Nuclei Of Superior Olivary Complex Research Articles

Ototoxicity-related changes in GABA immunolabeling within the rat inferior colliculus

Several studies suggest that hearing loss results in changes in the balance between inhibition and excitation in the inferior colliculus (IC). The IC is an integral nucleus within the auditory brainstem. The majority of ascending pathways from the lateral lemniscus (LL), superior olivary complex (SOC), and cochlear nucleus (CN) synapse in the IC before projecting to the thalamus and cortex. Many of these ascending projections provide inhibitory innervation to neurons within the IC. However, the nature and the distribution of this inhibitory input have only been partially elucidated in the rat. The inhibitory neurotransmitter, gamma aminobutyric acid (GABA), from the ventral nucleus of the lateral lemniscus (VNLL), provides the primary inhibitory input to the IC of the rat with GABA from other lemniscal and SOC nuclei providing lesser, but prominent innervation.There is evidence that hearing related conditions can result in dysfunction of IC neurons. These changes may be mediated in part by changes in GABA inputs to IC neurons. We have previously used gene micro-arrays in a study of deafness-related changes in gene expression in the IC and found significant changes in GAD as well as the GABA transporters and GABA receptors (Holt 2005). This is consistent with reports of age and trauma related changes in GABA (Bledsoe et al., 1995; Mossop et al., 2000; Salvi et al., 2000). Ototoxic lesions of the cochlea produced a permanent threshold shift. The number, intensity, and density of GABA positive axon terminals in the IC were compared in normal hearing and deafened rats. While the number of GABA immunolabeled puncta was only minimally different between groups, the intensity of labeling was significantly reduced. The ultrastructural localization and distribution of labeling was also examined. In deafened animals, the number of immuno gold particles was reduced by 78 % in axodendritic and 82 % in axosomatic GABAergic puncta. The affected puncta were primarily associated with small IC neurons. These results suggest that reduced inhibition to IC neurons contribute to the increased neuronal excitability observed in the IC following noise or drug induced hearing loss. Whether these deafness diminished inhibitory inputs originate from intrinsic or extrinsic CNIC sources awaits further study.

Developmental profile of microglia distribution in nuclei of the superior olivary complex.

In the brain, microglia are involved in immune responses and synaptic maturation. During early development, these cells invade the brain, proliferate, and morphologically mature to achieve coverage of the surrounding tissue with their fine processes. Their developmental proliferation overlaps with the postnatal development of neuronal circuits. Within the superior olivary complex (SOC), an auditory brainstem structure, microglia, and their early postnatal development have been documented. A quantification over the full developmental profile of the arrangement and morphological changes in single microglia cells is missing. Here, we used immunofluorescence labeling to quantify their distribution, morphological changes, and coverage during early and late postnatal development in the SOC of Mongolian gerbils. Microglia distributed rather homogenously within each nucleus with a bias to the nucleus borders at postnatal day (P) 5 and more centrally in the nucleus in mature stages. We found a nucleus-specific transient increase in microglia cell number and density reaching its peak at P17 with a subsequent decline to P55 values. Length and branching of microglia protrusions increased especially after P12. The stronger ramification together with the increase in cell density allows coverage of the surrounding tissue from P5 to mature stages, despite the large developmental increase in nucleus size. The transient increase in density during synaptic refinement in SOC nuclei suggests that microglia are important during the pruning period, compensating for developmental increase in tissue volume, and that in mature stages their main function appears tissue surveillance.

The nuclei of the lateral lemniscus: unexpected players in the descending auditory pathway.

