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At least 19 recordsLinked to original sources
PubMedDate not supplied

Reproductive strategies and developmental aspects in mormyrid and gymnotiform fishes.

Comparative data on the reproduction in captivity and ontogenetic development of six of mormyrid species and seven gymnotiform species are reported. Mormyrid fishes: egg diameter ranged from 1.8 (Petroephalus soudanensis) to 3.0 mm (Hippopotamyrus pictus. Campylomormyrus phantasticus); fecundity (egg number per spawning) from 28 to 215 (Pollimyrus isidori) and 121 to 1662 (Campylomormyrus cassaicus); spawning intervals from 5 to 20 days (Pollimyrus isidori) and 15 to 80 days (Campylomormyrus cassaicus). Pollimyrus isidori is the only species exhibiting parental care (in the male sex). Gymnotiform species: egg diameter ranged from 1.7 (Brachyhypopomus pinnicaudatus) to 3.0 mm (Sternopygus macrurus, Rhamphichthys sp., Gymnotus carapo); fecundity (egg number per spawning) from 1 to 105 (Apteronotus leptorhynchus) and from approximately 500 to approximately 1000 (Rhamphichthys sp.); spawning intervals from 2 to 5 days (Eigenmannia lineata) and from 20 to 41 days (Rhamphichthys sp.). Sternopyguls macrurus exhibited parental care (in the male sex) by guarding the eggs, whereas Gymnotus carapo revealed to be a mouthbreeder (guarding free embryos) in the male sex. Gonad maturation could be provoked by increase of conductivity alone in all mormyrid species tested and in several gymnotiform species. Four different stages of morphological development (hatchlings, larvae at beginning of exogenous feeding, juveniles, adults) are described in both taxa. The reproductive strategies of the gymnotiforms are considered more diverse than those of the mormyrids.

Animals↗
PubMed1995

Phylogenetic analysis of the South American electric fishes (order Gymnotiformes) and the evolution of their electrogenic system: a synthesis based on morphology, electrophysiology, and mitochondrial sequence data.

The order Gymnotiformes (South American electric fishes) is a fascinating assemblage of freshwater fishes that share the unusual ability to produce and sense electric fields used for electrolocation and social communication. In the last few decades, the electrogenic and electrosensory systems (EES) of these fish have served as an excellent model to study motor and sensory physiology in vertebrates. In an attempt to the evolution of characters associated with the EES in the group, we applied maximum-parsimony (MP), minimum-evolution (ME), and maximum-likelihood (ML) methods to analyze 302 aligned bases of the mitochondrial 12S rRNA and 416 bases of the mitochondrial 16S rRNA of 19 gymnotiform genera representing all six recognized families. Six catfish genera (order Siluriformes) were also sequenced and used as outgroups. The phylogenetic hypothesis resultant from molecular data analysis differs in some respects from previous hypotheses based on morphological studies. Our results were most informative within the family level, as we were unable to elucidate the relationships among deeper branches in this order with sufficient confidence by using molecular data alone. The phylogenetic information of both mitochondrial DNA segments appears to be affected by functional constraints, and the resultant topologies were sensitive to different weighting schemes and the algorithm used. Nonetheless, we found unanimous support for the following phylogenetic relationships: (1) the family Sternopygidae is an unnatural group, and Sternopygus is the sole representative of a unique lineage within the order; (2) the family Hypopomidae is not monophyletic; and (3) the order Gymnotiformes is composed of at least six natural clades: Sternopygus, family Apteronotidae, a new clade consisting of the remaining sternopygids, families Hypopomidae + Rhamphicthyidae, family Electrophoridae, and family Gymnotidae. By combining molecular, morphological, and physiological information, we propose a new hypothesis for the phylogeny of this group and suggest a new family Eigenmanniidae n. (order Gymnotiformes).

Animals↗
PubMed1993

Identification and localization of somatostatin-like immunoreactivity in the cerebellum of gymnotiform fish, Apteronotus leptorhynchus.

Previous studies have shown major mismatches between the localization of the neuropeptide somatostatin of its mRNA and the distribution of somatostatin receptors in the cerebellum of gymnotiform fish. By employing highly sensitive immunohistochemical techniques, we re-examined these mismatches in the gymnotiform fish Apteronotus leptorhynchus. We found somatostatin-like immunoreactivity in the corpus cerebelli, valvula cerebelli, eminentia granularis anterior, eminentia granularis posterior, and transitional zone. While labelled somata were predominantly restricted to a region near the boundary between the granule cell layers and molecular layers, immunopositive processes were distributed throughout large areas within the respective molecular layer. These immunopositive cells appear to represent a novel cell type in the cerebellum of gymnotiform fish.

