Ultrastructural organization of the retino-pretecto-olivary pathway in the rabbit: a combined WGA-HRP tracing and GABA immunocytochemical study.

The ultrastructural organization of the pretecto-olivary projection neurons within the nucleus of the optic tract and dorsal terminal accessory optic nucleus of rabbits was studied by using anti-GABA immunolabelling and retrograde transport of WGA-HRP. GABA-like immunoreactivity was determined with a postembedding colloidal gold technique. WGA-HRP was injected in the dorsal cap of the inferior olive. The WGA-HRP-labelled neurons were incubated with gold-substituted silver peroxidase. Neurons projecting to the inferior olive had large to medium-sized cell bodies and were GABA negative. In the nucleus of the optic tract, projection neurons are found in the rostral parts, while the majority of the local GABAergic interneurons are mainly found in the caudal parts. In the dorsal terminal nucleus both types of neurons are intermingled. The projection neurons were frequently in synaptic contact by GABAergic terminals. These neurons also receive retinal afferents indicating the existence of a two-step synaptic connection from the retina to the inferior olive. It is suggested that this class of projection neurons forms the "direction-selective" neurons that can be antidromically stimulated from the inferior olive. The GABAergic terminals on the identified projection neurons are of axonal origin (F-terminals), whereas presynaptic dendrites of interneurons (P-terminals) were seldom observed to be in synaptic contact with retrogradely labelled profiles. The strong input of GABA on direction-selective neurons indicates that GABA is directly involved in modulating retinal signals to the inferior olive.     

The distribution of glutamic acid decarboxylase immunoreactivity in the diencephalon of the opossum and rabbit

We have examined the distribution of neurons and terminals immunoreactive for glutamic acid decarboxylase (GAD) in the thalamus and adjacent structures of the opossum (Didelphis virginiana) and the rabbit and have compared this distribution with the distributions we described previously for the cat and bushbaby (Galago senegalensis). The significance of these experiments depends, first, on the fact that GAD is the synthetic enzyme for GABA, and therefore that GAD immunoreactivity is a marker for GABAergic inhibitory neurons, and second, on previous findings that suggest that GABAergic neurons in the dorsal thalamus are local circuit neurons. In both cat and Galago, GAD-immunoreactive neurons are distributed essentially throughout the entire thalamus. In the opossum, GAD neurons are chiefly confined to the dorsal lateral geniculate nucleus and the lateral extremity of the lateral posterior nucleus. The distribution of GAD neurons in the rabbit is intermediate between that found in the opossum on the one hand and cat and Galago on the other. Like opossum, about 25% of the neurons in the lateral geniculate nucleus of rabbit are GAD immunoreactive. Unlike opossum, however, as many as 18% of the cells in the ventral posterior nucleus of the rabbit are GAD immunoreactive, and scattered cells are also labeled in other thalamic areas, such as the medial geniculate and the lateral group. Aside from the findings in the dorsal thalamus, the chief observation is that GAD-immunoreactive neurons and/or terminals densely fill all principal targets of the optic tract, including the ventral lateral geniculate nucleus; the superficial gray layer of the superior colliculus; the anterior, posterior, and olivary pretectal nuclei; the nucleus of the optic tract; and the medial and lateral terminal nuclei of the accessory optic tract. These results support the idea first put forward by Cajal that local circuit neurons increase in number during the course of the evolution of complex mammalian brains. If we can assume that the conservative opossum retains characteristics reflecting an early stage of mammalian evolution, the results suggest that thalamic local circuit neurons arose first in the visual system and only later in evolution spread throughout the thalamus.     

