Projections to the caudolateral medulla from the pons,midbrain, and diencephalon in the cat

Previous research implicated neurons in the caudolateral medulla in the expression of estrous behaviors triggered by genital stimulation in the female cat. The present study identified descending pathways through which the activity of neurons in estrogen-concentrating cellular regions of the diencephalon and anterior brain stem could be transmitted to caudolateral medullary neurons. Cats received medullary injections of 50% horseradish peroxidase (HRP) in or around nucleus ambiguus. After 1 to 3 days, retrograde transport of HRP was demonstrated using tetramethyl benzidine as a chromogen. In the pons, labeled cells were most numerous in the ipsilateral parabrachial nuclei, the Kölliker-Fuse nucleus, and lateral tegmental field. In the midbrain, the central gray contained many labeled neurons bilaterally, especially at trochlear and caudal oculomotor nuclear levels. Labeled cells were also found in the midbrain reticular formation bilaterally and in the contralateral deep tectum and red nucleus. In the diencephalon, some labeled neurons were in lateral and periventricular hypothalamic regions, usually posteriorly, and many paraventricular nucleus neurons were labeled. The existence of central gray and deep tectal projections to the lateral medulla was also verified electrophysiologically by antidromic invasion. The substantial projection from the central gray to the caudolateral medulla provides a potential route for the activity of estrogen-concentrating neurons to be transmitted to cells involved in genitally triggered estrous responses because some central gray cells bind estrogen and the central gray also receives strong projections from hypothalamic estrogen-concentrating neuronal regions.     

Trigeminal and dorsal column nuclei projections to the anterior pretectal nucleus in the rat

The projections of the trigeminal (V) sensory nuclei (VSN) and the dorsal column nuclei (DCN) to the anterior pretectal nucleus (APT) of the rat were investigated by the use of anterograde and retrograde transport of wheat-germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Injections of WGA-HRP into the APT retrogradely labeled neurons in the contralateral VSN and DCN. The labeled neurons in the VSN were most concentrated in the rostral V subnucleus interpolaris (Vi), but were also found in caudal V subnucleus oralis (Vo). No labeled neurons were seen in V subnucleus caudalis. In the DCN, retrogradely labeled neurons were observed in rostral portions of both the cuneate (Cu) and gracile (Gr) nuclei. Injections of WGA-HRP into the rostral Vi or caudal Vo resulted in dense anterograde terminal labeling in the ventral two-thirds of the APT; the labeling was maximal in the ventromedial part of the caudal half of the APT and did not extend into its most rostral portion. Labeling resulting from injections of tracer into Cu or Gr was located primarily in the ventral half of the APT, was maximal in the mid-levels of the nucleus and extended into its rostral portions. These results indicate the existence of prominent somatosensory projections to the APT and are consistent with recent findings suggesting a role for the APT in sensorimotor integration.     

Tectal connections in Python reticulatus

The origins of the axons terminating in the mesencephalic tectum in Python reticulatus were examined by unilateral tectal injections of horseradish peroxidase. Kutrogradely labeled cells were observed bilaterally throughout the spinal cord in all subdivisions of the trigeminal system, with the exception of nucleus principalis, which showed labeled cells only on the ipsilateral side. Labeling of the reticular formation occurred bilaterally in nucleus reticular is interiormagnocellularis, nucleus reticularis lateralis, nucleus reticularis medius and the mesencephalic reticular formation. The tectum also receives bilateral projections from the dorsal tegmentaJ field, the nucleus of the lateral lemniscus and nucleus isthmi, and ipsilateral projections from nucleus profundus mesencephali. A few labeled cells were found ipsilaterally in the locus coeruleus and in nuclei vestibulares ventrolateralis and centromedialis. In the diencephalon labeled cells were observed ipsilaterally in nucleus ventrolateralis thalami, nucleus ventromedialis thalami, nucleus suprapeduncularis, and in the dorsal and ventral lateral geniculate nuclei. Bilateral labeling was observed in nucleus periventricularis hypo-thalami. Furthermore, labeling was ipsilaterally present in the ventral telen-cephalic areas. The tectum in Python reticulatus receives a wide variety of afferent connections which confirm the role of the tectum as an integration center of visual and exteroceptive information.     

