Dorsal Acoustic Stria Research Articles

A novel method for selectively labelling olivocochlear collaterals in the rat

Axons of olivocochlear neurons originate from the brainstem and project to the cochlea. A subpopulation, medial olivocochlear (MOC) neurons, also projects collateral branches to the cochlear nucleus. The precise targets of these collaterals are as yet unknown. Previous methods for labelling these collaterals include firstly, cochlear injections of retrograde tracers, but this is technically demanding and can also label afferent projections or secondly, labelling by injecting tracers into the nuclei of origin of MOC neurons. However, this latter method is non-specific because it also labels non-MOC projections. A technique was used to specifically label MOC collaterals, which involved injections of the tracer biocytin at the floor of the fourth ventricle and fixation 3 hours later. Biocytin injections resulted in labelled neurons in the ventral nucleus of the trapezoid body and rostral periolivary nucleus, confirming MOC axonal labelling. Labelled neurons in dorsal cochlear nucleus indicated labelling of the dorsal acoustic stria and these injections were discarded. After selective MOC labelling, collateral branches were found to innervate granule cell regions, medial edge and core of the ventral cochlear nucleus, as well as the dorsal cochlear nucleus, in agreement with previous data. Therefore we conclude that injections at the floor of the fourth ventricle provide a simple, rapid and specific technique for labelling the majority of MOC axons and their collaterals and this technique may assist in defining the precise neuronal targets of olivocochlear collaterals in cochlear nucleus.

Commissural axons of the mouse cochlear nucleus

The axons of commissural neurons that project from one cochlear nucleus to the other were studied after labeling with anterograde tracer. Injections were made into the dorsal subdivision of the cochlear nucleus in order to restrict labeling only to the group of commissural neurons that gave off collaterals to, or were located in, this subdivision. The number of labeled commissural axons in each injection was correlated with the number of labeled radiate multipolar neurons, suggesting radiate neurons as the predominant origin of the axons. The radiate commissural axons are thick and myelinated, and they exit the dorsal acoustic stria of the injected cochlear nucleus to cross the brainstem in the dorsal half, near the crossing position of the olivocochlear bundle. They enter the opposite cochlear nucleus via the dorsal and ventral acoustic stria and at its medial border. Reconstructions of single axons demonstrate that terminations are mostly in the core and typically within a single subdivision of the cochlear nucleus. Extents of termination range from narrow to broad along both the dorsoventral (i.e., tonotopic) and the rostrocaudal dimensions. In the electron microscope, labeled swellings form synapses that are symmetric (in that there is little postsynaptic density), a characteristic of inhibitory synapses. Our labeled axons do not appear to include excitatory commissural axons that end in edge regions of the nucleus. Radiate commissural axons could mediate the broadband inhibition observed in responses to contralateral sound, and they may balance input from the two ears with a quick time course.

Differentiation of simple spike waveforms in the hamster dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) consists of many cell types with different morphologies and properties. DCN cells belonging to different morphological classes are distinguished by differences in their physiological characteristics such as their spectral and temporal response patterns, their levels of spontaneous activity, and certain biophysical properties. Recent studies suggest that they may also exhibit different action potentials, such as simple and complex spikes. In the present study, we systematically examined the spike waveforms of spontaneously active DCN neurons using extracellular recording methods. Neurons were found to exhibit simple spikes consisting of trains of individual action potentials. Spikes fell into two discrete groups of opposite polarity, those with M-shaped and those with W-shaped waveforms. The shapes of these waveforms recorded from a given unit remained constant, despite large changes in amplitude that occurred as the electrode was moved along its axis of penetration. A quantitative analysis of the fine details of the waveforms demonstrated that, although the durations of W- and M-shaped spikes exhibited considerable variation, the variants within each category fell along a continuous gradient rather than into discrete subgroups. Both M- and W-shaped waveforms were found predominantly in the fusiform cell and deep layers, with smaller numbers found in the dorsal acoustic stria. Consideration of their depths of occurrence, their response properties, and levels of spontaneous activity of the recorded neurons suggests that W-shaped waveforms probably are associated with fusiform cells, whereas M-shaped spikes likely originate from more than one cell type.

