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.

Molecular, topographic, and functional organization of the cerebellar nuclei: analysis by three-dimensional mapping of the olivonuclear projection and aldolase C labeling.

The olivocerebellar climbing fiber projection pattern is closely correlated with the pattern of aldolase C expression in cerebellar Purkinje cells. Based on this expression pattern, the olivocerebellar projection can be classified into five "groups" of functional compartments. Each group originates from a subarea within the inferior olive that projects to multiple cortical stripes of Purkinje cells, all of which are either aldolase C positive or aldolase C negative. However, no equivalent compartmental organization has been demonstrated in the cerebellar nuclei (CN). Thus, in the CN of the rat, we systematically mapped the location of olivonuclear projections belonging to the five groups and determined their relationship to the expression of aldolase C in Purkinje cell axonal terminals. The CN were divided into caudoventral aldolase C-positive and rostrodorsal aldolase C-negative parts. The olivonuclear terminations from the five groups projected topographically to five separate compartments within the CN that partly crossed the traditional boundaries that define the fastigial, interposed, and dentate nuclei. Each compartment had mostly uniform cytoarchitecture and the same aldolase C expression (either positive or negative) that was found in the corresponding olivocortical projection. These results suggest a new view of the organization of the CN whereby the pattern of olivonuclear terminations links portions of different CN together. We propose that each compartment in the CN, along with its corresponding olivary subarea and cortical stripes, may be related to a different aspect of motor control.

Identification of aldolase C compartments in the mouse cerebellar cortex by olivocerebellar labeling.

Aldolase C (zebrin II) is expressed in Purkinje cells aligned in complicated longitudinal stripe-shaped compartments. The tight link between these aldolase C compartments and the topographic olivocerebellar projection to them has made it possible to identify each compartment as a target of a specific subarea of the inferior olive and thus as a functionally distinct entity in the rat. However, it is unknown whether the overall organization of aldolase C compartments is preserved in other mammals. In this study, we tried to clarify this organization in the mouse, which is more useful in genetic studies than the rat, by identifying each aldolase C compartment in terms of the olivocerebellar projection pattern. First, aldolase C compartments were reconstructed from serial sections throughout the cerebellar cortex. Aldolase C and olivocerebellar climbing fibers were then doubly labeled by small injections of biotinylated dextran amine into various areas of the inferior olive. Climbing fibers were topographically distributed on a specific linked pair of aldolase C compartments in the rostral and caudal cerebellum. The overall relationship between aldolase C compartments and the topographic olivocerebellar projection to them in the mouse was similar to that in the rat, except for some minor differences, suggesting that the aldolase C compartments and olivocerebellar projection are organized according to a common fundamental organization in the mouse and rat. This allowed the unequivocal identification of all aldolase C compartments in the mouse by referring to the definition and nomenclature in the rat.

Organization of the superior olivary complex in the guinea pig. I. Cytoarchitecture, cytochrome oxidase histochemistry, and dendritic morphology.

The superior olivary complex is a prominent component of the auditory system. It consists of the lateral and medial superior olivary nuclei and a large number of smaller cell groups known as the periolivary nuclei, which are sources of both ascending and descending projections. The goal of this study was to establish criteria for identifying the periolivary nuclei in the guinea pig. Use of Nissl stains, the Golgi impregnation technique, and cytochrome oxidase histochemistry allowed us to distinguish eleven periolivary nuclei on the basis of differences in the types of cells they contain, in the distribution of cell types, and in the cytochrome oxidase staining characteristics of both the cells and the neuropil. The nuclei, named according to their position with respect to the lateral and medial superior olivary nuclei, can be divided into four groups: (1) a lateral group comprising the lateral nucleus of the trapezoid body and the anterolateral and posteroventral periolivary nuclei, (2) a dorsal group comprising the dorsal and dorsolateral periolivary nuclei, (3) a ventral group comprising the ventral nucleus of the trapezoid body and the anteroventral, ventromedial and rostral periolivary nuclei, and (4) a medial group comprising the medial nucleus of the trapezoid body and the superior paraolivary nucleus. Cytological distinctions among the periolivary nuclei are consistent with other evidence that they serve different functions and highlight the need for detailed study of their connections, immunocytochemistry and physiological response properties.

Localization of glutamic-acid-decarboxylase-immunoreactive axon terminals in the inferior olive of the rat, with special emphasis on anatomical relations between GABAergic synapses and dendrodendritic gap junctions.