In the mammalian auditory pathway, the nuclei of the lateral lemniscus (NLL) are thought to be exclusively involved in the bottom-up transmission of auditory information. However, our repeated observation of numerous NLL neurons labeled after injection of retrograde tracers into the superior olivary complex (SOC) led us to systematically investigate with retrograde tracers the descending projections from the NLL to the SOC of the rat. We performed large injections of FluoroGold into the SOC to determine NLL contributions to descending projections, and focal injections of biotinylated dextran amine (BDA) to pinpoint the specific nuclei of the SOC innervated by each NLL. The SOC is innervated by thousands of neurons distributed across four nuclei or regions associated with the lateral lemniscus: the ipsilateral ventral and intermediate nuclei of the lateral lemniscus (VNLL and INLL); the medial paralemniscal region (PL) of both sides; and the ipsilateral semilunar nucleus (SLN), a previously unrecognized nucleus that wraps around the INLL dorsally, medially, and caudally and consists of small, flat neurons. In some experiments, at least 30% of neurons in the VNLL and INLL were retrogradely labeled. All nuclei of the SOC, except the medial and lateral superior olives, are innervated by abundant lemniscal neurons, and each SOC nucleus receives a unique combination of lemniscal inputs. The primary target of the projections from the VNLL is the ventral nucleus of the trapezoid body (VNTB), followed by the superior paraolivary nucleus (SPON), and the medial nucleus of the trapezoid body (MNTB). The INLL selectively innervates the VNTB. The PL innervates dorsal periolivary regions bilaterally. The SLN preferentially innervates the MNTB and may provide the first identified non-calyceal excitatory input to MNTB neurons. Our novel findings have strong implications for understanding acoustic information processing in the initial stages of the auditory pathway. Based on the proportion of lemniscal neurons involved in all the projections described, the NLL should be considered major players in the descending auditory pathway.

Characterization of the superior olivary complex of chimpanzees (Pan troglodytes) in comparison to humans

The superior olivary complex (SOC) is a collection of nuclei in the hindbrain of mammals with numerous roles in hearing, including localization of sound sources in the environment, encoding temporal and spectral elements of sound, and descending modulation of the cochlea. While there have been several investigations of the SOC in primates, there are discrepancies in the descriptions of nuclear borders and even the presence of certain cell groups among studies and species. Herein, we aimed to clarify some of these issues by characterizing the SOC from chimpanzees using Nissl staining, quantitative morphometry and immunohistochemistry. We found the medial superior olive (MSO) to be the largest of the SOC nuclei and the arrangement of its neurons and peri-MSO to be very similar to humans. Additionally, we found neurons in the medial nucleus of the trapezoid body (MNTB) to be immunopositive for the calcium binding protein calbindin. Further, most neurons in the MNTB, and some neurons in the lateral nucleus of the trapezoid body were associated with large, calretinin-immunoreactive calyx terminals. Together, these findings indicate the organization of the SOC of chimpanzees is organized very similar to the SOC in humans and suggests modifications to this region among species consistent with differences in head/body size, restricted hearing range and sensitivity to low frequency sounds.

Structural arrangement of auditory brainstem nuclei in the bats Phyllostomus discolor and Carollia perspicillata.

The structure of the mammalian auditory brainstem is evolutionarily highly plastic, and distinct nuclei arrange in a species-dependent manner. Such anatomical variability is present in the superior olivary complex (SOC) and the nuclei of the lateral lemniscus (LL). Due to the structure-function relationship in the auditory brainstem, the identification of individual nuclei supports the understanding of sound processing. Here, we comparatively describe the nucleus arrangement and the expression of functional markers in the auditory brainstem of the two bat species Phyllostomus discolor and Carollia perspicillata. Using immunofluorescent labeling, we describe the arrangement and identity of the SOC and LL nuclei based on the expression of synaptic markers (vesicular glutamate transporter 1 and glycine transporter 2), calcium-binding proteins, as well as the voltage-gated ion channel subunits Kv1.1 and HCN1. The distribution of excitatory and inhibitory synaptic labeling appears similar between both species and matches with that of other mammals. The detection of calcium-binding proteins indicates species-dependent differences and deviations from other mammals. Kv1.1 and HCN1 show largely the same expression pattern in both species, which diverges from other mammals, indicating functional adaptations in the cellular physiology of bat neurons.

Multiple Sources of Cholinergic Input to the Superior Olivary Complex.

The superior olivary complex (SOC) is a major computation center in the brainstem auditory system. Despite previous reports of high expression levels of cholinergic receptors in the SOC, few studies have addressed the functional role of acetylcholine in the region. The source of the cholinergic innervation is unknown for all but one of the nuclei of the SOC, limiting our understanding of cholinergic modulation. The medial nucleus of the trapezoid body, a key inhibitory link in monaural and binaural circuits, receives cholinergic input from other SOC nuclei and also from the pontomesencephalic tegmentum. Here, we investigate whether these same regions are sources of cholinergic input to other SOC nuclei. We also investigate whether individual cholinergic cells can send collateral projections bilaterally (i.e., into both SOCs), as has been shown at other levels of the subcortical auditory system. We injected retrograde tract tracers into the SOC in gerbils, then identified retrogradely-labeled cells that were also immunolabeled for choline acetyltransferase, a marker for cholinergic cells. We found that both the SOC and the pontomesencephalic tegmentum (PMT) send cholinergic projections into the SOC, and these projections appear to innervate all major SOC nuclei. We also observed a small cholinergic projection into the SOC from the lateral paragigantocellular nucleus of the reticular formation. These various sources likely serve different functions; e.g., the PMT has been associated with things such as arousal and sensory gating whereas the SOC may provide feedback more closely tuned to specific auditory stimuli. Further, individual cholinergic neurons in each of these regions can send branching projections into both SOCs. Such projections present an opportunity for cholinergic modulation to be coordinated across the auditory brainstem.