Axonal projection patterns of neurons in the secondary gustatory nucleus of channel catfish

The secondary gustatory nucleus of teleost fishes receives ascending fibers from the primary gustatory center in the medulla and sends efferent fibers to several nuclei in the inferior lobe of the diencephalon. Similar to the corresponding parabrachial nucleus in birds and mammals, the secondary gustatory nucleus of catfish consists of several cytoarchitectonically distinct subnuclei which receive input from different portions of the primary gustatory nuclei. However, it is unclear how the subnuclear organization relates to the processing of gustatory information in the hindbrain and the subsequent transmission of that information to the forebrain. To determine whether cells within different subnuclei of the secondary gustatory nucleus of channel catfish project to different diencephalic targets, single cells were intracellularly labeled with biocytin. Three subnuclei have been identified in the secondary gustatory nucleus: a medial subnucleus spanning most of the rostrocaudal extent of the nucleus, a central subnucleus and a dorsal subnucleus, the latter two located in the rostrolateral portion of the complex. Cells throughout the secondary gustatory nucleus typically possessed similar collateral projections to several nuclei in the inferior lobe, although four of the six cells filled in the medial subnucleus projected only to nucleus centralis. The only apparent subnucleus-specific projection pattern involved cells at the rostral edge of the secondary gustatory nucleus and in the secondary visceral nucleus. Axons of these cells terminated only in restricted portions of nucleus lobobulbaris. These results suggest that efferents from different subnuclei of the secondary gustatory nucleus of catfish, like those of the parabrachial nucleus of birds and mammals, do not possess simple, topographical projections to target nuclei in the diencephalon. © 1996 Wiley-Liss, Inc.     

Spinal and medullary input to the lateral cervical nucleus

The distributions of spinal and medullary cells projecting to the lateral cervical nucleus (LCN) have been investigated in young cats and dogs using the retrograde horseradish peroxidase (HRP) technique. Labeled spinal cells, whose axons contribute to the spinocervical tract (SCT), were found at all levels of the spinal cord ipsilateral to the injection sites. No significant differences were found between cat and dog, nor between cases with single injections at different levels of the LCN. SCT cells were found predominantly, in not exclusively, within lamina IV, with some extension into medial lamina V. No apparent mediolateral or dorsoventral density gradient was observed within lamina IV; cells of all sizes were labeled. Cells in cervical laminae I and V-VII were occasionally labeled; these, however, were considered to be propriospinal, supplying afferent fibers to the C_1–2 dorsal horn. Cells of origin of spinocerebellar fibers consistently remained unlabeled in cases with restricted HRP injections and minimal fiber damage in the dorsolateral funiculus (DLF) around the injection sites. These results, therefore, corroborate and refine the findings of electrophysiological studies of the SCT and the LCN. Labeled medullary cells were located in the caudoventral and rostral portions of the dorsal column nuclei (DCN; stellate and fusiform cells), the underlying n. medullae oblongatae centralis, subnucleus dorsalis (parvicellular medullary reticular formation), the marginal and magnocellular layers (both large and small cells) of the n. trigeminalis spinalis pars caudalis and also in pars interpolaris; a cluster of cells was also consistently labeled in the lateral reticular formation just ventral to pars caudalis. The projection from the DCN to the LCN was confirmed with the anterograde Nauta technique. Fiber degeneration was observed in the entire ipsilateral LCN, although it was less abundant than that observed in the adjacent C_1–2 dorsal horn. These results indicate that neurons in the rostral portions of the DCN not only may affect the input to the LCN (at the level of the dorsal horn), but also the output of the LCN itself. These data also suggest the possibility of both noxious and non-noxious facial input to the LCN.     

