Diversity in the structure of electrocommunication signals within a genus of electric fish, Apteronotus

Why the brown ghost chirps at night

Since the pioneering work by Moeller, Szabo, and Bullock, weakly electric fish have served as a valuable model for investigating spatial and social cognitive abilities in a vertebrate taxon usually less accessible than mammals or other terrestrial vertebrates. These fish, through their electric organ, generate low-intensity electric fields to navigate and interact with conspecifics, even in complete darkness. The brown ghost knifefish is appealing as a study subject due to a rich electric 'vocabulary', made by individually variable and sex-specific electric signals. These are mainly characterized by brief frequency modulations of the oscillating dipole moment continuously generated by their electric organ, and are known as chirps. Different types of chirps are believed to convey specific and behaviorally salient information, serving as behavioral readouts for different internal states during behavioral observations. Despite the success of this model in neuroethology over the past seven decades, the code to decipher their electric communication remains unknown. To this aim, in this study we re-evaluate the correlations between signals and behavior offering an alternative, and possibly complementary, explanation for why these freshwater bottom dwellers emit electric chirps. By uncovering correlations among chirping, electric field geometry, and detectability in enriched environments, we present evidence for a previously unexplored role of chirps as specialized self-directed signals, enhancing conspecific electrolocation during social encounters.

Supervised learning algorithm for analysis of communication signals in the weakly electric fish Apteronotus leptorhynchus

Signal analysis plays a preeminent role in neuroethological research. Traditionally, signal identification has been based on pre-defined signal (sub-)types, thus being subject to the investigator's bias. To address this deficiency, we have developed a supervised learning algorithm for the detection of subtypes of chirps-frequency/amplitude modulations of the electric organ discharge that are generated predominantly during electric interactions of individuals of the weakly electric fish Apteronotus leptorhynchus. This machine learning paradigm can learn, from a 'ground truth' data set, a function that assigns proper outputs (here: time instances of chirps and associated chirp types) to inputs (here: time-series frequency and amplitude data). By employing this artificial intelligence approach, we have validated previous classifications of chirps into different types and shown that further differentiation into subtypes is possible. This demonstration of its superiority compared to traditional methods might serve as proof-of-principle of the suitability of the supervised machine learning paradigm for a broad range of signals to be analyzed in neuroethology.

Mixed selectivity coding of sensory and motor social signals in the thalamus of a weakly electric fish

High-level neural activity often exhibits mixed selectivity to multivariate signals. How such representations arise and modulate natural behavior is poorly understood. We addressed this question in weakly electric fish, whose social behavior is relatively low dimensional and can be easily reproduced in the laboratory. We report that the preglomerular complex, a thalamic region exclusively connecting midbrain with pallium, implements a mixed selectivity strategy to encode interactions related to courtship and rivalry. We discuss how this code enables the pallial recurrent networks to control social behavior, including dominance in male-male competition and female mate selection. Notably, response latency analysis and computational modeling suggest that corollary discharge from premotor regions is implicated in flagging outgoing communications and thereby disambiguating self- versus non-self-generated signals. These findings provide new insights into the neural substrates of social behavior, multi-dimensional neural representation, and its role in perception and decision making.

The weakly electric fish, Apteronotus albifrons, actively avoids experimentally induced hypoxia

Anthropogenic environmental degradation has led to an increase in the frequency and prevalence of aquatic hypoxia (low dissolved oxygen concentration, DO), which may affect habitat quality for water-breathing fishes. The weakly electric black ghost knifefish, Apteronotus albifrons, is typically found in well-oxygenated freshwater habitats in South America. Using a shuttle-box design, we exposed juvenile A. albifrons to a stepwise decline in DO from normoxia (> 95% air saturation) to extreme hypoxia (10% air saturation) in one compartment and chronic normoxia in the other. On average, A. albifrons actively avoided the hypoxic compartment below 22% air saturation. Hypoxia avoidance was correlated with upregulated swimming activity. Following avoidance, fish regularly ventured back briefly into deep hypoxia. Hypoxia did not affect the frequency of their electric organ discharges. Our results show that A. albifrons is able to sense hypoxia at non-lethal levels and uses active avoidance to mitigate its adverse effects.

