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

Distribution of the cholinergic nuclei in the brain of the weakly electric fish, Apteronotus leptorhynchus: Implications for sensory processing

Acetylcholine acts as a neurotransmitter/neuromodulator of many central nervous system processes such as learning and memory, attention, motor control, and sensory processing. The present study describes the spatial distribution of cholinergic neurons throughout the brain of the weakly electric fish, Apteronotus leptorhynchus, using in situ hybridization of choline acetyltransferase mRNA. Distinct groups of cholinergic cells were observed in the telencephalon, diencephalon, mesencephalon, and hindbrain. These included cholinergic cell groups typically identified in other vertebrate brains, for example, motor neurons. Using both in vitro and ex vivo neuronal tracing methods, we identified two new cholinergic connections leading to novel hypotheses on their functional significance. Projections to the nucleus praeeminentialis (nP) arise from isthmic nuclei, possibly including the nucleus lateralis valvulae (nLV) and the isthmic nucleus (nI). The nP is a central component of all electrosensory feedback pathways to the electrosensory lateral line lobe (ELL). We have previously shown that some neurons in nP, TS, and tectum express muscarinic receptors. We hypothesize that, based on nLV/nI cell responses in other teleosts and isthmic connectivity in A. leptorhynchus, the isthmic connections to nP, TS, and tectum modulate responses to electrosensory and/or visual motion and, in particular, to looming/receding stimuli. In addition, we found that the octavolateral efferent (OE) nucleus is the likely source of cholinergic fibers innervating the ELL. In other teleosts, OE inhibits octavolateral hair cells during locomotion. In gymnotiform fish, OE may also act on the first central processing stage and, we hypothesize, implement corollary discharge modulation of electrosensory processing during locomotion.

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.

Hypothalamic Projections to the Optic Tectum in Larval Zebrafish

The optic tectum of larval zebrafish is an important model for understanding visual processing in vertebrates. The tectum has been traditionally viewed as dominantly visual, with a majority of studies focusing on the processes by which tectal circuits receive and process retinally-derived visual information. Recently, a handful of studies have shown a much more complex role for the optic tectum in larval zebrafish, and anatomical and functional data from these studies suggest that this role extends beyond the visual system, and beyond the processing of exclusively retinal inputs. Consistent with this evolving view of the tectum, we have used a Gal4 enhancer trap line to identify direct projections from rostral hypothalamus (RH) to the tectal neuropil of larval zebrafish. These projections ramify within the deepest laminae of the tectal neuropil, the stratum album centrale (SAC)/stratum griseum periventriculare (SPV), and also innervate strata distinct from those innervated by retinal projections. Using optogenetic stimulation of the hypothalamic projection neurons paired with calcium imaging in the tectum, we find rebound firing in tectal neurons consistent with hypothalamic inhibitory input. Our results suggest that tectal processing in larval zebrafish is modulated by hypothalamic inhibitory inputs to the deep tectal neuropil.

Weakly electric fish learn both visual and electrosensory cues in a multisensory object discrimination task

Weakly electric fish use electrosensory, visual, olfactory and lateral line information to guide foraging and navigation behaviors. In many cases they preferentially rely on electrosensory cues. Do fish also memorize non-electrosensory cues? Here, we trained individuals of gymnotiform weakly electric fish Apteronotus albifrons in an object discrimination task. Objects were combinations of differently conductive materials covered with differently colored cotton hoods. By setting visual and electrosensory cues in conflict we analyzed the sensory hierarchy among the electrosensory and the visual sense in object discrimination. Our experiments show that: (i) black ghost knifefish can be trained to solve discrimination tasks similarly to the mormyrid fish; (ii) fish preferentially rely on electrosensory cues for object discrimination; (iii) despite the dominance of the electrosense they still learn the visual cue and use it when electrosensory information is not available; (iv) fish prefer the trained combination of rewarded cues over combinations that match only in a single feature and also memorize the non-rewarded combination.

