Calretinin-like immunoreactivity in the optic tectum of the tench (Tinca tinca L.)

The distribution of calretinin-like immunopositive cells and fibers in the optic tectum of the tench (Tinca tinca) was studied by using a polyclonal antibody and the avidin-biotin-peroxidase technique. A clear laminated pattern of calretinin-like immunoreactivity was observed. The stratum periventriculare demonstrated a large number of strongly labeled cells whereas in the strata album centrale and griseum centrale, and at the boundary between the strata griseum centrale and fibrosum et griseum superficiale, some scarce, weakly immunostained cells were observed. No immunoreactive cells were seen in the strata fibrosum et griseum superficiale, opticum and marginale. Cells belonging to neuronal types X and XIV, previously characterized using Golgi impregnation, were found to be calretinin-like immunoreactive. Most calretinin-like immunopositive fibers were found in the strata fibrosum et griseum superficiale and opticum with a distribution pattern similar to retinotectal axons in these layers. In agreement with previous biochemical studies, our data suggest that, by contrast to all other classes of vertebrates, instead of calretinin and calbindin D-28k, only one protein is present in teleosts. Nevertheless, the calretinin-like immunostaining pattern in the teleost optic tectum was more complex than that previously described for calbindin D-28k. When compared to the calretinin-immunostaining in the rat superior colliculus, it is evident the presence in both amniotes and anamniotes of calretinin-immunopositive retinotectal axons. However, the distribution patterns of intrinsic calretinin-immunoreactive cells were different. Immunolabeled cells have been described in all layers of the superior colliculus, whereas the cells containing calretinin were restricted to the three deep strata of the tench optic tectum, a more similar distribution to what has been reported in the chick optic tectum.     

Immunohistochemical localization of nicotinic acetylcholine receptor subunits in the mesencephalon and diencephalon of the chick (Gallus gallus).

Monoclonal antibodies against two alpha-bungarotoxin-binding subunits (alpha 7 and alpha 8) of the nicotinic acetylcholine receptors (nAChRs) were used as immunohistochemical probes to map their distribution in the chick diencephalon and mesencephalon. The distribution of the alpha 7 and alpha 8 nAChR subunits was compared to the distribution of immunoreactivity produced by a monoclonal antibody against the beta 2 structural subunit of the nAChRs. Structures that contained high numbers of alpha 7-like immunoreactive (LI) somata included the intergeniculate leaflet, nucleus intercalatus thalami, nucleus ovoidalis, organum paraventricularis, nucleus rotundus, isthmic nuclei, nucleus trochlearis, oculomotor complex, nucleus interstitio-pretecto-subpretectalis, stratum griseum centrale of the optic tectum, and nucleus semilunaris. Neuropil staining for alpha 7-LI was intense in the nucleus dorsomedialis hypothalami, nucleus geniculatus lateralis ventralis, griseum tecti, isthmic nuclei, nucleus lentiformis mesencephali, nucleus of the basal optic root, and stratum griseum et fibrosum superficiale of the tectum. High numbers of alpha 8-LI somata were found in the stratum griseum et fibrosum superficiale of the tectum and the nucleus interstitio-pretecto-subpretectalis, and intense neuropil staining for alpha 8-LI was found in the dorsal thalamus, nucleus geniculatus lateralis ventralis, lateral hypothalamus, griseum et fibrosum superficiale of the tectum. High numbers of beta 2-LI somata were found only in the nucleus spiriformis lateralis, whereas neuropil staining for beta 2-LI was intense in the nucleus geniculatus lateralis ventralis, nucleus suprachiasmaticus, nucleus lateralis anterior, nucleus habenularis lateralis, area pretectalis, griseum tecti, nucleus lentiformis mesencephalis, nucleus externus, and nucleus interpeduncularis, and in the stratum griseum centrale, stratum griseum et fibrosum superficiale, and stratum opticum of the tectum. These results indicate that there are major disparities in the localization of the alpha-bungarotoxin-binding alpha 7 and alpha 8 nAChR subunits and the beta 2 structural nAChR subunit in the chick diencephalon and mesencephalon. These nAChR subunits appear, however, to coexist in several regions of the chick brain.     