Three dimensional reconstructions of the superior olivary complex from children with autism spectrum disorder

Auditory dysfunction is a common symptom of autism spectrum disorder (ASD) and ranges from decreased acuity to hypersensitivity where routine sounds may result in panic or anxiety. Irrespective of altered sensitivity, there is often additional difficulty of listening in background noise. Previous studies of post-mortem brain specimens from subjects with ASD have revealed consistent dysmorphology in the superior olivary complex (SOC). The medial superior olive (MSO) is the largest and most prominent nucleus in the human SOC. Our morphological studies have shown that in subjects with ASD, there are significantly fewer neurons in the MSO and surviving neurons are smaller, more round and have abnormal dendritic orientations. Based on these findings, we hypothesize that the SOC in subjects with ASD not only includes fewer neurons but that these nuclei occupy significantly less brain volume and demonstrate abnormal nuclear contours. We investigated this hypothesis by making 3D volume renderings of the SOC nuclei using Amira software. Subjects in this study include three neurotypical and seven age-matched (ages 2–11 years of age) children with ASD. Our 3D reconstructions and volume measurements of the SOC nuclei emphasize the drastic alterations in the size, volume and organization of the human SOC in ASD.

Quantitative 3D Reconstructions of the Human Superior Olive Complex are Consistent with Auditory Dysfunction in Autism Spectrum Disorder

Beyond the characteristic social, communication and behavioral aberrations associated with autism spectrum disorder (ASD), the vast majority of subjects with this condition have some degree of auditory dysfunction. The auditory deficits range from complete deafness to hypersensitivity but typically include difficulty listening to speech in the presence of background noise. Clinical testing of brainstem evoked potentials and stapedial reflexes in children and adults with ASD reveal lower thresholds but slower conduction times and right‐left ear asymmetries. Previous studies on post‐mortem brain specimens from our lab revealed drastic and consistent hypoplasia and dysmorphology in the auditory brainstem. The most drastic alterations were found in the medial superior olive (MSO), the largest and most prominent component of the superior olivary complex (SOC). Our morphological studies have shown that in subjects with ASD, there are significantly fewer neurons in the MSO and surviving neurons are smaller, more round and have an abnormal orientation. Based on these results, we hypothesized that the SOC in subjects with ASD not only includes fewer neurons but that these nuclei occupy significantly less brain volume and demonstrate abnormal nuclear contours. We investigated this hypothesis by obtaining donated brain tissue from an age‐matched series of subjects with ASD or no known neurological diagnosis (neurotypical; NT) through Autism BrainNet. Free‐floating tissue sections were mounted onto glass slides, stained for Nissl substance with Giemsa and sealed under coverslips. 3D volume renderings of the SOC nuclei were created in Amira. Subjects in this study include two NT subjects (ages 4 and 8, both female) and four age‐matched subjects with ASD (ages 2–11 years of age, all male). At autopsy, there was no difference in total brain weight between NT and ASD subjects (p=.42). Our 3D reconstructions reveal a drastic reduction in the total volume of the SOC in subjects with ASD (control = 6.8 mm3, ASD = 2.23 mm3; p<.005). In fact, all SOC nuclei were significantly smaller in ASD subjects expect for the ventral nucleus of the trapezoid body (p=.922). Together, these results support the brainstem as a core site of pathology in ASD and are consistent with previous clinical evaluations of hearing in subjects with ASD.Support or Funding InformationAutism BrainNet

