Electrocommunication signals alone are sufficient to increase neurogenesis in the brain of adult electric fish, Apteronotus leptorhynchus

Social interaction can have profound influences on the structure of the adult brain, but little is known about the precise stimulus feature found within social interaction that induces such brain plasticity. We examined the effects of social stimuli on cell addition and radial glial fiber formation in the brains of adult electric fish. These fish communicate primarily through weak, quasi-sinusoidal electric signals. Fish were housed in isolation, paired with another fish or exposed to only the electrocommunication signals of another fish for 7 days. After 3 days of exposure to these stimulus conditions, fish were injected with bromodeoxyuridine (BrdU) to mark newborn cells. We sacrificed the fish 4 days after BrdU injection and used immunohistochemistry to measure cell addition (BrdU+), the fraction of added cells that differentiated into neurons (BrdU+/NeuroTrace+) and the density of radial glia fibers (vimentin+) in the periventricular zone of the diencephalon. Fish that were exposed only to the electrocommunication signals of another fish and no other social stimuli had equivalent levels of cell addition and radial glial fiber density to fish that were housed with full social interaction and higher levels than fish housed in isolation. About 60% of the added cells differentiated into neurons; this fraction did not differ among treatment groups. Artificial sine wave electrical stimuli that mimicked electrocommunication signals were ineffective in increasing cell addition and glia fiber formation above those found in isolated fish. Thus, stimuli through a single modality are sufficient for inducing this brain plasticity, but the waveform or dynamic features of communication signals are crucial for the effect.     

Social novelty enhances brain cell proliferation,cell survival,and chirp production in an electric fish,Apteronotus leptorhynchus

For many animals, enriched environments and social interaction promote adult neurogenesis. However, in some cases, the effect is transient, and long‐term environmental stimuli have little benefit for neurogenesis. In electric fish, Apteronotus leptorhynchus, fish housed in pairs for 7 days show higher density of newborn brain cells (cell addition) than isolated fish, but fish paired for 14 days have rates of cell addition similar to isolated controls. We examined whether introduction of social novelty can sustain elevated levels of cell addition and prevent long‐term habituation to social interaction. We also monitored electrocommunication signals (“chirps”) as a measure of the behavioral response to social novelty. We paired fish for 14 days with one continuous partner (no social novelty), two sequential partners changed after 7 days (low novelty) or seven sequential partners changed every 2 days (high novelty). On Day 11, we injected fish with BrdU, sacrificed fish 3 days later and quantified BrdU labeling in the diencephalic periventricular zone. Fish exposed to no novelty had BrdU labeling similar to isolated fish. Fish with low novelty showed small increases in BrdU labeling and those with high novelty had much greater BrdU labeling. Similarly, chirp rates were greater in fish with low novelty than with no novelty and greatest yet in fish with high novelty. By varying the timing of novelty relative to BrdU injection, we showed that social novelty promoted both proliferation and survival of newborn cells. These results indicated that brain cell proliferation and survival is influenced more by social change than simply the presence of social stimuli. © 2012 Wiley Periodicals, Inc. Develop Neurobiol, 2013     

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.     

Automatic Realistic Real Time Stimulation/Recording in Weakly Electric Fish: Long Time Behavior Characterization in Freely Swimming Fish and Stimuli Discrimination

Weakly electric fish are unique model systems in neuroethology, that allow experimentalists to non-invasively, access, central nervous system generated spatio-temporal electric patterns of pulses with roles in at least 2 complex and incompletely understood abilities: electrocommunication and electrolocation. Pulse-type electric fish alter their inter pulse intervals (IPIs) according to different behavioral contexts as aggression, hiding and mating. Nevertheless, only a few behavioral studies comparing the influence of different stimuli IPIs in the fish electric response have been conducted. We developed an apparatus that allows real time automatic realistic stimulation and simultaneous recording of electric pulses in freely moving Gymnotus carapo for several days. We detected and recorded pulse timestamps independently of the fish’s position for days. A stimulus fish was mimicked by a dipole electrode that reproduced the voltage time series of real conspecific according to previously recorded timestamp sequences. We characterized fish behavior and the eletrocommunication in 2 conditions: stimulated by IPIs pre-recorded from other fish and random IPI ones. All stimuli pulses had the exact Gymontus carapo waveform. All fish presented a surprisingly long transient exploratory behavior (more than 8 h) when exposed to a new environment in the absence of electrical stimuli. Further, we also show that fish are able to discriminate between real and random stimuli distributions by changing several characteristics of their IPI distribution.     

