GABA and glycine in the central auditory system of the mustache bat: structural substrates for inhibitory neuronal organization

The distribution and morphology of neurons and axonal endings (puncta) immunostained with antibodies to gamma-aminobutyric acid (GABA) and glycine (Gly) were analyzed in auditory brainstem, thalamic, and cortical centers in the mustache bat. The goals of the study were (1) to compare and contrast the location of GABAergic and glycinergic neurons and puncta, (2) to determine whether nuclei containing immunoreactive neurons likewise have a similar concentration of puncta, (3) to assess the uniformity of immunostaining within a nucleus and to consider regional differences that were related to or independent of cytoarchitecture, and (4) to compare the patterns recognized in this bat with those in other mammals. There are nine major conclusions. (1) Glycinergic immunostaining is most pronounced in the hindbrain. (2) In the forebrain, GABA alone is present. (3) Some nuclei have GABAergic or glycinergic neurons exclusively; a few have neither. (4) Although there is sometimes a close relationship between the relative number of immunopositive neurons and the density of the puncta, just as often there is no particular correlation between them; this reflects the fact that many GABAergic and glycinergic neurons project beyond their nucleus of origin. (5) Even nuclei devoid of or with few GABAergic or glycinergic neurons contain relatively abundant numbers of puncta; some neurons receive axosomatic terminals of each type. (6) In a few nuclei there are physiological subregions with specific local patterns of immunostaining. (7) The patterns of immunostaining resemble those in other mammals; the principal exceptions are in nuclei that, in the bat, are hypertrophied (such as those of the lateral lemniscus) and in the medial geniculate body. (8) Cellular colocalization of GABA and Gly is specific to only a few nuclei. (9) GABA and glutamic acid decarboxylase (GAD) immunostaining have virtually identical distributions in each nucleus. Several implications follow. First, the arrangements of GABA and Gly in the central auditory system represent all possible patterns, ranging from mutually exclusive to overlapping within a nucleus to convergence of both types of synaptic endings on single neurons. Second, although both transmitters are present in the hindbrain, glycine appears to be dominant, and it is often associated with circuitry in which precise temporal control of aspects of neuronal discharge is critical. Third, the auditory system, especially at or below the level of the midbrain, contains significant numbers of GABAergic or glycinergic projection neurons. The latter feature distinguishes it from the central visual and somatic sensory pathways.

Localization of two high-threshold potassium channel subunits in the rat central auditory system

The firing pattern of auditory neurons is determined in part by the type of voltage-sensitive potassium channels expressed. The expression patterns for two high-threshold potassium channels, Kv3.1 and Kv3.3, that differ in inactivation properties were examined in the rat auditory system. The positive activation voltage and rapid deactivation kinetics of these channels provide rapid repolarization of action potentials with little effect on action potential threshold. In situ hybridization experiments showed that Kv3.3 mRNA was highly expressed in most auditory neurons in the rat brainstem, whereas Kv3.1 was expressed in a more limited population of auditory neurons. Notably, Kv3.1 mRNA was not expressed in neurons of the medial and lateral superior olive and a subpopulation of neurons in the ventral nucleus of the lateral lemniscus. These results suggest that Kv3.3 channels may be the dominant Kv3 subfamily member expressed in brainstem auditory neurons and that, in some auditory neurons, Kv3.1 and Kv3.3 may coassemble to form functional channels. The localization of Kv3.1 protein was examined immunohistochemically. The distribution of stained somata and neuropil varied across auditory nuclei and correlated with the distribution of Kv3.1 mRNA-expressing neurons and their terminal arborizations, respectively. The intensity of Kv3.1 immunoreactivity varied across the tonotopic map in the medial nucleus of the trapezoid body with neurons responding best to high-frequency tones most intensely labeled. Thus, auditory neurons may vary the types and amount of K(+) channel expression in response to synaptic input to subtly tune their firing properties.

