Transneuronal labeling of cochlear nucleus neurons by HRP-labeled auditory nerve fibers and olivocochlear branches in mice

Auditory brainstem mechanisms likely compensate for self-imposed peripheral inhibition

Feedback networks in the brain regulate downstream auditory function as peripheral as the cochlea. However, the upstream neural consequences of this peripheral regulation are less understood. For instance, the medial olivocochlear reflex (MOCR) in the brainstem causes putative attenuation of responses generated in the cochlea and cortex, but those generated in the brainstem are perplexingly unaffected. Based on known neural circuitry, we hypothesized that the inhibition of peripheral input is compensated for by positive feedback in the brainstem over time. We predicted that the inhibition could be captured at the brainstem with shorter (1.5 s) than previously employed long duration (240 s) stimuli where this inhibition is likely compensated for. Results from 16 normal-hearing human listeners support our hypothesis in that when the MOCR is activated, there is a robust reduction of responses generated at the periphery, brainstem, and cortex for short-duration stimuli. Such inhibition at the brainstem, however, diminishes for long-duration stimuli suggesting some compensatory mechanisms at play. Our findings provide a novel non-invasive window into potential gain compensation mechanisms in the brainstem that may have implications for auditory disorders such as tinnitus. Our methodology will be useful in the evaluation of efferent function in individuals with hearing loss.

Targets of olivocochlear collaterals in cochlear nucleus of rat and guinea pig

Descending auditory pathways can modify afferent auditory input en route to cortex. One component of these pathways is the olivocochlear system which originates in brainstem and terminates in cochlea. Medial olivocochlear (MOC) neurons also project collaterals to cochlear nucleus and make synaptic contacts with dendrites of multipolar neurons. Two broadly distinct populations of multipolar cells exist: T-stellate and D-stellate neurons, thought to project to inferior colliculus and contralateral cochlear nucleus, respectively. It is unclear which of these neurons receive direct MOC collateral input due to conflicting results between in vivo and in vitro studies. This study used anatomical techniques to identify which multipolar cell population receives synaptic innervation from MOC collaterals. The retrograde tracer Fluorogold was injected into inferior colliculus or cochlear nucleus to label T-stellate and D-stellate neurons, respectively. Axonal branches of MOC neurons were labeled by biocytin injections at the floor of the fourth ventricle. Fluorogold injections resulted in labeled cochlear nucleus multipolar neurons. Biocytin abundantly labeled MOC collaterals which entered cochlear nucleus. Microscopic analysis revealed that MOC collaterals made some putative synaptic contacts with the retrogradely labeled neurons but many more putative contacts were observed on unidentified neural targets. This suggest that both T- and D-stellate neurons receive synaptic innervation from the MOC collaterals on their somata and proximal dendrites. The prevalence of these contacts cannot be stated with certainty because of technical limitations, but the possibility exists that the collaterals may also make contacts with neurons not projecting to inferior colliculus or the contralateral cochlear nucleus.

Cholinergic responses of acoustically-characterized cochlear nucleus neurons: An in vivo iontophoretic study in Guinea pig

The responses of guinea pig cochlear nucleus neurons to in vivo iontophoretic application of various neurotransmitter agonists were recorded with extracellular multi-barrelled electrodes. Where possible, neurons were physiologically identified using strict criteria. Emphasis was placed on the action of cholinergic agonists in relation to the possible action of olivocochlear collateral innervation. Excitatory responses (increase in action potential firing) to glutamate were confirmed in a number of neuronal response types. Application of acetylcholine (ACh) or the broad spectrum cholinergic agonist carbachol produced reliable excitatory responses in about 47% of neurons (n = 29 out of 61 neurons). The remaining neurons were unresponsive to cholinergic agonists and no inhibitory responses were observed. Cholinergic responses were more common in dorsal cochlear nucleus (DCN) (73% of 30 neurons tested) than in ventral cochlear nucleus (VCN) (23% of 31 neurons). Of the total neuron sample in which cholinergic responses were investigated, 41 neurons were able to be categorized according to established acoustic response features. Excitatory responses to cholinergic agonists were seen in "Pauser-buildup" (Pb) and "Transient chopper" (Ct) response types. Primary-like neurons (PL and Pn) as well as "Onset chopper" (Oc) neurons (n = 6) were unresponsive to either ACh or carbachol. Oc neurons also did not show any effect on their acoustic responses. Robust cholinergic responses were also seen in several VCN and DCN neurons that were either unresponsive to sound, or had acoustic response properties that did not fit standard classification. The results suggest a relatively more robust cholinergic innervation of DCN compared to VCN. The excitatory cholinergic responses of some Ct neurons and the lack of effect on Oc neurons are consistent with previous results in mouse brain slice studies, but are in conflict with reports of medial olivocochlear collateral excitatory responses in onset-type neurons in vivo. The results also indicate that a number of neurons of unknown identity may also receive cholinergic input.

