Organization and development of brain stem auditory nuclei of the chicken: tonotopic organization of n. magnocellularis and n. laminaris.

Extracellular recordings of responses to tone-burst stimulation were used to determine the tonotopic organization of n. magnocellularis (NM) and n. laminaris (NL) in hatching chickens. NM cells show "primary-like" response patterns to ipsilateral stimulation, and are arranged in dorso-ventral isofrequency columns. Units responding to the highest frequency tones (about 4,100 Hz) are situated at the rostromedial pole of the medial division. Units with lower characteristic frequencies (CF's) are found at successively caudal and lateral sites, until extremely low CF's ( less than 500 Hz) are represented dorsoventrally in the daudolateral tail of the lateral division. No evidence was found of auditory input to the region which receives projections from the macula lagena. NL receives polarized, binaural, excitatory input. Units have similar CF's and thresholds to tones presented to either ear. The tonotopic organization in NL matches that found in NM--high CF's rostromedially and low CF's caudal and lateral. Quantitative procedures were developed for relating CF to the position of a unit within either nucleus. These analyses account for 79% and 89% of the frequency variance found within NM and NL, respectively, and predict the CF of a neuron by its position within each nucleus.     

Organization and development of brain stem auditory nuclei of the chicken: organization of projections from n. magnocellularis to n. laminaris.

The tonotopic and topographic organization of the bilateral projection from second-order auditory neurons of nucleus magnocellularis (NM) to nucleus laminaris (NL) was examined in young chickens. In one group of birds, the NM axons which innvervate the contralateral NL were severed by cutting the crossed dorsal cochlear tract at the midline. Heavy terminal degeneration in NL was confined to the neuropil area immediately ventral to the perikaryl lamina. Very little degeneration was seen in the dorsal neuropil region. In a second series of animals, the charactertistic frequency (CF) of cells in an area of NM was first determined by microelectrode recording techniques and then a small electrolytic lesion was made through the recording electrode. Following survival periods of 24-48 hours, the distribution of projections from the lesioned area to the ipsilateral and contralateral NL was examined using the Fink-Heimer method. As previously described in the pigeon, projections from NM terminate densely in the neuropil region immediately dorsal to the ipsilateral NL cell bodies and ventral to the perikaryl layer on the contralateral side, providing each NL neuron with segregated binaural innervation. Lesions in any area of the NM produced degeneration confined to a limited caudo-rostral and medio-lateral portion of both laminar nuclei. To investigate this topographic relationship, the cuado-rostral extents of the lesion in NM and of the resulting degeneration in both NL were determined. Linear regression and correlation analyses then related these positional values to each other and to the CF found at the center of each lesion. All correlations were highly significant and ranged from 0.78 between the position of the lesion in NM and CF to 0.91 between the caudo-rostral position of degeneration in the NL ipsilateral and contralateral to the lesion. It is concluded that neurons in NM project in a very discrete topographic, tonotopic and symmetrical fashion to NL on both sides of the brain, contributing to the binaural response properties and tonotopic organization of neurons in NL. The results also suggest that the organization of projections from NM to NL could provide a mechanism for the differential transmission delay required by a "place" model of low-frequency sound localization.     

Organization and development of brain stem auditory nuclei of the chicken: dendritic gradients in nucleus laminaris.

