Cochlear ablation alters acoustically induced c-fos mRNA expression in the adult rat auditory brainstem

Expression of c-fos mRNA was studied in the adult rat brain following cochlear ablations by using in situ hybridization. In normal animals, expression was produced by acoustic stimulation and was found to be tonotopically distributed in many auditory nuclei. Following unilateral cochlear ablation, acoustically driven expression was eliminated or decreased in areas normally activated by the ablated ear, e.g., the ipsilateral dorsal and ventral cochlear nuclei, dorsal periolivary nuclei, and lateral nucleus of the trapezoid body and the contralateral medial and ventral nuclei of the trapezoid body, lateral lemniscal nuclei, and inferior colliculus. These deficits did not recover, even after long survivals up to 6 months. Results also indicated that neurons in the dorsal cochlear nucleus could be activated by contralateral stimulation in the absence of ipsilateral cochlear input and that the influence of the contralateral ear was tonotopically organized. Results also indicated that c-fos expression rose rapidly and persisted for up to 6 months in neurons in the rostral part of the contralateral medial nucleus of the trapezoid body following a cochlear ablation, even in the absence of acoustic stimulation. This response may reflect a release of constitutive excitatory inputs normally suppressed by missing afferent input or changes in homeostatic gene expression related to sensory deprivation. Instances of transient, surgery-dependent increases in c-fos mRNA expression in the absence of acoustic stimulation were observed in the superficial dorsal cochlear nucleus and the cochlear nerve root on the ablated side.     

Free radicals from X-irradiated ''dry'' and hydrated lyophilized DNA as studied by electron spin resonance spectroscopy: analysis of spectral components between 77K and room temperature

In order to identify cytochemical traits relevant to understanding excitatory neurotransmission in brainstem auditory nuclei, we have analyzed in the dorsal cochlear nucleus the synaptic distribution of aspartate aminotransferase, glutamate, and vesicular zinc, three molecules probably involved in different steps of excitatory glutamatergic signaling. High levels of glutamate immunolabeling were found in three classes of synaptic endings in the dorsal cochlear nucleus, as determined by quantitation of immunogold labeling. The first type included auditory nerve endings, the second were granule cell endings in the molecular layer, and the third very large endings, better described as "mossy." This finding points to a neurotransmitter role for glutamate in at least three synaptic populations in the dorsal cochlear nucleus. The same three types of endings enriched in glutamate immunoreactivity also contained histochemically detectable levels of aspartate aminotransferase activity, suggesting that this enzyme may be involved in the synaptic handling of glutamate in excitatory endings in the dorsal cochlear nucleus. There was also extrasynaptic localization of the enzyme. Zinc ions were localized exclusively in granule cell endings, as determined by a Danscher-selenite method, suggesting that this ion is involved in the operation of granule cell synapses in the dorsal cochlear nucleus.     

Comparative morphometry of mammalian central auditory systems: variation in nuclei and form of the ascending system

The volumes of the ten largest subcortical auditory nuclei were measured individually in a sample of 53 mammals, including 16 Australian and four American marsupials. The nuclear sizes relative to the total of subcortical auditory tissue were normalized and then analyzed individually for statistically reliable deviations. The overall form of the entire system of ten nuclei and two nuclear sub-systems (cochlear nuclei, superior olives) were also analyzed for similarities and notable deviations among the animals. The results show that the absolute size of the auditory system varies more than 139-fold among the 53 mammals (with moles the smallest and humans the largest). Log auditory system volume and log brain weight are closely correlated (r = 0.903, p < 0.0001). Bats, kangaroo rats, marmosa opossums, and Norway rats have the largest auditory systems relative to their brain size, while humans have the smallest by far. The other primates also have auditory system/brain size ratios smaller than the sample average, suggesting that the condition in humans is one result of an expansion of non-auditory brain parts rather than a reduction of the auditory system over geological time. The relative sizes of the ten nuclei are well ordered, with the inferior colliculus the largest nucleus by far and medical superior olive the smallest. Because the size of the superior olives, collectively, is reliably related to the size of anteroventral cochlear nucleus (r = 0.744, p < 0.001), and not to the size of dorsal cochlear nucleus, the interconnectivity of the subcortical auditory system is probably a factor in the size of the nuclei. In its overall form, the subcortical auditory system is highly similar among mammals, with an average correlation across nuclei of 0.923. This high value means that the overall form of the system has been relatively stable over geological time. The animals with least deviation from the average form are ring-tailed possums, bandicoots, and yellow-bellied gliders, all marsupials. Those with the most unusual forms are mice, bats, and kangaroo rats, all placentals.     

