Muscle physiology changes induced by every other day feeding and endurance exercise in mice: effects on physical performance

Every other day feeding (EOD) and exercise induce changes in cell metabolism. The aim of the present work was to know if both EOD and exercise produce similar effects on physical capacity, studying their physiological, biochemical and metabolic effects on muscle. Male OF-1 mice were fed either ad libitum (AL) or under EOD. After 18 weeks under EOD, animals were also trained by using a treadmill for another 6 weeks and then analyzed for physical activity. Both, EOD and endurance exercise increased the resistance of animals to extenuating activity and improved motor coordination. Among the groups that showed the highest performance, AL and EOD trained animals, ALT and EODT respectively, only the EODT group was able to increase glucose and triglycerides levels in plasma after extenuating exercise. No high effects on mitochondrial respiratory chain activities or protein levels neither on coenzyme Q levels were found in gastrocnemius muscle. However, exercise and EOD did increase β-oxidation activity in this muscle accompanied by increased CD36 levels in animals fed under EOD and by changes in shape and localization of mitochondria in muscle fibers. Furthermore, EOD and training decreased muscle damage after strenuous exercise. EOD also reduced the levels of lipid peroxidation in muscle. Our results indicate that EOD improves muscle performance and resistance by increasing lipid catabolism in muscle mitochondria at the same time that prevents lipid peroxidation and muscle damage.     

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

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

Androgen modulates the kinetics of the delayed rectifying K+ current in the electric organ of a weakly electric fish

Weakly electric fish such as Sternopygus macrurus utilize a unique signal production system, the electric organ (EO), to navigate within their environment and to communicate with conspecifics. The electric organ discharge (EOD) generated by the Sternopygus electric organ is quasi-sinusoidal and sexually dimorphic; sexually mature males produce long duration EOD pulses at low frequencies, whereas mature females produce short duration EOD pulses at high frequencies. EOD frequency is set by a medullary pacemaker nucleus, while EOD pulse duration is determined by the kinetics of Na+ and K+ currents in the electric organ. The inactivation of the Na+ current and the activation of the delayed rectifying K+ current of the electric organ covary with EOD frequency such that the kinetics of both currents are faster in fish with high (female) EOD frequency than those with low (male) EOD frequencies. Dihydrotestosterone (DHT) implants masculinize the EOD centrally by decreasing frequency at the pacemaker nucleus (PMN). DHT also acts at the electric organ, broadening the EO pulse, which is at least partly due to a slowing of the inactivation kinetics of the Na+ current. Here, we show that chronic DHT treatment also slows the activation and deactivation kinetics of the electric organ's delayed rectifying K+ current. Thus, androgens coregulate the time-varying kinetics of two distinct ion currents in the EO to shape a sexually dimorphic communication signal.     

Opposing actions of 5HT1A and 5HT2-like serotonin receptors on modulations of the electric signal waveform in the electric fish Brachyhypopomus pinnicaudatus

Serotonin (5-HT) is an indirect modulator of the electric organ discharge (EOD) in the weakly electric gymnotiform fish, Brachyhypopomus pinnicaudatus. Injections of 5-HT enhance EOD waveform "masculinity", increasing both waveform amplitude and the duration of the second phase. This study investigated the pharmacological identity of 5-HT receptors that regulate the electric waveform and their effects on EOD amplitude and duration. We present evidence that two sets of serotonin receptors modulate the EOD in opposite directions. We found that the 5HT1AR agonist 8-OH-DPAT diminishes EOD duration and amplitude while the 5HT1AR antagonist WAY100635 increases these parameters. In contrast, the 5HT2R agonist alpha-Me-5-HT increases EOD amplitude but not duration, yet 5-HT-induced increases in EOD duration can be inhibited by blocking 5HT2A/2C-like receptors with ketanserin. These results show that 5-HT exerts bi-directional control of EOD modulations in B. pinnicaudatus via action at receptors similar to mammalian 5HT1A and 5HT2 receptors. The discordant amplitude and duration response suggests separate mechanisms for modulating these waveform parameters.     

Androgen correlates of socially induced changes in the electric organ discharge waveform of a mormyrid fish

Weakly electric fish from the family Mormyridae produce pulsatile electric organ discharges (EODs) for use in communication. For many species, male EODs are seasonally longer in duration than those of females, and among males, there are also individual differences in EOD duration. While EOD elongation can be induced by the administration of exogenous androgens, androgen levels have never before been assessed under natural or seminatural conditions. By simulating the conditions occurring during the breeding season in the laboratory, we provide evidence of a sex difference in EOD duration as well as document levels of circulating androgens in males. In this study, we analyzed the nature of social influences on male EOD duration and plasma androgen levels in Brienomyrus brachyistius. Individual males, first housed with a single female and then placed into social groups consisting of three males and three females, showed status-dependent changes in EOD duration. Top-ranking males experienced a relatively large increase in EOD duration. Second-ranking males experienced a more modest increase, and low-ranking males experienced a decrease in EOD duration. These changes were paralleled by differences in circulating levels of plasma 11-ketotestosterone (11-KT), but not testosterone, suggesting that the changes in EOD duration may have been mediated by changes in plasma 11-KT levels. Thus, it appears that EOD duration is an accurate indicator of male status, which is under social and hormonal control.     

