Mossy fiber projections from the cuneate nucleus to the cochlear nucleus in the rat

A reciprocal connection is known to exist between the cuneate nucleus, which is a first-order somatosensory nucleus, and the cochlear nucleus, which is a first-order auditory nucleus. We continued this line of study by investigating the fiber endings of this projection in the cochlear nucleus of rats using the neuronal tracer Phaseolus vulgaris leucoagglutinin in combination with ultrastructural and immunocytochemical analyses. In the cochlear nucleus, mossy fiber terminals had been described and named for their morphologic similarity to those in the cerebellum, but their origins had not been discovered. In the present study, we determined that the axonal projections from the cuneate region gave rise to mossy fiber terminals in the granule cell regions of the ipsilateral cochlear nucleus. The cuneate mossy fibers appear to be excitatory in nature, because they are filled with round synaptic vesicles, they make asymmetric synapses with postsynaptic targets, and they are labeled with an antibody to glutamate. The postsynaptic targets of the mossy fibers include dendrites of granule cells. This projection onto the granule cell interneuron circuit of the cochlear nucleus indicates that somatosensory cues are intimately involved with information processing at this early stage of the auditory system. © 1996 Wiley-Liss, Inc.     

Projections from the spinal trigeminal nucleus to the cochlear nucleus in the rat

The integration of information across sensory modalities enables sound to be processed in the context of position, movement, and object identity. Inputs to the granule cell domain (GCD) of the cochlear nucleus have been shown to arise from somatosensory brain stem structures, but the nature of the projection from the spinal trigeminal nucleus is unknown. In the present study, we labeled spinal trigeminal neurons projecting to the cochlear nucleus using the retrograde tracer, Fast Blue, and mapped their distribution. In a second set of experiments, we injected the anterograde tracer biotinylated dextran amine into the spinal trigeminal nucleus and studied the resulting anterograde projections with light and electron microscopy. Spinal trigeminal neurons were distributed primarily in pars caudalis and interpolaris and provided inputs to the cochlear nucleus. Their axons gave rise to small (1-3 microm in diameter) en passant swellings and terminal boutons in the GCD and deep layers of the dorsal cochlear nucleus. Less frequently, larger (3-15 microm in diameter) lobulated endings known as mossy fibers were distributed within the GCD. Ventrally placed injections had an additional projection into the anteroventral cochlear nucleus, whereas dorsally placed injections had an additional projection into the posteroventral cochlear nucleus. All endings were filled with round synaptic vesicles and formed asymmetric specializations with postsynaptic targets, implying that they are excitatory in nature. The postsynaptic targets of these terminals included dendrites of granule cells. These projections provide a structural substrate for somatosensory information to influence auditory processing at the earliest level of the central auditory pathways.     

Synapses formed by olivocochlear axon branches in the mouse cochlear nucleus

Cochlear nucleus branches of thick olivocochlear axons were labeled by injections of horseradish peroxidase into the spiral ganglion of the cochlear basal turn in mice. Six labeled axons were traced by light microscopy, and selected portions of seven branches were sectioned serially for electron microscopic examination. Axonal branches most frequently terminated near certain granule cell regions of the ventral cochlear nucleus. This article describes terminals, synapses, and postsynaptic elements of these olivocochlear branches. The olivocochlear branches had both terminal and en passant boutons that contained round vesicles and made asymmetric synapses with other neuronal processes. About a quarter of the synapses also possessed additional specializations, postsynaptic, or subjunctional bodies. Mossy terminals, a multisynaptic type of terminal commonly found in granule cell regions, were not found arising from any of the labeled branches. No somatic synapses were found, although contacts with cell bodies were occasionally observed. The predominant synaptic target of olivocochlear branches were what appeared to be dendrites of large diameter. At least some of these large dendrites received multiple synapses from a single labeled olivocochlear branch. The morphological characteristics of reconstructed dendrites suggest that multipolar cells might be predominant targets for the medial olivocochlear system in the cochlear nucleus. This was demonstrated in one case in which a large dendrite was followed to its cell body of origin.     

