GABAergic inputs to the nucleus rotundus (pulvinar inferior) of the pigeon (Columba livia)

The avian nucleus rotundus, a nucleus that appears to be homologous to the inferior/caudal pulvinar of mammals, is the major target of an ascending retino-tecto-thalamic pathway. Further clarification of the inputs to the rotundus and their functional properties will contribute to our understanding of the fundamental role of the ascending tectal inputs to the telencephalon in all vertebrates, including mammals. We found that the rotundus contains a massive plexus of glutamic acid decarboxylase (GAD)-immunoreactive axons using antibodies against GAD. The cells within the rotundus, however, were not immunoreactive for GAD. The retrograde tracer cholera toxin B fragment was injected into the rotundus to establish the location of the afferent neurons and determine the source of the gamma-aminobutyric acid (GABA) inputs into the rotundus. In addition to the recognized bilateral inputs from layer 13 of the tectum, we found intense retrograde labeling of neurons within the ipsilateral nuclei subpretectalis (SP), subpretectalis-caudalis (SPcd), interstitio-pretecto-subpretectalis (IPS), posteroventralis thalami (PV), and reticularis superior thalami (RS). All the neurons of the SP, SPcd, IPS, and PV were intensely GAD-immunoreactive. The neurons of layer 13 of the tectum were not immunoreactive for GAD. Following the destruction of the ipsilateral SP/IPS complex, we found a major reduction in the intensity of the GAD axonal immunoreactivity within the ipsilateral rotundus, but this destruction did not diminish the intensity of the GAD-immunoreactivity within the contralateral rotundus. Our studies indicated that the source of the massive GAD-immunoreactive plexus within the rotundus was from the ipsilateral SP, SPcd, IPS, and PV nuclei. These nuclei, in turn, received ipsilateral tectal input via collaterals of the neurons of layer 13 in the course of their projections upon the rotundus. We suggest that the direct bilateral tecto-rotundal projections are excitatory, whereas the indirect ipsilateral projections from the SP/IPS and PV are mainly inhibitory, possibly acting via a GABA-A receptor. © 1996 Wiley-Liss, Inc.     

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.     

An immunocytochemical analysis of the lateral geniculate complex in the pigeon (Columba livia).

The lateral geniculate complex (GL) of pigeons was investigated with respect to its immunohistochemical characteristics, retinal afferents, and the putative transmitters/modulators of its neurons. The distributions of serotonin-, choline acetyltransferase-, glutamic acid decarboxylase-, tyrosine hydroxylase-, neuropeptide Y- (NPY), substance P- (SP), neurotensin- (NT), cholecystokinin- (CCK), and leucine-enkephalin- (L-ENK) like immunoreactive perikarya and fibers were mapped. Retinal projections were studied following injections of Rhodamine-B-isothiocyanate into the vitreous. Transmitter-specific projections onto the visual Wulst and the optic tectum were studied by simultaneous double-labelling of retrograde tracer molecules and immunocytochemical labelling. The GL can be divided into three major subdivisions, the n. geniculatus lateralis, pars dorsalis (GLd; previously designated as the n. opticus principalis thalami, OPT), the n. marginalis tractus optici (nMOT), and the n. geniculatus lateralis, pars ventralis (GLv). All three subdivisions are retinorecipient. The GLd can be further subdivided into at least five components differing in their immunohistochemical characteristics: n. lateralis anterior (LA); n. dorsolateralis anterior thalami, pars lateralis (DLL), n. dorsolateralis anterior thalami, pars magnocellularis (DLAmc); n. lateralis dorsalis nuclei optici principalis thalami (LdOPT); and n. suprarotundus (SpRt). The LdOPT consists of an area of dense CCK-like and NT-like terminals of probable retinal origin. Three subnuclei (DLL, DLAmc, SpRt) were shown to project to the visual Wulst. Cholinergic and cholecystokinergic relay neurons participated in this projection. The nMOT occupies a position between the GLd and GLv and encircles the rostral pole of n. rotundus and the LA. It is characterized mainly by medium sized NPY-like perikarya which were shown to project onto the ipsilateral optic tectum. Bands of NPY-like fibers in the tectal layers 2, 4, and 7 could at least in part be due to this projection of the nMOT. Most of the antisera used revealed transmitter/modulator-specific fiber systems in the GLv which often showed a layer-specific distribution. Perikaryal labelling was only obtained with glutamic acid decarboxylase. On the basis of its chemoarchitectonics, topography, and connectional pattern, the GLd complex of pigeons is most directly equivalent to the mammalian GLd. However, although the different subdivisions of the avian GLd may represent functionally different channels within the thalamofugal pathway similar to the lamina-specific differentiation within the mammalian geniculostriate projection, direct comparison of subnuclei of birds and mammals is not justified at this time. The nMOT appears similar to the intergeniculate leaflet (IGL) and the avian GLv clearly corresponds in many features to the mammalian GLv.     