Animals↗
PubMed1999

Systematic biology of gymnotiform and mormyriform electric fishes: phylogenetic relationships, molecular clocks and rates of evolution in the mitochondrial rRNA genes

The phylogenetic relationships of both African and South American electric fish orders are reviewed at their intra-ordinal level taking into consideration recent studies in which cladistic principles have been employed. Several concordant topologies emerge from the different data sets, but some unsettled issues still remain. From the studies available, a consensus topology has been suggested for the Mormyriformes and for the Gymnotiformes. Subsequently, the evolutionary relationships of these two electric fish clades are considered within each respective superorder, i.e. in relation to the other osteoglossomorph and ostariophysan orders. The inter-ordinal phylogenies are used as a framework to test the molecular clock hypothesis with two gene fragments of the mitochondrial genome. Gymnotiformes, Siluriformes and Characiformes are accumulating mutations at the same pace in relation to their respective outgroups, but for all the other combinations of sister clades tested the molecular clock can be statistically rejected. Fossil records are then surveyed and used to calibrate absolute rates of genetic differentiation for each main lineage (orders) of both osteoglossomorphs and ostariophysans. The most conserved regions (stems) of the 12S and 16S gene fragments used are evolving at an average rate of 0.123 % 10(6 )years-1 for the osteoglossomorphs and 0.137 % 10(6 )years-1 for the ostariophysans, with no significant difference between these two values. The rate of mutation in the loops, the faster-evolving segments, estimated for closely related electric fish taxa is 0.82 % 10(6 )years-1 for four Brienomyrus species and 1.01 % 10(6 )years-1 for the four eigenmanniid genera. When the entire molecule (loops + stems) is considered, the rate of mutation in both mormyriforms and gymnotiforms converges to a rounded value of 0.23 % 10(6 )years-1.

Journal Article↗
PubMed2001

NADPH-diaphorase activity and nitric oxide synthase-like immunoreactivity colocalize in the electromotor system of four species of gymnotiform fish.

The electric organ discharge (EOD) of gymnotiform electric fish is controlled by a well-characterized neural circuit in the brainstem and spinal cord. NADPH-diaphorase (NADPH-d) activity was previously found in phase-locking and/or rapidly firing neurons in the electromotor and electrosensory systems of Apteronotus leptorhynchus [Turner and Moroz, 1995]. These findings suggested that nitric oxide synthase (NOS) is expressed in these neurons and may regulate their precise, high frequency firing. We extended these results by examining the distribution of both NADPH-d activity and NOS-like immunoreactivity (NOS-lir) in the electromotor systems of four gymnotiform species that differ in the frequency and modulation of their EODs. NOS-lir colocalized with NADPH-d staining throughout the electromotor system, indicating that NADPH-d is a faithful indicator of NOS in this system. The distribution of NOS-lir and NADPH-d was similar in the electromotor systems of all four species in this study, with one exception: NOS and NADPH-d staining was consistently less intense in pacemaker and relay cells in Sternopygus macrurus, which produces low frequency EODs, than in the three other species that produce higher frequency EODs. This species difference in NOS expression in the pacemaker nucleus may be related to species differences either in EOD frequency or in modulations of the EOD (e.g., the jamming avoidance response). In Apteronotus species, NOS-lir and NADPH-d were concentrated in bands along the axons of their nerve-derived electric organs. These bands corresponded to regions surrounded by little or no staining with a Schwann cell-specific antibody, suggesting that the NOS-positive regions lie near nodes of Ranvier. In Sternopygus and Eigenmannia, the innervated, posterior membranes of muscle-derived electrocytes were more intensely labeled for NADPH-d and NOS than inexcitable portions of the membrane. Thus, in both muscle- and nerve-derived electric organs, NOS is concentrated near excitable membranes. These results indicate that NOS is well-positioned within the electromotor system to regulate the frequency, precision, amplitude, and waveform of EODs.

Animals↗
PubMed1982

Peripheral organization and central projections of the electrosensory nerves in gymnotiform fish.