Glycine receptors in adult guinea pig brain stem auditory nuclei: regulation after unilateral cochlear ablation

In young adult guinea pigs, the effects of unilateral cochlear ablation were determined on the specific binding of [3H]strychnine measured in subdivisions of the cochlear nucleus (CN), the superior olivary complex, and the auditory midbrain, after 2, 7, 31, 60, and 147 postlesion days. Changes in binding relative to that in age-matched controls were interpreted as altered activity and/or expression of synaptic glycine receptors. Postlesion binding declined ipsilaterally in most of the ventral CN and in the lateral superior olive (LSO). Binding was modestly deficient in the ipsilateral dorsal CN and in the anterior part of the contralateral anteroventral CN. Binding was elevated in the contralateral LSO. Transient changes also occurred. Binding was elevated transiently, between 2 and 31 days, contralaterally in parts of the anteroventral CN, bilaterally in the medial superior olive (MSO), and bilaterally in most of the midbrain nuclei. Binding was deficient transiently, at 60 days, in most of the contralateral CN and bilaterally in the midbrain nuclei. The present findings, together with previously reported postlesion changes in glycine release, were consistent with persistently weakened glycinergic inhibitory transmission ipsilaterally in the ventral CN and the LSO and bilaterally in the dorsal CN. Glycinergic inhibitory transmission was strengthened in the contralateral LSO and transiently strengthened in the MSO bilaterally. A hypothetical model of the findings suggested that glycine receptor regulation may depend on excitatory and glycinergic input to auditory neurons. The present changes in glycine receptor activity may contribute to altered auditory functions, which often accompany hearing loss.

The distribution of splice variants of the NMDAR1 subunit mRNA in adult rat brain

Regional variation in the alternative splice forms of the NMDAR1 subunit mRNA was investigated by in situ hybridization in the adult rat brain, using radiolabelled splice-specific oligonucleotide probes. Each splice variant was detected in an individual distribution. The NMDAR1-a and NMDAR1-2 forms were widely and abundantly distributed throughout the brain, except for the inferior colliculus. The NMDAR1-b and NMDAR1-4 variants were located in similar patterns in fewer areas (e.g. parietal cortex, hippocampus CA3, thalamus, inferior colliculus, cerebellar granule cells). In contrast, the NMDAR1-1 forms were distributed in a pattern approximately complementary in the forebrain to that of NMDAR1-4 (weakly expressed in thalamus and inferior colliculus). The NMDAR1-3 variants were not abundant in any structure. Considerable overlap of the in situ hybridization images was noted, so all eight splice combinations are possible in heterogenous distributions. Correlation of the distribution of NMDAR1 mRNA splice forms with functional analyses of heteromeric recombinant receptors will be necessary to ascertain if alternative splicing of the NMDAR1 subunit can account for some of the known heterogeneity of endogenous NMDA receptors.

Common patterns of increased and decreased fos expression in midbrain and pons evoked by noxious deep somatic and noxious visceral manipulations in the rat

Immunohistochemical detection of the protein product (Fos) of the c-fos immediate early gene was used to study neuronal activation in the rostral pons and midbrain of halothane-anesthetised rats following noxious deep somatic or noxious visceral stimulation. In animals exposed only to halothane anesthesia, Fos-like immunoreactive (IR) neurons were located in the midbrain periaqueductal gray matter, tectum, and parabrachial nucleus. Following noxious stimulation of hindlimb muscle, knee joint, vagal cardiopulmonary, or peritoneal nociceptors, there was, compared to halothane-only animals, a significant increase in the numbers of Fos-like (IR) cells in the caudal ventrolateral periaqueductal gray and the intermediate gray lamina of the superior colliculus. Given the general agreement that increased Fos expression is a consequence of increased neuronal activity, the finding that a range of noxious deep somatic and noxious visceral stimuli evoked increased neuronal activity in a discrete, caudal ventrolateral periaqueductal gray region is consistent with previous suggestions that this region is an integrator of deep noxious evoked reactions. The noxious deep somatic and noxious visceral manipulations also evoked, compared to halothane-only animals, reductions in the numbers of Fos-like IR cells in the stratum opticum of the superior colliculus and the unlaminated portion of the external subnucleus of the inferior colliculus. To our knowledge this is the first report of reductions in Fos-expression in the tectum evoked by noxious stimulation. In separate experiments, the effects of noxious deep somatic and noxious visceral manipulations on arterial pressure and heart rate were measured. The noxious visceral manipulations evoked substantial and sustained falls in arterial pressure (15-45 mmHg), and heart rate (75-100 bpm), whereas the depressor and bradycardiac effects of the noxious deep somatic manipulations were weaker, not as sustained, or entirely absent. As similar distributions and numbers of both increased and decreased Fos-like IR cells were observed after each of the deep noxious manipulations, it follows that the deep noxious evoked increases and decreases in Fos expression were not secondary to the evoked depressor or bradycardiac effects.