Afferent and efferent connections of the optic tectum in the carp (Cyprinus carpio L.)

The afferent and efferent connections of the tectum opticum in the carp (Cyprinus carpio L.) were studied with the HRP method. Following iontophoretic peroxidase injections in several parts of the tectum anterograde transport of the enzyme revealed tectal projections to the lateral geniculate nucleus, dorsal tegmentum, pretectal nuclei, nucleus rotundus, torus longitudinalis, torus semicircularis, nucleus isthmi, contralateral tectum and to the mesencephalic and bulbar reticular formations.Tectal afferents were demonstrated by retrograde HRP transport in the area dorsalis pars centralis of the telencephalon, torus longitudinalis, torus semicircularis, nucleus isthmi, nucleus profundus mesencephali, several pretectal nuclei, dorsomedial and dorsolateral thalamic nuclei, nucleus of the posterior commissure, mesencephalic and bulbar reticular nuclei and nucleus ruber. Visuo-cerebellar circuitry was investigated by means of peroxidase injections in the various parts of the cerebellum. These experiments revealed indirect retino- and tecto-cerebellar pathways via the pretectal nuclei and the nucleus isthmi.     

Selective uptake and transport of label within three identified neuronal systems after injection of 3H-GABA into the pigeon optic tectum: an autoradiographic and Golgi study.

After injection of tritiated gamma-aminobutyric acid (GABA) into the pigeon optic tectum and thalamus it was possible to correlate certain aspects of the autoradiographic labeling pattern with observations made from Golgi material. Three neuronal, GABA specific systems were identified both from the uptake of the amino acid and from the subsequent and bidirectional in tracellular transport of labe. The first system derives from cell bodies within sublayer IIi the axons of which could be selectively labelled throughout their course within layer I and to the areas of termination within the pretectum and ventral thalamus. The radially ascending dendrites and axon collaterals of these neurons arbourised within sublayer IIf, and could be labelled in a retrograde fashion after tectal or thalamic injections. The second system was represented by small perikarya within sublayer IIc with locally and superficially directed dendrites and with a radially and deep directed axon from which an extensive axon collateral system arose. It was found possible to label these perikarya either directly or indirectly after tangential tectal injections which preferentially labelled the axons and terminals of these neurons within the deeper regions of the tectal cortex and resulted in the retrograde axonal movement of label to theoverlying cell bodies. A third system was found within sublayer IId, was horizontally organized and from a correlation with degeneration, other autoradiographic and Golgi preparations thought to be mainly dendritic in nature. The biochemical and anatomical implications of specific GABA uptake and subsequent transport of label are discussed and a model of the tectal cortex, based on the three proposed inhibitory systems and their relation to a number of tectal afferent inputs, considered.     

Structural organization of parallel information processing within the tectofugal visual system of the pigeon

Visual information processing within the ascending tectofugal pathway to the forebrain undergoes essential rearrangements between the mesencephalic tectum opticum and the diencephalic nucleus rotundus of birds. The outer tectal layers constitute a two-dimensional map of the visual surrounding, whereas nucleus rotundus is characterized by functional domains in which different visual features such as movement, color, or luminance are processed in parallel. Morphologic correlates of this reorganization were investigated by means of focal injections of the neuronal tracer choleratoxin subunit B into different regions of the nuclei rotundus and triangularis of the pigeon. Dependent on the thalamic injection site, variations in the retrograde labeling pattern of ascending tectal efferents were observed. All rotundal projecting neurons were located within the deep tectal layer 13. Five different cell populations were distinguished that could be differentiated according to their dendritic ramifications within different retinorecipient laminae and their axons projecting to different subcomponents of the nucleus rotundus. Because retinorecipient tectal layers differ in their input from distinct classes of retinal ganglion cells, each tectorotundal cell type probably processes different aspects of the visual surrounding. Therefore, the differential input/output connections of the five tectorotundal cell groups might constitute the structural basis for spatially segregated parallel information processing of different stimulus aspects within the tectofugal visual system. Because two of five rotundal projecting cell groups additionally exhibited quantitative shifts along the dorsoventral extension of the tectum, data also indicate visual field-dependent alterations in information processing for particular visual features.     

