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     

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.     

Ontogeny of the electric organ discharge and the electric organ in the weakly electric pulse fish Brachyhypopomus pinnicaudatus (Hypopomidae, Gymnotiformes)

I recorded the electric organ discharges (EODs) of 331 immature Brachyhypopomus pinnicaudatus 6–88 mm long. Larvae produced head-positive pulses 1.3 ms long at 7 mm (6 days) and added a second, small head-negative phase at 12 mm. Both phases shortened duration and increased amplitude during growth. Relative to the whole EOD, the negative phase increased duration until 22 mm and amplitude until 37 mm. Fish above 37 mm produced a “symmetric” EOD like that of adult females. I stained cleared fish with Sudan black, or fluorescently labeled serial sections with anti-desmin (electric organ) or anti-myosin (muscle). From day 6 onward, a single electric organ was found at the ventral margin of the hypaxial muscle. Electrocytes were initially cylindrical, overlapping, and stalk-less, but later shortened along the rostrocaudal axis, separated into rows, and formed caudal stalks. This differentiation started in the posterior electric organ in 12-mm fish and was complete in the anterior region of fish with “symmetric” EODs. The lack of a distinct “larval” electric organ in this pulse-type species weakens the hypothesis that all gymnotiforms develop both a temporary (larval) and a permanent (adult) electric organ. Accepted: 1 March 1997     

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     

The effects of simple objects on the electric field of Apteronotus

How might electric fish determine, from patterns of transdermal voltage changes, the size, shape, location, and impedance of a nearby object? I have investigated this question by measuring and simulating electric images of spheres and ellipsoids near an Apteronotus leptorhynchus. Previous studies have shown that this fish's electric field magnitude, and perturbations of the field due to objects, are complicated nonliner functions of distance from the fish. These functions become much simpler when distance is measured from the axes of symmetry of the fish and the object, instead of their respective edges. My analysis suggests the following characteristics of high frequency electric sense and electric images. 1. The shape of electric images on the fish's body is relatively independent of a spherical object's radius, conductivity, and rostrocaudal location. 2. An image's relative width increases linearly with lateral distance, and might therefore unambiguously encode object distance. 3. Only objects with very large dielectric constants cause appreciable phase shifts, and the degree of shift depends strongly on water conductivity. 4. Several parameters, such as the range of electric sense, may depend on the rostrocaudal location of an object. Large objects may be detectable further from the head than the tail, and conversely, small objects may be detectable further from the tail than head. 5. Asymmetrical objects produce different electric images, correlated with their cross-sections, for different orientations and phases of the electric field. 6. The steep attenuation with distance of the field magnitude causes spatial distortions in electric images, somewhat analogous to the perspective distortion inherent in wide angle optical lenses.     

Peripheral electrosensory imaging by weakly electric fish

Different species have developed different solutions to the problem of constructing a representation of the environment from sensory images projected onto sensory surfaces. Comprehension of how these images are formed is an essential first step in understanding the representation of external reality by a given sensory system. Modeling of the electrical sensory images of objects began with the discovery of electroreception and continues to provide general insights into the mechanisms of imaging. Progress in electric image research has made it possible to establish the physical basis of electric imaging, as well as methods to accurately predict the electric images of objects alone and as a part of a natural electric scene. In this review, we show the following. (1) The internal low resistance of the fish’s body shapes the image in two different ways: by funneling the current generated by the electric organ to the sensory surface, it increases the fields rostrally, thus enhancing the perturbation produced by nearby objects; and by increasing the projected image. (2) The electric fish’s self-generated currents are modified by capacitive objects in a distinctive manner. These modulations can be detected by different receptor types, yielding the possibility of “electric color.” (3) The effects of different objects in a scene interact with each other, generating an image that is different from the simple addition of the images of individual objects, thus causing strong contextual effects.     

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.     

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.     

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.     

