Visualization of HRP-filled axons in unsectioned, flattened optic tecta of frogs

In order to trace individual axons in the tectum, a curved structure, we have modified the HRP method of Adams for use on unsectioned, flattened tecta. Filled axons appear dark and uniformly filled and can be followed without the necessity for reconstructions from serial sections.     

Nucleus isthmi provides most tectal choline acetyltransferase in the frogRana pipiens

Up to 9 weeks following the removal of unilateral retinal input, choline acetyltransferase (ChAT) activity in the de-afferented tectal lobe is not significantly different from the intact tectal lobe. At 14 weeks, there is a 29% increase in the de-afferented side compared to the intact side. Following unilateral lesion of nucleus isthmi, ChAT activity in the tectal lobe ipsilateral to the lesion is approximately 30% of that measured in the contralateral lobe. Following bilateral n. isthmi lesion, ChAT activity in each tectal lobe is reduced by approximately 94% from intact tectal lobe controls. Thus, nucleus isthmi is the principal source of cholinergic input to the tectum.     

Intertectal commissural projection in the lizard Gallotia stehlini: Origin and midline topography

The retinotectal projection of reptiles is largely crossed. The intertectal commissure is an important pathway that interconnects directly the two sides of the optic tectum. The rostrocaudal topography of intertectal commissural fibers at the dorsal midplane was examined by means of the in vitro horseradish peroxidase (HRP) labelling technique in the lizard Gallotia stehlini. Unilateral large deposits of tracer in the optic tectum as well as smaller deposits restricted to one quadrant were used to map the intertectal fibers anterogradely. Most commissural axons reached the contralateral side grouped into a dense bundle at the transition between two structurally distinct parts of the midbrain dorsal midline. The smaller rostral zone relates laterally to the griseum tectale, whereas the larger caudal zone relates to the tectum. The intertectal fibers seem to converge on the rostralmost part of the latter midline region, even though they originate throughout the optic tectum. A rough rostrocaudal tectotopic order was detected at the midline. Retrogradely labelled neurons were best obtained by depositing HRP directly within the compact commissure at the midline. These belong to pyriform cells in the periventricular layers 3 and 5. Axons labelled from the tectum did not enter the posterior commissure nor the intervening commissural region related to the griseum tectale. © 1996 Wiley-Liss, Inc.     

Cytoarchitecture and fiber connections of the superficial pretectum in a teleost,Navodon modestus

Fiber connections of the so-called nucleus geniculatus lateralis (or the nucleus pretectalis superficialis pars parvocellularis) in a teleost, Navodon modestus, were examined by means of the horseradish peroxidase (HRP) tracing method. The nucleus receives fibers from the contralateral retina, ipsilateral optic tectum and nucleus isthmi, and projects bilaterally to the nucleus intermedius of Brickner and ipsilaterally to the optic tectum and raphe nuclei. The fiber connections suggest that the nucleus relays mainly visual information to the inferior lobe (hypothalamus) but not to the telencephalon. The nucleus is not a homologous structure to the lateral geniculate nucleus in other vertebrate classes.     

Retinotopic analysis of fiber pathways in amphibians. II. The frog Rana nigromaculata

The possibility of retinotopic organization of pathways of retinal fibers within the optic tract and the tectum of the frog was studied by selective labeling of the retinal fibers with horseradish peroxidase. Within the optic tract the pathways of the ventral, temporal and dorsal retinal fibers were ordered from the dorsal to ventral edges of the optic tract. The nasal retinal fibers ran along both the dorsal and ventral edges of the optic tract. The dorsal retinal fibers and the nasal retinal fibers which were located along the ventral edge of the optic tract entered the ventrolateral perimeter of the tectum and formed the lateral tract. The ventral retinal fibers and the nasal retinal fibers which were located along the dorsal edge of the optic tract entered the dorsomedial perimeter of the tectum and formed the dorsomedial tract. The temporal retinal fibers invaded the tectum directly at the diencephalo-tectal junction. The topography of fiber pathway observed for the frog was exactly the same as that seen in the newt, and seemed to be common to all amphibian species.     

