Androgens Alter the Tuning of Electroreceptors

Differential expression of steroid-related genes across electrosensory brain regions in two sexually dimorphic species of electric knifefish

The production of communication signals can be modulated by hormones acting on the brain regions that regulate these signals. However, less is known about how signal perception is regulated by hormones. The electrocommunication signals of weakly electric fishes are sexually dimorphic, sensitive to hormones, and vary across species. The neural circuits that regulate the production and perception of these signals are also well-characterized, and electric fishes are thus an excellent model to examine the neuroendocrine regulation of sensorimotor mechanisms of communication. We investigated (1) whether steroid-related genes are expressed in sensory brain regions that process communication signals; and (2) whether this expression differs across sexes and species that have different patterns of sexual dimorphism in their signals. Apteronotus leptorhynchus and Apteronotus albifrons produce continuous electric organ discharges (EODs) that are used for communication. Two brain regions, the electrosensory lateral line lobe (ELL) and the dorsal torus semicircularis (TSd), process inputs from electroreceptors to allow fish to detect and discriminate electrocommunication signals. We used qPCR to quantify the expression of genes for two androgen receptors (ar1, ar2), two estrogen receptors (esr1, esr2b), and aromatase (cyp19a1b). Four out of five steroid-related genes were expressed in both sensory brain regions, and their expression often varied between sexes and species. These results suggest that expression of steroid-related genes in the brain may differentially influence how EOD signals are encoded across species and sexes, and that gonadal steroids may coordinately regulate central circuits that control both the production and perception of EODs.

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.

Molecular tuning of electroreception in sharks and skates

Ancient cartilaginous vertebrates, such as sharks, skates, and rays, possess specialized electrosensory organs that detect weak electric fields and relay this information to the central nervous system^1–4. Sharks exploit this sensory modality for predation, whereas skates may also use it to detect signals from conspecifics⁵. Here we analyze shark and skate electrosensory cells to ask if discrete physiological properties could contribute to behaviorally-relevant sensory tuning. We show that sharks and skates use a similar low threshold voltage-gated calcium channel to initiate cellular activity but employ distinct potassium channels to modulate this activity. Electrosensory cells from sharks express specially adapted voltage-gated potassium channels that support large, repetitive membrane voltage spikes capable of driving near-maximal vesicular release from elaborate ribbon synapses. In contrast, skates use a calcium-activated potassium channel to produce small, tunable membrane voltage oscillations that elicit stimulus-dependent vesicular release. We propose that these sensory adaptations support amplified indiscriminate signal detection in sharks versus selective frequency detection in skates, potentially reflecting the electroreceptive requirements of these elasmobranch species. Our findings demonstrate how sensory systems adapt to suit an animal’s lifestyle or environmental niche through discrete molecular and biophysical modifications.

Genes linked to species diversity in a sexually dimorphic communication signal in electric fish

Sexually dimorphic behaviors are often regulated by androgens and estrogens. Steroid receptors and metabolism are control points for evolutionary changes in sexual dimorphism. Electric communication signals of South American knifefishes are a model for understanding the evolution and physiology of sexually dimorphic behavior. These signals are regulated by gonadal steroids and controlled by a simple neural circuit. Sexual dimorphism of the signals varies across species. We used transcriptomics to examine mechanisms for sex differences in electric organ discharges (EODs) of two closely related species, Apteronotus leptorhynchus and Apteronotus albifrons, with reversed sexual dimorphism in their EODs. The pacemaker nucleus (Pn), which controls EOD frequency (EODf), expressed transcripts for steroid receptors and metabolizing enzymes, including androgen receptors, estrogen receptors, aromatase, and 5α-reductase. The Pn expressed mRNA for ion channels likely to regulate the high-frequency activity of Pn neurons and for neuromodulator and neurotransmitter receptors that may regulate EOD modulations used in aggression and courtship. Expression of several ion channel genes, including those for Kir3.1 inward-rectifying potassium channels and sodium channel β1 subunits, was sex-biased or correlated with EODf in ways consistent with EODf sex differences. Our findings provide a basis for future studies to characterize neurogenomic mechanisms by which sex differences evolve.

Sensory plasticity: seasonal regulation of hearing by a potassium channel

Animals may vary in their responsiveness to sensory cues seasonally. The transcriptional regulation of a potassium channel in the hearing organ of a fish explains its seasonally varying sensitivity to acoustic frequencies in its courtship vocalization.