Dynamics of a neuronal pacemaker in the weakly electric fish Apteronotus

The precise timing of neuronal activity is critical for normal brain function. In weakly electric fish, the medullary pacemaker network (PN) sets the timing for an oscillating electric organ discharge (EOD) used for electric sensing. This network is the most precise biological oscillator known, with sub-microsecond variation in oscillator period. The PN consists of two principle sets of neurons, pacemaker and relay cells, that are connected by gap junctions and normally fire in synchrony, one-to-one with each EOD cycle. However, the degree of gap junctional connectivity between these cells appears insufficient to provide the population averaging required for the observed temporal precision of the EOD. This has led to the hypothesis that individual cells themselves fire with high precision, but little is known about the oscillatory dynamics of these pacemaker cells. As a first step towards testing this hypothesis, we have developed a biophysical model of a pacemaker neuron action potential based on experimental recordings. We validated the model by comparing the changes in oscillatory dynamics produced by different experimental manipulations. Our results suggest that this relatively simple model can capture a large range of channel dynamics exhibited by pacemaker cells, and will thus provide a basis for future work on network synchrony and precision.

Electroreception, electrogenesis and electric signal evolution

Electroreception, the capacity to detect external underwater electric fields with specialised receptors, is a phylogenetically widespread sensory modality in fishes and amphibians. In passive electroreception, a capacity possessed by c. 16% of fish species, an animal uses low-frequency-tuned ampullary electroreceptors to detect microvolt-range bioelectric fields from prey, without the need to generate its own electric field. In active electroreception (electrolocation), which occurs only in the teleost lineages Mormyroidea and Gymnotiformes, an animal senses its surroundings by generating a weak (< 1 V) electric-organ discharge (EOD) and detecting distortions in the EOD-associated field using high-frequency-tuned tuberous electroreceptors. Tuberous electroreceptors also detect the EODs of neighbouring fishes, facilitating electrocommunication. Several other groups of elasmobranchs and teleosts generate weak (< 10 V) or strong (> 50 V) EODs that facilitate communication or predation, but not electrolocation. Approximately 1.5% of fish species possess electric organs. This review has two aims. First, to synthesise our knowledge of the functional biology and phylogenetic distribution of electroreception and electrogenesis in fishes, with a focus on freshwater taxa and with emphasis on the proximate (morphological, physiological and genetic) bases of EOD and electroreceptor diversity. Second, to describe the diversity, biogeography, ecology and electric signal diversity of the mormyroids and gymnotiforms and to explore the ultimate (evolutionary) bases of signal and receptor diversity in their convergent electrogenic-electrosensory systems. Four sets of potential drivers or moderators of signal diversity are discussed. First, selective forces of an abiotic (environmental) nature for optimal electrolocation and communication performance of the EOD. Second, selective forces of a biotic nature targeting the communication function of the EOD, including sexual selection, reproductive interference from syntopic heterospecifics and selection from eavesdropping predators. Third, non-adaptive drift and, finally, phylogenetic inertia, which may arise from stabilising selection for optimal signal-receptor matching.