Nucleus rotundus in a snake,Thamnophis sirtalis: An analysis of a nonretinotopic projection

Nucleus rotundus, a tectorecipient thalamic nucleus in reptiles and birds, is described for the first time in a snake. The morphology of rotunda neurons and tectorotundal axons was studied at the light microscopic level by using anterograde and retrograde filling with horseradish peroxidase (HRP) Injections of HRP in the dorsal ventricular ridge retrogradely fill neurons in rotundus. Rotundus is situated centrally in the caudal diencephalon medial to the cell plate of the retinorecipient geniculate complex and ventrolateral to the lentiform thalamic nucleus. The dendrites of rotundal neurons are long and radiate, but are confined within the cytoarchitectonically defined borders of the nucleus. Injections of HRP into the optic tectum anterogradely fill axons that project to rotundus bilaterally via the tectothalamic tract. Small injections show that axons arising from a single tectal locus distribute to all sectors of rotundus. Thus, this projection may not be retinotopically organized. However, single axons reconstructed through serial sections form spatially restricted, sheetlike terminal fields that pass caudorostrally through the entire extent of rotundus. Several hypotheses on the functional significance of such organized but nonretinotopic visual projections are discussed.     

Tectothalamic visual projections in turtles: their cells of origin revealed by tracing methods

In two species of turtle (Emys orbicularis and Testudo horsfieldi), retrograde and anterograde tracer techniques were used to study projections from the optic tectum to the nucleus rotundus (Rot) and to the dorsal lateral geniculate nucleus (GLd). The ipsilateral Rot received the most massive tectal projections, stemming from numerous neurons located in the stratum griseum centrale (SGC). These neurons varied in size and shape, many of them having a wide zone of dendritic arborization within both the (SGC) and the stratum griseum et fibrosum superficiale (SGFS). Projections from the tectum to the GLd were ipsilateral, were extremely scarce, and arose from a small number of neurons of various shapes situated in the SGFS; these cells were, as a rule, smaller than those projecting to the Rot. For the most part, these neurons were radially oriented, with rather restricted dendritic arborizations in the most superficial sublayers of the SGFS; smaller numbers of projection neurons were horizontally oriented, with long dendrites branching throughout the layer. Some neurons located in the stratum griseum periventriculare (SGP) projected to both the Rot and the GLd. Most of these neurons had dendritic arborizations within the retinorecipient zone of the SGFS. We were unable to rule out the possibility that some cells projecting to the GLd were situated in the SGC. Both the GLd and the main body of the Rot did not contain neurons projecting to the optic tectum. Thalamic neurons projecting to the tectum were observed in the ventral lateral geniculate nucleus, the intergeniculate leaflet and the interstitial nuclei of the tectothalamic tract, and the nucleus of the decussatio supraoptica ventralis. The question of whether variation in the laminar organization of the tectorotundal and tectogeniculate projection neurons in reptiles, birds, and mammals may be related to different degrees of differentiation of the tectal layers is discussed.     

Projections of the retinorecipient pretectal nuclei in the pigeon (Columba livia)

