Retinofugal projections in a weakly electric gymnotid fish (Apteronotus leptorhynchus)

The sensory effects of light on the electric organ discharge rate of Gymnotus omarorum

Gymnotiformes are nocturnal fishes inhabiting the root mats of floating plants. They use their electric organ discharge (EOD) to explore the environment and to communicate. Here, we show and describe tonic and phasic sensory-electromotor responses to light distinct from indirect effects depending on the light-induced endogenous circadian rhythm. In the dark, principally during the night, inter-EOD interval histograms are bimodal: the main peak corresponds to the basal rate and a secondary peak corresponds to high-frequency bouts. Light causes a twofold tonic but opposing effect on the EOD histogram: (i) decreasing the main mode and (ii) blocking the high-frequency bouts and consequently increasing the main peak at the expense of removal of the secondary one. Additionally, light evokes phasic responses whose amplitude increases with intensity but whose slow time course and poor adaptation differentiate from the so-called novelty responses evoked by abrupt changes in sensory stimuli of other modalities. We confirmed that Gymnotus omarorum tends to escape from light, suggesting that these phasic responses are probably part of a global 'light-avoidance response'. We interpret the data within an ecological context. Fish rest under the shade of aquatic plants during the day and light spots due to the sun's relative movement alert the fish to hide in shady zones to avoid macroptic predators and facilitate tracking the movement of floating plant islands by wind and/or water currents.

A time-stamp mechanism may provide temporal information necessary for egocentric to allocentric spatial transformations

Learning the spatial organization of the environment is essential for most animals’ survival. This requires the animal to derive allocentric spatial information from egocentric sensory and motor experience. The neural mechanisms underlying this transformation are mostly unknown. We addressed this problem in electric fish, which can precisely navigate in complete darkness and whose brain circuitry is relatively simple. We conducted the first neural recordings in the preglomerular complex, the thalamic region exclusively connecting the optic tectum with the spatial learning circuits in the dorsolateral pallium. While tectal topographic information was mostly eliminated in preglomerular neurons, the time-intervals between object encounters were precisely encoded. We show that this reliable temporal information, combined with a speed signal, can permit accurate estimation of the distance between encounters, a necessary component of path-integration that enables computing allocentric spatial relations. Our results suggest that similar mechanisms are involved in sequential spatial learning in all vertebrates.

Finding their way around is an essential part of survival for many animals and helps them to locate food, mates and shelter. Animals have evolved the ability to form a 'map' or representation of their surroundings. For example, the electric fish Apteronotus leptorhynchus, is able to precisely learn the location of food and navigate there. It can do this in complete darkness by generating a weak electric field. As it swims, every object it encounters generates an ‘electric image’ that is detected on the skin and processed in the brain.

However, all the cues the fish comes across are from its own point of view – the information about its environment is processed with respect to its location. And yet, the map that it generates needs to be independent of the fish’s position – it has to work regardless of where the animal is. The way animals translate ‘self-centered’ experiences to form a general representation of their surroundings is not yet fully understood.

Now, Wallach et al. studied how internal brain maps are generated in A. leptorhynchus. Information about the fish's environment passes through a structure in the brain called the preglomerular complex. Measuring the activity of this region revealed that the preglomerular complex does not process much self-centered information. Instead, whenever the fish passed any object – regardless of where it was in relation to the fish – the event triggered a brief burst of preglomerular activity. The intensity of the activity depended on how recently the fish had encountered another object. This information, combined with the dynamics of the fish's movement, could be what allows the fish to convert a sequence of encounters into a general spatial map.

These findings could help to inform research on learning and navigation. Further research could also reveal whether other species, including humans, generate their mental maps in a similar way. This may be relevant for people suffering from diseases such as Alzheimer’s, in which a sense of orientation has become impaired.

The Mormyrid Optic Tectum Is a Topographic Interface for Active Electrolocation and Visual Sensing