Neural Processing of Communication Signals: The Extent of Sender-Receiver Matching Varies across Species of Apteronotus

As communication signal properties change, through genetic drift or selective pressure, the sensory systems that receive these signals must also adapt to maintain sensitivity and adaptability in an array of contexts. Shedding light on this process helps us to understand how sensory codes are tailored to specific tasks. In a species of weakly electric fish, Apteronotus albifrons, we examined the unique neurophysiological properties that support the encoding of electrosensory communication signals that the animal encounters in social exchanges. We compare our findings to the known coding properties of the closely related species Apteronotus leptorhynchus to establish how these animals differ in their ability to encode their distinctive communication signals. While there are many similarities between these two species, we found notable differences leading to relatively poor coding of the details of chirp structure occurring on high-frequency background beats. As a result, small differences in chirp properties are poorly resolved by the nervous system. We performed behavioral tests to relate A. albifrons chirp coding strategies to its use of chirps during social encounters. Our results suggest that A. albifrons does not exchange frequent chirps in a nonbreeding condition, particularly when the beat frequency is high. These findings parallel the mediocre chirp coding accuracy in that they both point to a reduced reliance on frequent and rich exchange of information through chirps during these social interactions. Therefore, our study suggests that neural coding strategies in the CNS vary across species in a way that parallels the behavioral use of the sensory signals.

Co-adaptation of electric organ discharges and chirps in South American ghost knifefishes (Apteronotidae)

Animal communication signals that simultaneously share the same sensory channel are likely to co-evolve to maximize the transmission of each signal component. Weakly electric fish continuously produce a weak electric field that functions in communication. Fish modulate the electric organ discharge (EOD) on short timescales to produce context-specific signals called chirps. EODs and chirps are simultaneously detected by electroreceptors and processed in the electrosensory system. We analyzed these signals, first to explore whether EOD waveform is encoded in the signal received by electroreceptors and then to examine how EODs and chirps interact to influence conspicuousness. Our findings show that gross discrimination of sinusoidal from complex EOD waveforms is feasible for all species, but fine discrimination of waveform may be possible only for species with waveforms of intermediate complexity. The degree of chirp frequency modulation and chirp relative decay strongly influenced chirp conspicuousness, but other chirp parameters were less influential. The frequency difference between the interacting EODs also strongly impacted chirp conspicuousness. Finally, we developed a method for creating hybrid chirp/EOD combinations to independently analyze the impact of chirp species, EOD species, and EOD difference frequency on chirp conspicuousness. All three components and their interactions strongly influenced chirp conspicuousness, which suggests that evolutionary changes in parameters of either chirps or EODs are likely to influence chirp detection. Examining other environmental factors such as noise created by fish movement and species-typical patterns of sociality may enrich our understanding of how interacting EODs affect the detection and discrimination of chirps across species.

Evolution and hormonal regulation of sex differences in the electrocommunication behavior of ghost knifefishes (Apteronotidae)

The ghost knifefishes (family Apteronotidae) are one of the most successful and diverse families of electric fish. Like other weakly electric fish, apteronotids produce electric organ discharges (EODs) that function in electrolocation and communication. This review highlights the diversity in the structure, function and sexual dimorphism of electrocommunication signals within and across apteronotid species. EOD frequency (EODf) and waveform vary as a function of species, sex and/or social rank. Sex differences in EODf are evolutionarily labile; apteronotid species express every pattern of sexual dimorphism in EODf (males>females; males<females; males=females). The direction and magnitude of sex differences in EODf are correlated across species and populations with the responsiveness of EODf to androgens and/or estrogens, which suggests that sex differences evolve through gains and/or losses of hormone sensitivity. During social interactions, apteronotids also modulate their EODs to produce motivational signals known as chirps. Chirp structure differs markedly across species, and many species produce two or more discrete chirp types with potentially different functions. The structure of chirps is sexually dimorphic in all apteronotid species, and chirping is influenced by gonadal steroids and by neuromodulators. Encoding of chirps by the electrosensory system depends on the social context created by the interactions of the EODs of signalers and receivers. Electrosensory systems may thus influence the evolution of signal structure and function, and neuromodulators may coordinately shape the production and reception of electrocommunication signals depending on social context.