Differential expression of the metabotropic glutamate receptor mGluR1alpha by neurons and axons in the cochlear nucleus: in situ hybridization and immunohistochemistry

mGluR1alpha is a metabotropic glutamate receptor involved in synaptic modifiability. A differential expression in specific neuronal types could reflect their different connections and response properties in central auditory processing. Using in situ hybridization and immunohistochemistry, we studied mGluR1alpha receptor expression throughout the cochlear nucleus. Robust labeling occurred in the dorsal cochlear nucleus and small cell shell, with less in the ventral cochlear nucleus. Among the most intensely labeled were the granule cells of the small cell shell. In the dorsal cochlear nucleus, most cell types expressed message and receptor protein, except granule cells. High levels of receptor were expressed by corn cells and cartwheel cells. The terminal dendrites and synaptic spines of cartwheel and fusiform cells contained receptor protein in the molecular layer, where they could synapse with parallel fibers. Fusiform dendrites also expressed mRNA for mGluR1alpha. The basal dendrites of fusiform cells contained receptor protein in the region where they receive cochlear nerve synapses. Immunostaining of terminal axons was prominent in the molecular layer and the small cell shell, where they were associated with synaptic nests, structures thought to provide long-term changes in excitability. Differential expression levels may reflect different functional requirements of specific cell types, including inhibitory interneurons, like corn cells and cartwheel cells, and excitatory interneurons, like granule cells in the small cell shell, which may participate in local circuits involved in modulatory or gating functions, such as stimulus enhancement or suppression. In presynaptic axons, mGluR1alpha may relate to the long-term signaling requirements of their modulatory functions.     

Sources of GABAergic input to the inferior colliculus of the rat

We have studied the GABAergic projections to the inferior colliculus (IC) of the rat by combining the retrograde transport of horseradish peroxidase (HRP) and immunohistochemistry for gamma-amino butyric acid (GABA). Medium-sized (0.06-0.14 microliter) HRP injections were made in the ventral part of the central nucleus (CNIC), in the dorsal part of the CNIC, in the dorsal cortex (DCIC), and in the external cortex (ECIC) of the IC. Single HRP-labeled and double (HRP-GABA)-labeled neurons were systematically counted in all brainstem auditory nuclei. Our results revealed that the IC receives GABAergic afferent connections from ipsi- and contralateral brainstem auditory nuclei. Most of the contralateral GABAergic input originates in the IC and the dorsal nucleus of the lateral lemniscus (DNLL). The dorsal region of the IC (DCIC and dorsal part of the CNIC) receives connections mostly from its homonimous contralateral region, and the ventral region from the contralateral DNLL. The commissural GABAergic projections originate in a morphologically heterogeneous neuronal population that includes small to medium-sized round and fusiform neurons as well as large and giant neurons. Quantitatively, the ipsilateral ventral nucleus of the lateral lemniscus is the most important source of GABAergic input to the CNIC. In the superior olivary complex, a smaller number of neurons, which lie mainly in the periolivary nuclei, display double labeling. In the contralateral cochlear nuclei, only a few of the retrogradely labeled neurons were GABA immunoreactive. These findings give us more information about the role of GABA in the auditory system, indicating that inhibitory inputs from different ipsi- and contralateral, mono- and binaural auditory brainstem centers converge in the IC.     

