Immunocytochemical localization of glutaminase-like and aspartate aminotransferase-like immunoreactivities in the rat and guinea pig hippocampus

There is considerable evidence that pathways of the hippocampus use an excitatory amino acids as transmitter. We have attempted to immunocytochemically identify excitatory amino acid neurons in the hippocampus of the rat and guinea pig using antiserum to glutaminase and antiserum to aspartate aminotransferase, which have been proposed as markers for aspartergic/glutamergic neurons. Glutaminase-like immunoreactivity was seen in granule cells in the dentate gyrus and fibers and puncta associated with the mossy fiber pathway in the hilus and stratum lucidum of the hippocampus. At the ultrastructural level, glutaminase-like immunoreactivity was observed in mossy fiber terminals in the stratum lucidum. Glutaminase-like immunoreactivity was also seen in pyramidal cells in regio inferior and regio superior and in cells in layer two of the entorhinal cortex. Schaffer collateral terminals, commissural fiber terminals and perforant pathway terminals were not seen at the light microscopic level. Glutaminase-like immunoreactivity is thus found in the cell bodies of proposed excitatory amino acid neurons of hippocampal pathways, but does not appear to label all terminals. Aspartate aminotransferase-like immunoreactivity was not seen in any cells, fibers or terminals in the rat or guinea pig hippocampus.     

Neonatal deafness results in degraded topographic specificity of auditory nerve projections to the cochlear nucleus in cats

We previously examined the early postnatal maturation of the primary afferent auditory nerve projections from the cat cochlear spiral ganglion (SG) to the cochlear nucleus (CN). In normal kittens these projections exhibit clear cochleotopic organization before birth, but quantitative data showed that their topographic specificity is less precise in perinatal kittens than in adults. Normalized for CN size, projections to the anteroventral (AVCN), posteroventral (PVCN), and dorsal (DCN) subdivisions are all significantly broader in neonates than in adults. By 6-7 postnatal days, projections are proportionate to those of adults, suggesting that significant refinement occurs during the early postnatal period. The present study examined SG projections to the CN in adult cats deafened as neonates by ototoxic drug administration. The fundamental organization of the SG-to-CN projections into frequency band laminae is clearly evident despite severe auditory deprivation from birth. However, when normalized for the smaller CN size in deafened animals, projections are disproportionately broader than in controls; AVCN, PVCN, and DCN projections are 39, 26, and 48% broader, respectively, than predicted if they were precisely proportionate to projections in normal hearing animals. These findings suggest that normal auditory experience and neural activity are essential for the early postnatal development (or subsequent maintenance) of the topographic precision of SG-to-CN projections. After early deafness, the basic cochleotopic organization of the CN is established and maintained into adulthood, but the CN is severely reduced in size and the topographic specificity of primary afferent projections that underlies frequency resolution in the normal central auditory system is significantly degraded.     

Bilirubin induces auditory neuropathy in neonatal guinea pigs via auditory nerve fiber damage

Bilirubin can cause temporary or permanent sensorineural deafness in newborn babies with hyperbilirubinemia. However, the underlying targets and physiological effects of bilirubin‐induced damage in the peripheral auditory system are unclear. Using cochlear functional assays and electron microscopy imaging of the inner ear in neonatal guinea pigs, we show here that bilirubin exposure resulted in threshold elevation in both compound action potential (CAP) and auditory brainstem response (ABR), which was apparent at 1 hr and peaked 8 hr after drug administration. The threshold elevation was associated with delayed wave latencies and elongated interwave intervals in ABR and CAP. At 72 hr postinjection, these measures returned to control levels, except for the CAP amplitude. Cochlear microphonics remained unchanged during the experiment. Morphological abnormalities were consistent with the electrophysiological dysfunction, revealing fewer auditory nerve fibers (ANFs) in the basal turn, myelin sheath lesions of spiral ganglion neurons (SGNs) and ANFs, and loss of type 1 afferent endings beneath inner hair cells (IHCs) without loss of hair cells at 8 hr posttreatment. Similar to the electrophysiological findings, morphological changes were mostly reversed 10 days after treatment, except for the ANF reduction in the basal turn. These results suggest that hyperbilirubinemia in neonatal guinea pigs impaired auditory peripheral neuromechanisms that targeted mainly the IHC synapses and the myelin sheath of SGNs and their fibers. Our observations indicate a potential connection between hyperbilirubinemia and auditory neuropathy. © 2012 Wiley Periodicals, Inc.     

