Electric interactions through chirping behavior in the weakly electric fish, Apteronotus leptorhynchus

The weakly electric fish Apteronotus leptorhynchus produces wave-like electric organ discharges distinguished by a high degree of regularity. Transient amplitude and frequency modulations (“chirps”) can be evoked in males by stimulation with the electric field of a conspecific. During these interactions, the males examined in this study produced six types of chirps, including two novel ones. Stimulation of a test fish with a conspecific at various distances showed that two electrically interacting fish must be within 10 cm of each other to evoke chirping behavior in the neighboring fish. The chirp rate of all but one chirp type elicited by the neighboring fish was found to be negatively correlated with the absolute value of the frequency difference between the two interacting fish, but independent of the sign of this difference. Correlation analysis of the instantaneous rates of chirp occurrence revealed two modes of interactions characterized by reciprocal stimulation and reciprocal inhibition. Further analysis of the temporal relationship between the chirps generated by the two fish during electric interactions showed that the chirps generated by one individual follow the chirps of the other with a short latency of approximately 500–1000 ms. We hypothesize that this “echo response” serves a communicatory function.     

Testosterone modulates female chirping behavior in the weakly electric fish,Apteronotus leptorhynchus

The weakly electric fish, Apteronotus leptorhynchus, produces a wave-like electric organ discharge (EOD) utilized for electrolocation and communication. Both sexes communicate by emitting chirps: transient increases in EOD frequency. In males, chirping behavior and the jamming avoidance response (JAR) can be evoked by an artificial EOD stimulus delivered to the water at frequencies 1–10 Hz below the animal's own EOD. In contrast, females rarely chirp in response to this stimulus even though they show consistent JARs. To investigate whether this behavioral difference is hormone dependent, we implanted females with testosterone (T) and monitored their chirping activity over a 5 week period. Our findings indicate that elevations in blood levels of T cause an enhancement of chirping behavior and a lowering of basal EOD frequency in females. Elevated blood levels of T also appear to modulate the quality of chirps produced by hormone treated females. The effects of T on female chirping behavior and basal EOD frequency appear specific, since the magnitude of the JAR was not affected by the hormonal treatment. These findings suggest that seasonal changes in circulating concentrations of T may regulate behavioral changes in female chirping behavior and basal EOD frequency.Abbreviations DHT dihydrotestosterone - E estradiol - EOD elecdric organ discharge - GSI gonadal size index - JAR jamming avoidance response - PPn prepacemaker nucleus - T testosterone     

Phase and amplitude maps of the electric organ discharge of the weakly electric fish,Apteronotus leptorhynchus

Summary The electric organ discharge (EOD) potential was mapped on the skin and midplane of several Apteronotus leptorhynchus. The frequency components of the EOD on the surface of the fish have extremely stable amplitude and phase. However, the waveform varies considerably with different positions on the body surface. Peaks and zero crossings of the potential propagate along the fish's body, and there is no point where the potential is always zero. The EOD differs significantly from a sinusoid over at least one third of the body and tail. A qualitative comparison between fish showed that each individual had a unique spatiotemporal pattern of the EOD potential on its body.The potential waveforms have been assembled into high temporal and spatial resolution maps which show the dynamics of the EOD. Animation sequences and Macintosh software are available by anonymous ftp (mordor.cns.caltech.edu; cd/pub/ElectricFish).We interpret the EOD maps in terms of ramifications on electric organ control and electroreception. The electrocytes comprising the electric organ do not all fire in unison, indicating that the command pathway is not synchronized overall. The maps suggest that electroreceptors in different regions fulfill different computational roles in electroreception. Receptor mechanisms may exist to make use of the phase information or harmonic content of the EOD, so that both spatial and temporal patterns could contribute information useful for electrolocation and communication.Abbreviations EOD electric organ discharge - EO electric organ - CV coefficient of variance     

Effect of temperature on spinal cord regeneration in the weakly electric fish, Apteronotus leptorhynchus

Temperature manipulation has been shown to significantly affect recovery after spinal cord injury in various mammalian model systems. Little has been known thus far about the impact of temperature on structural and functional recovery after central nervous system lesions in regeneration-competent, poikilotherm organisms. In the present study, we addressed this aspect using an established model of adult spinal cord regeneration, the weakly electric teleost fish Apteronotus leptorhynchus. We observed an overall beneficial effect of increased temperature on both structural and behavioral recovery after amputation of the caudal spinal cord. Fish kept at 30°C recovered the amplitude of the electric organ discharge at more than twice the rate observed in fish kept at 22°C, within the first 20 days post-injury. This improved recovery was supported by increased cell proliferation and decreased apoptosis levels in fish kept at 30°C. The high temperature appeared to have a direct inhibitory effect on apoptosis and to lead to a compression of the duration of the wave of post-lesion apoptosis. The latter effect was presumably induced through the acceleration of the metabolic rate, a phenomenon also supported by the observation that re-growth of the tail was significantly increased in fish kept at 30°C.     