Mauthner cell-initiated abrupt increase of the electric organ discharge in the weakly electric fishGymnotus carapo

Stimulation of the spinal cord of the electric fish Gymnotus carapo, evoked an abrupt increase in the discharge rate of the electric organ. At the maximum of this response, the rate increased an average of 26 ± 11.8%. The duration of the response was 4.9 ± 2.12 s; its latency was 10.4 ± 1.1 ms. Activation of the Mauthner axon played a decisive role in this phenomenon as indicated by the following: (1) recordings from the axon cap of the Mauthner cell demonstrated that the response was evoked if the Mauthner axon was antidromically activated and (2) a response that was similar to that produced by spinal cord stimulation, was elicited by intracellular stimulation of either Mauthner cell. Stimulation of the eighth nerve could also increase the discharge rate of the electric organ. The effect was greater if a Mauthner cell action potential was elicited. The findings described in the present report, indicate the existence of a functional connection between the Mauthner cell and the electromotor system in Gymnotus carapo. This connection may function to enhance the electrolocative sampling of the environment during Mauthner-cell mediated behaviors. This is a novel function for the Mauthner cell.Abbreviations EHP extrinsic hyperpolarizing potential - EOD electric organ discharge - M-AIR Mauthner initiated abrupt increase in rate - M-cell Mauthner cell - M-axon Mauthner axon - PM pacemaker nucleus - PM-cell pacemaker cell - PPn prepacemaker nucleus - SPPn sublemniscal prepacemaker nucleus     

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.     

Plasticity of the electric organ discharge waveform of the electric fish Brachyhypopomus pinnicaudatus I. Quantification of day-night changes

The electric organ discharge of the gymnotiform fish Brachyhypopomus pinnicaudatus is a biphasic waveform. The female's electric organ discharge is nearly symmetric but males produce a longer second phase than first phase. In this study, infrared-sensitive video cameras monitored the position of unrestrained fish, facilitating precise measurement of electric organ discharge duration and amplitude every 2 h for 24 h. Males (n=27) increased electric organ discharge duration by 37 ± 12% and amplitude by 24 ± 9% at night and decreased it during the day. In contrast, females (n=8) exhibited only minor electric organ discharge variation over time. Most of a male's increase occurred rapidly within the first 2–3 h of darkness. Electric organ discharge values gradually diminished during the second half of the dark period and into the next morning. Modulation of the second phase of the biphasic electric organ discharge produced most of the duration change in males, but both phases changed amplitude by similar amounts. Turning the lights off at mid-day triggered an immediate increase in electric organ discharge, suggesting modification of existing ion channels in the electric organ, rather than altered genomic expression. Exaggeration of electric organ discharge sex differences implies a social function. Daily reduction of duration and amplitude may reduce predation risk or energy expenditure. Accepted: 12 September 1998     

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.     

Social interaction and cortisol treatment increase cell addition and radial glia fiber density in the diencephalic periventricular zone of adult electric fish, Apteronotus leptorhynchus

In electric fish, Apteronotus leptorhynchus, both long-term social interaction and cortisol treatment potentiates chirping, an electrocommunication behavior that functions in aggression. Chirping is controlled by the diencephalic prepacemaker nucleus (PPn-C) located just lateral to the ventricle. Cells born in adult proliferative zones such as the periventricular zone (PVZ) can migrate along radial glial fibers to other brain regions, including the PPn-C. We examined whether social interactions or cortisol treatment influenced cell addition and radial glia fiber formation by (1) pairing fish (4 or 7 days) or (2) implanting fish with cortisol (7 or 14 days). Adult fish were injected with bromodeoxyuridine 3 days before sacrifice to mark cells that were recently added. Other fish were sacrificed after 1 or 7 days of treatment to examine vimentin immunoreactivity (IR), a measure of radial glial fiber density. Paired fish had more cell addition than isolated fish at 7 days, coinciding temporally with the onset of socially induced increase in chirping behavior. Paired fish also had higher vimentin IR at 1 and 7 days. For both cell addition and vimentin IR, the effect was regionally specific, increasing in the PVZ adjacent to the PPn-C, but not in surrounding regions. Cortisol increased cell addition at 7 days, correlating with the onset of cortisol-induced changes in chirping, and in a regionally specific manner. Cortisol for 14 days increased cell addition, and cortisol for 7 days increased vimentin IR but in a regionally non-specific manner. The correlation between treatment-induced changes in chirping and regionally specific increases in cell addition, and radial glial fiber formation suggests a causal relationship between such behavioral and brain plasticity in adults, but this hypothesis will require further testing.     