Connections of the torus semicircularis and oliva superior in the frog, Rana esculenta: a Phaseolus vulgaris leucoagglutinin labeling study

The afferent and efferent connections of the frog principal nucleus (TP) of torus semicircularis (TOS) and superior olive (SO) were examined by employing the anterograde and retrograde transport patterns of Phaseolus vulgaris leucoagglutinin (PHA-L). After injecting the tracer into these nuclei it was found that the TP projected to the ipsilateral posterior and central thalamic nuclei, all subdivisions of the bilateral TDS and the ipsilateral nucleus isthmi (NI). In the rhombencephalon the projection was restricted mainly to the contralateral SO and the cochlear nucleus (CN). Retrogradely labeled cells were found in most of the areas that contained anterogradely labeled terminals. The termination areas of the SO fibers were similar to the projections of fibers of TP origin in the diencephalic and in the mesencephalic auditory centers. A strong projection was followed into the contralateral SO; the CNs received fibers at both sides. Caudally to the SO the reticular formation, the spinal nucleus of the trigeminal nerve, the solitary nucleus and the dorsal column nuclei were supplied by the fibers of the SO origin. Retrogradely labeled cells were found in the TOS, tegmental nuclei, solitary nucleus, dorsal column nuclei and in the spinal nucleus of the trigeminal nerve. Our results indicate that the frog auditory pathway is more complex at the level of the secondary and tertiary fiber projections than has been previously recognized.     

The cochlear nuclei of snakes

The cochlear nuclei of three burrowing snakes (Xenopeltis unicolor, Cylindrophis rufus, and Eryx johni) and three non-burrowing snakes (Epicrates cenchris, Natrix sipedon, and Pituophis catenifer) were studied. The posterior branch of the statoacoustic nerve and its posterior ganglion were destroyed and the degenerated nerve fibers and terminals traced to primary cochlear nuclei in 13 specimens of Pituophis catenifer. All these snake species possess three primary and one secondary cochlear nuclei. The primary cochlear nuclei consist of a small nucleus angularis located at the cerebello-medullary junction and a fairly large nucleus magnocellularis forming a dorsal cap over the cephalic end of the alar eminence. Nucleus magnocellularis may be subdivided into a medially placed group of rounder cells, nucleus magnocellularis medialis, and a laterally placed group of more ovate and paler-staining cells, nucleus magnocellularis lateralis. A small but well-defined secondary nucleus which showed no degenerated nerve terminals after nerve root section, nucleus laminaris, underlies the cephalic part of both nucleus magnocellularis medialis and nucleus magnocellularis lateralis. Larger and better-developed cochlear nuclei were found in burrowing species than in non-burrowing species of snakes. Of the three burrowing species studied, Xenopeltis showed the greatest development of cochlear nuclei; Eryx cochlear nuclei were not quite as large but were better differentiated than in Xenopeltis; and Cylindrophis cochlear nuclei were fairly large but not as well developed nor as well differentiated as in either Xenopeltis or Eryx. The cochlear nuclei of the three non-burrowing snakes, Epicrates, Natrix, and Pituophis, were not as large nor as well developed as those of the burrowing snakes. There is some, but not complete, correlation between cochlear development and papilla basilaris length and number of hair cells. Thus, Xenopeltis and Eryx, with well-developed cochlear nuclei, have relatively long papillae basilares; but the boid, Epicrates, with less well-developed cochlear nuclei, has a fairly well-developed papilla basilaris. Cylindrophis, a burrowing species, shows only a moderate degree of cochear nuclei and papilla basilaris development. The non-burrowers, Natrix and Pituophis, have both small cochlear nuclei and relatively short papillae basilares.     