Weakly electric fish for biomonitoring water quality

Environmental pollution is a major issue that calls for suitable monitoring systems. The number of possible pollutants of municipal and industrial water grows annually as new chemicals are developed. Technical devices for pollutant detection are constructed in a way to detect a specific and known array of pollutants. Biological systems react to lethal or non-lethal environmental changes without pre-adjustment, and a wide variety have been employed as broad-range monitors for water quality. Weakly electric fish have proven particularly useful for the purpose of biomonitoring municipal and industrial waters. The frequency of their electric organ discharges directly correlates with the quality of the surrounding water and, in this way, concentrations of toxicants down to the nanomolar range have been successfully detected by these organisms. We have reviewed the literature on biomonitoring studies to date, comparing advantages and disadvantages of this test system and summarizing the lowest concentrations of various toxicants tested. Eighteen publications were identified investigating 35 different chemical substances and using six different species of weakly electric fish.

Adaptive hearing in the vocal plainfin midshipman fish: getting in tune for the breeding season and implications for acoustic communication

The plainfin midshipman fish (Porichthys notatus Girard, 1854) is a vocal species of batrachoidid fish that generates acoustic signals for intraspecific communication during social and reproductive activity and has become a good model for investigating the neural and endocrine mechanisms of vocal-acoustic communication. Reproductively active female plainfin midshipman fish use their auditory sense to detect and locate "singing" males, which produce a multiharmonic advertisement call to attract females for spawning. The seasonal onset of male advertisement calling in the midshipman fish coincides with an increase in the range of frequency sensitivity of the female's inner ear saccule, the main organ of hearing, thus leading to enhanced encoding of the dominant frequency components of male advertisement calls. Non-reproductive females treated with either testosterone or 17β-estradiol exhibit a dramatic increase in the inner ear's frequency sensitivity that mimics the reproductive female's auditory phenotype and leads to an increased detection of the male's advertisement call. This novel form of auditory plasticity provides an adaptable mechanism that enhances coupling between sender and receiver in vocal communication. This review focuses on recent evidence for seasonal reproductive-state and steroid-dependent plasticity of auditory frequency sensitivity in the peripheral auditory system of the midshipman fish. The potential steroid-dependent mechanism(s) that lead to this novel form of auditory and behavioral plasticity are also discussed.

Androgens enhance plasticity of an electric communication signal in female knifefish, Brachyhypopomus pinnicaudatus

Sex steroids were initially defined by their actions shaping sexually dimorphic behavioral patterns. More recently scientists have begun exploring the role of steroids in determining sex differences in behavioral plasticity. We investigated the role of androgens in potentiating circadian, pharmacological, and socially-induced plasticity in the amplitude and duration of electric organ discharges (EODs) of female gymnotiform fish. We first challenged female fish with injections of serotonin (5-HT) and adrenocorticotropic hormone (ACTH), and with social encounters with female and male conspecifics to characterize females' pre-implant responses to each treatment. Each individual was then implanted with a pellet containing dihydrotestosterone (DHT) concentrations of 0.0, 0.03, 0.1, 0.3, or 1.0 mg 10 g(-1) body weight. We then repeated all challenges and compared each female's pre- and post-implant responses. The highest implant dose enhanced EOD duration modulations in response to all challenge types, responses to male challenge were also greater at the second highest dose, and responses to ACTH challenge were enhanced in females receiving all but the smallest dose (and blank) implants. Alternatively, amplitude modulations were enhanced only during female challenges and only when females received the highest DHT dose. Our results highlight the differential regulation of EOD duration and amplitude, and suggest that DHT enhanced the intrinsic plasticity of the electrogenic cells that produce the EOD rather than modifying behavioral phenotypes. The relative failure of DHT to enhance EOD amplitude plasticity also implies that factors other than androgens are involved in regulating/promoting male-typical EOD circadian rhythms and waveform modulations displayed in social contexts.