Immunocytochemical and electron microscopic methods were used to examine the GABAergic innervation of the inferior olivary nucleus in adult rats. This neuronal system was visualized with an antibody against glutamic acid decarboxylase (GAD, EC 4.1.1.15), the GABA-synthesizing enzyme. A GAD-positive reaction product was encountered only in short segments of preterminal axons and in axon terminals. Their relative number per unit area of neuropil was very similar in all olivary subnuclei. Despite this homogeneity in density, obvious intraregional differences existed. Some regions were strongly immunoreactive (the "c" subgroup, the beta nucleus, and the mediolateral outgrowth of the medial accessory olive), whereas others were weakly labeled (the dorsomedial cell column and the central zones of the medial accessory and principal olives). The strongly immunoreactive areas contained the largest and most intensively labeled axon terminals. Areas of weak labeling were filled with small, weakly immunoreactive nerve terminals. Thus, variations in size and in intensity of labeling create a specific pattern of GABA innervation, revealed by an almost continuous gradient between the above-mentioned extremes. The GAD-positive axon terminals established conventional synapses with dendrites (94% of the samples) or with cell bodies (6%). The vast majority of these synapses were type II (84%) and only a small proportion formed type I synaptic contacts (16%), regardless of the nature of the postsynaptic element. Immunoreactive terminals were also involved in the complex synaptic arrangements--the glomeruli, which characterize the olivary neuropil. Within these formations, olivary neurons were electrotonically coupled through dendrodendritic gap junctions. There was a constant association between GAD-positive axon terminals and small dendritic appendages linked by gap junctions. This association was revealed not only by the systematic presence of immunolabeled terminals directly apposed to the dendritic appendages but, more importantly, by the frequent presence of type II synapses straddling both elements. These synapses were in close proximity to the low-resistance pathways represented by the gap junctions. The strategic location of these GABA synapses is discussed in relation to recent findings indicating the possibility of a synaptic modulation of the electrical coupling: the release of GABA, by increasing nonjunctional membrane conductance, could shunt the coupling between olivary neurons. The functional decoupling of selected gap junctions would be responsible for the spatial organization of the olivary electrotonic coupling.

Distribution of GABA-T-intensive neurons in the rat hindbrain.

The pharmacohistochemical method previously used to identify the distribution in rat brain of gamma-aminobutyric acid transaminase (GABA-T)-intensive neurons has been applied to the rat pons and medulla. The method involves systemic administration of the irreversible GABA-T inhibitor Gabaculine and the detection, 12 to 15 hours after the injection of the newly synthesized GABA-T by histochemical means. GABA-T-intensive neurons were found to be rich in the following hindbrain structures: inferior colliculus, nuclei of the raphe system, nuclei parabrachialis dorsalis and ventralis, nucleus cuneiformis, nucleus vestibularis medialis, nucleus tractus spinalis nervi trigemini, nucleus vagus, nucleus cochlearis, nucleus reticularis lateralis, nucleus ambiguus, nucleus cuneatus lateralis, inferior olive, and reticular formation of the pons and medulla. Neurons of the deep cerebellar nuclei and the rostral portion of the lateral vestibular nucleus were negative for GABA-T but were surrounded by granular staining indicative of impinging GABA-T-rich nerve endings. These results provide further support for the hypothesis that GABA neurons are far more GABA-T-intensive than other neurons in the central nervous system.

Comparative study of glutamate decarboxylase immunoreactive boutons in the mammalian inferior olive.

An antiserum raised against rat glutamate decarboxylase was used to map GABAergic boutons in the inferior olive of rabbit, cat, rhesus monkey, and human. A description of the human periolivary region is also included. The inferior olive of each species contained a dense GABAergic innervation, but immunostaining intensities varied among regions. These intensities were evaluated visually and photometrically, and the sizes and frequencies of occurrence of boutons in various olivary subnuclei were measured. The beta nucleus in all species was intensely immunostained and contained the largest boutons. The caudal subdivision of the dorsal accessory olive stained with a lower intensity than the beta nucleus, but contained similarly large GABAergic boutons. By visual analysis, the rostral subdivision and the subnucleus a of the medial accessory olive and the principal olive were stained with an intermediate intensity, and these regions contained small GABAergic boutons. Photometric analysis of focal regions of the neuropil, however, revealed species differences in teh staining intensity of the principal olive, which was lowest in rabbits and highest in primates. In all species, the lowest immunostaining intensity was observed in the subnucleus b of the medial accessory olive. Species variations in bouton sizes and regional staining intensities were observed in the dorsal cap and the dorsomedial cell column. The heterogeneous staining pattern and regional variation of bouton size argue for the existence of separate GABAergic projections to discrete regions of the inferior olive. Since glutamate decarboxylase immunostaining patterns in the olive are largely similar across species, the afferent projections producing these patterns may also be similar.