Electric organ discharge patterns during group hunting by a mormyrid fish

Weakly electric fish emit and receive low-voltage electric organ discharges (EODs) for electrolocation and communication. Since the discovery of the electric sense, their behaviours in the wild have remained elusive owing to their nocturnal habits and the inaccessible environments in which they live. The transparency of Lake Malawi provided the first opportunity to simultaneously observe freely behaving mormyrid fish and record their EODs. We observed a piscivorous mormyrid, Mormyrops anguilloides, hunting in small groups in Lake Malawi while feeding on rock-frequenting cichlids of the largest known vertebrate species flock. Video recordings yielded the novel and unexpected finding that these groups resembled hunting packs by being largely composed of the same individuals across days. We show that EOD accelerations accompany prey probing and size estimation by M. anguilloides. In addition, group members occasionally synchronize bursts of EODs with an extraordinary degree of precision afforded by the mormyrid echo response. The characteristics and context of burst synchronization suggest that it may function as a pack cohesion signal. Our observations highlight the potential richness of social behaviours in a basal vertebrate lineage, and provide a framework for future investigations of the neural mechanisms, behavioural rules and ecological significance of social predation in M. anguilloides.     

Production of aggressive electrocommunication signals to progressively realistic social stimuli in male Apteronotus leptorhynchus

Brown ghost knife fish, Apteronotus leptorhynchus, produce a continuous electric organ discharge (EOD) that they use for communication. While interacting aggressively, males also emit brief EOD modulations termed chirps. The simplicity of this behaior and its underlying neural circuitry has made it an important model system in neuroethology. Chirping is typically assayed by confining a fish in a tube (‘chirp chamber’) and presenting it with sine wave electrical stimuli that partially mimic EODs of other fish. We presented male fish with progressively more realistic social stimuli to examine whether some of the stimulus complexities during dyadic interaction influence the production of chirps. In a chirp chamber, fish chirped less to a recording of an EOD containing chirps than to a recording of an EOD alone and to sine wave stimuli. Free‐swimming fish chirped more to stimulus fish than to sine wave stimuli presented through electrodes. Fish chirped more when interacting directly than when interacting across a perforated barrier. Together, these studies demonstrate that the presence of chirps, electric field complexity, and/or non‐electric social stimuli are important in eliciting chirp production in brown ghosts.     

Electrocommunication and Social Behaviour in Marcusenius senegalensis (Mormyridae,Teleostei)

The electric organ discharges (EODs) of Marcusenius senegalensis, a West African freshwater fish, are bipolar pulses of short duration (220 ± SE 13 μs). In males (n = 10; 10.1–13.1 cm standard length — which is around the size of getting mature), the duration of EOD pulses was of significantly greater variance than in females (n = 9; 9.8–12.8 cm standard length). Male EODs also showed a tendency for a longer duration than female EODs. Groups of three as well as of 14 M. senegalensis formed temporary schools in a ‘naturally’ equipped 720-1 tank. While swimming slowly in a loose school during their nocturnal active phase, fish discharged in irregular long-short-long inter-EOD interval patterns. Near neighbours displayed a tendency to discharge in intervals of similar duration (nearest neighbour distance < 1/2 fish length). On removal of a plastic partition that had separated a pair of fish for at least 3 days, mutual threat displays followed by fighting were observed. During threatening, the fish alternated regularly between bursts of a high discharge rate and short discharge breaks; the rate of change was 4/s. The subdominant animal in a group of two was attacked frequently and often ceased discharging when the dominant fish approached. Courtship behaviour involving gonadally mature fish was accompanied by high-discharge-rate displays with intervals of constant duration in both fish, and the reciprocal display of ‘preferred’ EOD latencies in the 12 ms range. The results demonstrate electric communication by distinct inter-discharge interval patterns in the social behaviour of this mormyrid fish.     