Steroid-dependent auditory plasticity for the enhancement of acoustic communication: recent insights from a vocal teleost fish

The vocal plainfin midshipman fish (Porichthys notatus) has become an excellent model for identifying neural mechanisms of auditory perception that may be shared by all vertebrates. Recent neuroethological studies of the midshipman fish have yielded strong evidence for the steroid-dependent modulation of hearing sensitivity that leads to enhanced coupling of sender and receiver in this vocal-acoustic communication system. Previous work shows that non-reproductive females treated with either testosterone or 17beta-estradiol exhibit an increase in the degree of temporal encoding by the auditory saccular afferents to the dominant frequency content of male vocalizations produced during social-reproductive behaviors. The expanded frequency sensitivity of steroid treated females mimics the reproductive female's auditory phenotype and is proposed to improve the detection and localization of calling conspecific mates during the summer breeding season. This review focuses on the novel form of steroid-dependent auditory plasticity that is found in the adult midshipman fish and its association with the reproductive biology and behavior of this species. Evidence for midshipman reproductive-state and steroid-dependent auditory plasticity is reviewed and the potential mechanisms that lead to this novel form of adaptive plasticity are discussed.

Neuroendocrine and behavioral responses to weak electric fields in adult sea lampreys (Petromyzon marinus)

We characterized the behavioral and neuroendocrine responses of adult sea lampreys (Petromyzon marinus) to weak electric fields. Adult sea lampreys, captured during upstream spawning migration, exhibited limited active behaviors during exposure to weak electric fields and spent the most time attached to the wall of the testing arena near the cathode (-). For adult male sea lampreys, exposure to weak electric fields resulted in increased lamprey (l) GnRH-I mRNA expression but decreased lGnRH-I immunoreactivities in the forebrain, and decreased Jun (a neuronal activation marker) mRNA levels in the brain stem. Similar effects were not observed in the brains of female sea lampreys after weak electric field stimulation. The influence of electroreception on forebrain lGnRH suggests that electroreception may modulate the reproductive systems in adult male sea lampreys. The changes in Jun expression may be associated with swimming inhibition during weak electric field stimulation. The results for adult sea lampreys are the opposite of those obtained using parasitic-stage sea lampreys, which displayed increased activity during and after cathodal stimulation. Our results demonstrate that adult sea lampreys are sensitive to weak electric fields, which may play a role in reproduction. They also suggest that electrical stimuli mediate different behaviors in feeding-stage and spawning-stage sea lampreys.

Walter Heiligenberg: the jamming avoidance response and beyond

Walter Heiligenberg (1938-1994) was an exceptionally gifted behavioral physiologist who made enormous contributions to the analysis of behavior and to our understanding of how the brain initiates and controls species-typical behavioral patterns. He was distinguished by his rigorous analytical approach used in both behavioral studies and neuroethological investigations. Among his most significant contributions to neuroethology are a detailed analysis of the computational rules governing the jamming avoidance response in weakly electric fish and the elucidation of the principal neural pathway involved in neural control of this behavior. Based on his work, the jamming avoidance response is perhaps the best-understood vertebrate behavior pattern in terms of the underlying neural substrate. In addition to this pioneering work, Heiligenberg stimulated research in a significant number of other areas of ethology and neuroethology, including: the quantitative assessment of aggressivity in cichlid fish; the ethological analysis of the stimulus-response relationship in the chirping behavior of crickets; the exploration of the neural and endocrine basis of communicatory behavior in weakly electric fish; the study of cellular mechanisms of neuronal plasticity in the adult fish brain; and the phylogenetic analysis of electric fishes using a combination of morphology, electrophysiology, and mitochondrial sequence data.

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

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

Sonic and electric fish: at the crossroads of neuroethology and behavioral neuroendocrinology

Field and laboratory studies of weakly electric and sound-producing teleost fishes demonstrate how steroidal and non-steroidal hormones mediate the translation of neural events into behavior. The development of this research program has depended upon an interdisciplinary neuroethological approach that has characterized the neurophysiological properties of the motor and sensory pathways that lead to the production and detection of easily quantified highly stereotyped behaviors, namely, electric organ discharges (EODs) and vocalizations. Neuroethological studies of these teleosts have now integrated a behavioral neuroendocrinology approach that has provided several examples of how hormone-sensitive neurobiological traits contribute to adaptive behavioral plasticity in natural habitats. As such, these studies provide guideposts for comparable studies in other groups of teleosts and vertebrates in general.