Nucleus laminaris (NL) is a third-order auditory nucleus in the avian brain stem which receives spatially-segregated binaural inputs from the second-order magnocellular nuclei. The organization of dendritic structure in NL was examined in Golgi-impregnated brains from hatchling chickens. Quantitative analyses of dendritic size and number were made from camera lucida drawings of 135 neurons sampled from throughout the nucleus. The most significant results of this study may be summarized as follows: (1) The preponderant neuron in n. laminaris may be characterized as having a cylindrical-to-ovoid cell body, about 20 micrometer in diameter. The neurons comprising NL were found to be nearly completely homogeneous in issuing their dendrites in a bipolar fashion: one group of dendrites is clustered on the dorsal surface of the cells, the other group on the ventral. The dendrites of NL are contained within the glia-free neuropil surrounding the nucleus. From the rostromedial to the caudolateral poles of NL there is a gradient of increasing extension of the dendrites, increasing number of tertiary and higher-order dendrites, and increasing distance from the somata of the occurrence of branching. (2) The total dendritic size (sum of the dorsal) and ventral dendritic lengths of the cells) increases 3-fold from the rostromedial to the caudolateral poles of NL. About 50% of the variance in dendritic size is accounted for by the position of the cells in NL, and the gradient of dendritic size increase has the same orientation across NL as the tonotopic gradient of decreasing characteristic frequency in NL. (3) From the rostromedial pole to the caudolateral pole of NL there is an 11-fold decrease in the number of primary dendrites along a gradient coinciding with the length and frequency gradients. Sixty-six percent of the variance in dendrite number is accounted for by position in the nucleus. (4) The correlation of dorsal and ventral dendritic size on a cell-by-cell basis is not high (r = 0.47), indicating a fair amount of variability on the single-cell level. On the other hand, the average dorsal dendritic length within an isofrequency band in NL correlates very highly with the average ventral dendritic length. Thus, on an areal basis, the amount of dendritic surface area offered to the dorsal and ventral afferents is tightly regulated. (5) The dorsal and ventral dendrites have separate gradients of increasing length and number across NL. The dorsal gradients are skewed toward the rostrocaudal axis, while the ventral dendritic gradients are skewed mediolaterally. (6) There was no correlation between either dendritic size or number of primary dendrites and the size of the somata in NL, which remains relatively constant throughout the nucleus. Several hypotheses about the ontogenetic control of dendritic structure are examined in light of the above data. Of these, the hypotheses that the ontogeny of dendritic size and number is largely under afferent control receives a great deal of circumstantial support.     

Afferent influences on brain stem auditory nuclei of the chicken: effects of conductive and sensorineural hearing loss on n. magnocellularis

Nucleus magnocellularis is the avian homologue of the spherical cell region of the mammalian anteroventral cochlear nucleus. Its primary excitatory synaptic input is from large end bulbs of Held from the eighth nerve ganglion cells. We have examined the effects of three peripheral manipulations--middle ear ossicle (columella) removal (monaural and binaural), columella removal and oval window puncture (monaural), and monaural earplug--on cross-sectional cell area ("cell size") of second-order auditory neurons in n. magnocellularis of the chicken. Manipulations were performed between embryonic day 19 and posthatch day 4. Survival time was varied from 2 to 60 days. Air conduction and bone conduction thresholds were determined to assess for conductive and sensorineural hearing loss associated with each of these manipulations. Hair cell counts were made from basilar papillae of each experimental group. We found that a columella removal alone, which produced a 50-55-dB purely conductive hearing loss, was not associated with changes in cell size of n. magnocellularis neurons. Similarly, chronic monaural earplugging did not affect the cross-sectional area of these second-order auditory neurons. Conversely, a combined columella removal and oval window puncture, which produced a mixed hearing loss with a 15-40-dB sensorineural component was associated with an 18-20% reduction in n. magnocellularis cell area. Hair cell counts for experimental ears were not significantly different from control ears. These results, in conjunction with measurements of multiunit activity recorded in n. magnocellularis, suggest that manipulations which markedly attenuate extrinsic auditory stimulation, but do not result in chronic change in the average activity levels, also do not influence the size of n. magnocellularis cell bodies. On the other hand, a manipulation which influences overall activity levels, but does not result in degeneration of receptor cells, resulted in marked changes in n. magnocellularis cell size.     

Organization and development of the brain stem auditory nuclei of the chicken: primary afferent projections

The pattern of primary auditory projections to the brain stem of young chickens was investigated using terminal degeneration methods and orthograde transport of horseradish peroxidase (HRP) or tritiated amino acid. Of particular interest was the question of whether nucleus laminaris (NL) receives primary afferents. A study of silver-stained degeneration pattersn in nucleus magnocellularis (NM) and NL at three intervals following unilateral interruption of the cochlear nerve revealed that by 48 hours after the lesion, degenerating terminals were found only in the ipsilateral nucleus angularis (NA), NM and lagenar projection areas but not in NL. Five- and eight-day survival times, however, also revealed degeneration bilaterally in NL. The appearance of terminal degeneration in NL at the longer survival times is attributed to the previously-reported severe and rapid transneuronal degeneration of neurons in NM following deafferentation and not to the presence of cochlear nerve terminals in NL. Injection of HRP or tritiated proline into the basilar papilla produced patterns of labeling similar to that seen in the 2-day degeneration material; HRP reaction product or autoradiographic label were seen only in the ipsilateral NA and NM and in the ipsilateral projection areas of the macula lagena but not in either NL. The patterns of primary auditory projections revealed by the three methods were quite similar to each other and to that previously reported for the pigeon and confirm the conslucion that the laminar nucleus of chickens does not receive primary afferents.     