Synaptic interrelationships between the optic tectum and the ipsilateral nucleus isthmi in Rana pipiens

The nucleus isthmi is reciprocally connected to the ipsilateral optic tectum. Ablation of the nucleus isthmi compromises visually guided behavior that is mediated by the tectum. In this paper, horseradish peroxidase (HRP) histochemistry and electron microscopy were used to explore the synaptic interrelationships between the optic tectum and the ipsilateral nucleus isthmi. After localized injections of HRP into the optic tectum, there are retrogradely labeled isthmotectal neurons and orthogradely labeled fibers and terminals in the ipsilateral nucleus isthmi. These terminals contain round. Clear vesicles of medium diameter (40–52 nm). These terminals make synaptic contact with dendrites of nucleus isthmi cells. Almost half of these postsynaptic dendrites are retrogradely labeled, indicating that there are monosynaptic tectoisthmotectal connections. Localized HRP injection into the nucleus isthmi labels terminals primarily in tectal layers B, E, F, and 8. The terminals contain medium-sized clear vesicles and they form synaptic contacts with tectal dendrites. There are no instances of labeled isthmotectal terminals contacting labeled dendrites. Retrogradely labeled tectoisthmal neurons are contacted by unlabeled terminals containing medium-sized and small clear vesicles. Fifty-four percent of the labeled fibers connecting the nucleus isthmi and ipsilateral tectum are myelinated fibers (average diameter approximately 0.6 μm). The remainder are unmyelinated fibers (average diameter approximately 0.4 μm). © 1994 Wiley-Liss, Inc.     

Projections of the second cervical dorsal root ganglion to the cochlear nucleus in rats

Physiological, anatomical, and clinical data have demonstrated interactions between somatosensory and auditory brainstem structures. Spinal nerve projections influence auditory responses, although the nature of the pathway(s) is not known. To address this issue, we injected biotinylated dextran amine into the cochlear nucleus or dorsal root ganglion (DRG) at the second cervical segment (C2). Cochlear nucleus injections retrogradely labeled small ganglion cells in C2 DRG. C2 DRG injections produced anterograde labeling in the external cuneate nucleus, cuneate nucleus, nucleus X, central cervical nucleus, dorsal horn of upper cervical spinal segments, and cochlear nucleus. The terminal field in the cochlear nucleus was concentrated in the subpeduncular corner and lamina of the granule cell domain, where endings of various size and shapes appeared. Examination under an electron microscope revealed that the C2 DRG terminals contained numerous round synaptic vesicles and formed asymmetric synapses, implying depolarizing influences on the target cell. Labeled endings synapsed with the stalk of the primary dendrite of unipolar brush cells, distal dendrites of presumptive granule cells, and endings containing pleomorphic synaptic vesicles. These primary somatosensory projections contribute to circuits that are hypothesized to mediate integrative functions of hearing.     

Prefrontal cortical efferents in the rat synapse on unlabeled neuronal targets of catecholamine terminals in the nucleus accumbens septi and on dopamine neurons in the ventral tegmental area.

Physiological and pharmacological studies indicate that descending projections from the prefrontal cortex modulate dopaminergic transmission in the nucleus accumbens septi and ventral tegmental area. We investigated the ultrastructural bases for these interactions in rat by examining the synaptic associations between prefrontal cortical terminals labeled with anterograde markers (lesion-induced degeneration or transport of Phaseolus vulgaris leucoagglutinin; PHA-L) and neuronal processes containing immunoreactivity for the catecholamine synthesizing enzyme, tyrosine hydroxylase. Prefrontal cortical terminals in the nucleus accumbens and ventral tegmental area contained clear, round vesicles and formed primarily asymmetric synapses on spines or small dendrites. In the ventral tegmental area, these terminals also formed asymmetric synapses on large dendrites and a few symmetric axodendritic synapses. In the nucleus accumbens septi, degenerating prefrontal cortical terminals synapsed on spiny dendrites which received convergent input from terminals containing peroxidase immunoreactivity for tyrosine hydroxylase, or from unlabeled terminals. In single sections, some tyrosine hydroxylase-labeled terminals formed thin and punctate symmetric synapses with dendritic shafts, or the heads and necks of spines. Close appositions, but not axo-axonic synapses, were frequently observed between degenerating prefrontal cortical afferents and tyrosine hydroxylase-labeled or unlabeled terminals. In the ventral tegmental area, prefrontal cortical terminals labeled with immunoperoxidase for PHA-L were in synaptic contact with dendrites containing immunogold reaction product for tyrosine hydroxylase, or with unlabeled dendrites. These results suggest that: (1) catecholaminergic (mainly dopaminergic) and prefrontal cortical terminals in the nucleus accumbens septi dually synapse on common spiny neurons; and (2) dopaminergic neurons in the ventral tegmental area receive monosynaptic input from prefrontal cortical afferents. This study provides the first ultrastructural basis for multiple sites of cellular interaction between prefrontal cortical efferents and mesolimbic dopaminergic neurons.     