Retinal Projections to the Pretectum, Accessory Optic System and Superior Colliculus in Pigmented and Albino Ferrets

Retinal projections to the pretectal nuclei, accessory optic system and superior colliculus in pigmented and albino ferrets were studied using anterograde tracing techniques. Both Nissl- and myelin-stained material was used to identify the pretectal nuclei, nuclei of the accessory optic system and the layers of the superior colliculus. Following monocular injection of either horseradish peroxidase or rhodamine-B-isothiocyanate, four pretectal nuclei, including the nucleus of the optic tract, posterior pretectal nucleus, anterior pretectal nucleus and the olivary pretectal nucleus, could be identified to receive direct retinal input in both pigmented and albino strains. In the accessory optic system, retinal terminals were observed in the dorsal, lateral and medial terminal nuclei as well as in the interstitial nucleus of the superior fasciculus, posterior fibres. The retinal projection to the superior colliculus was found to innervate the three superficial layers. The retinal projections to the pretectal nuclei and nuclei of the accessory optic system in the pigmented animals were bilateral, although the label was most dense contralateral to the injected eye. Ipsilateral retinal projections to the pretectal nuclei and nuclei of the accessory optic system appeared to be absent in albino ferrets, i.e. they were invisible with our methods. In both pigmented and albino ferrets retinal terminals in the contralateral superior colliculus densely innervated the three superficial layers. In both strains the ipsilateral projection appeared as clusters which were absent in rostral and caudal poles. In pigmented animals the ipsilateral projection was much denser and more extensive than in albinos. Following injection of retrograde tracers into the brainstem at the level of the dorsal cap of the inferior olive, retrogradely labelled neurons in the pretectum were found in the ipsilateral nucleus of the optic tract. Their somata overlapped mainly with scattered retinal terminals close to the pretectal surface and rarely or not all with the deeper prominent terminal clusters. In the accessory optic system, inferior olive projecting neurons were observed in all four ipsilateral nuclei and fully coincided with the retino-recipient zones. In the superior colliculus, retrogradely labelled neurons were found contralateral to the injection site in the deep layers.     

Efferent projections of the intergeniculate leaflet and the ventral lateral geniculate nucleus in the rat

The intergeniculate leaflet (IGL) and the ventral lateral geniculate nucleus (VLG) are ventral thalamic derivatives within the lateral geniculate complex. In this study, IGL and VLG efferent projections were compared by using anterograde transport of Phaseolus vulgaris-leucoagglutinin and retrograde transport of FluoroGold. Projections from the IGL and VLG leave the geniculate in four pathways. A dorsal pathway innervates the thalamic lateral dorsal nucleus (VLG), the reuniens and rhomboid nuclei (VLG and IGL), and the paraventricular nucleus (IGL). A ventral pathway runs through the geniculohypothalamic tract to the suprachiasmatic nucleus and the anterior hypothalamus (IGL). A medial pathway innervates the zona incerta and dorsal hypothalamus (VLG and IGL); the lateral hypothalamus and perifornical area (VLG); and the retrochiasmatic area (RCA), dorsomedial hypothalamic nucleus, and subparaventricular zone (IGL). A caudal pathway projects medially to the posterior hypothalamic area and periaqueductal gray and caudally along the brachium of the superior colliculus to the medial pretectal area and the nucleus of the optic tract (IGL and VLG). Caudal IGL axons also terminate in the olivary pretectal nucleus, the superficial gray of the superior colliculus, and the lateral and dorsal terminal nuclei of the accessory optic system. Caudal VLG projections innervate the lateral posterior nucleus, the anterior pretectal nucleus, the intermediate and deep gray of the superior colliculus, the dorsal terminal nucleus, the midbrain lateral tegmental field, the interpeduncular nucleus, the ventral pontine reticular formation, the medial and lateral pontine gray, the parabrachial region, and the accessory inferior olive. This pattern of IGL and VLG projections is consistent with our understanding of the distinct functions of each of these ventral thalamic derivatives.     

Connections of the pretectum in the cat.

