that NGF-like molecules may have a neurotrophic function for acoustic ganglion cells. Transient expression of NGFRs in secretory cells of the vestibular endorgan and pillar cells in the organ of Corti implicate a role for neurotrophins in the differentiation of these epithelial cell types.

Animals↗

PubMed1991

Anatomical correlates of functional recovery in the avian inner ear following aminoglycoside ototoxicity.

Tucci and Rubel have demonstrated functional recovery of the chick cochlea following aminoglycoside ototoxicity. The cochleae of these same animals were examined by scanning electron microscopy (SEM) in order to further understand this recovery process. Hatchling chicks were given daily doses of gentamicin for 10 days. Auditory-evoked potential measurements and examination of the cochlea by scanning electron microscopy were performed after survival periods of 5 days to 20 weeks. After 5 days of gentamicin exposure, there was near complete basal hair cell loss associated with a high-frequency hearing loss. Apical progression of damage with a broad-band hearing loss occurred over 4 weeks. At 20-weeks, hair cell counts were normal with a small high-frequency hearing loss. Hair cell regeneration played a major role in the functional recovery of the cochlea.

Animals↗

PubMed1991

Hair cell regeneration in the avian cochlea: if it works in birds, why not in man?

Hearing loss caused by cochlear hair cell loss is the most common process afflicting the hearing impaired. Recent studies in the avian cochlea following ototoxic drug and noise damage have demonstrated a remarkable capacity for anatomical and functional recovery. Hair cell regeneration has been shown to play a major role in this recovery process. Future studies may one day make hair cell regeneration or transplantation possible in man.

Animals↗