Output systems of the dorsal column nuclei in the cat

Numerous authors have demonstrated that the dorsal column nuclear complex (DCN) is functionally heterogeneous and has multiple terminal targets throughout the neuroaxis. In order to increase understanding of the functional significance of DCN's divergent connections, the present study used single and double light microscopic retrograde tracing strategies in the cat to characterize the location and morphology of DCN neurons that project to different portions of the diencephalon, rostral mesencephalon and spinal cord. These neuronal populations were then compared with those (previously reported from this and other laboratories) that project to the caudal mesencephalon, pons, inferior olive and cerebellum. When the results are considered together, a tentative picture of DCN emerges in which a population of clustered neurons that project exclusively to VPL form a core that is surrounded by and infiltrated with neurons projecting to other parts of the nervous system. Although the neuronal populations projecting to each of the different targets were individually separable anatomically by their location and/or morphological characteristics, previously reported physiological and other anatomical evidence permitted a preliminary grouping of these populations into 3 main systems. The first, a sensory tactile and kinesthetic 'cortical' system, consisted of 3 components: a double core of round, clustered medium-sized neurons (one each in the gracile and cuneate nuclei) and a variform rostral group projecting to the ventroposterolateral nucleus (VPL), a ventral group of unclustered large round neurons in the middle cuneate nucleus and a dense group of neurons in nucleus Z projecting to VPL's border with the ventrolateral nucleus (VPL/VL), and a group of mainly small-sized neurons located between the clusters of neurons or in the thin dorsal rim around the caudal and middle portions of the double cores and a populous, variform rostral group projecting indirectly (and possibly directly) to the posterior group through the intercollicular region of the tectum. The second, a sensorimotor 'cerebellar' system, consisted of multiple, subtly separable populations of neurons with different morphological characteristics all of which were located in different parts of the complex region that surrounds the cores on all sides. These neurons projected to restricted portions of interconnected targets within the zona incerta, tectum, pretectum, red nucleus, pontine grey, pontine raphe, inferior olive, and cerebellum.(ABSTRACT TRUNCATED AT 400 WORDS)     

Descending connections from the brainstem to the spinal cord in the electric fish Eigenmannia. Quantitative description based on retrograde horseradish peroxidase and fluorescent-dye transport

The descending connections from the brainstem to the spinal cord in Eigenmannia sp. were demonstrated using the horseradish peroxidase (HRP) technique. The spinal cord was transected and HRP crystals were deposited in the cut. The point of transection was located at varying distances from the head in different specimens. In all experiments, cells were labeled in both the rhombencephalic and mesencephalic tegmentum. No labeled cells were found in the cerebellum, the lateral-line lobes, the torus semicircularis, the tectum mesencephali, the hypothalamus, the diencephalon or the telencephalon. Labeled neurons were found in the ventrolateral column, nucleus formation reticularis (NFR) inferior, NFR medius, NFR superior pars superior and pars suprema, NFR tegmenti mesencephali lateralis, nucleus vestibularis magnocellularis and nucleus fasciculi longitudinalis medialis. Furthermore, the Mauthner cells and the neurons of the pacemaker nucleus were filled with HRP granules. The neurons labeled were predominantly the large ones of more than 25 microns in diameter which are very conspicuous along the brainstem. The number of these neurons in the different nuclei varied from animal to animal, however, the number of labeled neurons increased monotonically at a similar rate in all brainstem nuclei with more rostrally located transection sites. In a second series, the number of neurons terminating in a small number of segments independent of absolute position along the body axis was assessed using two different fluorescent dyes. Within tolerable statistical limits, this number was found to be constant, corroborating the data obtained with HRP. A possible interpretation of the data is placed in the context of physiological data previously presented.     