Enzyme reaction rate studies in electromotor neurons of the weakly electric fish Apteronotus leptorhynchus

A histochemical analysis of reaction rates of a series of enzymes was performed in electromotor neurons of the weakly electric fish Apteronotus leptorhynchus. These neurons were selected because of their functional homogeneity. The high metabolic activity of these cells as well as their large size facilitate cytophotometric analysis in cryostat sections. Sections were incubated for the activity of hexokinase, glucose-6-phosphate dehydrogenase, succinate dehydrogenase, NADPH dehydrogenase, NADPH ferrihaemoprotein reductase and beta-hydroxybutyrate dehydrogenase. All media contained polyvinyl alcohol as tissue stabilizer and Nitro BT as final electron acceptor. Measurements were performed with a Vickers M85a cytophotometer. Linear relationships between the specific formation of formazan (test minus control reaction) and incubation time were obtained for all enzymes although some reactions showed an initial lag phase or an intercept with the ordinate. The relatively high activities of hexokinase, succinate dehydrogenase and the extremely low activity of hydroxybutyrate dehydrogenase indicate that energy is mainly supplied by glycolysis. Glucose-6-phosphate dehydrogenase showed a high activity whereas NADPH reductase and dehydrogenase activity were low in electromotor neurons, indicating that the NADPH generated is largely used for biosynthesis. Despite their synchronous firing pattern activity, electromotor neurons showed a considerable heterogeneity with respect to their metabolic activity.     

An in vitro physiological preparation of a vertebrate communicatory behavior: chirping in the weakly electric fish, Apteronotus

1. An in vitro preparation of the medullary pacemaker nucleus of the weakly electric fish Apteronotus leptorhynchus was studied which fires regularly and synchronously at the fish's characteristic frequency of electric organ discharge (EOD). Upon bipolar stimulation of tissue regions through which pass prepacemaker nucleus afferents to the pacemaker, a brief, transient increase in discharge frequency ensued at short-latency (Fig. 1A). 2. Intracellular recordings revealed that the acceleration was accompanied by a depolarization and decline in action potential amplitude. The magnitude of these changes was both phase- (Fig. 5) and amplitude-dependent, with the latter showing an evident threshold effect (Figs. 4 and 12). The response was reversibly blocked by high Mg2+ saline (Fig. 1B), and the magnitude of the accelerations showed marked facilitation during repeated stimulation (Fig. 6). 3. Optical and histological identification allowed characteristically different responses in the intracellular recordings to be attributed to the two cell types of the pacemaker nucleus: pacemaker and relay cells (Figs. 2 and 3). Similar responses have been observed at these respective recording locations in the intact animal during chirping (Dye and Heiligenberg 1987). 4. Simultaneous recordings of pairs of cells revealed a transient change in the phase relationship of firing during the accelerations which was most marked between relay and pacemaker cells (Fig. 7). These dual recordings also revealed that the relay cells depolarize and accelerate more than pacemaker cells (Fig. 10), suggesting that they are the principal effectors of this behavioral modulation. 5. Trains of pulses additionally elicited a long-lasting frequency elevation which occurred at a slightly higher threshold than the brief accelerations. This slow frequency change relaxed back to baseline following a biexponential time course which closely resembled that of a distinct behavior seen in intact fish, termed 'yodeling' (Dye 1987).     

Hormonal and body size correlates of electrocommunication behavior during dyadic interactions in a weakly electric fish, Apteronotus leptorhynchus

Brown ghost knife fish, Apteronotus leptorhynchus, produce sexually dimorphic, androgen-sensitive electrocommunication signals termed chirps. The androgen regulation of chirping has been studied previously by administering exogenous androgens to females and measuring the chirping response to artificial electrical signals. The present study examined the production of chirps during dyadic interactions of fish and correlated chirp rate with endogenous levels of one particular androgen, 11-ketotestosterone (11KT). Eight males and four females were exposed to short-term (5-min) interactions in both same-sex and opposite-sex dyads. Twenty-four hours after all behavioral tests, fish were bled for determination of plasma 11KT levels. Males and females differed in both their production of chirps and their ability to elicit chirps from other fish: males chirped about 20-30 times more often than females and elicited 2-4 times as many chirps as females. Among males, chirp rate was correlated positively with plasma 11KT, electric organ discharge frequency, and body size. Combined with results from experimental manipulation of androgen levels, these results support the hypothesis that endogenous 11KT levels influence electrocommunication behavior during interactions between two male fish.     