We have used anterograde autoradiographic and retrograde HRP techniques to investigate the efferent connections of the retinorecipient pretectal nuclei in the pigeon. In the accompanying paper we identified these nuclei in the pigeon as the nucleus lentiformis mesencephali--pars lateralis and pars medialis, the tectal gray, the area pretectalis, and pretectalis diffusus. Although there are reports of a few of the projections of these nuclei, they had not previously been the subject of a detailed study. We found that different cell types in the lentiformis mesencephali, pars medialis and the lentiformis mesencephali, pars lateralis have descending projections to different targets. These targets include the inferior olive, the cerebellum, the lateral pontine nucleus, the nucleus papillioformis, the nucleus of the basal optic root, the nucleus mesencephalicus profundus, pars ventralis, the nucleus principalis precommissuralis, and the stratum cellulare externum. We found that a few cells in the lentiformis mesencephali project to the medial pontine nucleus, but that a much heavier projection arises from the nucleus laminaris precommissuralis, which is medial to the nucleus lentiformis mesencephali, pars medialis. The tectal gray has predominantly ascending projections to the diencephalon. The nuclei that it projects to are the nucleus intercalatus thalami, the nucleus of the ventral supraoptic decussation, the nucleus posteroventralis, the ventral lateral geniculate nucleus, the nucleus dorsolateralis medialis, and the nucleus dorsolateralis anterior. The tectal gray also projects topographically to layers 4 and 8-13 of the optic tectum. Area pretectalis has both ascending and descending projections. It has ipsilateral ascending projections to the nucleus dorsolateralis anterior, pars magnocellularis, the nucleus lateralis anterior, and the nucleus ventrolateralis thalami. It has ipsilateral descending projections to the central gray, the nucleus of the basal optic root, pars dorsalis, the lateral pontine nucleus, and the deep layers of the optic tectum. It has contralateral projections to the area pretectalis, the nucleus Campi Foreli, the interstitial nucleus of Cajal, the nucleus of Darkschewitsch, the cerebellum, and the Edinger-Westphal nucleus. The efferent projections of pretectalis diffusus are limited. It projects contralaterally to the pretectalis diffusus, and ipsilaterally to the nucleus of the ventral supraoptic decussation, the lateral pons, and the cerebellum.4     

Visual-field-specific heterogeneity within the tecto-rotundal projection of the pigeon.

The organization of the tecto-rotundal projection of the pigeon was investigated by means of anterograde and retrograde tracing techniques. Besides the known organization in tecto-rotundal connectivity, this study additionally demonstrates major variations in the ascending projections of different tectal subfields. We show that the ventral tectum opticum (TO) has significantly more projections onto the nucleus rotundus (Rt) than dorsal tectal areas. This difference coincides with differential innervation densities of afferent fibres within rotundal subregions. While ventral tectal efferents project onto the ventral and central Rt, dorsal tectal efferents mainly arborize within limited areas between the central Rt and its dorsal cap, the nucleus triangularis. Thus, the ventral TO, representing the lower and frontal field of view, exhibits a quantitatively and spatially enhanced projection onto the Rt, as compared with the dorsal TO. The data presented here demonstrate a visual field-dependent projection pattern of ascending tectal outputs onto different rotundal domains. The data are consistent with behavioural studies, demonstrating tectofugal lesions to suppress visual stimulus analysis mainly within the frontal field of view.     

Retinal afferents to the tectum opticum and the nucleus opticus principalis thalami in the pigeon.

The retinal afferents of the tectum opticum and the n. opticus principalis thalami (OPT) were studied with fluorescent tracers in pigeons. Injections into the tectum opticum revealed topographically related areas of high density labelling in the contralateral retina. In these areas up to 15,000 cells/mm2 were labelled. After tectal injections the soma sizes of labelled retinal ganglion cells in the area centralis ranged from 5 to 23 microns with a mean of 7.5 microns. Afferents from the ipsilateral retina could not be demonstrated. Injections into the OPT labelled neurons throughout the retina without a clear topographical relation to the locus of injection. The density never exceeded 150 cells per mm2. The soma size range was 8 to 35 microns with a mean of 14.6 microns. Independently of the injection area within the OPT, the red field in the dorsotemporal retina was always extremely sparsely labelled. The number of labelled ganglion cells in this area never exceeded 25 neurons/mm2. After OPT injections the average density of labelling per unit area was six times higher in the yellow than in the red field. The results confirm previous reports of a massive and topographically organized retinal projection onto the optic tectum. The projection onto the OPT was clearly smaller and with the retrograde tracing techniques in use, an orderly topography has not been demonstrated. The paucity of red field projections onto the OPT suggests that the role of the thalamofugal pathway in binocular integration is very limited.     