Plasticity of electric organ discharge waveform in the South African Bulldog fish, Marcusenius pongolensis: tradeoff between male attractiveness and predator avoidance?

Background

In adult male Marcusenius pongolensis the duration of their Electric Organ Discharge (EOD) pulses increases with body size over lifetime (267 to 818 μs, field-measured). Spawning males have been observed to exhibit an additional, temporary pulse duration increase which probably betters their mating success but increases predation risk by electroreceptive catfish. We here study the question of how the additional pulse duration increase is triggered and for how long it persists, in an attempt to understand the compromise between opposing selective forces.

Results

Here, we demonstrate short-term plasticity in male EOD waveform in 10 captive M. pongolensis. An increase in EOD duration was experimentally evoked in two different ways: by exchanging the familiar neighbours of experimental subjects for stranger males that were separated by plastic mesh partitions, or by separating familiar fish by plastic mesh partitions introduced into their common tank. Both treatments evoked an increase of male EOD duration. Values exceeded those found in the non-reproductive season in nature. In one male the increase of EOD duration was 5.7 fold, from 356 μs to 2029 μs. An increase in EOD duration was accompanied by a high level of aggression directed against the neighbours through the plastic mesh. With conditions remaining constant, EOD duration receded to 38 – 50% of the maximum EOD duration after 10 weeks, or, more rapidly, when sensory contact between the fish was severely restricted by the introduction of a solid plastic wall.

Conclusion

The short-term increase of EOD duration evoked by experimental manipulation of sensory contact with conspecifics through the plastic mesh, as reported here, resembled the changes in EOD waveform that accompanied reproduction in two captive males. Plasticity of the male EOD in pulse duration seems to be an adaptation for (1) securing a higher fitness by a sexually "attractive" long-duration EOD, while (2) limiting the risk of detection by electroreceptive predators, such as the sharptooth catfish, by receding to a shorter EOD as soon as reproduction is over.     

Flowering responses to altered expression of phytochrome in mutants and transgenic lines of Arabidopsis thaliana (L.) Heynh.

The long-day plant Arabidopsis thaliana (L.) Heynh. flowers early in response to brief end-of-day (EOD) exposures to far-red light (FR) following a fluorescent short day of 8 h. FR promotion of flowering was nullified by subsequent brief red light (R) EOD exposure, indicating phytochrome involvement. The EOD response to R or FR is a robust measure of phytochrome action. Along with their wild-type (WT) parents, mutants deficient in either phytochrome A or B responded similarly to the EOD treatments. Thus, neither phytochrome A nor B exclusively regulated flowering, although phytochrome B controlled hypocotyl elongation. Perhaps a third phytochrome species is important for the EOD responses of the mutants and/or their flowering is regulated by the amount of the FR-absorbing form of phytochrome, irrespective of the phytochrome species. Overexpression of phytochrome A or phytochrome B resulted in differing photoperiod and EOD responses among the genotypes. The day-neutral overexpressor of phytochrome A had an EOD response similar to all of the mutants and WTs, whereas R EOD exposure promoted flowering in the overexpressor of phytochrome B and FR EOD exposure inhibited this promotion. The comparisons between relative flowering times and leaf numbers at flowering of the over-expressors and their WTs were not consistent across photoperiods and light treatments, although both phytochromes A and B contributed to regulating flowering of the transgenic plants.     

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

In several species of electric fish with a sex difference in their pulse-type electric organ discharge (EOD), the action potential-generating cells of the electric organ (electrocytes) of males are larger and more invaginated compared to females. Androgen treatment of females and juveniles produces a longer-duration EOD pulse that mimics the mature male EOD, with a concurrent increase in electrocyte size and/or membrane infolding. In Sternopygus macrurus, which generates a wave-type EOD, androgen also increases EOD pulse duration. To investigate possible morphological correlates of hormone-dependent changes in EOD in Sternopygus, we examined electric organs from both fish collected in the field, and untreated and androgen-treated specimens in the laboratory. The electrocytes are cigar shaped, with prominent papillae on the posterior, innervated end. Electrocytes of field-caught specimens were significantly larger in all parameters than were electrocytes of specimens maintained in the laboratory. EOD pulse duration and frequency were highly correlated, and were significantly different between the sexes in sexually mature fish. Nevertheless, no sex difference in electrocyte morphology was observed, nor did any parameters of electrocyte morphology correlate with EOD pulse duration or frequency. Further, whereas androgen treatment significantly lowered EOD frequency and broadened EOD pulse duration, there was no difference in electrocyte morphology between hormone-treated and control groups. Thus, in contrast to results from studies on both mormyrid and gymnotiform pulse fish, electrocyte morphology is not correlated with EOD waveform characteristics in the gymnotiform wave-type fish Sternopygus. The data, therefore, suggest that sex differences in EOD are dependent on changes in active electrical properties of electrocyte membranes. © 1992 John Wiley & Sons, Inc.     

