Zebrin II distinguishes the ampullary organ receptive map from the tuberous organ receptive maps during development in the teleost electrosensory lateral line lobe

In weakly electric gymnotiform teleosts, monoclonal antibody anti-zebrin II recognizes developing pyramidal cells in the ampullary organ-receptive medial segment of the medullary electrosensory lateral line lobe (ELL) and in the mechanoreceptive nucleus medialis. Developing pyramidal cells in the remaining three tuberous organ-receptive lateral ELL segments are unreactive. These results suggest that certain biochemical features of the ELL ampullary organ-receptive medial segment are more similar to the nucleus medialis than to the tuberous organ-receptive ELL segments, and support the hypothesis that the ampullary system evolved from mechanosensory precursors.     

Collateral sprouting in the electrosensory lateral line lobe of weakly electric teleosts (Gymnotiformes) following ricin ablation

Sprouted collateral axons were observed in the electrosensory lateral line lobe (ELL) of gymnotiform teleosts (Apteronotus leptorhynchus:) following the ablation of the supraorbital branch of the anterior lateral line nerve. Ablation was accomplished by using microinjections of the toxic lectin ricin. Sprouted axons were followed for up to 26 weeks postablation. Ricin exposure severely reduced axonal numbers and the peripheral electroreceptors in the region innervated by these fibers. To visualize sprouted fibers, intact lateral line afferent nerve branches were anterogradely labelled with the neuronal tract tracers horseradish peroxidase or cobalt chloride, or the monoclonal antibody Q26A3. Within the four somatotopically organized ELL segments, sprouted collaterals were first observed two weeks after ricin injection in the medial and centromedial segments, and four weeks postinjection in the centrolateral and lateral segments. Sprouting involved intrasegmental, horizontally directed axons from adjacent nerve branch terminal fields, and mixed intra-and extrasegmental, dorsally directed axons from the ELL deep fiber layer. The sprouting response was robust but variable in its timing, peaking between 6 and 12 weeks. Subsequently, the intrasegmental, horizontally directed fibers were retained but the mixed dorsally directed fibers, including all extrasegmental axons, were retracted. Therefore, this sprouting response appears to consist of a collateral overproduction followed by a selective axonal retraction. In our view, the most likely explanation for this axonal retraction is that the descending inputs from the isthmus and the cerebellum, as well as commissural fibers from the contralateral ELL, maintain established somatotopic relationships by eliminating somatotopically mismatched sprouted collaterals. © 1993 Wiley-Liss, Inc.     

Multiple electrosensory maps in the medulla of weakly electric gymnotiform fish. II. Anatomical differences

Both wave- and pulse-type species of weakly electric gymnotiform fish have 3 topographic maps of electroreceptive information in the electrosensory lateral line lobe (ELL). These maps receive identical input from trifurcating axons of phase- and amplitude-coding primary afferents (Carr et al., 1982; Heiligenberg and Dye, 1982). Physiological experiments in the ELL of the wave-type fish Eigenmannia show that the amplitude-coding pyramidal cells differ among maps with respect to receptive field size, sensitivity, rate of adaptation, and temporal-frequency response (Shumway, 1989). This study investigated morphological correlates of the physiological differences among maps. Estimates of primary afferent convergence in Eigenmannia, based on map size, cell counts, and areas of terminal fields from intracellularly filled P-type primary afferents, suggest a 2-fold increase in convergence in the lateral map relative to the centromedial map. Similar differences in convergence between maps are found in the wave-type species Apteronotus leptorhynchus and the pulse-type fish Hypopomus occidentalis. The lateral and centrolateral maps in Hypopomus, however, show an even greater difference in convergence. Comparison of the efferent projections of pyramidal cells among the different maps of Eigenmannia indicates that cells from the 3 maps terminate in the same laminae of the torus semicircularis, but the maps differ in the strength of projection to particular laminae. In both wave-type species, the abundance of a class of interneurons which receives descending input and inhibits pyramidal cells (interneurons of the ventral molecular layer) differs among maps; the centromedial map has 10 times fewer neurons of this type than the other 2 maps. Cytochrome oxidase studies in all 3 species demonstrated increased levels of activity in the lateral map, within the region receiving descending input from the cerebellum. These results suggest that the primary anatomical bases of the physiological differences among maps are differences in the amount of primary afferent convergence, coupled with differences in descending input.     