Effect of unilateral noise exposure on the tonotopic distribution of spontaneous activity in the cochlear nucleus and inferior colliculus in the cortically intact and decorticate rat

Effects of unilateral noise exposure on spontaneous activity (SA) in the anteroventral and dorsal cochlear nuclei (AVCN and DCN) and the central nucleus of the inferior colliculus (ICc) were studied in cortically intact and decorticate rats. SA was measured 1 week following exposure using uptake of 14C-labeled 2-deoxyglucose (2DG) in quiet. Optical density (OD) measurements were obtained in low- and high-frequency (LF and HF) areas of each nucleus. We refer to the ipsilateral AVCN and DCN (side of the noise-exposed ear) and the contralateral ICc as direct nuclei and to their opposite side counterparts as indirect nuclei. Noise exposure altered the tonotopic profile of SA in the direct pathway by causing a decrease in the ratio of HF OD to LF OD (HF/LF ratio). In intact animals, the decreased HF/LF ratio was due to decreased HF OD. In decorticate animals, it was due to decreased HF OD and increased LF OD, the latter occurring mainly in the DCN and ICc. Decorticate-intact differences may reflect corticofugal feedback inhibition. Lesion of the dorsal acoustic stria caused a substantial decrement of SA in the contralateral ICc. Furthermore, strong positive correlations between HF/LF ratios in the DCN, AVCN, and contralateral ICc suggest that the cochlear nucleus is a major contributor to SA in the ICc. Noise exposure had opposite and weaker effects on 2DG uptake in the indirect pathway that were attributed to crossed inhibition. Noise-induced changes in the tonotopic profile of SA may represent a neural correlate of tinnitus.

The effect of dorsal cochlear nucleus ablation on tinnitus in rats

A growing body of evidence implies that the dorsal cochlear nucleus (DCN) plays an important role in tinnitus. To test the hypothesis that the rostral output of the DCN is necessary for the experience of chronic tinnitus, the dorsal DCN and the dorsal acoustic stria of rats with psychophysical evidence of tinnitus was ablated. If the DCN plays a necessary role in the generation of chronic tinnitus, ablating the DCN should decrease the evidence of tinnitus in subjects previously shown to have tinnitus. Contrary to prediction, bilateral dorsal DCN ablation did not significantly ( n = 11, p = 0.707) affect the psychophysical evidence of tinnitus, and ipsilateral dorsal DCN ablation appeared to increase the evidence of tinnitus ( n = 9, p = 0.018) compared to pre-ablation performance. It was concluded that the DCN does not act as a simple feed-forward source of chronic tinnitus. Alternative hypotheses were considered, among them that elevated DCN activity following acoustic trauma triggers persistent pathological changes distributed across more than one level of the auditory system. In addition to serving as a trigger, the DCN may also modify the experience of tinnitus, since the evidence of tinnitus was enhanced by ipsilateral DCN ablation.

Connections of Purkinje cell axons of lobule X with vestibulospinal neurons projecting to the cervical cord in the rat.

Connections of Purkinje cell axons of lobule X (nodulus vermis) with vestibulospinal neurons have been demonstrated in the rat, by anterograde labeling of axons with biotinylated dextran (BD) injected into sublobule Xa and by retrograde labeling of neurons with cholera toxin subunit B (CTB) injected into cervical segments. Labeled terminals of Purkinje cell axons were numerous in the superior vestibular nucleus, the parvocellular (MVpc) and the caudal part (MVc) of the medial vestibular nucleus (MV), and group y. A limited number of labeled terminals were seen in the caudal part of the descending vestibular nucleus (DV). Occasional labeled terminals were seen in the lateral part of the lateral vestibular nucleus (LV) whereas few labeled terminals were seen in the magnocellular part of the MV (MVmc). Vestibulospinal neurons labeled from the C2 and C3 segments were seen bilaterally in the MVmc, MVpc, MVc, and DV, and ipsilaterally in the LV. CTB-labeled vestibulospinal neurons in contact with BD-labeled terminals of Purkinje cell axons were identified in the lateral part of the MVpc, near the border between the MVpc and MVmc, or close to the dorsal acoustic stria, and in the middle part of the MVc at its rostral level. The present study suggests that Purkinje cells of lobule X regulate the output of cervical-projecting vestibulospinal neurons in the MVpc and MVc.