The multiple functions of T stellate/multipolar/chopper cells in the ventral cochlear nucleus

Acoustic information is brought to the brain by auditory nerve fibers, all of which terminate in the cochlear nuclei, and is passed up the auditory pathway through the principal cells of the cochlear nuclei. A population of neurons variously known as T stellate, type I multipolar, planar multipolar, or chopper cells forms one of the major ascending auditory pathways through the brainstem. T Stellate cells are sharply tuned; as a population they encode the spectrum of sounds. In these neurons, phasic excitation from the auditory nerve is made more tonic by feedforward excitation, coactivation of inhibitory with excitatory inputs, relatively large excitatory currents through NMDA receptors, and relatively little synaptic depression. The mechanisms that make firing tonic also obscure the fine structure of sounds that is represented in the excitatory inputs from the auditory nerve and account for the characteristic chopping response patterns with which T stellate cells respond to tones. In contrast with other principal cells of the ventral cochlear nucleus (VCN), T stellate cells lack a low-voltage-activated potassium conductance and are therefore sensitive to small, steady, neuromodulating currents. The presence of cholinergic, serotonergic and noradrenergic receptors allows the excitability of these cells to be modulated by medial olivocochlear efferent neurons and by neuronal circuits associated with arousal. T Stellate cells deliver acoustic information to the ipsilateral dorsal cochlear nucleus (DCN), ventral nucleus of the trapezoid body (VNTB), periolivary regions around the lateral superior olivary nucleus (LSO), and to the contralateral ventral lemniscal nuclei (VNLL) and inferior colliculus (IC). It is likely that T stellate cells participate in feedback loops through both medial and lateral olivocochlear efferent neurons and they may be a source of ipsilateral excitation of the LSO.

Ventral cochlear nucleus responses to contralateral sound are mediated by commissural and olivocochlear pathways

In the normal guinea pig, contralateral sound inhibits more than a third of ventral cochlear nucleus (VCN) neurons but excites <4% of these neurons. However, unilateral conductive hearing loss (CHL) and cochlear ablation (CA) result in a major enhancement of contralateral excitation. The response properties of the contralateral excitation produced by CHL and CA are similar, suggesting similar pathways are involved for both types of hearing loss. Here we used the neurotoxin melittin to test the hypothesis that this "compensatory" contralateral excitation is mediated either by direct glutamatergic CN-commissural projections or by cholinergic neurons of the olivocochlear bundle (OCB) that send collaterals to the VCN. Unit responses were recorded from the left VCN of anesthetized, unilaterally deafened guinea pigs (CHL via ossicular disruption, or CA via mechanical destruction). Neural responses were obtained with 16-channel electrodes to enable simultaneous data collection from a large number of single- and multiunits in response to ipsi- and contralateral tone burst and noise stimuli. Lesions of each pathway had differential effects on the contralateral excitation. We conclude that contralateral excitation has a fast and a slow component. The fast excitation is likely mediated by glutamatergic neurons located in medial regions of VCN that send their commissural axons to the other CN via the dorsal/intermediate acoustic striae. The slow component is likely mediated by the OCB collateral projections to the CN. Commissural neurons that leave the CN via the trapezoid body are an additional source of fast, contralateral excitation.