Spontaneous modulations of the electric organ discharge in the weakly electric fish, Apteronotus leptorhynchus: a biophysical and behavioral analysis

Brown ghosts, Apteronotus leptorhynchus, are weakly electric gymnotiform fish whose wave-like electric organ discharges are distinguished by their enormous degree of regularity. Despite this constancy, two major types of transient electric organ discharge modulations occur: gradual frequency rises, which are characterized by a relatively fast increase in electric organ discharge frequency and a slow return to baseline frequency; and chirps, brief and complex frequency and amplitude modulations. Although in spontaneously generated gradual frequency rises both duration and amount of the frequency increase are highly variable, no distinct subtypes appear to exist. This contrasts with spontaneously generated chirps which could be divided into four "natural" subtypes based on duration, amount of frequency increase and amplitude reduction, and time-course of the frequency change. Under non-evoked conditions, gradual frequency rises and chirps occur rather rarely. External stimulation with an electrical sine wave mimicking the electric field of a neighboring fish leads to a dramatic increase in the rate of chirping not only during the 30 s of stimulation, but also in the period immediately following the stimulation. The rate of occurrence of gradual frequency rises is, however, unaffected by such a stimulation regime.     

Electrocommunication behaviour and non invasively-measured androgen changes following induced seasonal breeding in the weakly electric fish, Apteronotus leptorhynchus

Androgens are known to be involved in reproductive behaviours including courtship and aggression. According to the Challenge Hypothesis, androgen activity upregulates male reproductive behaviour seasonally and also modulates short term adaptation of these behaviours in response to social context. In the weakly electric fish, Apteronotus leptorhynchus, 11-ketotestosterone (11-KT) has been previously implicated in the regulation of electrocommunication behaviours that are believed to have roles in both aggression and courtship. Changes in male 11-KT levels were quantified using a non-invasive measurement technique alongside changes in electrocommunication behaviour following environmental cues that simulated the onset of the breeding season. Males showed an increase in mean electric organ discharge frequency (EODf), which is consistent with earlier results showing a female preference for high EODf. A subset of males with high initial EODfs showed increases in both 11-KT and EODf, which provides support for an EODf-based dominance hierarchy in this species. Males housed in social conditions and exposed to breeding conditioning also showed higher overall electric organ discharge frequencies and 11-KT compared to males housed in isolation. Evidence is presented that another type of electrocommunication signal previously implicated in courtship may also serve as an inter-male signal of submission. Our results are consistent with earlier observations that electrocommunication signals produced during inter-male aggression serve in deterring attacks, and their pattern of production further suggested the formation of a dominance hierarchy.     

Retreat site selection and social organization in captive electric fish, Apteronotus leptorhynchus

Gymnotiform fish use their electric organ discharge for electrolocation and communication. They are active nocturnally and seek retreat sites during the day. We examined retreat site selection in Apteronotus leptorhynchus by assessing their preference for retreat tubes that differed in opacity and dimension. Isolated fish preferred opaque to clear tubes, long and narrow diameter tubes to short, wide diameter tubes, and open-ended to closed tubes. We also assessed how groups of fish distributed themselves in tubes according to sex and electric organ discharge frequency under four conditions: (1) unlimited tube availability, (2) limited tube availability, (3) variation in tube opacity, and (4) variation in tube dimension. When tube availability was unlimited, fish generally preferred to occupy tubes alone. However, females, but not males, often cohabited tubes with consexuals. When tube availability was limited, females were more often than males found outside of tubes. When tubes varied by opacity and dimension, fish clustered most commonly in preferred tube types (opaque and long tubes). Males with the highest electric organ discharge frequencies usually occupied the most preferred tube type. Thus, fish have clear preferences in selecting retreat sites and groups of fish reveal their dominance relationships when presented with variation in retreat sites.     