Afferent influences on brain stem auditory nuclei of the chicken: presynaptic action potentials regulate protein synthesis in nucleus magnocellularis neurons

Studies of the avian auditory system indicate that neurons in nucleus magnocellularis (NM) and nucleus laminaris of young animals are dramatically altered by changes in the auditory receptor. We examined the role of presynaptic activity on these transneuronal regulatory events. TTX was used to block action potentials in the auditory nerve. TTX injections into the perilymph reliably blocked all neuronal activity in the cochlear nerve and NM. Far-field recordings of sound-evoked potentials revealed that responses returned within 6-12 hr after a single TTX injection. Changes in protein synthesis by NM neurons were measured by determining the incorporation of 3H-leucine using autoradiography. NM neurons on the side of the brain ipsilateral to the TTX injection were compared to normally active cells on the other side of the same tissue section. Grain counts over individual neurons revealed that a single injection of TTX produced a 40% decrease in grain density in ipsilateral NM neurons within 1.5 hr after the TTX injection. However, by 24 hr after a single TTX injection, grain densities were not different on the 2 sides of the brain. Continuous activity blockade for 6 hr caused the cessation of amino acid incorporation in a portion of NM neurons and a 15-20% decrease in the remaining neurons. These changes in amino acid incorporation are comparable to those following complete removal of the cochlea (Steward and Rubel, 1985). We also examined NM for neuron loss and soma shrinkage after blocking eighth nerve action potentials. TTX injected every 12 hr for 48 hr caused a 20% neuron loss and an 8% shrinkage of the remaining neurons. Similar reductions were found following cochlea removal (Born and Rubel, 1985). It is concluded that neuronal activity plays a major role in the maintenance of normal NM neurons. Furthermore, these results suggest that transneuronal morphological changes seen in neurons following deafferentation or alterations of sensory experience are a result of changes in the level of presynaptic activity.     

Organization and development of brain stem auditory nuclei in the chick: Ontogeny of postsynaptic responses

The onset of responsiveness to eighth nerve stimulation was examined in n. magnocellularis and n. laminaris, (second- and third-order neurons) of the chick brainstem auditory system. Extracellular microelectrode mapping techniques were used to examine postsynaptic responses in in vitro brainstem preparations. Two specific questions were addressed. First, what is the earliest time at which postsynaptic action potentials can be evoked in n. magnocellularis and n. laminaris by eighth nerve stimulation? Second, does responsiveness to eighth nerve stimulation develop along a spatial gradient in n. magnocellularis and, if so, how does this gradient compare with other developmental events observed in the chick auditory system? Postsynaptic responses in n. magnocellularis were first recorded at 11 days of incubation. Nucleus laminaris responses to direct stimulation of n. magnocellularis were also first recorded at 11 days, although n. laminaris responses to eighth nerve stimulation were not seen until 12 days of incubation. A gradient of response development within n. magnocellularis was indicated by mapping of responsive sites on days 11–13. At 11 days, responses to eighth nerve stimulation were restricted to the most anteromedial portion of n. magnocellularis. Between 11 and 13 days, cells in increasingly more posterolateral portions of n. magnocellularis became responsive. This is anteromedial-to-posterolateral gradient in n. magnocellularis is correlated with the basal-to-apical gradient of morphogenesis observed in the basilar papilla and morphogenetic gradients previously observed in n. magnocellularis and n. laminaris.     

Afferent influences on the development of the brain stem auditory nuclei of the chicken: Otocyst ablation