Data supporting phylogenetic reconstructions of the Neotropical clade Gymnotiformes

Data is presented in support of model-based total evidence (MBTE) phylogenetic reconstructions of the Neotropical clade of Gymnotiformes "Model-based total evidence phylogeny of Neotropical electric knifefishes (Teleostei, Gymnotiformes)" (Tagliacollo et al., 2016) [1]). The MBTE phylogenies were inferred using a comprehensive dataset comprised of six genes (5277 bp) and 223 morphological characters for an ingroup taxon sample of 120 of 218 valid species and 33 of the 34 extant genera. The data in this article include primer sequences for gene amplification and sequencing, voucher information and GenBank accession numbers, descriptions of morphological characters, morphological synapomorphies for the recognized clades of Gymnotiformes, a supermatrix comprised of concatenated molecular and morphological data, and computer scripts to replicate MBTE inferences. We also included here Maximum-likelihood and Bayesian topologies, which support two main gymnotiform clades: Gymnotidae and Sternopygoidei, the latter comprised of Rhamphichthyoidea (Rhamphichthyidae+Hypopomidae) and Sinusoidea (Sternopygidae+Apteronotidae).

Cryptic laminar and columnar organization in the dorsolateral pallium of a weakly electric fish

In the weakly electric gymnotiform fish, Apteronotus leptorhynchus, the dorsolateral pallium (DL) receives diencephalic inputs representing electrosensory input utilized for communication and navigation. Cell counts reveal that, similar to thalamocortical projections, many more cells are present in DL than in the diencephalic nucleus that provides it with sensory input. DL is implicated in learning and memory and considered homologous to medial and/or dorsal pallium. The gymnotiform DL has an apparently simple architecture with a random distribution of simple multipolar neurons. We used multiple neurotracer injections in order to study the microcircuitry of DL. Surprisingly, we demonstrated that the intrinsic connectivity of DL is highly organized. It consists of orthogonal laminar and vertical excitatory synaptic connections. The laminar synaptic connections are symmetric sparse, random, and drop off exponentially with distance; they parcellate DL into narrow (60 μm) overlapping cryptic layers. At distances greater than 100 μm, the laminar connections generate a strongly connected directed graph architecture within DL. The vertical connectivity suggests that DL is also organized into cryptic columns; these connections are highly asymmetric, with superficial DL cells preferentially projecting towards deeper cells. Our experimental analyses suggest that the overlapping cryptic columns have a width of 100 μm, in agreement with the minimal distance for strong connectivity. The architecture of DL and the expansive representation of its input, taken together with the strong expression of N-methyl-D-aspartate (NMDA) receptors by its cells, are consistent with theoretical ideas concerning the cortical computations of pattern separation and memory storage via bump attractors.

Neural maps in the electrosensory system of weakly electric fish

The active electrosense of weakly electric fish is evolutionarily and developmentally related to passive electrosensation and the lateral line system. It shows the most highly differentiated topographic maps of the receptor array of all these senses. It is organized into three maps in the hindbrain that are, in turn, composed of columns, each consisting of six pyramidal cell classes. The cells in each column have different spatiotemporal processing properties yielding a total of 18 topographic representations of the body surface. The differential filtering by the hindbrain maps is used by superimposed maps in the multi-layered midbrain electrosensory region to extract specific stimulus features related to communication and foraging. At levels beyond the midbrain, topographic mapping of the body surface appears to be lost.

Expression of the cannabinoid CB1 receptor in the gymnotiform fish brain and its implications for the organization of the teleost pallium

Cannabinoid CB1 receptors (CB1R) are widely distributed in the brains of many vertebrates, but whether their functions are conserved is unknown. The weakly electric fish, Apteronotus leptorhynchus (Apt), has been well studied for its brain structure, behavior, sensory processing, and learning and memory. It therefore offers an attractive model for comparative studies of CB1R functions. We sequenced partial AptCB1R mRNAs and performed in situ hybridization to localize its expression. Partial AptCB1R protein sequence was highly conserved to zebrafish (90.7%) and mouse (81.9%) orthologs. AptCB1R mRNA was highly expressed in the telencephalon. Subpallial neurons (dorsal, central, intermediate regions and part of the ventral region, Vd/Vc/Vi, and Vv) expressed high levels of AptCB1R transcript. The central region of dorsocentral telencephalon (DC(core) ) strongly expressed CB1R mRNA; cells in DC(core) project to midbrain regions involved in electrosensory/visual function. The lateral and rostral regions of DC surrounding DC(core) (DC(shell) ) lack AptCB1R mRNA. The rostral division of the dorsomedial telencephalon (DM1) highly expresses AptCB1R mRNA. In dorsolateral division (DL) AptCB1R mRNA was expressed in a gradient that declined in a rostrocaudal manner. In diencephalon, AptCB1R RNA probe weakly stained the central-posterior (CP) and prepacemaker (PPn) nuclei. In mesencephalon, AptCB1R mRNA is expressed in deep layers of the dorsal (electrosensory) torus semicircularis (TSd). In hindbrain, AptCB1R RNA probe weakly labeled inhibitory interneurons in the electrosensory lateral line lobe (ELL). Unlike mammals, only few cerebellar granule cells expressed AptCB1R transcripts and these were located in the center of eminentia granularis pars posterior (EGp), a cerebellar region involved in feedback to ELL.