Output systems of the dorsal column nuclei in the cat

Numerous authors have demonstrated that the dorsal column nuclear complex (DCN) is functionally heterogeneous and has multiple terminal targets throughout the neuroaxis. In order to increase understanding of the functional significance of DCN's divergent connections, the present study used single and double light microscopic retrograde tracing strategies in the cat to characterize the location and morphology of DCN neurons that project to different portions of the diencephalon, rostral mesencephalon and spinal cord. These neuronal populations were then compared with those (previously reported from this and other laboratories) that project to the caudal mesencephalon, pons, inferior olive and cerebellum. When the results are considered together, a tentative picture of DCN emerges in which a population of clustered neurons that project exclusively to VPL form a core that is surrounded by and infiltrated with neurons projecting to other parts of the nervous system.Although the neuronal populations projecting to each of the different targets were individually separable anatomically by their location and/or morphological characteristics, previously reported physiological and other anatomical evidence permitted a preliminary grouping of these populations into 3 main systems (see Fig. 8). The first, a sensory tactile and kinesthetic ‘cortical’ system, consisted of 3 components: (a) a double core of round, clustered medium-sized neurons (one each in the gracile and cuneate nuclei) and a variform rostral group projecting to the ventroposterolateral nucleus (VPL), (b) a ventral group of unclustered large round neurons in the middle cuneate nucleus and a dense group of neurons in nucleus Z projecting to VPL's border with the ventrolateral nucleus (VPL/VL), and (c) a group of mainly small-sized neurons located between the clusters of neurons or in the thin dorsal rim around the caudal and middle portions of the double cores and a populous, variform rostral group projecting indirectly (and possibly directly) to the posterior group through the intercollicular region of the tectum⁶⁹. The second, a sensorimotor ‘cerebellar’ system, consisted of multiple, subtly separable populations of neurons with different morphological characteristics all of which were located in different parts of the complex region that surrounds the cores on all sides. These neurons projected to restricted portions of interconnected targets³¹ within the zona incerta, tectum¹¹, pretectum, red nucleus, pontine grey⁸², pontine raphe⁷⁹, inferior olive^90,91 and cerebellum³⁴. The third, a feedback ‘spinal’ system, consisted of morphologically diverse, but predominantly large neurons located mainly ventrally between the cuneate and gracile nuclei projecting to the spinal cord.Thus, the cuneate and gracile nuclei not only appear to serve a role in tactile sensation and kinesthesia by topographically relaying precise cutaneous and proprioceptive information through VPL and VPL/VL to the cerebral cortex, but they also appear to serve complex, feedback-regulated roles in sensorimotor integration by systematically channeling and transmitting a variety of other types of somatic information to other parts of the nervous system.     

Serotoninergic innervation of the dorsal column nuclei and its relation to cytoarchitectonic subdivisions: An immunohistochemical study in cats and monkeys (Aotus trivirgatus)

The serotoninergic innervation of the dorsal column nuclei (DCN) was investigated in cats and owl monkeys (Aotus trivirgatus) with immunohistochemical methods. A dense network of serotonin-immunoreactive fibers was present in the reticular regions of DCN in cats, and in the pars triangularis of the cuneate nucleus and the peripheral and caudal regions of the gracile nucleus in owl monkeys. The cat's cluster regions and the monkey's rotund regions were more sparsely innervated. Electron microscopic examination showed that the labeled fibers were thin and unmyelinated. Vesicle-containing, terminal-like structures were small. They were in contact with dendrites, other terminals and cell bodies, but synapses were rare. The results demonstrate that the serotoninergic projection to the DCN in both cats and owl monkeys is heterogeneously distributed in a pattern that is faithfully related to the cytoarchitectonic subdivisions of the DCN. The densely innervated reticular regions in the DCN of cats and the corresponding regions in monkeys are predominantly involved in the processing of sensory information to the cerebellum, either directly, or indirectly through projections to the inferior olive, pontine gray, tectum, pretectum, red nucleus, or zona incerta. Thus, the present findings suggest that the serotoninergic innervation of the DCN is primarily related to the DCN's involvement in motor functions. © 1993 Wiley-Liss, Inc.     

Origins of projections from the inferior colliculus to the cochlear nucleus in guinea pigs

We used retrograde tracing techniques to examine the projections from the inferior colliculus to the cochlear nucleus in guinea pigs. Following injection of a retrograde tracer into one cochlear nucleus, labeled cells were found bilaterally in all subdivisions of the inferior colliculus. The majority of cells were located in the central nucleus and external cortex; relatively few cells were located in the dorsal cortex. Multipolar (stellate) cells were labeled in all subdivisions of the inferior colliculus. In the central nucleus, disk-shaped cells were also labeled. To determine whether individual collicular neurons send collateral projections to the cochlear nuclei on both sides, we injected different fluorescent tracers into left and right cochlear nuclei in the same animal. The inferior colliculi contained very few double-labeled cells, indicating that the projections to ipsilateral and contralateral cochlear nuclei originate from separate populations of cells.     