Distribution of Met-enkephalin, Leu-enkephalin, substance P, neuropeptide Y, FMRFamide, and serotonin immunoreactivities in the optic tectum of the Atlantic salmon (Salmo salar L.)

The distribution of the neuropeptides methionine- and leucine-enkephalins, substance P, FMRFamide, neuropeptide Y, and vasoactive intestinal peptide, as well as the biogenic amine serotonin was studied in the optic tectum of the Atlantic salmon by means of immunocytochemistry. Peroxidase-antiperoxidase and indirect immunofluorescence methods were used to compare the differential laminar distribution of each of these substances. Nine parts of the optic tectum were selected for analysis on frontal sections: median, dorsolateral, and ventrolateral areas at rostral, medial, and caudal levels. Methionine- and leucine-enkephalin immunoreactive fibers were found in discrete sublayers in the following strata: stratum opticum, stratum fibrosum et griseum superficiale, stratum griseum centrale, stratum, and album centrale. Most of the substance P-, serotonin-, and vasoactive intestinal peptide-immunoreactive fibers were found in the stratum album centrale, whereas the FMRFamide- and neuropeptide Y-immunoreactive fibers were more or less randomly distributed within most of the strata of the optic tectum. Neuropeptide Y-immunoreactive cell bodies were located in the stratum periventriculare. We suggest an extrinsic origin for most of the immunoreactive fibers observed in the optic tectum, except for the neuropeptide Y-immunoreactive fibers that probably originate in the periventricular neurons. Although retinal peptidergic input to the optic tectum has been proposed in other vertebrates, there is no evidence that any of the neuropeptidelike or serotonin immunoreactive fibers in the optic tectum of the salmon should be of retinal origin. Differences and similarities with the distribution of neuropeptides in the optic tectum in representatives of other vertebrate classes are discussed.     

The optic tectum of the carp: pyramidal neurons of the SFGS

We have studied the stratum fibrosum et griseum superficiale (SFGS) of the carp optic tectum with optic and electron microscopy. This stratum is a dense neuropil with disordered appearance, in which numerous neuronal bodies of different characteristics and variable distribution according to the tectal regions are intercalated, more abundant in the dorsomedial zones of the tectum. Within these neuronal types, the most characteristic of SFGS are the large pyramidals of vertical development. Such neurons shows an ascendant dendritic shaft, very developed in the stratum marginale, a thinner dendritic shaft in the basal pole and a descending axon that reaches the internal zones of the stratum griseum centrale. Glial elements are highly associated to the pyramidal neuron bodies. The synaptic contacts are abundant and of various types, specially on the spinous dendritic branches which lie in the stratum marginale.     

The visual connections of the adult flatfish, Achirus lineatus.

Metamorphosis in the flatfish is characterized by the migration of one eye around the dorsal surface of the head to a position adjacent to the other eye on the new top side of the animal. The visual connections of the adult flatfish, Achirus lineatus, were examined. Either the migrating or non-migrating eye was removed and the animal allowed to survive for one to three weeks. Alternate sections of the brain were stained by a modification of the Fink-Heimer technique, or with cresyl violet. The diencephalic visual connections of the flatfish were similar to those of other teleosts with contralateral projections to the nuclei corticalis, dorsomedialis thalami, pretectalis, and the corpus geniculatum laterale. The distribution of the retinal efferents to the optic tectum is unique in the flatfish. In the medial one-third of the tectum, terminal degeneration was found in three bands in the stratum opticum (SO) and the stratum griseum et fibrosum superficiale (sgfs). In the middle part of the tectum, two bands of degeneration remained over the sgfs. The lateral part of the tectum has only a very small amount of degeneration distributed radomly in scattered clusters over the deep SO and superficial sgfs. The Nissl preparations also reflected the differences between the medial and lateral parts of the tectum. Distinct layer was lacking in the medial tectum with a conspicuously absent large cell layer in the stratum griseum centrale (sgc). In contrast, the lateral tectum had a typical tectal stratification. Most notable were the large neurons of the sgc.     

