Retinal projections in the catfish,Mystus vittatus (Bloch) as revealed by tracer studies with horseradish peroxidase

Summary The retinal efferents of the catfish, Mystus vittatus, were investigated with the use of the horseradish peroxidase (HRP) technique. Most retinal fibres extended contralateral to the eye that had received HRP label, while a few fascicles projected to the ipsilateral side without decussation in the optic chiasma. The contralateral fibres projected to the suprachiasmatic nucleus, the nucleus opticus dorsolateralis, the nucleus of the posterior commissure, the nucleus geniculatus lateralis, pretectal nuclear complex, and to two layers of the optic tectum, i.e., stratum fibrosum et griseum superficiale and stratum griseum centrale. The accessory optic tract arose from the inner area of the optic tract and extended ventromedially to the accessory optic nucleus. The ipsilateral fascicles projected to almost all the above mentioned nuclei, but these projections were comparatively sparse. The ipsilateral retinal projection was restricted to the rostral tectum.     

Bilateral retinal projections in the black piranah (Serrasalmus niger)

Summary The retinal projections were studied in the black piranah (Serrasalmus niger) with degeneration and autoradiographic methods. The projections are bilateral to the hypothalamic optic nucleus, the dorsomedial optic nucleus, corpus geniculatum ipsum of Meader (1934) and the optic tectum. Unilateral, crossed projections were traced to the pretectal nucleus and the cortical nucleus. The visual system of the black piranah is exceptionally well developed but has retained many primitive features including the extensive bilateral projections.     

Retinofugal projections in the eel,Anguilla anguilla L. (Teleostei), visualized by the cobalt-filling technique

The retinofugal projections in the eel were studied by use of the cobalt-filling technique. The optic tract projects contralaterally to the hypothalamic optic nucleus, the anterior periventricular nucleus, the lateral geniculate nucleus, the dorsomedial optic nucleus, four pretectal recipient areas, the optic tectum, and the tegmentum. Small ipsilateral projections were demonstrated in the hypothalamic optic nucleus, the dorsomedial optic nucleus, and the optic tectum.     

Retinal projections in the chain pickerel (Esox niger Lesueur)

Summary The retinal projections inEsox niger, as determined with the aid of a modified cobalt-lysine method, are considerably more extensive in the diencephalon and pretectum than in other teleost fishes so far examined. Although most retinal axons terminate contralaterally, rare fibers can be traced to the same aggregates ipsilaterally. The retinohypothalamic projection appears larger than hitherto reported in teleosts, and the dorsomedial optic tract issues fibers to a series of cell clusters extending from the rostral thalamus to mid-torus levels. A retinal projection to a presumed ventrolateral optic nucleus (VLO) is described for the first time in a teleost. Other targets of retinal fibers include the nucleus geniculatus lateralis ipse of Meader (GLI), the pretectal nucleus (P), the cortical nucleus and a well-developed ventromedial optic nucleus (VMO). The projection to the optic tectum is principally to the stratum fibrosum et griseum superficiale (SFGS) and stratum marginale (SM), but a considerable number of axons also course through the stratum album centrale (SAC) before terminating there or piercing the stratum griseum centrale (SGC) and terminating in SFGS. Rare terminal arborizations of retinal fibers were also observed in stratum griseum centrale (SGS) and in the stratum griseum periventriculare (SGC) in restricted portions of the tectum. Because of the relatively large size of the visual structures inE. niger it is a potentially useful model for future experimental studies on the visual system.     

Brain-derived neurotrophic factor (BDNF)-like immunoreactivity localization in the retina and brain of Cichlasoma dimerus (Teleostei, Perciformes)

Brain-derived neurotrophic factor (BDNF) is a neurotrophin involved in the development and maintenance of vertebrate nervous systems. Although there were several studies in classical animal models, scarce information for fish was available. The main purpose of this study was to analyze the distribution of BDNF in the brain and retina of the cichlid fish Cichlasoma dimerus. By immunohistochemistry we detected BDNF-like immunoreactive cells in the cytoplasm and the nuclei of the ganglion cell layer and the inner nuclear layer of the retina. In the optic tectum, BDNF-like immunoreactivity was detected in the nucleus of neurons of the stratum periventriculare and the stratum marginale and in neurons of the intermediate layers. In the hypothalamus we found BDNF-like immunoreactivity mainly in the cytoplasm of the nucleus lateralis tuberis and the nucleus of the lateral recess. To confirm the nuclear and cytoplasm localization of BDNF we performed subcellular fractionation, followed by Western blot, detecting a 39 kDa immunoreactive-band corresponding to a possible precursor form of BDNF in both fractions. BDNF-like immunoreactivity was distributed in areas related with photoreception (retina), the integration center of retinal projections (optic tectum) and the control center of background and stress adaptation (hypothalamus). These results provide baseline anatomical information for future research about the role of neurotrophins in the adult fish central nervous system.     