The electrosensory system of weakly electric gymnotiform fish is described from the receptor distribution on the body surface to the termination of the primary afferents in the posterior lateral line lobe (PLLL). There are two types of electroreceptor(ampullary and tuberous) and a single type of lateral line mechanoreceptor (neuromast). Receptor counts in Apteronotus albifrons show that (1) neuromasts are distributed as in other teleosts; (2) ampullary receptors number 151 on one side of the head and 208 on one side of the body; (3) tuberous receptors were estimated to number 3,000-3,500 on one side of the head and 3,500-5,000 on one side of the body. The distribution of each receptor type is described. Each receptor is innervated by a single primary afferent. Electrosensory afferents have myelinated cell bodies in the ganglion of the anterior lateral line nerve (ALLN). The distribution of these ganglion cell diameters is strongly bimodal in Apteronotus and Eigenmannia: The smaller-diameter cells may be those which innervate ampullary electroreceptors, the larger-diameter tuberous electroreceptors. Transganglionic HRP transport techniques were used to determine the first-order connections of the anterior lateral line nerve in six species of gymnotiform fish. Small branches of the ALLN were labeled so as to determine the somatotopic organization in the PLLL. The PLLL is divided into four segments from medial to lateral, termed medial, centromedial, centrolateral, and lateral segments (Heiligenberg and Dye, '81). Representations of the head are found rostrally in each zone, and the trunk is mapped caudally in each zone. Thus there are four body maps in the PLLL. The medial segment receives ampullary input (Heiligenberg and Dye, '82) and maps the dorsoventral body axis mediolaterally, as does the tuberous centrolateral segment. The tuberous centromedial and lateral segments map the dorsoventral axis lateromedially. Thus the medial and centromedial segments meet belly to belly, the centromedial and centrolateral segments meet back to back, and the centrolateral and lateral segments meet belly to belly. Adjacent electrosensory maps within the PLLL are therefore always mirror images.

Afferent Pathways↗
PubMed1990

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.

Animals↗
PubMed1992

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.

Animals↗
PubMed1994

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 gamma-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 electrosensory 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.

Animals↗
PubMed1996

Sensory cues for the gradual frequency fall responses of the gymnotiform electric fish, Rhamphichthys rostratus.

The sensory cues for a less known form of frequency shifting behavior, gradual frequency falls, of electric organ discharges (EODs) in a pulse-type gymnotiform electric fish, Rhamphichthys rostratus, were identified. We found that the gradual frequency fall occurs independently of more commonly observed momentary phase shifting behavior, and is due to perturbation of sensory feedback of the fish's own EODs by EODs of neighboring fish. The following components were identified as essential features in the signal mixture of the fish's own and the neighbor's EOD pulses: (1) the neighbor's pulses must be placed within a few millisecond of the fish's own pulses, (2) the neighbor's pulses, presented singly at low frequencies (0.2-4 Hz), were sufficient, (3) the frequency of individual pulse presentation must be below 4 Hz, (4) amplitude modulation of the sensory feedback of the fish's own pulses induced by such insertions of the neighbor's pulses must contain a high frequency component: sinusoidal amplitude modulation of the fish's own EOD feedback at these low frequencies does not induce gradual frequency falls. Differential stimulation across body surfaces, which is required for the jamming avoidance response (JAR) of wave-type gymnotiform electric fish, was not necessary for this behavior. We propose a cascade of high-pass and low-pass frequency filters within the amplitude processing pathway in the central nervous system as the mechanism of the gradual frequency fall response.

Animals↗
PubMed1998

Detection of multiple stimulus features forces a trade-off in the pyramidal cell network of a gymnotiform electric fish's electrosensory lateral line lobe.

Modification of an existing neural structure to support a second function will produce a trade-off between the two functions if they are in some way incompatible. The trade-off between two such sensory functions is modeled here in pyramidal neurons of the gymnotiform electric fish's medullar electrosensory lateral line lobe (ELL). These neurons detect two electric stimulus features produced when a nearby object interferes with the fish's autogenous electric field: (1) amplitude modulation across a cell's entire receptive field and (2) amplitude variation within a cell's receptive field produced by an object's edge. A model of sensory integration shows that detection of amplitude modulation and enhancement of spatial contrast involve an inherent mechanistic trade-off and that the severity of the trade-off depends on the particular algorithm of sensory integration. Electrophysiology data indicate that of the two algorithms for sensory integration modeled here for the gymnotiform fish Brachyhypopomus pinnicaudatus, the algorithm with the better trade-off function is used. Further, the intrinsic trade-off within single cells has been surmounted by the replication of ELL into multiple electrosensory map segments, each specialized to emphasize different sensory features.