Optic tectum of the eastern garter snake, Thamnophis sirtalis. III. Morphology of intrinsic neurons

Extracellular iontophoretic injections of horseradish peroxidase and Golgi preparations were used to study the distribution and morphology of intrinsic neurons of the garter snake optic tectum. Four morphologically distinct classes of neurons were identified. The type A neuron is found throughout the retinorecipient tectal layers. It has a large, fusiform soma and infrequently branching dendrites that radiate in the horizontal plane and are studded with varicose appendages. An axon arises from the soma or proximal dendrite and gives rise to widely spreading branches that overlap the cell's dendritic field. The type B neuron has a small, spherical soma in sublayer b of the stratum fibrosum et griseum superficiale. Thick, varicose dendrites ascend from the soma and form a bushy arbor in the overlying sublayer a. A thin axon descends vertically from the soma and arborizes in vertical alignment with the cell's dendritic field in sublayer c of the stratum fibrosum et griseum superficiale and the upper third of the stratum griseum centrale. The type C neuron is a bipolar cell with a small, vertically fusiform soma situated at the upper border of the stratum griseum centrale. Thin, sparsely branching dendrites extend vertically into the superficial and central gray layers. An axon arises from the soma and courses ventrally into the stratum griseum centrale where it gives rise to a plexus of widely spreading branches that extend medially from the cell's dendritic field. The type D neuron is a small, stellate cell with a spherical soma and fine, appendage-laden dendrites that are restricted to the stratum griseum centrale. The axon of the type D cell courses in the central gray where it gives rise to widely spreading branches that extend laterally from the cell's dendritic field.     

The retinofugal pathways in the primitive African bony fish Polypterus senegalus (Cuvier, 1829)

Retinal projections were studied using Fink-Heimer and radioautographic methods in Polypterus senegalus, a species which is representative of a small group of African fresh-water bony fish often considered to be very primitive.The large optic nerve showed partial decussation at the chiasm. Two major contralateral tracts were observed: the axillary and marginal optic tracts, with the latter being subdivided posteriorly into the tractus opticus medialis and tractus opticus lateralis. The retina projected onto the: (1) hypothalamus (area optica postoptica); (2) thalamus (nucleus opticus dorsolateralis thalimi, nucleus dorsomedialis thalami, corpus geniculatum laterale, area optica dorsolateralis thalami, area optica ventrolateralis thalami); (3) pretectum (nuclei commissurae posterioris, pretectalis ventralis, pretectalis dorsalis); and (4) optic tectum (stratum marginale, stratum opticum, stratum griseum et fibrosum superficiale, stratum griseum et album centrale, stratum griseum et fibrosum periventriculare). Ipsilateral retinal projections were demonstrated to the same 4 levels and more precisely to the nucleus opticus dorsolateralis thalami, area optica dorsolaterale thalami, nucleus commissurae posterioris, stratum marginale and stratum griseum et album centrale. The existence of a retinal projection to the mesencephalic tegmentum is discussed.Comparing the primary optic system of Polypterus with that of other jawed vertebrates, and particularly with that of other bony fish, indicated that this species possesses a combination of characteristics which are both actinopterygian and sarcopterygian. The phylogenetic significance of this mozaic anatomical arrangement is discussed.     

Quantitative histological studies of the optic tectum in six species of Notropis and Cyprinella (Cyprinidae, Teleostei).

Significant differences in stratification and size of the visual layers of the optic tectum were found between three clear-water minnows (Notropis amabilis, N. boops, Cyprinella venustas) and three turbid-water minnows (N. atherinoides, N. bairdi, and C. lutrensis). Correlations among a variety of neural structures suggested the importance of stratum marginale (SM), stratum opticum (SO), and stratum fibrosum et griseum superficiale (SFGS), stratum griseum centrale (SGC) and stratum periventriculare (SPV) in vision, of stratum album centrale (SAC) and SGC for olfaction, and of SPV for the processing of acoustico-lateral information.     