Cytoarchitectural organization of the electromotor system in the electric catfish (Malapterurus electricus)

Summary The cytoarchitectural organization of the electromotor system of the electric catfish (Malapterurus electricus) was investigated in order to obtain insight into the neuronal reorganization accompanying the functional transition of a presumptive previous motor system to an electromotor system eliciting electric organ discharge. The electric catfish possesses two giant electromotoneurons situated within the rostral spinal cord. Intracellular dye injections have revealed the enormous extension of the dendritic tree of electromotoneurons. About 50 primary dendrites span the entire lateral funicle and intermediate grey matter, and reveal an extensive contralateral projection. The giant dendritic tree (1.2 mm in rostrocaudal direction) presumably receives inputs from all ascending and descending pathways of the spinal cord. Electromotoneurons and motoneurons receive the same type of fibre inputs, and electromotoneurons and interneurons are connected through common presynaptic elements. The innervation pattern of the electromotoneurons and spinal motoneurons is similar. Synaptic terminals with round synaptic vesicles often reveal chemical contacts and gap junctions. Furthermore, dendrites of the two electromotoneurons form juxtapositions (ephapses) with each other and also with spinal interneurons. Our results suggest that the two electromotoneurons are homologous to median (primary) spinal motoneurons and are the central structures of the electromotor system within the central nervous system of the electric catfish. A high capability of information processing can be attributed to the giant dendritic trees from functional considerations. This presumably enables the electromotoneurons to elicit an electric organ discharge in different behavioural contexts with a minimum of functional reorganization.     

Temporal patterning of electric organ discharges in the African electric catfish, Malapterurus electricus (Gmelin)

This paper is the first detailed analysis of situation-specific temporal patterning of electric organ discharges (EODs) in a strong electric fish. Using a resident-intruder paradigm EODs were recorded during interactions between dyads composed of Malapterurus electricus (Gmelin) and four different types of fish: (1) conspecifics; (2) large prey-type mid-water fish, goldfish ( Carassius auratus , Linnaeus 1758) and tilapia ( Oreochromis melanotheron , Rüppel, 1852); (3) a sympatric competitor, Polypterus palmas (Ayres 1850) and (4) a larger, threatening catfish, Clarias sp.
An analysis of the EODs emitted showed that in the presence of conspecifics the average EOD volley consisted of a single long-duration, low frequency train of EODs. The presence of the midwater fish (goldfish and Tilapia) elicited volleys consisting of two short trains, and P. palmas elicited long duration volleys with two trains and long inter-train intervals. Finally, an attacking Clarias resulted on average in volleys consisting of two high-frequency trains of EODs. With nonconspecific partner species resident electric catfish emitted volleys with more pulses, more trains that were longer in duration and higher in frequency than the EODs in volleys emitted by intruder electric catfish with the same species stimulus fish.     

The electromotor system of the electric catfish (Malapterurus electricus): A fine-structural analysis

Summary The electromotor system of the electric catfish (Malapterurus electricus) consists of two large ganglion cells situated in the spinal cord, two single axons containing electric nerves and two large electric organs with several million electroplaque cells. The small, irregularly stacked electroplaque cells possess at their center a crater-like indentation from which a stalk like protrusion arises. Many synaptic contacts derived from a single axon collateral are carried on lobe-like protrusions at the terminal knob of this stalk. The electric nerve consists of a large myelinated axon (diameter: 25 m) surrounded by many layers of connective tissue cells. The two ganglion cells (200 m in diameter) are rich in elements of the rough endoplasmic reticulum, Golgi apparatus and lysosomal structures. The cytoplasm of the soma changes its appearance towards the voluminous axon hillock (50 m in diameter) which these organelles do not enter. The cell soma is perforated in a tunnel-like manner by blood capillaries, axons and processes of glial cells. The cell soma and dendrites are covered with two types of synapse. One type forms mixed chemical and electrical (gap junctions) contacts with intermediate attachment plaques. The other type is only chemical in nature. This system may be useful in the study of an identified vertebrate giant neuron.     