Connections of the auditory midbrain in a teleost fish, Cyprinus carpio

Central auditory pathways were traced in Japanese carp, Cyprinus carpio, using electrophysiological mapping and HRP tract-tracing methods. Multiunit recordings made from the carp torus semicircularis, the major midbrain area for processing octavolateralis information, revealed a mediolateral segregation of auditory and lateral line sensory modalities. Iontophoretic injections of HRP were made into the medial torus to trace afferent and efferent projections of the carp auditory midbrain. Following unilateral HRP injections into the medial torus, retrogradely labeled neurons were observed within six nuclei of the carp medulla. Two octaval nuclei, the anterior octavus nucleus and descending octavus nucleus, contained HRP-filled neurons. Labeled neurons were also observed within the ipsilateral superior olive, scattered among fibers of both lateral lemnisci, and bilaterally within the medullary reticular formation. In addition, bilateral retrograde cell labeling was found within a group of Purkinje-like cells located adjacent to the IVth ventricle, just rostral to the level of the VIIIth nerve. Few labeled neurons were found within the nucleus medialis, a principal target for lateral line afferents within the medulla. At midbrain levels, retrogradely labeled neurons were observed within the contralateral torus semicircularis and the ipsilateral optic tectum. Three forebrain nuclei project to the carp auditory midbrain. Within the diencephalon, descending projections originate from the anterior tuberal nucleus, bilaterally, and from the ipsilateral central posterior thalamic nucleus. The ipsilateral caudal telencephalon also projects to the carp auditory midbrain via large multipolar neurons within area dorsalis pars centralis. Anterograde labeling of fibers and terminals revealed efferent projections of the carp auditory midbrain to the following targets: the ipsilateral superior olive, the ipsilateral medullary reticular formation, the deep layers of the optic tectum, the contralateral torus semicircularis, the anterior tuberal nucleus, and the central posterior thalamic nucleus. These results, together with recent studies of lateral line pathways in teleosts (Finger, '80, '82a), demonstrate that central auditory and lateral line pathways are anatomically distinct in the carp, at least from medullary to diencephalic levels. Furthermore, there are striking similarities in the organization of the central auditory pathways of the carp and those of amphibians and land vertebrates.     

Somatosensory projection to the mesencephalon: an anatomical study in the monkey

The terminal areas and cells of origin of the somatosensory projection to the mesencephalon in the monkey were investigated by the intraaxonal transport method. Following injection of wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) into the spinal enlargements, the lateral cervical nucleus (LCN), the dorsal column nuclei (DCN), or the spinal trigeminal nucleus, anterograde labeling was observed in several regions of the mid-brain. (1) Injection of tracer into the spinal enlargements resulted in dense terminal labeling in the parabrachial nucleus (PBN) and the periaqueductal gray matter (PAG); moderate termination was observed in the intercollicular nucleus (Inc), the intermediate and deep gray layers of the superior colliculus (SGI, SGP), the posterior pretectal nucleus (PTP), and the nucleus of Darkschewitsch (D); and scattered terminal fibers were seen in the cuneiform nucleus (CNF) and the pars compacta of the anterior pretectal nucleus (PTAc). The projections from the cervical enlargement to PAG, Inc, and the superior colliculus terminated more rostrally than those from the lumbar segments, indicating a somatotopic organization. (2) Terminal labeling after injection of tracer into LCN was found mainly in Inc, SGI, and SGP, but sparse labeling was also observed in the nucleus of the brachium of the inferior colliculus (BIN), PAG, PBN, PTP, and D. (3) The projection from DCN terminated densely in the external and pericentral nuclei of the inferior colliculus (ICX, ICP), Inc, SGI, SGP, PTP, PTAc, the nucleus ruber, and D, and weak terminal labeling was seen in BIN, PAG, and PBN. Comparisons of the anterograde labeling following injections involving both the gracile nucleus and the cuneate nucleus with that after injection restricted to the gracile nucleus alone suggested a somatotopic termination pattern in Inc, the superior colliculus, and the pretectal nuclei. (4) The patterns of projection from the laminar and alaminar parts of the spinal trigeminal nucleus differed: injection of tracer into the caudal part of the alaminar spinal trigeminal nucleus (nucleus interpolaris) resulted in dense anterograde labeling in SGI and SGP, moderate termination in Inc, and minor projections to PBN, PAG, and PTP, whereas after tracer injection into the laminar trigeminal nucleus (nucleus caudalis) terminal labeling was present only in PBN and PAG. Following injection of tracer into the midbrain terminal areas retrogradely labeled neurons were found in the spinal cord, LCN, DCN, and the spinal trigeminal nucleus, with the majority of labeled cells situated on the side contralateral to the injection site.(ABSTRACT TRUNCATED AT 400 WORDS)     