Development of the time coding pathways in the auditory brainstem of the barn owl

The barn owl's head grows after hatching, causing interaural distances to more than double in the first 3 weeks posthatch. These changes expose the bird to a constantly increasing range of interaural time cues. We have used Golgi and ultrastructural techniques to analyze the development of the connections and cell types of the nucleus magnocellularis (NM) and the nucleus laminaris (NL) with reference to the growth of the head. The time coding circuit is formed but immature at the time of hatching. In the month posthatch, the auditory nerve projection to the NM matures, and appears adult-like by posthatch day (P)21. NM neurons show a late growth of permanent dendrites starting at P6. Over the first month, these dendrites change in length and number, depending upon rostrocaudal position, to establish the adult pattern in which high best frequency neurons have few or no dendrites. These changes are not complete by P21, when NM neurons still have more dendrites than in the adult owl. The neurons of NL have many short dendrites before hatching. Their number is greatly reduced by P6, and then does not change during later development. Like NM neurons, NL neurons and dendrites grow in the first month posthatch, and at P21, NL dendrites are longer than those in the adult owl. Thus, the auditory brainstem circuits grow in the first month after hatching, but are not yet mature at the time the head reaches its adult size. © 1996 Wiley-Liss, Inc.     

Postnatal refinement of auditory nerve projections to the cochlear nucleus in cats

Studies of visual system development have suggested that competition driven by activity is essential for refinement of initial topographically diffuse neuronal projections into their precise adult patterns. This has led to the assertion that this process may shape development of topographic connections throughout the nervous system. Because the cat auditory system is very immature at birth, with auditory nerve neurons initially exhibiting very low or no spontaneous activity, we hypothesized that the auditory nerve fibers might initially form topographically broad projections within the cochlear nuclei (CN), which later would become topographically precise at the time when adult-like frequency selectivity develops. In this study, we made restricted injections of Neurobiotin, which labeled small sectors (300-500 microm) of the cochlear spiral ganglion, to study the projections of auditory nerve fibers representing a narrow band of frequencies. Results showed that projections from the basal cochlea to the CN are tonotopically organized in neonates, many days before the onset of functional hearing and even prior to the development of spontaneous activity in the auditory nerve. However, results also demonstrated that significant refinement of the topographic specificity of the primary afferent axons of the auditory nerve occurs in late gestation or early postnatal development. Projections to all three subdivisions of the CN exhibit clear tonotopic organization at or before birth, but the topographic restriction of fibers into frequency band laminae is significantly less precise in perinatal kittens than in adult cats. Two injections spaced > or = 2 mm apart in the cochlea resulted in labeled bands of projecting axons in the anteroventral CN that were 53% broader than would be expected if they were proportional to those in adults, and the two projections were incompletely segregated in the youngest animals studied. Posteroventral CN (PVCN) projections (normalized for CN size) were 36% broader in neonates than in adults, and projections from double injections in the youngest subjects were nearly fused in the PVCN. Projections to the dorsal division of the CN were 32% broader in neonates than in adults when normalized, but the dorsal CN projections were always discrete, even at the earliest ages studied.     

Unmyelinated axons of the auditory nerve in cats.

This paper describes some central terminations of type II spiral ganglion neurons as labeled by extracellular injections of horseradish peroxidase (HRP) into the auditory nerve of cats. After histological processing with diaminobenzidine, both thick (2-4 microns) and thin (0.5 microns) fibers of the auditory nerve were stained. Whenever traced, thick fibers always originated from type I spiral ganglion neurons and thin fibers always from type II ganglion neurons. Because the labeling of type II axons faded as fibers projected into the cochlear nucleus, this report is limited to regions of the ventral cochlear nucleus near the auditory nerve root. The central axons of type II neurons are unmyelinated, have simple yet variable branching patterns in the cochlear nucleus, and form both en passant and terminal swellings. Under the light microscope, most swellings are located in the neuropil but they are also found in the vicinity of cell bodies, nodes of Ranvier of type I axons, and blood vessels. Eighteen en passant swellings in the neuropil were located by light microscopy and resectioned for electron microscopy; two of these swellings exhibited ultrastructural features characteristic of chemical synapses. The data indicate that inputs from outer hair cells might be able to influence auditory processing in the cochlear nucleus through type II primary neurons.     