Transient CD15 expression reflects stages of differentiation and maturation in the human subcortical central auditory pathway

The expression of the terminal saccharide determinant CD15 (3[a1-3]-fucosyl-N-acetyl-lactosamine) was evaluated in the central auditory system of the human developing brain by using monoclonal antibodies against this epitope. CD15 immunoreactivity was first observed in the ventral cochlear nucleus at 10 weeks of gestation, whereas the dorsal cochlear nucleus became positive from 13 weeks of gestation. In both nuclei, the intensity of immunoreactivity increased until 16 weeks of gestation and lasted until 25 weeks of gestation. In the inferior colliculi, CD15 was poorly expressed in the central nucleus from 13 to 23 weeks of gestation and later with moderate levels until birth. Within the medial geniculate nucleus, a biphasic pattern of expression was observed with peaks around 14-17 and 21-24 weeks of gestation. Heterogeneous expression in the medial geniculate nucleus, which was associated either with neurons or the neuropil, allowed distinction of subnuclei. In many of the auditory pathway structures (e.g., ventral cochlear nucleus and central nucleus of the inferior colliculus), a heterogeneous pattern of CD15 expression in the form of repeating parallel bands, possibly related to tonotopic organization, became transiently apparent around 23 weeks of gestation, whereas in the magnocellular part of the medial geniculate nucleus, a striking modular or compartmental arrangement of immunoreactive structures (which could also be associated with tonotopic organization) was also noted at about 23 weeks of gestation. We propose that the initiation of CD15 expression in each nucleus heralds the appearance of functional contacts and that high levels of neuropil labeling are related to the formation of nonstabilized synaptic contacts. Thus, transient CD15 expression in the central auditory system is possibly correlated with phases of functional plasticity in this pathway.     

Inputs to a physiologically characterized region of the inferior colliculus of the young adult CBA mouse

Presbycusis is a sensory perceptual disorder involving loss of high-pitch hearing and reduced ability to process biologically relevant acoustic signals in noisy environments. The present investigation is part of an ongoing series of studies aimed at discerning the neural bases of presbycusis. The purpose of the present experiment was to delineate the inputs to a functionally characterized region of the dorsomedial inferior colliculus (IC, auditory midbrain) in young, adult CBA mice. Focal, iontophoretic injections of horseradish peroxidase were made in the 18-24 kHz region of dorsomedial IC of the CBA strain following physiological mapping experiments. Serial sections were reacted with diaminobenzidine or tetramethylbenzidine, counterstained and examined for retrogradely labeled cell bodies. Input projections were observed contralaterally from: all three divisions of cochlear nucleus; intermediate and dorsal nuclei of the lateral lemniscus (LL); and the central nucleus, external nucleus and dorsal cortex of the IC. Input projections were observed ipsilaterally from: the medial and lateral superior olivary nuclei; the superior paraolivary nucleus; the dorsolateral and anterolateral periolivary nuclei; the dorsal and ventral divisions of the ventral nucleus of LL; the dorsal and intermediate nuclei of LL; the central nucleus, external nucleus and dorsal cortex of the IC outside the injection site; and small projections from central gray and the medial geniculate body. These findings in young, adult mice with normal hearing can now serve as a baseline for similar experiments being conducted in mice of older ages and with varying degrees of hearing loss to discover neural changes that may cause age-related hearing disorders.     

Neurons with perineuronal sulfated proteoglycans in the mouse brain and spinal cord: their distribution and reactions to lectin Vicia villosa agglutinin and Golgi's silver nitrate

This study demonstrates that many neurons in the somatosensory cortex, cingulate cortex, retrosplenial cortex and hippocampal subiculum of the mouse brain are covered by sulfated proteoglycans which are intensely negative-charged and stained with cationic iron colloid, while being digested with hyaluronidase. Neurons with similar perineuronal proteoglycans are also recognized in the extrapyramidal system (superior colliculus, red nucleus, reticular formation, vestibular nuclei and cerebellar nuclei), in the secondary auditory system (cochlear nuclei, nucleus of trapezoid body, inferior colliculus and nucleus of lateral lemniscus), in the vestibulo-ocular reflex system (vestibular nuclei and extraocular motor nuclei), and in the pupillary reflex system. The neurons with perineuronal sulfated proteoglycans in the cerebral cortices and hippocampal subiculum are usually labeled with the lectin Vicia villosa agglutinin, though those in the cerebellar, vestibular and cochlear nuclei may not be reactive to this lectin. Double staining of the retrosplenial cortex, hippocampal subiculum and cerebellar nuclei with Golgi's silver nitrate and cationic iron colloid indicates that the perineuronal sulfated proteoglycans are identical with the Golgi's reticular coating or glial nets.     