Efferent projections of the posterior lateral line lobe in gymnotiform fish

The posterior lateral line lobe (PLLL) of gymnotoid fish has efferent projections to two midbrain regions: the nucleus praeeminentialis dorsalis (n.P.d.) and the torus semicircularis dorsalis (T.Sd.). Both ipsilateral and contralateral connections are present; the n.P.d. receives nearly equal input from both sides while the T.Sd. receives a stronger contralateral input. The PLLL projection to n.P.d. merely maps medial PLLL to ventral n.P.d. and lateral PLLL to dorsal n.P.d., thus preserving the separate topography and relative orientation of the four electrosensory maps found in the PLLL. Only PLLL pyramidal cells (basilar and nonbasilar pyramids) contribute to this projection. The four PLLL electrosensory maps converge onto T.Sd. so that they map the dorsal body surface onto medial T.Sd. and the ventral body surface onto lateral T.Sd. Pyramidal cells, spherical cells, and multipolar cells contribute to this projection. A small commiusural connection links homologous segments of the PLLL; these fibers arise from polymorphic cells of the PLLL.     

Multiple electrosensory maps in the medulla of weakly electric gymnotiform fish. I. Physiological differences

The electrosensory lateral line lobe in the weakly electric gymnotiform fish Eigenmannia contains 3 topographic maps of high-frequency (tuberous) electroreceptive information from the body surface. The maps receive identical primary afferent input since axonal collaterals of both amplitude- and phase-coding afferents project to all 3 maps (Heiligenberg and Dye, 1982). Response properties of the amplitude-coding pyramidal neurons in the multiple maps were investigated in order to determine whether the maps differ physiologically. Units in the lateral map have larger receptive fields and are more sensitive than units in the centromedial map. The former units respond more phasically and with shorter latencies to step changes in stimulus amplitude (measured from the stimulus onset to the maximum response). Although 75% of pyramidal cells in all maps show a center-surround receptive-field organization, the strength of the inhibitory surround varies among maps, tending to be weakest for units in the lateral map and strongest for units in the centromedial map. Pyramidal neurons also differ in their responses with respect to the temporal frequency of amplitude modulations; the majority of units in the lateral map prefer high temporal frequencies, while those in the centromedial map prefer low frequencies. These results suggest that the multiple electrosensory maps could provide the initial separation of spatial and temporal processing of sensory information, much as has been suggested for X and Y ganglion cells in the cat retina (Shapley and Perry, 1986). The centromedial map could provide high spatial contrast with correspondingly poor temporal resolution, while the more sensitive units in the lateral map could best provide information about temporal changes in stimulus amplitude.     

Receptive field organization across multiple electrosensory maps. I. Columnar organization and estimation of receptive field size

The electric fish Apteronotus leptorhynchus emits a high‐frequency electric organ discharge (EOD) sensed by specialized electroreceptors (P‐units). Amplitude modulations (AMs) of the EOD are caused by objects such as prey as well as by social interactions with conspecifics. The firing rate of P‐units is modulated by the AMs due to both objects and communication signals. P‐units trifurcate as they enter the medulla; they terminate topographically with three maps of the electrosensory lateral line lobe (ELL): the centromedial (CMS), centrolateral (CLS), and lateral (LS) segments. Within each map P‐units terminate onto the basal dendrites of pyramidal cells. Anterograde filling of P‐units and retrograde filling of the basal bushes of pyramidal cells were used to estimate their respective spreads and spacing in the three maps. These estimates were used to compute the receptive field structure of the pyramidal cells: receptive fields were small in CMS and very large in LS with intermediate values in CLS. There are several classes of pyramidal cells defined by morphological and functional criteria; these cells are organized into columns such that each column contains one member of each class and all cells within a column receive the same P‐unit input. J. Comp. Neurol. 516:376–393, 2009. © 2009 Wiley‐Liss, Inc.     