Divergence in androgen sensitivity contributes to population differences in sexual dimorphism of electrocommunication behavior

Weakly-electric fish (Apteronotidae) produce highly diverse electrocommunication signals. Electric organ discharges (EODs) vary across species, sexes, and in the magnitude and direction of their sexual dimorphism. Gonadal steroid hormones can modulate EODs, and differences in androgen sensitivity are hypothesized to underlie variation in the degree of sexual dimorphism across species. In this study, we asked whether variation in androgen sensitivity explained variation in sexual dimorphism of EODs within species, at the population level. We examined two populations of black ghost knifefish (Apteronotus albifrons), one from the Orinoco and the other from the Amazon River Basin. EOD frequency (EODf) and chirp rates were measured to characterize diversity in sexual dimorphism across populations. The magnitude of sexual dimorphism in EODf differed significantly across populations, and was more pronounced in the Orinoco population than in the Amazon population. Chirp rates were sexually monomorphic in both populations. 11-Ketotestosterone (11-kT) was administered over a two-week period to assess population differences in sensitivity to androgens. 11-kT masculinized EODf significantly more in the population with the greater degree of sexual dimorphism. 11-kT had no effect on the sexually monomorphic chirping rates. We conclude that population divergence in androgen sensitivity contributes to variation in sexual dimorphism of EODf in A. albifrons.

Sex and species differences in plasma testosterone and in counts of androgen receptor-positive cells in key brain regions of Sceloporus lizard species that differ in aggression

We studied neuroendocrine correlates of aggression differences in adults of two Sceloporus lizard species. These species differ in the degree of sex difference in aggressive color signals (belly patches) and in aggression: Sceloporus undulatus (males blue, high aggression; females white, low aggression) and Sceloporus virgatus (both sexes white, lower aggression). We measured plasma testosterone and counted cells expressing androgen receptor-like immunoreactivity to the affinity-purified polyclonal AR antibody, PG-21, in three brain regions of breeding season adults. Male S. undulatus had the highest mean plasma testosterone and differed significantly from conspecific females. In contrast, there was no sex difference in plasma testosterone concentrations in S. virgatus. Male S. undulatus also had the highest mean number of AR-positive cells in the preoptic area: the sexes differed in S. undulatus but not in S. virgatus, and females of the two species did not differ. In the ventral medial hypothalamus, S. undulatus males had higher mean AR cell counts compared to females, but again there was no sex difference in S. virgatus. In the habenula, a control brain region, the sexes did not differ, and although the sex by species interaction significant was not significant, there was a trend (p=0.050) for S. virgatus to have higher mean AR cell counts than S. undulatus. Thus hypothalamic AR cell counts paralleled sex and species differences in aggression, as did mean plasma testosterone levels in these breeding-season animals.

A central pacemaker that underlies the production of seasonal and sexually dimorphic social signals: anatomical and electrophysiological aspects

Our long-term goal is to approach the understanding of the anatomical and physiological bases for communication signal diversity in gymnotiform fishes as a model for vertebrate motor pattern generation. Brachyhypopomus gauderio emits, in addition to its electric organ discharge (EOD) at basal rate, a rich repertoire of rate modulations. We examined the structure of the pacemaker nucleus, responsible for the EOD rate, to explore whether its high output signal diversity was correlated to complexity in its neural components or regional organization. We confirm the existence of only two neuron types and show that the previously reported dorsal-caudal segregation of these neurons is accompanied by rostral-caudal regionalization. Pacemaker cells are grouped dorsally in the rostral half of the nucleus, and relay cells are mainly ventral and more abundant in the caudal half. Relay cells are loosely distributed from the center to the periphery of the nucleus in correlation to somata size. Our findings support the hypothesis that regional organization enables a higher diversity of rate modulations, possibly offering distinct target areas to modulatory inputs. Since no anatomical or electrophysiological seasonal or sexual differences were found, we explored these aspects from a functional point of view in a companion article.