The accessory optic system in a prosimian primate (Microcebus murinus): evidence for a direct retinal projection to the medial terminal nucleus

The accessory optic system (AOS) was studied in the prosimian primate, Microcebus murinus, by using intraocular injections of the anterograde tracers 3H-proline and horseradish peroxidase (HRP). Retinal fibers were found to terminate bilaterally in all three mesencephalic AOS nuclei as defined by Hayhow ('66, J. Comp. Neurol. 126:653-672). In contrast to previous reports in primates, we find that both the ventral and dorsal divisions of the medial terminal nucleus (MTN) receive projections from the retina. The ventral MTN is composed of a compact triangular group of cells, situated at the medial base of the cerebral peduncle, rostral to the rootlets of the third cranial nerve. The dorsal MTN extends dorsomedial to the substantia nigra and is composed of characteristic fusiform cells embedded in a fibrous neuropil. Although the cells of the dorsal MTN intermingle somewhat with the nigral cells, the nucleus is clearly distinguished by cyto- and myeloarchitectural features. The large lateral terminal nucleus (LTN) receives a dense projection from the retina and forms a prominent bulge on the lateral surface of the cerebral peduncle. The dorsal terminal nucleus (DTN) is located between the brachia of the superior and inferior colliculi, near the origin of the superior fasciculus of the accessory optic tract (AOT). This fasciculus is composed of anterior, middle, and posterior branches. In addition, a ventral group of fibers, corresponding to the inferior fasciculus of the AOT previously described in nonprimates, was identified in all planes of section. The results confirm the existence of a common plan of AOS organization in mammals.     

Transneuronal labeling with horseradish peroxidase in the visual system of the house fly

Horseradish peroxidase (HRP) is taken up by lesioned neurons in the fly CNS and passes from these into certain sets of adjoining neurons which are in synaptic contact. This transneuronal labeling resolves neurons in fine detail. Electron microscopy shows reaction product in secondary neurons associated with plasmalemma and microtubules. In some cases also vesicles were found containing reaction product. It is suggested that the transneuronal passage takes place in vivo.     

Transcellular transfer of HRP in the amphibian visual system

Unilateral intraocular injections of horseradish peroxidase (HRP) were made in the green tree frog, Hyla cinerea. Survival times ranged from 1 to 28 days. Control injections were placed in the orbit, peritoneum, or third ventricle. By 1 day after ocular injection anterogradely transported HRP was observed in the optic nerve and tract and in thalamic and midbrain retinal recipient zones. Retrograde filling of motor neurons was also observed by 1 day. At 3 days, HRP-positive magnocellular preoptic neurons became apparent. Finally, at 3-5 days post-injection, ependymal cells radially adjacent to HRP-positive neuropils, but not retrogradely filled cells, contained a small amount of reaction product. By 7 days these ependymal cells were densely filled and processes could be seen extending toward the neuropils. There was never evidence of HRP uptake by neurons in or adjacent to these HRP-positive neuropils. Neither retinal fibers nor ependymal cells were HRP-positive after any control injection or after processing uninjected material by the HRP histochemistry protocols. In contrast, motor cells were filled following orbital and peritoneal, but not ventricular, injections, suggesting blood-borne HRP reaching motor endplates could account for some of the motor neuron filling. Preoptic cells were filled after all control injections, demonstrating that they too could take up circulating HRP. The specificity of ependymal cell filling, however, suggests that anterogradely transported HRP can be released at the axon terminals and taken up specifically by ependymoglial cells.     

Tectal projection neurons to the retinopetal nucleus in the filefish

Following horseradish peroxidase (HRP) injections into the preoptic retinopetal nucleus (PRN), neurons in the ipsilateral optic tectum were labeled retrogradely. Labeled neurons exhibited a 'Golgi-like' appearance, somata of these neurons were pyriform or round, and most of them were located in the stratum album centrale (SAC) or the stratum periventriculare (SPV). These neurons had a long apical dendrite, which ramified in the upper-half of SGC into horizontally arborized dendritic fields. The main trunk of the apical dendrites also gave off several branches in the stratum fibrosum et griseum superficiale (SFGS) and reached the stratum opticum (SO). These neurons resemble the 'large pyriform neurons' of Vanegas et al. (Vanegas, H., Laufer, M. and Amat, J., The optic tectum of a perciform teleost. I. General configuration and cytoarchitecture, J. Comp. Neurol., 154 (1974) 43-60) except that in the tecto-PRN neurons the axons originates from the apical dendritic shaft at or near the level of the SAC. Judging from their dendritic patterns, the tectal neurons projecting indirectly to the retina may receive non-retinal inputs besides the retinal input.     