Segregation of electroreceptive and mechanoreceptive lateral line afferents in the hindbrain of chondrostean fishes

The anterior lateral line nerve (ALLN) in the chondrostean fishes (sturgeon and paddlefishes) consists of both fibers innervating ampullary electroreceptors and fibers innervating the mechanoreceptive neuromasts of the cephalic lateral line system. The fibers of the posterior lateral line nerve (PLLN) innervate only mechanoreceptive neuromasts on the body trunk. The ALLN enters the medulla via dorsal and ventral roots; the dorsal root projects to the dorsal octavolateralis nucleus (DON), whereas the ventral root and the PLLN project principally to the medial octavolateralis nucleus (MON). Previous studies in elasmobranchs have demonstrated that fibers of the dorsal root of the ALLN convey electrosensory information, and fibers of the ventral root are concerned with mechanoreceptive information. Electrophysiological and neuroanatomical methods are employed in this study in order to determine if there exists a similar segregation of electroreceptive and mechanoreceptive lateral line afferents within the chondrostean medulla. In specimens of shovelnose, Scaphirhynchus platorynchus, and Atlantic sturgeon, Acipenser oxyrhynchus, and paddlefish, Polyodon spathula, evoked potentials recorded from the hindbrain and elicited by electric fields reached maximum amplitude within the DON and decreased in amplitude through the cerebellar crest. Evoked potentials elicited by stimulation of the posterior lateral line nerve achieved maximum amplitude within the MON. Single and multiple unit recordings revealed that units within the DON responded only to electric field stimulation, whereas units recorded in the MON responded only to mechanical stimulation. Horseradish peroxidase implanted beneath isolated patches of ampullae in Polyodon revealed fibers innervating electroreceptors projecting to the DON via the dorsal root of the ALLN. These results demonstrate a segregation of electroreceptive and mechanoreceptive lateral line afferent fibers in the chondrostean hindbrain, similar to that seen in elasmobranchs. This supports the contention that the electrosensory systems of elasmobranchs and chondrosteans are homologous, and are derived from the common ancestor of elasmobranch and actinopterygian fishes.     

Lateral line receptors: where do they come from developmentally and where is our research going?

The lateral line system is composed of both mechanoreceptors, which exhibit little variation in structure between taxonomic groups, and electroreceptors, which exhibit considerably more variation. Cathodally sensitive ampullary electroreceptors are the primitive condition and are found in agnathans, chondrichthyans, and most osteichthyans. Aquatic amphibians also have ampullary electroreceptors for at least part of their life cycle. The more recently evolved anodally sensitive ampullary electroreceptors and tuberous electroreceptors are only found in four groups of teleost fishes. The basic ontogenetic unit of lateral line development is the dorsolateral placode. Primitively, there are six pairs of placodes, which pass through sequential stages of development into lateral line receptors. There is no question about the origin of primitive mechanoreceptors or electroreceptors, however, we do not have a good understanding of the origin of teleost mechanoreceptors and their ampullary or tuberous electroreceptors; do they come exclusively from dorsolateral placodes or from neural crest or even general ectoderm? A second intriguing lateral line question is how certain teleost fish groups evolved tuberous electroreceptors. Electroreception appears to have re-evolved at least twice in teleosts after being lost during the neopterygian radiation. It has been suggested that the development of tuberous electroreceptors might be due to changes in placodal patterning or a change in the general ectoderm that placodes arise from. Unfortunately, our understanding of lateral line origins in fishes is very sketchy, and, if we are to answer such an evolutionary question, we first need more complete information about lateral line development in a variety of fishes, which can then be combined with gene expression data to better interpret lateral line receptor development.     