Temporal and Binaural Properties in Dorsal Cochlear Nucleus and Its Output Tract

The dorsal cochlear nucleus (DCN) is one of three nuclei at the terminal zone of the auditory nerve. Axons of its projection neurons course via the dorsal acoustic stria (DAS) to the inferior colliculus (IC), where their signals are integrated with inputs from various other sources. The DCN presumably conveys sensitivity to spectral features, and it has been hypothesized that it plays a role in sound localization based on pinna cues. To account for its remarkable spectral properties, a DCN circuit scheme was developed in which three inputs converge onto projection neurons: auditory nerve fibers, inhibitory interneurons, and wide-band inhibitors, which possibly consist of Onset-chopper (Oc) cells. We studied temporal and binaural properties in DCN and DAS and examined whether the temporal properties are consistent with the model circuit. Interneurons (type II) and projection (types III and IV) neurons differed from Oc cells by their longer latencies and temporally nonlinear responses to amplitude-modulated tones. They also showed evidence of early inhibition to clicks. All projection neurons examined were inhibited by stimulation of the contralateral ear, particularly by broadband noise, and this inhibition also had short latency. Because Oc cells had short-latency responses and were well driven by broadband stimuli, we propose that they provide short-latency inhibition to DCN for both ipsilateral and contralateral stimuli. These results indicate more complex temporal behavior in DCN than has previously been emphasized, but they are consistent with the recently described nonlinear behavior to spectral manipulations and with the connectivity scheme deduced from such manipulations.

Role of acoustic striae in hearing: Reflexive responses to elevated sound-sources

This report is the fourth in a series describing the results of ablation-behavior experiments directed to the ascending output of the cochlear nuclei as it is conducted centrally within the acoustic striae. This fourth report focuses on the unique physiology of the fusiform or ‘output’ cells of the dorsal cochlear nucleus whose axons course through the dorsal acoustic stria (DAS). Because electrophysiological studies have shown that the cues for sensing the elevation of a sound source would seem to be best analyzed by the dorsal cochlear nucleus and projected centrally via its DAS, we tested normal cats and cats deprived of DAS for their ability to orient to elevated sources of broad-band noise. For behavioral testing, we made use of reflexive or unconditioned orienting responses to elevated sound sources using a similar method to one we have used previously for azimuth testing (Thompson GC, Masterton RB. Brainstem auditory pathways involved in reflexive head orientation to sound. J Neurophysiol 1978;41:1183–1202). The results show that cats deprived of their DAS do indeed have a marked deficit in their ability to orient to an elevated sound source. Further behavioral testing indicated that this deficit is not the secondary result of an attentional or peripheral motor deficit. Although the present results do not prove that the reflexive deficit is strictly auditory in nature, the deficit is notable in that it is the only one yet known to result from a lesion of the dorsal cochlear nucleus or its central projections.

Role of acoustic striae in hearing: discrimination of sound-source elevation

After years of systematic experimentation, we finally uncovered one thing the dorsal system contributes to hearing which the ventral system may not – the mechanism for orienting to an elevated sound source [Sutherland, D.P., Masterton, R.B., Glendenning, K.K. (1998) Behav. Brain Res. in press]. This paper follows up this one positive result on a historical background of uniformly negative results. The focus of this report is on the fusiform cells of the dorsal cochlear nucleus whose axons course through the dorsal acoustic stria (DAS). Because electrophysiological studies have shown that the cues for sensing the elevation of a sound source would seem to be best analyzed by the dorsal cochlear nucleus, we tested, behaviorally, normal cats and cats deprived of their DAS or intermediate acoustic stria, bilaterally or ipsilaterally (with or without their contralateral ear deafened), for their ability to orient to elevated sources of broad-band noise. For behavioral testing, we made use of a conventional shock-avoidance procedure. The results lead to the conclusion that DCN and DAS may play no role in learned elevation discriminations. This result builds on that of another of our papers which suggests that a deficit in reflexive discrimination of elevation is strictly auditory in nature [Sutherland, D.P., Masterton, R.B., Glendenning, K.K. (1998) Behav. Brain Res. in press].