The origins of descending spinal projections in lepidosirenid lungfishes

The origins of descending spinal projections in the lepidosirenid lungfishes were identified by retrograde transport of horseradish peroxidase (HRP) introduced into the rostral spinal cords of juvenile African (Protopterus annectans and Protopterus amphibians) and South American (Lepidosiren paradoxa) lungfishes. Standard HRP histochemistry revealed retrogradely labeled neurons in the nucleus of the medial longitudinal fasciculus, midbrain tegmentum, red nucleus, optic tectum, mesencephalic trigeminal nucleus, granule cell layer of the cerebellum, superior, middle, and inferior medullary reticular nuclei, magnocellular and descending octaval nuclei, region of the descending trigeminal tract, solitary complex, and the margins of the spinal gray matter anterior to the spinal HRP implant. A small number of retrogradely labeled neurons were also present in the ventral thalamus of Protopterus. A descending spinal projection from the forebrain was not evident in either genus of lepidosirenid lungfishes. The presence of projections to the spinal cord from the diencephalon, medial reticular formation of the midbrain and medulla, octaval (vestibular) nuclei, solitary complex, and probable nucleus of the descendin trigeminal tract in lungfishes and their overall similarity to comparable projections in other vertebrates suggest that these pathways are among those representative of the primitive pattern of descending spinal projections in vertebrates.     

Axonal transport in serotonin neurons of the midbrain raphe.

The projections of serotonin-containing neurons of the midbrain raphe nuclei (nucleus raphe dorsalis, nucleus centralis superior) are studied by analysis of axonal transport of labeled amino acids. These results are correlated with regional alterations of serotonin content following midbrain raphe lesions which produce significant serotonin depletion in nearly all regions of the central nervous system. Twenty-four hours following injection of 100 muCi [3H]proline, raphe neurons have taken up labeled material and transported it, presumably as protein, to telencephalon, diencephalon, brain stem, the cerebellum and the spinal cord. This transport appears to take place predominantly in serotonin neurons. After injection of 100 muCi [3H]5-HTP into nucleus raphe dorsalis or nucleus centralis superior, the pattern of regional distribution of transported material is very similar to that obtained with tritiated proline. Selective lesions of serotonin terminals with 5.6-DHT result in greatly diminished axonal transport of proteins to all telencephalic, diencephalic and mesencephalic areas as well as to cerebellum, pons-medulla and spinal cord. Unilateral destruction of the medial forebrain bundle results in significant reduction in axonal transport of labeled material to ipsilateral telencehalon and thalamus. These results provide further support for the view that serotonin neurons of the midbrain raphe nuclei project widely throughout the neuraxis to telencephalon, diencephalon, brain stem, cerebellum and spinal cord.     

Subcortical projections to the pontine nuclei in the cat

In a systematic attempt to trace all projections from the brainstem and diencephalon to the pontine nuclei of the cat, implantations and injections of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) or Fluoro-Gold were placed in the pontine nuclei of 21 cats. In most of the cases there was no evidence of spread of tracer outside the pontine nuclei. Retrogradely labeled cells in the brainstem and diencephalon were carefully mapped and counted. The number labeled cells in the brainstem and diencephalon ranged from 24 in cases with very small implantations to 3,490 in cases with large injections in the pontine nuclei (counts from every fifth section). The labeled cells are located bilaterally with an ipsilateral preponderance. After large injections, 25-38% of the labeled cells were located in the brainstem reticular formation, 10-16% in the pretectal nuclei, 10-15% in the hypothalamus, 7-9% in the zona incerta, 3-9% in the fields of Forel, 4-5% in the nucleus locus coeruleus, 3-5% in the ventral lateral geniculate body, 2-4% in the superior colliculus, 3% in the periaqueductal gray, and 14-15% in other parts of the brainstem. Judging from cases with small tracer deposits entirely confined to the pontine nuclei, there appear to be two types of subcortical inputs: Projections from the reticular formation, the nucleus locus coeruleus, the periaqueductal gray, and the raphe nuclei are widespread, presumably reaching all parts of the pontine nuclei, while projections from a diversity of other sources are localized, reaching limited parts of the pontine nuclei only or predominantly.     