The effects of embryonic deafferentation on the morphological development of the avian cochlear nuclei, n. angularis (NA) and n. magnocellularis (NM), were investigated. The right otocyst was surgically removed from chick embryos at 55 to 60 hours of incubation and the subsequent development of total volume, neuron number, and neuron cross-sectional area were studied with quantitative methods in animals sacrificed at 2-day intervals between embryonic days 9 and 19 and at 28 days posthatching. The development of NA and NM is severely affected by otocyst ablation. Between embryonic days 9 and 19, a large group of NA neurons in the medioventral portion of the nucleus on the perated side moves to an ectopic ventromedial position, while the remainder of this nucleus stays in its normal dorsolateral position. Beginning about day 13 of incubation, the normal increase in the volume of NA and the size of its neurons becomes progressively retarded and 40% of its neurons are lost. The growth of NM is also retarded after day 11 of incubation and the growth of mean neuron size is retarded after day 15. There is a 30% loss of neurons in NM which begins after embronic day 11. The results indecate the primary cochlear fibers make a critical contribution to the growth and maintenance of their target neurons. The absence of this facilitative influence following otocyst ablation becomes apparent just at the time synapses would normally be formed between the promary auditory afferents and the brain stem auditory neurons. The abnormal movement of neurons in nucleus angularis to an ectopic position after otocyst ablation suggests that primary auditory afferents may serve to stabilize the position of their target cells within the developing brain.     

Ontogeny of tonotopic organization of brain stem auditory nuclei in the chicken: implications for development of the place principle

The morphological development of the cochlea begins in the base or midbasal region and spreads toward the apex. In adults, the base responds maximally to high-frequency sounds and lower frequencies are represented progressively toward the apex. This predicts that responses to sound should occur initially to high frequencies and gradually change to include lower frequencies. Paradoxically, animals respond first to relatively low frequencies and last to high frequencies. We have previously proposed that this discrepancy results from an ontogenetic change in spatial coding of frequency along the cochlea (Rubel et al., '76). According to this model, only the basal end of the cochlea transduces sound early in development but it responds to low frequencies. During maturation the representation of low and midrange frequencies shifts apically and the base becomes responsive to high frequencies. This hypothesis predicts that the tonotopic organization within the central nervous system should change during development; neurons at any given location within an auditory nucleus should become maximally responsive to successively higher frequency sounds during development. In the present study this prediction was tested by using microelectrode recording procedures to map the tonotopic organization of nucleus magnocellullaris (NM) and nucleus laminaris (NL), first- and second-order auditory nuclei, in chickens at three ages: embryonic day 17, 1 day posthatch, and 2-4 weeks posthatch. The characteristic frequencies of neurons having the same anatomical location were quantitatively compared across ages. The tonotopic order in NM and NL was similar at all ages; responses to high-frequency sounds were recorded anteromedially and lower frequencies were located progressively more caudolaterally. However, there was a striking quantitative change in tonotopic organization. Neurons at a given location in both nuclei became maximally responsive to progressively higher frequencies during development. The characteristic frequencies of neurons in embryos and newly hatched chicks averaged, respectively, 1.00 (+/- 0.06, S.E.M.) and 0.34 (+/- 0.04) octaves lower than their predicted adult values. All regions in both nuclei showed a statistically significant increase in characteristic frequency during development except the most posterolateral (low-frequency) sector. Too few neurons were recorded from this region to be able to reliably estimate characteristic frequency. These results support the hypothesis that the spatial coding of frequency along the cochlea shifts during development.(ABSTRACT TRUNCATED AT 400 WORDS)     

Afferent influences on brain stem auditory nuclei of the chicken: neuron number and size following cochlea removal

The consequences of cochlea removal on neuron number and soma cross-sectional area were examined in the second order auditory nucleus (n. magnocellularis) of chickens. Both the age of the subjects at the time of cochlea (basilar papilla) removal (1-66 weeks) and the survival period (1-45 days) were varied. Neuron number and soma cross-sectional area were determined from Nissl stained sections. Additional material was processed to examine the relationship of ganglion cell loss to changes in n. magnocellularis. Neuron number decreased by 25-30% and soma cross-sectional area decreased by 10-20% ipsilateral to the cochlea removal in chickens operated on during the first 6 weeks after hatching. In contrast, in chickens operated on at 66 weeks posthatch neuron number decreased less than 10% and there was no change in soma area. The changes were rapid, being nearly complete 2 days after cochlea removal. An initial change (1 and 2 days after surgery) observed in animals operated on up to 6 weeks posthatch was the presence of a large number of neurons in which no Nissl substance could be detected. These results demonstrate an age-dependent change in the susceptibility of NM neurons to deafferentation. This change is not temporally related to other measures of functional maturation of the auditory system.     