Ultrastructural localization of dynorphin in the dentate gyrus in human temporal lobe epilepsy: a study of reorganized mossy fiber synapses

Substantial reorganization of mossy fibers from granule cells of the dentate gyrus occurs in a high percentage of humans with medically intractable temporal lobe epilepsy. To identify these fibers and determine their ultrastructural features in human surgical specimens, we used preembedding immunoperoxidase labeling of dynorphin A, an opioid peptide that is abundant in normal mossy fibers. In electron microscopic preparations, dynorphin A immunoreactivity was highly associated with dense core vesicles and was localized predominantly in axon terminals in the inner molecular layer of the dentate gyrus, although some dynorphin-labeled dense core vesicles were also observed in dendritic shafts and spines. The labeled terminal profiles were numerous, and, whereas they varied greatly in size, many were relatively large (2.3 microm in mean major diameter). The terminals contained high concentrations of clear round vesicles and numerous mitochondrial profiles, formed distinct asymmetric synapses, often had irregular shapes, and, thus, exhibited many features of normal mossy fiber terminals. The dynorphin-labeled terminals formed synaptic contacts primarily with dendritic spines, and some of these spines were embedded in large labeled terminals, suggesting that they were complex spines. The labeled terminals frequently formed multiple synaptic contacts with their postsynaptic elements, and perforated postsynaptic densities, with and without spinules, were present at some synapses. These findings suggest that the reorganized mossy fiber terminals in humans with temporal lobe epilepsy form abundant functional synapses in the inner molecular layer of the dentate gyrus, and many of these contacts have ultrastructural features that could be associated with highly efficacious synapses.     

Immunocytochemical localization of GABA in the cochlear nucleus of the guinea pig

The immunocytochemical distribution of gamma-aminobutyric acid (GABA) was determined in the cochlear nucleus of the guinea pig using affinity-purified antibodies made against GABA conjugated to bovine serum albumin. Light microscopic immunocytochemistry shows immunoreactive puncta, which appear to be GABA-positive presynaptic terminals, distributed throughout the cochlear nucleus. In the ventral cochlear nucleus, these puncta are often found around unlabeled neuronal cell bodies. While occasional labeled small cells are found in the ventral cochlear nucleus, most GABA-immunoreactive cell bodies are present in the superficial layers of the dorsal cochlear nucleus. Based on size and shape, immunoreactive cells in the dorsal cochlear nucleus are divided into 3 classes: medium round cells with diameters averaging 16 microns, small round cells with average diameters of 9 microns and small flattened cells with major and minor diameters averaging 11 and 6 microns, respectively. Labeled fusiform and granule cells are not seen. A similar distribution of label was seen using antibodies against glutamic acid decarboxylase. Electron microscopic immunocytochemistry of the anteroventral cochlear nucleus shows GABA immunoreactive boutons containing oval/pleomorphic synaptic vesicles on cell bodies and dendrites. Other major classes of terminals, including those with small round, large round and flattened synaptic vesicles are unlabeled.     