Neural connections of the torus semicircularis in the adult Zebrafish

The torus semicircularis (TS) of teleosts is a key midbrain center of the lateral line and acoustic sensory systems. To characterize the TS in adult zebrafish, we studied their connections using the carbocyanine tracers applied to the TS and to other related nuclei and tracts. Two main TS nuclei, central and ventrolateral, were differentiable by their afferent connections. From central TS, (TSc) numerous toropetal cells were labeled bilaterally in several primary octaval nuclei (anterior, magnocellular, descending, and posterior octaval nuclei), in the secondary octaval nucleus, in the caudal octavolateralis nucleus, and in the perilemniscular region. In the midbrain, numerous toropetal cells were labeled in the contralateral TSc. In the diencephalon, toropetal cells labeled from the TSc were observed ipsilaterally in the medial prethalamic nucleus and the periventricular posterior tubercle nucleus. TSc toropetal neurons were also labeled bilaterally in the hypothalamic anterior tuberal nucleus (ATN) and ipsilaterally in the parvicellular preoptic nucleus but not in the telencephalon. Tracer application to the medial octavolateralis nucleus revealed contralateral projections to the ventrolateral TS (TSvl), whereas tracer application to the secondary octaval nucleus labeled fibers bilaterally in TSc and neurons in rostral TSc. The TSc sends ascending fibers to the ipsilateral lateral preglomerular region that, in turn, projects to the pallium. Application of DiI to the optic tectum labeled cells and fibers in the TSvl, whereas application of DiI to the ATN labeled cells and fibers in the TSc. These results reveal that the TSvl and TSc are mainly related with the mechanosensory lateral line and acoustic centers, respectively, and that they show different higher order connections.     

Dorsal cochlear nucleus projections to the inferior colliculus in the cat: A light and electron microscopic study

In the Present report retrograde and anterograde labeling techniques are used to study the Projections of the dorsal cochlear nucleus (DCN) to the inferior colliculus in the cat. Horseradish peroxidase (HRP) or wheat germ agglutinin (WGA-HRP) injections into the inferior colliculus produce large numbers of labeled neurons in the DCN on the opposite side. Labeled cells with projections to the colliculus are identified as fusiform and giant cells and are organized into rostrocaudal bands. The axons of these DCN neurons are labeled by anterograde transport of ³H-leucine and/or proline and studied in light and electron microscopic autoradiographs. Axons from the DCN terminate within the central nucleus of the inferior colliculus in densely labeled, rostrocaudally oriented bands. Less heavily labeled extensions of these bands are found in the deepest layer of the dorsal cortex, and light labeling is found adjacent to the bands in the central nucleus and in the ventrolateral nucleus. Cells in the dorsomedial DCN project to the most ventromedial part of the central nucleus while progressively more ventrolateral cells in the DCN project to more dorsolateral parts of the central nucleus. This present evidence suggests that the DCN sends afferents to only two of the four subdivisions of the central nucleus. Within these regions, the axons from the DCN form terminal boutons or boutons de passage characterized by medium-sized, round synaptic vesicles. The labeled endings nearly always make asymmetric synaptic contacts on the dendrites of disc-shaped and stellate cells in the central nucleus. A few axosomatic contacts are found on one particular cell type, possibly the stellate variety. The results support the hypothesis that each subdivision of the central nucleus receives afferents from a different set of cell types in the auditory nuclei of the lower brainstem. The banding patterns of the efferent cells in the cochlear nucleus and the axons within the central nucleus suggest that these inputs are congruent to the fibrodendritic layers of the central nucleus and may contribute to tonotopic organization in the central nucleus. Finally, the results suggest that each of the two major classes of cells in the central nucleus receives different patterns of inputs from the DCN. These morphological differences could contribute to different electrophysiological responses to the sound stimuli by these cells.     

Auditory projections from the cochlear nucleus to pontine and mesencephalic reticular nuclei in the rat.