The connections of the pretectal complex in the cat have been examined by anatomical methods which utilize the anterograde axonal transport of tritiated proteins or the retrograde axonal transport of the enzyme horseradish peroxidase. Following injections of tritiated amino acids into the eye, label can be seen in the contralateral and ipsilateral nucleus of the optic tract and olivary nucleus where it appears as two or three finger-like strips. Following large injections of tritiated amino acids into the pretectal complex transported label accumulates ipsilaterally in a region dorsolateral to the red nucleus, the central and pericentral divisions of the tegmental reticular nucleus, the intermediate layers of the superior colliculus, the nucleus of Darkschewitch, the thalamic reticular nucleus, zona incerta and fields of Forel, the central lateral nucleus, the pulvinar nucleus and the ventral lateral geniculate nucleus. Contralaterally label accumulates in the nucleus of the posterior commissure, the interstitial nucleus of Cajal, the anterior, posterior and medial pretectal nuclei, and the ventral lateral geniculate nucleus From smaller injections, more or less well confined to single nuclei, the following patterns of connections are demonstrated. The nucleus of the optic tract projects to the ipsilateral ventral lateral geniculate nucleus and pulvinar nucleus and to the contralateral nucleus of the posterior commissure. The anterior pretectal nucleus projects to the ipsilateral central lateral nucleus, the reticular nucleus, zona incerta, fields of Forel, the region dorsolateral to the red nucleus and to the contralateral anterior pretectal nucleus. The posterior pretectal nucleus seems to project only to the ipsilateral reticular nucleus and zona incerta. The central tegmental fields deep to the pretectum project to the tegmental reticular nucleus of the brainstem. When the injection involves the nucleus of the posterior commissure label is seen in the ipsilateral nucleus of Darkschewitch, and in the contralateral nucleus of the posterior commissure and interstitial nucleus of Cajal but no nucleus of the pretectum could be positively identified as projecting to any of the motor nuclei of cranial nerves III, IV, and VI. Following large injections of horseradish peroxidase into the pretectal complex, labeled cells are seen in the superficial layers of the ipsilateral superior colliculus, in the ipsilateral ventral lateral geniculate nucleus, reticular nucleus and zona incerta and in the contralateral anterior, medial and posterior pretectal nuclei, nucleus of the optic tract and ventral lateral geniculate nucleus.     

Dendritic morphology of projection neurons in the cat pretectum

The distribution and dendritic morphology of neurons in the cat pretectal nuclear complex were analyzed with respect to their projection to the ipsilateral dorsal lateral geniculate nucleus (LGNd) and the ipsilateral inferior olive (IO). Single and double retrograde tracing techniques were combined with intracellular injections of either horseradish peroxidase into electrophysiologically identified pretectal neurons or Lucifer Yellow into retrogradely labeled somata. Pretectal cells afferent to the LGNd were located in the nucleus of the optic tract (NOT), adjacent dorsal terminal nucleus of the accessory optic system (DTN), and posterior pretectal nucleus (NPP). Cells projecting to the IO were also distributed throughout the NOT-DTN and dorsal part of the NPP. Separate tracer injections (fluorogold and horseradish peroxidase [HRP] or granular blue) into the LGNd and the IO showed considerable overlap of labeled neurons in the NOT and dorsal NPP. Double-labeled neurons, however, were not observed after double tracer injections into LGNd and IO. Partial topographical segregation of the two populations was observed along the dorsoventral axis because LGNd-projecting neurons exhibited maximum density ventral to that of IO neurons. Pretectal cells to the LGNd had cell body diameters between 16 and 48 μm. Somatic shapes varied between fusiform and multipolar with considerable overlap between these two morphological appearances. Neurons projecting to the IO exhibited similar cell body sizes and their morphology also varied from fusiform to multipolar. Quantitative analysis of dendritic field size and orientation, number and order of dendritic arborizations, and symmetry of the dendritic tree revealed no statistically significant difference between the two neuronal populations. Hence, neurons of the two populations cannot be unequivocally identified just from the dendritic morphology. By contrast, dendritic morphology was correlated with the topographical location of either cell type within the pretectal nuclei rather than projection. Thus, the morphological appearance of neurons located dorsally predominantly was fusiform while neurons located ventrally mostly were multipolar. © 1996 Wiley-Liss, Inc.     