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.

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.

From oscillators to modulators: behavioral and neural control of modulations of the electric organ discharge in the gymnotiform fish, Apteronotus leptorhynchus

The brown ghost (Apteronotus leptorhynchus) is a weakly electric gymnotiform fish that produces wave-like electric organ discharges distinguished by their enormous degree of regularity. Transient modulations of these discharges occur both spontaneously and when stimulating the fish with external electric signals that mimic encounters with a neighboring fish. Two prominent forms of modulations are chirps and gradual frequency rises. Chirps are complex frequency and amplitude modulations lasting between 20 ms and more than 200 ms. Based on their biophysical characteristics, they can be divided into four distinct categories. Gradual frequency rises consist of a rise in discharge frequency, followed by a slow return to baseline frequency. Although the modulatory phase may vary considerably between a few 100 ms and almost 100 s, there is no evidence for the existence of distinct categories of this type of modulation signal. Stimulation of the fish with external electric signals results almost exclusively in the generation of type-2 chirps. This effect is independent of the chirp type generated by the respective individual under non-evoked conditions. By contrast, no proper stimulation condition is known to evoke the other three types of chirps or gradual frequency rises in non-breeding fish. In contrast to the type-2 chirps evoked when subjecting the fish to external electric stimulation, the rate of spontaneously produced chirps is quite low. However, their rate appears to be optimized according to the probability of encountering a conspecific. As a result, the rate of non-evoked chirping is increased during the night when the fish exhibit high locomotor activity and in the time period following external electric stimulation. These, as well as other, observations demonstrate that both the type and rate of modulatory behavior are affected by a variety of behavioral conditions. This diversity at the behavioral level correlates with, and is likely to be causally linked to, the diversity of inputs received by the neurons that control chirps and gradual frequency rises, respectively. These neurons form two distinct sub-nuclei within the central posterior/prepacemaker nucleus in the dorsal thalamus. In vitro tract-tracing experiments have elucidated some of the connections of this complex with other brain regions. Direct input is received from the optic tectum. Indirect input arising from telencephalic and hypothalamic regions, as well as from the preoptic area, is relayed to the central posterior/prepacemaker nucleus via the preglomerular nucleus. Feedback loops may be provided by projections of the central posterior/prepacemaker nucleus to the preglomerular nucleus and the nucleus preopticus periventricularis.

Distribution of calcium/calmodulin-dependent kinase 2 in the brain of Apteronotus leptorhynchus

Antibodies directed against the mammalian alpha and beta subunits of calcium/calmodulin-dependent kinase 2 (CaMK2) and brain dissection were used for immunoblot analysis of these proteins in various brain regions of Apteronotus leptorhynchus. Western blots revealed that the CaMK2alpha antibody labeled a single band of the expected molecular mass (approximately 50 kDa) for this enzyme in rat cortex and electric fish brain. CaMK2alpha was enriched in fish forebrain and hypothalamus and also strongly expressed in midbrain sensory areas. Western blots revealed that CaMK2beta antibodies labeled bands in an appropriate molecular mass range (approximately 58-64 kDa) for this enzyme in mammalian cortex and electric fish brain. However, a higher molecular mass band (approximately 80 kDa) was also labeled; because all these bands were eliminated by preadsorbtion with the CaMK2-derived peptide antigen, they may all represent CaMK2beta-like isoforms. We mapped the brain distribution of CaMK2 isoforms with emphasis on the electrosensory system. CaMK2alpha was present at high density in dorsal forebrain, hypothalamic nuclei, torus semicircularis, and tectum. It was also enriched in discrete fiber tracts in forebrain, diencephalon, and rhombencephalon. CaMK2beta-like isoforms were enriched in ventral forebrain, hypothalamic nuclei, torus semicircularis and the reticular formation. Unlike CaMK2alpha, CaMK2beta -like isoforms were predominantly present in cell bodies and rarely found in fiber tracts or neuropil. In the electrosensory lateral line lobe, CaMK2alpha was restricted to specific feedback fibers, i.e., tractus stratum fibrosum and its terminal field in the ventral molecular layer. In contrast, CaMK2beta-like isoforms were enriched in somata and dendrites of pyramidal cells and granular interneurons.