Mauthner cell-evoked synaptic actions on pacemaker medullary neurons of a weakly electric fish

The present study was designed to examine the synaptic events in neurons of the pacemaker nucleus of Gymnotus carapo during the increase in rate of the electric organ discharge following activation of Mauthner cells. Pacemaker and relay cells were investigated using intracellular recordings which were performed under two different conditions: (1) with the pacemaker nucleus spontaneously discharging and (2) after its activity was abolished by anesthesia. Mauthner axon activation induced an increase in the rate of pacemaker cell discharges. This response was accompanied by an increase in the slope of the pacemaker potential (up to 110%) and a depolarization of these cells. The discharges of relay cells followed one to one those of pacemaker cells. In contrast to that observed in pacemaker cells, only brief depolarizing antidromic effects could be evoked in relay cells after Mauthner axon activation. In quiescent pacemaker cells, Mauthner cell activation induced a prolonged (up to 500 ms) depolarizing potential with an average amplitude of 1.92 ± 0.82 mV; its latency was 4.43 ± 1.14 ms. Our data indicate that, within the pacemaker nucleus, the population of pacemaker cells is the only target for Mauthner cell-evoked, short-latency excitatory synaptic actions. Accepted: 1 March 1997     

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 500–1000 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 κA‐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 μO‐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. © 2005 Wiley Periodicals, Inc. J Neurobiol, 2006     

Discrimination of objects through electrolocation in the weakly electric fish,Gnathonemus petersii

Summary Three weakly electric fish (Gnathonemus petersii) were force-choice trained in a two-alternative procedure to discriminate between objects differing in their electrical characteristics. The objects were carbon dipoles in plexiglass tubing (length 2.5 cm, diameter 0.6 cm). Their electrical characteristics could be changed by varying the impedance of an external circuit to which they were connected (Fig. 1). In one (the capacitance dipole) the resistance was very low(< 3 ) and the capcitance variable. In the other (the resistance dipole) the resistance was variable and the capacitance low (<50 pF).Capacitances from several hundred pF (lower thresholds, Fig. 2) to several hundred nF (upper thresholds, Fig. 3) could be discriminated from both insulators and good conductors. In all cases the reward-negative stimulus was the capacitance dipole, which was avoided by all fish spontaneously. Thresholds were defined at 70% correct choices.The fish were then tested for their ability to discriminate between one object with a given capacitance and another with resistances varying from 3 to 200 k. The capacitance dipole continued to be the negative stimulus throughout. All 3 fish avoided it in at least 80% of the trials at each stimulus combination (Fig. 4). This result suggests that Gnathonemus perceives the capacitance and the resistance of objects differentially.The effect of the dipole-objects as well as some natural objects on the local EOD was recorded differentially very close to the fish's skin (Fig. 5). The amplitude of the local EODs was affected by all types of objects as they approached the skin. However, the waveform was changed only by capacitance dipoles and some natural objects (Figs. 6 and 7). It appears that the fish perceive not only intensity changes in the local EOD but wave-form deformations as well and can thus distinguish objects of different complex impedances.Abbreviations EOD electric organ discharge - f _max maximal spectral frequency - GP Gnathonemus petersii - LFS local filtered signal - PMA probing motor act - S+ positive stimulus - S negative stimulus     

Time coding in the midbrain of mormyrid electric fish. II. Stimulus selectivity in the nucleus exterolateralis pars posterior

The anterior and posterior exterolateral nuclei (ELa and ELp) of the mormyrid midbrain are thought to play a critical role in the temporal analysis of the electric discharge waveforms of other individuals. The peripheral electroreceptors receiving electric organ discharges (EODs) of other fish project through the brainstem to ELa via a rapid conducting pathway. EODs, composed of brief, but stereotyped waveforms are encoded as a temporal pattern of spikes. From previous work, we know that phase locking is precise in ELa. Here it is shown that evoked potentials recorded from ELp show a similar high degree of phase locking, although the evoked potentials last much longer. Single-unit recordings in ELp reveal two distinct populations of neurons in ELp: type I cells are responsive to voltage step functions, and not tuned for stimulus duration; type II cells are tuned to a specific range of stimulus durations. Type II cells are less responsive than type I cells, tend to respond with bursts of action potentials rather than with single spikes, have a longer latency, show weaker time locking to stimuli, and are more sensitive to stimulus polarity and amplitude. The stimulus selectivity of type II cells may arise from convergence of type I cell inputs. Despite the loss of rapid conduction between ELa and ELp, analysis of temporal features of waveforms evidently continues in ELp, perhaps through a system of labeled lines. Accepted: 25 June 1997     

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.