Diameters and terminal patterns of retinofugal axons in their target areas: an HRP study in two teleosts (Sebastiscus and Navodon)

Studies in various vertebrate classes, particularly amphibians and mammals, have revealed that retinal ganglion cells with different functional properties project by means of axons of correspondingly different diameters onto specific target regions. Whether a similar pattern exists in teleosts is partly investigated in the present study. HRP was injected into the optic nerve of Sebastiscus and Navodon. The calibers of intraretinal HRP-labeled axons were classed as fine (ca. 0.8 micron), medium (ca. 1.3 micron), and coarse (ca. 2.5 microns). The calibers of HRP-labeled retinofugal axons were then determined in their target areas, and these can be summarized as follows: Optic hypothalamus: fine, medium. Lateral geniculate nucleus: fine. Dorsolateral thalamic nucleus: fine, medium. Area pretectalis: fine. Nucleus of the posterior commissure: fine, medium. Area ventralis lateralis, contralateral: fine, medium, coarse; ipsilateral: coarse. Optic tectum, stratum opticum: fine, medium; stratum fibrosum et griseum superficiale: fine, medium, coarse, segregated in sublayers; stratum album centrale: fine, medium, coarse. Therefore, fine fibers were found to reach all target areas except the ipsilateral area ventralis lateralis, and these were the only fibers found in the lateral geniculate nucleus, area pretectalis, and stratum griseum centrale of the optic tectum. Coarse fibers, on the other hand, were found only in the area ventralis lateralis and the optic tectum (stratum fibrosum et griseum superficiale and stratum album centrale). Terminal patterns of these fibers were also studied. Most fine fibers take tortuous courses giving off a few branches and terminate with many varicosities, and medium and coarse fibers give off several finer branches and terminate with bulbous swellings. The physiological significance of these findings is discussed. In addition, retrogradely labeled (retinopetal) cells were found in the olfactory bulb and the area ventralis pars ventralis of the telencephalon, as well as in the preoptic area and the dorsolateral thalamic nucleus.     

Multiple-site optical recording for characterization of functional synaptic organization of the optic tectum of rainbow trout

To map the functional synaptic organization over a wide area in the optic tectum, we directly monitored two-dimensional propagation of postsynaptic depolarization evoked by firing of retinotectal afferents in optic tectum slices prepared from rainbow trout (Oncorhynchus mykiss), using a voltage-sensitive dye and a photodiode array system. The postsynaptic responses to afferent stimulation first propagated in the stratum opticum and stratum fibrosum et griseum superficiale in an anterograde fashion in the afferents and then expanded vertically into the deep layers. This vertical propagation appeared to occur along a bundle-like structure that corresponded well with a cluster of neurons whose somata are located in the stratum periventriculare. Pharmacological studies showed that these postsynaptic responses were mediated by ionotropic glutamate receptors. On the other hand, the optical signals appeared to consist of at least two components (a transient signal and a slow signal). The second transient signal summated with the first slow signal by paired stimulation, suggesting that the transient and slow signals originated from different cell types. Taken together, these results showed that the functional synaptic organization of the teleost optic tectum comprises of two depolarization-signal propagating paths along a horizontal layer structure and a vertical bundle-like structure and that these synaptic responses occur via glutamatergic transmission.     

Long-term potentiation in the goldfish optic tectum

Field potential recordings were made in the primary retinal synaptic area, the stratum fibrosum et griseum superficiale (SFGS), of an in vitro goldfish optic tectum preparation. Stimulation of the optic nerve at frequencies of 1 and 5 Hz produced a long-term potentiation (LTP) of the synaptic response which developed gradually. No potentiation was seen with lower or higher frequencies. These results demonstrate a significant LTP with a slow time course and restricted low-frequency dependence.     