Immunocytochemical localization of glutamate-, glutaminase- and aspartate aminotransferase-like immunoreactivity in the rat deep cerebellar nuclei

Although the anatomy and the connectivity of the deep cerebellar nuclei have been well documented, little is known about the neurotransmitter systems mediating cerebellar efferent pathways. The present study utilizes immunohistochemical procedures in conjunction with a novel monoclonal antibody specific for carbodiimide-fixed glutamate and polyclonal antisera against glutaminase (GLNase) and aspartate aminotransferase (AATase) to examine the presence of putative excitatory amino acid transmitters in neurons of the deep cerebellar nuclei. Carbodiimide-fixed glutamate-like, GLNase-like and AATase-like immunoreactivities were observed in neurons of the lateral, posterior interpositus, anterior interpositus and medial deep cerebellar nuclei. More neurons were stained with AATase antiserum than with the GLNase antiserum or the monoclonal antibody. These results suggest glutamate, GLNase and AATase are present in neurons of the deep cerebellar nuclei and raise the possibility that glutamate may be an excitatory transmitter in these structures.     

Morphology of labeled afferent fibers in the guinea pig cochlea

Cochlear afferent and efferent fibers in the guinea pig were labeled by focal extracellular injections of horseradish peroxidase into the spiral ganglion of the basal turn. The morphology and pattern of termination of these fibers were studied by light microscopy. Fibers labeled by injections into the peripheral side of the ganglion could be grouped on the basis of their courses and terminations in the cochlea into two classes of afferent fibers, two classes of efferent (olivocochlear) fibers, and other presumably autonomic fibers. This paper describes the characteristics of labeled afferent fibers and their parent ganglion cells. Peripheral afferent fibers were grouped into two major classes: thick (mean diameter 1.7 micron) radial fibers projecting in a primarily radial fashion from the spiral ganglion and terminating on single inner hair cells and thin (mean diameter 0.5 micron) outer spiral fibers that spiral basalward in the organ of Corti to terminate on outer hair cells, usually in one row. For outer spiral fibers, the number of outer hair cells contacted and the length of the terminal region depend on the row of outer hair cells contacted, with third-row fibers forming, on the average, the most extensive region of termination. Within the spiral ganglion, two types of ganglion cells could be distinguished: type-I ganglion cells of large size (mean soma area = 216 microns 2) with a ratio of central process diameter to peripheral process diameter greater than one and type-II ganglion cells of smaller size (mean soma area = 100 microns 2) and a central to peripheral process ratio near one. In three cochleae in which injections were made central to the ganglion, 11 type-I ganglion cells have been traced to radial fibers contacting inner hair cells and eight type-II ganglion cells have been traced to outer spiral fibers contacting outer hair cells. Thus the afferent innervation of the guinea pig cochlea is similar to the pattern described in other mammals, in which there is separate innervation of the inner and outer hair cells by the two types of ganglion cells. The central axons of both types of ganglion cells were traced individually through serial sections of a block of tissue containing the cochlea, the auditory nerve, and the cochlear nucleus. They followed similar courses in the auditory nerve, and the axons followed into the cochlear nucleus bifurcated in similar regions of the interstitial portion.(ABSTRACT TRUNCATED AT 400 WORDS)     

Localization of AMPA-selective glutamate receptors in the auditory brainstem of the barn owl

AMPA receptor subunit-specific antibodies were used to determine if the distribution of excitatory amino acid receptors in the owl's auditory brainstem and midbrain nuclei reflected specializations for temporal processing. Each auditory nucleus displays characteristic levels of immunostaining for the AMPA receptor subunits GluR1–4, with high levels of the subtypes which exhibit rapid desensitization (GluR4 and 2/3). In the auditory brainstem, levels of GluR2/3 and GluR4 were very high in the cochlear nucleus magnocellularis and the nucleus laminaris. The different cell types of the cochlear nucleus angularis and the superior olive were characterized by heterogeneous GluR2/3 and 4 immunostaining. GluR1 levels were very low or undetectable. In the lemniscal nuclei, most neurons contained low levels of GluR1, and dense GluR2/3 and GluR4 immunoreactivity, with high levels of GluR4 in the dendrites. Levels of GluR4 were higher in the anterior portion of the ventral nucleus of the lateral lemniscus. The divisions of the inferior colliculus could be distinguished on the basis of GluR1–4 immunoreactivity, with high levels of GluR4 and moderate levels of GluR1 in the external nucleus. No major differences were observed between the pathways for encoding time and sound level cues. J. Comp. Neurol. 378:239–253, 1997. © 1997 Wiley-Liss, Inc.     