Regional distribution of neurotrophin receptors in the developing auditory brainstem

Neuron survival and axonal regeneration become severely limited during early postnatal development. In conjunction with our recent organotypic analysis of regeneration in the auditory midbrain, we wished to determine whether neurotrophins could serve as a trophic substance during the postnatal period. Therefore, the current study examines the development of three neurotrophin receptor tyrosine kinases (TrkA, TrkB, and TrkC) in the gerbil auditory brainstem. Immunoreactivity to TrkA, the nerve growth-factor receptor, was observed in nonneuronal cells during the first two postnatal weeks. In the cochlear nucleus of mature animals, however, there was a TrkA-positive neuronal subpopulation. In contrast, immunoreactivity to TrkB and TrkC (the receptors for brain-derived neurotrophic factor and neurotrophin-3, respectively) displayed a widespread distribution in the auditory brainstem. At postnatal day 0, TrkB and TrkC staining was virtually absent from auditory nuclei, although immunopositive neurons were present in the mesencephalic trigeminal nucleus. By postnatal day 7, TrkB- and TrkC-positive neurons were present in most brainstem auditory nuclei. At postnatal day 15, TrkB immunoreactivity was observed throughout the inferior colliculus (IC), the cochlear nucleus, the medial and lateral nuclei of the trapezoid body, and the lateral superior olive, whereas TrkC labeled only a subpopulation of neurons within the central nucleus of the IC. The TrkB immunoreactivity was present on both neuronal somata and dendrites, whereas TrkC was generally restricted to cell bodies. At postnatal day 30, TrkB immunostaining was observed on most neurons of the IC. The medial and lateral nuclei of the trapezoid body displayed extremely strong TrkB staining, followed by the cochlear nucleus. In contrast, the TrkC immunostaining was decreased dramatically by postnatal day 21. Observations at the ultrastructural level confirmed a neuronal localization of TrkB and TrkC. Immunostaining for both receptors was restricted largely to the postsynaptic density of synaptic profiles in both dendrites and somata. In summary, this study illustrates a differential pattern of immunoreactivity between three neurotrophin receptors during development. The general increase of TrkB expression is well correlated with the onset of sound-evoked activity in this system, and its synaptic localization suggests that it may be involved in the modulation or maintenance of postsynaptic physiology.     

Role of acoustic striae in hearing: discrimination of sound-source elevation

After years of systematic experimentation, we finally uncovered one thing the dorsal system contributes to hearing which the ventral system may not -- the mechanism for orienting to an elevated sound source [Sutherland, D.P., Masterton, R.B., Glendenning, K.K. (1998) Behav. Brain Res. in press]. This paper follows up this one positive result on a historical background of uniformly negative results. The focus of this report is on the fusiform cells of the dorsal cochlear nucleus whose axons course through the dorsal acoustic stria (DAS). Because electrophysiological studies have shown that the cues for sensing the elevation of a sound source would seem to be best analyzed by the dorsal cochlear nucleus, we tested, behaviorally, normal cats and cats deprived of their DAS or intermediate acoustic stria, bilaterally or ipsilaterally (with or without their contralateral ear deafened), for their ability to orient to elevated sources of broad-band noise. For behavioral testing, we made use of a conventional shock-avoidance procedure. The results lead to the conclusion that DCN and DAS may play no role in learned elevation discriminations. This result builds on that of another of our papers which suggests that a deficit in reflexive discrimination of elevation is strictly auditory in nature [Sutherland, D.P., Masterton, R.B., Glendenning, K.K. (1998) Behav. Brain Res. in press].     

