The effect of unilateral basilar papilla removal upon nuclei laminaris and magnocellularis of the chick examined with [3H]2-deoxy-D-glucose autoradiography

Proteomic analyses of nucleus laminaris identified candidate targets of the fragile X mental retardation protein

The avian nucleus laminaris (NL) is a brainstem nucleus necessary for binaural processing, analogous in structure and function to the mammalian medial superior olive. In chickens (Gallus gallus), NL is a well-studied model system for activity-dependent neural plasticity. Its neurons have bipolar extension of dendrites, which receive segregated inputs from two ears and display rapid and compartment-specific reorganization in response to unilateral changes in auditory input. More recently, fragile X mental retardation protein (FMRP), an RNA-binding protein that regulates local protein translation, has been shown to be enriched in NL dendrites, suggesting its potential role in the structural dynamics of these dendrites. To explore the molecular role of FMRP in this nucleus, we performed proteomic analysis of NL, using micro laser capture and liquid chromatography tandem mass spectrometry. We identified 657 proteins, greatly represented in pathways involved in mitochondria, translation and metabolism, consistent with high levels of activity of NL neurons. Of these, 94 are potential FMRP targets, by comparative analysis with previously proposed FMRP targets in mammals. These proteins are enriched in pathways involved in cellular growth, cellular trafficking and transmembrane transport. Immunocytochemistry verified the dendritic localization of several proteins in NL. Furthermore, we confirmed the direct interaction of FMRP with one candidate, RhoC, by in vitro RNA binding assays. In summary, we provide a database of highly expressed proteins in NL and in particular a list of potential FMRP targets, with the goal of facilitating molecular characterization of FMRP signaling in future studies.

Astrocyte-secreted factors modulate the developmental distribution of inhibitory synapses in nucleus laminaris of the avian auditory brainstem

Nucleus laminaris (NL) neurons in the avian auditory brainstem are coincidence detectors necessary for the computation of interaural time differences used in sound localization. In addition to their excitatory inputs from nucleus magnocellularis, NL neurons receive inhibitory inputs from the superior olivary nucleus (SON) that greatly improve coincidence detection in mature animals. The mechanisms that establish mature distributions of inhibitory inputs to NL are not known. We used the vesicular GABA transporter (VGAT) as a marker for inhibitory presynaptic terminals to study the development of inhibitory inputs to NL between embryonic day 9 (E9) and E17. VGAT immunofluorescent puncta were first seen sparsely in NL at E9. The density of VGAT puncta increased with development, first within the ventral NL neuropil region and subsequently throughout both the ventral and dorsal dendritic neuropil, with significantly fewer terminals in the cell body region. A large increase in density occurred between E13–15 and E16–17, at a developmental stage when astrocytes that express glial fibrillary acidic protein (GFAP) become mature. We cultured E13 brainstem slices together with astrocyte-conditioned medium (ACM) obtained from E16 brainstems and found that ACM, but not control medium, increased the density of VGAT puncta. This increase was similar to that observed during normal development. Astrocyte-secreted factors interact with the terminal ends of SON axons to increase the number of GABAergic terminals. These data suggest that factors secreted from GFAP-positive astrocytes promote maturation of inhibitory pathways in the auditory brainstem.

Astrocyte-secreted factors modulate a gradient of primary dendritic arbors in nucleus laminaris of the avian auditory brainstem

Neurons in nucleus laminaris (NL) receive binaural, tonotopically matched input from nucleus magnocelluaris (NM) onto bitufted dendrites that display a gradient of dendritic arbor size. These features improve computation of interaural time differences, which are used to determine the locations of sound sources. The dendritic gradient emerges following a period of significant reorganization at embryonic day 15 (E15), which coincides with the emergence of astrocytes that express glial fibrillary acidic protein (GFAP) in the auditory brainstem. The major changes include a loss of total dendritic length, a systematic loss of primary dendrites along the tonotopic axis, and lengthening of primary dendrites on caudolateral NL neurons. Here we have tested whether astrocyte-derived molecules contribute to these changes in dendritic morphology. We used an organotypic brainstem slice preparation to perform repeated imaging of individual dye-filled NL neurons to determine the effects of astrocyte-conditioned medium (ACM) on dendritic morphology. We found that treatment with ACM induced a decrease in the number of primary dendrites in a tonotopically graded manner similar to that observed during normal development. Our data introduce a new interaction between astrocytes and neurons in the auditory brainstem and suggest that these astrocytes influence multiple aspects of auditory brainstem maturation.

Compartment-specific regulation of plasma membrane calcium ATPase type 2 in the chick auditory brainstem

Calcium signaling plays a role in synaptic regulation of dendritic structure, usually on the time scale of hours or days. Here we use immunocytochemistry to examine changes in expression of plasma membrane calcium ATPase type 2 (PMCA2), a high-affinity calcium efflux protein, in the chick nucleus laminaris (NL) following manipulations of synaptic inputs. Dendrites of NL neurons segregate into dorsal and ventral domains, receiving excitatory input from the ipsilateral and contralateral ears, respectively, via nucleus magnocellularis (NM). Deprivation of the contralateral projection from NM to NL leads to rapid retraction of ventral, but not the dorsal, dendrites of NL neurons. Immunocytochemistry revealed symmetric distribution of PMCA2 in two neuropil regions of normally innervated NL. Electron microscopy confirmed that PMCA2 localizes in both NM terminals and NL dendrites. As early as 30 minutes after transection of the contralateral projection from NM to NL or unilateral cochlea removal, significant decreases in PMCA2 immunoreactivity were seen in the deprived neuropil of NL compared with the other neuropil that continued to receive normal input. The rapid decrease correlated with reductions in the immunoreactivity for microtubule-associated protein 2, which affects cytoskeleton stabilization. These results suggest that PMCA2 is regulated independently in ventral and dorsal NL dendrites and/or their inputs from NM in a way that is correlated with presynaptic activity. This provides a potential mechanism by which deprivation can change calcium transport that, in turn, may be important for rapid, compartment-specific dendritic remodeling.

Distribution of glial-associated proteins in the developing chick auditory brainstem

In the avian brainstem, nucleus magnocellularis (NM) projects bilaterally to nucleus laminaris (NL) in a pathway that facilitates sound localization. The distribution of glia during the development of this pathway has not previously been characterized. Radial glia, astrocytes, and oligodendrocytes facilitate many processes including axon pathfinding, synaptic development, and maturation. Here we determined the spatiotemporal expression patterns of glial cell types in embryonic development of the chick auditory brainstem using glial-specific antibodies and histological markers. We found that vimentin-positive processes are intercalated throughout the NL cell layer. Astrocytes are found in two domains: one in the ventral neuropil region and the other dorsolateral to NM. GFAP-positive processes are primarily distributed along the ventral margin of NL. Astrocytic processes penetrate the NL cell layer following the onset of synaptogenesis, but before pruning and maturation. The dynamic, nonoverlapping expression patterns of GFAP and vimentin suggest that distinct glial populations are found in dorsal versus ventral regions of NL. Myelination occurs after axons have reached their targets. FluoroMyelin and myelin basic protein (MBP) gradually increase along the mediolateral axis of NL starting at E10. Multiple GFAP-positive processes are directly apposed to NM-NL axons and MBP, which suggests a role in early myelinogenesis. Our results show considerable changes in glial development after initial NM-NL connections are made, suggesting that glia may facilitate maturation of the auditory circuit.