Postnatal development of GAP-43 immunoreactivity in the auditory brainstem of the rat

Extensive data link the growth associated protein GAP-43 to axonal elongation and synapse formation during development and in plastic responses of nervous tissue. We have studied the changing levels of GAP-43 expression in the auditory brainstem nuclei of the developing rat by applying immunocytochemical techniques. By the first postnatal day (P1), GAP-43 was expressed at high concentrations in all subdivisions of the cochlear nuclear complex and the superior olivary complex. At this stage, neuropil structures recognized by the antibody did not show any varicosities on cellular processes in all these regions. By P8, the texture of the stain has turned markedly more granular, a pattern likely to reflect the formation of presynaptic endings. A predominantly granular distribution of GAP-43 has developed by P12. At that time, the staining intensity is markedly reduced compared to the levels of the newborn. By P16, the auditory brainstem nuclei have lost most of their GAP-43 immunoreactivity, but a distinct level of staining persisted into adulthood in all of them. This staining was restricted to boutons, which are thought to be presynaptic terminals. We conclude that a moderate but apparently relevant potential for plasticity is retained in these auditory structures. Should the patterns of neural signals, mediated by the inner ear, change during adulthood, the central structures appear to be able to respond with the formation of altered synaptic connectivity. J. Comp. Neurol. 382:104-115, 1997. © 1997 Wiley-Liss Inc.     

大鼠三叉—小脑投射神经元表达磷酸激活的谷氨酰胺酶——双标法研究

磷酸激活的谷氨酰胺酶 (phosphateactivatedglutaminase,PAG)可作为谷氨酸能神经元的标志物。采用逆行束路追踪技术和PAG免疫荧光组织化学法 ,研究脑干三叉核团复合体中向小脑袢状小叶Crus1区投射神经元是否呈PAG样免疫反应阳性。结果表明 :将荧光金 (Fluoro Gold ,FG)注入小脑袢状小叶Crus1区后 ,在三叉神经感觉主核 (Pr5 )、三叉神经脊束核极间亚核 (Sp5I)和吻侧亚核背内侧部 (Sp5DM)、三叉间核 (I5 )以及三叉神经运动核与上橄榄核之间的一个小区域 (nucleusJ)中均可见到FG逆行标记细胞。标记细胞出现在上述核团的双侧 ,但以同侧为主。PAG免疫荧光组织化学反应结果显示 :许多FG标记神经元呈强PAG样免疫阳性 ,其中FG/PAG双标神经元占FG逆行标记神经元总数的 73%～ 94 %。结果提示 :脑干三叉神经核团中的谷氨酸能神经元可能参与触觉信息从面口部向小脑的传递。     

大鼠三叉-小脑投射神经元表达磷酸激活的谷氨酰胺酶——双标法研究

磷酸激活的谷氨酰胺酶(phosphate activated glutaminase,PAG)可作为谷氨酸能神经元的标志物.采用逆行束路追踪技术和PAG免疫荧光组织化学法,研究脑干三叉核团复合体中向小脑袢状小叶Crusl区投射神经元是否呈PAG样免疫反应阳性.结果表明:将荧光金(Fluoro-Gold,FG)注入小脑袢状小叶Crusl区后,在三叉神经感觉主核(Pr5)、三叉神经脊束核极间亚核(Sp5I)和吻侧亚核背内侧部(Sp5DM)、三叉间核(15)以及三叉神经运动核与上橄榄核之间的一个小区域(nucleus J)中均可见到FG逆行标记细胞.标记细胞出现在上述核团的双侧,但以同侧为主.PAG免疫荧光组织化学反应结果显示:许多FG标记神经元呈强PAG样免疫阳性,其中FG/PAG双标神经元占FG逆行标记神经元总数的73%～94%.结果提示:脑干三叉神经核团中的谷氨酸能神经元可能参与触觉信息从面口部向小脑的传递.     

Plasticity of the auditory brainstem: Effects of cochlear ablation on GAP-43 immunoreactivity in the rat

