Regeneration of electroreceptors in weakly electric fish

Weakly electric teleost fish possess two classes of electroreceptors: tuberous and ampullary organs. Ampullary organs are used for detecting prey, while tuberous organs detect the fish's own electric organ discharges (EODs) and those of conspecifics. EOD frequency varies among individuals within a species and a fish's tuberous receptors are sharply tuned to its own EOD frequency. In young, small fish both tuberous and ampullary afferents innervate only single organs. As fish grow new receptor cells are added to each organ and it divides into two daughter organs. This process continues resulting in numerous organs in a cluster; the afferent nerve innervates all the organs in a cluster. When a patch of skin is removed new skin grows back complete with new receptor organs of both classes. From our studies we have shown that: (1) new organs are found only in the presence of nerve fibres; (2) their morphological development during regeneration is similar to their normal development; (3) organs divide rapidly giving rise to daughter organs until each afferent fibre innervates the correct number of organs for a fish of its size; (4) receptor cells are broadly tuned below the EOD frequency of a given fish and they gradually increase their tuned frequency and sharpness of tuning until they become correctly tuned to that EOD frequency; (5) the correct matching of receptor tuning to EOD frequency occurs in fish in which the spinal cord has been severed or with lesions of the medullary pacemaker nucleus, thereby eliminating the EOD and any possible 'calibration' signal; and (6) basal and capsule cells of receptor organs in the intact skin around the wound divide after skin damage and are a possible source of precursor cells for new receptor organs.

Cyclic AMP modulates electrical signaling in a weakly electric fish

Many species of electric fish show diurnal or socially elicited variation in electric organ discharge amplitude. In Sternopygus macrurus, activation of protein kinase A by 8-bromo-cAMP increases electrocyte sodium current magnitude. To determine whether the behavioral plasticity in electric organ discharge amplitude is controlled by electrocyte biophysical properties, we examined whether the effects of phosphorylation on ion currents in the electric organ translate directly into electric organ discharge changes. We injected the electric organ of restrained fish with 8-bromo-cAMP and monitored the electric organ discharge. The effect of protein kinase A activation on electrocyte action potentials was examined in isolated electric organ using two-electrode current clamp. Electric organ discharge and action potential amplitude and pulse duration increased in response to 8-bromo-cAMP. Pulse and action potential duration both increased by about 25%. However, the increase in electric organ discharge amplitude (approximately 400%) was several-fold greater than the action potential amplitude increase (approximately 40%). Resting membrane resistance decreased in electrocytes exposed to 8-bromo-cAMP. We propose that in the Thevenin equivalent circuit of the electric organ a moderate increase in action potential amplitude combined with a decrease in internal resistance produces a greater voltage drop across the external resistance (the water around the fish), accounting for the large increase in the externally recorded electric organ discharge.

Electrocommunication signals in female brown ghost electric knifefish, Apteronotus leptorhynchus

Female communication behaviors are often overlooked by researchers in favor of male behaviors, which are usually more overt and easier to elicit. Very little is known about female electrocommunication behaviors in brown ghost knifefish, a weakly electric wavetype Gymnotiform fish. Most behavioral studies have focused on males, and fish are usually restrained and played a stimulus near their own electric organ discharge frequency to evoke chirps (abrupt short-term frequency rises) or the jamming avoidance response. Our study focuses on categorizing and describing spontaneous and evoked electric organ discharge modulations in free-swimming female fish that were either electrically coupled to tanks containing a conspecific (male or female), or left isolated. Cluster analysis of signals produced under isolated and social conditions revealed three categories of rises: short rise, medium rise and long rise; and one category of frequency decrease (dip). Females produce significantly more short rises when electrically coupled to tanks containing lower-frequency females, and produce more long rises when electrically coupled to tanks containing males. Short rises may have an intrasexual aggressive function, while long rises may serve as an advertisement of status or reproductive condition in intersexual interactions.

Androgens alter electric organ discharge pulse duration despite stability in electric organ discharge frequency

Weakly electric fish in the genus Sternopygus emit a sinusoidal, individually distinct, and sexually dimorphic electric organ discharge (EOD) that is used in electrolocation and communication. Systemically applied androgens decrease EOD frequency, which is set by a medullary pacemaker nucleus, and increase pulse duration, which is determined by the cells of the electric organ (the electrocytes), in a coordinated fashion. One possibility is that androgens broaden the EOD pulse duration by acting on the pacemaker neurons, thereby effecting a change in pacemaker firing frequency, and that the change in EOD pulse duration is due to an activity-dependent process. To determine whether androgens can alter pulse duration despite a stable pacemaker nucleus firing frequency, we implanted small doses of dihydrotestosterone in the electric organ. We found that androgen implants increased EOD pulse duration, but did not influence EOD frequency. In addition, using immunocytochemistry, we found that electrocytes label positively with an androgen receptor antibody. While it is not known on which cells androgens act directly, together these experiments suggest that they likely act on the electrocytes to increase EOD pulse duration. Since pulse duration is determined by electrocyte action potential duration and ionic current kinetics, androgens may therefore play a causative role in influencing individual variation and sexual dimorphism in electrocyte electrical excitability, an important component of electrocommunicatory behavior.

Evolution and divergence of sodium channel genes in vertebrates

Invertebrate species possess one or two Na+ channel genes, yet there are 10 in mammals. When did this explosive growth come about during vertebrate evolution? All mammalian Na+ channel genes reside on four chromosomes. It has been suggested that this came about by multiple duplications of an ancestral chromosome with a single Na+ channel gene followed by tandem duplications of Na+ channel genes on some of these chromosomes. Because a large-scale expansion of the vertebrate genome likely occurred before the divergence of teleosts and tetrapods, we tested this hypothesis by cloning Na+ channel genes in a teleost fish. Using an approach designed to clone all of the Na+ channel genes in a genome, we found six Na+ channel genes. Phylogenetic comparisons show that each teleost gene is orthologous to a Na+ channel gene or gene cluster on a different mammalian chromosome, supporting the hypothesis that four Na+ channel genes were present in the ancestors of teleosts and tetrapods. Further duplications occurred independently in the teleost and tetrapod lineages, with a greater number of duplications in tetrapods. This pattern has implications for the evolution of function and specialization of Na+ channel genes in vertebrates. Sodium channel genes also are linked to homeobox (Hox) gene clusters in mammals. Using our phylogeny of Na+ channel genes to independently test between two models of Hox gene evolution, we support the hypothesis that Hox gene clusters evolved as (AB) (CD) rather than [D[A(BC)]].