Effects of lithium and deafferentation on expression of glycogen synthase kinase-3beta, NFkappaB, beta-catenin and pCreb in the chick cochlear nucleus

The avian brainstem serves as a useful model to answer the question of how afferent activity influences the viability of target neurons. Approximately 20-30% of neurons in the avian cochlear nucleus, nucleus magnocellularis (NM) die following deafferentation (i.e., deafness produced by cochlea removal). Interestingly, Bcl-2 mRNA (but not protein) is upregulated in 20-30% of NM neurons following deafferentation. We have recently shown that chronic treatments of lithium upregulates the neuroprotective protein Bcl-2 and increases neuronal survival following deafferentation. The pathways leading to the upregulation of Bcl-2 expression following these two manipulations are unknown. The present experiments examine changes in glycogen synthase kinase-3 beta (Gsk-3beta), and transcription factors nuclear factor kappaB (NFkappaB), beta-catenin, and pCreb following lithium administration and following deafferentation. These molecules are known to be influenced by lithium and to regulate Bcl-2 expression in other model systems. Lithium decreased immunolabeling for Gsk-3beta and increased expression for all three transcription factors. Deafferentation, however, did not alter Gsk-3beta or NFkappaB, resulted in lower beta-catenin expression, but did increase pCreb immunoreactivity. While it is possible that pCreb is a common link in the regulation of Bcl-2 following these two manipulations, the timing and distribution of pCreb labeling suggests that it is not the sole determinant of Bcl-2 upregulation following deafferentation. It is likely that the regulation of Bcl-2 gene expression by lithium and by deafferentation involves different molecular pathways.

Afferent regulation of oxidative stress in the chick cochlear nucleus

The chick auditory brain stem has been a useful model system for examining the afferent-dependent signals that regulate postsynaptic neurons. Like other sensory systems, compromised afferent input results in rapid death and atrophy of postsynaptic neurons. The present paper explores the possible contributions of an oxidative stress pathway in determining neuronal fate following deafferentation. Levels of reactive oxygen species, lipid damage measured by 4-hydroxynonenal formation, and a compensatory reactive oxygen species-induced response regulated by glutathione s transferase M1 and the reactive oxygen species-sensitive transcriptional factor, nuclear respiratory factor 1 were examined. Unilateral cochlea removal surgery was performed on young posthatch chicks. Labeling in the cochlear nucleus, nucleus magnocellularis, on opposite sides of the same tissue sections were compared by densitometry. The results showed a dramatic increase in reactive oxygen species in the deafferented nucleus magnocellularis by 6 h following cochlea removal. This increase in reactive oxygen species was accompanied by lipid damage and a compensatory upregulation of both glutathione s transferase M1 and nuclear respiratory factor 1. Double-labeling revealed that glutathione s transferase M1 expression was highest in neurons that were likely to survive deafferentation, as assessed immunocytochemically with Y10b, a marker for ribosomal integrity. Together, these data suggest reactive oxygen species are generated and a compensatory detoxifying pathway is upregulated in the first few hours following deafferentation. This is consistent with the hypothesis that oxidative stress plays a role in determining whether a given neuron survives following deafferentation.

Activity-dependent regulation of the potassium channel subunits Kv1.1 and Kv3.1

Afferent activity, especially in young animals, can have profound influences on postsynaptic neuronal structure, function and metabolic processes. Most studies evaluating activity regulation of cellular components have examined the expression of ubiquitous cellular proteins as opposed to molecules that are specialized in the neurons of interest. Here we consider the regulation of two proteins (voltage-gated potassium channel subunits Kv1.1 and Kv3.1) that auditory brainstem neurons in birds and mammals express at uniquely high levels. Unilateral removal of the avian cochlea leads to rapid and dramatic reduction in the expression of both proteins in the nucleus magnocellularis (NM; a division of the avian cochlear nucleus) neurons as detected by immunocytochemistry. Uniform downregulation of Kv1.1 was reliable by 3 hours after cochlea removal, was sustained through 96 hours, and returned to control levels in the surviving neurons by 2 weeks. The activity-dependent changes in Kv3.1 appear to be bimodal and are more transient, being observed at 3 hours after cochlea removal and recovering to control levels within 24 hours. We also explored the functional properties of Kv1.1 in NM neurons deprived of auditory input for 24 hours by whole-cell recordings. Low-threshold potassium currents in deprived NM neurons were not significantly different from control neurons in their amplitude or sensitivity to dendrotoxin-I, a selective K+ channel antagonist. We conclude that the highly specialized abundant expression of Kv1.1 and 3.1 channel subunits is not permanently regulated by synaptic activity and that changes in overall protein levels do not predict membrane pools.

Influence of auditory deprivation upon the tonopic organization in the inferior colliculus: a Fos immunocytochemical study in the rat

The frequency organization in the inferior colliculus of neonatally-deafened rats was investigated using electrical stimulation of the cochlea and immunoreactivity for Fos as a marker of neuronal activity. An electrode implanted either at the base or at the apex of the right cochlea delivered a unique 45-min stimulation at two different level intensities and at two time points, i.e. either at 4 weeks or at 4 months. In 4-week-old rats stimulated at 5x threshold, a site-for-site organization was observed since basal or apical stimulation induced a strong labelling in the ventro-medial or in the dorsolateral part of the left inferior colliculus, respectively. In 4-month-old rats, stimulation of the base induced an extremely weak Fos labelling without any specific location in the left inferior colliculus while stimulation of the apex induced a diffuse labelling with two discrete bands being distinguishable in the left inferior colliculus. In 4-week-old rats stimulated at 15x threshold, basal stimulation elicited a diffuse Fos-like immunoreactivity in the left inferior colliculus while apical stimulation yielded a response restricted to the dorsal part of the left inferior colliculus. In 4-month-old rats, no response was detected in the left inferior colliculus after stimulation of the basal part of the cochlea. Stimulation of the apex could still induce a labelling in the dorsolateral left inferior colliculus. Thus, the inferior colliculus exhibits an adult-like tonotopic organization early on independently of any acoustic stimulation. Prolonged absence of auditory input dramatically alters this organization in the inferior colliculus, especially for high frequencies. From a clinical standpoint, these results could argue for early implantation in deaf children.

Auditory system development: primary auditory neurons and their targets

The neurons of the cochlear ganglion transmit acoustic information between the inner ear and the brain. These placodally derived neurons must produce a topographically precise pattern of connections in both the inner ear and the brain. In this review, we consider the current state of knowledge concerning the development of these neurons, their peripheral and central connections, and their influences on peripheral and central target cells. Relatively little is known about the cellular and molecular regulation of migration or the establishment of precise topographic connection to the hair cells or cochlear nucleus (CN) neurons. Studies of mice with neurotrophin deletions are beginning to yield increasing understanding of variations in ganglion cell survival and resulting innervation patterns, however. Finally, existing evidence suggests that while ganglion cells have little influence on the differentiation of their hair cell targets, quite the opposite is true in the brain. Ganglion cell innervation and synaptic activity are essential for normal development of neurons in the cochlear nucleus.