In the adult brain, expression of the growth associated protein GAP-43 may serve as an indicator of synaptic remodeling. We have studied localization and time course of the re-expression of GAP-43 following deafening through cochlear ablation. As a consequence of unilateral cochlear lesioning, a substantial increase in the expression of GAP-43 was observed in the neuropil of all subnuclei of the ipsilateral cochlear nuclear complex. This expression of GAP-43 occurred in well-defined fibers and boutons. In the ventral cochlear nuclei, boutons immunoreactive for GAP-43 were often localized on cell bodies. However, they were found only on selected subpopulations of cochlear nucleus neurons, i.e., on cell bodies containing glutamate or calretinin immunoreactivity, but apparently not on GABAergic neurons. Olivocochlear neurons must have been axotomized by the operation. Following cochlear ablation, a dramatic re-expression of GAP-43 occurred in cell bodies of the ipsilateral lateral superior olive but not in the ventral nucleus of the trapezoid body. Position and number of these cells suggested that most, if not all, of them serve the lateral olivocochlear bundle. However, although axon collaterals are given off by certain types of olivocochlear neurons, a direct involvement of the immunoreactive cell bodies in the emergence of GAP-43 in the cochlear nucleus is not obvious. A transient rise of GAP-43 immunoreactivity that could not be attributed to axotomized neurons was observed in the contralateral dorsal cochlear nucleus and in the ipsilateral inferior colliculus. Given the functional significance attributed to GAP-43, we conclude that the sudden loss of spiral ganglion cells leads to a reactive synaptogenesis in complex patterns across several auditory brainstem nuclei. J. Comp. Neurol. 382:116-138, 1997. © 1997 Wiley-Liss Inc.     

Changes in the tonotopic map of the dorsal cochlear nucleus in hamsters with hair cell loss and radial nerve bundle degeneration

Hamsters were exposed to an intense tone (10 kHz) at levels and durations sufficient to cause hair cell loss and radial nerve bundle degeneration. A previous study reported changes in the tonotopic map of the dorsal cochlear nucleus (DCN) in hamsters with tone-induced stereocilia loss. Such changes appear similar to those observed by others in the auditory nerve following acoustic trauma, and suggest that the map alterations have a peripheral origin. However, the potential for tonotopic map reorganization after more severe lesions involving cellular degeneration in the cochlea has not yet been determined. The purpose of the present study was to determine how the tonotopic map of the DCN appears in animals with severe cochlear injury involving hair cell loss and radial nerve bundle degeneration. Neural population thresholds and tonotopic organization were mapped over the surface of the DCN in normal unexposed animals and those showing tone-induced lesions. The results indicate that cochlear lesions characterized mainly by radial bundle degeneration in a restricted portion of the organ of Corti cause changes in a corresponding region of the tonotopic map which reflect primarily changes in the shape and thresholds of neural tuning curves. In many cases the center of the lesion was represented in the DCN as a distinct characteristic frequency (CF) gap in the tonotopic map in which responses were either extremely weak or absent. In almost all cases the map area representing the center of the lesion was bordered by an expanded region of near-constant CF, a feature superficially suggestive of map reorganization (i.e., plasticity). However, these expanded map areas had abnormal tip thresholds and showed other features suggesting that their CFs had been shifted downward by distortion and deterioration of their original tips. Such changes in neural tuning following tone-induced loss of anatomical input to the central auditory pathway are similar to those observed in our previous study and by others in the auditory nerve following less severe acoustic trauma, and thus would seem to have a peripheral origin. Thus, changes in the DCN tonotopic map can be explained by peripheral modifications and do not seem to involve plastic changes (i.e., reorganization).     

Immunocytochemical localization of aspartate aminotransferase and glutaminase immunoreactivities in the cerebellum

The distributions of glutaminase and aspartate aminotransferase were studied immunocytochemically in the cerebellum of the guinea pig and the rat. In the granule cell layer, both antibodies gave a similar staining pattern. Granule cell bodies were labeled, but staining was also found to lie outside the cell body, associated with what appear to be synaptic processes. In the molecular and Purkinje cell layers, aspartate aminotransferase was concentrated in stellate and basket cell bodies and in terminal baskets beneath Purkinje cells. Glutaminase, however, was not concentrated in these structures.     

Development of the cochlear innervation of the dorsal cochlear nucleus of the hamster