Receptive field organization across multiple electrosensory maps. II. Computational analysis of the effects of receptive field size on prey localization

The electric fish Apteronotus leptorhynchus emits a high‐frequency electric organ discharge (EOD) sensed by specialized electroreceptors (P‐units) distributed across the fish's skin. Objects such as prey increase the amplitude of the EOD over the underlying skin and thus cause an increase in P‐unit discharge. The resulting localized intensity increase is called the electric image and is detected by its effect on the P‐unit population; the electric image peak value and the extent to its spreads are cues utilized by these fish to estimate the location and size of its prey. P‐units project topographically to three topographic maps in the electrosensory lateral line lobe (ELL): centromedial (CMS), centrolateral (CLS), and lateral (LS) segments. In a companion paper I have calculated the receptive fields (RFs) in these maps: RFs were small in CMS and very large in LS, with intermediate values in CLS. Here I use physiological data to create a simple model of the RF structure within the three ELL maps and to compute the response of these model maps to simulated prey. The Fisher information (FI) method was used to compute the optimal estimates possible for prey localization across the three maps. The FI predictions were compared with behavioral studies on prey detection. These comparisons were used to frame alternative hypotheses on the functions of the three maps and on the constraints that RF size and synaptic strength impose on weak signal detection and estimation. J. Comp. Neurol. 516:394–422, 2009. © 2009 Wiley‐Liss, Inc.     

Development of the electrosensory nervous system in Eigenmannia (Gymnotiformes): I. The peripheral nervous system

The nerves of the anterior lateral line system in embryonic and larval stages of the weakly electric gymnotiform fish Eigenmannia were visualized by injection of the fluorescent marker DiI into the primordium of the anterior (ALLN) and posterior (PLLN) lateral line nerves. Examination of developmental series reveals that the nerve fibers that innervate the electrosensory and mechanosensory components of the anterior lateral line system are present before the first mechanoreceptors and electroreceptors have differentiated. This suggests that nerve fibers might induce the formation of lateral line receptors. Whereas the innervation of the mechanoreceptive system is already established at an early stage, the afferent innervation of electroreceptors continues to arborize in the periphery, presumably by following pioneer axon pathways. The earliest recognizable stage of the anterior lateral line nerve ganglion (ALLNG) is evident 2 days after spawning. The ganglion shows two germinal cell masses that develop into the supraorbital-infraorbital and the hyomandibular placodes. The supraorbital-infraorbital placode forms the dorsal part of the ALLNG; the hyomandibular placode forms the ventral part of the ALLNG. Counts of ALLNG cells in embryonic, larval, and adult stages of Eigenmannia show that, at each stage examined, the number of ganglion cells is always significantly larger than the number of mechanoreceptors and electroreceptor units in the periphery. During development, the distribution of ALLNG cell diameters shifts from a unimodal distribution in juveniles to a bimodal distribution in adults, peaking at 8 microns and 18 microns. These results suggest that tuberous electroreceptive organs, which are innervated by the large ALLNG cells, may not be functional prior to day 18. Our results further suggest that the number of ALLNG cells correlates with the rate of induction of lateral line receptors in the periphery.     