Olivocochlear neurons in the squirrel monkey brainstem.

Centrifugal projections from the brain to the cochlea have been well described in rodents and cats. In order to gain a better understanding of the general mammalian features of this efferent projection system--the olivocochlear (OC) system--we have begun to extend its description to other mammalian orders, particularly primates. This report describes the origin, cellular morphology, and cholinergic nature of OC neurons in squirrel monkey. Olivocochlear neurons were identified after cochlear injection and subsequent retrograde transport of one of the tracers, horseradish peroxidase, True Blue, or Diamidino Yellow. One series of sections was processed to demonstrate the tracer and an adjacent series was processed to demonstrate acetylcholinesterase (AChE). In some cases, a series of sections was immunohistochemically processed to identify the presence of choline acetyltransferase (CAT), the synthesizing enzyme for acetylcholine. Approximately 1,700-1,800 OC neurons were contained in five distinct regions surrounding the major nuclei of the superior olivary complex (SOC), namely: dorsal to medial superior olive (MSO); between MSO and lateral superior olive (LSO); lateral to LSO; medial to SOC; and in the ventral nucleus of the trapezoid body (VTB). These neurons were larger in the regions dorsal to MSO, lateral to LSO, and within VTB; they tended to be smaller in the regions between MSO and LSO and medial to SOC. Neuronal shapes varied among regions and included oval, elongate, round, and multipolar cells. In further support of their cholinergic nature as implied by AChE reactivity, OC neurons also stained positively for the cholinergic marker, CAT.

Acetylcholinesterase staining in subdivisions of the cat's inferior olive.

The present paper describes a unique distribution of true AChE activity in the IO. In the dorsal accessory olive three areas with high AChE activity can be distinguished. The medial accessory olive can be subdivided into a caudal part which shows rostro-caudally directed bands with different enzymatic activity, and a rostral part which shows a more uniform, medium activity. In the nucleus beta and the dorso-medial cell column, AChE activity is low. The ventral and dorsal lamellae of the principal olive contain areas with high, medium, and low activity. The dorsal cap is strongly positive, while the ventrolateral outgrowth is negative for AChE. Enzyme distribution cannot be fully explained on the base of the known afferent and efferent connections with the IO. However, histochemical results provide evidence that generally supports a subdivision of the IO that mirrors these connections (Brodal, '40; Armstrong et al., '74' Boesten and Voogd, '75; Groenewegen et al., '75).

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

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

Nitric oxide/cyclic guanosine monophosphate pathway in the peripheral and central auditory system of the rat.

The neuronal isoform of nitric oxide synthase (nNOS) and soluble guanylate cyclase (sGC) were localized in the cochlea, the cochlear nucleus (CN), and the superior olivary complex (SOC) of Fisher 344 rats. In the cochlea, nNOS was identified in spiral ganglion cells by using nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry and in situ hybridization. NADPH-diaphorase staining also was detected in blood vessels of the modiolus. By using immunohistochemistry against cyclic guanosine monophosphate, cochlear sGC activity was localized to pericytes in the spiral ligament as well as nerve fibers innervating outer hair cells. In the lower auditory brainstem, nNOS was localized to principal cells of the medial nucleus of the trapezoid body (MNTB) with NADPH-diaphorase histochemistry and in situ hybridization. NADPH-diaphorase activity also was observed in the lateral and medial superior olive (LSO and MSO, respectively), the superior periolivary nucleus (SPN), the ventral and lateral nuclei of the trapezoid body (VNTB and LNTB, respectively), and the ventral cochlear nucleus (VCN). Transcripts of the beta-subunit of sGC were localized in rat brainstem by using in situ hybridization. mRNA for sGC was expressed in neurons within the SPN, LSO, MSO, LNTB, MNTB, VNTB, and VCN. Highest levels of sGC expression were seen in the SPN. These results suggest that the NO/cGMP pathway is involved in both the ascending and descending pathways of the auditory brainstem.

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.

Regional study of acid hydrolases and lysosomal membrane properties in the normal human brain at various ages.

Acid hydrolases and lysosomal membrane properties were studied at various ages in the normal human brain. In CSF and four brain regions, the inferior olive, the cerebellar cortex, the caudate nucleus and the frontal cortex were thus beta-galactosidase, beta-glucosidase, alpha-mannosidase, hexosaminidase and acid phosphatase biochemically quantitated at ages varying between 2 and 89 years of age. Also the membrane latency for acid phosphatase was studied in these regions. No major regional quantitative differences were found with regard to the enzymes studied. Their kinetic properties were also defined. There appeared to exist a regional and intra-areal variation in lysosomal membrane permeability. There was, however, no age related increase in total enzyme contents. The possibility significance of these findings are discussed with reference to the aging process.