Animals↗
PubMedDate not supplied

Substance P-like immunoreactivity in the brain of the gymnotiform fish Apteronotus leptorhynchus: presence of sex differences.

The distribution of substance P-like immunoreactivity (SPli) was charted in the brain of the gymnotiform fish Apteronotus leptorhynchus, and correlated with the circuitry underlying intraspecific electrocommunication. Cell bodies were found predominantly in the lateral hypothalamus and in certain paraventricular organs: nucleus preopticus periventricularis, anterior subdivision; anterior hypothalamus; nucleus posterioris periventricularis; nucleus recessus lateralis, medial subdivision 2; nucleus recessus posterioris and nucleus recessus lateralis, lateral subdivision. Cell bodies were also found in the rostral olfactory nucleus, ventral telencephalon (ventral and central subdivisions), the habenula, the vagal sensory and motor nuclei and in the subtrigeminal nucleus. The distribution of SPli fibers was similar in some respects to that reported for other vertebrates. SPli was found in the rhombencephalon associated with vagal afferent fibers and in the funicular nucleus (possibly related to nociception). In the diencephalon and midbrain SPli fibers were found in the habenular-interpeduncular tract, in the hypothalamus and pituitary. SPli fibers were also found in preoptic and forebrain areas. The most striking result was the sexually dimorphic SPli innervation of certain hypothalamic and septal nuclei, and of the prepacemaker nucleus (PPn), a diencephalic cell group which controls communication ('chirping') in gymnotiforms. The PPn and septal/hypothalamic nuclei were densely innervated by SPli in males but devoid of SPli in females.

Animals↗
PubMed1994

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.

Animals↗
PubMed2000

Sex chromosome evolution in fish. II. Second occurrence of an X1X2Y sex chromosome system in Gymnotiformes.

A multiple sex chromosome system of the X1X1X2X2:X1X2Y type is reported to occur in the fish species Brachyhypopomus pinnicaudatus (Gymnotiformes, Hypopomidae), being the second occurrence of this sex chromosome system in Gymnotiformes and the fifth among Neotropical freshwater fish. The possible origin of this system was hypothesized to be a centric fusion, which occurred in an ancestral form, of two medium-sized acrocentrics, giving origin to the metacentric neo-Y. Heterochromatic DAPI-positive regions were visualized in the pericentromeric region of all the chromosomes, including the Y-chromosome. In-situ hybridization with (TTAGGG)n (all-human-telomeres probe) did not detect any telomeric interstitial regions (ITS), indicating a possible loss of terminal segments of the chromosomes involved in the neo-Y formation.

Animals↗
PubMed1996

The dorsal filament of the weakly electric Apteronotidae (Gymnotiformes; Teleostei) is specialized for electroreception.

The Apteronotidae, a family of weakly electric fish from South America (Gymnotiformes), possess a structure called the dorsal filament with an unknown function and evolutionary origin. This study compared the gross anatomy of the dorsal filament of 13 species of apteronotids and used light microscopy to examine the filaments of Adontosternarchus balaenops, Apteronotus albifrons, and Apteronotus leptorhynchus. The dorsal filament is an unscaled, thin, tapering structure attached to a mid-dorsal groove on the posterior half of the fish's back. The interior of the filament is a gelatinous mucopolysaccharide matrix (connective tissue) containing blood vessels and a bilateral nerve in which nearly all the afferents are large (8-10 mu m) and heavily myelinated. The location of the anterior origin of the filament varies from 0.48 to 0.66 of the body length, posterior to the snout, in 13 species. The filament is covered with hundreds of large-type tuberous electroreceptors and some ampullary receptors, at approximately the same density and ratio as those on the nearby back. The morphology of the large-type tuberous receptors and their afferents suggests that they are phase-coding T-units. A double layer of epithelial cells separates the ventral side of the filament from the groove in the trunk of the fish, except at the anterior origin where the interior of the filament is continuous with the body. This specialized double epithelium could provide a high resistance barrier to electrical current. This study was unable to distinguish between two hypotheses: that the dorsal filament is a modified adipose fin (as suggested previously), retained only in this family of Gymnotiformes; or that it is a uniquely derived character of the Apteronotidae.