Response Classes in the Dorsal Cochlear Nucleus and Its Output Tract in the Chloralose-Anesthetized Cat

Neurons in the dorsal cochlear nucleus (DCN) can be classified into three major physiological classes on the basis of responses to pure tone and broadband noise stimuli. A circuit diagram that associates these classes with different cell types has been proposed. According to this proposal, type II cells are inhibitory interneurons that respond well to tones and poorly to broadband noise, type IV cells are projection neurons with the opposite behavior, and type III cells are an inhomogeneous class with intermediate properties. To test the associations proposed, I compared the response type distribution in the DCN with its output tract, the dorsal acoustic stria (DAS), in chloralose-anesthetized cats. Axonal recordings in the DAS showed type III and IV responses as in DCN, but no type II responses. Compared with reports in decerebrate animals, fewer type IV neurons were encountered having sustained inhibition that generated strongly nonmonotonic responses to tones in both DCN and DAS. The presence of type II responses in the nucleus, but not in the output tract, offers strong support for the proposed association with DCN interneurons. On the other hand, the distinction between type III and IV responses needs refinement because the differences are only graded and because both types of responses occur in DAS, which shows that they are both associated with projection neurons.

Ionotropic and metabotropic glutamate receptors show unique postsynaptic, presynaptic, and glial localizations in the dorsal cochlear nucleus.

The dorsal cochlear nucleus (DCN) is a major brain center for integration of auditory information, and excitatory amino acid neurotransmission plays a central role in the processing of this information. In this study, the distribution of glutamate receptors was examined with preembedding immunocytochemistry, using 14 antibodies to ionotropic (GluR1, GluR2/3, GluR4, GluR5-7, GluR6/7, KA2, NR1, NR2A/B, delta 1/2) and metabotropic (mGluR1 alpha, mGluR2/3, mGluR5) glutamate receptor subtypes. Each of these antibodies produced a specific immunolabeling pattern, including a variety of postsynaptic, presynaptic, and glial localizations. Some antibodies showed widespread distribution patterns, notably the antibodies to the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits, GluR2 and GluR3, and the N-methyl-D-aspartate (NMDA) receptor subunit, NR1. In contrast, antibodies to other glutamate receptor subunits produced more restricted distribution patterns, especially that to GluR1, which stained the outer neuropil of the DCN, cartwheel cells, and a small population of presumptive interneurons associated with the dorsal acoustic stria, but produced little or no staining in fusiform cells or deep DCN neurons. Staining of the postsynaptic density and membrane of the granule cell-parallel fiber/cartwheel cell spins synapse was most prevalent with delta 1/2 and mGluR1 alpha antibodies. A unique pattern of staining was found with mGluR2/3 antibody--with staining concentrated in Golgi cells and unipolar brush cells of the middle to deep DCN. Distribution of some glutamate receptors in the DCN shows similarities to that of the cerebellum, where delta 2 and mGluR1 alpha may modulate neurotransmission at parallel fiber synapses, while mGluR2 and/or mGluR3 may modulate mossy terminal function.

Dynamics of rabbit vestibular nucleus neurons and the influence of the flocculus.

1. We recorded single vestibular nucleus neurons shown by electrical stimulation to receive floccular inhibition [flocculus receiving neurons (FRNs)] and/or to project toward midbrain motoneuronal pools [midbrain projecting neurons (MPNs)] in awake, head-fixed rabbits during compensatory eye movements. Stimuli included head rotation in the light, head rotation in the dark, and rotation of an optokinetic drum about the animal. We employed sinusoidal and triangular position profiles in the 0.05- to 0.8-Hz frequency band. We also examined transient responses to step changes in eye position. 2. We found identified vestibular nucleus cells (i.e., FRN/non-MPNs, FRN/MPNs, and non-FRN/MPNs) in the parvocellular and magnocellular portions of the medial vestibular nucleus, at the rostrocaudal level of the dorsal acoustic stria. 3. All identified vestibular nucleus neurons were excited during ipsilateral (relative to side of recording) head rotation and contralateral eye rotation. 4. The neuronal firing rates could be related to eye position and its time derivatives, and that relationship could be approximated by a two-pole, one-zero linear transfer function. As with abducens neurons, a more detailed approximation requires inclusion of two nonlinearities-a hysteresis and a variable sensitivity term that increases as eye movement amplitude decreases. 5. When the vestibuloocular reflex is suppressed by a conflicting full-field visual stimulus [visual vestibular conflict condition (VVC)], vestibular nucleus neuron modulation is largely suppressed. The remaining modulation is motoric in nature, because it can be related to the residual eye movements. Cells with "sensory vestibular signals," i.e., cells whose modulation during VVC correlates better with head rotation than eye movement, were not encountered. 6. We examined the dependence of firing rate parameters on stimulus modality. All neurons exhibited increased phase lead with respect to abducens nucleus neurons during stimuli involving head rotation. This finding could indicate that vestibular-derived inputs are inhomogeneously distributed on premotor neurons and that the studied premotor population receives a stronger vestibular input than another premotor group, not recorded in the current experiments. 7. FRNs and non-FRNs were similar in their qualitative response to the fast phases, the applicability of the two-pole, one-zero transfer function, hysteresis, and the amplitude nonlinearity. 8. FRNs differed from non-FRNs in having a phase advanced firing rate at all stimulus frequencies during visual and vestibular stimuli. The phase difference suggests that one role of the rabbit flocculus is to regulate phase of the net premotor signal.