Hereditary visual impairment in a new mutant strain of chicken, GSN/1

Visual pathways of a new mutant of chicken (GSN/1) with hereditary visual impairment were ophthalmologically, electrophysiologically, and histopathologically examined. Clinically, GSN/1 chickens drooped and had decreased locomotor activity. As an indicator of visual acuity, the spatial frequency characteristics of GSN/1 chickens showed poor scores at high frequency. No abnormal findings were observed ophthalmoscopically in the fundus of them. The amplitudes of a and b waves of the electroretinogram in GSN/1 chickens revealed no abnormal findings for xenon flash stimuli with different intensities. However, responses of the visual evoked potential in GSN/1 chickens were insensitive to xenon flash stimuli. Histologically, the retina of GSN/1 chickens was slightly hypoplastic and the retinal ganglion cells decreased in number, although there were no degenerative or reactive changes. The optic tectum, especially the stratum opticum and the stratum griseum et fibrosum superficiale were hypoplastic and contained reduced numbers of optic nerve fibers. The sublayers of the stratum griseum et fibrosum superficiale were disorganized, in which axons of the optic nerve were distributed irregularly. These findings suggest that visual impairment observed in the new mutant GSN/1 chicken may be related to developmental defects in the visual pathways, especially in the optic tectum.     

Electron microscope histochemistry of acetylcholinesterase distribution in the optic tectum of teleosts.

An ultrastructural analysis was made on acetylcholinesterase (AChE) localization in the optic tectum of two teleosts, the goldfish and the catfish. Electron microscope histochemistry reveals several details on synthesis, distribution and possible sites of utilization of the enzyme in the different tectal layers. The results show that AChE is synthesized by all the neuronal types present in the optic tectum. The final localization of the enzyme is the result of its synthesis in cell bodies, its storage and transport along dendrites and its release in extracellular spaces. The differences in AChE localization between the two teleosts examined mainly derive from differential enzyme release in the corresponding layers of the optic tectum. Cholinergic synapses cannot be precisely identified by means of AChE histochemistry, but the layers in which maximum release of enzyme in the extracellular spaces occurs most likely correspond to areas where cholinergic mechanisms are operating. In this connection some interesting differences of AChE localization in the superficial tectal layers (stratum marginale, stratum opticum and stratum fibrosum et griseum superficiale) are discussed. Electron microscopic histochemistry of AChE confirms its usefulness in better understanding some links between the anatomical and functional organization of complex neural structures.     

Neuronal organization of the optic tectum in the river lamprey, Lampetra japonica: a Golgi study.

The neuronal and laminar organization of the optic tectum (OT) in the river lamprey was studied using the rapid Golgi method. Based primarily on the distribution pattern of the dendrites, the OT neurons were divided into vertical, horizontal and stellate neurons. The river lamprey OT shows a laminar structure consisting of eight concentric strata. The stratum ependymale consists of several rows of ependymal cells. The stratum cellulare periventriculare contains one to two rows of vertical neurons. The stratum fibrosum periventriculare is thin and contains a few vertical neurons. The stratum cellulare et fibrosum internum consists of several alternating cellular and fibrous layers: a large variety of vertical and horizontal neurons are distributed in this stratum. The stratum fibrosum centrale consists of compact horizontal fiber bundles, among which a few horizontal neurons are disseminated. In the stratum cellulare et fibrosum externum, numerous fibers run horizontally in a loosely organized plexus; various types of vertical, horizontal and stellate neurons are distributed among these fibers. The stratum opticum is the main terminal area of the optic nerve, and contains stellate and horizontal neurons. The stratum marginale is a thin layer and consists of sparse populations of vertical and horizontal neurons. Besides the above outer to inner laminar structure, the OT is divided into medial (m-OT) and lateral parts (1-OT), based primarily on the distribution pattern of the dendrites. The dendrites of neurons in the m-OT are distributed almost exclusively within the OT. On the other hand, the dendrites of some neurons in the 1-OT extended into the confines of the torus semicircularis (TS), and conversely, the dendrites of some neurons in the TS are distributed in the 1-OT. These findings are discussed in relation to the neuronal and laminar organization of the OT in other lamprey species and to recent hodological studies.     