Spatial acuity and prey detection in weakly electric fish

It is well-known that weakly electric fish can exhibit extreme temporal acuity at the behavioral level, discriminating time intervals in the submicrosecond range. However, relatively little is known about the spatial acuity of the electrosense. Here we use a recently developed model of the electric field generated by Apteronotus leptorhynchus to study spatial acuity and small signal extraction. We show that the quality of sensory information available on the lateral body surface is highest for objects close to the fish's midbody, suggesting that spatial acuity should be highest at this location. Overall, however, this information is relatively blurry and the electrosense exhibits relatively poor acuity. Despite this apparent limitation, weakly electric fish are able to extract the minute signals generated by small prey, even in the presence of large background signals. In fact, we show that the fish's poor spatial acuity may actually enhance prey detection under some conditions. This occurs because the electric image produced by a spatially dense background is relatively “blurred” or spatially uniform. Hence, the small spatially localized prey signal “pops out” when fish motion is simulated. This shows explicitly how the back-and-forth swimming, characteristic of these fish, can be used to generate motion cues that, as in other animals, assist in the extraction of sensory information when signal-to-noise ratios are low. Our study also reveals the importance of the structure of complex electrosensory backgrounds. Whereas large-object spacing is favorable for discriminating the individual elements of a scene, small spacing can increase the fish's ability to resolve a single target object against this background.     

EOD modulations of brown ghost electric fish: JARs, chirps, rises, and dips.

Weakly electric "wave" fish make highly regular electric organ discharges (EODs) for precise electrolocation. Yet, they modulate the ongoing rhythmicity of their EOD during social interactions. These modulations may last from a few milliseconds to tens of minutes. In this paper we describe the different types of EOD modulations, what they may signal to recipient fish, and how they are generated on a neural level. Our main conclusions, based on a species called the brown ghost (Apteronotus leptorhynchus) are that fish: (1) show sexual dimorphism in the signals that they generate; (2) make different signals depending on Whether they are interacting with a fish of the opposite sex or, within their own sex, to a fish of that which is dominant or subordinate to it; (3) are able to assess relative dominance from electrical cues; (4) have a type of plasticity in the pacemaker nucleus, the control center for the EOD, that occurs after stimulation of NMDA receptors that causes a long-lasting (tens of minutes to hours) change in EOD frequency; (5) that this NMDA receptor-dependent change may occur in reflexive responses, like the jamming avoidance response (JAR), as well as after certain long-lasting social signals. We propose that NMDA-receptor dependent increases in EOD frequency during the JAR adaptively shift the EOD frequency to a new value to avoid jamming by another fish and that such increases in EOD frequency during social encounters may be advantageous since social dominance seems to be positively correlated with EOD frequency in both sexes.     

Insight into the mechanisms of neuronal processing from electric fish

Weakly electric fish use their electric fields to locate objects and communicate with each other. Their electric discharges vary with species, gender, and social status. This variation is mediated by steroid and peptide hormones that influence ion currents through changes in gene expression or phosphorylation state. Understanding how electric fish decode the perturbations of their electric fields that result from interactions with the discharges of other fish or prey is illuminating general mechanisms of neuronal processing. Their central sensory circuits are specialized to process amplitude modulated signals, to detect microsecond variations in spike timing, and are dynamically reconfigured depending on the stimulus parameters.     

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.     

Circadian and Social Cues Regulate Ion Channel Trafficking

Electric fish generate and sense electric fields for navigation and communication. These signals can be energetically costly to produce and can attract electroreceptive predators. To minimize costs, some nocturnally active electric fish rapidly boost the power of their signals only at times of high social activity, either as night approaches or in response to social encounters. Here we show that the gymnotiform electric fish Sternopygus macrurus rapidly boosts signal amplitude by 40% at night and during social encounters. S. macrurus increases signal magnitude through the rapid and selective trafficking of voltage-gated sodium channels into the excitable membranes of its electrogenic cells, a process under the control of pituitary peptide hormones and intracellular second-messenger pathways. S. macrurus thus maintains a circadian rhythm in signal amplitude and adapts within minutes to environmental events by increasing signal amplitude through the rapid trafficking of ion channels, a process that directly modifies an ongoing behavior in real time.     