Avian brainstem neurogenesis is stimulated during cochlear hair cell regeneration

Unlike mammals, adult avians are able to regenerate cochlear sensory hair cells following injury. Brainstem auditory neurons in chicken nucleus magnocellularis (NM), which receive their sole excitatory afferent input from the cochlea, were examined for evidence of mitosis during ototoxin-induced loss and regeneration of cochlear hair cells. Using tritiated thymidine as a mitotic marker in tissue processed for autoradiography and counterstained with thionin, labeled NM neurons and glia were counted from chickens killed 16 days after gentamicin or saline injections. Newly generated NM neurons were observed during cochlear hair cell regeneration. More labeled neurons were observed in the experimental chickens, but a few were also seen in the control chickens. We predicted labeled NM neurons would be found solely in the rostral high frequency region, given the gentamicin-induced high frequency cochlear hair cell loss and regeneration. However, the labeled NM neurons were located throughout the tonotopic axis of the nucleus. The total number of labeled neurons was lower than predicted. Many labeled NM glia were observed in experimental and control chickens. Labeled cells were also observed throughout the chicken brainstem and cerebellum in both experimental and control chickens, indicating great potential for CNS plasticity. Results in NM indicate the avian auditory system is capable of regenerating brainstem auditory neurons in addition to the previously well-established capability of regenerating cochlear hair cells in response to ototoxic injury. Recovery of both central and peripheral auditory components will be necessary to restore hearing damaged by noise or ototoxic drugs.

Rapid deafferentation-induced upregulation of bcl-2 mRNA in the chick cochlear nucleus

Neuronal survival in developing animals is often dependent on afferent activity. In the posthatch chick, approximately 30% of the neurons in the avian cochlear nucleus, nucleus magnocellularis (NM) die following elimination of VIIIth nerve activity. The factors that influence death or survival of an individual NM neuron are largely unknown. Previous studies indicate that both cell death and cell survival mechanisms compete to determine cell fate. One factor that has been shown to suppress cell death cascades in a variety of systems is bcl-2. If this gene product plays a role in regulating cell survival following deafferentation, then one might expect the expression of this gene to be influenced by removal of afferent input. In the present study, in situ hybridization revealed a rapid and transient increase in bcl-2 mRNA in NM neurons following deafferentation. Enhanced bcl-2 mRNA expression was observed at 6 and 12 h following deafferentation, but not at 3 or 24 h. Surprisingly, the upregulation of bcl-2 mRNA was limited to a subpopulation (20-30%) of deafferented neurons corresponding to the number of NM neurons that eventually die following cochlea removal. The robust and rapid upregulation of this gene suggests that cell death cascades regulated by bcl-2 may be initiated following deafferentation.

Nucleus magnocellularis and nucleus laminaris in Belgian Waterslager and normal strain canaries

Belgian Waterslager (BWS) canaries are characterized by a mean 30% loss of hair cells in the basilar papilla compared to other canaries, and a corresponding increase in behavioral auditory thresholds. In spite of the large number of missing and damaged sensory cells, there is on average only a 12% reduction in the number of fibers in the VIIIth nerve. In this study, we examined cell number and size, and volume of auditory nuclei, specifically in nucleus magnocellularis and nucleus laminaris in Belgian Waterslager canaries. While the overall anatomical structure and organization of these nuclei and the total number of cells in the non-BWS and BWS canaries were comparable, BWS canaries showed a significant decrease in the volume of the auditory nuclei that was attributed to a reduction in cell size. These results provide further evidence in favor of a role of the sensory epithelium in the maintenance of central auditory structures.

Cochlear electrical stimulation: influence of age of implantation on Fos immunocytochemical reactions in inferior colliculi and dorsal cochlear nuclei of the rat

The influence of age at the time of implantation of a stimulating electrode unilaterally in the inner ear on central auditory pathways was investigated in rats deafened shortly after birth. Immunoreactivity for Fos served as a functional marker of neuronal activity. Electrodes were implanted in the left cochlea of rats aged 3 weeks or 4 months. Stimulation lasted 45 minutes, then rats were sacrificed and tissues processed for immunocytochemistry. The younger animals showed significantly more neurons with Fos immunoreactivity bilaterally in the dorsal cochlear nuclei (DCN) and inferior colliculi (IC) than the older rats or control animals with normal hearing receiving the same stimulation. Activity was more prominent in the left DCN and right IC. The results show that electrical stimulation of the inner ear is more effective in younger animals in eliciting gene expression associated with development of a functional network in the auditory pathways. This suggests that deaf children should be provided with cochlear implants as early as possible.

Effects of conductive hearing loss on gerbil central auditory system activity in silence

Animal models of conductive hearing loss (CHL) show altered structure and function in the central auditory system (CAS), particularly following unilateral deprivation. Assessment of neuronal activity as measured by 2-deoxyglucose (2-DG) uptake following CHL has been reported by two groups of investigators, with different findings. Woolf and colleagues [Brain Res. 274 (1983) 119] found that 2-DG uptake increased in the cochlear nucleus ipsilateral to the CHL, while Tucci et al. [Laryngoscope 109 (1999) 1359] found a decrease in 2-DG uptake in the ipsilateral cochlear nucleus. One significant difference between the protocols in the two studies was that, in the first study, animals were maintained in silence following 2-DG injection, whereas in the Tucci et al. study, animals were exposed to sound. The current study was designed to replicate the protocol used by Woolf et al. Young adult gerbils underwent unilateral malleus removal with bilateral canal ligation (n=6) or a sham procedure (n=7) 48 h prior to 2-DG administration and sacrifice. Optical density measurements were made from CAS nuclei. 2-DG uptake decreased in the ipsilateral cochlear nucleus and contralateral inferior colliculus, and in nuclei of the superior olivary complex bilaterally, supporting the finding that CHL is associated with a decrease in CAS neuronal activity.

Expression of NMDA, AMPA and GABA(A) receptor subunit mRNAs in the rat auditory brainstem. I. Influence of early auditory deprivation

Impact of early post-natal deafening on auditory pathways was investigated in newborn rats deafened by daily amikacin injections from P7 to P16 inducing a complete destruction of the organ of Corti. The expression of mRNAs encoding N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) and gamma-aminobutyric acid type A (GABA(A)) receptor subunits was then studied by in situ hybridization in the dorsal and ventral cochlear nucleus and in the central nucleus of the inferior colliculus (CNIC). Early post-natal deafening decreased bilaterally the expression of mRNAs encoding NR1, NR2a, NR2b and flop isoforms of AMPA receptors. On the contrary, it increased the expression of mRNAs encoding some GABA(A) subunits (alpha1, beta1, gamma2) and flip isoforms of AMPA receptors. These changes were more pronounced in cochlear nuclei than in CNIC. They suggest that auditory sensation is essential in the normal development of central auditory pathways.

Neuronal death, not axonal degeneration, results in significant gliosis within the cochlear nucleus of adult chickens

Injury to the central nervous system initiates a series of events that leads to neuronal cell death and glial activation. Astrocytes respond to damage and disease by becoming hyperplastic and hypertrophied. This 'reactive gliosis' is also accompanied by the upregulation of the intermediate filament protein glial fibrillary acidic protein, the release of growth factors and the formation of the glial scar. However, the signaling cascades which regulate these events, and the molecular mechanisms that give rise to this diverse response, have not been fully elucidated. For example, the role played by degenerating neurons vs. degenerating axons in the activation of astrocytes remains to be determined. To investigate the influence of neuronal cell death vs. axonal degeneration on gliosis, the current study examines the astrocyte response to cochlea removal in two different breeds of adult chickens, one of which exhibits neuronal cell death within the brainstem nucleus magnocellularis (NM) following the lesion and one which does not. Our results indicate that degeneration of NM neurons leads to large increases in both glial proliferation and hypertrophy, while eighth nerve degeneration without NM cell death results in very small increases in glial proliferation.