N-methyl-D-aspartate receptor 1 mRNA distribution in the central nervous system of the weakly electric fish Apteronotus leptorhynchus

We have isolated a partial cDNA for the N-methyl-D-aspartate (NMDA) receptor 1 (NMDAR1) subunit from an Apteronotus leptorhynchus brain cDNA library. The A. leptorhynchus cDNA fragment, which corresponds to nucleotides 135-903 within the 5' region of the rat NR1 mRNA, encodes 252 amino acids that are >80% identical to the homologous segments of the rat, human, and duck NR1 proteins. RNAse protection assays revealed that the A. leptorhynchus NR1 mRNA was highly enriched in the forebrain and hypothalamus, with lesser amounts in the brainstem, and very low levels in the cerebellum. In situ hybridization also demonstrated that neurons in the pallial forebrain were highly enriched in NR1 transcripts. High levels of NR1 mRNA were found in pyramidal cells within the optic tectum and octavolateral regions. Pyramidal cells of the electrosensory lateral line lobe had the highest levels of expression, and the NR1 mRNA was found to be selectively enriched in their apical dendrites.

Tectal input to the central posterior/prepacemaker nucleus of weakly electric fish, Apteronotus leptorhynchus: an in vitro tract-tracing study

The weakly electric fish Apteronotus leptorhynchus produces electric organ discharges which are highly stable in waveform and frequency. Short-term modulations of these discharges, typically displayed during social interactions, are controlled by the prepacemaker nucleus (PPn). Neurons of this thalamic cell group intermingle with cells of the central posterior nucleus (CP) to form a complex called 'CP/PPn'. By employing in vitro tract-tracing techniques, we have, in the present investigation, demonstrated that this complex receives input from the tectum opticum. The tectal input is mediated by varicose fibers forming an elongated stripe at the ventral rim of the CP/PPn. As suggested by retrograde tracing from the CP/PPn, this projection is likely to arise from 'multipolar cells with an ascending axon' previously characterized in a Golgi study [14]. As this tectal cell type has been shown to be predominantly driven by electrosensory stimuli [6], information arising from these cells may be used in controlling modulations of the electric organ discharges.

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.

Localization of nicotinamide adenine dinucleotide phosphate-diaphorase activity in electrosensory and electromotor systems of a gymnotiform teleost, Apteronotus leptorhynchus

The distribution of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) activity was determined in electrosensory and electromotor systems of the weakly electric gymnotiform teleost Apteronotus leptorhynchus as an indicator of putative nitric oxide synthase-containing cells. NADPH-d activity was detected in electroreceptors and in afferent nerves of both ampullary and type I and type II tuberous organs. All cell bodies within the anterior lateral line nerve ganglion were positive for NADPH-d activity, as were the primary afferent axons and termination fields in the medullary electrosensory lateral line lobe. In the corpus cerebelli and valvula cerebelli, NADPH-d label was present in Purkinje cell somata, mossy fiber synaptic glomeruli, granule cells, and parallel fibers. In the midbrain, NADPH-d activity was apparent in layer VIIIB of the torus semicircularis dorsalis and in electrosensory laminae of the optic tectum. NADPH-d was particularly associated with diencephalic electrosensory and electromotor nuclei, including the prepacemaker nucleus, the nucleus subelectrosensorius, and the central posterior nucleus of the thalamus. Intense NADPH-d activity was present in pacemaker and relay cells of the medullary pacemaker nucleus but was absent from a novel class of smaller cells in this structure. Relay cell axons and spinal electromotor neurons and their axons within the electric organ were positive for NADPH-d activity. These results indicate that putative nitric oxide synthase-containing neurons in Apteronotus are localized preferentially to electrosensory and electromotor structures, suggesting a role for nitric oxide in determining the activity of cells involved in detecting or generating weakly electric fields.