Observations on the food and feeding habits of the African electric catfish Malapterurus electricus (Gmelin)

The diet and feeding habits of the African electric catfish Malapterurus electricus in their natural habitat in Lake Kainji, and in the River Niger, downstream of Kainji dam, Nigeria, have been described and compared. The study showed the electric catfish is a voracious piscivore. It feeds on cichlids, clupeids, schilbeids and other available fish species. From the size distribution, numbers and composition of the small prey fish species examined in electric catfish stomachs, it was inferred that the powerful high-frequency electric organ discharge volleys serve as major predatory mechanism.     

Semicystic spermatogenesis and biflagellate spermatozoon ultrastructure in the Nile electric catfish Malapterurus electricus (Teleostei: Siluriformes: Malapteruridae)

Spermatogenesis and spermatozoon ultrastructure in the Nile electric catfish Malapterurus electricus are described using scanning and transmission electron microscopy. Although the testis organization conforms to the ‘unrestricted’ spermatogonial type, the species has a rare type of spermatogenesis not previously described among catfishes, ‘semicystic’, in which the cyst ruptures before the spermatozoon stage. Spermiogenesis also involves some peculiar features such as condensation of the chromatin in the posterior part of the nucleus to form a compact electron‐dense mass with some irregular electron‐lucent lacunae, while the uppermost part of the nucleus is a loose electron‐lucent area, absence of the nuclear rotation and, as a consequence, the centriolar complex and the initial segment of each flagellum arise directly in a position perpendicular to the basal pole of the nucleus, and occurrence of numerous vesicles in the midpiece. In addition, spermiogenesis includes migration of the diplosome and mitochondria to the basal pole of the nucleus, formation of two moderate nuclear fossae, each of which contains the centriolar complex, development of two independent flagella and elimination of the excess cytoplasm. The mature spermatozoon has a more or less round head with no acrosome or acrosomal vesicle, a long midpiece with numerous mitochondria and vesicles and two long tails or flagella having the classical axoneme structure of 9 + 2 microtubular doublet pattern and with no lateral fins and membranous compartment. These findings suggest that the ultrastructural features of spermiogenesis and spermatozoa of M. electricus are synapomorphies of types I and II spermiogenesis and spermiogenesis is closely similar to the type described in the Nile catfish Chrysichthys auratus.     

Projections of the optic tectum and the mesencephalic nucleus of the trigeminal nerve in the tegu lizard (Tupinambis nigropunctatus)

Summary Fibers undergoing Wallerian degeneration following tectal lesions were demonstrated with the Nauta and Fink-Heimer methods and traced to their termination. Four of the five distinct fiber paths originating in the optic tectum appear related to vision, while one is related to the mesencephalic nucleus of the trigeminus. The latter component of the tectal efferents distributes fibers to 1) the main sensory nucleus of the trigeminus, 2) the motor nucleus of the trigeminus, 3) the nucleus of tractus solitarius, and 4) the intermediate gray of the cervical spinal cord.The principal ascending bundle projects to the nucleus rotundus, three components of the ventral geniculate nucleus and the nucleus ventromedialis anterior ipsilaterally, before it crosses in the supraoptic commissure and terminates in the contralateral nucleus rotundus, ventral geniculate nucleus and a hitherto unnamed region dorsal to the nucleus of the posterior accessory optic tract.Fibers leaving the tectum dorso-medially terminate in the posterodorsal nucleus ipsilaterally and the stratum griseum periventriculare of the contralateral tectum. The descending fiber paths terminate in medial reticular cell groups and the rostral spinal cord contralaterally and in the torus and the lateral reticular regions ipsilaterally. The ipsilateral fascicle also issues fibers to the magnocellular nucleus isthmi.     

The Topographical Distribution of Alanine, Aspartate, γ-Aminobutyric Acid, Glutamate, Glutamine, and Glycine in the Pigeon Optic Tectum and the Effect of Retinal Ablation

Abstract: The concentrations of alanine, aspartate, γ-aminobutyric acid, glutamine, glutamate, and glycine were measured in the pigeon optic nerve and in the individual tectal layers. Characteristic topographical distribution patterns were observed for the different amino acids. After unilateral retinal ablation, the concentration of aspartate and glutamate was decreased in the nerve and contralateral tectum. The reduction was restricted to the superficial part of the tectum, which receives a direct retinal input. The maximal loss was measured in the first two layers, where aspartate was reduced by 51% and glutamate by 75% in comparison with the ipsilateral side 4 weeks after ablation. The results favor a special role for aspartate and glutamate in pigeon retino-tectal afferents.     

The ultrastructure of somatic neuromuscular junctions in the tick Amblyomma variegatum (Fabr.)