Sex steroids and communication signals in electric fish: a tale of two species

Weakly electric fish are good model animals to study the evolution of interspecific and sexual differences in communication signals. This is because the neural circuits producing these signals are simple and conserved among related species while the signals are highly species-specific, sexually-dimorphic, and under hormonal control. Here we focus on two related species of weakly electric gymnotiform fish that emit a wave-type discharge. These species differ in the direction of the sexual dimorphism of their electric organ discharge (EOD) frequencies and their propensity to produce aggressive communication signals called 'chirps'. Brown ghost (Apteronotus leptorhynchus) males produce high frequency EODs while females produce low frequency EODs. When presented with an EOD mimic, males chirp frequently, while females seldom chirp. By contrast, black ghost (A. albifrons) males discharge at lower EOD frequencies than females, and there is no sex difference in chirping in this species. Accordingly, non-aromatizable androgens raise EOD frequency in brown ghosts, but lower it in black ghosts. Androgens induce chirping in female brown ghosts, but do not increase the propensity to chirp in female black ghosts. Thus, the difference in sexually-dimorphic communication signals between these two species can be explained by differences in their responses to sex steroids. Future studies will elucidate how the neural circuits generating these signals are differentially sensitive to steroids in these species.

Parvocells: a novel interneuron type in the pacemaker nucleus of a weakly electric fish

Gymnotiform weakly electric fish produce electric organ discharges (EODs) that function in electrolocation and communication. The command signal for the EOD is produced by the medullary pacemaker nucleus, which contains two well-characterized neuron types: pacemaker cells and relay cells. In this study, we characterized a third neuron type in the pacemaker nucleus. These neurons, which we have named parvocells, were smaller (7-15 microm in diameter) than relay and pacemaker cells. The parvocells were labeled with an antibody against the neuronal calcium-binding protein, parvalbumin, and were not labeled with several glial-specific antibodies. Parvocells had one to three fine processes that often terminated at the periphery of relay and pacemaker cell bodies. The parvalbumin-positive terminals of the parvocells colocalized with immunoreactivity for SV-2, suggesting that the parvocells form chemical synapses on the relay and pacemaker cells. Parvalbumin-positive neurons are frequently gamma-aminobutyric acid (GABA)ergic or glycinergic, and the cytoplasm of the parvocell somata was immunoreactive with a glycine antibody. Antibodies against glycine receptors and gephyrin, however, did not label any cells in the pacemaker nucleus, suggesting that the pacemaker nucleus does not contain glycine or GABA((A)) receptors. Electron microscopy revealed gap junctions between the membranes of parvocells and adjacent terminal-like structures. Furthermore, neurobiotin injected into individual pacemaker or relay cells labeled parvocells as well as other pacemaker and relay cells, demonstrating that the parvocells are dye-coupled to the other neuron types in the pacemaker nucleus. These findings indicate that the parvocells are histochemically distinct from relay and pacemaker cells and that they receive electrotonic inputs from and make chemical synapses back onto pacemaker and relay cells. Further study is needed to investigate the function of these neurons in regulating the EOD.

Coregulation of voltage-dependent kinetics of Na(+) and K(+) currents in electric organ

The electric organ cells of Sternopygus generate action potentials whose durations vary over a fourfold range. This variation in action potential duration is the basis for individual variation in a communication signal. Thus, action potential duration must be precisely regulated in these cells. We had observed previously that the inactivation kinetics of the electrocyte Na(+) current show systematic individual variation. In this study, using a two-electrode voltage clamp, we found that the voltage-dependent activation and deactivation kinetics of the delayed rectifying K(+) current in these cells covary in a graded and predictable manner across fish. Furthermore, when Na(+) and K(+) currents were recorded in the same cell, their voltage-dependent kinetics were highly correlated. This finding illustrates an unprecedented degree of coregulation of voltage-dependent properties in two molecularly distinct ionic channels. Such a coregulation of ionic channels is uniquely observable in a cell specialized to generate individual differences in electrical activity and in which the results of biophysical control mechanisms are evident in behaving animals. We propose that the precise coregulation of the voltage-dependent kinetics of multiple ionic currents may be a general mechanism for regulation of membrane excitability.

Pharmacological characterization of ionic currents that regulate the pacemaker rhythm in a weakly electric fish

Electric organ discharge (EOD) frequency in the brown ghost knifefish (Apteronotus leptorhynchus) is sexually dimorphic, steroid-regulated, and determined by the discharge rates of neurons in the medullary pacemaker nucleus (Pn). We pharmacologically characterized ionic currents that regulate the firing frequency of Pn neurons to determine which currents contribute to spontaneous oscillations of these neurons and to identify putative targets of steroid action in regulating sexually dimorphic EOD frequency. Tetrodotoxin (TTX) initially reduced spike frequency, and then reduced spike amplitude and stopped pacemaker activity. The sodium channel blocker muO-conotoxin MrVIA also reduced spike frequency, but did not affect spike amplitude or production. Two potassium channel blockers, 4-aminopyridine (4AP) and kappaA-conotoxin SIVA, increased pacemaker firing rates by approximately 20% and then stopped pacemaker firing. Other potassium channel blockers (tetraethylammonium, cesium, alpha-dendrotoxin, and agitoxin-2) did not affect the pacemaker rhythm. The nonspecific calcium channel blockers nickel and cadmium reduced pacemaker firing rates by approximately 15-20%. Specific blockers of L-, N-, P-, and Q-type calcium currents, however, were ineffective. These results indicate that at least three ionic currents-a TTX- and muO-conotoxin MrVIA-sensitive sodium current; a 4AP- and kappaA-conotoxin SIVA-sensitive potassium current; and a T- or R-type calcium current-contribute to the pacemaker rhythm. The pharmacological profiles of these currents are similar to those of currents that are known to regulate firing rates in other spontaneously oscillating neural circuits.

Development and regeneration of the electric organ

The electric organ has evolved independently from muscle in at least six lineages of fish. How does a differentiated muscle cell change its fate to become an electrocyte? Is the process by which this occurs similar in different lineages? We have begun to answer these questions by studying the formation and maintenance of electrocytes in the genus Sternopygus, a weakly electric teleost. Electrocytes arise from the fusion of fully differentiated muscle fibers, mainly those expressing fast isoforms of myosin. Electrocytes briefly co-express sarcomeric proteins, such as myosin and tropomyosin, and keratin, a protein not found in mature muscle. The sarcomeric proteins are subsequently down-regulated, but keratin expression persists. We investigated whether the maintenance of the electrocyte phenotype depends on innervation. We found that, after spinal cord transection, which silences the electromotor neurons that innervate the electrocytes, or destruction of the spinal cord, which denervates the electrocytes, mature electrocytes re-express sarcomeric myosin and tropomyosin, although keratin expression persists. Ultrastructural examination of denervated electrocytes revealed nascent sarcomeres. Thus, the maintenance of the electrocyte phenotype depends on neural activity.