Current focusing and steering: modeling, physiology, and psychophysics

Current steering and current focusing are stimulation techniques designed to increase the number of distinct perceptual channels available to cochlear implant (CI) users by adjusting currents applied simultaneously to multiple CI electrodes. Previous studies exploring current steering and current focusing stimulation strategies are reviewed, including results of research using computational models, animal neurophysiology, and human psychophysics. Preliminary results of additional neurophysiological and human psychophysical studies are presented that demonstrate the success of current steering strategies in stimulating auditory nerve regions lying between physical CI electrodes, as well as current focusing strategies that excite regions narrower than those stimulated using monopolar configurations. These results are interpreted in the context of perception and speech reception by CI users. Disparities between results of physiological and psychophysical studies are discussed. The differences in stimulation used for physiological and psychophysical studies are hypothesized to contribute to these disparities. Finally, application of current steering and focusing strategies to other types of auditory prostheses is also discussed.

Contralateral effects and binaural interactions in dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) receives afferent input from the auditory nerve and is thus usually thought of as a monaural nucleus, but it also receives inputs from the contralateral cochlear nucleus as well as descending projections from binaural nuclei. Evidence suggests that some of these commissural and efferent projections are excitatory, whereas others are inhibitory. The goals of this study were to investigate the nature and effects of these inputs in the DCN by measuring DCN principal cell (type IV unit) responses to a variety of contralateral monaural and binaural stimuli. As expected, the results of contralateral stimulation demonstrate a mixture of excitatory and inhibitory influences, although inhibitory effects predominate. Most type IV units are weakly, if at all, inhibited by tones but are strongly inhibited by broadband noise (BBN). The inhibition evoked by BBN is also low threshold and short latency. This inhibition is abolished and excitation is revealed when strychnine, a glycine-receptor antagonist, is applied to the DCN; application of bicuculline, a GABAA-receptor antagonist, has similar effects but does not block the onset of inhibition. Manipulations of discrete fiber bundles suggest that the inhibitory, but not excitatory, inputs to DCN principal cells enter the DCN via its output pathway, and that the short latency inhibition is carried by commissural axons. Consistent with their respective monaural effects, responses to binaural tones as a function of interaural level difference are essentially the same as responses to ipsilateral tones, whereas binaural BBN responses decrease with increasing contralateral level. In comparison to monaural responses, binaural responses to virtual space stimuli show enhanced sensitivity to the elevation of a sound source in ipsilateral space but reduced sensitivity in contralateral space. These results show that the contralateral inputs to the DCN are functionally relevant in natural listening conditions, and that one role of these inputs is to enhance DCN processing of spectral sound localization cues produced by the pinna.

Effects of cochlear ablation on choline acetyltransferase activity in the rat cochlear nucleus and superior olive

Using microdissection and quantitative microassay, choline acetyltransferase (ChAT) activity was mapped in the cochlear nucleus (CN) and in the source nuclei of the olivocochlear bundle, the lateral superior olive and ventral nucleus of the trapezoid body. In control rats, gradients of ChAT activity were found within the major subdivisions of the CN and in the lateral superior olive. These gradients correlated with the known tonotopic organizations, with higher activities corresponding to locations representing higher sound frequencies. No gradient was found in the ventral nucleus of the trapezoid body. In rats surviving 7 days or 1 or 2 months after cochlear ablation, ChAT activity was increased 1 month after ablation in the anteroventral CN by 30-50% in most parts of the lesion-side and by 40% in the contralateral ventromedial part. ChAT activity in the lesion-side posteroventral CN was increased by approximately 40-50% at all survival times. Little change was found in the dorsal CN. Decreases of ChAT activity were also found ipsilaterally in the lateral superior olive and bilaterally in the ventral nucleus of the trapezoid body. Our results suggest that cholinergic neurons are involved in plasticity within the CN and superior olive following cochlear lesions.