Cochlear nerve projections to the small cell shell of the cochlear nucleus: The neuroanatomy of extremely thin sensory axons

Labeling cochlear nerve fibers in the inner ear of chinchillas with biotinylated dextran polyamine was used to trace the thin fibers (Type II), which likely innervate outer hair cells. These axons, 0.1–0.5 μm in diameter, were distinguished from the thicker Type I, fibers innervating inner hair cells, and traced to small‐cell clusters in the cochlear nucleus. This study provided two major new insights into the outer hair cell connections in the cochlear nucleus and the potential significance of very thin axons and synaptic nests, which are widespread in the CNS. 1) EM serial reconstructions of labeled and unlabeled material revealed that Type II axons rarely formed synapses with conventional features (vesicles gathered at junctions). Rather, their endings contained arrays of endoplasmic reticulum and small spherical vesicles without junctions. 2) Type II axons projected predominantly to synaptic nests, where they contacted other endings and dendrites of local interneurons (small stellate and mitt cells, but not granule cells). Synaptic nests lacked intrinsic glia and, presumably, their high‐affinity amino acid transporters. As functional units, nests and their Type II inputs from outer hair cells may contribute to an analog processing mode, which is slower, more diffuse, longer‐lasting, and potentially more plastic than the digital processors addressed by inner hair cells. Synapse 33:83–117, 1999. © 1999 Wiley‐Liss, Inc.     

Evidence for a cortical projection to the magnocellular basal nucleus in the rat: an electron microscopic axonal transport study

A potential reciprocal projection from the cerebral cortex to the nucleus basalis was studied in the rat using a new stabilization method to adapt tetramethylbenzidine-horseradish peroxidase histochemistry for electron microscopy. Following insular or cingulate cortical injections of wheat germ agglutinin-horseradish peroxidase conjugate, anterogradely labeled axon terminals were seen making symmetric synaptic contacts with retrogradely labeled nucleus basalis neurons. Labeled axon terminals contained round vesicles. Most of such contacts were located on distal dendrites, although a small number of synapses on proximal dendrites and cell somata were seen as well. These findings suggest that there is a reciprocal, excitatory projection from the cerebral cortex to the nucleus basalis in the rat.     

Synaptic connections of a periodontal primary afferent neuron within the subnucleus oralis of the cat

The central axon of a primary afferent neuron that responded to light mechanical stimulation of the lower premolar teeth in a fast adapting fashion was intra-axonally injected with horseradish peroxidase in the cat. The labeled terminals within the rostrodorsomedial (Vo.r) and dorsomedial (Vo.dm) parts of subnucleus oralis were examined electron microscopically. The labeled ending had pale axoplasm, contained clear spherical synaptic vesicles, and formed multiple synapses with dendrites and/or unlabeled axonal endings with pleomorphic vesicles (P-endings). In these synaptic contacts, the labeled primary ending was presynaptic to dendrites and postsynaptic to P-endings. Labeled endings simultaneously synapsing with both dendrites and P-endings were more frequent in Vo.dm (28%) than in Vo.r (8.3%).     

Ultrastructural features of sprouted mossy fiber synapses in kindled and kainic acid-treated rats

The mossy fiber pathway in the dentate gyrus undergoes sprouting and synaptic reorganization in response to seizures. The types of new synapses, their location and number, and the identity of their postsynaptic targets determine the functional properties of the reorganized circuitry. The goal of this study was to characterize the types and proportions of sprouted mossy fiber synapses in kindled and kainic acid-treated rats. In normal rats, synapses labeled by Timm histochemistry or dynorphin immunohistochemistry were rarely observed in the supragranular region of the inner molecular layer when examined by electron microscopy. In epileptic rats, sprouted mossy fiber synaptic terminals were frequently observed. The ultrastructural analysis of the types of sprouted synapses revealed that 1) in the supragranular region, labeled synaptic profiles were more frequently axospinous than axodendritic, and many axospinous synapses were perforated; 2) sprouted mossy fiber synaptic terminals formed exclusively asymmetric, putatively excitatory synapses with dendritic spines and shafts in the supragranular region and with the soma of granule cells in the granule cell layer; 3) in contrast to the large sprouted mossy fiber synapses in resected human epileptic hippocampus, the synapses formed by sprouted mossy fibers in rats were smaller; and 4) in several cases, the postsynaptic targets of sprouted synapses were identified as granule cells, but, in one case, a sprouted synaptic terminal formed a synapse with an inhibitory interneuron. The results demonstrate that axospinous asymmetric synapses are the most common type of synapse formed by sprouted mossy fiber terminals, supporting the viewpoint that most sprouted mossy fibers contribute to recurrent excitation in epilepsy.     