The distribution of tachykinin binding sites in the brain of an electric fish (Apteronotus leptorhynchus)

We mapped the distribution of tachykinin binding sites utilizing quantitative autoradiography of iodinated substance P and eledoisin as prototypic ligands for neurokinin-1 (NK1) and neurokinin-3 (NK3) receptors, respectively. The two ligands produced highly heterogenous and quantitatively different patterns of specific binding, suggesting that they revealed different tachykinin receptor subtypes. Although [125I]substance P and [125I]eledoisin binding were correlated in most brain regions, the binding of substance P was usually denser. [125I]substance P binding and substance P-like immunoreactivity were reasonably correlated in most brain areas, although discrepancies were found in some nuclei. Dense [125I]substance P binding was found in most areas of the subpallium and in parts of the pallium related to the olfactory system, as well as in the glomerular layer of the olfactory bulb. Moderate to dense binding of both ligands was observed in preoptic area, hypothalamus, habenula, parts of the thalamus and preglomerular complex. Especially noteworthy was the presence of [125I] substance P binding in the diencephalic prepacemaker nucleus, a region involved in the control of electroncommuncatory behavior. Substance P-like immunoreactivity is sexually dimorphic in certain diencephalic nuclei, including the prepacemaker nucleus (Weld and Maler, 1992); no obvious difference was seen between [125I]substance P or [125I]eledoisin binding in the brains of male versus female fish. In the mesencephalon striking laminar patterns of binding were seen in the torus semicircularis dorsalis and the optic tectum. Dense binding was also noted in the raphé nuclei, the locus ceruleus and the sensory nucleus of the vagus. Although binding of substance P in the electrosensory lateral line lobe and nucleus preeminentialis was light, it was distributed in a discrete fashion, suggesting a role of substance P in electrosensory processing.

The distribution of somatostatin binding sites in the brain of gymnotiform fish, Apteronotus leptorhynchus

The neuropeptide somatostatin (SS) and its binding sites display a wide distribution in the central nervous system of vertebrates. By employing semi-quantitative autoradiography, we identified such binding sites in the brain of the weakly electric fish Apteronotus leptorhynchus (Gymnotiformes, Teleostei). Whereas (SS1) binding sites for the octapeptide analogue Tyr3-SMS-201-995 appear to be absent in the gymnotiform brain, (SS2) binding sites for the analogue [Tyr0-D-Trp8]-somatostatin-14 were found in many brain regions and showed a similar distribution to that observed by other authors in the amphibian and mammalian central nervous system. Telencephalon While binding in the ventral telencephalon was typically low, all cell groups of the dorsal portion displayed a high degree of binding. The highest density of binding sites was found in the dorsal and caudal subdivision 2 of the dorsomedial telencephalon. Diencephalon Many cell groups of the diencephalon showed a medium to high degree of binding density. The highest level was seen in the habenula. Mesencephalon All layers of the optic tectum contained a medium number of binding sites, except the stratum marginale. In the torus semicircularis, the different layers displayed distinct binding density. While laminae 7-8 showed the highest degree of binding, the lowest density was found in lamina 6. Rhombencephalon Binding was generally low or absent in the tegmentum. Low levels of binding density were observed in the electrosensory lateral line lobe. Cerebellum Extremely high levels of binding were found in the eminentia granularis medialis and the eminentia granularis posterior. Throughout most regions of the brain, the relative density of binding sites and the relative amount of somatostatin immunoreactivity in fibres, as determined in previous studies, were in good agreement.

Immunohistochemical localization of ryanodine binding proteins in the central nervous system of gymnotiform fish

The ryanodine receptor, an integral membrane protein of the sarcoplasmic reticulum in muscle, embodies a high conductance channel permeable to calcium ions. Recent studies have identified ryanodine-binding proteins in avian and mammalian central nervous systems. These neuronal ryanodine receptors appear to function as Ca2+ channels which may gate the release of Ca2+ from caffeine-sensitive intracellular pools in neurons. In the present investigation, we employed monoclonal antibodies against ryanodine-binding proteins of avian muscle cells to the brain of weakly electric gymnotiform fish. Immunoprecipitation and Western blot analysis revealed two isoforms in the fish brain, with molecular weights comparable to those of avian and fish muscle ryanodine-binding proteins. By employing immunohistochemical techniques, we mapped these proteins in fish brain. Ryanodine receptor-like immunoreactivity was found in nerve cell bodies as well as dendrites and axonal processes. The ryanodine-binding protein is distributed throughout the neuraxis in specific cell types of the gymnotiform brain. In the telencephalon, immunoreactive cells were found in the glomerular layer of the olfactory bulb, in the supracommissural subdivision of the ventral telencephalon, and in the intermediate rostral subdivision of the ventral telencephalon. In the diencephalon, immunoreactive cells or fibers were observed in the nucleus prethalamicus and the habenula, within the nucleus at the base of the optic tract and the adjacent dorsal tegmental nucleus, the pretectal nuclei A and B, and the nucleus electrosensorius. In addition, immunopositive cells were seen in several nuclei of the hypothalamus, with the inferior and lateral subdivision of the nucleus recessus lateralis displaying the highest concentration of neurons. In the mesencephalon, the optic tectum contained the greatest number of immunopositive cells. In the rhombencephalon, labelling was seen in the nucleus of the lateral valvula, central gray, lateral tegmental nucleus, in boundary cells of the nucleus praeminentialis, efferent octavolateral nucleus, an area adjacent to the medial edge of the lateral reticular nucleus, nucleus medialis, and electrosensory lateral line lobe. As in avian brain, cerebellar Purkinje cells were positive for ryanodine-binding protein, although only subsets of Purkinje cells were labelled.