Seasonal changes in frequency tuning and temporal processing in single neurons in the frog auditory midbrain

Frogs rely on acoustic signaling to detect, discriminate, and localize mates. In the temperate zone, reproduction occurs in the spring, when frogs emerge from hibernation and engage in acoustically guided behaviors. In response to the species mating call, males typically show evoked vocal responses or other territorial behaviors, and females show phonotactic responses. Because of their strong seasonal behavior, it is possible that the frog auditory system also displays seasonal variation, as evidenced in their vocal control system. This hypothesis was tested in male Northern leopard frogs by evaluating the response characteristics of single neurons in the torus semicircularis (TS; a homolog of the inferior colliculus) to a synthetic mating call at different times of the year. We found that TS neurons displayed a seasonal change in frequency tuning and temporal properties. Frequency tuning shifted from a predominance of TS units sensitive to intermediate frequencies (700-1200 Hz) in the winter, to low frequencies (100-600 Hz) in the summer. In winter and early spring, most TS neurons showed poor, or weak, time locking to the envelope of the amplitude-modulated synthetic call, whereas in late spring and early summer the majority of TS neurons showed robust time-locked responses. These seasonal differences indicate that neural coding by auditory midbrain neurons in the Northern leopard frog is subject to seasonal fluctuation.

Neuroethology and life history adaptations of the elasmobranch electric sense

The electric sense of elasmobranch fishes (sharks and rays) is an important sensory modality known to mediate the detection of bioelectric stimuli. Although the best known function for the use of the elasmobranch electric sense is prey detection, relatively few studies have investigated other possible biological functions. Here, we review recent studies that demonstrate the elasmobranch electrosensory system functions in a wide number of behavioral contexts including social, reproductive and anti-predator behaviors. Recent work on non-electrogenic stingrays demonstrates that the electric sense is used during reproduction and courtship for conspecific detection and localization. Electrogenic skates may use their electrosensory encoding capabilities and electric organ discharges for communication during social and reproductive interactions. The electric sense may also be used to detect and avoid predators during early life history stages in many elasmobranch species. Embryonic clearnose skates demonstrate a ventilatory freeze response when a weak low-frequency electric field is imposed upon the egg capsule. Peak frequency sensitivity of the peripheral electrosensory system in embryonic skates matches the low frequencies of phasic electric stimuli produced by natural fish egg-predators. Neurophysiology experiments reveal that electrosensory tuning changes across the life history of a species and also seasonally due to steroid hormone changes during the reproductive season. We argue that the ontogenetic and seasonal variation in electrosensory tuning represent an adaptive electrosensory plasticity that may be common to many elasmobranchs to enhance an individual's fitness throughout its life history.

Evidence of a Hopf bifurcation in frog hair cells

The membrane potential of hair cells in the low-frequency hearing organ of the bullfrog, the amphibian papilla, sinusoidally oscillates at small amplitude in the absence of acoustical input. We stimulate the cell with a series of periodic currents close to this natural frequency and observe that its current-to-voltage transfer function is compressively nonlinear, having a large gain for small stimuli and a smaller gain for larger currents. Along with the spontaneous oscillation, this implies that the cell is poised close to a dynamical instability such as a Hopf bifurcation, because distant from the instability the transfer function becomes linear. The cell's frequency selectivity is enhanced for small stimuli. Simulations show that the cell's membrane capacitance is effectively reduced due to a current gain provided by this dynamical instability. We propose that the Hopf resonance is widely used by transducer cells on the sensory periphery to achieve small-signal amplification.

Androgen-induced changes in the response dynamics of ampullary electrosensory primary afferent neurons

Male stingrays use their ampullary electroreceptors to locate mates, but the effect of gonadal androgens on electrosensory encoding during the reproductive season is unknown. We tested the hypothesis that gonadal androgens induce neurophysiological changes in the electrosense of male Atlantic stingrays. During the primary androgen increase in wild males, the electrosensory primary afferent neurons show an increase in discharge regularity, a downshift in best frequency (BF) and bandpass, and a greater sensitivity to low-frequency stimuli from 0.01 to 4 Hz. Experimental implants of dihydrotestosterone in male stingrays induced a similar lowered BF and bandpass and increased average neural sensitivity to low-frequency stimuli (0.5-2 Hz) by a factor of 1.5. Primary afferents from long ampullary canals (>3 cm) were more sensitive and had a lower bandpass and BF than did afferents from short canals (<2 cm). We propose that these androgen-induced changes in the frequency response properties of electrosensory afferents enhance mate detection by male stingrays and may ultimately increase the number of male reproductive encounters with females. Furthermore, differences in primary afferent sensitivity among short and long canals may facilitate detection, orientation, and localization of conspecifics during social interactions.