Characterization of the superior olivary complex of Canis lupus domesticus

The superior olivary complex (SOC) is a collection of brainstem auditory nuclei which play essential roles in the localization of sound sources, temporal coding of vocalizations and descending modulation of the cochlea. Notwithstanding, the SOC nuclei vary considerably between species in accordance with the auditory needs of the animal. The canine SOC was subjected to anatomical and physiological examination nearly 50 years ago and was then virtually forgotten. Herein, we aimed to characterize the nuclei of the canine SOC using quantitative morphometrics, estimation of neuronal number, histochemistry for perineuronal nets and immunofluorescence for the calcium binding proteins calbindin and calretinin. We found the principal nuclei to be extremely well developed: the lateral superior olive (LSO) contained over 20,000 neurons and the medial superior olive (MSO) contained over 15,000 neurons. In nearly all non-chiropterian terrestrial mammals, the MSO exists as a thin, vertical column of neurons. The canine MSO was folded into a U-shaped contour and had associated with the ventromedial tip a small, round collection of neurons we termed the tail nucleus of the MSO. Further, we found evidence within the LSO, MSO and medial nucleus of the trapezoid body (MNTB) for significant morphological variations along the mediolateral or rostrocaudal axes. Finally, the majority of MNTB neurons were calbindin-immunopositive and associated with calretinin-immunopositive calyceal terminals. Together, these observations suggest the canine SOC complies with the basic plan of the mammalian SOC but possesses a number of unique anatomical features.

Differential morphology of the superior olivary complex of Meriones unguiculatus and Monodelphis domestica revealed by calcium-binding proteins.

In mammals, the superior olivary complex (SOC) of the brainstem is composed of nuclei that integrate afferent auditory originating from both ears. Here, the expression of different calcium-binding proteins in subnuclei of the SOC was studied in distantly related mammals, the Mongolian gerbil (Meriones unguiculatus) and the gray short-tailed opossum (Monodelphis domestica) to get a better understanding of the basal nuclear organization of the SOC. Combined immunofluorescence labeling of the calcium-binding proteins (CaBPs) parvalbumin, calbindin-D28k, and calretinin as well as pan-neuronal markers displayed characteristic distribution patterns highlighting details of neuronal architecture of SOC nuclei. Parvalbumin was found in almost all neurons of SOC nuclei in both species, while calbindin and calretinin were restricted to specific cell types and axonal terminal fields. In both species, calbindin displayed a ubiquitous and mostly selective distribution in neurons of the medial nucleus of trapezoid body (MNTB) including their terminal axonal fields in different SOC targets. In Meriones, calretinin and calbindin showed non-overlapping expression patterns in neuron somata and terminal fields throughout the SOC. In Monodelphis, co-expression of calbindin and calretinin was observed in the MNTB, and hence both CaBPs were also co-localized in terminal fields within the adjacent SOC nuclei. The distribution patterns of CaBPs in both species are discussed with respect to the intrinsic neuronal SOC circuits as part of the auditory brainstem system that underlie the binaural integrative processing of acoustic signals as the basis for localization and discrimination of auditory objects.

Changes in the immunohistochemical localization of the glycine receptor in the superior olivary complex of adult circling mice.

Circling mice is a mutant model of spontaneous deafness exhibiting degenerated spiral ganglion cells in the cochlea and loss of organ of Corti. The balance between glycinergic inhibition and glutamatergic excitation in the lateral superior olive (LSO) is essential for the detection of inte-raural level differences. Long term weakening of glycinergic synaptic inhibition in the LSO may lead to the downregulation of synaptic release of glycine in dorsal cochlear nucleus and downregulation of postsynaptic glycine receptor (GlyR) activity in the LSO, which may contribute to hearing loss. The present study utilized an immunohistochemical method to assess changes in GlyR immunoreactivity (IR) and the cell number in the superior olivary complex (SOC) of heterozygote (+/cir) and homozygote (cir/cir) circling mice. A significant decrease in the IR was observed in all nuclei of the SOC of homozygous mice. Loss of GlyR immunoreactive cells and a decrement in cell size was also observed in the homozygotes. A decrease in the GlyR IR in the neurons and neuropils, cell number and size of the cir/cir, may lead to profound changes in inhibitory transmission and the functional properties in the SOC nuclei. Therefore, the functional loss of inhibitory neurotransmitters in the brainstem may result in deafness of adult cir/cir mice.