Embryogenesis of arborization pattern and topography of individual axons in N. laminaris of the chicken brain stem

This study examined the development of individual axon terminal fields in n. laminaris (NL) of the chicken brainstem. In their mature form axons from the nucleus magnocellularis (NM), second-order auditory neurons in the chicken brainstem, project bilaterally onto the NL. Axons from the ipsilateral and contralateral NM neurons form spatially segregated, elongated arbors in the dorsal and ventral neuropil of NL, respectively. The long axes of these arbors correspond to physiologically defined isofrequency bands. To assess the development of this stereotyped arborization pattern, 6-17-day embryonic chicken brain stems were maintained in vitro while injecting horseradish peroxidase into small groups of axons. Three-dimensional reconstructions were made from serial sections and projected onto a cartesian plane for quantitative analyses. At embryonic day 6 (E6), the ventral axons already course beneath the recently migrated NL neurons. The arrival of the dorsal NM axon branches is delayed and their paths are indirect. They first loop dorsally into the the ventricular layer, where they seem to make specific connections with migrating NL neurons and use these as guides to their appropriate positions in the NL. During the period from E9 to E17 the dorsal and ventral terminal fields become similar, each adopting properties of the other's initial pattern. The dorsal terminal fields extend to form bands similar to the early ventral terminal fields, while the ventral terminal fields narrow and appear to shift position in order to achieve the tonotopic specificity characteristic of the early dorsal terminal fields. The results show that a complex, mature pattern of neuronal connections can be formed during development by the combination and reorganization of two simple patterns--each shaped, in turn, by its respective axonal trajectory.     

Afferent influences on brain stem auditory nuclei of the chicken: time course and specificity of dendritic atrophy following deafferentation

The time course and specificity of the changes in dendritic morphology following deafferentation were examined in nucleus laminaris of young chickens. The dendrites of nucleus laminaris neurons are segregated into dorsal and ventral domains, which are innervated separately from the ipsilateral and contralateral nucleus magnocellularis, respectively. Transection of the crossed dorsal cochlear tract deafferents the ventral dendrites of nucleus laminaris bilaterally without interrupting the matching input to the dorsal dendrites. In 10-day-old chicks, atrophy of the ventral dendrites began immediately after transecting the tract; the ventral dendrites were 10% shorter by 1 hour and 16% shorter by 2 hours after deafferentation. The length of the ventral dendrites progressively decreased over the next 2 weeks, resulting in at least a 60% loss of ventral dendrite 16 days after surgery. The dorsal dendrites of the same cells, whose afferents remained intact, did not change in length during the time course of this study. However, 16 days after the lesion, spines appeared on the normally smooth dorsal and ventral dendrites. The time course of dendritic atrophy and its restriction to the deafferented postsynaptic surface are related to possible mechanisms by which afferents regulate and maintain their target neurons.     

Afferent influences on brain stem auditory nuclei of the chicken: changes in succinate dehydrogenase activity following cochlea removal

We have examined one of the metabolic consequences of unilateral cochlea (basilar papilla) removal in the chick brain stem auditory system. We assessed changes in succinate dehydrogenase (SDH), a mitochondrial enzyme involved in energy metabolism, in neurons of second-order n. magnocellularis (NM) and third-order n. laminaris (NL). Chickens undergoing surgery at 10 days of age were perfused 4 hours to 35 days postlesion. Chickens 6 or 66 weeks of age at cochlea removal were examined 1 or 8 days after surgery. In all groups, cryostat sections were prepared for SDH histochemistry or Nissl staining. In normal chickens, NM cell bodies and NL neuropil contain SDH reaction product. In young birds, the density of SDH reaction product in NM shows a rapid biphasic response to cochlea removal. From 8 to 60 hours postlesion, density increases ipsilateral to cochlea removal; for survival times of 3-35 days, SDH density decreases in ipsilateral NM. In NL, no changes were observed until 3 days after cochlea removal. Then we observed a long-lasting decrease in density of SDH reaction product in the neuropil regions receiving input from the deafferented NM. All of these changes are age-dependent in that they were observed only following cochlea removal on or before 6 weeks of age.     