Responses of ventral cochlear nucleus neurons to contralateral sound after conductive hearing loss

Conductive hearing loss (CHL) is an attenuation of signals stimulating the cochlea, without damage to the auditory end organ. It can cause central auditory processing deficits that outlast the CHL itself. Measures of oxidative metabolism show a decrease in activity of nuclei receiving input originating at the affected ear but, surprisingly, an increase in the activity of second-order neurons of the opposite ear. In normal hearing animals, contralateral sound produces an inhibitory response to broadband noise in approximately one third of ventral cochlear nucleus (VCN) neurons. Excitatory responses also occur but are very rare. We looked for changes in the binaural properties of neurons in the VCN of guinea pigs at intervals immediately, 1 day, 1 wk, and 2 wk after the induction of a unilateral CHL by ossicular disruption. CHL was always induced in the ear ipsilateral to the VCN from which recordings were made. The main observations were as follows: 1) ipsilateral excitatory thresholds were raised by at least 40 dB; 2) contralateral inhibitory responses showed a small but statistically significant immediate decrease followed by an increase, returning to normal by 14 days; and 3) there was a large increase in the proportion of units with excitatory responses to contralateral BBN. The increase was immediate and lasting. The latencies of the excitatory responses were at least 6 ms, consistent with activation by a path involving several synapses and inconsistent with cross talk. The latencies and rate-level functions of contralateral excitation were similar to those seen occasionally in normal hearing animals, suggesting an upregulation of an existing pathway. In conclusion, contralateral excitatory inputs to the VCN exist, which are not normally effective, and can compensate rapidly for large changes in afferent input.

Superior olivary contributions to auditory system plasticity: Medial but not lateral olivocochlear neurons are the source of cochleotomy-induced GAP-43 expression in the ventral cochlear nucleus

A unilateral cochlear lesion induces expression of the growth and plasticity-associated protein 43 (GAP-43) in fibers and their varicosities on specific types of postsynaptic profiles in the ventral cochlear nucleus (VCN), suggesting the induction of synaptic remodeling. One candidate population from which GAP-43 might emerge was neurons of the lateral olivocochlear (LOC) system residing in the lateral superior olive (LSO). Upon cochleotomy, these neurons express GAP-43 mRNA and GAP-43 protein. However, retrograde axonal tracing with Fast Blue or biotinylated dextran amine from VCN revealed that the number of 6.8 +/- 1.3 neurons in the whole ipsilateral LSO labeled in normal adult rats was distinctly small and did not rise after cochleotomy. Concluding that LOC neurons cannot be the source of GAP-43 in the VCN, we reinvestigated the pattern of GAP-43 in situ hybridization and found that, after cochleotomy, shell neurons in the regions surrounding the LSO and medial olivocochlear (MOC) neurons in the ventral nucleus of the trapezoid body up-regulated GAP-43 mRNA. We then lesioned these regions by means of stereotaxic injections of kainic acid. Destruction of shell neurons preceding an ipsilateral cochleotomy did not change the emergence of GAP-43 immunoreactivity in the VCN. However, if the contralateral MOC system was lesioned, the rise of GAP-43 immunoreactivity in VCN on the side of the cochleotomy was significantly reduced. We conclude that, after cochlear dysfunction, MOC neurons are the major (if not exclusive) source of synaptic reorganization in the VCN that could possibly entail compensatory activation of the affected ascending auditory pathway.

Effects of trigeminal ganglion stimulation on unit activity of ventral cochlear nucleus neurons

The trigeminal ganglion sends a projection to the granule and magnocellular regions of the ventral cochlear nucleus (VCN; [J Comp Neurol 419 (2000) 271]), as well as to the cochlea ([Neuroscience 79 (1997) 605; Neuroscience 84 (1998a) 559]). We investigated the effects of electrically stimulating the trigeminal ganglion on unit responses in the guinea-pig VCN. Responses consisted of one, two or more phases of excitation, sometimes followed by a longer inhibitory phase. The latencies to the first excitation peak ranged between 5 and 17 ms from the onset of stimulation. These responses were preceded by a slow wave potential evoked by the stimulation. Applying kainic acid, which eliminates VIIIth nerve responses, diminished the firing rates of VCN units to trigeminal stimulation, and increased their first spike latencies. Cochlear destruction had a similar effect. The responses in VCN evoked by trigeminal ganglion stimulation therefore appear to result from direct stimulation of the trigeminal ganglion-cochlear nucleus pathway, as well as modulation by the trigeminal ganglion-cochlear pathway. Alternatively, a reduction in spontaneous rate of VCN neurons by removal of VIIIth nerve input could explain the decreased response to trigeminal stimulation after cochlear manipulations. The modulation of firing rate in second order auditory neurons by first order somatosensory neurons could influence central auditory targets and may be involved in generating or modulating perceptions of phantom sounds which can be modified by manipulations of somatic regions of the head and neck ("somatic tinnitus").