The topographical organization of descending projections from the central nucleus of the inferior colliculus in guinea pig

We describe the descending projections from the central nucleus of the inferior colliculus (CNIC) in guinea pig. Focal injections of the tracer biocytin, made in physiologically defined frequency regions of the CNIC, labelled laminated axonal terminal fields in the ipsilateral dorsal nucleus of the lateral lemniscus, and bilaterally in the ventral nucleus of the trapezoid body and the dorsal cochlear nucleus. Labelling was also present in the rostral periolivary nucleus, but we could not distinguish a clear border between the terminal fields in this nucleus and those in the ventral nucleus of the trapezoid body. Labelling observed in the ventral nucleus of the lateral lemniscus, and to a lesser extent in the dorsal nucleus of the lateral lemniscus, was accompanied by retrogradely labelled somata and therefore we cannot conclude unequivocally that the CNIC projects to these lemniscal nuclei. Where the labelling was ordered topographically, its position varied as a function of the best frequency at the injection site. High-frequency regions in the CNIC project to the medial parts of the ventral nucleus of the trapezoid body and dorsal cochlear nucleus, while low-frequency regions in the CNIC project to the lateral parts of the ventral nucleus of the trapezoid body and dorsal cochlear nucleus. Additional axonal labelling with terminal boutons, but with no apparent topographical arrangement, was present in the ipsilateral horizontal cell group, sagulum, and also bilaterally in the superficial granule cell layer of the ventral cochlear nucleus and layer 2 of the dorsal cochlear nucleus. Our findings are consistent with the existence of tonotopically organised feedback projections from the CNIC to the brainstem nuclei that project to it.     

Neonatal sound deprivation affects brain stem auditory nuclei

CBA/J mice deprived of airborne sound stimulation during postnatal development have smaller globular cells in the ventral cochlear nucleus and smaller neurons in the medial nucleus of the trapezoid body than do normal control mice. The sound deprivation in these mice is similar to that experienced by persons with pure congenital conductive hearing losses. Even more profound central neural changes were found in auditory nuclei in the brain stem of a congenitally sensorineural deaf human.     

Serotonin transporter messenger RNA in the developing rat brain: early expression in serotonergic neurons and transient expression in non-serotonergic neurons

Serotonin has been shown to affect the development of the mammalian nervous system. The serotonin transporter is a major factor in regulating extracellular serotonin levels. Using in situ hybridization histochemistry the rat serotonin transporter messenger RNA was localized during embryogenesis, the first four weeks postnatally and adulthood. Three general classes of serotonin transporter messenger RNA expression patterns were observed: (i) early detection with continued expression through adult age, (ii) transient expression colocalized with vesicular monoamine transporter 2 messenger RNA but with no detectable tryptophan hydroxylase immunoreactivity, and (iii) transient expression in the apparent absence of both vesicular monoamine transporter 2 messenger RNA and tryptophan hydroxylase immunoreactivity. For example, hybridization for serotonin transporter messenger RNA was strong in serotonin cell body-containing areas beginning early in gestation, and remained intense through adulthood. Immunoreactivity for tryptophan hydroxylase, the rate-limiting enzyme in serotonin synthesis, was completely overlapping with the presence of serotonin transporter messenger RNA in raphe nuclei postnatally. Sensory relay systems including the ventrobasal nucleus (somatosensory), lateral and medial geniculate nuclei (visual and auditory, respectively) as well as trigeminal, cochlear and solitary nuclei were representative of the second class of observations. In general, the limbic system expressed serotonin transporter messenger RNA in the third pattern with various limbic structures differing in the timing of expression. Septum, olfactory areas and the developing hippocampus contained serotonin transporter messenger RNA early in the developing brain. Other regions such as cingulate and frontopolar cortex exhibited hybridization peri- and postnatally, respectively. Several hypothalamic nuclei and pituitary transiently expressed serotonin transporter messenger RNA either postnatally or perinatally, respectively. If the observed patterns correlate with functional protein expression, distinct classes of serotonin transporter messenger RNA expression may reflect different functional roles for the serotonin transporter and serotonin, itself. Since the serotonin transporter is a target for a number of addictive substances including cocaine and amphetamine derivatives as well as antidepressants, transient expression of the serotonin transporter might suggest a window of vulnerability of associated cells to fetal drug exposure. Re-uptake, storage and re-release from non-serotonergic neurons might serve as a feedback mechanism from target neurons to serotonergic neurons. Alternatively, the transient expression of serotonin transporter messenger RNA may reflect critical periods important for tight regulation of extracellular serotonin in several brain regions, and may indicate previously unappreciated roles for serotonin as a developmental cue.     