Adipose Tissue↗
PubMed1999

Neurogenesis, cell death and regeneration in the adult gymnotiform brain.

Gymnotiform fish, like all teleosts examined thus far, are distinguished by their enormous potential for the production of new neurons in the adult brain. In Apteronotus leptorhynchus, on average 10(5) cells, corresponding to approximately 0.2 % of the total population of cells in the adult brain, are in S-phase within any period of 2 h. At least a portion of these newly generated cells survive for the rest of the fish's life. This long-term survival, together with the persistent generation of new cells, leads to a continuous growth of the brain during adulthood. Zones of high proliferative activity are typically located at or near the surface of the ventricular, paraventricular and cisternal systems. In the central posterior/ prepacemaker nucleus, for example, new cells are generated, at very high rates, in areas near the wall of the third ventricle. At least some of these cells differentiate into neurons, express immunoreactivity against the neuropeptide somatostatin and migrate into more lateral areas of this complex. Approximately 75 % of all new brain cells are generated in the cerebellum. In the corpus cerebelli and the valvula cerebelli, they are produced in the molecular layers, whereas in the eminentia granularis the newborn cells stem from proliferation zones in the pars medialis. Within the first few days of their life, these cells migrate towards specific target areas, namely the associated granule cell layers. At least some of them develop into granule neurons. The high proliferative activity is counterbalanced by apoptosis, a mechanism that resembles the processes known from embryonic development of the vertebrate brain. Apoptosis also appears to be used as an efficient mechanism for the removal of cells damaged through injury in the brain of adult Apteronotus leptorhynchus. Since apoptosis is not accompanied by the side effects known from necrosis, this 'clean' type of cell death may, together with the enormous proliferative activity in the brain, explain, at least partially, the tremendous capability of teleost fish to replace damaged neurons with newly generated ones. One factor that appears to play a major role in the generation of new cells and in their further development is the neuropeptide somatostatin. In the caudal cerebellum of the gymnotiform brain, somatostatin-binding sites are expressed, at extremely high densities, at sites corresponding to the areas of origin, migration and differentiation of the newborn cells. This pattern of expression resembles the expression pattern in the rat cerebellum, where somatostatin immunoreactivity and somatostatin-binding sites are transiently expressed at the time when the granule cells of the cerebellum are generated. Moreover, after mechanical lesions of the corpus cerebelli, the expression of somatostatin-like immunoreactivity is tremendously increased in several cell types (presumably astrocytes, microglia and granule cell neurons) near the path of the lesion; the time course of this expression coincides with the temporal pattern underlying the recruitment of new cells incorporated at the site of the lesion.

Animals↗
PubMedDate not supplied

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.

Animals↗
PubMedDate not supplied

Environmental and hormonal influences upon EOD waveform in gymnotiform pulse fish.

Temperature is a major variable that affects all biological systems. Environmental temperature determines animal geographical distribution and activity, and influences their reproductive cycle, particularly within the temperate zone. Temperature, as a physical parameter, also strongly affects excitable tissues. The hypothesis of temperature as the most important environmental cue for the onset of breeding in gymnotiform pulse fish of the temperate zone is supported by: (a) a clear temporal correlation that was observed in the wild between water temperature and sexual maturity, and (b) the induction of gonadal maturation and sexual differences after acclimation at high temperature (28 degrees C) in the laboratory. Temperature sensitivity of EOD waveform (described in Brachyhypopomus pinnicaudatus and Gymnotus carapo) is characterized by the decrease of the EOD's late head-negative phase as temperature increases. This phenomenon depends on electrocyte properties since: (a) experimentally induced changes of discharge rate at constant temperature generate smaller EOD distortion, and (b) the effect of temperature upon EOD also depends on water conductivity. Temperature sensitivity of EOD waveform is negatively correlated with gonadal maturity in Brachyhypopomus pinnicaudatus. High temperature sensitivity was observed during the non-breeding season, whereas low temperature sensitivity was recorded during the breeding season. Temperature sensitivity of EOD waveform in both Brachyhypopomus pinnicaudatus and Gymnotus carapo was modulated by: (a) testosterone treatment (100 microg/g) that decreased temperature sensitivity, and (b) acclimation at high temperature (28 degrees C, 1 month) that also decreased temperature sensitivity. Temperature is probably acting through the neuroendocrine system, and ultimately interacting with steroid hormones in their effects upon EOD waveform.

Animals↗