Optic tectum of the eastern garter snake, Thamnophis sirtalis. II. Morphology of efferent cells

Tectal efferent neurons were retrogradely filled from extracellular injections of horseradish peroxidase (HRP) into pathways efferent from the tectum. Tectorotundal neurons have cylindrical dendritic trees, 80-100 microns in diameter, that extend vertically across the central and superficial tectal layers. Apical and basal dendrites are laden with complex appendages. The axon gives rise to an intratectal, collateral arbor that extends horizontally into the stratum griseum centrale beyond the cell's dendritic tree. The parent axon exits the tectum laterally in the tectothalamic tract. Tectogeniculate neurons also have narrow, radially oriented, and highly branched apical dendrites, but their basal dendrites are infrequently branched and lack appendages. An intratectal axon collateral forms a small, spherical arbor overlapping the apical dendrites in sublayer c of the stratum fibrosum et griseum superficiale. The parent axon ascends vertically and just below the stratum opticum turns rostrad to follow the optic fibers to the diencephalon. Tectoisthmi neurons have small somata and thin, radial dendrites that arborize below the pial surface in the stratum zonale. An intratectal axon collateral forms a spatially restricted arbor ventral to the soma in register with the dendritic tree. Tectoisthmobulbar neurons have dendrites that arborize extensively in sublayer a of the stratum fibrosum et griseum superficiale. The axon exits the tectum without collateralizing and joins a small-caliber component of the ventral tectobulbar tract. Ipsilateral tectobulbar neurons have stellate dendritic fields, 150-250 microns in diameter, that are restricted to the deep layers of the tectum. Sparsely branched dendrites are appendage-free but bear many short, fine spicules. The axon initially ascends from the soma and recurves into the stratum album centrale without collateralizing before joining a medium-caliber component of the ventral tectobulbar tract. Crossed tectobulbar neurons have large, stellate dendritic trees with diameters ranging from 200 to 500 microns. Like ipsilateral tectobulbar neurons, their dendrites are appendage-free but bear spicules. Their thick-caliber axons exit the tectum without collateralizing and course deep in the stratum album centrale to reach the dorsal tectobulbar tract.     

Identification of retinotectal projection pathway in the deep tectal laminae in the chick

The optic tectum is a visual center of nonmammalian vertebrates that receives retinal fibers in a retinotopic manner. It has been accepted that retinal fibers project to some superficial laminae of the tectum, but do not go through lamina g of stratum griseum et fibrosum superficiale (SGFS). By a novel fiber-tracing method, we found a novel pathway of retinal fibers that run through deep laminae of the tectum. The retinal fibers that would run through the newly identified pathway first run caudally along the medial edge after invading the tectum, turn laterally, and extend toward the lateral side through the deep pathway. The deep pathway runs through stratum album centrale and stratum fibrosum periventriculare. The fibers that run through the deep pathway do not enter the stratum opticum, where the conventional retinal fibers run. As development proceeds, these fibers decrease and disappear by the adult stage. By the new method, we found that some of the conventional retinal fibers transiently run through lamina g of SGFS and invade laminae h/i. In conclusion, we found distinct but transient retinal fiber pathway in the deep tectal laminae, which have not been thought to be retinorecipient.     