The African weakly electric fish Gnathonemus petersii is capable of cross-modal object recognition using its electric sense or vision. Thus, object features stored in the brain are accessible by multiple senses, either through connections between unisensory brain regions or because of multimodal representations in multisensory areas. Primary electrosensory information is processed in the medullary electrosensory lateral line lobe, which projects topographically to the lateral nucleus of the torus semicircularis (NL). Visual information reaches the optic tectum (TeO), which projects to various other brain regions. We investigated the neuroanatomical connections of these two major midbrain visual and electrosensory brain areas, focusing on the topographical relationship of interconnections between the two structures. Thus, the neural tracer DiI was injected systematically into different tectal quadrants, as well as into the NL. Tectal tracer injections revealed topographically organized retrograde and anterograde label in the NL. Rostral and caudal tectal regions were interconnected with rostral and caudal areas of the NL, respectively. However, dorsal and ventral tectal regions were represented in a roughly inverted fashion in NL, as dorsal tectal injections labeled ventral areas in NL and vice versa. In addition, tracer injections into TeO or NL revealed extensive inputs to both structures from ipsilateral (NL also contralateral) efferent basal cells in the valvula cerebelli; the NL furthermore projected back to the valvula. Additional tectal and NL connections were largely confirmatory to earlier studies. For example, the TeO received ipsilateral inputs from the central zone of the dorsal telencephalon, torus longitudinalis, nucleus isthmi, various tegmental, thalamic and pretectal nuclei, as well as other nuclei of the torus semicircularis. Also, the TeO projected to the dorsal preglomerular and dorsal posterior thalamic nuclei as well as to nuclei in the torus semicircularis and nucleus isthmi. Beyond the clear topographical relationship of NL and TeO interconnections established here, the known neurosensory upstream circuitry was used to suggest a model of how a defined spot in the peripheral sensory world comes to be represented in a common associated neural locus both in the NL and the TeO, thereby providing the neural substrate for cross-modal object recognition.

An ascending visual pathway to the dorsal telencephalon through the optic tectum and nucleus prethalamicus in the yellowfin goby Acanthogobius flavimanus (Temminck & Schlegel, 1845)

Dual visual pathways reaching the telencephalon appear to be an ancient vertebrate trait, but some teleost fish seem to possess only one pathway via the optic tectum. We undertook the present study to determine if and when this loss occurred during evolution. Tracer injection experiments to the optic nerve, the optic tectum, and the dorsal telencephalon were performed in the present study, to investigate ascending visual pathways to the dorsal telencephalon in an acanthopterygian teleost, the yellowfin goby Acanthogobius flavimanus (Temminck & Schlegel, 1845). We confirmed the presence of a nucleus prethalamicus (PTh) in the goby, which has been convincingly identified only in holocentrids, suggesting that this nucleus is present in other acanthopterygians. We found that the optic tectum projects to the PTh bilaterally. The PTh projects in turn to the dorsal telencephalon, ipsilaterally. These results suggest that the yellowfin goby possesses only an extrageniculate-like pathway, while a geniculate-like pathway could not be identified. This situation is common with that of holocentrids and may be a character common in acanthopterygians. It is possible that a geniculate-like system was lost in the common ancestor of acanthopterygians, although the scenario for the evolution of ascending visual systems in actinopterygians remains uncertain due to the lack of precise knowledge in a number of actinopterygian taxons.

Visual Capability of the Weakly Electric Fish Apteronotus albifrons as Revealed by a Modified Retinal Flat-Mount Method

Apteronotus albifrons (Gymnotiformes, Apteronotidae) is well known to have a sophisticated active electrosense system and is commonly described as having poor vision or being almost blind. However, some studies on this species suggest that the visual system may have a role in sensing objects in the environment. In this study, we investigated the visual capabilities of A. albifrons by focusing on eye morphology and retinal ganglion cell distribution. The eyes were almost embedded below the body surface and pigmented dermal tissue covered the peripheral regions of the pupil, limiting the direction of incoming light. The lens was remarkably flattened compared to the almost spherical lenses of other teleosts. The layered structure of the retina was not well delineated and ganglion cells did not form a continuous sheet of cell bodies. A newly modified retinal flat-mount method was applied to reveal the ganglion cell distribution. This method involved postembedding removal of the pigment epithelium of the retina for easier visualization of ganglion cells in small and/or fragile retinal tissues. We found that ganglion cell densities were relatively high in the periphery and highest in the nasal and temporal retina, although specialization was not so high (approx. 3:1) with regard to the medionasal or mediotemporal axis. The estimated highest possible spatial resolving power was around 0.57 and 0.54 cycles/degree in the nasal and temporal retina, respectively, confirming the lower importance of the visual sense in this species. However, considering the hunting nature of A. albifrons, the relatively high acuity of the caudal visual field in combination with electrolocation may well be used to locate prey situated close to the side of the body.

Organization of the gymnotiform fish pallium in relation to learning and memory: II. Extrinsic connections