Spatial organization of the pigeon tectorotundal pathway: an interdigitating topographic arrangement

The retinotectofugal system is the main visual pathway projecting upon the telencephalon in birds and many other nonmammalian vertebrates. The ascending tectal projection arises exclusively from cells located in layer 13 of the optic tectum and is directed bilaterally toward the thalamic nucleus rotundus. Although previous studies provided evidence that different types of tectal layer 13 cells project to different subdivisions in Rt, apparently without maintaining a retinotopic organization, the detailed spatial organization of this projection remains obscure. We reexamined the pigeon tectorotundal projection using conventional tracing techniques plus a new method devised to perform small deep-brain microinjections of crystalline tracers. We found that discrete injections involving restricted zones within one subdivision retrogradely label a small fraction of layer 13 cells that are distributed throughout the layer, covering most of the tectal representation of the contralateral visual field. Double-tracer injections in one subdivision label distinct but intermingled sets of layer 13 neurons. These results, together with the tracing of tectal axonal terminal fields in the rotundus, lead us to propose a novel "interdigitating" topographic arrangement for the tectorotundal projection, in which intermingled sets of layer 13 cells, presumably of the same particular class and distributed in an organized fashion throughout the surface of the tectum, terminate in separate regions within one subdivision. This spatial organization has significant consequences for the understanding of the physiological and functional properties of the tectofugal pathway in birds.     

Central neural connections of the pineal organ and retina in the teleost Gasterosteus aculeatus L

The relations of the central neural connections of the pineal organ to those of the retinae of the lateral eyes were investigated in the three-spined stickleback, Gasterosteus aculeatus L. (Teleostei), by anterograde and retrograde transport of horseradish peroxidase (HRP). HRP was applied to the crushed pineal stalk and/or injected into the left or the right eye. Both pineal and retinal efferents project to area praetectalis, dorsal and ventral thalamic areas, and dorsal tegmentum. The most notable overlapping occurs in nucleus commissurae posterioris of area praetectalis. Pineal efferents also innervate the habenular nuclei and dorsal hypothalamus, while retinal efferents innervate rostral hypothalamus, ventrolateral thalamus, and tectum opticum. A small number of retinofugal axons recross and innervate the ipsilateral nucleus anterioris periventricularis and area praetectalis. After intraocular HRP injections, labeled perikarya were located both in retinofugal terminal areas and in areas not receiving direct retinal input, such as the telencephalic nucleus olfactoretinalis, deep tectal layers, and an area rostroventral to nucleus dorsolateralis thalami. No neurons afferent to the pineal organ were demonstrated. The close association of pineal efferents with retinofugal and possible retinopetal elements is in accordance with the view that both systems are potential neural mediators of photoperiodic events in the teleostean circadian system.     

Second tectofugal pathway in a songbird (Taeniopygia guttata) revisited: Tectal and lateral pontine projections to the posterior thalamus,thence to the intermediate nidopallium

Birds are almost always said to have two visual pathways from the retina to the telencephalon: thalamofugal terminating in the Wulst, and tectofugal terminating in the entopallium. Often ignored is a second tectofugal pathway that terminates in the nidopallium medial to and separate from the entopallium (e.g., Gamlin and Cohen [1986] J Comp Neurol 250:296–310). Using standard tract‐tracing and electroanatomical techniques, we extend earlier evidence of a second tectofugal pathway in songbirds (Wild [1994] J Comp Neurol 349:512–535), by showing that visual projections to nucleus uvaeformis (Uva) of the posterior thalamus in zebra finches extend farther rostrally than to Uva, as generally recognized in the context of the song control system. Projections to “rUva” resulted from injections of biotinylated dextran amine into the lateral pontine nucleus (PL), and led to extensive retrograde labeling of tectal neurons, predominantly in layer 13. Injections in rUva also resulted in extensive retrograde labeling of predominantly layer 13 tectal neurons, retrograde labeling of PL neurons, and anterograde labeling of PL. It thus appears that some tectal neurons could project to rUva and PL via branched axons. Ascending projections of rUva terminated throughout a visually responsive region of the intermediate nidopallium (NI) lying between the nucleus interface medially and the entopallium laterally. Lastly, as shown by Clarke in pigeons ([1977] J Comp Neurol 174:535–552), we found that PL projects to caudal cerebellar folia. J. Comp. Neurol. 524:963–985, 2016. © 2015 Wiley Periodicals, Inc.     