Somatotopy within the medullary electrosensory nucleus of the little skate, Raja erinacea

The dorsal octavolateral nucleus is the primary electrosensory nucleus in the elasmobranch medulla. We have studied the topographic organization of electrosensory afferent projections within the dorsal nucleus of the little skate, Raja erinacea, by anatomical (HRP) and physiological experiments. The electrosensory organs (ampullae of Lorenzini) in skates are located in four groups on each side of the body, and each group is innervated by a separate ramus of the anterior lateral line nerve (ALLN). Transganglionic transport of HRP in individual rami demonstrated that electroreceptor afferents in each ramus project to a separate, nonoverlapping division of the central zone of the ipsilateral dorsal nucleus. These divisions, which are distinct areas separated by compact cell plates, are somatopically arranged. The volume of each division of the dorsal nucleus that is related to a single ramus is proportional to the number of ampullae innervated by the ramus, but not to the body surface area on which the receptors are distributed. Nearly one-half of the nucleus is devoted to electrosensory inputs from the buccal and superficial ophthalmic ampullae concentrated in a small area on the ventral surface of the head rostral to the mouth. Multiple and single unit recordings demonstrated that adjacent cells in the nucleus have similar receptive fields on the body surface and revealed a detailed point-to-point somatotopy within the nucleus. With threshold stimuli most single units have ipsilateral receptive fields made up by excitatory inputs from 2-5 ampullary organs. The somatotopy within the mechanosensory medial nucleus, also revealed by the HRP fills of individual ALLN rami, appears less rigid than that in the dorsal nucleus, as extensive overlap is present in the terminal fields of separate ALLN rami.     

Light and electron microscopical studies on the spherical neurons in the electrosensory lateral line lobe of the gymnotiform fish,Sternopygus

Spherical cells are a principal cell type of the electrosensory lateral line lobe (ELLL) and play a crucial role in the jamming avoidance response (JAR) behavior. Since Sternopygus, a low frequency gymnotiform genus, does not display a JAR we searched for spherical cells in its ELLL. While present in Sternopygus, spherical cells differed remarkedly from those in the high-frequency gymnotiforms, Eigenmannia and Apteronotus. This study reveals species-characteristic differences in the morphology and synaptology of the spherical cell, a projection neuron located in the deep neuropil layer (DNL) of the ELLL. In contrast to the adendritic spherical cell of other species, the spherical neuron in Sternopygus exhibits an extensive basilar dendrite that extends into the primary electroreceptive afferent zone, the deep fiber layer (DFL). In Sternopygus, these neurons are distributed evenly across the full length of each tuberous subdivision, with cell densities highest in the centrolateral subdivision. At the ultrastructural level, the contacts on the soma, proximal, and distal dendrite of the spherical neuron in Sternopygus are asymmetrical chemical synapses, quite distinct from the electrotonic gap junctions found on the spherical neurons of other species.     

Correlating gamma-aminobutyric acidergic circuits and sensory function in the electrosensory lateral line lobe of a gymnotiform fish

Electric fish generate an electric field, which they sense with cutaneous electroreceptors. Electroreceptors project topographically onto the medullary electrosensory lateral line lobe (ELL). The ELL of gymnotiform electric fish is divided into four segments specialized to detect different aspects of the electrosensory input; it is also laminated with separate laminae devoted to electroreceptive input, interneurons, projection neurons, and feedback input. We have utilized antisera to glutamic acid decarboxylase (GAD) and γ-aminobutyric acid (GABA) to map the distribution of GABAergic cells and fibers in the ELL of the gymnotiform fish, Apteronotus leptorhynchus. Six types of GABAergic interneurons are found in ELL: Type 2 granular cells (granular layer) project to pyramidal cells; polymorphic cells (pyramidal cell layer) project to the non-GABAergic type 1 granular cells; ovoid cells (deep neuropil layer) project bilaterally upon basilar dendrites of pyramidal cells; multipolar cells (deep neuropil layer) project bilaterally, probably to dendrites and neurons within the deep neuropil layer; and neurons of the ventral molecular layer and stellate cells (molecular layer) project to apical dendrites of pyramidal cells. GABAergic bipolar cells in the nucleus praeminentialis, a rhombencephalic structure devoted to feedback in the electrosepsory system, project in relatively diffuse fashion to pyramidal cells. We hypothesize that the various GABAergic circuits of the ELL can be correlated with specific functions: type 2 granular cells with adaptation, size of receptive field center, and gain; polymorphic cells and type 1 granular cells with regulation of surround inhibition; ovoid cells with common mode rejection; and neurons of the ventral molecular layer with adaptive gain control. The feedback GABAergic input from bipolar cells of n. praeminentialis to pyramidal cells may be part of a searchlight mechanism similar to the one postulated for thalamocortical systems. © 1994, Wiley-Liss, Inc.     