This study describes the extrinsic connections of the dorsal telencephalon (pallium) of gymnotiform fish. We show that the afferents to the dorsolateral and dorsomedial pallial subdivisions of gymnotiform fish arise from the preglomerular complex. The preglomerular complex receives input from four clearly distinct regions: (1) descending input from the pallium itself (dorsomedial and dorsocentral subdivisions and nucleus taenia); (2) other diencephalic nuclei (centroposterior, glomerular, and anterior tuberal nuclei and nucleus of the posterior tuberculum); (3) mesencephalic sensory structures (optic tectum, dorsal and ventral torus semicircularis); and (4) basal forebrain, preoptic area, and hypothalamic nuclei. Previous studies have implicated the majority of the diencephalic and mesencephalic nuclei in electrosensory, visual, and acousticolateral functions. Here we discuss the implications of preglomerular/pallial electrosensory-associated afferents with respect to a major functional dichotomy of the electric sense. The results allow us to hypothesize that a functional distinction between electrocommunication vs. electrolocation is maintained within the input and output pathways of the gymnotiform pallium. Electrocommunication information is conveyed to the pallium through complex indirect pathways that originate in the nucleus electrosensorius, whereas electrolocation processing follows a conservative pathway inherent to all vertebrates, through the optic tectum. We hypothesize that cells responsive to communication signals do not converge onto the same targets in the preglomerular complex as cells responsive to moving objects. We also hypothesize that efferents from the dorsocentral (DC) telencephalon project to the dorsal torus semicircularis to regulate processing of electrocommunication signals, whereas DC efferents to the tectum modulate sensory control of movement.

Optic chiasm in the species of order Clupeiformes, family Clupeidae: optic chiasm of Spratelloides gracilis shows an opposite laterality to that of Etrumeus teres

In most teleost fishes, the optic nerves decussate completely as they project to the mesencephalic region. Examination of the decussation pattern of 25 species from 11 different orders in Pisces revealed that each species shows a specific chiasmic type. In 11 species out of the 25, laterality of the chiasmic pattern was not determined; in half of the individuals examined, the left optic nerve ran dorsally to the right optic nerve, while in the other half, the right optic nerve was dorsal. In eight other species the optic nerves from both eyes branched into several bundles at the chiasmic point, and intercalated to form a complicated decussation pattern. In the present study we report our findings that Spratelloides gracilis, of the order Clupeiformes, family Clupeidae, shows a particular laterality of decussation: the left optic nerve ran dorsally to the right (n = 200/202). In contrast, Etrumeus teres, of the same order and family, had a strong preference of the opposite (complementary) chiasmic pattern to that of S. gracilis (n = 59/59), revealing that these two species display opposite left–right optic chiasm patterning. As far as we investigated, other species of Clupeiformes have not shown left–right preference in the decussation pattern. We conclude that the opposite laterality of the optic chiasms of these two closely related species, S. gracilis and E. teres, enables investigation of species-specific laterality in fishes of symmetric shapes.

Clustered phylogenetic distribution of nucleus rostrolateralis among ray-finned fishes

Nucleus rostrolateralis, which was named for its location in the rostrolateral part of the diencephalon of neopterygian fishes, has been identified in a variety of species based on position, cytoarchitecture, hodology, and/or histochemistry. The phylogenetic distribution of the nucleus is highly sporadic, however. Due to this distribution, nucleus rostrolateralis cannot be regarded as phylogenetically homologous, but it might be an example of syngeny, or generative homology, which applies to characters that have the same genetic and/or developmental basis inherited from a common ancestor, whether or not the character itself has a phylogenetic distribution congruent with a monophyletic taxon--i.e., in general terms, an example of either phylogenetic homology or parallelism. To test whether the nucleus occurs in closely related taxonomic clusters, as might be expected for a character with a shared generative basis, a number of species of cyprinids and atherinomorphs (both teleost taxa) were examined for its presence. Many of the species examined appear to lack the nucleus, but a clustered occurrence of it was found within both taxa. Within cyprinids, nucleus rostrolateralis occurs in all three members examined of the Subfamily Rasborinae. Within atherinomorphs, it occurs in both members examined of the Tribe Poeciliini (of the Subfamily Poeciliinae, Family Poeciliidae) and in the one member examined of the Family Anablepidae. The clustered occurrence of nucleus rostrolateralis supports the hypothesis that it is an example of syngeny. Its postulated shared generative basis appears to derive from the common ancestor of the entire neopterygian radiation despite the rare occurrence of the character itself.

Afferent and efferent connections of the diencephalic prepacemaker nucleus in the weakly electric fish, Eigenmannia virescens: interactions between the electromotor system and the neuroendocrine axis

The afferent and efferent connections of the gymnotiform central posterior nucleus of the dorsal thalamus and prepacemaker nucleus (CP/PPn) were examined by retrograde and anterograde transport of the small molecular weight tracer, Neurobiotin. The CP/PPn was identified by physiological assay and received a local iontophoretic injection of Neurobiotin. Retrogradely labeled somata were observed in the ventral telencephalon, hypothalamus, and the pretectal nucleus electrosensorius. Anterogradely labeled fibers were traced from the CP/PPn to the ventral telencephalon, the hypothalamus, the neuropil immediately adjacent to the most rostral subdivision of the nucleus electrosensorius, the optic tectum, and the pacemaker nucleus. Retrograde transport of tracer following injections into the ventral telencephalon, preoptic area, lateral hypothalamus, tectum, and pacemaker nucleus confirmed these efferent targets. A rostromedial subarea of the CP/PPn can be identified that projects to basal forebrain regions and to a lateral region of the CP/PPn that contains afferents to the pacemaker. Many of the targets, which are connected with the CP/PPn, have been linked to reproductive behavior or neuroendocrine control in other fishes. A comparative analysis reveals that the efferent pathways of the CP/PPn appear similar and may be homologous to efferent pathways of some components of the auditory thalamus among tetrapods.