Tonotopic changes in 2-deoxyglucose activity in chick cochlear nucleus during hair cell loss and regeneration

Following cochlear ablation, auditory neurons in the central nervous system (CNS) undergo alterations in morphology and function, including neuronal cell death. The trigger for these CNS changes is the abrupt cessation of eighth nerve fiber activity. Gentamicin can cause ototoxic damage to cochlear hair cells responsible for high frequency hearing. In birds, these hair cells can regenerate. Therefore, gentamicin causes a partial, yet reversible insult to the ear. It is not known how this partial hair cell damage affects excitatory input to the cochlear nucleus. We examined chick cochlear nucleus activity during hair cell loss and regeneration by measuring 2-deoxyglucose (2DG) uptake. Normal animals showed a rostral to caudal gradient of 2DG activity, with higher activity in caudal regions. When hair cells are damaged (2, 5 days), 2DG uptake is decreased in cochlear nucleus. When hair cells regenerate (9, 16, 28 days), 2DG uptake returns to control levels. This decrease and subsequent return of activity only occurs in the rostral, high frequency region of the cochlear nucleus. No changes are seen in the caudal, low frequency region. These results suggest that changes in activity of cochlear nucleus occur at a similar time course to anatomical changes in the cochlea.

Histochemical mapping of the substrate for brain-stimulation reward with glycogen phosphorylase

Glycogen phosphorylase is the enzyme that regulates glycogenolysis and it appears that there is a relationship between central levels of glycogen and neuronal activity, which is influenced by a variety of neurotransmitters. In the present study, glycogen phosphorylase histochemistry was used to correlate changes in metabolic activity in response to rewarding lateral hypothalamic stimulation. Rats were allowed to self-stimulate for 1 h per day for ten consecutive days following which postmortem phosphorylase a activity was examined. Significant differences in optical density between the stimulated and contralateral hemispheres were found in three of the eight analyzed structures, two of which, the diagonal band of Broca and the caudate nucleus, showed a greater density of glycogen phosphorylase a on the stimulated side and the third, the habenula, had greater contralateral activity. In conclusion, our data suggest that glycogen phosphorylase activity is a viable but not weighty marker of energy alterations induced by chronic exposure to intracranial self-stimulation, and that it is generally consistent with the patterns revealed by other metabolic indices such as cytochrome oxidase and 2-deoxyglucose autoradiography.

Conductive hearing loss results in a decrease in central auditory system activity in the young gerbil

OBJECTIVES/HYPOTHESIS

The impact of childhood conductive HL (CHL) on development of auditory function has long been debated. The present study was conducted to define and compare the consequences of CHL and cochlear ablation (CA) in young and adult animals, using 2-deoxyglucose (2-DG) uptake as a measure of metabolic activity. It was hypothesized that, for both ages, CHL would result in a decrease in activity in the major ascending central auditory system pathway of the manipulated ear, but that this decrease would be significantly less than that observed with CA.

STUDY DESIGN

Sham-controlled study of metabolic effects of CHL during sound stimulation.

METHODS

Gerbils (aged 21 days or adult), underwent malleus removal, CA, or a sham procedure. Young animals survived either 48 hours or 3 weeks; adults survived 3 weeks. Each age/survival CHL group contained eight animals; otherwise, each group (CA and sham) contained five animals, for a total number of 54. At the appropriate survival time, animals were given an intracardiac injection of 14C-2-DG, and sacrificed under anesthesia after 45 minutes of exposure to normal laboratory sounds. Tissue sections were prepared for exposure to x-ray film for optical density measurements, and alternate sections stained for identification of nuclei. Measurements from auditory nuclei of experimental animals were corrected against an unaffected control area (abducens nucleus) and compared with measurements taken from animals in the sham group. Auditory evoked potential thresholds to both air- and bone-conducted stimuli were obtained in a second group of neonatal and adult animals.

RESULTS

Both CHL and CA resulted in a marked decrease in 2-DG uptake in the major ascending projection of the manipulated ear, in both the neonatal and adult animals. In young animals, effects of CHL and CA were similar. Effects of CHL in adult animals were less marked and significantly different from either effects of CHL in young animals or effects of CA in adult animals. HL following malleus removal only was purely conductive and ranged from 38 to 55 dB across frequency.

CONCLUSIONS

Results suggest that, particularly in young animals, a unilateral CHL may have profound effects on metabolic activity in the central auditory system.

Learning-induced physiological plasticity in the thalamo-cortical sensory systems: a critical evaluation of receptive field plasticity, map changes and their potential mechanisms

The goal of this review is to give a detailed description of the main results obtained in the field of learning-induced plasticity. The review is focused on receptive field and map changes observed in the auditory, somatosensory and visual thalamo-cortical system as a result of an associative training performed in waking animals. Receptive field (RF) plasticity, 2DG and map changes obtained in the auditory and somatosensory system are reviewed. In the visual system, as there is no RF and map analysis during learning per se, the evidence presented are from increased neuronal responsiveness, and from the effects of perceptual learning in human and non human primates. Across sensory modalities, the re-tuning of neurons to a significant stimulus or map reorganizations in favour of the significant stimuli were observed at the thalamic and/or cortical level. The analysis of the literature in each sensory modality indicates that relationships between learning-induced sensory plasticity and behavioural performance can, or cannot, be found depending on the tasks that were used. The involvement (i) of Hebbian synaptic plasticity in the described neuronal changes and (ii) of neuromodulators as "gating" factors of the neuronal changes, is evaluated. The weakness of the Hebbian schema to explain learning-induced changes and the need to better define what the word "learning" means are stressed. It is suggested that future research should focus on the dynamic of information processing in sensory systems, and the concept of "effective connectivity" should be useful in that matter.

Breed differences in deafferentation-induced neuronal cell death and shrinkage in chick cochlear nucleus

Removal of functional presynaptic input can result in a variety of changes in postsynaptic neurons in the central nervous system, including altered metabolism, changes in neuronal cell size, and even death of the postsynaptic cell. Age-dependent neuronal cell death and shrinkage has been documented in second order auditory neurons in the chick brainstem (nucleus magnocellularis, NM) following cochlea removal (Born and Rubel, 1985. J. Comp. Neurol. 231, 435-445). Here we examined whether the extent of neuronal cell death and shrinkage is also breed-dependent. We performed unilateral cochlea removal on both hatchling and adult birds of either a broiler breed (Arbor Acres Cross) or egg layer breed (Hy-Line, H and N) and killed birds one week later. Changes in neuronal cell number and cross sectional area were determined from Nissl-stained sections. We observed 25% neuronal cell loss and a 15-20% decrease in neuronal cross sectional area after cochlea removal in either broiler or egg layer hatchling birds. In adult birds, however, neuronal cell loss is breed-dependent. Adult egg layer birds lose an average of 37% of NM neurons after cochlea removal, while adult broiler birds show no cell loss. In both breeds of adult birds, cochlea removal results in a 20% decrease in neuronal cross sectional area. These results suggest that analysis of differences between breeds as well as ages of birds will prove fruitful in determining how afferent input controls neuronal survival and metabolism.