The optic tectum of the dogfish Scyliorhinus canicula L.: a Golgi study

The optic tectum of the dogfish Scyliorhinus canicula L. was studied by using the methods of Nissl, reduced silver nitrate, Golgi-aldehyde, and Golgi-Cox. Six layers and eight types of neurons were recognized. These cell types are not restricted to one layer; in fact some are found in all six tectal layers. The types of cells found are A) monopolar, B) triangular, C) radial bipolar, D) horizontal fusiform, E) large tectal, F) small tectal, G) pyriform, and H) stellate cells. In at least six of the cell types a series of dendritic specializations can be observed, such as spines in the form of "drumsticks" and thin varicose appendages, similar to those reported previously in the optic tecta of amphibians and teleosts. The optic tectum of the dogfish shows a degree of complexity comparable to that of amphibians and teleosts.

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.

Development of the electrosensory nervous system of Eigenmannia (gymnotiformes): II. The electrosensory lateral line lobe, midbrain, and cerebellum

The somatotopically and functionally organized electrosensory system of gymnotiform teleosts provides a model for the study of the formation of ordered nerve connections. This paper describes the development of the major electrosensory nuclei within the hind- and midbrain. All three main electrosensory nuclei--the electrosensory lateral line lobe (ELL), dorsal torus semicircularis (torus), and tectum--grow by adding cells at their caudolateral borders. Toral and tectal germinal zones arise from lateral ventricular outpocketings that either completely or partially close by maturity. In the ELL before day 5 postspawning, germinal cells form from an initial periventricular germinal zone, then migrate to the caudolateral border of the hindbrain and begin dividing. The ELL grows from two main germinal zones, one for the medial segment, and one for the three lateral tuberous segments. Within each ELL germinal zone, newly formed cells arise from two areas: granular cells arise from a ventral subzone, pyramidal cells are generated more dorsally. Granular cells remain in situ, whereas pyramidal cells may migrate rostromedially. Cells begin differentiating as soon as they are formed. Spherical and pyramidal cells send ascending axons into the internal plexiform layer by day 14-18 and the ELL gradually begins to assume its mature laminar appearance. The ELL grows caudally, preceding the caudal lobe of the cerebellum, which will eventually lie over and fuse with it. Primary electrosensory afferents enter the ELL by day 6; incoming afferents form four fascicles within the ELL, suggesting the formation of separate ELL segments. Unlabelled projections between labelled fields from a single nerve branch filled with HRP on day 7 suggest that somatotopic order is already present at this early age. In the periphery, receptor addition is unordered, occurring along nerve branch pathways. Meanwhile the ELL adds cells in an orderly fashion at its caudolateral border. This suggests that primary afferents shift position caudally with growth to maintain their somatotopic relationships. Because all three central nuclei are in topographic register and grow by adding cells caudally, during growth ELL efferents to the torus and toral efferents to the tectum may utilize passive mechanisms, such as fiber-fiber interactions, to guide axons.