Glutamate dehydrogenase in olivopontocerebellar atrophies: leukocytes, fibroblasts, and muscle mitochondria.

Glutamate dehydrogenase (GDH) activity was 68% of control values in leukocyte homogenates of 11 patients with dominant olivopontocerebellar atrophies (OPCA) and 46% in muscle mitochondria of 4 patients with dominant OPCA. In three patients with recessive OPCA and in one sporadic patient, muscle GDH was lower than in controls. However, muscle GDH activity was normal in one of two dominant patients in the same family and decreased in the other, and patients' activities overlapped with lower control values. Plasma glutamate levels were significantly higher in dominant patients than in controls after glutamate challenge. GDH activity may be partially altered at the mitochondrial level in a subgroup of OPCA patients.

Olivary hypertrophy: histochemical demonstration of hydrolytic enzymes.

Of four patients with palatal myoclonus, three had infarcts resulting from atherosclerosis, and one had cerebral emboli from a left atrial myxoma. Three specimens showed lesions in the brainstem and bilateral hypertrophy of the inferior olivary nuclei; the fourth revealed unilateral olivary changes caused by an infarct in the contralateral dentate nucleus. After incubation for acetylcholinesterase, neuropilar and capillary wall staining were absent or much reduced, but there was increased denisty of reaction product in the neuronal cell bodies and in numerous tortuous dendrites. Methods for acid phosphatase showed strong activity in the dendrites and glomeruloid structures of the diseased olives. Reactions for nonspecific esterase indicated dendritic expansion and reduced staining density in nerve cell bodies, but augmented glial reactivity.

Localization of enkephalin-like immunoreactivity in acetylcholinesterase-positive cells in the guinea-pig lateral superior olivary complex that project to the cochlea.

Olivocochlear fibers have been demonstrated to have acetylcholinesterase-positive staining both in brainstem and cochlea. Olivocochlear fibres in the cochlea have also been determined to contain enkephalin-like immunoreactivity. In this study, we first determined the source of olivocochlear fibers in the guinea-pig using horseradish peroxidase and wheat germ agglutinin in retrograde transport studies. These cells were then examined for enkephalin-like immunoreactivity followed by acetylcholinesterase staining on the same sections to determine which cells and fibers showed staining for both. It was found that cells in the guinea-pig lateral superior olive that project to the cochlea have both enkephalin-like immunoreactivity staining and acetylcholinesterase-positive staining. Cells in other areas giving rise to olivocochlear fibers showed only acetylcholinesterase staining. These results suggest that there is co-localization of enkephalin and acetylcholine in a population of olivocochlear cells and fibers.

Cytochrome oxidase activity in the auditory system of the mouse: a qualitative and quantitative histochemical study.

Detailed qualitative and quantitative determinations of cytochrome oxidase activity in the central auditory system of BALB/cJ mice were obtained at the light microscopic level. Cytochrome oxidase activity was determined using quantitative densitometry calibrated with standards of spectrophotometrically assayed enzymatic activity. This was done together with a cobalt-intensified histochemical procedure using fresh-frozen brains without perfusion-fixation. The resulting method showed improved sensitivity and allowed quantification of histochemical labeling as actual enzyme activity units. Adjacent sections were processed for either Nissl, fiber or Golgi stains to correlate the histochemical labeling with tissue morphology. The more peripheral auditory nuclei showed primarily somatic labeling with specific cell types showing predominant reactivity. However, higher auditory structures, including the inferior colliculus, medial geniculate and auditory cortex, showed predominantly neuropil reactivity. Comparison of mean cytochrome oxidase activities for the 27 auditory regions quantified revealed a trend for decreasing activity from the brainstem to the forebrain in central lemniscal structures. The extra-lemniscal auditory regions at each level showed lower activity than the corresponding lemniscal regions. The regions with the higher activity values showed around 10 times the labeling density of the white matter, indicating the high sensitivity of the method. The darkly labeling auditory structures were clearly delineated from surrounding neural regions, supporting the concept that basal levels of oxidative metabolic capacity are larger for the auditory system. It was concluded that the quantitative approach to cytochrome oxidase histochemistry may be applied successfully to the mouse brain. The normative data presented may be used as a starting point for other investigations of the effects of experimental manipulations on the metabolic activity of the auditory system.

The distribution and the modulation of tyrosine hydroxylase immunoreactivity in the lateral olivocochlear system of the guinea-pig.