Immunohistochemical localization of brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor in the superior olivary complex of mice after radiofrequency exposure

Raising health concerns about the biological effects from radiofrequency exposure, even with conflicting results, has prompted calls for formulation of a guideline of the biological safety level. Given the close proximity between a mobile phone and the ear, it has been suggested that the central auditory system may be detrimentally influenced by radiofrequency exposure. In the auditory system, neurotrophins are important in the regulation of neuron survival, especially mammalian cochlear neurons. Neurotrophic factors like brain-derived neurotrophic factor (BDNF) and glial-derived neurotrophic factor (GDNF) present in the auditory system are responsible for the maintenance of auditory neurons. BDNF and GDNF may protect against acoustic trauma and prevent from hearing defect. The present study applied radiofrequency at a specific absorption rate (SAR) of 1.6W/kg (E1.6) or 0W/kg group to determine the distribution of BDNF and GDNF in the nuclei of superior olivary complex (SOC). In the E1.6 group, significant decrements of BDNF immunoreactivity (IR) were noted in the lateral superior olive, medial superior olive, superior paraolivary nucleus and medial nucleus of the trapezoid body. GDNF IR was also significantly decreased (p<0.001) in all SOC nuclei of the E1.6 group. The decrease in the IR of these neurotrophic factors in the SOC of the E1.6 group suggests a detrimental effect of RF exposure in the auditory nuclei.

Metabolic Maturation of Auditory Neurones in the Superior Olivary Complex

Neuronal activity is energetically costly, but despite its importance, energy production and consumption have been studied in only a few neurone types. Neuroenergetics is of special importance in auditory brainstem nuclei, where neurones exhibit various biophysical adaptations for extraordinary temporal precision and show particularly high firing rates. We have studied the development of energy metabolism in three principal nuclei of the superior olivary complex (SOC) involved in precise binaural processing in the Mongolian gerbil (Meriones unguiculatus). We used immunohistochemistry to quantify metabolic markers for energy consumption (Na+/K+-ATPase) and production (mitochondria, cytochrome c oxidase activity and glucose transporter 3 (GLUT3)). In addition, we calculated neuronal ATP consumption for different postnatal ages (P0–90) based upon published electrophysiological and morphological data. Our calculations relate neuronal processes to the regeneration of Na+ gradients perturbed by neuronal firing, and thus to ATP consumption by Na+/K+-ATPase. The developmental changes of calculated energy consumption closely resemble those of metabolic markers. Both increase before and after hearing onset occurring at P12–13 and reach a plateau thereafter. The increase in Na+/K+-ATPase and mitochondria precedes the rise in GLUT3 levels and is already substantial before hearing onset, whilst GLUT3 levels are scarcely detectable at this age. Based on these findings we assume that auditory inputs crucially contribute to metabolic maturation. In one nucleus, the medial nucleus of the trapezoid body (MNTB), the initial rise in marker levels and calculated ATP consumption occurs distinctly earlier than in the other nuclei investigated, and is almost completed by hearing onset. Our study shows that the mathematical model used is applicable to brainstem neurones. Energy consumption varies markedly between SOC nuclei with their different neuronal properties. Especially for the medial superior olive (MSO), we propose that temporally precise input integration is energetically more costly than the high firing frequencies typical for all SOC nuclei.

Embryonic origins of the mouse superior olivary complex

Many areas of the central nervous system are organized into clusters of cell groups, with component cell groups exhibiting diverse but related functions. One such cluster, the superior olivary complex (SOC), is located in the ventral auditory brainstem in mammals. The SOC is an obligatory contact point for most projection neurons of the ventral cochlear nucleus and plays central roles in many aspects of monaural and binaural information processing. Despite their important interrelated functions, little is known about the embryonic origins of SOC nuclei, due in part to a paucity of developmental markers to distinguish individual cell groups. In this report, we present a collection of novel markers for the developing SOC nuclei in mice, including the transcription factors FoxP1, MafB, and Sox2, and the lineage-marking transgenic line En1-Cre. We use these definitive markers to examine the rhombic lip and rhombomeric origins of SOC nuclei and demonstrate that they can serve to uniquely identify SOC nuclei and subnuclei in newborn pups. The markers are also useful in identifying distinct nuclear domains within the presumptive SOC as early as embryonic day (E) 14.5, well before morphological distinction of individual nuclei is evident. These findings indicate that the mediolateral and dorsoventral position of SOC nuclei characteristic of the adult brainstem is established during early neurogenesis.