Inositol 1,4,5-trisphosphate receptor localization in the brain of a weakly electric fish (Apteronotus leptorhynchus) with emphasis on the electrosensory system

Inositol 1,4,5-trisphosphate is a widespread intracellular second messenger that mobilizes intracellular Ca2+ stores. The inositol 1,4,5-trisphosphate receptor involved is associated with the endoplasmic reticulum in neurons. In mammalian brain, inositol 1,4,5-trisphosphate receptor-containing neurons are found in many diverse regions, with cerebellar Purkinje cells containing the highest density of these receptors. We used immunohistochemical methods to identify the distribution of inositol 1,4,5-trisphosphate receptor-containing neurons in the brain of the weakly electric fish and Western blotting to confirm that a protein similar to the inositol 1,4,5-trisphosphate receptor of mammalian brain was recognized in the fish brain. In the telencephelon, the dorsal forebrain regions had low amounts of inositol 1,4,5-trisphosphate receptor. In the diencephalon, only the nucleus tuberis posterior was moderately immunoreactive. In the mesencephalon, only the optic tectum contained cells with intense immunoreactivity, similar to our findings for the ryanodine receptor (G.K.H. Zupanc, J.A. Airey, L. Maler, J. Sutko, and M.H. Ellisman, 1992, J. Comp. Neurol. 325:135-151), which also mobilizes intracellular calcium. In the rhombencephalon, a subset of the pyramidal cells of the electrosensory lateral line lobe contained inositol 1,4,5-trisphosphate receptor. These cells have been shown to contain ryanodine receptor (Zupanc et al., 1992). However, unlike the ryanodine receptor, the distribution of inositol 1,4,5-trisphosphate receptor in these cells is constrained to the soma and proximal dendrites. This compartmentalization may indicate the limit of the range of second-messenger action. Other regions containing immunoreactive cells were the nucleus praeminentialis dorsalis (multipolar and boundary cells), nucleus medialis and crista cerebellaris, and the cerebellum, whose Purkinje cells were the most intensely labeled. The functional implications of inositol 1,4,5-trisphosphate receptor localization in the electrosensory lateral line lobe are discussed.

The distribution of GABA-immunoreactive neurons in the brain of the silver eel (Anguilla anguilla L.)

The distribution of GABA-immunoreactivity was studied in the brain of the silver eel (Anguilla anguilla) by means of antibodies directed against GABA. Immunoreactive neuronal somata were distributed throughout the brain. Positive perikarya were detected in the internal cellular layer of the olfactory bulb, and in all divisions of the telencephalon, the highest density being observed along the midline. Numerous GABA-reactive cell bodies were found in the diencephalon, particularly in the preoptic and tuberal regions of the hypothalamus, and the dorsolateral, dorsomedial and ventromedial thalamic nuclei. In the optic tectum, the majority of GABA-positive cell bodies were located in the periventricular layer. A number of immunolabeled cell bodies were observed in different tegmental structures, notably the torus semicircularis. In the cerebellum, the Purkinje cells were either very intensely or very weakly immunoreactive. In the rhombencephalon, reactive cell bodies were observed in the eminentia granularis, the valvula cerebellaris, the octavolateral nucleus, the lobus vagus and in the vagal and glossopharyngeal motor nuclei. Intensely immunoreactive axons and terminals were observed in the external granular layer and internal cellular layer of the olfactory bulb. In the telencephalon, the highest density of reactive fibres and boutons was found in the fields of the medial wall. Many immunolabeled fibres were seen in the medial and lateral forebrain bundles. In the diencephalon, intense labelling of fibres and terminals were observed in the nuclei situated close to the midline. In the optic tectum the highest density of reactive fibres was seen in the sfgs, the layer to which the retina projects massively. Finally, in the rhombencephalon the strongest labelling of neurites was observed in the nuclei of the raphé, the nucleus octavocellularis magnocellularis and the nuclei of the IXth and Xth cranial nerves. The GABAergic system of the eel, which is well developed, appears to be generally comparable to that described in tetrapod vertebrates.