Evidence for loss and recovery of chick brainstem auditory neurons during gentamicin-induced cochlear damage and regeneration

It is well documented that damage to the chick cochlea caused by acoustic overstimulation or ototoxic drugs is reversible. Second-order auditory neurons in nucleus magnocellularis (NM) are sensitive to changes in input from the cochlea. However, few experiments studying changes in NM during cochlear hair cell loss and regeneration have been reported. Chicks were given a single systemic dose of gentamicin, which results in maximal hair cell loss in the base of the cochlea after 5 days. Many new hair cells are present by 9 days. These new hair cells are mature but not completely recovered in organization by 70 days. We counted neurons in Nissl-stained sections of the brainstem within specific tonotopic regions of NM, comparing absolute cell number between gentamicin- and saline-treated animals at both short and long survival times. Our data suggest that neuronal number in rostral NM parallels hair cell number in the base of the cochlea. That is, after a single dose of gentamicin, we see a loss of both cochlear hair cells and NM neurons early, followed by a recovery of both cochlear hair cells and NM neurons later. These results suggest that neurons, like cochlear hair cells, can recover following gentamicin-induced damage.

Single unit recordings in the auditory nerve of congenitally deaf white cats: morphological correlates in the cochlea and cochlear nucleus

It is well known that experimentally induced cochlear damage produces structural, physiological, and biochemical alterations in neurons of the cochlear nucleus. In contrast, much less is known with respect to the naturally occurring cochlear pathology presented by congenital deafness. The present study attempts to relate organ of Corti structure and auditory nerve activity to the morphology of primary synaptic endings in the cochlear nucleus of congenitally deaf white cats. Our observations reveal that the amount of sound-evoked spike activity in auditory nerve fibers influences terminal morphology and synaptic structure in the anteroventral cochlear nucleus. Some white cats had no hearing. They exhibited severely reduced spontaneous activity and no sound-evoked activity in auditory nerve fibers. They had no recognizable organ of Corti, presented >90% loss of spiral ganglion cells, and displayed marked structural abnormalities of endbulbs of Held and their synapses. Other white cats had partial hearing and possessed auditory nerve fibers with a wide range of spontaneous activity but elevated sound-evoked thresholds (60-70 dB SPL). They also exhibited obvious abnormalities in the tectorial membrane, supporting cells, and Reissner's membrane throughout the cochlear duct and had complete inner and outer hair cell loss in the base. The spatial distribution of spiral ganglion cell loss correlated with the pattern of hair cell loss. Primary neurons of hearing-impaired cats displayed structural abnormalities of their endbulbs and synapses in the cochlear nucleus which were intermediate in form compared to normal and totally deaf cats. Changes in endbulb structure appear to correspond to relative levels of deafness. These data suggest that endbulb structure is significantly influenced by sound-evoked auditory nerve activity.

Intensity and frequency functions of [14C]2-deoxyglucose labelling in the central nucleus of the inferior colliculus in the cat

The frequency organization of the central nucleus of the inferior colliculus (ICC) in the anesthetised cat was quantitatively mapped using [14C]2-deoxyglucose. From a standardised rostrocaudal region of the ICC, the position of peak selective labelling along the tonotopic axis closely conformed to the reported tonotopic organization of this nucleus. The position of the peak was found not to significantly change its position along the tonotopic axis with increasing stimulus intensity. However, the amplitude of peak uptake and width of selective labelling were shown to monotonically increase with increase in stimulus intensity. The increase in width of selective labelling, about the position of peak uptake, showed a slight asymmetry toward the high-frequency regions of the ICC. A 2-DG frequency-position function for the ICC, similar to that for the cochlea, enabled the width of 2-DG bands to be expressed in terms of their frequency spread along the tonotopic axis. This inturn enabled 2-DG tuning curves to be plotted which, when compared to electrophysiologically determined tuning curves, showed marked similarities. The minimum threshold and width (Q10) of these 2-DG tuning curves fell within the range reported for single units in the cat auditory pathway.

The three-dimensional frequency organization of the inferior colliculus of the cat: a 2-deoxyglucose study

The 3-dimensional (3-D) functional organization of the cat's inferior colliculus (IC) was examined using the 2-deoxyglucose method. Animals were dichotically stimulated with pure tone stimuli at an intensity of 80 dB SPL. Autoradiographic sections from these animals, cut in the three standard planes, were serially reconstructed to reveal the 3-D topography of the isofrequency sheets of labelling. In all 3-D reconstructions, the isofrequency sheets extend rostrocaudally through the IC with the rostral aspect of the sheet being situated more ventral than its caudal aspect. In the mediolateral dimensions, sheets are angled at between 40 degrees and 60 degrees to the horizontal, running from a dorsomedial to a ventrolateral position. The low-frequency sheets (0.5 and 2 kHz) are dorsolaterally convex and situated in the dorsolateral region of the IC. The 4 and 10 kHz isofrequency sheets have a helical structure and are situated in the mid-region of the IC. The high-frequency sheets (20 and 30 kHz) are dorsolaterally concaved and situated in the ventromedial region of the IC. The topography of these isofrequency sheets generally agree with, and extended our knowledge of, the tonotopic organization of the IC as derived from electrophysiological studies. The functional organization revealed by the 2-deoxyglucose method only partially correlated with the neural laminae in the anatomical models of the IC proposed by Rockel and Jones [J. Comp. Neurol. 147 (1973) 11-60] and Oliver and Morest [J. Comp. Neurol. 222 (1984) 237-264]. It is therefore concluded that the neural laminar organization of the IC may not be a necessary substrate for the tonotopic organization seen the IC.

Morphological changes in the cochlear nucleus of congenitally deaf white cats

Investigations in animal models and humans have indicated that congenital deafness produces degenerative changes in the central auditory pathway. The cochlear nucleus is the first central structure that receives cochlear input, and may be considered the origin of ascending auditory pathways. In this context, we studied congenitally deaf white cats, who express early onset cochlear receptor loss, in order to assess the nature of structural changes in cells of the cochlear nucleus. It is conceivable that pathologic alterations in higher auditory structures are transneuronally distributed through this nucleus. The cochlear nuclei of nonwhite cats with normal hearing were compared to those of deaf white cats exhibiting hearing loss in excess of 70 dB SPL. The cochlear nuclei of the deaf white cats were smaller in volume by roughly 50%, with the ventral and dorsal divisions being equally affected. Cell body silhouette area was determined for spherical bushy cells of the anteroventral cochlear nucleus (AVCN), pyramidal cells of the dorsal cochlear nucleus (DCN), sensory neurons from the principal trigeminal nucleus, and motoneurons of the facial nucleus. We found no statistical difference in neuronal cell body size between nonauditory neurons of these two groups of cats, whereas auditory neurons of deaf white cats were 30.8-39.4% smaller than those of normal cats. These data imply that neuronal changes in congenitally deaf cats are specific to the auditory pathway. Although cochlear nucleus volume loss was uniform for both divisions, there was a differential effect on cell density: AVCN cell density increased by 40%, whereas DCN cell density was relatively unaffected (10% increase). Astrocyte density was also greater in the AVCN (52%) compared to that in the DCN (5%). These observations reveal a differential impact on cells in the cochlear nucleus to congenital deafness, suggesting selective processing impairment at this level. If similar patterns of degeneration occur in humans, such pathologies may underlie reduced processing of input from cochlear implants in congenitally deaf adults.