Plasticity of the electric organ discharge: implications for the regulation of ionic currents

Weakly electric fish emit electric organ discharges (EODs) to locate objects around themselves and for communication. The EOD is generated by a simple hierarchically organized, neurophysiologically accessible circuit, the electromotor system. A number of forms of plasticity of the EOD waveform are initiated by social or environmental factors and mediated by hormones or neurotransmitters. Because the behavior itself is in the form of electric discharges, behavioral observations easily lead to testable hypotheses about the biophysical bases of these plasticities. This allows us to study ionic channels in their native cellular environments, where the regulation of various parameters of these currents have obvious functional consequences. In this review, we discuss three types of plasticity: a rapidly occurring, long-lasting, N-methyl-d-aspartate (NMDA)-receptor-dependent increase in baseline firing frequency of neurons in the pacemaker nucleus that underlies a readjustment of the baseline EOD frequency after long bouts of the jamming avoidance response; a rapidly occurring diurnal change in amplitude and duration of the EOD pulse that depends in part on modulation of the magnitude of the electrocyte Na+ current by a protein kinase; and a slowly occurring, hormonally modulated tandem change in pacemaker firing frequency and in the duration of the EOD pulse in which changes in EOD pulse duration are mediated by coordinated shifts in the activation and inactivation kinetics of the electrocyte Na+ and K+ currents.

The role of androgens in female vertebrates

The role of androgens in vertebrate females has been overlooked until recently. We examine the functional significance of androgens in females by reviewing studies that document relatively high levels of circulating plasma androgens, androgen receptors, or androgen-metabolizing enzymes in females. Among the mechanisms of androgenic action identified are enhanced neuron survival, stimulation of muscle satellite cell proliferation, alteration of ion current kinetics, and release of somatostatin. These mechanisms are not sex specific and thus we hypothesize that androgens play a significant role in normal female development. We encourage study in this nontraditional research area.

Model of P- and T-electroreceptors of weakly electric fish

To clarify the microscopic mechanisms by which P- and T-receptors encode amplitude modulation and zero crossing time of jamming signals, we present a model of P- and T-receptors based on their physiological and anatomical properties. The model consists of a receptor cell, supporting cells, and an afferent nerve fiber. The basal membrane of the receptor cell includes voltage-sensitive Ca2+ channels, Ca(2+)-activated K+ channels, and leak channels of Na+, K+, and Cl-. The driving force of potential change under stimulation is generated by the voltage-sensitive Ca2+ channels, and the suppressing force of the change is generated by Ca(2+)-activated K+ channels. It has been shown that in T-receptor cells the driving force is much stronger than the suppressing force, whereas in P-receptor cells the driving force is comparable with the suppressing force. The difference in various kinds of response properties between P- and T-receptors have been consistently explained based on the difference in the relative strengths of the driving and suppressing forces between P- and T-receptor cells. The response properties considered are encoding function, probability of firing of afferent nerve, pattern of damped oscillation, shape of tuning curves, values of the optimum frequency, and response latency.

Subthreshold oscillations and resonant frequency in guinea-pig cortical neurons: physiology and modelling

1. Intracellular recordings were made from neurons in slices from guinea-pig frontal cortex. In 50% of the cells, sustained subthreshold voltage oscillations were evoked by long (> 6 s) depolarizing pulses. The peak-to-peak amplitude of these oscillations was less than 5 mV and the frequency was voltage dependent, increasing with depolarization from 4 (near rest) to 20 Hz (at 30 mV depolarization). 2. The impedance-frequency relationship of both oscillating and non-oscillating cells was studied by intracellular injection of sinusoidal current with linearly changing frequency. In most cells, a peak in the impedance magnitude (resonant behaviour) was observed at depolarized levels. The frequency of the peak impedance (peak frequency) increased with depolarization from 3 (near rest) to 15 Hz (at 30 mV depolarization). 3. Application of TTX (10(-6) M) significantly decreased the impedance magnitude near the peak frequency. The subthreshold oscillations, however, as well as the action potentials, were fully blocked by TTX. On the other hand, TEA (15 mM) and Cs+ (5 mM) abolished both the subthreshold oscillations and the resonant behaviour. Replacing Ca2+ with Co2+ (5 mM) or Ni2+ (1 mM) did not abolish the subthreshold oscillations. The peak in the frequency-response curve was only slightly reduced. 4. An isopotential membrane model, consisting of a leak current, a fast persistent sodium current, a slow non-inactivating potassium current (with the kinetics of the M-current) and membrane capacitance, is sufficient to produce both voltage oscillations and resonant behaviour. The kinetics of the K+ current by itself is sufficient to produce resonance behaviour. The Na+ current amplifies the peak impedance magnitude and is essential for the generation of subthreshold oscillation. The model correctly predicted the behaviour of the frequency response before and after TTX and TEA application, as well as the relation between the expected passive impedance and the experimental impedance. 5. We speculate that the tendency of the neurons to generate voltage signals at a certain frequency (as a result of the subthreshold oscillations) and to preferentially respond to inputs arriving at the same frequency (the resonance behaviour) promotes population activity at that preferred frequency.