Giant cells of the dorsal cochlear nucleus of mice: intracellular recordings in slices.

1. In slices of the murine cochlear nuclear complex, intracellular recordings were made from five giant cells that were identified by intracellular labeling with biocytin. Giant cells form one of the two output pathways of the dorsal cochlear nucleus (DCN). Understanding how neuronal circuits and intrinsic electrical properties interact to control the firing of giant cells is a step toward understanding what acoustic information is conveyed through these cells. 2. Cell bodies of the labeled giant cells lay in the deep layer of the DCN. Dendrites, widespread both along the isofrequency axis and along the tonotopic axis, occupied mainly the deep layer, but some distal ends strayed into the molecular layer. Axons of giant cells were large, varying between 1 and 2 microns diam, and left through the dorsal acoustic stria. They were not observed to branch in the cochlear nuclei. 3. Giant cells fired large, overshooting action potentials that were followed by two afterhyperpolarizations. The first brought the membrane potential below rest, independent of the strength of injected current. The more variable second one produced either an undershoot or an inflection in the membrane potential between action potentials. 4. In each of the five labeled giant cells, shocks to the nerve root or to the anteroventral cochlear nucleus (AVCN) evoked a monosynaptic excitatory postsynaptic potential and two tandem inhibitory postsynaptic potentials (IPSPs) in the first 10 ms. Later IPSPs followed after latencies of between 10 and 50 ms. Monosynaptic excitation was usually cut short by the inhibition. 5. Strychnine, at 1 microM, blocked all IPSPs in the one giant cell tested, indicating that inhibitory input to this giant cell from circuits intrinsic to the cochlear nuclear complex was glycinergic. 6. The location of afferents was mapped for two giant cells. Both excitatory and inhibitory inputs to giant cells could be driven by the local application of glutamate to many loci in the AVCN and posteroventral cochlear nucleus, indicating that the ventral cochlear nucleus VCN contains interneurons that are monosynaptically or polysynaptically connected to giant cells. 7. An interpretation consistent with the results is that giant cells are excited by auditory nerve fibers and are inhibited by tuberculoventral cells. Giant cells may also be excited by granule or T stellate cells.

Development of GAD-immunoreactivity in the dorsal cochlear nucleus of the hamster and cat: light and electron microscopic observations

Physiologic and pharmacologie evidence suggests that inhibitory influences are active in the mammalian dorsal cochlear nucleus (DCN) by the onset of hearing, while anatomical evidence suggests that inhibitory synapses are not present until days or weeks later. One inhibitory neurotransmitter in the DCN is gamma aminobutyric acid (GABA) and its presence can be indexed by immunohistochemical localization of its synthetic enzyme glutamic acid decarboxylase (GAD). The present study investigated the ingrowth and synapse formation of GAD-immunoreactive inputs in the DCN of cat and hamster. GAD-immunoreactive puncta are present in the DCN of the cat at birth and of the hamster on postnatal day (PND) 3. Thus, the present data correlate well with the physiologic and pharmacologie evidence. In both species the first labelled puncta are near the dorsal acoustic stria and may originate from efferent axons in the stria. Several days later a band of labelled puncta is found in the fusiform cell layer. This location is equivalent to the termination zone of cartwheel cells, GAD- immunoreactive interneurons in the DCN. Based on this spatiotemporal sequence in the appearance of GAD-immunoreactive puncta, we suggest that sources of GABA extrinsic to the DCN mature first, followed by intrinsic sources.