The development of cochlear fibers and terminals in the dorsal cochlear nucleus of the hamster was studied with light and electron microscopic techniques. Like the dorsal cochlear nucleus of most other mammals, the dorsal cochlear nucleus of the adult hamster is a laminated structure. Three distinct layers can be identified in cresyl-violet-stained sections: the molecular layer, the fusiform cell layer, and the deep layer. The deep layer consists of a superficial zone, free of large cell bodies, and a deep zone which contains the somas of giant cells. Horseradish peroxidase and degeneration studies reveal that the cochlear fibers ramify throughout the deep and fusiform cell layers of the adult hamster but do not enter the molecular layer. In the electron microscope, three types of terminals that contact the fusiform and the giant cells can be distinguished. Only one type of terminal (type LR) degenerates after cochlear ablation and is, therefore, thought to be of cochlear origin. Type LR terminals are found throughout the deep and fusiform cell layers and contact the somas of giant and fusiform cells, as well as their intermingled dendrites in the deep layer. In Golgi-impregnated material, cochlear fibers are not found in the dorsal cochlear nucleus of the neonatal hamster, although they have entered the ventral cochlear nucleus. Ingrowth of cochlear fibers into the dorsal cochlear nucleus occurs over the first postnatal week and one-half. A spatial gradient is evident during the ingrowth of the fibers in that they invade the dorsomedial parts of the dorsal cochlear nucleus before they invade the ventrolateral parts. In all parts of the nucleus, the fibers enter the deepest layer and grow progressively more superficially. In the electron microscope, the first appearance of type LR terminals at each depth lags behind the ingrowth of the fibers by about two days. In hamsters, fibers from the basal turns of the cochlea terminate in the dorsomedial dorsal cochlear nucleus, while fibers from the apical turns terminate in the ventrolateral dorsal cochlear nucleus (DCN). The dorsomedial to ventrolateral gradient in the ingrowth of the cochlear fibers into the DCN indicates that the fibers from the basal turn are the first to arrive. Several components of the mammalian cochlea have been shown to mature at the base of the cochlea before they mature at the apex. The present study suggests that maturation gradients in the cochlear nucleus parallel those observed in the cochlea.     

Fos-like immunoreactivity in central auditory neurons of the mouse

Fos-like immunoreactivity was used to study sound-induced activation of neurons in the auditory brainstem. Immunoreactivity was assayed with a polyclonal antibody to Fos. In response to 6-kHz tone bursts, the pattern of staining was a band of immunoreactive neurons positioned at the tonotopically appropriate position within the cochlear nucleus and the inferior colliculus. The band was narrow at low sound pressure levels but wider along the tonotopic axis at higher sound levels. In response to noise bursts, the pattern was broader and often extended throughout the auditory nuclei. Often within this broad pattern were “sub-bands” of immunostained neurons, interspersed with bands of unstained neurons. With increasing sound pressure levels above 35--55 dB, the number of Fos-like immunoreactive neurons increased for the cochlear nucleus, superior olivary complex, and inferior colliculus. In the cochlear nucleus and inferior colliculus, the stained cells were small, and hence their activity would be difficult to sample in electrophysiological studies. In the medial nucleus of the trapezoid body, the stained neurons had larger somata and other characteristics of principal cells. Anesthesia with Nembutal or Avertin, but not with ketamine or urethane, decreased the number of Fos-like immunoreactive neurons in the cochlear nucleus. The different anesthetics produced more variable results in the inferior colliculus. In anesthetized, monaurally stimulated animals, the presence of staining in the contralateral cochlear nucleus indicates that some Fos-like immunoreactivity may be mediated by descending or commissural systems. These observations indicate that Fos assays are useful for studying the pattern of neuronal activation in the auditory system and may also be useful in studying the descending auditory pathways. © 1995 Wiley-Liss, Inc.     

Immunocytochemical Evidence that Glutamate is a Neurotransmitter in the Cochlear Nerve: A Quantitative Study in the Guinea-pig Anteroventral Cochlear Nucleus

The large so-called type I afferents of the cochlear nerve carry the majority of the auditory input from the cochlea to the cochlear nuclei in the brainstem. These fibres are excitatory and previous studies have suggested they may use glutamate as their neurotransmitter. In the present investigation therefore, antibodies to glutamate and to the glutamate precursor, glutamine, were applied to resin sections of perfusion-fixed brains and of in vitro brain slices subjected to depolarizing levels of potassium before fixation to study glutamate handling and synaptic release. Ultrathin sections were labelled by the immunogold technique, and the immunoreactivity was quantified by recording the density of gold particles over the various tissue profiles. Non-primary, presumably inhibitory, terminals and glial processes were used as reference structures. The cochlear primary terminals proved to be strongly immunoreactive for glutamate. The density of glutamate labelling was higher in primary terminals than in non-primary ones, and lowest in glial processes. The ratio between the mean glutamate and glutamine labelling densities was also higher in primary terminals than in non-primary ones, and lowest in glial processes in each case. In the primary terminals, the glutamate immunoreactivity was higher over vesicle-containing regions than over vesicle-free regions, whilst glutamine was evenly distributed throughout. The in vitro brain slices showed a potassium-induced, partly calcium-dependent depletion of glutamate from the primary terminals but not from the non-primary ones. These observations strongly support the conclusion that glutamate is a neurotransmitter of type I cochlear afferents.