Mu-opioid receptor is located on the plasma membrane of dendrites that receive asymmetric synapses from axon terminals containing leucine-enkephalin in the rat nucleus locus coeruleus

We have recently shown, by using immunoelectron microscopy, that the mu-opioid receptor (μOR) is prominently distributed within noradrenergic perikarya and dendrites of the nucleus locus coeruleus (LC), many of which receive excitatory-type (i.e., asymmetric) synaptic contacts from unlabeled axon terminals. To characterize further the neurotransmitter present in these afferent terminals, we examined in the present study the ultrastructural localization of an antipeptide sequence unique to the μOR in sections that were also dually labeled for the opioid peptide leucine-enkephalin (L-ENK). Immunogold-silver labeling for μOR was localized to extrasynaptic portions of the plasma membranes of perikarya and dendrites. The μOR-labeled dendrites were usually postsynaptic to axon terminals containing heterogeneous types of synaptic vesicles and forming asymmetric synaptic specializations characteristic of excitatory-type synapses. The majority of these were immunolabeled for the endogenous opioid peptide L-ENK. Some μOR-labeled dendrites received synaptic contacts from unlabeled axon terminals in fields containing L-ENK immunoreactivity. In such cases, the μOR-labeled dendrites were in proximity to L-ENK axon terminals that contained intense peroxidase labeling within large dense core vesicles along the perimeter of the axoplasm. These results indicate that L-ENK may be released by exocytosis from the dense core vesicles and diffuse within the extracellular space to reach μOR sites on the postsynaptic dendrite or dendrites of other neighboring neurons. The present study also reveals that unlabeled terminals apposed to μOR-labeled dendrites may contain other opioid peptides, such as methionine-enkephalin. These data demonstrate several sites where endogenous opioid peptides may interact with μOR receptive sites in the LC and may provide an anatomical substrate for the LC's involvement in mechanisms of opiate dependence and withdrawal. © 1996 Wiley-Liss, Inc.     

An electron microscopic study of synaptic organization in the medial superior olive of normal and experimental chinchillas

The medial superior olive (MSO) was studied in normal animals to determine the types of synaptic endings and their distribution over the surface of MSO neurons. Unilateral lesions were made in the anteroventral cochlear nucleus (AVCN) of experimental animals to determine the source of at least one synaptic type in the MSO. The surfaces of MSO neurons in normal animals were studded with three distinct types of synaptic endings distinguished mainly by the size of their synaptic vesicles. There were endings with large vesicles, 510 Å in mean diameter; endings with small vesicles, 380 Å; and endings with vesicles intermediate in size. 435 Å. The large vesicle ending typically was greater than 2 μm in maximum diameter. It appeared as the termination of a myelinated axon or as a swollen portion of a node and made multiple asymmetrical synapses. Large vesicle endings occurred exclusively on dendrites where they formed 85% of the synaptic endings. Small vesicle endings typically were less than 2 μm in diameter. They appeared as the termination of a fine unmyelinated axon and made only one symmetrical synapse. Small vesicle boutons occurred infrequently over the entire neuronal surface. Intermediate vesicle synaptic endings were similar to large vesicle endings except that they were present only on the cell body, axon hillock, and proximal portions of the dendrites where they formed most of the synapses. In AVCN lesioned animals degenerating myelinated axons and large vesicle synaptic endings were distributed to the lateral dendrites of the ipsilateral MSO and medial dendrites of the contralateral one. In addition, a few degenerating axons and large vesicle endings were found among the ipsilateral medial dendrites. The changes in the degenerating endings were characterized by an early proliferation of neurofilaments and swelling of the endings followed by collapse of the endings and increase in electron density, disappearance of filaments and synaptic vesicles, and phagocytosis of the degenerated endings by reactive glial cells. No degenerative changes were observed in the small and intermediate vesicle endings. The results of this study indicate that the more numerous large vesicle endings presynaptic to the MSO dendrites are the axon terminals of neurons in the AVCN. The persistence after lesions of the small and intermediate vesicle endings suggests that they arise from as yet unidentified sources.     