An electron microscope study of the cortico-pretecto-olivary projection in the cat by a combined degeneration and horseradish peroxidase tracing technique

The anterior pretectal nucleus (PTA) of the cat was observed electron microscopically after motor cortical ablation and horseradish peroxidase (HRP) injection into the inferior olive of the same animal. Direct synaptic connections were found between degenerated cortico-pretectal axon terminals and dendrites of pretecto-olivary projection neurons retrogradely labeled with HRP in the ventrolateral part of the PTA. Therefore, this combined method revealed that the PTA is a relay station of the cortico-olivary projection.     

Combined GABA-immunocytochemistry and TMB-HRP histochemistry of pretectal nuclei projecting to the inferior olive in rats, cats and monkeys

Cells in the pretectal nucleus of the optic tract (NOT) of rats, cats and monkeys were retrogradely labeled with horseradish peroxidase (HRP) stereotaxically injected into the inferior olive (IO). A procedure for stabilizing the tetramethylbenzidine (TMB)-HRP reaction product was used to visualize combined TMB-HRP and immunohistochemically localized gamma-aminobutyric acid (GABA) in the same sections. Positive GABAergic reaction product was found to be restricted to smaller-size intrinsic neurons. Larger NOT cells projecting to the IO were consistently free of GABA reaction product and, in addition, appeared to be contacted by relatively few GABAergic terminals.     

Oxygen toxicity of hippocampal tissue in vitro

The anterior pretectal nucleus (PTA) of the cat was observed electron microscopically after motor cortical ablation and horseradish peroxidase (HRP) injection into the inferior olive of the same animal. Direct synaptic connections were found between degenerated cortico-pretectal axon terminals and dendrites of pretecto-olivary projection neurons retrogradely labeled with HRP in the ventrolateral part of the PTA. Therefore, this combined method revealed that the PTA is a relay station of the cortico-olivary projection.     

Visual pretectal neurons projecting to the dorsal lateral geniculate nucleus and pulvinar nucleus in the cat

We observed morphological subtypes of visual pretectal neurons ascending to the dorsal thalamus, following injections of wheat germ agglutinin conjugated to horseradish peroxidase into the dorsal lateral geniculate nucleus (LGNd) or the pulvinar nucleus. These neurons are composed of fusiform cells and small-sized multipolar cells in the olivary pretectal nucleus, superficial horizontal cells, fusiform cells, small-, medium- and large-sized multipolar cells in the optic tract nucleus, and small- and medium-sized multipolar cells in the posterior pretectal nucleus. When somal size of the neurons projecting to the LGNd was compared to the size of neurons projecting to the pulvinar, the neuronal groups were not identical.     

An autoradiographic study of the projections from the lateral geniculate body of the rat.

The projections from the lateral geniculate body of the rat were followed using the technique of autoradiography after injections of [3H] proline into the dorsal and/or ventral nuclei of this diencephalic structure. Autoradiographs were prepared from either frozen or paraffin coronal sections through the rat brain. The dorsal nucleus of the lateral geniculate projected via the optic radiation to area 17 of the cerebral cortex. There was also a slight extension of label into the zones of transition between areas 17, 18 and 18a. The distribution of silver grains in the various layers of the cerebral cortex was analyzed quantitatively and showed a major peak of labeling in layer IV with minor peaks in outer layer I and the upper half and lowest part of layer VI. The significance of these peaks is discussed in respect to the distribution of geniculocortical terminals in other mammalian species. The ventral nucleus of the lateral geniculate body had 5 major projections to brain stem structures both ipsilateral and contralateral to the injected nucleus. There were two dorsomedial projections: (1) a projection to the superior colliculus which terminated mainly in the medial third of the stratum opticum, and (2) a large projection via the superior thalamic radiation which terminated in the ipsilateral pretectal area; a continuation of this projection passed through the posterior commissure to attain the contralateral pretectal area. The three ventromedial projections involved: (1) a geniculopontine tract which coursed through the basis pedunculi and the lateral lemniscus to terminate in the dorsomedial and dorsolateral parts of the pons after giving terminals to the lateral terminal nucleus of the accessory optic tract, (2) a projection via Meynert's commissure to the suprachiasmatic nuclei of both sides of the brain stem as well as to the contralateral ventral lateral geniculate nucleus and lateral terminal nucleus of the accessory optic tract, and (3) a medial projection to the ipsilateral zona incerta. The results obtained in these experiments are contrasted with other data on the rat's central visual connections to illustrate the importance of these connections in many subcortical visual functions.     