Reexpression of myogenic proteins in mature electric organ after removal of neural input

The electric organ (EO) of the weakly electric fish Sternopygus macrurus derives from striated myofibers that fuse and suppress many muscle properties. Mature electrocytes are larger than muscle fibers, do not contain sarcomeres, or express myosin heavy chain (MHC) or tropomyosin. Furthermore, electrocytes express keratin, a protein not expressed in muscle. In S. macrurus the EO is driven continuously at frequencies higher than those of the intermittently active skeletal muscle. The extent to which differences in EO and muscle phenotype are accounted for by activity patterns, or innervation per se, was determined by assessing the expression of MHC, tropomyosin, and keratin 2 and 5 weeks after the elimination of (1) activity patterns by spinal transection or (2) all synaptic input by denervation. Immunohistochemical analyses showed no changes in muscle fiber phenotypes after either experimental treatment. In contrast, the keratin-positive electrocytes revealed an upregulation of MHC and tropomyosin. Nearly one-third of all electrocytes expressed MHC (35%) and tropomyosin (25%) 2 weeks after spinal transection, whereas approximately two-thirds (61%) expressed MHC 2 weeks after denervation. After 5 weeks of denervation or spinal transection, all electrocytes contained MHC and tropomyosin. Newly formed sarcomere clusters also were observed in denervated electrocytes. The MHC expressed in electrocytes corresponded to that present in a select population of muscle fibers, i.e., type II fibers. Thus, the elimination of electrical activity or all synaptic input resulted in a partial reversal of the electrocyte phenotype to an earlier developmental stage of its myogenic lineage.

Phenotypic conversion of distinct muscle fiber populations to electrocytes in a weakly electric fish

In most groups of electric fish, the electric organ (EO) derives from striated muscle cells that suppress many muscle phenotypic properties. This phenotypic conversion is recapitulated during regeneration of the tail in the weakly electric fish Sternopygus macrurus. Mature electrocytes, the cells of the electric organ, are considerably larger than the muscle fibers from which they derive, and it is not known whether this is a result of muscle fiber hypertrophy and/or fiber fusion. In this study, electron micrographs revealed fusion of differentiated muscle fibers during the formation of electrocytes. There was no evidence of hypertrophy of muscle fibers during their phenotypic conversion. Furthermore, although fish possess distinct muscle phenotypes, the extent to which each fiber population contributes to the formation of the EO has not been determined. By using myosin ATPase histochemistry and anti-myosin heavy chain (MHC) monoclonal antibodies (mAbs), different fiber types were identified in fascicles of muscle in the adult tail. Mature electrocytes were not stained by the ATPase reaction, nor were they labeled by any of the anti-MHC mAbs. In contrast, mature muscle fibers exhibited four staining patterns. The four fiber types were spatially arranged in distinct compartments with little intermixing; peripherally were two populations of type I fibers with small cross-sectional areas, whereas more centrally were two populations of type II fibers with larger cross-sectional areas. In 2- and 3-week regenerating blastema, three fiber types were clearly discerned. Most (> 95%) early-forming electrocytes had an MHC phenotype similar to that of type II fibers. In contrast, fusion among type I fibers was rare. Together, ultrastructural and immunohistochemical analyses revealed that the fusion of muscle fibers gives rise to electrocytes and that this fusion occurs primarily among the population of type II fibers in regenerating blastema.

Behavioral actions of androgens and androgen receptor expression in the electrocommunication system of an electric fish, Eigenmannia virescens

Gymnotiform electric fish emit an electric organ discharge (EOD) that, in many species, is sexually dimorphic and used in gender recognition. The glass knife fish Eigenmannia has been a major neuroethological model system, and there have been reports that its EOD is sexually dimorphic. However, no study has examined the role of steroids in establishing this dimorphism. We tested the effect of three androgens, testosterone, dihydrotestosterone, and 11-ketotestosterone (11kT), on the EOD of Eigenmannia virescens. Implants of any of these three androgens induced a decrease in EOD frequency, consistent with several studies showing that males have a lower EOD frequency than females. In a separate experiment, we found that 11kT treatment also increases EOD pulse duration, but has no effect on the duty cycle, the relative duration of the positive and negative phases of the wave, or the harmonic content of the EOD. Using immunocytochemistry, we found that the cells of the electric organ (the electrocytes) that produce the EOD and control its pulse duration label positively with an antibody to the androgen receptor. These experiments indicate that androgens decrease the firing frequency of the medullary pacemaker and alter the ion current kinetics in the electrocytes. Moreover, the presence of nuclear androgen receptors in electrocytes suggests that androgens act directly on the electric organ to modify EOD pulse duration.

Diversity of sexual dimorphism in electrocommunication signals and its androgen regulation in a genus of electric fish, Apteronotus

Gymnotiform electric fish emit an electric organ discharge that, in several species, is sexually dimorphic and functions in gender recognition. In addition, some species produce frequency modulations of the electric organ discharge, known as chirps, that are displayed during aggression and courtship. We report that two congeneric species (Apteronotus leptorhynchus and A. albifrons) differ in the expression of sexual dimorphism in these signals. In A. leptorhynchus, males chirp more than females, but in A. albifrons chirping is monomorphic. The gonadosomatic index and plasma levels of 11-ketotestosterone were equivalent in both species, suggesting that they were in similar reproductive condition. Corresponding to this difference in dimorphism, A. leptorhynchus increases chirping in response to androgens, but chirping in A. albifrons is insensitive to implants of testosterone, dihydrotestosterone or 11-ketotestosterone. Species also differ in the sexual dimorphism and androgen sensitivity of electric organ discharge frequency. In A. leptorhynchus, male discharge at higher frequencies than females, and androgens increase electric organ discharge frequency. In A. albifrons, males discharge at lower frequencies than females, and androgens decrease electronic organ discharge frequency. Thus, in both chirping and electric organ discharge frequency, evolutionary changes in the presence or direction of sexual dimorphism have been accompanied and perhaps caused by changes in the androgen regulation of the electric organ discharge.

The effects of steroid hormones on electrical activity of excitable cells

Steroid hormones influence the electrical activity of many neurons and effectors by regulating the transcription of their ion channels and neurotransmitter receptors, or by modulating the activity of their channels and receptors through second messenger-coupled membrane receptors, or both. In this article, four cell types with known functions and distinct electrical activities are focused on to illustrate how different steroids act synergistically with, or in opposition to, each other to modulate specific electrical phenomena such as spontaneous regular firing (GH3 cells, a pituitary cell line), action potential duration (electric organ cells), and intrinsic excitability and sensitivity to neurotransmitters (GnRH and opioidergic neurons).These examples illustrate how steroids might influence electrical activity in neurons involved in more complex central circuits.