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.     

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

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

Communication in the weakly electric fish Sternopygus macrurus

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

Mast cell hyperplasia in the skin of Dsg4-deficient hypotrichosis mice, which are long-living mutants of lupus-prone mice

Desmosomal cadherins are essential cell adhesion molecules expressed in the epidermis. We identified a mutation of a cadherin superfamily member, namely, desmoglein 4 (Dsg4), in early onset of death (EOD)( hage ) mice with hypotrichosis. The mutation was induced by the insertion of an early transposon II-beta into intron 8 of Dsg4. Mast cell hyperplasia was observed in the skin of EOD( hage ) mice. The abnormally expanded population of lpr T cells, i.e., CD4(-)CD8(-)B220(+)Thy1.2(+) alphabetaT cells, in the splenocytes of EOD mice was reduced in EOD( hage ) mice. Therefore, it was suspected that the long-living mutant EOD( hage ) mice were selected from lupus-prone EOD mice because of their immunological immaturity. These findings clearly indicate that Dsg4 is an important molecule for the formation of hair follicles and hypothesize that unorganized hyperplastic hair follicles in anagen due to the Dsg4 mutation provide niches for mast cell precursors in the skin.     

Testosterone and 11-ketotestosterone have different regulatory effects on electric communication signals of male Brachyhypopomus gauderio

The communication signals of electric fish can be dynamic, varying between the sexes on a circadian rhythm and in response to social and environmental cues. In the gymnotiform fish Brachyhypopomus gauderio waveform shape of the electric organ discharge (EOD) is regulated by steroid and peptide hormones. Furthermore, EOD amplitude and duration change on different timescales and in response to different social stimuli, suggesting that they are regulated by different mechanisms. Little is known about how androgen and peptide hormone systems interact to regulate signal waveform. We investigated the relationship between the androgens testosterone (T) and 11-ketotestosterone (11-KT), the melanocortin peptide hormone α-MSH, and their roles in regulating EOD waveform of male B. gauderio. Males were implanted with androgen (T, 11-KT, or blank), and injected with α-MSH before and at the peak of androgen effect. We compared the effects of androgen implants and social interactions by giving males a size-matched male stimulus with which they could interact electrically. Social stimuli and both androgens increased EOD duration, but only social stimuli and 11-KT elevated amplitude. However, no androgen enhanced EOD amplitude to the extent of a social stimulus, suggesting that a yet unidentified hormonal pathway regulates this signal parameter. Additionally, both androgens increased response of EOD duration to α-MSH, but only 11-KT increased response of EOD amplitude to α-MSH. Social stimuli had no effect on EOD response to α-MSH. The finding that EOD amplitude is preferentially regulated by 11-KT in B. gauderio may provide the basis for independent control of amplitude and duration.     

Strain differences in the induction of mono-oxygenase activity in mouse skin by topical clobetasol propionate: evidence of a role for the hr locus

The effect of the topical application of clobetasol propionate on cutaneous ethoxycoumarin O'dealkylation (EOD) has been studied in various strains of mice. Clobetasol propionate markedly increased cutaneous EOD activity in adult hairless mice only. Similar treatment of adult haired C57BL/6J mice, or adult haired DBA/2J mice had no significant effect on cutaneous EOD activity. In contrast 3 methylcholanthrene induced cutaneous EOD activity in both hairless and C57 strains to a far greater extent than in the DBA strain. EOD activity in hairless mice non-responsive to polycyclic hydrocarbons, derived by selective breeding of hairless and DBA strains was induced by clobetasol propionate to a similar extent to that observed in responsive hairless strains. Hepatic EOD activity was not induced by clobetasol propionate in any of the strains tested. Strain differences in the induction of EOD by clobetasol propionate were not related to differences in either the concentration of cytosolic glucocorticoid receptor in the skin, the dissociation constant of the cytosolic receptor, or differences in percutaneous absorption. Polycyclic hydrocarbons did not compete with triaminolone acetonide for binding to the cytosolic glucocorticoid receptor. Strain differences in the induction of EOD activity by clobetasol propionate appear therefore not to be related to strain differences in either the Ah receptor of the glucocorticoid receptor, but to be regulated by the hr locus.     