Sources of electrical transients in tectal neuropil of the frog,Rana pipiens

We have studied the outer neuropil layers in frog tectum where the ummyelinated optic nerve fibers terminate. At any point the neuropil an extracellular microelectrode records several different visually evoked electrical transients, distinct by size and shape. When classified by shape alone, each transient falls into one of 3 distinct classes. Some of these transients are binocularly driven, as originally described by Finch and Collett. The aggregate of the receptive fields of all the elements recorded at a single point defines a multiunit receptive field (MURF). Each MURF is characteristically oval, and divided into 3 sections along its long axis. Each section represents the aggregate of the receptive fields associated with one class of transient; i.e. transients belonging to only one specific class can be evoked by stimulating that part of the visual field corresponding to the appropriate section of the MURF. All of the MURFs mapped by recording in a single tectum are radially arranged in visual space about a central point, or ‘visual pole’. Several conclusions are made. First, the two larger types of transient are generated postsynaptically by electrically active dendritic elements, specifically the beaded dendritic appendages of tectal neurons. The smallest type of transient is of presynaptic origin. Second, these tectal elements have a local and global anatomical order across the tectum, which accounts for both the tripartite structure of the MURFs and their radial arrangement about a visual pole. Third, since the large transients are of postsynaptic origin, genuine recordings of single retinal ganglion cell (RGC) activity can be made only in the optic nerve or retina itself. Fourth, information is conveyed over the unmyelinated optic nerve fibers at pulse rates as high as 80/s and is transsynaptically effective at such rates. Finally, the electrically active tectal dendritic elements, with their highly organized spatial arrangement, are an important component of the frog's visual processing apparatus, instead of being merely relays or repeaters.     

Topography of regenerating optic fibers in goldfish traced with local wheat germ injections into retina: evidence for discontinuous microtopography in the retinotectal projection

A small injection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was injected into dorsal or ventral peripheral retina in normal goldfish and in goldfish with prior optic nerve crush. Serial sections of tectum were subsequently taken for horseradish peroxidase (HRP) histochemistry 18 hours after injection and studied with light microscopy and densitometric reconstructions. In normals, a small, sharply delineated patch of product 200-300 microns wide was observed at the appropriate medial or lateral periphery of tectum. This product filled the entire SFGS, the main optic termination layer, and fell off abruptly at its edges. No labelling was detected in the optic pathways. In regenerates at about 20 days after nerve crush, these retinal injections yielded product that was dispersed across 1,000 microns or more of tectum but not in a uniform fashion. The densest product was biased toward the appropriate tectal position while product of intermediate density was mainly distributed along a path from the anterior end of tectum to this region. Product in the inappropriate half of tectum was much lighter and typically fiberlike in appearance. By about 40 days, product had condensed considerably at roughly the correct region of tectum but it was not as sharply delimited as in normals. Dense label occupied a single area about twice that of normals and exhibited flanking regions of light label extending for several hundred micrometers. At 59-148 days, a further condensation was observed but into more than one patch of product. The patches were of variable size and consisted of sharply delimited dense product which filled the entire SFGS at each position. Morphologically, these patches bore a remarkable resemblance to the ocular dominance columns previously seen in this system.     

Anatomy and physiology of saccadic burst neurons in the alert squirrel monkey. II. Inhibitory burst neurons