The diencephalon of the Pacific herring, Clupea harengus: retinofugal projections to the diencephalon and optic tectum

The pattern of retinofugal projections to nuclei in the diencephalon and to the optic tectum was analyzed with horseradish peroxidase and autoradiographic methods in Clupea harengus, a clupeomorph teleost, for comparison with osteoglossomorph, elopomorph, and euteleost teleosts and with non-teleost actinopterygians. Most retinal fibers decussate in the optic chiasm and project to nuclei in the preoptic area, ventral and dorsal thalamus, posterior tuberculum, synencephalon, and pretectum, as well as to the accessory optic nuclei and optic tectum. Some ipsilateral projections do not decussate in the optic chiasm, while others decussate and recross via the supraoptic (minor) and posterior commissures. The pattern of projections is similar to that seen in other actinopterygian fishes with several exceptions. The terminal field usually present lateral to nucleus anterior in the dorsal thalamus is extremely reduced despite the relatively large size of the nucleus. A dense terminal field lies within the cell plate of nucleus corticalis in the pretectum rather than dorsal to it. The tectal hemisphere is composed of two distinct lobules, and the dorsal optic tract projects to the more rostromedial lobule while the ventral optic tract projects to the more caudolateral lobule. The lack of a significant projection to nucleus anterior and the lobular morphology of the optic tectum appear to be apomorphic for Clupea. Other features of the pattern of retinal projections are also analyzed in actinopterygian fishes including Clupea, and several hypotheses are advanced as to which traits are plesiomorphic for actinopterygians and/or for teleosts.

Anatomical organization of the hypophysiotrophic systems in the electric fish, Apteronotus leptorhynchus

The organization of afferents to the pituitary was investigated by applying DiI crystals to the pituitary or pituitary stalk of the gymnotiform electric fish, Apteronotus leptorhynchus. Most hypophysiotrophic cells were found in the hypothalamus and were distributed throughout its rostrocaudal extent: nucleus preopticus periventricularis, pars anterior and posterior; suprachiasmatic nucleus; anterior, dorsal, ventral, lateral, and caudal hypothalamic nuclei; and nucleus tuberis lateralis, pars anterior and posterior. In addition a small number of retrogradely labeled cells were found in the ventral telencephalon (area ventralis, pars ventralis) and, most surprisingly, in a thalamic nucleus (nucleus centralis posterioris). The nucleus preopticus periventricularis pars posterior and the anterior hypothalamic nucleus appear to correspond to the parvicellular and magnocellular divisions of the nucleus preopticus of other teleosts. Integration of these results with immunohistochemical localization of monoamines and neuropeptides in the apteronotid brain suggests many homologies between the hypophysiotrophic nuclei of teleosts and other vertebrates, including mammals. Apteronotus communicates electrically during agonistic and sexual interactions. There are numerous anatomical links between the hypophysiotrophic systems and the brain areas related to electrocommunication.

Structure and function of neurons in the complex of the nucleus electrosensorius of the gymnotiform fish Eigenmannia: detection and processing of electric signals in social communication

The complex of the diencephalic nucleus electrosensorius (nE) provides an interface between the electrosensory processing performed by the torus semicircularis and the control of specific behavioral responses. The rostral portion of the nE comprises two subdivisions that differ in the response properties and projection patterns of their neurons. First, the nEb, which contains neurons that are driven almost exclusively by beat patterns generated by the interference of electric organ discharges (EODs) of similar frequencies. Second, the area medial to the nEb, comprising the lateral pretectum (PT) and the nE-acusticolateralis region (nEar, 1 B-D), which contains neurons excited predominantly by EOD interruptions, signals associated with aggression and courtship. Neurons in the second area commonly receive convergent inputs originating from ampullary and tuberous electroreceptors, which respond to the low-frequency and high-frequency components of EOD interruptions, respectively. Projections of these neurons to hypothalamic areas linked to the pituitary may mediate modulations of a fish's endocrine state that are caused by exposure to EOD interruptions of its mate.

Somatostatin-like immunoreactivity in the brain of an electric fish (Apteronotus leptorhynchus) identified with monoclonal antibodies