It was previously shown that tyrosine hydroxylase (TH) immunoreactivity in the terminals of the lateral efferents of the cochlea is decreased by acoustic trauma and that sound preconditioning counteracted this decrease [Hear Res 174 (2002) 124]. Here we identify those neurons in the lateral olivocochlear system (LOC) in the brainstem that regulates the peripheral expression of TH in the cochlea. By employing retrograde tracing techniques, dextran-labeled neurons were found predominantly in the ipsilateral LOC system including lateral superior olive (LSO), and the surrounding periolivary regions (dorsal periolivary nucleus [DPO], dorsolateral periolivary nucleus [DLPO], lateral nucleus of trapezoid body [LNTB]). Employing immunocytochemistry, it was found that a control group had 35% of the ipsilateral LOC neurons positively stained with TH. Of the total population of TH neurons, 77% were double-stained (TH and dextran) in the LOC system. Acoustic trauma decreased the number of TH positive neurons in the LSO and the surrounding DLPO, and caused a reduction of TH fiber immunolabeling in these regions. Changes were not found in the DPO or the LNTB after acoustic trauma. Sound conditioning protected against the decrease of TH immunolabeling by acoustic trauma and increased the fiber staining for TH in the LSO and DLPO, but not in the DPO or the LNTB. These results provide evidence that TH positive neurons are present in the LOC system in the guinea-pig. It is now demonstrated that protection against acoustic trauma by sound conditioning has a central component that is governed by TH in the LSO and the surrounding periolivary DLPO region.

Conductive hearing loss results in changes in cytochrome oxidase activity in gerbil central auditory system.

Conductive hearing loss (CHL) restricts auditory input to an intact peripheral auditory system. Effects of deprivation on the central auditory system (CAS) have been debated, although a number of studies support the hypothesis that CHL can cause modification of CAS structure and function. The present study was designed to test the hypothesis that unilateral CHL results in a decrease in cytochrome oxidase (CO) activity in CAS nuclei that receive major afferent input from the affected ear. Gerbils at postnatal day 12 (P21) or 6-8 weeks underwent left unilateral CHL (malleus removal), cochlear ablation, or a sham surgical procedure. After a survival time of 48 hours or 3 weeks, animals were sacrificed and tissue was processed for cytochrome oxidase histochemistry. Optical density (OD) measurements were made from individual neurons in the anteroventral cochlear nucleus (AVCN) and from medial and lateral dendritic fields in the medial superior olivary nucleus (MSO), the lateral superior olivary nucleus, and the inferior colliculus. The width of the CO-stained neuropil in MSO was also measured as an estimate of dendritic length. OD measures were corrected to neutral areas of the brain. Cochlear ablation caused significant decreases in CO activity in left lower brainstem nuclei, particularly in adult animals. Following CHL, a significant decrease in CO activity was observed in the ipsilateral AVCN and a significant increase was observed in the contralateral AVCN. Cochlear ablation resulted in decreased width of MSO neuropil containing dendrites that receive primary input from the ablated ear. CHL resulted in a significant increase in the width of MSO neuropil on both sides of the brain in the P21 animals that survived 3 weeks but not in P21 animals that survived only 48 hours or in the adult animals. Unilateral CHL is associated with changes in CO activity in the AVCN and may affect MSO dendritic length in younger animals.

Immunohistochemical evaluation of cholinergic neurons in the rat superior olivary complex.

The cholinergic system in the rat superior olivary complex (SOC) was evaluated by immunohistochemistry for choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) and histochemistry for acetylcholinesterase (AChE). ChAT-positive somata were found mostly in the lateral superior olive (LSO) and ventral nucleus of the trapezoid body (VNTB). In the LSO, there were both rostral-caudal and medial-lateral gradients in concentration of ChAT-positive somata; the highest concentration was in the middle of the rostral-caudal extent and the most medial part. The estimated total number of ChAT-positive neurons in the LSO was similar to previous estimates of the total number of lateral olivocochlear neurons. Two groups of ChAT-positive somata were found in the VNTB: a dorsolateral group of larger, multipolar, and more darkly labeled neurons and a ventromedial group of smaller, oval, and more lightly labeled neurons, which was about 5 times as numerous. There was a caudal-to-rostral increase in number of neurons in each group. VAChT immunoreactivity, predominantly localized in puncta, was seen in LSO, VNTB, and LNTB, and, to a lesser extent, in other parts of the SOC. VAChT-positive somata were also found in the VNTB and medial LSO. This distribution pattern of VAChT was generally similar to that of ChAT. AChE labeling had a similar appearance to ChAT labeling in the VNTB but differed in the LSO, where AChE labeling was lighter and associated more with neuropil than with somata.