Transdifferentiation of muscle to electric organ: regulation of muscle-specific proteins is independent of patterned nerve activity

Transdifferentiation is the conversion of one differentiated cell type into another. The electric organ of fishes transdifferentiates from muscle but little is known about how this occurs. To begin to address this question, we studied the expression of muscle- and electrocyte-specific proteins with immunohistochemistry during regeneration of the electric organ. In the early stages of regeneration, a blastema forms. Blastemal cells cluster, express desmin, fuse into myotubes, and then express alpha-actinin, tropomyosin, and myosin. Myotubes in the periphery of the blastema continue to differentiate as muscle; those in the center grow in size, probably by fusing with each other, and lose their sarcomeres as they become electrocytes. Tropomyosin is rapidly down-regulated while desmin, alpha-actinin, and myosin continue to be diffusely expressed in newly formed electrocytes despite the absence of organized sarcomeres. During this time an isoform of keratin that is a marker for mature electrocytes is expressed. One week later, the immunoreactivities of myosin disappears and alpha-actinin weakens, while that of desmin and keratin remain strong. Since nerve fibers grow into the blastema preceding the appearance of any differentiated cells, we tested whether the highly rhythmic nerve activity associated with electromotor input plays a role in transdifferentiation and found that electrocytes develop normally in the absence of electromotor neuron activity.

Estrogen modifies an electrocommunication signal by altering the electrocyte sodium current in an electric fish, Sternopygus

Many species of electric fish emit sexually dimorphic electrical signals that are used in gender recognition. In Sternopygus, mature females produce an electric organ discharge (EOD) that is higher in frequency and shorter in pulse duration than that of mature males. EOD pulse duration is determined by ion currents in the electrocytes, and androgens influence EOD pulse duration by altering the inactivation kinetics of the electrocyte sodium current. We examined whether estrogen modulates the female-specific EOD and, if so, whether it regulates EOD pulse duration by acting on the same androgen-sensitive ion current in the electrocytes. We implanted gonadectomized Sternopygus with either empty SILASTIC capsules (control), one capsule filled with estradiol-17beta (E2; low dose), or three capsules of E2 (high dose). Twelve days after implantation, E2-treated fish had plasma E2 levels approximately 3.3-fold (low dose) or approximately 7.1-fold (high dose) higher than controls. After implantation, both E2-treated groups had higher EOD frequency and shorter EOD pulse duration than controls and their own preimplantation values. Through immunocytochemistry, we identified immunoreactive estrogen receptors in the nuclei of electrocytes, indicating that these cells are directly responsive to estrogen. In addition, voltage-clamp studies showed that E2 affected the electrocyte ion currents kinetics: the sodium inactivation time constant was significantly lower in E2-treated fish than in controls. Thus, sexual dimorphism in the electrocommunication signal results, at least in part, from estrogens and androgens acting in opposite directions on the same ion current in the electrocytes.

Protein kinase A activation increases sodium current magnitude in the electric organ of Sternopygus

The inactivation kinetics of the Na+ current of the weakly electric fish Sternopygus are modified by treatment with androgens. To determine whether phosphorylation could play a role in this effect, we examined whether activation of protein kinase A by 8 bromo cyclic AMP (8 Br cAMP) altered voltage-dependent properties of the current. Using a two-electrode voltage-clamp procedure, we found no effect of 8 Br cAMP on inactivation kinetics or other voltage-dependent properties of the Na+ current of the electrocytes. However, treatment with 8 Br cAMP did produce a dose-dependent increase in the Na+ current compared with saline controls: 17.6% at 100 microM, 42.4% at 1 mM, and 43.1% at 5 mM. This effect was blocked by 30 microM H89, a PKA inhibitor, indicating that the observed effect was attributable to 8 Br cAMP activation of PKA. We conclude that androgen-induced changes in Na+ current inactivation are not mediated by PKA and suggest that PKA-mediated increases in Na+ current underlie increases in the amplitude of the electric organ discharge observed in social interactions or with changes in water conductance.

Differential expression of proteins in muscle and electric organ, a muscle derivative

The electric organ of electric fish develops from a myogenic lineage. We have used immunohistochemistry and immunoblotting to determine which features of the muscle phenotype are retained and whether any new ones are expressed in mature electrocytes of the electric fish Sternopygus. The muscle-specific intermediate filament desmin was found throughout the electrocytes, and different desmin antibodies detected molecules with different subcellular distributions. Western blots confirm that these antibodies recognize a protein of MW = 53 kD, the molecular weight of desmin. Other muscle proteins were also present within electrocytes: Actin and sarcomeric alpha-actinin were found within the subsynaptic membrane beneath the plasmalemma of the electrocytes, and talin and acetylcholine receptors were detected both at the innervated posterior face and at the non-innervated anterior face. This was confirmed using rhodamine-conjugated alpha-bungarotoxin. Neither myosin heavy chain nor tropomyosin was present in electrocytes. Finally, we detected within electrocytes a type I acidic keratin that forms a filamentous meshwork within each cell. Immunoblots corroborate this result: A keratin-positive doublet of MW = 50 kD and 57 kD was found in both electrocytes and skin. Myosin, actin, talin, tropomyosin, desmin, alpha-actinin, and acetylcholine receptor, but not keratin, were all expressed in fish skeletal muscle fibers. Thus, electrocytes retain some muscle-specific proteins, do not express others, and in addition, express a non-muscle protein.

Tuberous electroreceptor organs form in denervated regenerating skin of a weakly electric fish

Weakly electric fish use tuberous electroreceptor organs to detect their own electric fields. We investigated the role of innervation upon regeneration and differentiation of tuberous electroreceptor organs. The left, infraorbital, anterior lateral line nerve of brown ghosts (Apteronotus leptorhynchus) was sectioned, and the proximal stump was dipped in ricin to prevent regrowth. Immediately after denervation, a piece of cheek skin (approximately 0.5 cm2) was removed bilaterally to induce skin regeneration. After survival periods of 3, 4, or 5 weeks, regenerated skin from the left (denervated) and the right (reinnervated) sides was removed and processed for immunocytochemistry or electron microscopy. Tuberous electroreceptor organs were present in regenerated reinnervated, as well as regenerated denervated skin patches at all survival times. With increased time after skin removal, the number of fully differentiated organs increased in the reinnervated regenerated skin while the number of organs with degenerating receptor cells or entirely devoid of receptor cells increased in the denervated regenerated skin. These results suggest that innervation is not essential for tuberous electroreceptor organ development, but that it is necessary for complete sensory cell differentiation and long-term survival.