Sex differences in the electrocommunication signals of the electric fish Apteronotus bonapartii

The South American weakly-electric knifefish (Apteronotidae) produce highly diverse and readily quantifiable electrocommunication signals. The electric organ discharge frequency (EODf), and EOD modulations (chirps and gradual frequency rises (GFRs)), vary dramatically across sexes and species, presenting an ideal opportunity to examine the proximate and ultimate bases of sexually dimorphic behavior. We complemented previous studies on the sexual dimorphism of apteronotid communication signals by investigating electric signal features and their hormonal correlates in Apteronotus bonapartii, a species which exhibits strong sexual dimorphism in snout morphology. Electrocommunication signals were evoked and recorded using a playback paradigm, and were analyzed for signal features including EOD frequency and the structure of EOD modulations. To investigate the androgenic correlates of sexually dimorphic EOD signals, we measured plasma concentrations of testosterone and 11-ketotestosterone. A. bonapartii responded robustly to stimulus playbacks. EODf was sexually monomorphic, and males and females produced chirps with similar durations and amounts of frequency modulation. However, males were more likely than females to produce chirps with multiple frequency peaks. Sexual dimorphism in apteronotid electrocommunication signals appears to be highly evolutionarily labile. Extensive interspecific variation in the magnitude and direction of sex differences in EODf and in different aspects of chirp structure suggest that chirp signals may be an important locus of evolutionary change within the clade. The weakly-electric fish represent a rich source of data for understanding the selective pressures that shape, and the neuroendocrine mechanisms that underlie, diversity in the sexual dimorphism of behavior.

Neural heterogeneity and efficient population codes for communication signals

Efficient sensory coding implies that populations of neurons should represent information-rich aspects of a signal with little redundancy. Recent studies have shown that neural heterogeneity in higher brain areas enhances the efficiency of encoding by reducing redundancy across the population. Here, we study how neural heterogeneity in the early stages of sensory processing influences the efficiency of population codes. Through the analysis of in vivo recordings, we contrast the encoding of two types of communication signals of electric fishes in the most peripheral sensory area of the CNS, the electrosensory lateral line lobe (ELL). We show that communication signals used during courtship (big chirps) and during aggressive encounters (small chirps) are encoded by different populations of ELL pyramidal cells, namely I-cells and E-cells, respectively. Most importantly, we show that the encoding strategy differs for the two signals and we argue that these differences allow these cell types to encode specifically information-rich features of the signals. Small chirps are detected, and their timing is accurately signaled through stereotyped spike bursts, whereas the shape of big chirps is accurately represented by variable increases in firing rate. Furthermore, we show that the heterogeneity across I-cells enhances the efficiency of the population code and thus permits the accurate discrimination of different quality courtship signals. Our study shows the importance of neural heterogeneity early in a sensory system and that it initiates the sparsification of sensory representation thereby contributing to the efficiency of the neural code.

Electrocommunication signals in free swimming brown ghost knifefish, Apteronotus leptorhynchus

Brown ghost knifefish, Apteronotus leptorhynchus, are a species of weakly electric fish that produce a continuous electric organ discharge (EOD) that is used in navigation, prey capture and communication. Stereotyped modulations of EOD frequency and amplitude are common in social situations and are thought to serve as communication signals. Of these modulations, the most commonly studied is the chirp. This study presents a quantitative analysis of chirp production in pairs of free-swimming, physically interacting male and female A. leptorhynchus. Under these conditions, we found that in addition to chirps, the fish commonly produce a second signal type, a type of frequency rise called abrupt frequency rises, AFRs. By quantifying the behaviours associated with signal production, we find that Type 2 chirps tend to be produced when the fish are apart, following periods of low aggression, whereas AFRs tend to be produced when the fish are aggressively attacking one another in close proximity. This study is the first to our knowledge that quantitatively describes both electrocommunication signalling and behavioural correlates on a subsecond time-scale in a wave-type weakly electric fish.