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.     

Development of the lateral line system in the shovelnose sturgeon

The lateral line systems of aquatic amphibians and all chondrichthyan and osteichthyan fish present a similar array of mechanoreceptors. However, electroreceptors, the second major component of the lateral line system, have clearly undergone more significant evolutionary change. Chondrichthyans and non-neopterygian fish possess primitive ampullary organ electroreceptors, whereas significantly different 'new' ampullary organs and tuberous electroreceptors are found in a few groups of teleosts (mormyrids, gymnotids and some catfish). The pairing of mechano- and electroreceptors in the lateral line system, as well as the morphologically and physiologically distinct electroreceptors of teleosts have inspired several recent studies on the origin and evolution of the lateral line receptors. We described the development of the lateral line system in sturgeon (Scaphirhynchus platorynchus) as part of an outgroup analysis of lateral line development in three taxa: vertebrates that have both mechanoreceptive neuromasts and primitive electroreceptors; neopterygian fish that only have mechanoreceptors; and teleosts that have re-evolved new electroreceptors. Development in Scaphirhynchus was consistent with previously studied taxa in that the lateral line system developed from a series of six dorsolateral placodes. Interestingly, we found that the octaval placode was bound rostrally and caudally by large placodal fields, out of which the six lateral line placodes arose. This finding supports recent suggestions for a common placodal primordium for all placodes. Each of the six placodes gave rise to the lateral line nerves before elongating into sensory ridges, which contained neuromast primordia. The ampullary organ fields of Scaphirhynchus arose from the lateral zones of the anterodorsal, anteroventral, otic and supratemporal sensory ridges, which is also consistent with recently studied taxa. Comparisons of the lateral line system of Scaphirhynchus and close relatives, Acipenser and Polyodon, indicate that variation in some aspects of lateral line receptor numbers and distribution are related to changes in head morphology and feeding strategy, whereas other changes, such as a reduction in receptor number without a change in placode field size, indicate changes in placode development.     

Postembryonic changes in the peripheral electrosensory system of a weakly electric fish: addition of receptor organs with age

The organization of the peripheral electrosensory system of the cheek was studied in an age-graded series of Sternopygus dariensis in Nissl-stained sections and silver-stained whole mounts of skin. As in other gymnotoids, both ampullary and tuberous electroreceptors are present. Small fish have only one ampullary organ or tuberous organ per axon, and the number of receptor organs per axon increases with age in both ampullary and tuberous systems. Large fish may have up to ten tuberous organs per axon, although the distribution of tuberous organs per axon is bimodal with one peak occurring at a single receptor organ per axon and the other peak shifting upward in relation to the age of the fish. The ampullary system adds receptor organs at a faster rate and a large fish may have 20 ampullary organs per axon. With increasing size, the number of sensory receptor cells in each organ remains constant for both types of electroreceptors. Evidence is presented for addition of new electroreceptor units by de novo production in small fish and increases in the number of organs in existing electroreceptor units by division of previously formed organs in medium-sized and large fish. As the surface area of the skin increases with growth, the density of electroreceptor units decreases and, although new receptor organs are still being added to existing receptor units, no generation of new receptor units occurs in medium-sized to large fish.     