Glutamate receptor phenotypes in the auditory brainstem and mid-brain of the developing rat

Glutamate receptors mediate most excitatory synaptic transmission in the adult vertebrate brain, but their activation in developing neurons also influences developmental processes. However, little is known about the developmental regulation of the subunits composing these receptors. Here we have studied age-dependent changes in the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) and N-methyl-D-aspartate (NMDA) receptor subunits in the cochlear nucleus complex (CN), the superior olivary complex (SOC), the nuclei of the lateral lemniscus, and the inferior colliculus of the developing rat. In the lateral superior olive, the medial nucleus of the trapezoid body, and the ventral nucleus of the lateral lemniscus, the distribution of AMPA receptor subunits changed drastically with age. While GluR1 and GluR2 subunits were highly expressed in the first 2 postnatal weeks, GluR4 staining was detectable only thereafter. GluR1 and GluR2 immunoreactivities rapidly decreased during the third postnatal week, with the GluR1 subunits disappearing from most neurons. In contrast, the adult pattern of the distribution of AMPA receptor subunits emerged gradually in most of the other auditory nuclei. Thus, progressive as well as regressive events characterized AMPA receptor development in some nuclei, while a monotonically maturation was seen in other regions. In contrast, the staining patterns of NMDA receptor subunits remained stable or only decreased during the same period. Although our data are not consistent with a generalized pattern of AMPA receptor development, the abundance of GluR1 subunits is a distinctive feature of early AMPA receptors. As similar AMPA receptors are present during plasticity periods throughout the brain, neurons undergoing synaptic and structural remodelling might have a particular need for these receptors.

GAD- and GABA-immunoreactivity in the ascending auditory pathway of horseshoe and mustached bats

A comparative study of the immunostain to antibodies directed against glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) in the ascending auditory pathway was carried out in horseshoe bats (Rhinolophus rouxi) and mustached bats (Pteronotus parnellii). In both species GAD/GABA-positive puncta (presumed axonal boutons) and GAD/GABA-positive cells were found in the cochlear nucleus, the superior olivary complex, the nuclei of the lateral lemniscus the inferior colliculus, and the medial geniculate body. General features of the immunostaining pattern in the auditory pathway agree with observations in other mammals. Quantitative analysis of puncta distribution shows that many auditory centers are characterized by subregional differences in puncta density and distribution. This indicates local differences in putatively inhibitory input related to connectivity and tonotopic organization. The following species characteristic features were found: 1) The dorsal non-laminated portion of the dorsal cochlear nucleus in horseshoe bats lacks the GAD/GABA-immunoreactive cells typical for the ventral laminated portion and the dorsal cochlear nucleus of other species. Clearly, a cytoarchitectonic specialization is accompanied by a loss of putatively GABAergic local inhibitory circuits. 2) The ventral division of the medial geniculate body of the mustached bat lacks GAD/GABA-immunopositive cells. Such cells are present in the horseshoe bat and other mammals. This finding implies functional differences in the organization of the medial geniculate body within the same mammalian order.

Distribution of calcium-binding protein calbindin-D28k in the auditory system of adult and developing rats

Calbindin-D28k (CaBP) is a calcium-binding protein, which appears to be involved in the buffering of free intracellular calcium and may thereby contribute to calcium homeostasis. This study attempted to determine the distribution pattern of CaBP immunoreactivity in the central auditory system of adult rats and during development, when calcium ions play key roles in several aspects of nerve cell function. It was found that most steps during CaBP development occur postnatally in the central auditory system. With the exception of the lateral superior olive, the ventral and the intermediate nuclei of the lateral lemniscus, and the auditory cortex, which already express CaBP prenatally, CaBP immunoreactivity is not present before postnatal day 2 (P2). Development proceeds until about P24, when the pattern characteristic of adult animals can be seen. There was no detectable sequence in CaBP development from lower to higher stations in the auditory pathway, i.e., the different nuclei appear to express CaBP independently of each other, indicating that intrinsic, rather than peripheral, maturation processes may predominantly influence CaBP expression. Neurons in four brainstem nuclei (the lateral superior olive, the ventral and intermediate nuclei of the lateral lemniscus, and the central nucleus of the inferior colliculus) express CaBP only transiently. In these nuclei, CaBP immunoreactivity peaks between P6 and P18, which coincides with the period of synapse stabilization. Therefore, CaBP may play a specific role during neuronal development, by buffering the concentration of intracellular free Ca2+, which may be necessary for modification of synaptic efficiency.