Sources of subcortical GABAergic projections to the superior colliculus in the cat

The goal of this study was to identify GABAergic input to the cat superior colliculus from neurons located in the caudal diencephalon, mesencephalon, pons and medulla. Cells efferent to the superior colliculus were labeled retrogradely with the tracer horseradish peroxidase, and an antibody to gamma-aminobutyric acid was used to label GABAergic neurons in the same sections. The results indicate that neurons in several distinct areas of the caudal diencephalon and brainstem are both immunocytochemically labeled for GABA and retrogradely labeled with horseradish peroxidase. These areas include zona incerta, nucleus of the posterior commissure, anterior and posterior pretectal nuclei, nucleus of the optic tract, superior colliculus, cuneiform nucleus, subcuneiform area, substantia nigra pars reticulata and pars lateralis, periparabigeminal area, external nucleus of the inferior colliculus, the area ventral to the external nucleus of the inferior colliculus, mesencephalic reticular formation, dorsal and ventral nuclei of the lateral lemniscus, and the perihypoglossal nucleus. The role that such diverse inhibitory input to the superior colliculus might play, particularly in influencing eye movements, is discussed.     

Projection from the pretectal nuclei to the dorsal lateral geniculate nucleus in the cat: a wheat germ agglutinin-horseradish peroxidase study

To study the projection from the pretectum to the dorsal lateral geniculate nucleus (LGNd) in the cat, we used anterograde and retrograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Special attention was directed to the retinotopic maps of the pretectum and LGNd. Multiple restricted injections were made into different parts of the pretectum or LGNd. The pretectogeniculate pathway terminates mostly in the medial interlaminar nucleus (MIN) and layers A and A1, and to some extent in the lamina C within the ipsilateral LGNd. The lateral part of the nucleus of the optic tract (NTO) receives afferents from the superior retina, and the medial part of NTO and posterior pretectal nucleus (NPP) receives afferents from the inferior retina. There is no topographic organization in the retinal projection to the olivary pretectal nucleus (NOL). The lateral part of NTO projects ipsilaterally to the rostral portion of LGNd, which receives afferents from the superior retina. The medial part of NTO projects ipsilaterally to the caudal portion of LGNd, which receives afferents from the inferior retina. The NOL projects to all laminar parts of LGNd, ipsilaterally. The NPP projects largely to the ipsilateral MIN, which receives afferents from the pericentral and peripheral retina. These results suggest that similar parts of the retinotopic maps present in the pretectum and LGNd are connected.     

The pretectal complex of the cat: cells of origin of projections to the pulvinar nucleus

The pretectal projection to the pulvinar nucleus in the cat was examined ussing the retrograde transport of wheat germ agglutinin—horseradish peroxidase. These data show that both visual and non-visual areas of the pretectal complex contribute to the projection. Specifically, large numbers of labeled neurons are located within the pretectal olivary nucleus with a substantial number of labeled neurons observed within the nucleus of the optic tract. Labeled neurons are also located within the medial, anterior and posterior nuclei, but not to the degree observed in the other pretectal nuclei. Morphometric analysis of labeled and Nissl-stained neurons indicate that the pretectopulvinar pathway is not correlated to any single cell size.     