Opposing actions of androgen and estrogen on in vitro firing frequency of neuronal oscillators in the electromotor system

The South American knifefish (Apteronotus leptorhynchus), or brown ghost, produces a high-frequency (600-1000 Hz) sinusoidal electric organ discharge (EOD) with males discharging at higher frequencies than females. In addition, each fish has a unique EOD frequency within the frequency range of its gender. The electromotor circuit responsible for EOD production consists of a medullary pacemaker nucleus (PMN) and spinal electromotor neurons (EMNs). In vitro spinal slice recording showed that, similar to the PMN, EMNs fire spontaneously at rates near the EOD frequency of each fish. The persistence of firing 2 weeks after high spinal transaction demonstrated that spontaneous firing rate was intrinsic to the EMNs and was not dependent on presynaptic input. We confirmed that 11-ketotestosterone (11 kT) raised and 17-beta-estradiol (E2) lowered the EOD frequency of intact fish. Because electromotor cells fire spontaneously near EOD, frequency, we investigated whether these steroids affect endogenous firing rates. Steroid implants were made in normal or spinally transected fish. Two weeks later, PMNs of normal fish and EMNs of transected fish were recorded in vitro. 11 kT increased and E2 decreased the intrinsic firing rate of neurons in the PMN and the EMNS. Hormones shifted the intrinsic firing rates of EMNS, although they were synaptically isolated during the hormone exposure.

Hormonal modulation of communication signals in electric fish

Steroid hormones have profound effects on vertebrate behaviors. Yet, in most cases we can neither identify the neurons responsible for particular behaviors nor how steroids alter their excitability. Electric fish emit sexually dimorphic electric organ discharges (EODs) the waveform of which may be altered by steroid treatment. These communication signals are generated by a medullary pacemaker nucleus (PMN) composed of two cell types. The output neurons of the PMN synapse on spinal cord electromotoneurons which then innervate the electric organ. The PMN receives input from only two sources, and these are responsible for brief modulations that occur during social interactions. The small numbers and stereotyped electrical behaviors of the cells in the EOD-generating circuitry and their access for biophysical recording makes them a convenient model system to study the actions of steroids on identified cells.

Individual variation in and androgen-modulation of the sodium current in electric organ

Electric fish of the genus Sternopygus produce a sinusoidal electric organ discharge (EOD) of low frequencies in males, high frequencies in females, and overlapping and intermediate frequencies in juveniles. Correspondingly, the cells of the electric organ, the electrocytes, generate action potentials which are of long duration in mature males, short duration in females, and intermediate duration in immatures. The androgen dihydrotestosterone (DHT) lowers EOD frequency and increases electrocyte action potential duration. We examined the electrocytes under voltage clamp to determine whether variations in the kinetic properties of the Na+ current might underlie these phenomena. We found that the fast inactivation time constants of the peak Na+ current (0 mV) ranged from 0.5 to 4.7 msec and varied systematically with EOD frequency and action potential duration. Voltage dependence of steady-state inactivation also varied with EOD frequency with the midpoint of inactivation being more positive in fish with low EOD frequencies. There was no correlation between the voltage at which the Na+ current activates, voltage at peak current, reversal potential, rate of recovery from inactivation, or TTX sensitivity and EOD frequency. We tested whether DHT influenced Na+ current inactivation by recording from electrocytes before and after juvenile fish of both sexes were implanted with a DHT-containing or empty capsule. We found that inactivation time constants were significantly slower in DHT implanted, but not control, fish. This is the first observation of functionally relevant individual variation in the kinetics of a Na+ current and the first demonstration that the kinetics of a Na+ current may be modulated by an androgen.

Effects of denervation upon receptor cell survival and basal cell proliferation in tuberous electroreceptor organs of a weakly electric fish

Weakly electric fish generate electric fields for the purposes of electrolocation and communication. These fields are detected by specialized receptor organs: the tuberous organs. In the present study we investigated the effects of denervation upon receptor cell survival and progenitor (basal) cell proliferation rate. The left, infraorbital, anterior lateral line nerve of brown ghosts (Apteronotus leptorhynchus) was sectioned, and the proximal stump was dipped in ricin to prevent regrowth. In groups of four, the animals were given two daily injections of the cell proliferation marker bromodeoxyuridine (BrdU) for 2 days at 1, 2, 3, or 4 weeks following denervation. At the completion of the BrdU injection schedule, a piece of cheek skin, rostroventral to the eye, was removed from the left (denervated) and the right (intact) sides and processed for light microscopy or immunocytochemistry. Our results show: (1) there is progressive receptor cell death and tuberous organ degeneration following denervation; (2) basal cell proliferation increases steadily with time after denervation and tuberous organ degeneration; and (3) despite denervation, some proliferating basal cells differentiate into receptor cells, but these new receptor cells eventually die. These results suggest that innervation is essential for tuberous electroreceptor cell survival and that the rate at which basal cells proliferate is regulated by receptor cell health, locally released factors, or both.

Conductances contributing to the action potential of Sternopygus electrocytes

In Sternopygus macrurus, electrocyte action potential duration determines the electric organ discharge pulse duration. Since the electric organ discharge is a sexually-dimorphic behavior under the control of steroid hormones, and because electrocyte action potential durations can range from 3-14 ms, the electrocytes provide a unique opportunity to study how sex steroids regulate membrane excitability. In this study, the voltage-sensitive ionic currents of electrocytes were identified under current- and voltage-clamp as a prelude to further studies on their regulation by sex steroid hormones. Bath application of TTX completely abolished the spike and eliminated an inward current under voltage clamp, indicating that the action potential is due primarily to a sodium current. Calcium-free saline had no effect on spike waveform or voltage-clamp currents, indicating that neither calcium nor calcium-dependent currents contribute to the action potential. Application of potassium channel blocking agents, such as tetraethylammonium and cesium ions, caused changes in the spike which, together with voltage-clamp results, indicate the presence of two potassium currents: an inward rectifier and a classical delayed rectifier. In addition, these cells have a large, presumably voltage-insensitive, chloride current. Differences in one or more of these currents could be responsible for the range of action potential durations found in these cells and for the steroid-mediated changes in spike duration.

Bromodeoxyuridine labeling reveals a class of satellite-like cells within the electric organ

When the electric organ (EO) of weakly electric fish is amputated, a blastema forms from which new EO and muscle cells arise. However, the progenitor cells that contribute to the blastema are unknown. We studied regeneration of the electric organ in Sternopygus to answer this question. The EO of this species is composed of electrocyte cells surrounded by peripheral bundles of muscle fibers. Fish were injected with 5'-bromodeoxyuridine (BrdU) 24 h after amputating the terminal portion of the EO. At this time, a population of small cells were labeled in the extracellular matrix between electrocytes and muscle fibers. These cells did not label in control fish injected with saline or in nonamputated BrdU-injected fish. For the first 6 days postamputation, increasing numbers of BrdU-labeled cells appeared at the wound margin. A blastema formed 6 days after amputation and contained numerous BrdU-labeled cells. At 10 days postamputation, clusters of BrdU-positive cells were seen throughout the wound margin and proximal blastema. At 14 days, BrdU-labeled nuclei were present within developing electrocytes. Labeling alternate sections with MF20 antimyosin and AE1 anticytokeratin antibodies confirmed that BrdU-positive multinucleate cells coexpress myosin and cytokeratin epitopes, diagnostic of newly regenerated electrocytes. Electron micrographs reveal that the small cells surrounding muscles and electrocytes are similar; they contain an elongate nucleus, are largely devoid of cytoplasm, and possess few organelles. This morphology and evidence of myogenic potential suggests that these cells are satellite cells.