Development of tectal neurons in the perciform teleost Haplochromis burtoni. A Golgi study

The differentiation of the tectum mesencephali of Haplochromis burtoni (Teleostei, Cichlidae) was studied using a modified Golgi rapid impregnation. The analysis concentrated on the gradient of differentiation of four neuronal types, type I, IIIu, VI and XII, in 15-day-old larvae. The following developmental steps taken by these neuronal types are identified: (1) morphogenesis and growth are largely independent developmental events. Tectal neurons first develop their typical dendritic morphology. The tectal lamination, as indicated by the spatial relationships of the dendrites of tectal neurons, is acquired already in 15-day-old larvae. Subsequently the neurons grow to their adult size. Intersegments of dendrites elongate considerably. Dendritic and axonal reorganization and/or intersegmental growth may take place. (2) The teleost cell types I and VI show variable positions of their perikaryon in 15-day-old larvae, but not in adults. It is suggested that they translocate their perikaryon inside their stem dendrite, while their dendrites are already well developed.

Ganglion cell arrangement and axonal trajectories in the anterior lateral line nerve of the weakly electric fish Apteronotus leptorhynchus (Gymnotiformes)

To determine the organizational principles underlying the peripheral electrosensory nervous system of weakly electric gymnotiform teleosts we labelled each of the four anterior lateral line nerve branches with HRP. We determined the position of labelled cell bodies within the ganglion and followed anterogradely filled fibers to their termination sites in one of the four somatotopic maps in the electroreceptive lateral line lobe (ELL). Within the ganglion, cell bodies exhibit a loose somatotopy based on nerve branch position: trunk electroreceptors have their cell bodies located in the caudal ganglion; cell bodies to the head receptors are rostral. Cell bodies to the head exhibit a rough dorsoventral polarity, supraorbital cells tend to be located dorsally, infraorbital cells centrally, and mandibular cells ventrally. Despite this general somatotopy there is substantial overlap (up to 30%) of cell bodies among regions. There appears to be no rostrocaudal topography within nerve branch regions. Iontophoretic WGA-HRP injected into the medial segment of the ELL retrogradely labelled cell bodies that innervate ampullary organs. These cell bodies were dispersed throughout the ganglion, indicating that cell bodies do not cluster by receptor type. Peripherally directed axons from the ganglion appear to undergo an active reorganization in order to form the nerve branches. Within nerve branches, axons to a particular area of skin do not cluster together. Centrally from the ganglion, axons retain the position of their cell body until they reach the ELL border. Once in the ELL, fibers become sorted in the deep fiber layer according to receptor type and the map they terminate in. This reorganization involves rearrangement of fascicles and axons within fascicles. In toto, proceeding from peripheral to central, the electrosensory periphery loses at least a portion of its receptor topography in the distal nerve and ganglion and then acquires both a functional and somatotopic organization after reaching the ELL; conceptually it is torn down and rebuilt again. From an ontogenetic perspective, axonal growth occurs from the ganglion outward; the fact that ganglion cell bodies are not highly organized while the receptors they innervate and their central processes are suggests that active axonal guidance mechanisms are involved.

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.

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

The eyes of weakly electric gymnotid fish are poorly developed in comparison to those of most diurnal teleosts. The tectum and pretectum, despite their usual association with the visual system, are large and well differentiated in gymnotids. We have studied retinal projections in gymnotids in order to define the visual components of the mesencephalon and diencephalon and thus allow comparison with other teleosts in which retinofugal fibers have been extensively mapped. Retinofugal projections reported in this work are based on the anterograde transport of conjugated wheat germ agglutinin horseradish peroxidase, following injection into the posterior chamber of the eye of Apteronotus leptorhynchus (brown ghost knife fish). The results show a remarkable similarity to those of non-electroreceptive teleosts. Although the optic nerves appear to cross completely at the optic chiasm, close scrutiny shows a slender recrossing fascicle which continues from the contralateral tractus opticus medialis through the rostroventral hypothalamus to reach the ipsilateral side, providing a scanty projection to the n. opticus hypothalamicus, n. anterior periventricularis, n. dorsolateralis thalami, and n. commissurae posterioris. A few fibers ascend via the tractus opticus dorsomedialis to the rostral dorsomedial part of the stratum fibrosum et griseum superficiale of the ipsilateral tectum. The main body of the retinal projections in Apteronotus are to the following contralateral target areas: preoptic area, n. opticus hypothalamicus, n. anterior periventricularis, n. dorsolateralis thalami, n. pretectalis, area pretectalis, n. corticalis, n. commissurae posterioris, n. geniculatus lateralis, area and n. ventrolateralis thalami, caudal dorsal tegmentum and the tectum opticum. The retinotectal projection is modest in comparison to that of more vision dependent fish and terminates mainly in the upper half of the stratum fibrosum et griseum superficiale; hardly any retinal fibers reach the caudalmost tectum.