Statistical quantification of 24-hour and monthly variabilities of spontaneous otoacoustic emission frequency in humans

Previous evidence has suggested a relationship between spontaneous otoacoustic emissions (SOAEs) and established, biological cycles, although detailed statistical quantifications of the suggested relationships do not exist in the literature. In an attempt to statistically quantify the purported circadian and monthly influences on this phenomenon, two experiments were undertaken. The first experiment was conducted over eight weeks, investigating 31 SOAEs recorded from eight women and two men. Time series statistical analysis examined whether daily, weekly, and/or monthly cycles characterized SOAE frequency variability. Results yielded a significant monthly cycle for the majority of SOAEs recorded from the women but for none of the SOAEs recorded from the men. These results suggest the possibility that SOAE frequency fluctuation in women may be entrained to the monthly menstrual cycle. In the second experiment, hourly SOAE frequency stability was examined over a 24-h period to ascertain the nature of the daily frequency variation as precisely as possible. Four SOAEs from two subjects were examined, and time series analysis of these data included (1) modelling the autocorrelation structure of the measurements, (2) resolving each 24-h series of measurements into cyclical components of various periodicities, and (3) testing the statistical significance of given cycles within the spectrum of each series. Findings included a significant 24-h variability of frequency for each SOAE, suggesting the possibility of a circadian influence on frequency fluctuation. Results from the two experiments provide quantitative evidence supporting a hypothetical relationship between SOAEs and established, biological cycles.

Conductances contributing to the action potential of Sternopygus electrocytes

In Sternopygus macrurus, electrocyte action potential duration determines the electric organ discharge pulse duration. Since the electric organ discharge is a sexually-dimorphic behavior under the control of steroid hormones, and because electrocyte action potential durations can range from 3-14 ms, the electrocytes provide a unique opportunity to study how sex steroids regulate membrane excitability. In this study, the voltage-sensitive ionic currents of electrocytes were identified under current- and voltage-clamp as a prelude to further studies on their regulation by sex steroid hormones. Bath application of TTX completely abolished the spike and eliminated an inward current under voltage clamp, indicating that the action potential is due primarily to a sodium current. Calcium-free saline had no effect on spike waveform or voltage-clamp currents, indicating that neither calcium nor calcium-dependent currents contribute to the action potential. Application of potassium channel blocking agents, such as tetraethylammonium and cesium ions, caused changes in the spike which, together with voltage-clamp results, indicate the presence of two potassium currents: an inward rectifier and a classical delayed rectifier. In addition, these cells have a large, presumably voltage-insensitive, chloride current. Differences in one or more of these currents could be responsible for the range of action potential durations found in these cells and for the steroid-mediated changes in spike duration.

Circadian and menstrual rhythms in frequency variations of spontaneous otoacoustic emissions from human ears

This paper reports hourly and daily monitoring of the frequencies of spontaneous otoacoustic emissions. Regular circadian variations in frequency were found in two of three subjects. Consistent monthly variations, in step with the menstrual cycle, were seen in three of four women. The circadian cycle typically showed a rise in frequency of 0.6-1% while asleep and a similar fall while awake. The monthly cycle typically saw frequencies rise and fall by 0.4-0.6%, reaching a minimum near the onset of menstruation, and rising to a peak close to ovulation. A review of the literature revealed that certain cardiovascular parameters such as arterial blood pressure follow, over both daily and menstrual cycles, a broadly similar time course to SOAE frequency. Further experiments produced data supporting a relationship between blood pressure and SOAE frequency, and it is therefore suggested that much of the circadian-linked, menstrual-linked, and background variation in SOAE frequency may arise from cardiovascular changes. A likely causal mechanism, involving cerebrospinal fluid, is discussed.