Distribution of ω-Conotoxin GVIA binding sites in teleost cerebellar and electrosensory neurons

The distribution of ω-Conotoxin GVIA (CgTx) binding sites was used to localize putative N-type Ca²⁺ channels in an electrosensory cerebellar lobule, the eminentia granularis pars posterior, and in the electrosensory lateral line lobe of a gymnotiform teleost (Apteronotus leptorhynchus). The binding sites for CgTx revealed by an anti-CgTx antibody had a consistent distribution on somatic and dendritic membranes of specific cell types in both structures. The distribution of CgTx binding was unaffected by co-incubation with nifedipine or Aga Toxin IVA, blocking agents for L- and P-type Ca²⁺ channels, respectively. Incubation with CgTx in the presence of varying levels of extracellular Ca²⁺ altered the number but not the cell types exhibiting immunolabel. A punctate immunolabel was detected on somatic membranes of granule and stellate cell interneurons in both the eminentia granularis pars posterior and the electrosensory lateral line lobe. Punctate CgTx binding sites were also present on spherical cell somata and on the large presynaptic terminals of primary afferents that terminate on spherical cells in the electrosensory lateral line lobe. No label was detected in association with distal dendritic membranes of any cell class, or with parallel fibers in the respective molecular layers. Binding sites for CgTx in the eminentia granularis are consistent with the established role for N-type Ca²⁺ channels in cell migrations, an activity which is known to persist in this layer in adult Apteronotus. The distribution of labeled stellate cells with respect to topographic maps in the electrosensory lateral line lobe further suggest that N-type Ca²⁺ channels are expressed in relation to functional activity across these sensory maps. © 1996 Wiley-Liss, Inc.     

Acutely isolated and cultured cells from the electrosensory lateral line lobe of a gymnotiform teleost

The present study established the morphological and immunocytochemical criteria necessary to identify neuronal and nonneuronal cells after dissociating select regions of the medullary electrosensory lateral line lobe of adult weakly electric fish (Apteronotus Zeptorhynchus). Cells dissociated from the pyramidal cell body layers of the centromedial and lateral segments exhibited similar characteristics in the acutely dissociated preparation and up to 14 days in culture. Basilar and nonbasilar pyramidal cells were tentatively identified according to a bipolar or monopolar process extension, and polymorphic cells by the extension of three or more processes and positive immunoreactivity for gamma-aminobutyric acid. Nonneuronal cells were identified by the pattern of process arborization and positive immunolabel for gamma-aminobutyric acid or glial fibrillary acidic protein. Neuronal cells increased in total number over the first 4 days and could appear for the first time on any day in culture. Individual pyramidal cells could maintain their morphology from the time of dissociation and over several days in culture. Pyramidal cell processes were phenotypically similar to apical and basal dendrites found in situ but were reduced in size and in the degree of process branching. These results indicate that dissociated adult apteronotid neurons can maintain a morphology sufficiently similar to that found in situ as to allow tentative identification, opening up a wide range of possibilities for studying the electrophysiological and regenerative properties of electrosensory neurons. © 1995 Wiley-Liss, Inc.     

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.     

The development of lateral-line receptors in Eigenmannia (Teleostei, Gymnotiformes). II. The electroreceptive lateral-line system

Weakly electric fish of the genus Eigenmannia were induced to spawn in conditions simulating the tropical rainy season. The skin of embryos of different ages was prepared for histological examination, and whole animals were examined by various histological methods and scanning electron microscopy. It was found that the electrosensory system develops after the first mechanoreceptive lines have formed. The tuberous and ampullary organs initially form adjacent to the lines of the lateral-line system. The tuberous organs develop at a rate 5 times higher than that of the ampullary organs. The rate of development for both classes of electroreceptors is 4 times higher on the head than on the trunk. The first tuberous organs develop on the head at day 7 and on the trunk at day 8. They increase in number and size during the growth of the fish. The ampullary organs begin to form on the head and on the most rostral part of the trunk at day 8. They are deeply sunk into the corium and have the same number of receptor cells as in adults. There are both ampullary and tuberous organs within fields of receptors that are innervated by a single nerve branch.