Distribution of GABAA, GABAB, and glycine receptors in the central auditory system of the big brown bat, Eptesicus fuscus

Quantitative autoradiographic techniques were used to compare the distribution of GABAA, GABAB, and glycine receptors in the subcortical auditory pathway of the big brown bat, Eptesicus fuscus. For GABAA receptors, the ligand used was 35S-t-butylbicyclophosphorothionate (TBPS) for GABAB receptors, 3H-GABA was used as a ligand in the presence of isoguvacine to block binding to GABAA sites; for glycine, the ligand used was 3H-strychnine. In the subcortical auditory nuclei there appears to be at least a partial complementarity in the distribution of GABAA receptors labeled with 35S-TBPS and glycine receptors labeled with 3H-strychnine, GABAA receptors were concentrated mainly in the inferior colliculus (IC) and medial geniculate nucleus, whereas glycine receptors were concentrated mainly in nuclei below the level of the IC. Within the IC, there was a graded spatial distribution of 35S-TBPS binding; the most dense labeling was in the dorsomedial region, but very sparse labeling was observed in the ventrolateral region. There was also a graded spatial distribution of 3H-strychnine binding. The most dense labeling was in the ventral and lateral regions and the weakest labeling was in the dorsomedial region. Thus, in the IC, the distribution of 35S-TBPS was complementary to that of 3H-strychnine. GABAB receptors were distributed at a low level throughout the subcortical auditory nuclei, but were most prominent in the dorsomedial part of the IC.

Parvalbumin, calbindin D-28k, and calretinin immunoreactivity in the ascending auditory pathway of horseshoe bats

In the subcortical auditory system of Rhinolophus rouxi, antibodies directed against the calcium-binding proteins parvalbumin, calbindin D-28k, and calretinin yield partly overlapping and partly complementary labeling patterns which are described in detail for each nucleus. The most general features of the labeling patterns are that: 1) Parvalbumin is a potent marker for large and heterogeneous populations of cells and puncta (presumed axon terminals) throughout the auditory pathway. 2) Immunostaining with the monoclonal calbindin-antiserum was typically absent or sparse in most auditory brainstem centers, but prominent in auditory nerve fibers and in cells of the medial geniculate body (MGB). 3) Calretinin label is abundant but more restricted to subsets of auditory nuclei or subpopulations of cells than parvalbumin. 4) Calcium-binding proteins are useful markers to define particular subregions or cell types in auditory nuclei: for example, i) different labeling patterns are obtained within the nuclei of the lateral lemniscus and adjacent tegmental zones; ii) in the inferior colliculus both calbindin- and calretinin-antisera yield similar regional specific staining patterns, but label different cell types; iii) subregions of the medial geniculate body have characteristic profiles of calcium-binding proteins; and iv) analyses of different nuclei showed that there is no simple common denominator for cells characterized by the expression of particular calcium-binding proteins, nor does labeling correspond in a straightforward way with specific functional systems. 5) there are profound differences between the calbindin labeling patterns seen in Rhinolophus and those in other mammals.

Regional distribution of extracellular 5-hydroxytryptamine and 5-hydroxyindoleacetic acid in the brain of freely moving rats

The extracellular concentrations of 5-hydroxytryptamine (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) have been determined in six brain areas of awake rats (frontal cortex, striatum, hypothalamus, hippocampus, inferior colliculus, and raphe nuclei) using intracerebral microdialysis. The extracellular levels of 5-HT showed no significant differences among the brain regions studied. The tissue levels of 5-HT and 5-HIAA as well as the extracellular concentration of 5-HIAA were significantly higher in raphe nuclei. The regional distribution of tissue and extracellular 5-HIAA were very similar, suggesting that extracellular 5-HIAA depends mainly on the output from the intracellular compartment. On the other hand, extracellular 5-HT and tissue 5-HT showed a different distribution pattern. The tissue/extracellular ratio for 5-HT ranged from 739 in frontal cortex to 2,882 in raphe, whereas it only amounted to 1.8-3.6 for 5-HIAA. The relationship between the present results and the density of 5-HT uptake sites in these areas is discussed.