Aminergic projections to cochlear nucleus via descending auditory pathways

The cochlear nucleus (CN) receives descending input from a variety of auditory nuclei. Descending inputs from the superior olive in particular have been well described, especially those of olivocochlear neurons, which terminate ultimately in the cochlea. It has been demonstrated that olivocochlear neurons receive serotonergic and noradrenergic inputs and thus form a route by which the aminergic system may modulate cochlear mechanisms. Since olivocochlear neurons send collaterals into the CN, it is possible that they also from a route by which the aminergic systems modulate CN processes. The goal of the current study was to determine if neurons in the superior olive that projected to the CN received serotonergic or noradrenergic inputs. The retrograde tracer WGAapoHRP-Au was injected into the CN of cats. The brainstems were silver-enhanced to visualize the tracer and then immunohistochemically processed with antibodies raised against serotonin or dopamine-beta-hydroxylase (DBH) to label serotonergic or noradrenergic fibers, respectively. The sections were viewed with high power light microscopy to determine if the retrogradely labeled neurons were contacted by serotonin- or DBH-immunoreactive varicosities. Retrogradely labeled cells were observed in auditory brainstem nuclei known to project to the CN including the superior olivary complex and inferior colliculus bilaterally and the opposite CN. In these regions, retrogradely labeled neurons were closely associated with serotonin- and/or DBH-immunoreactive varicosities. Assuming a synaptic relationship between the projection neurons and varicosities, these results indicate that the serotonergic and noradrenergic systems innervate the descending pathways to the CN. Since the serotonergic and noradrenergic systems modulate their targets based on level of arousal, these results support the theory that descending systems are involved in selective attention.

Cholinergic modulation of stellate cells in the mammalian ventral cochlear nucleus

The main source of excitation to the ventral cochlear nucleus (VCN) is from glutamatergic auditory nerve afferents, but the VCN is also innervated by two groups of cholinergic efferents from the ventral nucleus of the trapezoid body. One arises from collaterals of medial olivocochlear efferents, and the other arises from neurons that project solely to the VCN. This study examines the action of cholinergic inputs on stellate cells in the VCN. T stellate cells, which form one of the ascending auditory pathways to the inferior colliculus, and D stellate cells, which inhibit T stellate cells, are distinguished electrophysiologically. Whole-cell recordings from stellate cells in slices of the VCN of mice demonstrate that most T stellate cells are excited by cholinergic agonists through three types of receptors, whereas all D stellate cells tested were insensitive to cholinergic agonists. Nicotinic excitation in T stellate cells has two components. The faster component was blocked by alpha-bungarotoxin and methyllycaconitine, suggesting that receptors contained alpha7 subunits; the slower component was insensitive to both. Muscarinic receptors excite T stellate cells by blocking a voltage-insensitive, "leak" potassium conductance. Our results suggest that cholinergic efferent innervation enhances excitation by sounds of T stellate cells, opposing the inhibitory action of cholinergic innervation in the cochlea that is conveyed indirectly through the glutamatergic afferents. The inhibitory action of D stellate cells on their targets is probably not affected by cholinergic inputs. Excitation of T stellate cells by cholinergic efferents would be expected to enhance the encoding of spectral peaks in noise.

Origins of projections from the inferior colliculus to the cochlear nucleus in guinea pigs

We used retrograde tracing techniques to examine the projections from the inferior colliculus to the cochlear nucleus in guinea pigs. Following injection of a retrograde tracer into one cochlear nucleus, labeled cells were found bilaterally in all subdivisions of the inferior colliculus. The majority of cells were located in the central nucleus and external cortex; relatively few cells were located in the dorsal cortex. Multipolar (stellate) cells were labeled in all subdivisions of the inferior colliculus. In the central nucleus, disk-shaped cells were also labeled. To determine whether individual collicular neurons send collateral projections to the cochlear nuclei on both sides, we injected different fluorescent tracers into left and right cochlear nuclei in the same animal. The inferior colliculi contained very few double-labeled cells, indicating that the projections to ipsilateral and contralateral cochlear nuclei originate from separate populations of cells.