Malformation of the human superior olive in autistic spectrum disorders

Autistic spectrum disorders (ASD) comprise a continuum of psychosocial disorders clinically characterized by social difficulties, impaired communication skills and repetitive behavioral patterns. Despite the prevalence of ASD, the neurobiology of this disorder is poorly understood. However, abnormalities in neuronal morphology, cell number and connectivity have been described throughout the autistic brain. Further, there is ample evidence that auditory dysfunction is a common feature of autism. Our preliminary investigation of neuronal morphology in the auditory brainstem of individuals with ASD focused on the medial superior olive (MSO) and revealed that neurons in this region were significantly smaller and rounder than in controls. In this report, we expand our investigation to examine all nuclei within the human superior olivary complex (SOC), an important auditory brainstem center. We examine neuronal morphology and neuronal number in four control (average age=15years) and 9 autistic brains (average age=15years). This detailed investigation supports our previous descriptions of the MSO, and also reveals significant dysmorphology in five other SOC nuclei. Moreover, we provide evidence of a consistent and significant decrease in the number of SOC neurons in the autistic brain. Our studies implicate an extensive malformation of the auditory brainstem in the hearing and language difficulties in individuals with ASD. The results from this investigation suggest that neonatal testing of auditory function may aid in the identification of individuals with ASD earlier than presently possible.

Neurochemistry of Olivocochlear Neurons in the Hamster

The present study was conducted to characterize the superior olivary complex (SOC) of the lower brain stem in the pigmented Djungarian hamster Phodopus sungorus. Using Nissl-stained serial cryostat sections from fresh-frozen brains, we determined the borders of the SOC nuclei. We also identified olivocochlear (OC) neurons by retrograde neuronal tracing upon injection of Fluoro-Gold into the scala tympani. To evaluate the SOC as a putative source of neuronal nitric oxide synthase (nNOS), arginine-vasopressin (AVP), oxytocin (OT), vasoactive intestinal polypeptide (VIP), or pituitary adenylate cyclase-activating polypeptide (PACAP) that were all found in the cochlea, we conducted immunohistochemistry on sections exhibiting retrogradely labeled neurons. We did not observe AVP-, OT-, or VIP-immunoreactivity, neither in OC neurons nor in the SOC at all, revealing that cochlear AVP, OT, and VIP are of nonolivary origin. However, we found nNOS, the enzyme responsible for nitric oxide synthesis in neurons, and PACAP in neuronal perikarya of the SOC. Retrogradely labeled neurons of the lateral olivocochlear (LOC) system in the lateral superior olive did not contain PACAP and were only infrequently nNOS-immunoreactive. In contrast, some shell neurons and some of the medial OC (MOC) system exhibited immunofluorescence for either substance. Our data obtained from the dwarf hamster Phodopus sungorus confirm previous observations that a part of the LOC system is nitrergic. They further demonstrate that the medial olivocochlear system is partly nitrergic and use PACAP as neurotransmitter or modulator.

Shift from depolarizing to hyperpolarizing glycine action occurs at different perinatal ages in superior olivary complex nuclei

The inhibitory transmitters glycine and GABA undergo a developmental shift from depolarizing to hyperpolarizing action (D/H-shift). To analyse this shift in functionally related nuclei of the rat superior olivary complex (SOC), we employed voltage-sensitive dye recordings in auditory brainstem slices. Complementarily, we analysed single neurons in gramicidin perforated-patch recordings. Our results show a differential timing of the D/H-shift in the four SOC nuclei analysed. In the medial superior olive (MSO), the shift occurred at postnatal day (P) 5-9. In the superior paraolivary nucleus (SPN), it occurred between embryonic day (E) 18 and P1. No D/H-shift was observed in the medial nucleus of the trapezoid body (MNTB) until P10. This is in line with the finding that most of the patched MNTB neurons displayed glycine-induced depolarizations between P0-9. While no regional differences regarding the D/H-shift were found within the MSO, SPN, and MNTB, we observed such differences in the lateral superior olive (LSO). All LSO regions showed a D/H-shift at P4-5. However, in the high-frequency regions, hyperpolarizations were large already at P6, yet amplitudes of this size were not present until P8 in the low-frequency regions, suggesting a delayed development in the latter regions. Our physiological results demonstrate that D/H-shifts in SOC nuclei are staggered in time and occur over a period of almost two weeks. Membrane-associated immunoreactivity of the Cl- outward transporter KCC2 was found in every SOC nucleus already at times when glycine was still depolarizing. This implies that the mere presence of KCC2 does not correlate with functional Cl- outward transport.