Pharmacological characterization of ionic currents that regulate high-frequency spontaneous activity of electromotor neurons in the weakly electric fish, Apteronotus leptorhynchus

The neural circuit that controls the electric organ discharge (EOD) of the brown ghost knifefish (Apteronotus leptorhynchus) contains two spontaneous oscillators. Both pacemaker neurons in the medulla and electromotor neurons (EMNs) in the spinal cord fire spontaneously at frequencies of 500-1,000 Hz to control the EOD. These neurons continue to fire in vitro at frequencies that are highly correlated with in vivo EOD frequency. Previous studies used channel blocking drugs to pharmacologically characterize ionic currents that control high-frequency firing in pacemaker neurons. The goal of the present study was to use similar techniques to investigate ionic currents in EMNs, the other type of spontaneously active neuron in the electromotor circuit. As in pacemaker neurons, high-frequency firing of EMNs was regulated primarily by tetrodotoxin-sensitive sodium currents and by potassium currents that were sensitive to 4-aminopyridine and kappaA-conotoxin SIVA, but resistant to tetraethylammonium. EMNs, however, differed from pacemaker neurons in their sensitivity to some channel blocking drugs. Alpha-dendrotoxin, which blocks a subset of Kv1 potassium channels, increased firing rates in EMNs, but not pacemaker neurons; and the sodium channel blocker muO-conotoxin MrVIA, which reduced firing rates of pacemaker neurons, had no effect on EMNs. These results suggest that similar, but not identical, ionic currents regulate high-frequency firing in EMNs and pacemaker neurons. The differences in the ionic currents expressed in pacemaker neurons and EMNs might be related to differences in the morphology, connectivity, or function of these two cell types.     

Social interactions and cortisol treatment increase the production of aggressive electrocommunication signals in male electric fish,Apteronotus leptorhynchus

Brown ghost knife fish, Apteronotus leptorhynchus, continually emit a weakly electric discharge that serves as a communication signal and is sensitive to sex steroids. Males modulate this signal during bouts of aggression by briefly (approximately 15 ms) increasing the discharge frequency in signals termed "chirps." The present study examined the effects of short-term (1-7 days) and long-term (6-35 days) male-male interaction on the continuous electric organ discharge (EOD), chirping behavior, and plasma levels of cortisol and two androgens, 11-ketotestosterone (11KT) and testosterone. Males housed in isolation or in pairs were tested for short-term and long-term changes in their EOD frequency and chirping rate to standardized sinusoidal electrical stimuli. Within 1 week, chirp rate was significantly higher in paired fish than in isolated fish, but EOD frequency was equivalent in these two groups of fish. Plasma cortisol levels were significantly higher in paired fish than in isolated fish, but there was no difference between groups in plasma 11KT levels. Among paired fish, cortisol levels correlated positively with chirp rate. To determine whether elevated cortisol can cause changes in chirping behavior, isolated fish were implanted with cortisol-filled or empty Silastic tubes and tested for short-term and long-term changes in electrocommunication signals and steroid levels. After 2 weeks, fish that received cortisol implants showed higher chirp rates than blank-implanted fish; there were no difference between groups in EOD frequency. Cortisol implants significantly elevated plasma cortisol levels compared to blank implants but had no effect on plasma 11KT levels. These results suggest that male-male interaction increases chirp rate by elevating levels of plasma cortisol, which, in turn, acts to modify neural activity though an 11KT-independent mechanism.     

Distribution of Kv1-like potassium channels in the electromotor and electrosensory systems of the weakly electric fish Apteronotus leptorhynchus

The electromotor and electrosensory systems of the weakly electric fish Apteronotus leptorhynchus are model systems for studying mechanisms of high-frequency motor pattern generation and sensory processing. Voltage-dependent ionic currents, including low-threshold potassium currents, influence excitability of neurons in these circuits and thereby regulate motor output and sensory filtering. Although Kv1-like potassium channels are likely to carry low-threshold potassium currents in electromotor and electrosensory neurons, the distribution of Kv1 alpha subunits in A. leptorhynchus is unknown. In this study, we used immunohistochemistry with six different antibodies raised against specific mammalian Kv1 alpha subunits (Kv1.1-Kv1.6) to characterize the distribution of Kv1-like channels in electromotor and electrosensory structures. Each Kv1 antibody labeled a distinct subset of neurons, fibers, and/or dendrites in electromotor and electrosensory nuclei. Kv1-like immunoreactivity in the electrosensory lateral line lobe (ELL) and pacemaker nucleus are particularly relevant in light of previous studies suggesting that potassium currents carried by Kv1 channels regulate neuronal excitability in these regions. Immunoreactivity of pyramidal cells in the ELL with several Kv1 antibodies is consistent with Kv1 channels carrying low-threshold outward currents that regulate spike waveform in these cells (Fernandez et al., J Neurosci 2005;25:363-371). Similarly, Kv1-like immunoreactivity in the pacemaker nucleus is consistent with a role of Kv1 channels in spontaneous high-frequency firing in pacemaker neurons. Robust Kv1-like immunoreactivity in several other structures, including the dorsal torus semicircularis, tuberous electroreceptors, and the electric organ, indicates that Kv1 channels are broadly expressed and are likely to contribute significantly to generating the electric organ discharge and processing electrosensory inputs.     