Classical conditioning modifies cytochrome oxidase activity in the auditory system

The effects of excitatory classical conditioning on cytochrome oxidase activity in the central auditory system were investigated using quantitative histochemistry. Rats in the conditioned group were trained with consistent pairings of a compound conditional stimulus (a tone and a light) with a mild footshock, to elicit conditioned suppression of drinking. Rats in the pseudorandom group were exposed to pseudorandom presentations of the same tone, light and shock stimuli without consistent pairings. Untrained rats in a naive group did not receive presentations of the experimental stimuli. The findings demonstrated that auditory fear conditioning modifies the metabolic neuronal responses of the auditory system, supporting the hypothesis that sensory neurons are responsive to behavioural stimulus properties acquired by learning. There was a clear distinction between thalamocortical and lower divisions of the auditory system based on the differences in metabolic activity evoked by classical conditioning, which lead to an overt learned behavioural response versus pseudorandom stimulus presentations, which lead to behavioural habituation. Increases in cytochrome oxidase activity indicated that tone processing is enhanced during associative conditioning at upper auditory structures (medial geniculate nucleus and secondary auditory cortices). In contrast, metabolic activation of lower auditory structures (cochlear nuclei and inferior colliculus) in response to the pseudorandom presentation of the experimental stimuli suggest that these areas may be activated during habituation to tone stimuli. Together these findings show that mapping the metabolic activity of cytochrome oxidase with quantitative histochemistry can be successfully used to map regional long-lasting effects of learning on brain systems.

Cloning, expression and pharmacology of the mouse 5-HT(4L) receptor

Since most of our knowledge on pharmacological properties of brain 5-HT4 receptors have been discussed for mouse colliculi neurons, we cloned the corresponding receptor using the RT-PCR approach. As expected, the homology with the already cloned rat 5-HT(4L) receptor was high, revealing only 16 differences at the amino-acid level. One of the differences, proline75 in mouse, alanine75 in the already published rat sequences was not confirmed. Therefore this proline is part of the consensus sequence present in all 5-HT receptor transmembrane domain II (LVMP). Comparing the affinities of 11 agonists and five antagonists for the cloned mouse receptor (5-HT(4L))expressed in LLCPK1 and the corresponding receptor in mouse colliculi shows an excellent correlation. The transfected mouse 5-HT(4L) receptor stimulated cAMP production. When expressed at high density, it exhibited intrinsic activity. In contrast to the previously described distribution, we found that mRNA encoding for both the short (5-HT(4S))and the long form (5-HT(4L)) of 5-HT4 receptors are expressed in all mouse and rat brain areas.

Distribution of calcium-binding protein immunoreactivities in the guinea pig auditory brainstem

This study was intended to provide an overview of the distribution of calcium-binding proteins in the rodent auditory brainstem. We based our observations on immunohistochemical material obtained in the guinea pig, a species widely used in auditory research in which a mapping of calcium-binding proteins in the auditory brainstem is still missing. Differences in the amounts of these proteins throughout the auditory brainstem were further analyzed semiquantitatively. Parvalbumin was present in most neurons and their axon terminals throughout the ascending auditory brainstem. Nuclei that surround the main relay nuclei of the ascending auditory pathway lacked labeling. Calretinin staining was prominent in spherical and globular cells of the cochlear nucleus, in their axon terminals in the superior olivary complex, and in principal cells of the medial superior olive. Measures of optical densities showed that auditory neurons involved in sound localization had the highest calretinin labeling levels. Calbindin D-28k was present in cartwheel cells of the dorsal cochlear nucleus, in almost all neurons of the medial nucleus of the trapezoid body, and in globular cells in the ventral nucleus of the lateral lemniscus. The labeling patterns for calretinin and calbindin D-28k were non-overlapping throughout the auditory brainstem. This was also evident in the ventral nucleus of the lateral lemniscus where calbindin D-28k-immunoreactive terminals were found in the medial portion, while the calretinin-immunoreactive terminals were observed in the lateral portion. This study presents the first direct and comprehensive comparison of these three calcium-binding proteins in the auditory brainstem of a rodent. Each antibody yields a unique staining pattern that provides a basis for further defining neuronal populations. In addition, since their axons are also selectively stained, auditory nuclei can further be compartmentalized based on different terminal fields. These immunoreactivities have provided clues to the complex structure of the auditory brainstem.

Glutamate in the inferior colliculus plays a critical role in audiogenic seizure initiation