Electrophysiological and intracellular labelling studies in the cat have identified a population of saccadic burst neurons in the medullary reticular formation that have an inhibitory, monosynaptic projection to the contralateral abducens nucleus. In the present study, intraaxonal recording and injection of horseradish peroxidase were used to identify and characterize the corresponding population of inhibitory burst neurons (IBNs) in the alert squirrel monkey. Squirrel monkey IBNs are located in the reticular formation ventral and caudal to the abducens nucleus and project contralaterally to the abducens. Additional contralateral projections are present to the vestibular nuclei, the nucleus prepositus, and the pontine and medullary reticular formation rostral and caudal to the abducens. All neurons fire a burst of spikes during saccades and are silent during fixation. In most neurons the burst begins 5-15 msec before saccade onset. The number of spikes in the saccadic burst is linearly related to the amplitude of the component of the saccade in the neuron's on-direction. Linear relationships also exist between burst duration and saccade duration and between firing frequency and instantaneous eye velocity. For all neurons, the on-direction is in the ipsilateral hemifield, with a vertical component that may be either upward or downward. Neurons with projections to the vertically related descending and superior vestibular nuclei tend to have on-directions with larger vertical components than neurons that lack these projections. These results, together with those on excitatory burst neurons reported in the preceding paper, demonstrate a reciprocal organization of burst neuron input to the abducens in the monkey similar to that found in the cat and indicate a major role for these neurons in generating the oculomotor activity in motoneurons as well as in other classes of premotor neurons.     

Visual system of the channel catfish (Ictalurus punctatus): II. The morphology associated with the multiple optic papillae and retinal ganglion cell distribution

The retinal organization associated with the multiple optic papillae of the catfish Ictalurus punctatus was examined. In each retina from ten to 17 papillae form an oval ring (which is wider dorsoventrally than mediolaterally). The dorsalmost papilla in this ring lies at the center of the retina. In addition, up to seven small papillae lie within the ring. Bundles of fibers leave the neural retina via the papillae. These bundles remain separate as they pass through the distal portions of the neural retina and then merge before passing through the choroid. Bundles running through dorsal papillae receive fibers from a roughly wedge-shaped retinal area; bundles running through ventral papillae receive fibers from a small area of central retina and a disproportionately large area of peripheral retina. A band of high ganglion cell density was observed extending between the nasal and temporal poles of the retina. No correlation was found between the retinal areas contributing fibers to the bundles of axons emerging from individual papillae and the areas of high cell density. Furthermore, no correlation was found between the average area of the retinal ganglion cells and the ganglion cell density. From HRP preparations and axon counts we estimate that each retina of 95-mm catfish contains about 50,000 ganglion cells.     

Retinal ganglion cell terminals in the hamster superior colliculus: an ultrastructural study.

The distribution, cross-sectional area, and presynaptic and postsynaptic characteristics of retinal ganglion cell axon terminals in the superior colliculus of normal adult female Syrian hamsters were investigated by quantitative ultrastructural techniques. After an intravitreal injection of horseradish peroxidase, most labelled axon terminals were found in the stratum griseum superficiale and stratum opticum of the contralateral superior colliculus. However, a small proportion (approximately 2%) of retinal ganglion cell axon terminals were located in deeper layers of the superior colliculus between the stratum opticum and the periaqueductal grey matter. Terminals were smaller in the upper two-thirds of the stratum griseum superficiale than in the lower one-third of this layer, the stratum opticum, and the stratum griseum intermedium. Presynaptic characteristics such as the length and number of contacts and the postsynaptic neuronal domains (somata, dendritic spines, or shafts) contacted by retinal ganglion cell axons in the superior colliculus were similar in all layers.     

Anterograde axoplasmic transport of peroxidase in the rat visual system after intraocular neurotoxin injection

The effect of intraocular injection of the neurotoxin, kainic acid, on the retinofugal pathway has been examined by anterograde transport of horseradish peroxidase. Although less peroxidase was present in all the primary optic centers after the intraocular injection of kainic acid that in the controls, the area occupied by the retinal terminals remained fairly constant except for the pretectal region where there is a smaller area of peroxidase precipitate. These results suggest that certain ganglion cells are killed by kainic acid but some others survive and their terminals are present in the majority of the terminal fields.     