Plasticity of the superior olivary complex

The superior olivary complex (SOC) is part of the auditory brainstem of the vertebrate brain. Residing ventrally in the rhombencephalon, it receives sensory signals from both cochleae through multisynaptic pathways. Neurons of the SOC are also a target of bilateral descending projections. Ascending and descending efferents of the SOC affect the processing of auditory signals on both sides of the brainstem and in both organs of Corti. The pattern of connectivity indicates that the SOC fulfills functions of binaural signal integration serving sound localization. But whereas many of these connectional features are shared with the inferior colliculus (with the important exception of a projection to the inner ear), cellular and molecular investigations have shown that cells residing in SOC are unique in several respects. Unlike those of other auditory brainstem nuclei, they specifically express molecules known to be involved in development, plasticity, and learning (e.g., GAP-43 mRNA, specific subunits of integrin). Moreover, neurons of the SOC in adult mammals respond to various kinds of hearing impairment with the expression of plasticity-related substances (e.g., GAP-43, c-Jun, c-Fos, cytoskeletal elements), indicative of a restructuring of auditory connectivity. These observations suggest that the SOC is pivotal in the developmental and adaptive tuning of binaural processing in young and adult vertebrates.

Trigeminal ganglion innervates the auditory brainstem

A neural connection between the trigeminal ganglion and the auditory brainstem was investigated by using retrograde and anterograde tract tracing methods: iontophoretic injections of biocytin or biotinylated dextran-amine (BDA) were made into the guinea pig trigeminal ganglion, and anterograde labeling was examined in the cochlear nucleus and superior olivary complex. Terminal labeling after biocytin and BDA injections into the ganglion was found to be most dense in the marginal cell area and secondarily in the magnocellular area of the ventral cochlear nucleus (VCN). Anterograde and retrograde labeling was also seen in the shell regions of the lateral superior olivary complex and in periolivary regions. The labeling was seen in the neuropil, on neuronal somata, and in regions surrounding blood vessels. Retrograde labeling was investigated using either wheatgerm agglutinin-horseradish peroxidase (WGA-HRP), BDA, or a fluorescent tracer, iontophoretically injected into the VCN. Cells filled by retrograde labeling were found in the ophthalmic and mandibular divisions of the trigeminal ganglion. We have previously shown that these divisions project to the cochlea and middle ear, respectively. This study provides the first evidence that the trigeminal ganglion innervates the cochlear nucleus and superior olivary complex. This projection from a predominantly somatosensory ganglion may be related to integration mechanisms involving the auditory end organ and its central targets.

Synapses from labeled type II axons in the mouse cochlear nucleus

This study investigates the ultrastructure and central targets in the cochlear nucleus of axonal swellings of type II primary afferent neurons. Type II axons comprise only 5-10% of the axons of the auditory nerve of mammals, but they alone provide the afferent innervation of the outer hair cells. In this study, type II axons were labeled with horseradish peroxidase, and serial-section electron microscopy was used to examine their swellings in: (1) the granule-cell lamina at its boundary with posteroventral cochlear nucleus, (2) the rostral anteroventral cochlear nucleus, and (3) the auditory nerve root. Only some (18%) of the type II terminal and en-passant swellings formed synapses. The synapses were asymmetric and contained clear round synaptic vesicles, suggesting that they are excitatory. Type II synapses were compared to those from type I fibers providing the afferent innervation of the inner hair cells. Type II synapses tended to have slightly smaller and fewer synaptic vesicles, had a greater proportion of the membrane apposition accompanied by a postsynaptic density, and often had densities that were discontinuous or 'perforated'. In all cochlear nucleus regions examined, the postsynaptic targets of type II synapses had characteristics of dendrites; in most cases these dendrites could not be traced to their cell bodies of origin. Some evidence suggests, however, that targets may include granule cells, spherical cells, and other cells in the nerve root. These results suggest afferent information from outer hair cells reaches diverse regions and targets within the cochlear nucleus.