The immunohistochemical localization of somatostatin-like immunoreactive (SSir) cells and fibers in the brain of the gymnotiform teleost (Apteronotus leptorhynchus) was investigated using well-characterized monoclonal antibodies directed against somatostatin-14 and -28. Large populations of SSir neurons occur in the basal forebrain, diencephalon and rhombencephalon and a dense distribution of fibers and terminal fields is found in the ventral, dorsomedial and dorsolateral telencephalon, hypothalamus, centralis posterior thalamus, subtrigeminal nucleus, the motor nucleus of vagus and in the ventrolateral medulla. Immunoreactive neurons in the forebrain are concentrated mainly in the ventral telencephalic areas, the region of the anterior commissure and entopeduncular nucleus. In a fashion similar to the large basal telencephalic cells of other species, the cells of the rostral nucleus entopeduncularis have a significant projection to the dorsal telencephalon. The preoptic region and the peri- and paraventricular hypothalamic nuclei are richly endowed with SSir cells; some of these cells contribute fibres to the pituitary stalk and gland. In the thalamus, only the n. centralis posterior stands out for the density of SSir cells and terminals; these cells appear to project to the prepacemaker nucleus, thus suggesting an SS influence on electrocommunication. In the mesencephalon most SSir cells occur in the optic tectum, torus semicircularis and interpeduncular nucleus. The rhombencephalic SSir cells have a wider distribution (central gray, raphe, sensory nuclei, reticular formation, electrosensory lateral line lobe and surrounding the central canal). The results of this study show the presence of SS in various sensory systems, electromotor system and specific hypothalamic nuclei, suggesting a modulatory role in the processing of sensory information, electrocommunication, endocrine and motor activities.

An atlas of the brain of the electric fish Apteronotus leptorhynchus

This atlas consists of a set of six macrophotographs illustrating the important external landmarks of the apteronotid brain, as well as 54 transverse levels through the brain stained with cresyl violet. There are 150 microns between levels and the scales have 1 mm divisions (100 microns small divisions). In general the neuroanatomy of this brain is similar to that of other teleosts except that all parts known to be concerned with electroreception are greatly hypertrophied (electrosensory lateral line lobe, nucleus praeminentialis, caudal lobe of the cerebellum, torus semicircularis dorsalis, optic tectum and nucleus electrosensorius). There are other regions of this brain which are hypertrophied or which have not been described in other teleosts, but which are not known to be directly linked to the electrosensory/electromotor system; these regions are mentioned in the accompanying text.

The distribution of excitatory amino acid binding sites in the brain of an electric fish, Apteronotus leptorhynchus

The distribution of three types of exitatory amino acid receptors was examined in the brain of a high frequency weakly electric fish, Apteronotus leptorhynchus, by localizing the binding sites of ligands selective for mammalian kainic acid (KA), quisqualate (AMPA) and N-methyl-D-aspartate (NMDA) receptors. All three binding sites were densest within the forebrain and in certain hypothalamic nuclei (nucleus tuberis anterior, inferior lobe). The core of the dorsal forebrain (dorsal centralis) had a very high density of NMDA binding sites and only moderate levels of AMPA and KA binding sites, while this was reversed for the dorsolateral forebrain. The AMPA and NMDA binding sites were found throughout the brain while KA binding sites were relatively restricted and were absent from most of the brainstem. The cerebellar molecular layer contained a very high density of KA and AMPA binding sites but almost no NMDA binding sites; the granular layer had a low density of AMPA and NMDA binding sites but was lacking in KA binding sites. All three types of binding sites were found within the electromotor system (nucleus electrosensorius and prepacemaker nucleus) at sites where the iontophoresis of glutamate causes species-specific behaviours. KA binding sites were found at only two sites along the electrosensory afferent pathways: (1) in the molecular layer of the electrosensory lateral line lobe, associated with a feedback pathway emanating from granule cells of the overlying cerebellum, and (2) in the lateral nucleus praeminentialis dorsalis, associated with a descending pathway emanating from the torus semicircularis. NMDA and AMPA binding sites are found throughout the electrosensory pathways. Within the electrosensory lateral line lobe the NMDA binding sites were predominantly associated with the feedback pathways terminating in its molecular layer and not with the deep neuropil layer containing primary electroreceptor afferents.

Structural and functional organization of a diencephalic sensory-motor interface in the gymnotiform fish, Eigenmannia