Summary Injections of large doses of horseradish peroxidase (HRP) into the telencephalon of the squirrel fish (Holocentrus rufus) revealed the first anatomical evidence for a visual thalamo-telencephalic projection in a teleost. The central optic nucleus of the thalamus appears to be the only visual thalamic nucleus projecting to the telencephalon in this species. Since the central optic nucleus has a large tectal input but not a direct one from the retina, it is suggested that a retino-geniculo-telencephalic pathway does not exist in this species. Acknowledgements. The author is grateful to Drs. J. Maldonado, Dietrich Meyer and Henning Scheich for encouragement and support in this endeavor. The study was supported by: National Institutes of Health grant EY-02014 and EY-03264, a NIH grant to Dr. José del Castillo and the German Science Foundation (SFB 45)     

Retinofugal and retinopetal connections in the upside-down catfish (Synodontis nigriventris)

Summary The retinofugal and retinopetal connections in the upside-down catfish Synodontis nigriventris were studied by use of the horseradish-peroxidase (HRP) techniques, autoradiography, and degeneration-silver methods. An unusual retinal projection to the torus semicircularis as well as projections to the retina from three different sources in the brain are described. After intra-ocular injections of HRP, labeled cells were found in the optic tectum, the dorsomedial optic nucleus and one of the pretectal nuclei. These new findings support the basic hypothesis (i) that neuronal connections are more extensive in primitive brains, and (ii) that the evolutionary development of more complex brains involves the loss of some selected connections.     

Anterograde labelling from the optic nerve reveals multiple central targets in the teleost,Lethrinus chrysostomus (Perciformes)

Summary Cobaltous-lysine is transported anterogradely from the optic nerve of the teleost, Lethrinus chrysostomus (Lethrinidae, Perciformes). The marginal optic tract is labelled in longtitudinal bands of light and dark staining fibres which persists caudally within the ventral division but not in the dorsal division. This species possesses multiple central targets in the contralateral preoptic, diencephalic, pretectal, periventricular and tectal regions of the brain. In addition, a greater subdivision of the marginal optic tract is found to project to various nuclei. Ipsilateral projections are found in the suprachiasmatic nucleus and in the region of the horizontal commissure. Projections are also found in the telencephalic region of the nucleus olfactoretinalis and the thalamic region of the nucleus thalamoretinalis. The retinotopicity of some of these nuclei, found in previous studies, is discussed in relation to the possibility of specific sub-populations of retinal ganglion cells having different central targets.Abbreviations used in the Text and Figures A nucleus anteriorthalami - AO accessory optic nucleus - AOT accessory optic tract - AxOT axial optic tract - BO nucleus of the basal optic root - C cerebellum - HCv ventral division of horizontal commissure - I nucleus intermedius thalami - IL inferior lobe - MdOT medial optic tract - MO medulla oblongata - MOTd dorsal division of the marginal optic tract - MOTi intermediate division of the marginal optic tract - MOtv ventral division of the marginal optic tract - O olfactory bulb - OT optic tract - PC nucleus pretectalis centralis - PCo posterior commissure - Pd nucleus pretectalis dorsalis - PG preglomerular complex - PPd nucleus pretectalis periventricularis, pars dorsalis - PPv nucleus pretectalis periventricularis, pars ventralis - PSm nucleus pretectalis superficial pars magnocellularis - PSp nucleus pretectalis superficialis, pars parvocellularis - Sn suprachiasmatic nucleus - TEL telencephalon - TeO optic tectum - TL torus longtitudinalis - TrOlfR tractus olfactoretinalis - VCg granular layer of the valvula cerebelli - VCm molecular layer of the valvula cerebelli - VM nucleus medialis thalami - VL nucleus ventrolateralis thalami - VMdOT ventro-medial optic tract     

Retinal projections in sockeye salmon smolts (Oncorhynchus nerka)

Summary The retinal projections in 2-year-old salmon smolt (Oncorhynchus nerka) are significantly different from those observed in other teleosts examined to date in that the projections are more extensive. Very noticeable are extensive projections to most of the dorsal thalamus, to all layers of the optic tectum, and into the periaqueductal gray of the torus semicircularis. The salmon smolt has bilateral retinal projections to the diencephalon and pretectum. A small retinal projection to the lateral habenular nucleus has not been described previously. Although these findings suggest striking differences in retinal projections among teleosts, this variation may relate to age differences since the previously studied teleosts were adults.     