Synaptic input to cochlear nucleus dendrites that receive medial olivocochlear synapses

Axons of olivocochlear neurons originate in the superior olivary complex and project to the cochlea. Along their course, medial olivocochlear axons give off branches to the cochlear nucleus. We labeled these branches with horseradish peroxidase and used electron microscopy to determine their target dendrites. Target dendrites were of two classes: "large" dendrites and "varicose" dendrites. Using serial sections, we reconstructed the dendrites and, in addition to the labeled olivocochlear input, we determined the synaptic profile of unlabeled inputs onto the dendrites. We classified the terminals on the basis of the shape and size of their synaptic vesicles. On large dendrites, the predominant type of unlabeled terminal had small round (SmRnd) vesicles. These terminals are likely to be excitatory, and some of them may originate from unlabeled medial olivocochlear branches. On varicose dendrites, the predominant type of terminal had pleomorphic vesicles. These terminals are likely to be inhibitory. They may be from descending inputs that arise in higher centers. A final type of terminal onto large dendrites exhibited signs of neuronal degeneration, possibly because the cell body of origin was damaged during the injection procedure. These terminals often had long, perforated synaptic densities and may originate from type II primary afferents. Thus, medial olivocochlear efferents and type II afferents, which both contact outer hair cells in the periphery, appear to synapse onto the same targets in the cochlear nucleus. In contrast, where examined, the target dendrites did not receive terminals with large vesicles from afferents that contact inner hair cells. Thus, target neurons appear to function in a neural circuit associated more closely with outer than with inner hair cells.

Neuron labeling by extracellular delivery of horseradish peroxidase in vivo: a method for studying the local circuitry of projection and interneurons at physiologically characterized sites

An anatomical method is described that yields individual neurons with continuously labeled dendrites and axons following the extracellular deposition of horseradish peroxidase (HRP) at neurophysiological recording sites in vivo. The method is a logical evolution of previous methods for iontophoretic delivery of HRP: Parameters critical to the ultimate concentration of HRP at the labeling site are reduced by an order of magnitude relative to standard practice. In successful cases one neuron or two in the immediate vicinity (50 microns) of recording sites is/are labeled. Labeling of other processes traversing the injection site, if any, is subliminal at highest light microscopic magnification. Due to the labeling of so few cells and the absence of other labeled processes, dendritic trees and local axonal arbors can be reconstructed without ambiguity. In addition to recovering neurons at sites characterized with physiological (e.g., sensory) stimuli, the method offers the further advantage of being fully compatible with subsequent electron microscopy. Both large (> 20 microns) and small (approximately 8 microns) neuron types and glia have been labeled.

Development and organization of the ocular motor nuclei in the larval sea lamprey, Petromyzon marinus L.: an HRP study

Retrograde transport of horseradish peroxidase (HRP) after its application into the orbit was used to investigate the development of the different ocular motor nuclei in larvae of the sea lamprey (Petromyzon marinus) and to identify their regions of origin. In the smallest larvae studied (10-19 mm in length), the oculomotor and abducens neurons were ipsilateral to the site of HRP application, whilst trochlear neurons were contralateral. These motoneurons did not have dendritic processes. In larvae more than 19 mm in length, both ipsilateral and contralateral components were found in the oculomotor and trochlear nuclei; dendrites were present, and their length and branching increased with larval age. An adult-like pattern of topographic organization and dendritic arborization was reached in larvae of about 45-60 mm in length. In oculomotor neurons, medial dendrites appear first, then dorsolateral dendrites, and finally ventral dendrites. Similarly, in trochlear neurons ventral and ventrolateral dendrites develop first, followed by dorsal dendrites that course either to the caudal optic tectum or to the terminal fields of the octaval and lateral line nerves in the cerebellar plate. Dorsal and ventral dendrites of the abducens neurons arise at the same time, but dorsal dendrites attain an adult-like morphology earlier. A few motoneurons showed ventricular attachments in larvae longer than 40 mm. The significance of these processes and their possible usefulness as a marker for the regions of origin of the ocular motor nuclei are discussed. Finally, the results presented here indicate that differentiation of the ocular motor nuclei in larval lampreys precedes and is independent of the maturation of the eye at transformation.