Weakly electric fish as model systems for studying long-term steroid action on neural circuits

Weakly electric fish generate electric organ discharges (EODs) that are species-specific and often sexually-dimorphic. The waveform or frequency of an EOD can be altered by treating a fish with sex steroid hormones. The EOD is controlled by a few discrete nuclei in the brainstem and spinal cord and a muscle-derived electric organ. The organizational simplicity and steroid-sensitivity of the electromotor system make it a premier system for investigating how sex steroids modulate behavior, neural circuitry, and ion channels. In addition, the diversity of EOD patterns in the many species of electric fish provide a wealth of material in which to examine the evolution of sexual dimorphisms in the nervous system.

Electric organ morphology of Sternopygus macrurus, a wave-type, weakly electric fish with a sexually dimorphic EOD

In several species of electric fish with a sex difference in their pulse-type electric organ discharge (EOD), the action potential-generating cells of the electric organ (electrocytes) of males are larger and more invaginated compared to females. Androgen treatment of females and juveniles produces a longer-duration EOD pulse that mimics the mature male EOD, with a concurrent increase in electrocyte size and/or membrane infolding. In Sternopygus macrurus, which generates a wave-type EOD, androgen also increases EOD pulse duration. To investigate possible morphological correlates of hormone-dependent changes in EOD in Sternopygus, we examined electric organs from both fish collected in the field, and untreated and androgen-treated specimens in the laboratory. The electrocytes are cigar shaped, with prominent papillae on the posterior, innervated end. Electrocytes of field-caught specimens were significantly larger in all parameters than were electrocytes of specimens maintained in the laboratory. EOD pulse duration and frequency were highly correlated, and were significantly different between the sexes in sexually mature fish. Nevertheless, no sex difference in electrocyte morphology was observed, nor did any parameters of electrocyte morphology correlate with EOD pulse duration or frequency. Further, whereas androgen treatment significantly lowered EOD frequency and broadened EOD pulse duration, there was no difference in electrocyte morphology between hormone-treated and control groups. Thus, in contrast to results from studies on both mormyrid and gymnotiform pulse fish, electrocyte morphology is not correlated with EOD waveform characteristics in the gymnotiform wave-type fish Sternopygus. The data, therefore, suggest that sex differences in EOD are dependent on changes in active electrical properties of electrocyte membranes.

Communication in the weakly electric fish Sternopygus macrurus. II. Behavioral test of conspecific EOD detection ability

A classical conditioning paradigm was used to test the ability of Sternopygus macrurus to detect EOD-like stimuli (sine waves) of different frequencies. The behavioral tuning curves were quite close in shape to tuning curves based on single-unit recordings of T units, although the sensitivity at all frequencies was much greater. The behavioral curves showed notches of greatly reduced sensitivity when the test frequency was equal to, or twice the EOD frequency. The EOD of each of the fish was eliminated by lesioning the medullary pacemaker nucleus, and the fish were retested. The resulting tuning curves were nearly the same in shape as those of the EOD-intact individuals, but the PMN-lesioned fish showed an overall reduction of sensitivity of 30 dB. The EOD appears to enhance sensitivity by placing the summed stimulus (test stimulus+fish's EOD) at an amplitude where T units are maximally sensitive to small temporal modulations in the fish's own EOD. Peripheral tuning appears to limit the ability of males to detect the EOD of females, since these are, on average, an octave higher in frequency than the male EOD, while the peak sensitivity of the male occurs 5-10 Hz above its own EOD frequency.

Electric organ discharge frequency and plasma sex steroid levels during gonadal recrudescence in a natural population of the weakly electric fish Sternopygus macrurus

1. Sternopygus macrurus were collected in Venezuela during the period of gonadal recrudescence in early or late dry season. Electric organ discharge (EOD) frequencies were recorded, blood samples were taken for analysis of steroid titers, and gonads were taken for determination of reproductive condition. 2. Mean EOD frequencies were significantly lower in males than in females in all samples. EOD frequency was inversely correlated with body length in males in late, but not early, dry season, and these parameters were never correlated in females. 3. Plasma levels of testosterone (T) and 11-ketotestosterone (11-KT), but not estradiol-17 beta (E2), were inversely correlated with EOD frequency in males. No 11-KT was observed in plasma of females, and plasma levels of T and E2 in females were comparable to those of males. Neither T nor E2 were correlated with EOD frequency in females. 4. Testes collected in late dry season were more mature than those from early dry season; androgen levels and EOD frequency were correlated with testicular maturity. Ovaries collected in early dry season were immature, while those from late dry season were more mature. There was no relationship between EOD frequency and stage of ovarian development. 5. These results suggest that plasma androgens modulate EOD frequency in males during the reproductive season and that plasma E2 has little relationship to EOD frequency in either sex.

Chronic androgen treatment increases action potential duration in the electric organ of Sternopygus

The quasi-sinusoidal electric organ discharge (EOD) of the weakly electric fish Sternopygus is involved in communication and orientation. Each monophasic pulse of the low-intensity EOD is a compound action potential (AP) from the simultaneously firing electrocytes of the electric organ. EOD frequency is lower and EOD pulse duration longer in sexually mature males than in sexually mature females; exogenous androgen lowers EOD frequency and increases EOD pulse duration. In order to determine the contribution of single electrocyte spikes to the entire EOD pulse, APs were induced by intracellular current injection in single electrocytes of isolated pieces of electric organ. Each AP looks very similar to the externally recorded EOD pulses, and AP duration (APD) is significantly correlated with EOD pulse duration (r = 0.48; p less than 0.0005). The APD is slightly longer when compared to the EOD pulse duration, but this difference is likely due to the stimulation paradigm. Fish treated with dihydrotestosterone showed a decrease in EOD frequency, increase in EOD pulse duration, and corresponding increase in APD; control fish showed random, insignificant changes in EOD wave form and APD. Evidence presented here shows that changes in the passive membrane properties are unlikely to be responsible for the APD increase. The possibility is discussed that androgens act directly upon the electric organ, ultimately altering the ionic currents that produce the AP.