Projections of the lateral reticular nucleus to the cochlear nucleus in rats

The lateral reticular nucleus (LRN) resides in the rostral medulla and caudal pons, is implicated in cardiovascular regulation and cranial nerve reflexes, and gives rise to mossy fibers in the cerebellum. Retrograde tracing data revealed that medium-sized multipolar cells from the magnocellular part of the LRN project to the cochlear nucleus (CN). We sought to characterize the LRN projection to the CN using BDA injections. Anterogradely labeled terminals in the ipsilateral CN appeared as boutons and mossy fibers, and were examined with light and electron microscopy. The terminal field in the CN was restricted to the granule cell domain (GCD), specifically in the superficial layer along the anteroventral CN and in the granule cell lamina. Electron microscopy showed that the smallest LRN boutons formed 1-3 synapses, and as boutons increased in size, they formed correspondingly more synapses. The largest boutons were indistinguishable from the smallest mossy fibers, and the largest mossy fiber exhibited 15 synapses. Synapses were asymmetric with round vesicles and formed against thin dendritic profiles characterized by plentiful microtubules and the presence of fine filopodial extensions that penetrated the ending. These structural features of the postsynaptic target are characteristic of the terminal dendritic claw of granule cells. LRN projections are consistent with known organizational principles of non-auditory inputs to the GCD.     

Morphology and origin of axonal endings in nucleus laminaris of the chicken

The axonal endings on the somata and dendrites of third-order auditory neurons in nucleus laminaris (NL) were measured and classified in thin-sectioned material from adult chickens. Two methods were used to deter-mine which ending types arise from second-order auditory neurons in nu-cleus magnocellularis (NM): (1) degeneration of axonal endings in NL after transection of the crossed dorsal cochlear tract (CTrX) carrying the axons of each NM to the contralateral NL, and (2) injection of horseradish per-oxidase (HRP) into NM or the CTrX to label endings in NL by anterograde transport. About 42% of the perikaryl surface of NL neurons and 63% of the dendritic surface are apposed by axon terminals arising from NM; these endings are also prevalent on the axon hillock and initial segment. The NM endings are characterized by round, clear synaptic vesicles-distributed at an average density of 76/μm2 behind small punctate synaptic junctions with thick synaptic densities. These endings degenerate preferentially after tran-section of the CTrX and are the only endings consistently labeled after HRP injections. About 31% of the perikaryl surface of NL and 10% of the distal dendritic surface are apposed by a prominent non-NM axonal ending type This ending is characterized by a synaptic vesicle density of 135/ μm2 a single large area of synaptic contact which bears very slight densities on both pre- and postsynaptic membranes. On the perikaryon, about 80% of these non-NM terminals end on the cell body proper, with the rest found on proximal dendrites or capping the short somatic processes or spines occasionally seen in NL. The distribution of the two ending types does not differ significantly along the posterior-to-anterior axis of NL.     

Synaptic organization of projections from basal forebrain structures to the mediodorsal thalamic nucleus of the rat.