The diencephalic nucleus electrosensorius (nE) of gymnotiform fish comprises a series of finely tuned neuronal filters for control of the jamming avoidance response (JAR) and probably other electromotor tasks as well. The nE receives electrosensory input from the dorsal torus semicircularis (TSd) and octavolateral input from the ventral torus (TSv). The nE, in turn, projects to various hypothalamic and thalamic nuclei, including the prepacemaker nucleus (PPn), which can modulate the frequency of electric organ discharges (EODs) via its unique input to the medullary pacemaker nucleus. Four subdivisions of the nE can now be recognized: 1) The beat-related area (nEb)--a rostral cluster of tightly packed cells which receives TSd input and projects to the inferior lobe, anterior tuberal nucleus, anterior thalamic nucleus, central posterior thalamic nucleus, and PPn. The nEb contains neurons responsive to beat patterns caused by jamming stimuli. Stimulation of the nEb with L-glutamate, however, fails to induce any EOD-frequency shift. 2) The area causing EOD-frequency rises (nE increases)--a horizontal band of cells at the dorsal aspect of the caudal nE which receives TSd input and projects to the PPn and vicinity and to the cerebellum; nE increases stimulation induces slow EOD-frequency rises characteristic of the JAR. Responses of these cells to jamming stimuli are not yet known. 3) The area causing EOD-frequency falls (nE decreases)--a horizontal band of cells at the ventral aspect of the caudal nE which receives TSd input and projects only to the PPn and vicinity; nE decreases stimulation induces slow EOD-frequency falls characteristic of the JAR. The responses of these cells to jamming stimuli are not yet known. 4) The acousticolateral region (nEar)--a complex medial region of the nE which receives input predominantly from the ventral torus and projects to the inferior lobe, anterior tuberal nucleus, central posterior thalamic nucleus, PPn, and cerebellum; the sensory and motor properties of this region are not known in detail, although auditory and mechanosensory responses have been recorded here. Projections to the PPn and its vicinity suggest direct control of electromotor behaviors by the nE, whereas thalamic and hypothalamic projections may provide a substrate for electrosensory influences on neuroendocrine and motivational control centers. The optic tectum projects strongly to the pretectum and various other diencephalic nuclei in the vicinity of the nE, but it does not innervate the nE itself. Accordingly, ablation of the tectum does not affect the performance of the JAR.

Catecholaminergic systems in the brain of a gymnotiform teleost fish: an immunohistochemical study

The localization of catecholamines (CA) in the brain of Apteronotus leptorhynchus was studied with immunohistochemical techniques using antibodies to the enzymes tyrosine hydroxylase (TH), dopamine B-hydroxylase (DBH), phenylethanolamine-N-methyltransferase (PNMT), and the neurotransmitter dopamine (DA). Telencephalic TH and DA immunoreactive (ir) neurons were located in the following structures: olfactory bulb, area ventralis telencephali partes ventralis, centralis, dorsalis, and intermediate. Diencephalic TH ir neurons were distributed in: nucleus preopticus periventricularis pars anterior, floor of preoptic recess, n. suprachiasmaticus, n. preopticus periventricularis pars posterior, n. anterior periventricularis, area ventralis lateralis, rostral region of posterior periventricular nucleus (paraventricular organ of other authors), periventricular nucleus of posterior tuberculum, n. recessus lateralis, n. tuberis lateralis pars anterior, and n. tuberis posterior. Although most diencephalic TH ir structures were also DAir, the posterior periventricular nucleus, n. recessus lateralis pars medialis, n. recessus posterioris, and ventral region of nucleus lateralis tuberis pars anterior showed differences in the distribution of TH and DA immunoreactivity. The rhombencephalic structures contained cell groups with different combinations of catecholamines as follows: TH and DBH ir neurons in the isthmic tegmentum (locus coeruleus); TH and DBH ir cells in the rostral medullary tegmentum ventral to VIIth nerve; TH and PNMT ir cells in the sensory nucleus of the vagus nerve; TH, DBH, and PNMT ir cells in the dorsal medullary tegmentum, TH and DBH ir cells in the dorsomedian postobecular region, ventral to the descending trigeminal tract and lateral to the central canal at medullospinal levels. This study shows that: (1) with few exceptions TH and DA ir coincides, (2) gymnotiforms possess similar DBH ir rhombencephalic groups, but additional telencephalic and rhombencephalic TH ir groups, and PNMT ir cells that were not reported previously in teleosts, and (3) the presence of CAergic fibers in the electrosensory system supports findings of their modulatory function in communication and aggression.

The organization of afferent input to the caudal lobe of the cerebellum of the gymnotid fish Apteronotus leptorhynchus

The caudal lobe of the cerebellum of the high frequency gymnotid fish Apteronotus leptorhynchus is that region of the cerebellum lying lateral to the posterolateral sulcus. It consists of three granular masses--the eminentia granularis posterior pars lateralis, a transitional zone T, and the eminentia granularis posterior pars medialis--with their associated molecular layers. We have used the retrograde transport of wheat germ agglutinin conjugated horseradish peroxidase to study the afferent input to the various subdivisions of the caudal lobe. Each granular mass receives different types of input. Eminentia granularis posterior pars lateralis receives a massive bilateral input from an isthmic nucleus, nucleus praeeminentialis, concerned with descending control of the electrosensory system and from a rhombencephalic nucleus, the lateral reticular nucleus, which itself receives a major spinal input. In addition eminentia granularis posterior receives lesser input from other pretectal, (N. at base of dorsomedial optic tract, pretectal complex "B") mesencephalic (dorsal tegmental N., nucleus raphe dorsalis), isthmic (bed N. of praeeminentialis-cerebellaris tract, locus coeruleus) and rhombencephalic nuclei (lateral tegmental N., eurydendroid cells, octaval N., perihypoglossal N., paramedian reticular N., medullary reticular formation, medullary raphe, efferent octavolateralis N., inferior olive, and funicular N.). The input from nucleus praeeminentialis dorsalis is mapped topographically onto eminentia granularis posterior with respect to their rostro-caudal location. We could not define any topography in the mapping of the dorso-ventral body axis upon eminentia granularis posterior; small injections of WGA-HRP produced several small clusters of labeled cells within nucleus praeeminentialis dorsalis which does suggest a more complex organization of this projection. Zone T receives most of its input from the ipsilateral VIIIth nerve ganglion cells and certain pretectal nuclei, but it also receives a small input from nucleus praeeminentialis dorsalis. Eminentia granularis posterior pars medialis receives minor input from a small pretectal nucleus and a small ventral diencephalic nucleus, this region appears to receive its major input from eurydendroid cells of eminentia granularis posterior. The molecular layer associated with each granular mass receives contralateral input from separate clusters of inferior olivary cells. In addition the eurydendroid cells (cerebellar output neurons) of eminentia granularis posterior pars lateralis receive a substantial direct input from cells located in the medial aspect of nucleus praeeminentialis dorsalis.