Two distinct populations of tectal neurons have unique connections within the retinotectorotundal pathway of the pigeon (Columba livia)

The tectofugal pathway is a massive ascending polysynaptic pathway from the tectum to the thalamus and then to the telencephalon. In birds, the initial component of this pathway is known as the tectorotundal pathway; in mammals, it is known as the tectopulvinar pathway. The avian tectorotundal pathway is highly developed; thus, it provides a particularly appropriate model for exploring the fundamental properties of this system in all amniotes. To further define the connectivity of the tectorotundal projections of the tectofugal pathway, we injected cholera toxin B fragment into various rotundal divisions, the tectobulbar projection, and the ventral supraoptic decussation of the pigeon. We found intense bilateral retrograde labeling of neurons that stratified within layer 13 and, in certain cases, granular staining in layer 5b of the optic tectum. Based on these results, we propose that there are two distinct types of layer 13 neurons that project to the rotundus: 1) type I neurons, which are found in the outer sublamina of layer 13 (closer to layer 12) and which project to the anterior and centralis rotundal divisions, and 2) type II neurons, which are found in the inner sublamina of layer 13 (closer to layer 14) and which project to the posterior and triangularis rotundal divisions. Only the labeling of type I neurons produced the granular dendritic staining in layer 5b. An additional type of tectal neuron was also found that projected to the tectobulbar system. We then injected Phaseolus vulgaris-leucoagglutinin in the optic tract and found that the retinal axons terminating within tectal layer 5b formed narrow radial arbors (7–10 μm in diameter) that were confined to layer 5b. Based on these results, we propose that these axons are derived from a population of small retinal ganglion cells (4.5–6.0 μm in diameter) that terminate on the distal dendrites of type I neurons. This study strongly indicated the presence of a major bilateral oligosynaptic retinotectorotundal pathway arising from small retinal ganglion cells projecting to the rotundus with only a single intervening tectal neuron, the proposed type I neuron. We suggest that a similar organization of retinotectopulvinar connections exist in reptiles and in many mammals. J. Comp. Neurol. 387:449–465, 1997. © 1997 Wiley-Liss, Inc.     

Topographic and morphometric effects of bilateral embryonic eye removal on the optic tectum and nucleus isthmus of the leopard frog

Rana pipiens were raised through metamorphosis after extirpation of both eye primordia at Shumway embryonic stage 17 (Shumway '40). The visual connections between the isthmic nuclei and the optic tectum were examined in these animals using horseradish peroxidase (HRP) histochemistry. Isthmo-tectal projections are normally aligned with the primary retinotectal map. We asked whether these connections would develop normal topographic organization in the absence of normal retinal input. HRP was formed into a solid pellet (? 200–500 μm diameter) and inserted into one tectal lobe on the tip of a fine metal probe. The procedure produced relatively restricted retrograde label in somas and dendrites in both isthmi nuclei. In the nucleus isthmus ipsilateral to the tectal lobe receiving the HRP pellet, processes of tecto-isthmi neurons were labeled by anterograde transport. The topography of the isthmo-tectal and tecto-isthmic projections were identical in the developmentally enucleated animals and in normal frogs, even though eye removal severely reduced the volume of the optic tecta and the isthmi nuclei. Thus our analyses indicate that retinal contacts do not play an active role in the development of the positional or polarity cues that are involved in “mapping” projections between central visual nuclei. These results are discussed in the context of peripheral specification of central connections and in terms of models that have recently been proposed to explain the development of the retinotectal system.     

Organization of the tectofugal visual pathway in the pigeon: a retrograde transport study.