The synaptic organization of the mediodorsal thalamic nucleus (MD) in the rat was studied with the electron microscope, and correlated with the termination of afferent fibers labeled with wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Presynaptic axon terminals were classified into four categories in MD on the basis of the size, synaptic vesicle morphology, and synaptic membrane specializations: 1) small axon terminals with round synaptic vesicles (SR), which made asymmetrical synaptic contacts predominantly with small dendritic shafts; 2) large axon terminals with round vesicles (LR), which established asymmetrical synaptic junctions mainly with large dendritic shafts; 3) small to medium axon terminals with pleomorphic vesicles (SMP), which formed symmetrical synaptic contacts with somata and small-diameter dendrites; 4) large axon terminals with pleomorphic vesicles (LP), which made symmetrical synaptic contacts with large dendritic shafts. Synaptic glomeruli were also identified in MD that contained either LR or LP terminals as the central presynaptic components. No presynaptic dendrites were identified. In order to identify terminals arising from different sources, injections of WGA-HRP were made into cortical and subcortical structures known to project to MD, including the prefrontal cortex, piriform cortex, amygdala, ventral pallidum and thalamic reticular nucleus. Axons from the amygdala formed LR terminals, while those from the prefrontal and insular cortex ended exclusively in SR terminals. Fibers labeled from the piriform cortex formed both LR and SR endings. Based on their morphology, all of these are presumed to be excitatory. In contrast, the axons from the ventral pallidum ended as LP terminals, and those from the thalamic reticular nucleus formed SMP terminals. Both are presumed to be inhibitory. At least some terminals from these sources have also been identified as GABAergic, based on double labeling with anterogradely transported WGA-HRP and glutamic acid decarboxylase (GAD) immunocytochemistry.     

Central projections of auditory nerve fibers in the barn owl.

The central projections of the auditory nerve were examined in the barn owl. Each auditory nerve fiber enters the brain and divides to terminate in both the cochlear nucleus angularis and the cochlear nucleus magnocellularis. This division parallels a functional division into intensity and time coding in the auditory system. The lateral branch of the auditory nerve innervates the nucleus angularis and gives rise to a major and a minor terminal field. The terminals range in size and shape from small boutons to large irregular boutons with thorn-like appendages. The medial branch of the auditory nerve conveys phase information to the cells of the nucleus magnocellularis via large axosomatic endings or end bulbs of Held. Each medial branch divides to form 3-6 end bulbs along the rostrocaudal orientation of a single tonotopic band, and each magnocellular neuron receives 1-4 end bulbs. The end bulb envelops the postsynaptic cell body and forms large numbers of synapses. The auditory nerve profiles contain round clear vesicles and form punctate asymmetric synapses on both somatic spines and the cell body.     

Chemical anatomy of excitatory endings in the dorsal cochlear nucleus of the rat: Differential synaptic distribution of aspartate aminotransferase,glutamate, and vesicular zinc

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. J. Comp. Neurol. 399:341–358, 1998. © 1998 Wiley-Liss, Inc.     

Status epilepticus-induced hilar basal dendrites on rodent granule cells contribute to recurrent excitatory circuitry

Mossy fiber sprouting into the inner molecular layer of the dentate gyrus is an important neuroplastic change found in animal models of temporal lobe epilepsy and in humans with this type of epilepsy. Recently, we reported in the perforant path stimulation model another neuroplastic change for dentate granule cells following seizures: hilar basal dendrites (HBDs). The present study determined whether status epilepticus-induced HBDs on dentate granule cells occur in the pilocarpine model of temporal lobe epilepsy and whether these dendrites are targeted by mossy fibers. Retrograde transport of biocytin following its ejection into stratum lucidum of CA3 was used to label granule cells for both light and electron microscopy. Granule cells with a heterogeneous morphology, including recurrent basal dendrites, and locations outside the granule cell layer were observed in control preparations. Preparations from both pilocarpine and kainate models of temporal lobe epilepsy also showed granule cells with HBDs. These dendrites branched and extended into the hilus of the dentate gyrus and were shown to be present on 5% of the granule cells in pilocarpine-treated rats with status epilepticus, whereas control rats had virtually none. Electron microscopy was used to determine whether HBDs were postsynaptic to axon terminals in the hilus, a site where mossy fiber collaterals are prevalent. Labeled granule cell axon terminals were found to form asymmetric synapses with labeled HBDs. Also, unlabeled, large mossy fiber boutons were presynaptic to HBDs of granule cells. These results indicate that HBDs are present in the pilocarpine model of temporal lobe epilepsy, confirm the presence of HBDs in the kainate model, and show that HBDs are postsynaptic to mossy fibers. These new mossy fiber synapses with HBDs may contribute to additional recurrent excitatory circuitry for granule cells.