Social spacing in the mormyrid fish Gnathonemus petersii (Pisces): A multisensory approach

The sensory basis of group cohesion in the weak-electric fish Gnathonemus petersii was investigated in a circular tank with groups of four fish each, interacting through a wide-meshed plastic screen with intact or operated conspecifics, or with other stimulus objects. We confined these stimuli to one or two peripheral holding compartments. The response measures were obtained from the free swimming fish and included (1) the time the fish spent together as a group, (2) the time they spent in front of the holding compartments, (3) the circular distribution of the fish's positions, and (4) the mean nearest neighbour distances. Under empty compartment conditions, four different groups were tested, consisting of either (1) intact, electrically active fish, or (2) electrically ‘silent’ fish (with their electric organ surgically rendered inoperative), or (3) blind, or (4) ‘silent’ and blind animals. The loss of either sensory modality, vision or feedback from electric organ discharge, led to changes of comparable size, decreasing the time spent as a group and increasing the mean nearest neighbour distance. In fish lacking both modalities, group cohesion was further impaired. With stimuli present in one or both holding compartments, the strength of social attraction depended on the nature of the stimulus: the more intact stimulus conspecifics were present, the more densely did the fish group in front of the stimulus compartment. ‘Wired-in’ electric organ discharges (simulating waveform and intensity) and electrically ‘silent’ fish were equally attractive, but only half as attractive as intact fish. Blind free swimming fish aggregated with intact and also with ‘silent’ conspecifics. Under dim light conditions, group cohesion was predominantly, though not exclusively, affected by electrosensory feedback from the electric organ discharge and visual input. Mechanical and olfactory cues may also be involved.     

The impact of social status on the erythrocyte beta-adrenergic response in rainbow trout, Oncorhynchus mykiss

The objective of the present investigation was to determine whether chronic increases in circulating cortisol concentrations, resulting from the occupation of subordinate status in rainbow trout social hierarchies, resulted in an enhancement of the erythrocyte adrenergic response. Rainbow trout (Oncorhynchus mykiss) were confined in fork length matched pairs for 6 h, 18 h, 48 h or 5-7 days, and social status was assigned through observations of behaviour. Erythrocyte adrenergic responsiveness, determined in vitro as changes in water content following incubation with the beta-adrenoreceptor agonist isoproterenol, was significantly greater in subordinate than dominant fish at 48 h of social interactions but not after 5-7 days, nor when assessed as changes in extracellular pH (pHe). However, the activity of the Na+/H+ exchanger (beta-NHE), assessed in vitro as the pHe change following incubation with the permeable cyclic AMP analogue 8-bromo-cyclic AMP, was significantly lower in subordinate fish. The number of erythrocyte membrane-bound adrenergic receptors (Bmax) was significantly higher in subordinate than dominant fish at 48 h, but had decreased by 5-7 days to a value that was not significantly different from that for dominant fish. The apparent dissociation constant (KD) of these receptors was not significantly impacted by either social status or interaction time. Finally, the relative expressions of beta-3b adrenergic receptor (AR) and beta-NHE mRNA were determined using real-time PCR and were found to be minimally affected by social rank. Relative to a control group, beta-3b AR mRNA was significantly up-regulated in both dominant and subordinate trout at all time periods, whereas the expression of beta-NHE was in general significantly down-regulated. Unlike the situation in rainbow trout treated with exogenous cortisol, elevations in circulating cortisol resulting from low social status did not "pre-adapt" the erythrocyte adrenergic response, but rather may have served to offset the potentially adverse effects elicited by plasma catecholamines, which were elevated during social hierarchy formation.     