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 periventriculare, 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.

The optic tectum of the gymnotiform electric fish, Eigenmannia: labeling of physiologically identified cells

A total of 47 tectal neurons of the weakly electric fish, Eigenmannia, were studied physiologically and labelled by intracellular injection of Lucifer Yellow. With the exception of two cell types, all cells could be classified in accordance with the Golgi studies of Sas and Maler. The dominant stimulus modality of neurons was correlated with their laminar location. Neurons of the stratum opticum only responded to visual stimuli, such as modulations of the light level or the motion of an object. They showed, however, no directional preferences for motion. Neurons of the stratum griseum centrale were predominantly driven by electrosensory stimuli, most often those associated with the movement of an object, and generally were very sensitive to the direction of motion. Integration of different sensory modalities was found in neurons with dendrites invading laminae with different sensory inputs. In addition, small axons of interneurons appear to relay information across laminae. Large multipolar neurons in the deep tectum responded to the motion of objects, often preferring a particular direction of motion. Some of these large multipolar neurons of the deep tectum also discriminated the sign of the frequency difference between a mimic of a neighbor's sinusoidal electric organ discharge and the animal's own signal. These neurons are potential candidates for the control of the jamming avoidance response. These neurons were morphologically indistinguishable from large multipolar neurons of the deep tectum that either responded to moving objects or to acoustical stimuli. Individual large cells of the deep tectum project to various targets (Fig. 1) and probably contribute to the control of different behavioral responses. This suggests that the nature of such responses would then depend upon the constitution of sets of neurons recruited by a given stimulus situation, and the role of individual tectal neurons would neither be particularly specific nor very significant.

The optic tectum of gymnotiform teleosts Eigenmannia virescens and Apteronotus leptorhynchus: a Golgi study

Golgi, Nissl, Bielschowsky and cholinesterase techniques have been used to analyze the optic tectum of the weakly electric teleost fish Eigenmannia virescens and Apteronotus leptorhynchus. Six layers are readily distinguished: a fairly thick stratum marginale, a narrow stratum opticum and stratum fibrosum et griseum superficiale, a well-developed stratum griseum centrale, a stratum album centrale and a compact stratum periventriculare. Fifty-six neuronal types are present. In regard to comparative aspects of tectal organization, it became apparent that although most neuronal types are similar to those reported in other teleostean fish, there are certain obvious differences such as: pyramidal cell somata not confined to stratum fibrosum et griseum superficiale, but also clustered in the adjacent stratum opticum, presenting stratified or diffuse basilar dendritic arbors; and a change from vertical to oblique and almost horizontal neuronal orientation in the ventral and caudal tectum. The presence of pyramidal cells with aligned and misaligned apical and basal dendritic fields. A cell of stratum griseum centrale with an ascending axon to stratum opticum. A special projection type of fusiform cell of stratum griseum centrale, with an efferent axon of somatic origin. A cell rich stratum griseum centrale, with a wider variety of multipolar and bipolar cell population than reported in other teleosts. Fourteen types of pyriform cells are present, four of which are efferent. Our observations are suggestive of regional differences in regard to the caudalmost tectum in Apteronotus: presumably this is related to the extremely sparse retinal input to this part of the tectum. A close functional correlation has been found between some multipolar and pyriform cells identified in our material with similar cells reported by Rose and Heiligenberg as multisensory cells, following recordings and horseradish peroxidase fillings of these cells. Based on the observation of patchy torus semicircularis input to stratum fibrosum et griseum superficiale, disjunct from the retinal input to this layer, it is proposed that perhaps this arrangement is the result of competition for synaptic targets during development.