Comparative neurology of the optic tectum in ray-finned fishes: patterns of lamination formed by retinotectal projections

Retinotectal projections were studied in 33 different species of Actinopterygii, the ray-finned fishes, with horseradish peroxidase and cobalt tracing techniques. The distribution of retinorecipient layers in the contralateral optic tectum was analyzed. In addition, the degree of differentiation of the stratum periventriculate, and the presence of ipsilateral retinotectal projections was examined. Retinofugal fibers are labeled in the stratum opticum (SO), stratum fibrosum et griseum superficiale (SFGS), stratum griseum centrale (SGC), stratum album centrale (SAC) and stratum periventriculare (SPV). Some species lack the projection to the SO, others lack the projection to the SGC, and a third group of fishes lack both projections. Five different patterns of retinorecipient tectal strata are distinguished. These patterns correlate with the species' taxonomic position. Evolutionary trends of tectal lamination and retinotectal innervation are described. The retinotectal projection patterns provide a useful indicator of phylogenetic relationships. Some of our data suggest different relationships between actinopterygian species than hitherto believed.     

Retinofugal and retinopetal projections in the cichlid fish Astronotus ocellatus

The retinofugal and retinopetal projections of the cichlid fish Astronotus ocellatus were studied by applying cobaltous-lysine to the optic nerve. Retinal axons terminate bilaterally in a preoptic-suprachiasmatic region between the base of the third ventricle and the anterior genu of the horizontal commissure and among periventricular cells along the sides of the ventricle. Other retinal axons innervate the tuberal region of the hypothalamus anterior to the infundibulum. Targets innervated in the pretectum include nucleus lateralis geniculatus and dorsal, medial, and ventral pretectal nuclei. Three other targets (nucleus opticus dorsolateralis, nucleus opticus commissurae posterior, nucleus opticus ventrolateralis) are innervated by fibers that leave the medial edge of the dorsal optic tract. Two other targets (basal optic nucleus and accessory optic nucleus) are innervated by fibers from the ventral optic tract. These retinal projections are similar to those previously reported for goldfish in an experiment that used the cobaltous-lysine method (Springer and Gaffney, J. Comp. Neurol. 203:401-424, '81). Retinotectal optic axons were found in a superficial lamina just above the stratum opticum, in the stratum opticum, in three layers of the stratum fibrosum et griseum superficiale, in a lamina just beneath the stratum fibrosum et griseum superficiale, and in the stratum album centrale just above the stratum periventriculare. This result is similar to that previously reported for goldfish; however, the spatial relationships between the various retinorecipient laminae differ for goldfish and Astronotus ocellatus. Efferents to the retina originate in two nuclei and both project contralaterally. The first is the nucleus olfactoretinalis, which is located ventrally between the olfactory lobe and telencephalon. It consists of about 400 cells, of which, approximately 200 cells project to the retina. The second retinopetal nucleus, nucleus thalamoretinalis, is a diffuse group of about 200 cells that project to the retina.     

Periventricular efferent neurons in the optic tectum of rainbow trout

The efferent connections and axonal and dendritic morphologies of periventricular neurons were examined in the optic tectum of rainbow trout to classify periventricular efferent neurons in salmonids. Among the target nuclei of tectal efferents, tracer injections to the following four structures labeled periventricular neurons: the area pretectalis pars dorsalis (APd), nucleus pretectalis superficialis pars magnocellularis (PSm), nucleus ventrolateralis of torus semicircularis (TS), and nucleus isthmi (NI). Two types of periventricular neurons were labeled by injections to the APd. One of them had an apical dendrite ramifying at the stratum fibrosum et griseum superficiale (SFGS), with an axon that bifurcated into two branches at the stratum griseum centrale (SGC), and the other had an apical dendrite ramifying at the SGC. Two types of periventricular neurons were labeled after injections to the TS. One of them had an apical dendrite ramifying at the boundary between the stratum opticum (SO) and the SFGS, and the other had dendritic branches restricted to the stratum album centrale or stratum periventriculare. Injections to the PSm and NI labeled periventricular neurons of the same type with an apical dendrite ramifying at the SO and a characteristic axon that split into superficial and deep branches projecting to the PSm and NI, respectively. This cell type also possessed axonal branches that terminated within the tectum. These results indicate that periventricular efferent neurons can be classified into at least five types that possess type-specific axonal and dendritic morphologies. We also describe other tectal neurons labeled by the present injections.