Phylogenetic comparative analysis of electric communication signals in ghost knifefishes (Gymnotiformes: Apteronotidae)

Electrocommunication signals in electric fish are diverse, easily recorded and have well-characterized neural control. Two signal features, the frequency and waveform of the electric organ discharge (EOD), vary widely across species. Modulations of the EOD (i.e. chirps and gradual frequency rises) also function as active communication signals during social interactions, but they have been studied in relatively few species. We compared the electrocommunication signals of 13 species in the largest gymnotiform family,Apteronotidae. Playback stimuli were used to elicit chirps and rises. We analyzed EOD frequency and waveform and the production and structure of chirps and rises. Species diversity in these signals was characterized with discriminant function analyses, and correlations between signal parameters were tested with phylogenetic comparative methods. Signals varied markedly across species and even between congeners and populations of the same species. Chirps and EODs were particularly evolutionarily labile, whereas rises differed little across species. Although all chirp parameters contributed to species differences in these signals, chirp amplitude modulation, frequency modulation (FM) and duration were particularly diverse. Within this diversity,however, interspecific correlations between chirp parameters suggest that mechanistic trade-offs may shape some aspects of signal evolution. In particular, a consistent trade-off between FM and EOD amplitude during chirps is likely to have influenced the evolution of chirp structure. These patterns suggest that functional or mechanistic linkages between signal parameters(e.g. the inability of electromotor neurons increase their firing rates without a loss of synchrony or amplitude of action potentials) constrain the evolution of signal structure.

Stimulus frequency differentially affects chirping in two species of weakly electric fish: implications for the evolution of signal structure and function

During social interactions, apteronotid electric fish modulate their electric organ discharges (EODs) to produce transient communication signals known as chirps. Chirps vary widely across species and sex in both number and structure. In Apteronotus leptorhynchus, males chirp far more than females and their chirps have greater frequency modulation than those of females. High-frequency chirps are produced by males most often in response to female-like electric signals. As such, they have been hypothesized to function in courtship. The more common low-frequency chirps, produced by both males and females in response to same-sex signals, are hypothesized to function as aggressive signals. To determine whether the two chirp types in the closely related Apteronotus albifrons have similar functions, we stimulated chirping in male and female A. leptorhynchus and A. albifrons with playbacks simulating the EODs of same-sex versusopposite-sex conspecifics. As in A. leptorhynchus, male and female A. albifrons produced low-frequency chirps most often to same-sex signals. Unlike A. leptorhynchus, however, A. albifrons also produced more high-frequency chirps to same-sex stimuli than to opposite-sex stimuli. This suggests that high-frequency chirps in A. albifrons,unlike those in A. leptorhynchus, may not function as courtship signals and that the function of similar chirp types has diversified in Apteronotus. Examples such as this, in which the function of a communication signal has changed in closely related species, are rare. The electrocommunication signals of apteronotids may thus provide a remarkable opportunity to investigate the evolutionary interactions of signal structure and function.

Interval coding. I. Burst interspike intervals as indicators of stimulus intensity

Short interspike intervals such as those that occur during burst firing are hypothesized to be distinct features of the neural code. Although a number of correlations between the occurrence of burst events and aspects of the stimulus have been identified, the relationship between burst characteristics and information transfer is uncertain. Pyramidal cells in the electrosensory lobe of the weakly electric fish, Apteronotus leptorhynchus, respond to dynamic broadband electrosensory stimuli with bursts and isolated spikes. In the present study, we mimic synaptic input during sensory stimulation by direct stimulation of electrosensory pyramidal cells with broadband current in vitro. The pyramidal cells respond to this stimulus with burst interspike intervals (ISIs) that are reliably and precisely correlated with the intensity of stimulus upstrokes. We found burst ISIs must differ by a minimum of 2 ms to discriminate, with low error, differences in stimulus intensity. Based on these results, we define and quantify a candidate interval code for the processing of sensory input. Finally, we demonstrate that interval coding is restricted to short ISIs such as those generated in burst events and that the proposed interval code is distinct from rate and timing codes.