Afferent influences on brain stem auditory nuclei of the chicken: cessation of amino acid incorporation as an antecedent to age-dependent transneuronal degeneration

Previous studies of the avian auditory system have revealed that removal of the peripheral receptor (the cochlea) leads to a transneuronal degeneration of auditory relay neurons in nucleus magnocellularis (NM) of the brain stem. An early manifestation of the degeneration which can be observed within 12 hours is a decrease of histochemical staining for RNA (Nissl staining); such a decrease could reflect an alteration in protein synthetic activity within the NM neurons. The present study evaluates this possibility by determining whether the cochlea removal led to an alteration incorporation of protein precursors in the target neurons which exhibit transneuronal degeneration and if so, how early the changes appeared. The cochlea was removed unilaterally in seventeen 10-day-old chicks and two 66-week-old mature chickens, and incorporation of protein precursors was evaluated in the neurons of NM at 0.5, 1.5, 3, 6, 12, and 24 hours following the cochlea removal. Each chick received an intravenous injection of 3H leucine, and was allowed to survive for 30 minutes after the injection of precursor. The brains were then prepared for autoradiography. The extent of incorporation by neurons in NM was determined by counting grains overlying each cell body and determining grain density/micrometers2 of neuron cross-sectional area. We found that auditory relay neurons whose synaptic inputs have been silenced exhibit dramatic decreases in protein synthesis within 30 minutes after removal of the cochlea; leucine incorporation was reduced by about 50%. In chicks sacrificed 3 to 24 hours after removal of the cochlea, some neurons (about 1/3) were entirely unlabeled despite heavy labeling of their neighbors and heavy labeling of all NM neurons on the opposite side of the brain. The remaining neurons exhibited about a 15% reduction in incorporation in comparison with the cells in the contralateral (control) NM. While the decreases in incorporation were apparent at all survival intervals, there was no consistent decrease in Nissl staining until 6 hours after cochlea removal. There were no changes in protein precursor incorporation following removal of the cochlea in adult birds, a result which is in keeping with the relative absence of transneuronal degeneration following removal of the cochlea at maturity. The results suggest a very rapid transneuronal regulation of protein metabolism within target neurons in young animals, perhaps by activity-related events.     

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.     

The raphe nuclei of the rabbit brain stem.

The raphe nuclei of the rabbit brain stem were found in the midline and adjacent reticular formation of the medulla, pons, and mesencephalon. Nuclei raphe obscurus, pallidus, and magnus were located in the medulla. Nucleus raphe pontis and the caudal portion of nuclei raphe dorsalis and centralis superior were present in the pons. The rostral portion of nuclei raphe dorsalis and centralis superior, and nuclei linearis caudalis and intermedius were present in the msencephalon. Wings of neurons extended from the midline clusters of raphe neurons into the adjacent reticular formation. These wings of neurons contained serotonergic perikarya which were cytoarchitecturally indistinguishable from the midline neurons. A detailed localization of these nuclei is presented in atlas form. These raphe nuclei contained heterogeneous populations of neurons which varied in the size, shape and density of the cell bodies. In addition, the dendritic branching, specific orientation of dendrites, and appearance of spines were distinct for each of the raphe nuclei. Individual raphe nuclei often contained several subpopulations of neurons characterized by unique spatial configuration and orientation. The main morphological similarities of the raphe nuclei are location in or adjacent to the midline, the presence of serotonergic cell bodies in all raphe nuclei except the linear nuclei, and heterogeneous cell populations.     

Protein kinase C regulation of glycine and gamma-aminobutyric acid release in brain stem auditory nuclei

Protein kinase C (PKC) can regulate transmitter release in several brain areas. We determined if PKC could regulate the electrically evoked release of radiolabeled glycine (Gly) and gamma-aminobutyric acid (GABA) in dissected samples of several brain stem auditory nuclei, such as the major subdivisions of the cochlear nucleus (CN) and the main nuclei of the superior olivary complex (SOC). The PKC activators, phorbol 12,13-diacetate (PDA) or phorbol 12,13-dibutyrate (PDBu) (3 microM), elevated the release by 1.4- to 2.0-fold. The PKC inhibitor, Ro31-8220 (50 nM), did not alter the release in most of the tissues but blocked the stimulatory effects of PDA and PDBu. This suggested that PKC positively regulates glycinergic and GABAergic release in the sampled nuclei. In the dorsal CN (DCN), Ro31-8220 elevated the release of [(14)C]Gly by 23%, suggesting that PKC negatively regulates glycinergic release in a proportion of DCN synapses. We also determined if PKC could regulate release after unilateral cochlear ablation (UCA). In the anteroventral (AVCN) and posteroventral (PVCN) CN and in the lateral (LSO) and medial (MSO) superior olive, the stimulatory effects of PDBu declined after this lesion and Ro31-8220 failed to alter release. Since UCA failed to alter release in these tissues, the stability of the release correlated with the lack of regulatory capacity of PKC. In the DCN and the medial nucleus of the trapezoid body (MNTB), the stimulatory effects of PDBu persisted after UCA. We previously demonstrated a postablation decline of Gly release in the DCN and elevated GABA release in the MNTB. Treatment of these tissues with Ro31-8220 reversed these changes in release. These findings suggested that PKC regulation persisted in the DCN and MNTB after UCA. Moreover, endogenous regulatory mechanisms activated after UCA probably act through PKC to alter release in these tissues. Thus, limiting PKC activation or activity might ameliorate pathological symptoms that accompany hearing loss and that stem from these plasticities in the DCN and MNTB.     