A computational model of afferent neural activity from the cochlea to the dorsal acoustic stria

The first comprehensive computational model of the precortical mammalian auditory system to include afferent neural processing up to the level of the dorsal acoustic stria (DAS) is described. The model consists of two scissile stages simulating (1) the cochlea and auditory nerve (AN) and (2) the dorsal cochlear nucleus (DCN). The model derives its input from a 128-channel cochlear filterbank. Cochlear transduction, rectification, logarithmic compression, and two-tone suppression functions are performed at the first stage of the simulation. The 512 artificial neurons employed model the cell at the level of transmembrane potential and have interconnections that follow closely those reported in recent anatomical and physiological studies of the cat AN and DCN. The responses of the model to pure-tone stimuli (at various sound-pressure levels) and noise stimuli (at various levels and bandwidths) are reported in detail and compare well with published results. The model is being used to investigate the representation of initial English stop consonants (differing in voice-onset time) in the DAS; this work is briefly described.

Morphology and physiology of cells in slice preparations of the dorsal cochlear nucleus of mice.

Horseradish peroxidase (HRP) was injected into cells from which intracellular recordings were made in slices of the dorsal cochlear nucleus (DCN) in order to correlate physiology with morphology. In general, the morphology of cells labeled intracellularly with HRP corresponded to those made with Golgi impregnations in mice and other mammals. The following cells were labeled: one granule cell, four cartwheel cells, eight fusiform cells, two other cells in the fusiform cell layer, and two tuberculoventral association cells in the deep layers of the DCN. The axon of the granule cell runs parallel to isofrequency laminae with collaterals branching perpendicularly and running along the tonotopic axis. The cartwheel cells have dendrites in the molecular layer that are densely covered with spines. The axon of one cell terminates just dorsally to the cell body. Fusiform cells have the characteristic spiny, apical and smooth, basal dendrites. The basal dendrites are conspicuously oriented parallel to isofrequency laminae. Axons of the fusiform cells exit through the dorsal acoustic stria without branching. The two tuberculoventral association cells in the deep DCN have axons that terminate both in the deep DCN, within the same isofrequency lamina that contains the cell body, and in the ventral cochlear nucleus (VCN). Intracellular recordings from 11 of these cells show that they cannot be distinguished on the basis of their responses to intracellularly injected current. All cell types fired large action potentials that were followed by a fast and a slower undershoot, distinguishing them from cells of the VCN but not from one another. Most cells responded to shocks of the auditory nerve root with early EPSPs and later IPSPs. The latencies of EPSPs show that some were monosynaptic and others polysynaptic. That there was no systematic relationship between the latencies of EPSPs and the cell types from which they were recorded shows that shocks to the nerve root may have activated more than just the large, myelinated, auditory nerve fibers.

Descending projections from the superior olivary complex to the cochlear nucleus of the cat.

Subdivisions of the cochlear nuclear complex give rise to a number of discrete projections to certain cell groups of the superior olivary complex and also received substantial descending projections from the periolivary nuclei. In the present study, we sought to determine by means of retrograde transport of horseradish peroxidase (HRP), and anterograde transport of radiolabeled protein, if the periolivary nuclei give rise to discrete projections to the various subdivisions of the cochlear nuclear complex. Following medium to large injections of HRP into the cochlear nucleus, irrespective of location, labeled cells were found in all periolivary nuclei bilaterally. In every case more than 40% of the labeled cells were found in the lateral nucleus of the trapezoid body on the same side and the ventral nucleus of the trapezoid body of both sides. Other periolivary nuclei contributing more than 5% of the total number of cells in individual cases were the contralateral lateral nucleus of the trapezoid body and the ipsilateral anterolateral and dorsal periolivary nuclei. Injections of tritiated leucine into periolivary nuclei gave rise to axonal labeling to the trapezoid body and the dorsal acoustic stria, usually bilaterally, and to terminal labeling that was widely distributed within the cochlear nuclear complex. In several cases with small injections, particularly in the lateral nucleus of the trapezoid body, the projections from the periolivary nuclei to the anteroventral and dorsal cochlear nuclei connected areas described as having similar best-frequency representation. The autoradiographic data corroborated the main results from the HRP experiments and provided additional information permitting these conclusions: the projections from the periolivary nuclei to the cochlear nuclear complex are organized tonotopically, at least in part; each periolivary nucleus (and perhaps individual cells), projects widely throughout the cochlear nuclear complex; the pattern of termination of projections from different periolivary nuclei to a given region of the cochlear nuclear complex are similar, as seen in autoradiograms, and the lateral and dorsal periolivary nuclei project mainly ipsilaterally, while the medial periolivary nuclei project bilaterally with a contralateral bias. The magnitude of these projections and their widespread distribution within the cochlear nuclear complex would suggest an important role for the descending projections in the normal functioning of the cochlear nucleus.