In birds, superficial laminae of the optic tectum receive a massive retinal input; the tectum in turn projects upon the nucleus rotundus thalami, which then sends its efferents to the ectostriatal core of the telencephalon. To examine the detailed organization of this principal ascending visual pathway, small injections of the marker horseradish peroxidase (HRP) were placed in various sites throughout the ectostriatum (E) or nucleus rotundus (Rt) in pigeons. Analysis of the resulting patterns of retrograde labeling indicates the tectofugal pathway to be comprised of at least five different channels. Cells which lie at various depths in the stratum griseum centrale (SGC) of the tectum project upon distinct subdivisions of nucleus rotundus. Anterior portions of Rt receive input from superficial-most cells in the SGC, while medial and more caudal portions of Rt are projected upon by deeper SGC neurons. A ventral subdivision of Rt was found to receive its primary input from two pretectal nuclei. Additional inputs to all portions of Rt arise from nucleus reticularis superior thalami. The various subdivisions of rotundus in turn project upon distinct portions of the ectostriatum. Thus, the segregation between the different input classes into Rt is largely retained at the telencephalic level. In contrast, the nucleus triangularis, a dorso-medial extension of Rt which receives its input from the deepest of all SGC neurons, sends its efferents to all parts of the ectostriatum.     

Differential sensitivities of the two visual pathways of the chick to labelling by fluorescent retrograde tracers.

This study investigates the neurone structure-specific differences of sensitivities of fluorescent tracers. The tracers were used for retrograde labelling of contralateral projections in the two visual pathways of the chick. Rhodamine B Isothiocyanate (RITC), Fluorogold (FG) and True blue (TB) were injected into either the visual Wulst (thalamofugal pathway) or the nucleus rotundus (Rt; tectofugal pathway) and the retrogradely labelled neurones in the nucleus geniculatus lateralis pars dorsalis (GLd) or the optic tectum, respectively, were counted. Differential retrograde labelling in the two pathways was observed. In the thalamofugal pathway, both the contralateral and ipsilateral GLd cells were labelled by all three tracers (RITC, FG and TB). However, in the tectofugal pathway, whereas RITC labelled both the ipsilateral and contralateral tectal neurones, FG or TB labelled effectively only the ipsilateral tectal neurones. It was clear that FG and TB were taken up by the nerve endings and transported part-way along the axon but failed to be transported to the cell bodies of the contralateral tectal neurones. In addition, red beads and green beads were also injected into Rt and the differential labelling was also observed. Red beads labelled both ipsilateral and contralateral tectal neurones but green beads labelled only the ipsilateral tectal neurones. Since the contralateral tectal projections consist of divergent axon collaterals, the present study suggests that various retrograde tracers are not transported in these axon collaterals to label cell bodies. The contralaterally projecting neurones in the thalamofugal pathway are not axon collaterals and they were labelled by all of the tracers used.     

Optic tectum of the eastern garter snake, Thamnophis sirtalis. V. Morphology of brainstem afferents and general discussion

Brainstem neurons that project to the optic tectum of the eastern garter snake were identified by retrograde transport of horseradish peroxidase. The distribution and morphology of tectal afferent axons from the thalamus, pretectum, nucleus isthmi, and midbrain reticular formation were then studied by anterograde transport of horseradish peroxidase. Diencephalic projections to the tectum arise from the ventral lateral geniculate complex ipsilaterally and the ventrolateral nucleus, suprapeduncular nucleus, and nucleus of the ventral supraoptic decussation bilaterally. Three pretectal groups (the lentiform thalamic nucleus, the lentiform mesencephalic-pretectal complex and the geniculate pretectal nucleus) give rise to heavy, bilateral tectal projections. Small neurons in nucleus isthmi and large reticular neurons in nucleus lateralis profundus mesencephali also give rise to bilateral projections. Caudal to the tectum, projections arise bilaterally from the pontine and medullary tegmentum, nuclei of the lateral lemniscus, the posterior colliculus, and the sensory trigeminal nucleus. A small contralateral projection arises from the medial vestibular complex. Tectal afferents from the thalamus, pretectum, nucleus isthmi, and midbrain reticular formation had characteristic morphologies and laminar distributions within the tectum. However, these afferents fall into two groups based on their spatial organization. Afferents from the thalamus and nucleus isthmi arise from small neurons with spatially restricted, highly branched dendritic trees. Their axons terminate in single, highly branched and bouton-rich arbors about 100 micron in diameter. By contrast, afferents from the midbrain reticular formation and the pretectum arise from large neurons with long, radiate, and sparsely branched dendritic trees. Their axons course parallel to the tectal surface and emit numerous collateral branches that are distributed widely through the mediolateral and rostrocaudal extent of either the central or superficial gray layers. Each collateral bears several small, spatially disjunct clusters of boutons.