Time course of structural changes in regenerating electroreceptors of a weakly electric fish

We examined the regenerating electroreceptors of the weakly electric fish Sternopygus by light and electron microscopy to search for possible structural correlates of known physiological changes that occur during regeneration (Zakon: J. Neurosci. 6(11):3297-3308, 1986) and to compare them with developing electroreceptors in larval fish (Vischer: Brain Behav. Evol. 33:223-236). Nine days after removal of a patch of cheek skin, new skin had filled the wound and undifferentiated precursor cell clusters were located in the epidermis just above the dermis. Nerve fibers were present near most, but not all, cell clusters. A few recognizable tuberous and ampullary precursor organs were seen at this time. Tuberous organs were composed of a group of large cells surrounded by smaller cells without a lumen and showed the beginning of a cellular plug. Ampullary organs appeared as a ball of cells with a small lumen opening into a nascent canal. Degenerating cells were found within organs, and sometimes entire organs degenerated. These were not innervated. By 2 weeks the large cells of the tuberous organ were developing into sensory cells, while the smaller cells were forming the capsule wall and the underlying basal cells. The characteristic tuberous organ canal filled with loosely packed epidermal cells was evident. The sensory cells of the ampullary organs were visible within the epithelial layer at the base of the lumen, and the large synaptic discs were beginning to form. The sensory cells and postsynaptic terminals contained numerous vesicles. The presynaptic vesicles, which appear in normal receptor cells, remained throughout regeneration and presumably underlie transmitter release. The postsynaptic vesicles appeared transiently in large numbers but declined to adult values by 4 weeks. We presume that these may serve a trophic role. By 3 weeks, organs generally appeared mature and began dividing into daughter organs. The formation of individual receptor organs during regeneration is similar to that observed in development. Receptor organs continued dividing until the appropriate number of organs per afferent was reached for the size of the fish. Although the organization of the receptors appeared generally normal, there were a few anomalies. Some afferents sent sprouts into the epidermis, and, as a result of such sprouting, some of these afferents innervated multiple organs over a greater distance than normal. This was first seen early in regeneration and persisted for as long as 5 months.(ABSTRACT TRUNCATED AT 400 WORDS)

Human chorionic gonadotropin-induced shifts in the electrosensory system of the weakly electric fish, Sternopygus

Sternopygus macrurus of both sexes were injected with human chorionic gonadotropin (hCG) or saline. Electric organ discharge (EOD) frequency rose after hCG injections in females and gradually declined to baseline levels over the next few weeks. EOD changes in males were more complex and variable; most males showed an initial minor rise in EOD frequency followed by a larger decrease, or simply a decrease. hCG treatment also resulted in a rise in electroreceptor best frequency and shortened electric organ pulse duration in females, and had the opposite effect on these parameters in males. The saline-injected controls showed no changes in any of these parameters. Levels of testosterone (T) and 11-ketotestosterone, but not estrogen (E), were elevated in males preceding the fall in EOD frequency, whereas neither T nor E changed significantly in females before EOD frequency increases. Saline injections caused a drop in T in the male control group and had no effect in the female control group. We presume that the effect of hCG on the electrosensory system of males is mediated via androgens. Whether the effects of hCG on females are mediated by slight increases in circulating levels of gonadal steroids, the release of hormones other than T or E, or are due to direct effects on the nervous system is not known.

The effects of postembryonic receptor cell addition on the response properties of electroreceptive afferents

The weakly electric fish Sternopygus detects electric fields with a receptor organ called a tuberous electroreceptor. Previous studies have shown that an electroreceptive afferent fiber innervates a single organ in small fish and that as fish grow some of these organs divide, giving rise to daughter organs; these divide in turn to produce a cluster of organs. All of the organs in a cluster are innervated by the original afferent, which sprouts new terminals to accommodate them. Other organs, however, seldom divide. Thus, the distribution of the number of tuberous organs per afferent becomes increasingly bimodal with fish body length (Zakon, 1984a). In order to investigate the effect of organ addition on neural coding within the afferent fiber, activity was recorded from single units within the anterior branch of the anterior lateral line nerve in fish of a range of sizes. It was found that, as fish increase in body length, the best frequency, sharpness of tuning, and sensitivity increase in a subset of afferents, while others remain essentially unchanged. This results in an increasingly bimodal distribution of these physiological measures with increasing fish body length. These results suggest that the afferents that innervate multiple receptor organs are more sensitive and possess higher best frequencies than those that innervate 1 or 2 organs. This was confirmed by dye injections of electroreceptive fibers with Lucifer yellow.(ABSTRACT TRUNCATED AT 250 WORDS)

The medullary pacemaker nucleus is unnecessary for electroreceptor tuning plasticity in Sternopygus

Tuberous electroreceptors of the weakly electric fish Sternopygus macrurus are closely tuned to the frequency of electric organ discharge (EOD), which is determined by a medullary pacemaker nucleus (PMN). Previous studies have demonstrated that androgens lower the frequency of PMN discharge and concomitantly lower the best frequencies (BFs) of electroreceptors. In order to determine if the PMN serves as an internal reference for the hormone-mediated returning of electroreceptors, the PMN was lesioned and the change in mean BF was measured for dihydrotestosterone (DHT)-implanted or control animals. DHT-implanted fish showed the characteristic lowering of mean electroreceptor BF by approximately 25%, a significant change compared with controls (p less than 0.01, Mann-Whitney). This result indicates that the PMN is not necessary for the hormone-mediated shift of electroreceptor tuning. In a related study, the contribution of the PMN to the genesis of tuning in regenerating electroreceptors was examined by removing a patch of cheek skin from PMN-lesioned fish. Regenerating electroreceptors became sharply tuned to the previous EOD frequency by 6 weeks in the same fashion as regenerating receptors in intact fish. In addition, intact receptors from PMN-lesioned fish remained tuned for up to 160 d. Together, these results demonstrate that the pacemaker nucleus is unnecessary for the maintenance, development, or hormone-mediated shift of receptor tuning.

A simple, reliable and inexpensive silver stain for nerve fibers in bleached skin

A procedure for silver staining is described which leads to the selective and reliable impregnation of nerve fibers in bleached skin of vertebrates and invertebrates. In combination with osmium, the protocol enhances the staining of secondary sensory cells of mechanosensory and electrosensory organs so that the innervation pattern of each organ and the number of sensory cells per organ can easily be evaluated. The technique can be also used for staining nerve fibers in whole embryos.

Variation in the mode of receptor cell addition in the electrosensory system of gymnotiform fish

Age-related changes in ampullary and tuberous receptor organ morphology were studied in six species of gymnotiform weakly electric fish. Cheek skin was silver-stained, whole-mounted, and viewed under Nomarski differential interference contrast optics. The ampullary receptor units of all species show an increasing number of receptor organs per afferent fiber with fish size, presumably the result of addition of newly formed receptor organs. Ampullary units composing over a dozen organs were observed in large specimens of a few species. Receptor cells were also added in the tuberous receptor system of all species, but in different ways. As previously reported for Sternopygus, small specimens of Eigenmannia had only a single tuberous receptor organ per afferent. Fish of increasing size retained a population of afferents that innervated only a single receptor organ and, in addition, had a population of afferents that innervated a cluster of receptor organs. The mean number of receptor organs per cluster increased in fish of increasing size. In addition, the mean number of sensory receptor cells per organ increased. New organs presumably derive from older ones, which divide under the stimulus of continued addition of new receptor cells. Apteronotus, Adontosternarchus, and Hypopomus all added more receptor cells to their tuberous organs. In these species, every afferent innervated only a single tuberous organ and there was no indication of division of receptor organs. Gymnorhamphichthys and Gymnotus were intermediate in that they added new receptor cells to each receptor organ, and, in larger fish, these were segregated into discrete patches within a single receptor organ. It is likely that the addition of new receptor cells aids in increasing sensitivity of both ampullary and tuberous receptors as fish grow.