Collateral projections of gamma-aminobutyric acid positive neurons from the lateral superior nucleus to the cochlea and cochlear nucleus in guinea pigs

Collateral projections of gamma-aminobutyric acid (GABA) neurons from the lateral superior olivary nucleus (LSO) to the cochlea and cochlear nuclei in the guinea pigs were studied by injection of two retrograde fluorescent neuronal tracers. For experiments, fast blue (FB) was injected into the scala tympani of one cochlea and diamidine yellow (DY) was injected into cochlear nuclei of the same side. The results showed that the FB-labelled cells and DY-labelled cells constituted approximately 80.8% and 12.4%, respectively; FB and DY double-labelled cells constituted about 6%; FB and DY labelled cells with GABA constituted about 0.7% in the ipsilateral LSO. In the contralateral LSO, the FB and DY labelled cells were less than those of ipsilateral LSO and no FB-DY double-labelled cells could be found. Our results suggest that there are collateral projections of GABA neurons from ipsilateral LSO to the organ of Corti and cochlear nuclei in the guinea pig, even though the numbers are few. The results also show that the efferent projections to the cochlea and cochlear nuclei generally come from two different auditory neuronal nuclei.     

Synapses from labeled type II axons in the mouse cochlear nucleus

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

Distribution of pre-pro-glucagon and glucagon-like peptide-1 receptor messenger RNAs in the rat central nervous system

Glucagon-like peptide-1 (GLP-1) is derived from the peptide precursor pre-pro-glucagon (PPG) by enzymatic cleavage and acts via its receptor, glucagon-like peptide-1 receptor (GLP-1R). By using riboprobes complementary to PPG and GLP-1R, we described the distribution of PPG and GLP-1R messenger RNAs (mRNAs) in the central nervous system of the rat. PPG mRNA-expressing perikarya were restricted to the nucleus of the solitary tact or to the dorsal and ventral medulla and olfactory bulb. GLP-1R mRNA was detected in numerous brain regions, including the mitral cell layer of the olfactory bulb; temporal cortex; caudal hippocampus; lateral septum; amygdala; nucleus accumbens; ventral pallium; nucleus basalis Meynert; bed nucleus of the stria terminalis; preoptic area; paraventricular, supraoptic, arcuate, and dorsomedial nuclei of the hypothalamus; lateral habenula; zona incerta; substantia innominata; posterior thalamic nuclei; ventral tegmental area; dorsal tegmental, posterodorsal tegmental, and interpeduncular nuclei; substantia nigra, central gray; raphe nuclei; parabrachial nuclei; locus ceruleus, nucleus of the solitary tract; area postrema; dorsal nucleus of the vagus; lateral reticular nucleus; and spinal cord. These studies, in addition to describing the sites of GLP-1 and GLP-1R synthesis, suggest that the efferent connections from the nucleus of the solitary tract are more widespread than previously reported. Although the current role of GLP-1 in regulating neuronal physiology is not known, these studies provide detailed information about the sites of GLP-1 synthesis and potential sites of action, an important first step in evaluating the function of GLP-1 in the brain. The widespread distribution of GLP-1R mRNA-containing cells strongly suggests that GLP-1 not only functions as a satiety factor but also acts as a neurotransmitter or neuromodulator in anatomically and functionally distinct areas of the central nervous system.