Distribution of GABAergic neurons and terminals in the auditory system of the barn owl

Antisera to GAD (glutamic acid decarboxylase) and GABA were used to determine the distribution of GABAergic cells and terminals in the brainstem and midbrain auditory nuclei of the barn owl. The owl processes time and intensity components of the auditory signal in separate pathways, and each pathway has a distinctive pattern of GAD- and GABA-like immunoreactivity. In the time pathway, all the cells of the cochlear nucleus magnocellularis and nucleus laminaris receive perisomatic GABAergic terminals, and small numbers of GABAergic neurons surround both nuclei. The ventral nucleus of the lateral lemniscus (anterior division) contains both immunoreactive terminals and some GABAergic neurons. In the intensity pathway, dense immunoreactive terminals are distributed throughout the cochlear nucleus angularis, which also contains a small number of GABAergic neurons. The superior olive contains two GABAergic cell types and immunoreactive terminals distributed throughout the neuropil. All the neurons of the nucleus of the lateral lemniscus (ventral part) appear to be GABAergic, and this nucleus also contains a moderate number of immunoreactive terminals. Immunoreactive terminals are distributed throughout the neuropil of the ventral nucleus of the lateral lemniscus (posterior division), whereas multipolar and small fusiform GABAergic neurons predominate in the dorsal regions of the nucleus. The time and intensity pathways combine in the inferior colliculus. The central nucleus of the inferior colliculus contains a larger number of fusiform and stellate GABAergic neurons and a dense plexus of immunoreactive terminals, whereas the external nucleus contains slightly fewer immunoreactive cells and terminals. The superficial nucleus contains dense, fine immunoreactive terminals and a small number of GABAergic neurons.     

Two visual pathways to the telencephalon in the nurse shark (Ginglymostoma cirratum). I. Retinal projections

The central projections of the retina in the nurse shark were studied by anterograde transport of horseradish peroxidase and tritiated proline. With regard to efferent retinal fibres, both techniques gave completely identical results. Projections were found to pretectal area, dorsal thalamus, basal optic nucleus, and optic tectum, all at the contralateral side. The retinal target cells in the dorsal thalamus are restricted to the ventrolateral optic nucleus and the posterior optic nucleus. No evidence was found for an earlier-reported projection to the lateral geniculate nucleus. The present findings show that the ventrolateral optic nucleus exhibits homological features of the dorsal lateral geniculate nucleus in other vertebrate groups, whereas the lateral geniculate nucleus of the nurse shark is much more comparable to the nucleus rotundus of teleosts and birds and would be more appropriately so named. The application of the HRP technique also allowed us to study afferents to the retina by retrograde transport of tracer. Retrogradely labeled cells were observed in the contralateral optic tectum and are apparently similar to those reported for teleosts and birds.     

A topographically organized gamma-aminobutyric acid projection from the ventral pallidum to the nucleus accumbens in the rat

Anatomical and electrophysiological studies have indicated that a reciprocal projection from the ventral pallidum back to the nucleus accumbens exists and has functional relevance. In this study, the topographical projection from the ventral pallidum to the nucleus accumbens was examined by using retrograde tracing with fluoro-gold iontophoresed in subcompartments of the nucleus accumbens in rats combined with either in situ hybridization for glutamic acid decarboxylase and preproenkephalin mRNA or substance P immunoreactivity. Deposits made into the medial nucleus accumbens preferentially labeled neurons in the medial ventral pallidum, while deposits into the dorsolateral nucleus accumbens, at or lateral to the anterior commissure, labeled primarily cells in the dorsal and lateral ventral pallidum. A mediolateral to rostrocaudal topography was also observed, with the medial deposits preferentially labeling cells in rostral ventral pallidum and the lateral deposits resulting in retrogradely labeled cells in the ventral pallidum below the crossing of the posterior anterior commissure (subcommissural) as well as below the globus pallidus (sublenticular). The majority of cells retrogradely labeled with fluoro-gold were double-labeled for glutamic acid decarboxylase mRNA. In contrast, very few retrogradely labeled neurons in the ventral pallidurn were double labeled for mRNA for preproenkephalin. These data demonstrate a topographically organized projection from the ventral pallidum, to the nucleus accumbens that is primarily γ-aminobutyric acid (GABA)-ergic and reciprocal to the GABAergic projection from the nucleus accumbens to the ventral pallidum. © 1994 Wiley-Liss, Inc.     