We investigated projections from the cochlear nucleus in the rat using the anterograde tracer Phaseolus vulgaris-leucoagglutinin. We focused on nuclei in the brainstem which are not considered to be part of the classical auditory pathway. In addition to labeling in auditory nuclei, we found presumed terminal fibers in 4 pontine and mesencephalic areas: (1) the pontine nucleus (PN), which receives bilateral projections from the antero- and posteroventral cochlear nuclei; (2) the ventrolateral tegmental nucleus (VLTg), which receives a contralateral projection from the rostral portion of the anteroventral cochlear nucleus; (3) the caudal pontine reticular nucleus (PnC), which receives bilateral input originating predominantly in the dorsal cochlear nucleus; and (4) the lateral paragigantocellular nucleus (LPGi), which receives projections from all subdivisions of the cochlear nuclei. In the VLTg and PnC, anterogradely labeled varicose axons were often found in close apposition to the primary dendrites and somata of large reticular neurons. Injections of the retrograde fluorescent tracer Fluoro-Gold into the VLTg demonstrated that the neurons of origin are mainly located contralaterally in the rostral anteroventral cochlear nucleus and in the cochlear root nucleus. The relevance of these auditory projections for short-latency audio-motor behaviors and acoustically elicited autonomic responses is discussed.     

Changes in the topographically organized connections between the nucleus isthmi and the optic tectum after partial tectal ablation in adult goldfish

The projection of the nucleus isthmi to the ipsilateral optic tectum was examined in normal goldfish. This was compared to the projection in animals in which the entire visual field had been induced to compress onto a rostral half tectum by caudal tectal ablation. The isthmo-tectal projection was examined by making localized injections of horseradish peroxidase into the optic tecta and observing the patterns of labeled cells within the nucleus isthmi. The teleost nucleus isthmi consists of a cell sparse medulla covered by a cellular cortex, which is thick on the rostral, medial, and dorsal surfaces of the nucleus. Almost all isthmic cells projecting to the tectum were located in the area of thick cortex. In normal fish, rostral tectal injections labeled cells in the rostroventral portion of the thick cortex; injections midway in the rostrocaudal tectal axis labeled more caudodorsally located cells, and caudal tectal injections labeled cells a little further caudally in extreme dorsal cortex. The rostroventral to caudodorsal isthmic axis was therefore seen to project rostrocaudally along the tectum. This topography contrasts somewhat with the situation seen in amphibia where the rostrocaudal tectal axis receives projections from the rostrocaudal isthmic axis. In fish with half-tectal ablations, injections near the caudal edge of the half tectum (at a site that had originally been midtectal) labeled cells that had previously projected to caudal tectum. Rostral tectal injections in fish with compression of the visual field gave a normal pattern of labeled isthmic cells. The results indicate that a topographically ordered isthmo-tectal projection exists in goldfish that may be induced to compress onto a half tectum.     

Single axonal characterization of trigeminocerebellar projection patterns in the mouse

The cerebellar projection from the trigeminal nuclear complex is one of the major populations of the cerebellar inputs. Although this projection is essential in cerebellar functional processing and organization, its morphological organization has not been systematically clarified. The present study addressed this issue by lobule-specific retrograde neuronal labeling and single axonal reconstruction with anterograde labeling. The cerebellar projection arose mainly from the interpolaris subdivision of the spinal trigeminal nucleus (Sp5I) and the principal trigeminal sensory nucleus (Pr5). Although crus II, paramedian lobule, lobule IX, and simple lobule were the major targets, paraflocculus, and other lobules received some projections. Reconstructed single trigeminocerebellar axons showed 77.8 mossy fiber terminals on average often in multiple lobules but no nuclear collaterals. More terminals were located in zebrin-negative or lightly-positive compartments than in zebrin-positive compartments. While Pr5 axons predominantly projected to ipsilateral crus II, Sp5I axons projected either predominantly to crus II and paramedian lobule often bilaterally, or predominantly to lobule IX always ipsilaterally. Lobule IX-predominant-type Sp5I neurons specifically expressed Gpr26. Gpr26-tagged neuronal labeling produced a peculiar mossy fiber distribution, which was dense in the dorsolateral lobule IX and extending transversely to the dorsal median apex in lobule IX. The projection to the cerebellar nuclei was observed in collaterals of ascending Sp5I axons that project to the diencephalon. In sum, multiple populations of trigeminocerebellar projections showed divergent projections to cerebellar lobules. The projection was generally complementary with the pontine projection and partly matched with the reported orofacial receptive field arrangement.