Effects of social isolation on growth,stress response,and immunity of zebrafish

Stressful housing conditions like social isolation have been shown to profoundly affect the physiology and health of various organisms which is rarely addressed in fish species. In the present study, we used a shoaling species, zebrafish, to investigate the stress reactivity of grouped and individually housed fish. We also hypothesized if isolation is a stressful condition may disrupt growth performance and innate immune response of individuals. To this end, fish were housed individually (social isolated treatment) or in groups of five fish (control treatment) for 60 days. Growth indices of fish were not affected by social isolation. Sixty-day social isolation did insignificant effect on baseline cortisol levels of specimens; however, individually housed zebrafish showed lower plasma cortisol to chasing stress than the control grouped fish. On the contrary, exposure to predator caused higher cortisol levels in social isolated fish. Serum lysozyme activity of isolated individuals was significantly lower than control fish, but activity of serum complement remained unchanged. Our results represent evidences that zebrafish experienced social isolation showed broad changes in physiological and immunological functions which may affect the quality of life.     

Communication by harmonization of electric organ discharge frequencies by Eigenmannia virescens (Sternopygidae, Pisces)

Spectrum analysis of the combined electric organ discharges of confined, interacting Eigenmannia virescens individuals indicated 1) the relative frequency position of each individual in the group's "chord" was invariably conserved, i.e. the fish do not cross each other in frequency; 2) the ratios of 2:3 and 3:4 between the frequencies of two individuals were conserved over intervals of many days; 3) changes in group size induced frequency shifts that were often quite dramatic. It is suggested that this "chord" may function in the communication of information regarding a group's spatial position and its social rank relations.     

Species-specific differences in sensorimotor adaptation are correlated with differences in social structure

Here, we report a species difference in the strength and duration of long-term sensorimotor adaptation in the electromotor output of weakly electric fish. The adaptation is produced by changes in intrinsic excitability in the electromotor pacemaker nucleus; this change is a form of memory that correlates with social structure. A weakly electric fish may be jammed by a similar electric organ discharge (EOD) frequency of another fish and prevents jamming by transiently raising its own emission frequency, a behavior called the jamming avoidance response (JAR). The JAR requires activation of NMDA receptors, and prolonged JAR performance results in long-term frequency elevation (LTFE) of a fish’s EOD frequency for many hours after the jamming stimulus. We find that LTFE is stronger in a shoaling species (Eigenmannia virescens) with a higher probability of encountering jamming conspecifics, when compared to a solitary species (Apteronotus leptorhynchus). Additionally, LTFE persists in Eigenmannia, whereas, it decays over 5–9 h in Apteronotus.     

Electric Organ Discharge Displays during Social Encounter in the Weakly Electric Fish Brienomyrus niger L. (Mormyridae)

We investigated the electric organ discharge (EOD) activity of the mormyrid fish Brienomyrus niger during social encounters. The fish were contained in porous ceramic shelters and tested alone and in pairs in an experimental tank designed to restrict communication to the electrosensory modality. We moved one fish toward and away from a stationary conspecific, beginning at a distance known to be outside the range of communication (250 cm). Baseline EOD activity was recorded prior to interaction and categorized as ‘variable’, ‘regular’, and ‘scallop’. When moved closer together, the fish modulated this baseline activity in four ways: (1) At 100–130 cm apart, the stationary fish emitted a maximum of sudden EOD rate increases which defined the outer limit of its communication range. (The associated Electric Field Gradient was 1 μV/cm). (2) Long EOD cessations, which we called social silence, lasted from 5–130 s and occurred most frequently when the fish were 36 to 55 cm apart (EFG: 100 μV/cm). The duration of social silence was negatively correlated (r = ? 0.862) with the responding fish's size, and was independent of the partner's sex and size. Fish whose EOD baseline pattern was ‘scallop’ were least likely to fall electrically silent, and those that were categorized as ‘regular’ or ‘variable’ were most likely to cease discharging. (3) Within electrolocation range, fish ‘regularized’ their EOD activity while the partner was ‘silent’ (EFG: 1 mV/cm). (4) Following long EOD cessations the fish resumed discharging with characteristic EOD rebound patterns. The possible ethological significance of these findings is discussed.     

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