The emergence of tuning in newly generated tuberous electroreceptors

Tuning curves of afferent electroreceptive fibers in the anterior lateral line nerve of the weakly electric fish, Sternopygus macrurus, indicate that the tuberous electroreceptors of each individual are well-tuned to its own electric organ discharge (EOD) frequency. In order to study how receptor tuning may develop, new receptor organs were induced to form in regenerating cheek skin, and their tuning properties were compared with those of intact receptors from the same fish. At 3 weeks after the onset of regeneration, new receptors of a given fish were broadly tuned with best frequencies (BFs) lower than that fish's EOD frequency and the BFs of its own intact tuberous receptors. Three weeks later, regenerated receptors of the same fish were indistinguishable from intact receptors in BF, although tuning curves were occasionally slightly broader than normal. To determine if the presence of an ongoing electric field is necessary for the genesis of proper tuning, receptors were allowed to regenerate in fish deprived of their EODs. At 6 weeks, tuning curves of these receptors also had BFs that were tuned similarly to intact receptors and to each individual's characteristic EOD frequency (determined by recordings of the pacemaker nucleus in the medulla). Thus, as regenerating receptors mature, they gradually become more sharply tuned and tuned to progressively higher frequencies until reaching the correct BF, which matches the EOD frequency; however, tuning to the appropriate EOD frequency occurs without reference to the ongoing electric field.

Evidence for a direct effect of androgens upon electroreceptor tuning

Tuberous electroreceptors of individual wave type weakly electric fish are tuned to the fundamental frequency of that fish's electric organ discharge (EOD). EOD frequency and receptor best frequency (BF) are both lowered following systemic injection of 5-alpha-dihydrotestosterone (DHT). A previous study (Meyer et al. 1984) showed that the effect of DHT on the EOD generating circuitry was independent of an ongoing EOD and suggested that its effect on electroreceptor tuning was indirect, possibly mediated by the electric field. We have continued these studies to determine the factors which influence electroreceptor tuning. Baseline recordings of EOD frequency, receptor oscillations, and single afferent tuning curves were taken. After fish were electrically silenced by spinal cord transection they were injected daily with either DHT or saline or were implanted with either DHT-filled or empty silastic capsules. As previously reported, the EOD frequency (determined from pacemaker nucleus recordings) was lowered in DHT-treated, transected fish and increased in control fish. Similarly, receptor tuning was lowered in the DHT-treated, silenced fish. Oscillation frequencies decreased in both treated and control groups, but significantly more in the hormone group. Single afferent best frequencies were lowered in both DHT groups and raised in their respective control groups. In another series of experiments exogenous electric fields capable of driving receptors in a 1-to-1 phase-locked manner were placed around silenced fish. We were unable to elicit any shift in pacemaker frequency or electroreceptor tuning regardless of stimulus field geometry. Four transected fish were injected with DHT and placed in exogenous electric fields of higher frequency than their original EOD. Even in the presence of a higher frequency electric field, DHT lowered EOD frequency and afferent BF. We conclude that androgens produce effects both on the EOD generating circuitry, probably at the level of the pacemaker nucleus, and on electroreceptors, probably, ultimately, on receptor cell membrane conductances. These effects occur in parallel allowing the two parameters to remain well matched. In contrast to former predictions, exogenous electric fields alone appear unable to shift receptor tuning.

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.

Reorganization of connectivity in amphibian central auditory system following VIIIth nerve regeneration: time course

In the frog, most neurons in the primary (dorsal medullary nucleus, DMN) and secondary (superior olivary nucleus, SO) auditory nuclei have V-shaped tuning curves, almost as narrowly tuned as those recorded in the nerve. Thus, the innervation pattern is such that if more than one excitatory afferent innervates a postsynaptic cell, they must all possess similar best frequencies (BFs). Similarly, binaural cells in these nuclei display matched frequency selectivities when acoustically stimulated via either ear. The VIIIth nerve was unilaterally severed and allowed to regenerate back into the DMN. At various postoperative intervals, extracellular single-unit recordings were made in the SO contralateral to the regenerated nerve, as that nucleus receives its dominant excitatory input (approximately 80%) from the contralateral side. Recordings were also made in the SO of a number of unoperated control animals. Functional reinnervation commenced between 4 and 5 wk postoperatively and by 6 wk, a normal innervation density, as judged by physiological criteria, was achieved. Single units of any best frequency represented within the frog's two auditory papillae could be recorded during earliest reinnervation. In general, the tuning curves of both monaural and binaural cells were V shaped in the 6 wk regenerates. Although many tuning curves were narrowly tuned (Q10dB greater than 1.0) as in unoperated animals, some were very broadly tuned (Q10dB less than 0.5). The mean Q10dB value for all contralaterally excited cells was 1.45 +/- 0.77 (SD), which was significantly lower than that of SO units in unoperated frogs (Q10dB = 1.66 +/- 0.52 (SD)). Binaural cells often had mismatched BFs and tuning curves. By 8 wk after nerve transection, tuning curves were as narrow as in unoperated animals (Q10dB = 1.64 +/- 0.68 (SD)), and the BFs of binaural cells evinced a greater match than at 6 wk. By 12 wk postoperatively, V-shaped tuning curves were still as narrow as in controls (Q10dB = 1.71 +/- 0.69 (SD)), and the tuning curves and BFs of binaural cells were well matched again. At all postoperative intervals, about 10% of the tuning curves in the SO of regenerates were W shaped. This was never seen in normal animals. The return of narrow V-shaped tuning curves in the majority of neurons and the recurrence of matched binaural cells in the SO are interpreted as evidence of specificity for potential postsynaptic targets in the DMN by regenerating auditory afferents.

Androgens Alter the Tuning of Electroreceptors

Weakly electric fish possess electroreceptors that are tuned to their individual electric organ discharge frequencies. One genus, Sternopygus, displays both ontogenetic and seasonal shifts in these frequencies, possibly because of endocrine influences. Systemic treatment with androgens lowers the discharge frequencies in these animals. Concomitant with these changes in electric organ discharge frequencies are decreases in electroreceptor best frequencies; hence the close match between discharge frequency and receptor tuning is maintained. These findings indicate that the tuning of electroreceptors is dynamic and that it parallels natural shifts in electric organ discharge frequency.