Morphologic and Neurochemical Abnormalities in the Auditory Brainstem of the Genetically Epilepsy‐prone Hamster (GPG/Vall)

This study was performed to evaluate whether audiogenic seizures, in a strain of genetically epilepsy-prone hamsters (GPG/Vall), might be associated with morphologic alterations in the cochlea and auditory brainstem. In addition, we used parvalbumin as a marker of neurons with high levels of activity to examine changes within neurons. Cochlear histology as well as parvalbumin immunohistochemistry were performed to assess possible abnormalities in the GPG/Vall hamster. Densitometry also was used to quantify levels of parvalbumin immunostaining within neurons and fibers in auditory nuclei. In the present study, missing outer hair cells and spiral ganglion cells were observed in the GPG/Vall hamster. In addition, an increase was noted in the size of spiral ganglion cells as well as a decrease in the volume and cell size of the cochlear nucleus (CN), the superior olivary complex nuclei (SOC), and the nuclei of the lateral lemniscus (LL) and the inferior colliculus (IC). These alterations were accompanied by an increase in levels of parvalbumin immunostaining within CN, SOC, and LL neurons, as well as within parvalbumin-immunostained fibers in the CN and IC. These data are consistent with a cascade of atrophic changes starting in the cochlea and extending along the auditory brainstem in an animal model of inherited epilepsy. Our data also show an upregulation in parvalbumin immunostaining in the neuropil of the IC that may reflect a protective mechanism to prevent cell death in the afferent sources to this nucleus.

Developmental pattern of three vesicular glutamate transporters in the rat superior olivary complex

Vesicular glutamate transporters (VGLUTs) mediate the packaging of the excitatory neurotransmitter glutamate into synaptic vesicles. Three VGLUT subtypes have been identified so far, which are differentially expressed in the brain. Here, we have investigated the spatiotemporal distribution of the three VGLUTs in the rat superior olivary complex (SOC), a prominent processing center, which receives strong glutamatergic inputs and which lies within the auditory brainstem. Immunoreactivity (ir) against all three VGLUTs was found in the SOC nuclei throughout development (postnatal days P0-P60). It was predominantly seen in axon terminals, although cytoplasmic labeling also occurred. Each transporter displayed a characteristic expression pattern. In the adult SOC, VGLUT1 labeling varied from strong in the medial nucleus of the trapezoid body, lateral superior olive, and medial superior olive (MSO) to moderate (ventral and lateral nuclei of the trapezoid body) to faint (superior paraolivary nucleus). VGLUT2-ir was moderate to strong throughout the SOC, whereas VGLUT3 was only weakly expressed. These results extend previous reports on co-localization of VGLUTs in the auditory brainstem. As in the adult, specific features were seen during development for all three transporters. Intensity increases and decreases occurred with both VGLUT1 and VGLUT3, whereas VGLUT2-ir remained moderately high throughout development. A striking result was obtained with VGLUT3, which was only transiently expressed in the different SOC nuclei between P0 and P12. A transient occurrence of VGLUT1-immunoreactive terminals on somata of MSO neurons was another striking finding. Our results imply a considerable amount of synaptic reorganization in the glutamatergic inputs to the SOC and suggest differential roles of VGLUTs during maturation and in adulthood.

Projections from the auditory cortex to the superior olivary complex in guinea pigs.

We used anterograde tracing techniques to characterize projections from auditory cortex to the superior olivary complex (SOC) in guinea pigs. Large injections of fluorescent or biotinylated dextrans into the temporal cortex labeled many axons in the SOC. Labeled boutons were most numerous in the ventral nucleus of the trapezoid body, with additional boutons in all other olivary nuclei. The distribution of boutons was similar in the ipsilateral and contralateral SOC; however, the contralateral SOC had markedly fewer axons and boutons. Similar patterns of labeling were also observed following injections confined to primary auditory cortex or the dorsocaudal auditory field. Cortical axons in many of the SOC nuclei share numerous morphological features, suggesting that individual axons may innervate multiple nuclei and have widespread effects. In addition, some nuclei contain axons with branching or termination patterns unique to that nucleus; these axons may represent focused projections with effects limited to individual SOC nuclei. Given the many projections of SOC nuclei, cortico-olivary projections are in a position to modify the activity of many brainstem auditory circuits.