Alterations of excitant amino acid (EAA) action are implicated in seizure susceptibility in the genetically epilepsy-prone rat (GEPR). The inferior colliculus (IC) is critical for audiogenic seizure (AGS) initiation in the GEPR. The present study observed that bilateral microinjection into the IC of L-canaline, a glutamate synthesis inhibitor, decreased AGS severity in the GEPR and also decreased potassium-evoked release of glutamate from IC slices. Bilateral microinjection of NMDA receptor antagonists, 2-amino-7-phosphonoheptanoate (AP7) or 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonate (CPP) into IC blocked AGS, and an antagonist at non-NMDA EAA receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), also blocked AGS. NMDA receptor antagonists were 5-200 times more effective than CNQX. Microinjection of a non-competitive NMDA receptor antagonist, dizocilpine (MK-801), into IC had little effect except with very high doses. Microinjection of CPP or AP7 into the IC blocked AGS at considerably lower doses as compared to pontine reticular formation (PRF). However, MK-801 attenuated AGS when microinjected into PRF at doses that were ineffective in IC. Systemically administered CPP blocked AGS and significantly reduced IC neuronal firing in the behaving GEPR, suggesting an important action of systemically administered NMDA receptor antagonists on brainstem auditory nuclei critical to AGS. The present results support a critical role for glutamate acting, in part, through NMDA receptors in IC in initiation of AGS.

The distribution of fos immunoreactivity in rat brain following freezing and escape responses elicited by electrical stimulation of the inferior colliculus

Several sources of evidence indicate that the inferior colliculus also integrates acoustic information of an aversive nature besides its well-known role as a relay station for auditory pathways. Gradual increases of the electrical stimulation of this structure cause in a hierarchical manner alertness, freezing and escape behaviors. Independent groups of animals implanted with bipolar electrodes into the inferior colliculus received electrical stimulation at one of these aversive thresholds. Control animals were submitted to the same procedure but no current was applied. Next, analysis of Fos protein expression was used to map brain areas activated by the inferior colliculus stimulation at each aversive threshold and in the controls. Whereas alertness elicited by stimulation of the inferior colliculus did not cause any significant labeling in any structure studied in relation to the respective control, electrical stimulation applied at the freezing threshold increased Fos-like immunoreactivity in the central amygdaloid nucleus and entorhinal cortex. In contrast, escape response enhanced Fos-like immunoreactivity in the nucleus cuneiform and the dorsal periaqueductal gray matter of the mesencephalon. This evidence supports the notion that freezing and escape behaviors induced by electrical stimulation of the inferior colliculus activate different neural circuitries in the brain. Both defensive behaviors caused significant expression of c-fos in the frontal cortex, hippocampus and basolateral amygdaloid nucleus. This indistinct pattern of c-fos distribution may indicate a more general role for these structures in the modulation of fear-related behaviors. Therefore, the present data bring support to the notion that amygdala, dorsal hippocampus, entorhinal cortex, frontal cortex, dorsal periaqueductal gray matter and cuneiform nucleus altogether play a role in the integration of aversive states generated at the level of the inferior colliculus.

GABA and its related enzymes in the lower auditory system of the guinea pig

The possible existence of GABA-transmitter neurons in the lower auditory system of the guinea pig has been investigated by means of three different experimental approaches: (1) the regional distribution of GABA and its related enzymes, (2) the subcellular distribution of glutamate decarboxylase, and (3) the effect of selected nerve lesions on glutamate decarboxylase concentrations in the auditory nuclei. Within the regions investigated considerable variations in glutamate decarboxylase activity and GABA concentration were found, with the highest values observed in the inferior colliculus. The dorsal cochlear nucleus also contained significant amounts of both glutamate decarboxylase and GABA, in addition to high concentrations of GABA transaminase. The subcellular distribution of glutamate decarboxylase was bimodal in both the cochlear nucleus and inferior colliculus with most enzyme activity recovered in the soluble and synaptosomal fractions. Neither end organ (cochlea) nor trapezoid body lesions induced a significant loss of glutamate decarboxylase activity in either the cochlear nucleus or inferior colliculus. The results suggest the presence of short axon GABAergic interneurons in the cochlear nucleus, most of which appear to terminate within the dorsal cochlear nucleus.

Evidence for glutamatergic projections from the cochlear nucleus to the superior olive and the ventral nucleus of the lateral lemniscus