A new tectal afferent nucleus of the infrared sensory system in the medulla oblongata of crotaline snakes

The existence of an infrared sensory neuron group with ascending fibers which directly reach the optic tectum in Crotaline snakes was confirmed with three methods. (1) With the retrograde horseradish peroxidase (HRP) method, labeled neurons were not found within the nucleus descendens lateralis nervi trigemini (DLV), but in an unnamed cell group located immediately ventral to the DLV of the contralateral side at the transitional portion between the nucleus oralis (DVo) and the nucleus interpolaris (DVi). This unnamed cell group, which was seen only in the Crotalinae, was provisionally called the ‘new nucleus’. (2) Normal brain series of 15 species were stained by the methods of Bodian-Otsuka, Klüver-Barrera and Nissl staining to compare the cytoarchitecture of the medulla oblongata. The ‘new nucleus’ was found only in species belonging to the Crotalinae. This nucleus was situated in fiber tracts which appeared to correspond to the lemniscus spinalis and tractus spino-cerebellaris of the reptilian medulla oblongata, and contained medium-sized multipolar or fusiform neurons. (3) In an electrophysiological study 16 single units responding unimodally to an infrared stimulus were recorded. Three of these recording sites were determined with Pontamine sky blue marking to be near or within the ‘new nucleus’.     

Retinotopic analysis of fiber pathways in the regenerating retinotectal system of the adult newt cynops pyrrhogaster

Retinotopic analysis of the pathways of regenerating retinal fibers within the optic tract and in the tectum of an adult newt was performed by selective labeling of the retinal fibers with horseradish peroxidase. At the tenth week of regeneration, all the regenerating retinal fibers from different retinal quadrants had terminal arbors nearly at the parts of the tectum innervated normally by those quadrants. The pathways for individual retinal fibers, however, were greatly disorganized within the optic tract and did not show any retinotopic ordered geography. The most rostral segregation of pathways of regenerating fibers was observed at the diencephalo-tectal junction. THe temporal retinal fibers invaded the tectum directly, while the dorsal, ventral and nasal retinal fibers generally shifted toward the dorsomedial or the lateral direction, as if they traced the dorsomedial or the lateral tracts formed in normal newt. The direction of the shifting of fiber pathways, however, did not depend on the origins of retinal fibers within retinal circumference, but depended on the location of fibers with in the optic tract. As a result, a large number of regenerating fibers reached their normal sites of innervation within the tectum via anomalous routes. These mis-routed fibers did not form branches or terminal arbors at ectopic parts within the tectum.     

Cortical and subcortical afferents to the amygdala of the rhesus monkey (Macaca mulatta)

The afferent projections to the primate amygdala were studied using horseradish peroxidase. The potential advantages of this technique are discussed compared with those previously used to determine amygdaloid afferents. The findings indicate that certain agranular or dysgranular cortical regions may project directly to the amygdala: in particular, the orbital frontal cortex, anterior cingulate gyrus, subcallosal gyrus, temporal pole and anterior insula. These projections probably terminate predominantly in either the lateral or accessory basal nuclei. Other cortical projections from the inferotemporal and superior temporal gyri are described. Evidence was found for a heavy projection from the superior temporal sulcus to the lateral nucleus. Subcortical afferents were found from the hypothalamus, substantia innominata, diagonal band, thalamus, periaqueductal central gray, peripeduncular nucleus and from a band of cells extending medially from the peripeduncular nucleus to the midline, just ventral to the thalamus. In the thalamus, labelled cells were restricted to the non-specific nuclei, and were common in the rostral midline nuclei. No projection was observed from the dorsomedial nucleus of the thalamus. We discuss the implications of these results for interpreting the functions of the amygdala.     

Retinotectal terminals in the superior colliculus of the rabbit: a light and electron microscopic analysis

The projection from the retina to the controlateral superior colliculus was studied light and electron microscopically by means of anterogradely transported horseradish peroxidase and tetramethylbenzidine histochemistry as well as light microscopically by experimental degeneration and [3H]-leucine autoradiography. Labeled boutons were found in stratum zonale (SZ) and in stratum griseum superficiale (SGS), but not in stratum opticum (S0). The number of boutons was maximal in a narrow zone in SZ about 25 to 100 micron below the surface. The labeled boutons contained numerous round vesicles and predominantly pale mitochondria. They usually formed asymmetrical synapses and contacted dendrites or boutons. Occasionally, labeled boutons were observed whose cytological features were different from those generally associated with retinotectal axons. In general, labeled boutons in SZ contained fewer mitochondria than those from SGS. Labeled myelinated axons were found throughout SGS, in the lowest part of SZ, and in SO. In upper and middle SGS they were small while in lower SGS and SO also large fibers were found.