Bilaterally recorded brain stem auditory evoked responses. Their asymmetric abnormalities and lesions of the brain stem.

Simultaneous bilateral recordings (C3 to A1 and C4 to A2) of brain stem auditory evoked responses have been studied in 67 supratentorial lesions, nine midbrain lesions, 21 intrinsic pontine lesions, and 23 extrinsic compressions of the pons. The responses in supratentorial lesions showed completely normal records. In midbrain lesions, wave V was specifically altered. As wave 1 has been shown to be a far-field seventh nerve potential, and wave V the midbrain potential, waves II to IV can be inferred to originate in the central auditory pathway between the seventh nerve and the midbrain. Alterations of waves II to IV correlated well with localization of pontine lesions, and asymmetric alterations of the bilaterally recorded responses were associated with unilateral lesions of the brain stem auditory pathway and/or lesions of the crossed auditory projections.     

Baclofen and the brain stem auditory evoked potential

Baclofen, a commonly used antispastic drug, is believed to block the release of excitatory amino-acid neurotransmitters. Auditory distortions are one side effect of this drug. Peak 1 of the brain stem auditory evoked potential of the cat was not affected by intravenously applied baclofen (2 to 3 mg/kg). Peaks 2, 4, and 5, however, were strongly suppressed or blocked. The results indicate a basis for the occasional auditory side effects of baclofen and suggest that the transmitter of the auditory nerve is an excitatory amino acid.     

AMPA receptor binding in adult guinea pig brain stem auditory nuclei after unilateral cochlear ablation

This study determined if an asymmetric hearing loss, due to unilateral cochlear ablation, could induce the regulation of intracellular AMPA receptors in brain stem auditory nuclei. In young adult guinea pigs, the high-affinity specific binding of [(3)H]AMPA was measured in the cochlear nucleus (CN), the superior olivary complex (SOC), and the auditory midbrain at 2-147 postlesion days. After correction for tissue shrinkage, changes in specific binding relative to that in age-matched unlesioned controls were interpreted as altered numbers and/or activity of intracellular AMPA receptors. In the CN, transient elevations and/or deficits in binding were evident in most regions, which usually recovered by 147 days. However, persistently deficient binding was evident ipsilaterally in the anterior part of the anteroventral CN (AVCNa). In the SOC, transient elevations in binding were evident at 2 days in the medial limb of the lateral superior olive (LSOmed) and the medial superior olive. Between 7 and 147 days, most SOC nuclei exhibited transient, temporally synchronized postlesion deficits in binding. However, late in the survival period, deficits persisted ipsilaterally in the LSOmed and the lateral (LSOlat) limb of the lateral superior olive. In the midbrain, transient elevations and/or deficits in binding were evident in the dorsal nucleus of the lateral lemniscus as well as in the central and dorsal nucleus of the inferior colliculus. A persistent deficit was evident in the intermediate nucleus of the lateral lemniscus. The findings implied that auditory neurons contain regulatory mechanisms that control the numbers and/or activity of intracellular AMPA receptors. Regulation was induced by cochlear nerve destruction and probably by changes in the excitation of glutamatergic neurons. Many of the regulatory changes were transient, except in the ipsilateral AVCNa and LSO, where postlesion downregulations were persistent. The downregulation in the ipsilateral AVCNa was probably induced directly by the loss of cochlear nerve endings. However, other regulatory changes may have been induced by signals carried on pathways emerging from the ipsilateral CN and on centrifugal auditory pathways.