This study attempts to determine if projections ascending from the guinea pig cochlear nucleus (CN) could be glutamatergic and/or aspartatergic. Multiple radio frequency lesions were made to ablate the right CN. The ablation was verified histologically. To identify the principal targets of CN efferents, silver impregnation methods were used to localize the preterminal degeneration of fibers in transverse sections of the brainstem 5 and 7 days after CN ablation. CN efferents projected heavily to the lateral superior olive (LSO) ipsilaterally, the medial superior olive (MSO) bilaterally, and contralaterally to the medial (MNTB) and ventral (VNTB) nuclei of the trapezoid body, the ventral (VNLL) and intermediate nuclei of the lateral lemniscus and the central nucleus of the inferior colliculus (ICc). There were smaller projections to the lateral nucleus of the trapezoid body ipsilaterally, the dorsal and dorsomedial periolivary nuclei bilaterally, and the dorsal nucleus of the lateral lemniscus contralaterally. There were sparse projections to the VNLL and ICc ipsilaterally and the CN contralaterally, and a very sparse projection to the contralateral LSO. To determine if CN efferents were glutamatergic and/or aspartatergic, the fresh brainstem was sectioned transversely and samples of the LSO, MSO, MNTB, VNLL, and ICc were taken to measure the electrically evoked release and the uptake of D-[3H]Asp and [14C]Gly or [14C]GABA 3-5 days after the CN ablation. The release studies suggest that only certain of the histologically identified projections ascending from the CN may be glutamatergic and/or aspartatergic. CN ablation depressed D-[3H]Asp release in the MSO bilaterally and in the contralateral MNTB and VNLL, suggesting that the CN efferents to these nuclei may use glutamate or aspartate as a transmitter. It was unclear whether a marginal depression of D-[3H]Asp release in the ipsilateral LSO reflected the presence of glutamatergic CN projections to this nucleus. D-[3H]Asp release in the ICc was unaffected, suggesting that CN efferents to this nucleus may not be glutamatergic. There were no deficits in D-[3H]Asp uptake. [14C]Gly release from the LSO and MSO was unchanged. [14C]Gly uptake was unchanged in the MSO and depressed only in the contralateral LSO, possibly reflecting subnormal uptake activity in endings contributed by contralateral MNTB cells that had lost their CN efferents.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Serotonergic innervation of the auditory brainstem of the Mexican free-tailed bat,Tadarida brasiliensis

Anatomical and electrophysiological evidence suggests that serotonin alters the processing of sound in the auditory brainstem of many mammalian species. The Mexican free-tailed bat is a hearing specialist, like other microchiropteran bats. At the same time, many aspects of its auditory brainstem are similar to those in other mammals. This dichotomy raises an interesting question regarding the serotonergic innervation of the bat auditory brainstem: Is the serotonergic input to the auditory brainstem similar in bats and other mammals, or are there specializations in the serotonergic innervation of bats that may be related to their exceptional hearing capabilities? To address this question, we immunocytochemically labeled serotonergic fibers in the brainstem of the Mexican free-tailed bat, Tadarida brasiliensis. We found many similarities in the pattern of serotonergic innervation of the auditory brainstem in Tadarida compared with other mammals, but we also found two striking differences. Similarities to staining patterns in other mammals included a higher density of serotonergic fibers in the dorsal cochlear nucleus and in granule cell regions than in the ventral cochlear nucleus, a high density of fibers in some periolivary nuclei of the superior olive, and a higher density of fibers in peripheral regions of the inferior colliculus compared with its core. The two novel features of serotonergic innervation in Tadarida were a high density of fibers in the fusiform layer of the dorsal cochlear nucleus relative to surrounding layers and a relatively high density of serotonergic fibers in the low-frequency regions of the lateral and medial superior olive.

Development of calretinin immunoreactivity in the brainstem auditory nuclei of the barn owl (Tyto alba)

The early development of calretinin immunoreactivity (CR-IR) was described in the auditory nuclei of the brainstem of the barn owl. CR-IR was first observed in the auditory hindbrain at embryonic day (E17) and a day later (E18) in the inferior colliculus. In each of the auditory nuclei studied, CR-IR did not develop homogeneously, but began in the regions that map high best frequencies in the adult barn owl. In the hindbrain, CR-IR was first observed in the rostromedial regions of the cochlear nucleus magnocellularis and the nucleus laminaris, and in the dorsal regions of the nucleus angularis and in the nucleus of the lateral lemniscus. In the inferior colliculus, CR-IR began in the ventral region of the central core. The edge of these gradients moved along the future tonotopic axes during the development of all nuclei studied, until adult patterns of CR-IR were achieved about a week after hatching.