Selective labeling of [3H]2-deoxy-D-glucose in the snake trigeminal system: basal and infrared-stimulated conditions

[3H]2-Deoxy-D-glucose (2-DG) and high-resolution autoradiography were employed to investigate labeling patterns of the trigeminal and infrared sensory system in a crotaline snake, the pit viper (Trimeresurus flavoviridis). Following intracardiac injection of 9.25 MBq [3H]2-DG, neurons in the nucleus of the lateral descending trigeminal tract (LTTD), nucleus reticularis caloris (RC), nucleus trigemini mesencephalicus, nucleus trigemini motorius, and trigeminal ganglia were labeled in various degrees after the pit organ had been removed (basal condition). This revealed that a higher rate of glucose utilization occurred in these nuclei than in the common sensory trigeminal nuclei, which lacked labeling entirely. When a pit was stimulated periodically with an infrared stimulus for 45 min, the difference in percentage of labeled cells was ipsilaterally increased by 12.84% in large cells of the LTTD and by 7.55% in the RC, as compared with the contralateral, basal-condition side. These slight changes indicate a small increase of glucose consumption during infrared reception. On the other hand, the small cells in the LTTD showed labeling that did not change with stimulation, suggesting that 2-DG uptake in inhibitory interneurons is relatively constant.

GABAergic terminals in nucleus magnocellularis and laminaris originate from the superior olivary nucleus

The auditory brainstem nuclei, angularis (NA), magnocellularis (NM), and laminaris (NL) of the chicken, Gallus, contain terminals that stain for antibodies against the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA). Some of these terminals originate from cells surrounding nucleus magnocellularis. Results from this study indicate that the majority of the GABAergic terminals found in NA, NM and NL originate from the superior olivary nucleus (SON). Injections of cholera toxin and horseradish peroxidase show that superior olivary nucleus (SON) neurons, which respond to pure tones, project bilaterally to NA, NM, and NL. NA and NL are reciprocally connected with the SON. More NA cells project to the SON than NL cells. While SON neurons project to NM, NM neurons do not project axons back to the SON. The configuration of SON terminals in NA, NM and NL matches the pattern of GABA-immunoreactive puncta seen in these three nuclei: they surround individual NM cells, congregate in the dendritic neuropil of NL, and blanket the NA. The data indicate that NA, NM and NL may be affected by two different inhibitory cell types: local interneurons and SON neurons. Patterns of connectivity described in this report suggest that the activity of NA cells could influence NM and NL cell physiology. Specifically, increases in NA cell activity could augment the effects of GABAergic SON neurons on NM and NL. Hence, binaural perception in the chicken may be more dependent upon changes in intensity cues than previously believed.

Metabolic mapping of the forelimb motor system in the rat: local cerebral glucose utilization following execution of forelimb movements mainly involving proximal musculature

The present study was undertaken to establish a metabolic map of forelimb motor pathways under conditions of physiological activation. For that purpose, we used the [14C]2-deoxy-D-glucose (2-DG) method to identify forebrain and midbrain centers showing an increase in 2-DG uptake in animals trained to execute specific lever-pressing movements with the right forelimb. Following repetitive execution of these movements, principally involving proximal (shoulder, elbow, and wrist) muscles, increases in 2-DG uptake were found contralaterally in several neocortical or subcortical centers. The largest left-right differences in local cerebral glucose utilization (LCGU) were found in a central region of the sensorimotor cortex composed of the caudal part of area 3 of the frontal cortex (Fr3; p < 0.01), the intermediate part of area 1 of Fr (Fr1; p < 0.01), and the forelimb cortical area (p < 0.04). Fr3 was the brain center with the highest differences in left-right LCGU. This central region of the sensorimotor cortex seems to correspond closely to the caudal forelimb area of Neafsey et al. (1986). Intermediate left-right differences in LCGU were found (1) in the just-adjoining rostral-medial areas of the motor cortex involving the intermediate part of area 2 of Fr (Fr2; p < 0.01) and the rostral part of Fr1 (p < 0.04), and (2) in the rostral part of area 1 of the parietal cortex (Par1; p < 0.01) and the caudal part of area 2 of Par (Par2; p < 0.05), both corresponding to forelimb representation. Weak (not statistically significant) left-right differences in LCGU were found in the rostral parts of Fr2 and Fr3, in the caudal parts of Fr2 and Fr1, in the hindlimb cortical area, and in the caudal part of Par1 and the rostral part of Par2. In the remaining cortical areas (cingulate; agranular and granular retrosplenial; temporal; and occipital), there was practically no difference in left-right 2-DG uptake. In addition, increased 2-DG uptake was present contralaterally in several subcortical motor-related centers. In those centers in which a somatomotor map has been established (caudate putamen, ventral lateral and ventral posterolateral thalamic nuclei, and red nucleus), increased 2-DG uptake was found in regions corresponding to forelimb representation.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid increase in mitochondrial volume in nucleus magnocellularis neurons following cochlea removal

Second-order auditory neurons in nucleus magnocellularis (NM) of the chick brainstem undergo a series of rapid metabolic changes following unilateral cochlea removal, culminating in the death of 25% of NM neurons. Within hours of cochlea removal, ipsilateral NM neurons show marked increases in histochemical staining for the mitochondrial enzymes succinate dehydrogenase and cytochrome oxidase. We investigated corresponding ultrastructural changes in NM neurons by preparing animals undergoing unilateral cochlea removal for transmission electron microscopy. We quantified changes in NM mitochondrial volume by stereological methods and qualitatively compared mitochondrial morphology between NM neurons destined to survive and those destined to die after cochlea removal. Within hours of cochlea removal, ipsilateral NM neurons show striking increases in mitochondrial volume (84% at 6 hours and 236% at 12 hours after cochlea removal compared to unoperated, control animals). At 2 week survival times, ipsilateral NM neurons contain fewer mitochondria than contralateral neurons. Surprisingly, anesthesia alone causes short-term increases in NM mitochondrial volume. Animals anesthetized with pentobarbital and ketamine and sacrificed 6 or 12 hours later showed a 45% increase in mitochondrial volume compared to previously unanesthetized animals. NM neurons destined to die within days of cochlea removal can be identified within several hours after deafferentation by the appearance of their ribosomes. We observed qualitative differences in mitochondrial morphology in dying neurons. Mitochondria in neurons destined to die consistently showed mitochondrial swelling and vacuolization indicative of metabolic dysfunction. Similar mitochondrial changes have been reported when mitochondria take up excess calcium. Ultrastructural changes in NM after cochlea removal display features of both programmed and pathological cell death, in which increased intracellular calcium is thought to play a role.

Plastic changes in ipsi-contralateral differences of auditory cortex and inferior colliculus evoked potentials after injury to one ear in the adult guinea pig

In normal adult guinea pigs, evoked potentials recorded at the ipsilateral auditory cortex to monaural high-frequency acoustic stimuli present higher thresholds and lower amplitudes than at the contralateral cortex; in the inferior colliculus, such ipsi-contralateral differences (ICDs) are smaller than in the auditory cortex. Changes in the ICDs were studied after opposite ear injury. Following quasi-complete hair cell destruction induced by sisomicin injection into the contralateral inner ear, threshold ICDs almost disappeared after about two to six days and ipsilateral amplitudes progressively increased in two to three weeks. The occurrence of ICDs at higher auditory centers revealed in this study, indicates peculiar processing of high frequency stimuli in normal guinea pigs. The alteration of ICDs after opposite ear impairment provides a new possibility to study the auditory plasticity in adult animals.