Glucocorticoid receptor blockade inhibits brain cell addition and aggressive signaling in electric fish, Apteronotus leptorhynchus

When animals are under stress, glucocorticoids commonly inhibit adult neurogenesis by acting through glucocorticoid receptors (GRs). However, in some cases, conditions that elevate glucocorticoids promote adult neurogenesis, and the role of glucocorticoid receptors in these circumstances is not well understood. We examined the involvement of GRs in social enhancement of brain cell addition and aggressive signaling in electric fish, Apteronotus leptorhynchus. In this species, long-term social interaction simultaneously elevates plasma cortisol, enhances brain cell addition and increases production of aggressive electrocommunication signals (“chirps”). We implanted isolated and paired fish with capsules containing nothing (controls) or the GR antagonist, RU486, recorded chirp production and locomotion for 7 d, and measured the density of newborn cells in the periventricular zone. Compared to isolated controls, paired controls showed elevated chirping in two phases: much higher chirp rates in the first 5 h and moderately higher nocturnal rates thereafter. Treating paired fish with RU486 reduced chirp rates in both phases to those of isolated fish, demonstrating that GR activation is crucial for socially induced chirping. Neither RU486 nor social interaction affected locomotion. RU486 treatment to paired fish had a partial effect on cell addition: paired RU486 fish had less cell addition than paired control fish but more than isolated fish. This suggests that cortisol activation of GRs contributes to social enhancement of cell addition but works in parallel with another GR-independent mechanism. RU486 also reduced cell addition in isolated fish, indicating that GRs participate in the regulation of cell addition even when cortisol levels are low.     

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.     

Serotonergic activation of 5HT1A and 5HT2 receptors modulates sexually dimorphic communication signals in the weakly electric fish Apteronotus leptorhynchus

Serotonin modulates agonistic and reproductive behavior across vertebrate species. 5HT_1A and 5HT_1B receptors mediate many serotonergic effects on social behavior, but other receptors, including 5HT₂ receptors, may also contribute. We investigated serotonergic regulation of electrocommunication signals in the weakly electric fish Apteronotus leptorhynchus. During social interactions, these fish modulate their electric organ discharges (EODs) to produce signals known as chirps. Males chirp more than females and produce two chirp types. Males produce high-frequency chirps as courtship signals; whereas both sexes produce low-frequency chirps during same-sex interactions. Serotonergic innervation of the prepacemaker nucleus, which controls chirping, is more robust in females than males. Serotonin inhibits chirping and may contribute to sexual dimorphism and individual variation in chirping. We elicited chirps with EOD playbacks and pharmacologically manipulated serotonin receptors to determine which receptors regulated chirping. We also asked whether serotonin receptor activation generally modulated chirping or more specifically targeted particular chirp types. Agonists and antagonists of 5HT_1B/1D receptors (CP-94253 and GR-125743) did not affect chirping. The 5HT_1A receptor agonist 8OH-DPAT specifically increased production of high-frequency chirps. The 5HT₂ receptor agonist DOI decreased chirping. Receptor antagonists (WAY-100635 and MDL-11939) opposed the effects of their corresponding agonists. These results suggest that serotonergic inhibition of chirping may be mediated by 5HT₂ receptors, but that serotonergic activation of 5HT_1A receptors specifically increases the production of high-frequency chirps. The enhancement of chirping by 5HT_1A receptors may result from interactions with cortisol and/or arginine vasotocin, which similarly enhance chirping and are influenced by 5HT_1A activity in other systems.     