Interconnections among nuclei of the subcortical visual shell: The intergeniculate leaflet is a major constituent of the hamster subcortical visual system

The intergeniculate leaflet (IGL), a major constituent of the circadian visual system, is one of 12 retinorecipient nuclei forming a “subcortical visual shell” overlying the diencephalic–mesencephalic border. The present investigation evaluated IGL connections with nuclei of the subcortical visual shell and determined the extent of interconnectivity between these nuclei. Male hamsters received stereotaxic, iontophoretic injections of the retrograde tracer, cholera toxin β fragment, or the anterograde tracer, Phaseolus vulgaris-leucoagglutin, into nuclei of the pretectum (medial, commissural, posterior, olivary, anterior, nucleus of the optic tract, posterior limitans), into the superior colliculus, or into the visual thalamic nuclei (lateral posterior, dorsal lateral geniculate, intergeniculate leaflet, ventral lateral geniculate). Retrogradely labeled cell bodies identified nuclei with afferents projecting to the site of injection, whereas the presence of anterogradely labeled fibers with terminals revealed brain nuclei targeted by neurons at the site of injection. The IGL projects bilaterally to all nuclei of the visual shell except the lateral posterior and dorsal lateral geniculate nuclei. The IGL also has afferents from the same set of nuclei, except the nucleus of the optic tract. The extensive bilateral efferent projections distinguish IGL from the ventral lateral geniculate nucleus. The superior colliculus, commissural pretectal, olivary pretectal, and posterior pretectal nuclei also project bilaterally to the majority of subcortical visual nuclei. The IGL has a well-established role in circadian rhythm regulation, but there is as yet no known function for it in the larger context of the subcortical visual system, much of which is involved in oculomotor control. J. Comp. Neurol. 396:288–309, 1998. © 1998 Wiley-Liss, Inc.     

Comparative topography of projections from the mesodiencephalic junction to the inferior olive, vestibular nuclei, and upper cervical cord in the cat

Distributions of neurons located in the central rostral mesencephalon and caudal diencephalon that project to the upper cervical spinal cord, vestibular nuclei, or inferior olive were studied in the cat by using retrograde axonal transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Afferent sources to all of these targets were observed in the interstitial nucleus of Cajal (INC), the region surrounding the fasciculus retroflexus (PF), and the nucleus of the fields of Forel (NFF). Three-dimensional reconstruction revealed differences in densities of cells projecting from these common areas. Spinal projecting cells were present in slightly greater numbers in the caudal two-thirds of the INC, whereas those projecting to the vestibular complex were more numerous in the rostral two-thirds of this nucleus. A relatively smaller number of olivary projecting cells were dispersed throughout the INC. Olivary afferent sources outnumber those with spinally directed or vestibularly directed axons in the PF region. In the fields of Forel, cells projecting to the vestibular nuclei or inferior olive were concentrated medially, whereas cells projecting to the spinal cord appeared both medially and laterally. Each type of afferent source was also seen in the nucleus of the posterior commissure and the posterior ventral lateral hypothalamic area. Unique sources of afferents to the inferior olive were observed in the parvicellular red nucleus (ipsilateral to the injections) and the anterior and posterior pretectal nuclei. A large number of labeled neurons was seen in the nucleus of Darkschewitsch after injections of tracer into the inferior olive, but this projection did not appear to be unique, as small numbers of labeled cells were also seen after injections into the cervical spinal cord. The Edinger-Westphal nucleus and the adjacent somatic oculomotor nucleus contained cells which projected separately to the spinal cord or the vestibular complex, and the superior colliculus contained cells which projected separately to the contralateral spinal cord or the contralateral inferior olive. In this study, it was also noted that neurons in the medial terminal nucleus of the accessory optic tract projected to the ipsilateral inferior olive or to the contralateral vestibular complex. These differences in locations and densities of cells projecting to the cervical spinal cord, vestibular complex, and inferior olive may underlie functional specializations in these areas in relation to vertical eye and head movement control and to neural systems controlling postural adjustments accompanying limb movements.