Retinal projections in European Salamandridae

Summary The retinal projections to the brain were studied in three species of European Salamandridae using anterograde transport of horseradish peroxidase and autoradiography. The results obtained were basically identical for all species and confirmed earlier findings on the fiber supply to the preoptic nucleus and the basal optic neuropil. In the anterior thalamus projections to three distinct terminal fields are clearly visible: (i) the diffusely stained corpus geniculatum thalamicum, (ii) the neuropil of Bellonci, pars lateralis, and (iii) a dorsomedial terminal field, the neuropil of Bellonci, pars medialis. Caudal to these terminal fields is an almost terminal-free region, the lateral neuropil. In the posterior thalamus a medial terminal field, the uncinate field, and a laterally located terminal field, the posterior thalamic neuropil, are distinguishable. The tectum opticum displays as many as four dense layers of retinofugal fibers and terminals in the rostral part and, in addition, a more densely stained strip of neuropil running from rostral to caudal over the tectum. The extent of ipsilateral fibers is greater than previously reported in other urodele species. They supply the medial and the lateral parts of the neuropil of Bellonci, the uncinate field, and reach the tectum opticum via the medial optic tract. Further, they form terminals in the innermost optic fiber layer throughout the rostral half of the ipsilateral tectum. A small proportion of ipsilateral fibers contributes very sparsely to all other thalamic terminal fields, leaving only the caudal part of the tectum and several layers of the rostral tectum completely free of a direct retinofugal fiber supply.     

Retinofugal projections in salamanders of the family Plethodontidae

Summary A comparison of the retinofugal projections in 14 species of plethodontid salamanders by means of the horseradish peroxidase (HRP) technique revealed almost identical contralateral projections. In all species studied three optic tracts were found. Behind the chiasma opticum the basal optic tract runs to the peduncle region, there forming the basal optic neuropil. The marginal optic tract courses from the chiasma over the thalamus to the tectum opticum where it covers the entire surface. In the anterior thalamus the marginal optic tract innervates the neuropil Bellonci-pars lateralis and the corpus geniculatum thalamicum, and more caudally the neuropil posterior thalami. The medial optic tract supplies the neuropil Bellonci-pars lateralis and pars medialis in the anterior thalamus from where it runs medial to the marginal optic tract as a separate tract to the uncinate field in the posterior thalamus.The ipsilateral projections show differences among the species studied, although the global organization remains constant. The differences mainly concern the marginal optic tract which varies from being weakly labeled and restricted to the rostral part of the tectum opticum, to being heavily labeled and innervating the entire tectum to its caudal edge. Species with the heaviest ipsilateral projections all belong to the plethodontid tribe Bolitoglossini, all of which show direct development, a highly projectile tongue, rather frontally oriented eyes and excellent depth perception. In these species the thalamic ipsilateral projection areas are equal in size and shape to the contralateral one. The ipsilateral projections to the tectum show two distinct layers, a superficial and a deep one, which intermingle with the contralateral projections. The two other ipsilateral tracts do not differ significantly among the plethodontid species: the medial optic tract is always heavily and the basal optic tract always weakly labeled.     

Identification and morphometric evaluation of the synapses of optic nerve afferents in the optic tectum of the axolotl (Ambystoma mexicanum)

Summary The terminals of retinal afferents in the tectum of the axolotl have been identified ultrastructurally using techniques of horseradish peroxidase-filling and degeneration. The mitochondria in filled structures show a characteristic electron-lucent matrix. After both eyes have been removed, terminals with light mitochondria disappear from the area known to receive an optic input. In this area the presence of light mitochondria is almost always diagnostic of the retinal origin of a bouton. The synapses are similar to those assumed to be of retinal origin in other vertebrates. Detailed morphometric analysis has been carried out on identified optic synapses in the optic tectum of the axolotl.     

Excitatory and inhibitory receptive fields of tectal cells are differentially modified by magnocellular and parvocellular divisions of the pigeon nucleus isthmi

It has been known that magnocellular and parvocellular divisions of the pigeon nucleus isthmi exert excitatory and inhibitory actions on tectal cells, respectively. The present study shows that injection of N-methyl-D-aspartate into the parvocellular division results in an increase in responsive strength and extent of the inhibitory receptive fields, which expand into the excitatory receptive fields of tectal cells. This injection concurrently leads to a decrease in responsiveness and extent of the excitatory fields. On the other hand, injection of acetylcholine into the magnocellular division enhances visual responsiveness, although the excitatory field is not obviously changed in extent. Meanwhile, strength and extent of the inhibitory fields are decreased by acetylcholine. The excitatory and inhibitory fields are reduced in both strength and extent by magnocellular and parvocellular injection of lidocaine, respectively. It suggests that isthmic inputs from both parvocellular and magnocellular divisions converge onto the same tectal cells, and the magnocellular and parvocellular subnuclei can modulate excitatory and inhibitory receptive fields of tectal cells, respectively, with some interactions between both fields. Accepted: 1 March 2000