Altered malate dehydrogenase activity in nucleus magnocellularis of the chicken following cochlea removal

The metabolism of second order auditory neurons in nucleus magnocellularis (NM) in the chick brainstem can be profoundly altered when excitatory input from the cochlea is removed. Within hours of cochlea removal, NM neurons show an increase in histochemical staining for the Kreb's cycle enzyme succinate dehydrogenase (SDH), followed in several days by decreases in SDH staining. We examined the activity of another Kreb's cycle enzyme, malate dehydrogenase (MDH) using a histochemical stain and a sensitive quantitative biochemical assay for comparison. We found changes in MDH staining similar in time course and magnitude to those of SDH; within 4 h of cochlea removal, MDH activity increases in ipsilateral NM neurons. By 9 days after cochlea removal MDH activity decreases, although not to the same degree as seen with SDH. Biochemical measurements of MDH activity also showed an early increase in activity in ipsilateral NM, followed at 9 days survival by a decrease in activity. Biochemical measurements of the activity of other enzymes in NM may be useful in further defining the metabolic consequences of deafferentation.

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.

Cochleotopic selectivity of a multichannel scala tympani electrode array using the 2-deoxyglucose technique

The 2-deoxyglucose (2-DG) technique was used to study the cochleotopic selectivity of a multichannel scala tympani electrode array in four cats with another acting as an unstimulated control. Each animal was unilaterally deafened and a multichannel electrode array inserted 6 mm into the scala tympani. Thresholds to electrical stimulation were determined by recording electrically evoked auditory brainstem responses (EABRs). Each animal was injected with 2-DG, and electrically stimulated using bipolar electrodes located either distal or proximal to the round window. The contralateral ear was stimulated with acoustic tone pips at frequencies that matched the electrode place. Stimulation of both distal and proximal bipolar electrodes at 3 x EABR threshold, evoked localized 2-DG labelling in both ipsilateral cochlear nucleus (CN) and the contralateral inferior colliculus (IC), which was very similar in orientation and breadth to labelling evoked by the contralateral tone pips. The cochleotopic position of labelling to proximal stimulation was located in the 24-26 kHz region of each structure, whereas the distal labelling was located around 12 kHz. Distal stimulation at 10 x EABR threshold produced very broad 2-DG labelling in IC centered around the 12 kHz place. The present 2-DG results clearly illustrate cochleotopic selectivity using multichannel bipolar scala tympani electrodes. The extent of this selectivity is dependent on electrical stimulus levels. The 2-DG technique has great potential in evaluating the efficacy of new electrode array designs.

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.

Unilateral olfactory deprivation: effects on succinate dehydrogenase histochemistry and [3H]leucine incorporation in the olfactory mucosa

Surgically closing one external naris reduces airflow through one half of the nasal cavity, decreasing the access of odors to the receptor sheet. In rats, unilateral naris occlusion performed near birth results in large reductions in the size of the olfactory bulb, the primary central relay, when examined 30 days later. Previous research has demonstrated that there is a rapid reduction in [3H]2-deoxyglucose (2-DG) and [3H]leucine uptake in the bulb within hours after naris closure. The present study examined whether similar rapid changes could be observed in the sensory periphery. Pups occluded on P1 and examined on P3 with succinate dehydrogenase histochemistry exhibited reduced staining on the closed side of the nasal cavity, suggesting occlusion results in reductions in mucosal metabolism. Larger differences in staining were observed in pups examined at P6. [3H]Leucine incorporation was quite similar on both sides of the nasal septum as late as 30 days post occlusion, suggesting less dramatic changes in protein synthesis. The results suggest that naris closure does indeed have rapid effects on mucosal function, but indicate that the changes are different than those observed in the bulb.

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.

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)

Cytochrome oxidase response to cochlea removal in chicken auditory brainstem neurons

Changes in cytochrome oxidase (CO) activity were studied in the chick brainstem auditory nuclei, n. magnocellularis (NM) and n. laminaris (NL), following unilateral cochlea removal. Chickens aged 10 days or 56 weeks underwent unilateral cochlea removal. Following survival periods of 30 minutes to 14 days for the 10-day-old birds and 6 hours or 14 days for the 56-week-old birds, the animals were perfused with paraformaldehyde/glutaraldehyde fixative. Cryostat sections of the brainstem were then prepared for CO histochemistry. Microdensitometry was used to quantify the difference in CO staining in NM and NL ipsilateral and contralateral to the cochlea removal. Since the cochlea projects to the ipsilateral NM, the contralateral NM was used as a within-animal control. In normal chickens, NM cell bodies and the cell bodies and dendrites of NL neurons stain darkly for CO in both young and adult birds. In 10-day-old birds, there is no significant change in CO staining in NM from 30 minutes to 3 hours after cochlea removal. Then, a rapid biphasic change in CO staining was found in the ipsilateral NM. An increase in staining was observed 6 to 24 hours postoperatively, followed by a decrease in CO staining at 3- to 14-day survival times. In the 56-week-old birds, no increases in CO staining were observed 6 hours after cochlea removal, but a decrease in CO staining was found 14 days postoperatively. In NL, no changes were observed until 3 days (10-day-old birds) or 14 days (56-week-old birds) after cochlea removal. Then a decrease in CO staining was observed in the dendritic and glial/fiber regions of NL containing axons from the deafferented NM. Thus it appears that afferent input has a regulatory effect on the oxidative metabolism of neurons in the chicken auditory brainstem nuclei, an effect that differs with the age of the animal at the time of afferent manipulation.

Patterns of functional innervation in the auditory nuclei of the chick brainstem following early unilateral removal of the otocyst

Neurons in the third-order auditory nuclei in the brainstem of chicks (nuclei laminares, NL) receive functional innervation from the ipsilateral and contralateral second-order nuclei (nuclei magnocellulares, NM) which is restricted to the dorsal and ventral dendrites respectively. This pattern of innervation in NL is established by embryonic stage 40 (day 15 of incubation). We have examined the distribution of this innervation in both NL at this age or older in embryos from which one otocyst had been removed or damaged on day 3 of incubation. The distribution of functional synapses was determined by analysis of the changes in polarity of field potentials evoked by electrical stimulation of either the ipsilateral or contralateral NM. The distribution of field potential polarity in NL of 40% of recordings in operated embryos and in all sham-operated embryos was the same as that observed in unoperated embryos. However, in the remaining operated embryos, the time course of the field potentials and the changes in the polarity of the responses as the recording electrode penetrated NL were abnormal. The abnormal complexity of responses and the abnormal distribution of field potential amplitude in NL in the operated embryos suggests that loss or damage to the first-order auditory innervation can result in (a) the formation of novel, functional synapses between second-order auditory neurons, and/or (b) disruption of processes that produce segregated innervation of the dendrites of the third-order auditory neurons.

Blood flow changes in chicken brain stem auditory nuclei following cochlea removal

Cochlea removal results in rapid and persistent metabolic and morphological changes in avian brain stem auditory nuclei. Because such changes in the central nervous system are often associated with changes in local blood flow, we examined blood flow in second-order auditory nucleus magnocellularis (NM) and third-order nucleus laminaris (NL). The diffusible tracer [14C]-iodoantipyrine was infused intravenously into 20- to 26-day-old chickens either 30 min or 6 h after unilateral cochlea removal. This tracer rapidly equilibrates between blood and tissue in proportion to local blood flow. Unoperated animals served as controls. Thirty seconds after tracer infusion, brains were removed and frozen. Cryostat sections were prepared for quantitative film autoradiography. Blood flow in normal and deafferented areas within NM and NL was compared. Nucleus magnocellularis receives its only excitatory input from the ipsilateral cochlea via the eighth nerve. Axons from NM bifurcate and project to the ipsilateral dorsal dendritic region of NL (NLd) and the contralateral ventral dendritic region of NL (NLv). Thirty minutes after cochlea removal, blood flow in ipsilateral NM decreases by 30%. This decrease persists at 6 hours. Blood flow in NL does not change in accordance with the pattern of afferent input from NM. Rather, blood flow in NLd and NLv ipsilateral to cochlea removal is significantly decreased 6 h post lesion. These results are in contrast to the pattern of morphological and metabolic changes observed in NL after cochlea removal.