Effects of GABA-mediated inhibition on direction-dependent frequency tuning in the frog inferior colliculus

Earlier studies from our laboratory have shown that the frequency selectivity of neurons in the frog inferior colliculus is direction dependent. The goal of this study was to test the hypotheses that gamma-aminobutyric acid or GABA (but not glycine)-mediated synaptic inhibition was responsible for the direction-dependence in frequency tuning, and that GABA acted through creation of binaural inhibition. We performed single unit recordings and investigated the unit's free-field frequency tuning, and/or the unit's response to the interaural level differences (under dichotic stimulation), before and during local applications of antagonists specific to gamma-aminobutyric acid A and glycine receptors. Our results showed that application of bicuculline produced a broadening of free-field frequency tuning depending on sound direction, i.e., more pronounced at azimuths at which the unit exhibited narrower frequency tuning under the pre-drug condition, thereby typically abolishing direction dependence in tuning. Application of strychnine produced no change in frequency tuning. The results from dichotic stimulation further revealed that bicuculline typically elevated and/or flattened the unit's interaural-level-difference response function, indicating a reduction in the strength of binaural inhibition. Our study provides evidence that gamma-aminobutyric acid-mediated binaural inhibition is important for direction dependence in frequency tuning.

Extracortical descending projections to the rat inferior colliculus

Feedback controlling is an important element in the sensory processing in the auditory system. It has been long recognized that the inferior colliculus (IC) sends direct ascending projections to the medial geniculate body (MGB), but receives feedback regulation from the auditory cortex. In the present study we probed the shorter extracortical projections to the IC, including the direct descending pathway from the MGB. In the rat, the fluorescence retrograde tracers Fluorogold, True Blue or Rhodamine latex microspheres were injected into the IC, and the auditory thalamus and surrounding regions were examined for fluorescent neurones. We did not find any retrograde labelling in the ventral division of the MGB. However, retrogradely labelled neurones were found in the medial and suprageniculate nuclei of the MGB. We also observed densely packed groups of fluorescent neurones in the peripeduncular nucleus and numerous labelled neurones in the nucleus of the brachium of the IC. The existence of a direct descending pathway to the IC from at least some auditory thalamic nuclei challenges the perception of the colliculo-thalamic relationship as one-way traffic and suggests more direct involvement of the auditory thalamus in the feedback regulation of the incoming acoustic signals.

Characterization of neuronal subsets surrounded by perineuronal nets in the rhesus auditory brainstem

The distribution of perineuronal nets and the potassium channel subunit Kv3.1b was studied in the subdivisions of the cochlear nucleus, the medial nucleus of the trapezoid body, the medial and lateral superior olivary nuclei, the lateral lemniscal nucleus and the inferior colliculus of the rhesus monkey. Additional sections were used for receptor autoradiography to visualize the patterns of GABAA and GABAB receptor distribution. The Kv3.1b protein and perineuronal nets [visualized as Wisteria floribunda agglutinin (WFA) binding] were revealed, showing corresponding region-specific patterns of distribution. There was a gradient of labelled perineuronal nets which corresponded to that seen for the intensity of Kv3.1b expression. In the cochlear nucleus intensely and faintly stained perineuronal nets were intermingled, whereas in the medial nucleus of the trapezoid body the pattern changed to intensely stained perineuronal nets in the medial part and weakly labelled nets in its lateral part. In the inferior colliculus, intensely labelled perineuronal nets were arranged in clusters and faintly labelled nets were arranged in sheets. Using receptor autoradiography, GABAB receptor expression in the anterior ventral cochlear nucleus was revealed. The medial part of the medial nucleus of the trapezoid body showed a high number of GABAA binding sites whereas the lateral part exhibited more binding sites for GABAB. In the inferior colliculus, we found moderate GABAB receptor expression. In conclusion, intensely WFA-labelled structures are those known to be functionally involved in high-frequency processing.

Patterns of calcium-binding proteins in human inferior colliculus: identification of subdivisions and evidence for putative parallel systems

The subdivisions of human inferior colliculus are currently based on Golgi and Nissl-stained preparations. We have investigated the distribution of calcium-binding protein immunoreactivity in the human inferior colliculus and found complementary or mutually exclusive localisations of parvalbumin versus calbindin D-28k and calretinin staining. The central nucleus of the inferior colliculus but not the surrounding regions contained parvalbumin-positive neuronal somata and fibres. Calbindin-positive neurons and fibres were concentrated in the dorsal aspect of the central nucleus and in structures surrounding it: the dorsal cortex, the lateral lemniscus, the ventrolateral nucleus, and the intercollicular region. In the dorsal cortex, labelling of calbindin and calretinin revealed four distinct layers.Thus, calcium-binding protein reactivity reveals in the human inferior colliculus distinct neuronal populations that are anatomically segregated. The different calcium-binding protein-defined subdivisions may belong to parallel auditory pathways that were previously demonstrated in non-human primates, and they may constitute a first indication of parallel processing in human subcortical auditory structures.