Choline acetyltransferase-immunoreactive cochlear efferent neurons in the chick auditory brainstem

Cholinergic neurons in the chick auditory brainstem were studied with the aid of an antiserum to choline acetyltransferase (ChAT), the biosynthetic enzyme for acetylcholine. ChAT-immunoreactive (ChAT-I) neurons were found in a ventrolateral and a dorsomedial cell group. The ventrolateral group is a rostrocaudally directed column of cells that surround the superior olive (SO), are ventromedial to the ventral facial nucleus (VIIv), and are lateral to the nucleus pontis lateralis (PL) as far rostrally as the nucleus subceruleus ventralis. Cells in the dorsomedial group were found in the pontine reticular formation medial to the dorsal facial nucleus and lateral to the abducens nerve root. Occasionally, small ChAT-I cells were found in the crossed dorsal cochlear tract and in the medial vestibular nucleus near the dorsal border of the caudal nucleus magnocellularis (NM). No ChAT-I neurons or fibers were observed in NM, nucleus angularis, nucleus laminaris, in the nuclei of the lateral lemniscus, or in the nucleus mesencephalicus lateralis pars dorsalis. To determine which cholinergic neurons project to the cochlea, a double-labeling technique was used combining ChAT-I and the retrograde transport of biotinylated dextran amine (BDA) from the inner ear. Double-labeled cells were found bilaterally in both the ventrolateral and dorsomedial cell groups, with the exception of large ChAT-I cells dorsal to the SO, which do not appear to project to the cochlea. Cholinergic cells that project to the cochlea were classified into three morphological groups: multipolar, elongate, and round-to-oval. Both the ventrolateral and the dorsomedial cell groups appear to have a mixture of these different cell types. The average somal area of cholinergic cochlear efferents was 246 microns 2. Only about 70% of the cochlear efferent neurons, however, are cholinergic.

Morphological correlations between spontaneously discharging primary vestibular afferents and vestibular nucleus neurons in the cat

Synaptic connections between physiologically classified primary vestibular afferents (PVAs) and their target vestibular nucleus (VN) neurons were examined by a combination of intra-axonal staining and electron microscopic techniques. PVAs originating from the horizontal semicircular canal were electrophysiologically classified as either regular- or irregular-type based on the regularity of their spontaneous discharge patterns, and were intra-axonally labeled with horseradish peroxidase (HRP). HRP-labeled PVAs of both types had many swellings along their course that contacted VN neurons. These swellings contained spherical synaptic vesicles and showed asymmetric postsynaptic specialization. Target VN neurons of both types of PVAs were distributed primarily in the superior, medial, and inferior VN. Irregular-type PVAs made more axosomatic contacts than did regular-type PVAs. The soma size of target VN neurons and the number of terminal boutons per target VN neuron were larger for irregular-type PVAs than for regular-type PVAs. Large VN neurons (presumably kinetic neurons) were innervated exclusively by irregular-type PVAs. Small VN neurons were innervated by PVAs of the regular-type and the irregular-type. These results demonstrate that there is a correlation between the physiological properties and morphological characteristics of PVAs and their target VN neurons.

Origin and migration of trochlear, oculomotor and abducent motor neurons in Petromyzon marinus L

The development of the ocular motor system was studied in 3- to 6-year old larval lampreys with two different retrograde tracers. Motor neurons of the oculomotor and trochlear nuclei are situated closely to one another in younger larvae. Cases in which only trochlear neurons were labelled revealed trochlear motor neurons scattered from the midbrain tegmentum through the anterior medullary velum. We believe this distribution reflects the place of final mitosis (midbrain tegmentum) and subsequent migration (anterior medullary velum) of lamprey trochlear motor neurons. Evidence is also presented for contralateral migration of oculomotor motor neurons and for ventrolateral migration of abducent motor neurons. The distances covered by migrating ocular motor neurons range from 100 to 150 microns in small larvae; these are distances that could be covered easily during the several years duration of larval development in lampreys.