Glycine immunoreactivity in the brainstem auditory and vestibular nuclei of the guinea pig.

An immunohistochemical study was performed on the brainstem of the guinea pig, using a specific antibody against glycine. Glycine-like immunoreactivity was observed in stellate and multipolar neurons in the cochlear nucleus, in the medial and lateral nuclei of the trapezoid body and in the ventromedial periolivary cell group. No immunoreactive neurons were found in the vestibular nuclei. Positive fibre tracts were observed mainly in dorsal acoustic stria and lateral lemniscus. The results are consistent with electrophysiological and anatomical data from the literature concerning the response pattern in the fusiform layer of the dorsal cochlear nucleus and the phenomenon of binaural inhibition in the superior olivary complex.

Electron microscopic features of physiologically characterized, HRP-labeled fusiform cells in the cat dorsal cochlear nucleus.

We report on the anatomy and physiology of three fusiform cells in the dorsal cochlear nucleus (DCN) of the cat. The extra- and intracellular responses of these cells to pure tones showed features typical of the cell type. Peristimulus time histograms (PSTHs) were usually of the pauser or buildup configuration with chopping behavior noted in certain instances. Intracellular records during stimulus presentations revealed sustained depolarizations for the duration of the tone followed by a prolonged after-hyperpolarization (AHP). On rare occasions, a hyperpolarization corresponding to the pause region of the PSTH was noted. Occasionally, a stimulus-induced depolarization would be maintained after stimulus offset. Rebound excitation was also observed after the AHP. Morphologically, all three cells showed the standard fusiform cell features at the light microscopic level. The cell body gave rise to apical and basal dendritic trees. The apical tree branched frequently and displayed numerous spines distally. The basal tree had fewer branches and fewer, more irregular appendages. The axon originated from the cell body and gave rise to one or more collaterals before leaving the nucleus via the dorsal acoustic stria (DAS). At the electron microscopic (EM) level, the axon collaterals may terminate on a variety of cell types in the DCN, including fusiform cells. Their vesicles are round and the terminals closely resemble many unlabeled terminals seen on the cell body and apical and basal dendrites of our labeled fusiform cells. Terminals containing round vesicles, believed to be eighth nerve terminals, were found, with one exception, only on the basal dendrites. The spine-laden, distal apical dendrites received primarily terminals containing round vesicles, presumed to originate from the unmyelinated axons of granule cells. The cell body and unmyelinated initial segment received mostly terminals containing pleomorphic and flat vesicles, which also made up a large percentage of the dendritic input. Some relevant correlations, between the distribution of synaptic terminals and the observed physiology, may be possible.

Physiological response properties of cells labeled intracellularly with horseradish peroxidase in cat dorsal cochlear nucleus.

The physiology and morphology of fusiform cells in the dorsal cochlear nucleus were studied using extracellular and intracellular recording and intracellular injection of horseradish peroxidase. Fusiform cells displayed a variety of responses to tone pips presented at the characteristic frequency; most often these cells exhibited the pauser/buildup pattern defined in earlier studies. The response pattern of each neuron was dependent on frequency and sound-pressure level. Tone pips evoked short-lasting depolarizations of about 10 mV and long-lasting hyperpolarizations of about 10 mV in cells whose resting potentials were -50 to -65 mV. The time courses of both the excitation and the inhibition depended on frequency and sound-pressure level. Generally the depolarization was sustained for the duration of the tone pip, whereas the hyperpolarization could last as long as 600 ms after the end of the tone pip. Often a neuron exhibited a sustained chopper pattern after microelectrode impalement. This was probably a result of a decrease in membrane potential which altered the relative effectiveness of the excitatory and inhibitory inputs. The large, bitufted fusiform cells had many apical dendrites, which branched one to five times and were covered with spines, and fewer basal dendrites, which exhibited little branching and had few appendages. The morphology of fusiform cells varied systematically as a function of location within the dorsal cochlear nucleus. Response patterns for tone pips were not exclusive to individual cell types as two nonfusiform cells were found to exhibit a buildup pattern. Axons of injected neurons left the nucleus via the dorsal acoustic stria and 14 of 15 had collaterals within the dorsal cochlear nucleus.