Structure and sexual dimorphism of the electrocommunication signals of the weakly electric fish, Adontosternarchus devenanzii

Electrocommunication signals of electric fish vary across species, sexes and individuals. The diversity of these signals and the relative simplicity of the neural circuits controlling them make them a model well-suited for studying the mechanisms, evolution and sexual differentiation of behavior. In most wave-type gymnotiform knifefishes, electric organ discharge (EOD)frequency and EOD modulations known as chirps are sexually dimorphic. In the most speciose gymnotiform family, the Apteronotidae, EOD frequency is higher in males than females in some species, but lower in males than females in others. Sex differences in EOD frequency and chirping, however, have been examined in only three apteronotid species in a single genus, Apteronotus. To understand the diversity of electrocommunication signals, we characterized these behaviors in another genus, Adontosternarchus. Electrocommunication signals of Adontosternarchus devenanzii differed from those of Apteronotus in several ways. Unlike in Apteronotus, EOD frequency was not sexually dimorphic in A. devenanzii. Furthermore,although A. devenanzii chirped in response to playbacks simulating conspecific EODs, the number of chirps did not vary with different stimulus frequencies. A. devenanzii chirps also differed in structure from Apteronotus chirps. Whereas Apteronotus species produce functionally distinct chirp types differing in frequency modulation (FM), A. devenanzii produced only high-frequency chirps that had either single or multiple frequency peaks. Males produced more multi-peaked chirps than females. Thus, the temporal structure of chirps, rather than the amount of FM, delineated chirp types in A. devenanzii. Our results demonstrate that the structure, function and sexual dimorphism of electrocommunication signals are evolutionary labile in apteronotids and may be useful for understanding the diversity of sexually dimorphic behavior.

Distribution and function of potassium channels in the electrosensory lateral line lobe of weakly electric apteronotid fish

Potassium channels are one of the fundamental requirements for the generation of action potentials in the nervous system, and their characteristics shape the output of neurons in response to synaptic input. We review here the distribution and function of a high-threshold potassium channel (Kv3.3) in the electrosensory lateral line lobe of the weakly electric fish Apteronotus leptorhynchus, with particular focus on the pyramidal cells in this brain structure. These cells contain both high-threshold Kv3.3 channels, as well as low-threshold potassium channels of unknown molecular identity. Kv3.3 potassium channels regulate burst discharge in pyramidal cells and enable sustained high frequency firing through their ability to reduce an accumulation of low-threshold potassium current.

Sex and species differences in neuromodulatory input to a premotor nucleus: A comparative study of substance P and communication behavior in weakly electric fish

Many electric fish species modulate their electric organ discharges (EODs) to produce transient social signals that vary in number and structure. In Apteronotus leptorhynchus, males modulate their EOD more often than females, whereas in Apteronotus albifrons, males and females produce similar numbers of modulations. Sex differences in the number of EOD modulations in A. leptorhynchus are associated with sex differences in substance P in the diencephalic nucleus that controls transient EOD modulations, the CP/PPn. These sex differences in substance P have been hypothesized to regulate sex differences in the production of EOD modulations. To comparatively test this hypothesis, we examined substance P immunoreactivity in the CP/PPn of male and female A. leptorhynchus and A. albifrons. Because the number of EOD modulations is sexually monomorphic in A. albifrons, we predicted no sex difference in substance P in the CP/PPn of this species. Contrary to this prediction, male A. albifrons had significantly more substance P in the CP/PPn than females. This suggests that sex differences in substance P are not sufficient for controlling sex differences in the number of EOD modulations. Modulation structure (frequency excursion and/or duration), however, is also sexually dimorphic in A. leptorhynchus and is another possible behavioral correlate of the sexually dimorphic distribution of substance P. The present study found pronounced sex differences in the structure of EOD modulations in A. albifrons similar to those in A. leptorhynchus. Thus, sex differences in substance P may influence sex differences in the structure, rather than the number, of EOD modulations.