Afferent regulation of neurons in the brain stem auditory system

We have reviewed a series of experiments which begin to examine the cellular events underlying afferent regulation of neuronal structure. Our initial interest in such experiments stemmed from a desire to understand the cellular nature of experiential influences on brain development. While this remains a long-range goal, it's elusive nature has become increasingly apparent; how will we know when such a goal is achieved? On the other hand, it has become increasingly clear that by approaching this question as a subset of the larger problem of tissue interactions regulating nervous system structure and function, some progress is possible. In this respect, understanding afferent regulation is part and parcel of understanding "competition." Both exemplify the fact that we are dealing with a dynamic system, where changes in the balance of extracellular factors result in a cascade of events defining a new "steady state." Unfortunately, most of our methods are limited to taking "snap-shots" of a few parameters and attempting to reconstruct an epic. Our analyses of the postsynaptic events following cochlea removal have only scratched the surface. They are beginning to reveal myriad cellular processes that are dramatically altered by changing the balance of synaptic activity, or "synaptic drive," in a neuronal system. We have been continually struck by the rapidity of these postsynaptic changes when the manipulations are performed on immature animals. While the kinetics of metabolic and structural events we have studied do not yet match those of ionic events involved in information transmission, the two classes of intercellular communication are coming much closer. Some neuromodulators can alter synaptic currents for up to many seconds, and we have shown that altering afferent activity can cause changes in protein synthesis within a few minutes. The merging of these two classes of phenomena should come as no surprise since our studies and many others have definitively linked a variety of metabolic and structural events to changes in the synaptic drive between two neurons. On the other hand, this progress does highlight the need for increased attention to the short-term changes following manipulations of afferent activity. Hopefully such studies will lead to an understanding of the intracellular chain of events responsible for the regulation of neuronal form. A second area of interest has been the age restrictions on the events we have studied.(ABSTRACT TRUNCATED AT 400 WORDS)

Cochlear ablation in deafness mutant mice: 2-deoxyglucose analysis suggests no spontaneous activity of cochlear origin

Deafness mutant mice show no stimulus-related cochlear potentials as well as abnormal electrically-evoked responses recorded from the inferior colliculus. Abnormal spontaneous activity in the auditory periphery could result in abnormal development and/or maintenance of the central auditory pathways. We therefore assessed spontaneous activity of cochlear origin in the central nuclei of the mutants by ablating one cochlea and subsequently using the 2-deoxyglucose (2DG) technique to study metabolic activity. Any asymmetries in labeling in a given nucleus should be due to spontaneous activity in the cochlear nerve on the unoperated side. In control animals (+/dn mice undergoing unilateral cochlea ablation), statistically significant decreased 2DG labeling was observed in the ipsilateral PVCN and AVCN, and contralateral MNTB and IC; all receive primary excitatory input from the ablated ear. No significant differences in labeling between right and left sides were observed in any of the nuclei studied in the mutant animals. These findings suggest that there is no spontaneous activity of cochlear origin in these mutants, even though many cochlear nerve fibers and spiral ganglion cells survive.

Auditory pathway and auditory activation of primary visual targets in the blind mole rat (Spalax ehrenbergi): I. 2-deoxyglucose study of subcortical centers

The blind mole rat Spalax ehrenbergi is a subterranean rodent that shows striking behavioral, structural, and physiological adaptations to fossorial life including highly degenerated eyes and optic nerves and a behavioral audiogram that indicates high specialization for low-frequency hearing. A 2-deoxyglucose functional mapping of acoustically activated structures, in conjunction with Nissl/Klüver-Barrera-stained material, revealed a typical mammalian auditory pathway with some indications for specialized low-frequency hearing such as a poorly differentiated lateral nucleus and a well-developed medial nucleus in the superior olive complex. The most striking finding was a marked 2-deoxyglucose labeling of the dorsal lateral geniculate body and of cortical regions that correspond to visual areas in sighted rodents. The results render the blind mole rat a good model system for studying natural neural plasticity and intermodal compensation. In this report, we confine ourselves to the subcortical levels. The cortical level will be dealt comprehensively in a following paper.

Decreased protein synthesis in cochlear nucleus following developmental auditory deprivation. Use of vascular saline perfusion to improve small tissue sample analysis

The incorporation of tritiated leucine was used as an index of protein synthesis in the cochlear nucleus (CN) of mice unilaterally (right side) hearing deprived throughout the period of hearing development. Right-left differences in radiolabel concentration were measured by scintillation counting of whole tissue homogenates. To improve upon the detection of small differences in radiolabel incorporation, the brain was perfused with saline prior to removal of CN tissue and the results compared with the standard nonperfusion method of tissue collection. Statistical analyses demonstrated the perfusion significantly reduced the acid soluble (unbound) label in CN without affecting the amount of protein bound label. Furthermore, a significant right side decrease in leucine incorporation was seen with the perfusion treatment, but not in the nonperfused treatment. This demonstrated that developmental auditory deprivation led to a decrease in protein synthesis at maturity. The results also demonstrated that mechanisms for leucine uptake were not impaired and the decrease in protein synthesis was not due to reduced availability of precursor amino acid. Thus, the use of saline perfusion prior to tissue collection facilitated the identification of protein synthesis differences that were unidentified by the traditional method.

Auditory brainstem of the ferret: effects of unilateral cochlear lesions on cochlear nucleus volume and projections to the inferior colliculus

Unilateral lesions of the right cochlea were made in ferrets aged postnatal day (P)12 to P93. The extent of the lesions was assessed by counting remaining hair cells and ganglion cells in midmodiolar sections through the lesioned cochleas and by comparison with a sample of unlesioned cochleas. The neural effects of the lesions were assessed by measuring the volume of each cochlear nucleus (CN) and by counting the number of neurons in each CN that were retrogradely labeled following injections of WGA-HRP in the left inferior colliculus (IC). Survival times between lesioning and injection of the tracer ranged from 11 to 98 days. CN volume and projections to the IC were also measured in a sample of normal adult ferrets and in normal infants aged P39 to P80. Cochlear lesions resulted in a reduction of the volume of the CN on the lesioned side, relative to the other CN, in animals of all ages and survival times. The extent of the CN volume reduction was negatively correlated with the number of remaining cochlear ganglion cells. However, even where the number of ganglion cells was within the normal range, significant volume reductions occurred. The ventral CN was more severely affected by the lesions than the dorsal CN, but no difference was found between the anteroventral and posteroventral divisions of the nucleus. There was no significant difference in the extent of CN volume reductions between animals of different ages or survival times. Lesions of the right cochlea in younger animals (P14 to P24) resulted, after 90 days survival, in an increase in the number of left CN neurons projecting to the left IC. No significant increase was seen following lesions in older (P90) ferrets or following short (11 or 30 days) survival times in young (P14 to P24) ferrets. The extent of the increase in the ipsilateral CN-IC projection was not related to the number of remaining ganglion cells or to the division of the CN examined. Lesions did not affect the contralateral CN-IC projection. We conclude that cochlear lesions in infant ferrets can alter auditory brainstem morphology and connectivity. The dependence of these alterations on the age of the animal, survival time following lesion, and extent of the lesion varies markedly with the index examined.