Dynamics and stimulus-dependence of pacemaker control during behavioral modulations in the weakly electric fish,Apteronotus

1. Weakly electric fish generate around their bodies low-amplitude, AC electric fields which are used both for the detection of objects and intraspecific communication. The types of modulation in this signal of which the high-frequency wave-type gymnotiform, Apteronotus, is capable are relatively few and stereotyped. Chief among these is the chirp, a signal used in courtship and agonistic displays. Chirps are brief and rapid accelerations in the normally highly regular electric organ discharge (EOD) frequency. 2. Chirping can be elicited artificially in these animals by the use of a stimulus regime identical to that typically used to elicit another behavior, the jamming avoidance response (JAR). The neuronal basis for the JAR, a much slower and lesser alteration in EOD frequency, is well understood. Examination of the stimulus features which induce chirping show that, like the JAR, there is a region of frequency differences between the fish's EOD and the interfering signal that maximally elicits the response. Moreover, the response is sex-specific with regard to the sign of the frequency difference, with females chirping preferentially on the positive and most males on the negative Df. These features imply that the sensory mechanisms involved in the triggering of these communicatory behaviors are fundamentally similar to those explicated for the JAR. 3. Additionally, two other modulatory behaviors of unknown significance are described. The first is a non-selective rise in EOD frequency associated with a JAR stimulus, occurring regardless of the sign of the Df. This modulation shares many characteristics with the JAR. The second behavior, which we have termed a 'yodel', is distinct from and kinetically intermediate to chirping and the JAR. Moreover, unlike the other studied electromotor behaviors it is generally produced only after the termination of the eliciting stimulus.     

Intracellular recording in the medullary pacemaker nucleus of the weakly electric fish,Apteronotus, during modulatory behaviors

1. The weakly electric gymnotiform fish, Apteronotus leptorhynchus, can be induced to perform a variety of modulations of its quasi-sinusoidal, electric organ discharge (EOD) in acute physiological preparations. These modulations, many of which are communicatory in function, include the jamming avoidance response (JAR). We have recorded intracellularly from neurons of the medullary pacemaker nucleus which is responsible for maintaining the ongoing EOD frequency during these modulatory behaviors. 2. We have used dye-filled microelectrodes to characterize single cell morphology of the two types of cells in the pacemaker nucleus (relay and pacemaker cells) and to localize anatomically the site of the differing responses we see during frequency modulations. We have also recorded with KCl-filled electrodes and attributed these data to cell type and location on the basis of characteristic behavior during these modulations. 3. Much of our data deals with chirps, brief accelerations of the EOD frequency lasting 10 to 14 ms. We see distinct patterns of activity in the pacemaker nucleus corresponding to different anatomical locations: the relay cell soma and axon, and the pacemaker cell soma and axon. Most of these loci show a marked rise in baseline voltage during the acceleration in spike frequency. The most unusual of these is the pacemaker cell axon which displays an often extreme decline in spike amplitude concurrent with the chirp (Fig. 7A). 4. 'Yodeling' (Dye 1987) appears to involve similar, characteristic changes in the pattern of firing as those seen during chirping. Similar quantitative analyses suggest that the JAR involves a different mechanism, however.     

The electric organ discharges of the gymnotiform fishes: I. Apteronotus leptorhynchus

We present high temporal and spatial resolution maps in 3-dimensions of the electric field vector generated by the weakly electric fish, Apteronotus leptorhynchus. The waveforms and harmonic composition of the electric organ discharge (EOD) are variable around the fish but highly stable over long times at any position. We examine the role of harmonics on the temporal and spatial characteristics of the EOD, such as the slew rate and rostral-to-caudal propagation. We also explore the radial symmetry of the fish's field. There are major differences in the direction of the electric field vector at the head and caudal body. In the caudal part of the fish, the electric field vector rotates during the EOD cycle. However, rostral of the pectoral fin, the field magnitude and sign oscillate while maintaining relatively constant orientation. We discuss possible functional ramifications of these electric field patterns to electrolocation, communication, and electrogenesis.Abbreviations EOD electric organ discharge - EO electric organ - RMS root mean square - ADC analog-to-digital converter     

Cutaneous electrical oscillation in a weakly electric fish, Gymnarchus niloticus

An African electric fish, Gymnarchus niloticus. ceases its electric organ discharge for a prolonged time in response to external electrical signals. During the cessation of electric organ discharges from the electric organ, a weak sinusoidal signal (approximately 0.1 mV cm(-1)) near the fish's previous discharge frequency was recorded near the body. The oscillatory potentials at all points on the body surface were synchronized and had a complex spatial distribution. The source of the potential was determined to be within the dermal tissue. Electroreceptive central neurons that responded to a moving target near the fish with normal electric organ discharges also responded to the same target when the electric organ discharge was interrupted and the potential from the skin existed. This result suggests that the fish may be able to electrolocate objects without the discharge from the electric organ.