A temporal analysis of testosterone-induced changes in electric organs and electric organ discharges of mormyrid fishes

The electric organ discharge (EOD) of several species of mormyrid fishes within the genus Brienomyrus is sexually dimorphic during the breeding season: the duration of the male's EOD is much longer than the duration of the female's (for a review see Hopkins, 1986). The mormyrid used here, Brienomyrus sp., exhibits similar alterations in the duration of the triphasic EOD after treatment with testosterone, as do other members of this genus (for reviews see Bass, 1986a,b). In this experiment, animals were intraperitoneally implanted with pellets of either 11-ketotestosterone or 17 a-methyltestosterone, and the time course of the changes in the duration of each of the three phases of the EOD were quantified. Additionally, the time course of changes in the morphology of the electric organ, after testosterone treatment, was also quantified using electron microscopic techniques. The results suggest that the change in the duration of the first phase of the EOD is due exclusively to the change in the thickness of the electrocyte body: this is consistent with a model proposed by Bennett and Grundfest (1961) for the electrogenesis of a triphasic EOD. Changes in the duration of the second and third phases of the EOD are highly correlated with the changes in the surface area of the posterior and anterior faces of the electrocyte, respectively. The results support the hypothesis that gonadal steroid hormone-induced changes in the EOD are due to structural changes in the electrocyte's membranes, and that all of the observed changes in the discharge of this system can be explained by the action of steroid hormones on the peripheral target cells (electrocytes).     

Discovery of a novel small molecule, 1-ethoxy-3-(3,4-methylenedioxyphenyl)-2-propanol, that induces apoptosis in A549 human lung cancer cells

A novel small molecule, 1-ethoxy-3-(3,4-methylenedioxyphenyl)-2-propanol (EOD), was synthesized in our laboratory. Previously, we reported pharmacological properties of EOD, triggering apoptosis in Human umbilical vein endothelial cells (HUVECs). Here, we further investigated the effects of EOD on the growth of A549 human lung cancer cells. EOD treatment induced apoptosis in A549 cells via up-regulating the expression of P53 protein, blocking cell cycle partly at G1 phase, and ultimately activating caspase-3. In contrast, caspase-8 might be irrelevant to EOD-triggered apoptosis. This study indicated that EOD might be a potential chemopreventive agent for lung cancer. The work would encourage us to add more novel compounds to our 'library' of small molecules derived through modern synthetic organic chemistry, and would drive us to determine the proteins that the compounds target.     

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.     

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.     

Observations on the electric organ discharge of two skate species (Chondrichthyes: Rajidae) and its relationship to behaviour

Synopsis The electric organ discharge (EOD) of the little skate,Raja erinacea and winter skate,R. ocellata was recorded both from isolated individuals and from small groups using methods that allowed for the identification of individuals producing EODs. Pulse duration, train lengh, frequency, and pulse patterns are characterized and correlated with behaviour. The two species,R. erinacea andR. ocellata, were found to have characteristically different EOD pulse durations of 70 ms and 217 ms respectively. Isolated skates rarely discharged whereas groups of skates were found to discharge regularly. The EOD was evoked by tactile prodding, physical contact with other skates and electrical stimulation. Skates also discharged reflexively in response to an artificially induced head-positive DC stimulus, sine wave and monopolar square pulses. During approach and contact, skates responded to each other with interacting EOD displays. EOD interaction and pulse duration differences between other species suggest a possible intra-specific communication function of the EOD inRaja.     

Communication in the weakly electric fish Sternopygus macrurus

There is a sexual dimorphism in the frequency of the quasi-sinusoidal electric organ discharge (EOD) of Sternopygus macrurus, with males, on average, an octave lower. EODs are detected by tuberous electroreceptor organs, which exhibit V-shaped frequency tuning with maximal sensitivity near the fish's own EOD frequency. This would seem to limit the ability of a fish to detect the EODs of opposite-sex conspecifics. However, electroreceptor tuning has always been based on single-frequency stimulation, while actual EOD detection involves the addition of a conspecific EOD to the fish's own. In the present study, recordings were made from single electroreceptive units while the fish were stimulated with pairs of sine waves: one (S1) representing the fish's own EOD added to a second (S2) representing a conspecific EOD. T unit response was easily predicted by assuming that the electroreceptor acts as a linear filter in series with a threshold-sensitive spike initiator. P unit response was more complex, and unexpectedly high sensitivity was found for frequencies of S2 well displaced from the fish's EOD frequency. For both P and T units, detection thresholds for S2 were much lower when added to S1, than when presented alone.