The effects of androgens and estrogen on the external morphology and electric organ discharge waveform of Gnathonemus petersii (Mormyridae, Teleostei)

The effects of androgens and estrogen on the external morphology and electric organ discharge (EOD) waveform in Gnathonemus petersii, a weakly discharging electric fish, were investigated. Following preimplant data collection, juvenile and adult fish were gonadectomized and implanted with silastic capsules containing either high or low doses of testosterone (T), dihydrotestosterone (DHT), estradiol-17 beta (E2), or cholesterol. One group of fish was treated with high doses of DHT + E2. Radioimmunoassays revealed that low-dose implants resulted in plasma T levels comparable to and high-dose implants about sixfold greater than those found in adult males imported during breeding season. High-dose E2 implants resulted in higher plasma E2 levels in adults than those in juveniles. At either dose, both androgens induced male-like indentations in the dorsal margin of the anal fin of juveniles and adult females by 4 weeks postimplant. Both low and high doses of T decreased the peak power spectrum frequency (PPSF) of Fourier transformations of EODs and increased the durations of phases 2 and 3 of the EOD in juveniles and adults, but the high doses caused more rapid and profound effects. The two doses of T caused opposite effects on the durations of phases 1 and 4 juveniles. The low dose of T decreased the durations of phases 1 and 4, while the high dose increased them. In adults, the high dose of T increased the duration of phase 1, but had inconsistent effects on the duration of phase 4. Total EOD durations were increased by both doses of T in juveniles, while adults showed inconsistent effects possibly due to individual variability in hormone sensitivity. Compared to T, DHT exerted similar, but less dramatic effects on all measures, but only at high doses. E2 significantly increased adult PPSFs, the first such finding in a mormyrid species. E2 had no effects on juvenile PPSFs, or on adult or juvenile EOD phase durations. The effects of DHT + E2 on PPSF and phases 2 and 3 were similar to those of DHT alone. These findings demonstrate quantifiable steroid-dependent plasticity in the durations of individual phases of EODs in an electric fish and are the first to show that the external morphology in Gnathonemus petersii is androgen-dependent. The results are discussed with regard to methodological considerations and hormone studies involving sex differences in EODs reported for this and other species.

Human chorionic gonadotropin-induced shifts in the electrosensory system of the weakly electric fish, Sternopygus

Sternopygus macrurus of both sexes were injected with human chorionic gonadotropin (hCG) or saline. Electric organ discharge (EOD) frequency rose after hCG injections in females and gradually declined to baseline levels over the next few weeks. EOD changes in males were more complex and variable; most males showed an initial minor rise in EOD frequency followed by a larger decrease, or simply a decrease. hCG treatment also resulted in a rise in electroreceptor best frequency and shortened electric organ pulse duration in females, and had the opposite effect on these parameters in males. The saline-injected controls showed no changes in any of these parameters. Levels of testosterone (T) and 11-ketotestosterone, but not estrogen (E), were elevated in males preceding the fall in EOD frequency, whereas neither T nor E changed significantly in females before EOD frequency increases. Saline injections caused a drop in T in the male control group and had no effect in the female control group. We presume that the effect of hCG on the electrosensory system of males is mediated via androgens. Whether the effects of hCG on females are mediated by slight increases in circulating levels of gonadal steroids, the release of hormones other than T or E, or are due to direct effects on the nervous system is not known.

Hormone-induced and maturational changes in electric organ discharges and electroreceptor tuning in the weakly electric fish Apteronotus

Plasticity in the frequency of the electric organ discharge (EOD) and electroreceptor tuning of weakly electric fish was studied in the genus Apteronotus. Both hormone-induced and maturational changes in EOD frequency and electroreceptor tuning were examined. Apteronotus is different from all other steroid-responsive weakly electric fish in that estradiol-17 beta, rather than androgens, induces discharge frequency decreases. This result can account for the 'reversed' discharge frequency dimorphism found in Apteronotus in which, counter to all other known sexually dimorphic electric fish, females have lower discharge frequencies than males. Studies of electroreceptor tuning in Apteronotus indicate that electroreceptors are closely tuned to the frequency of the EOD. This finding was noted not only in adult animals, but also in juvenile animals shortly after the onset of their EODs. Tuning plasticity in Apteronotus, as in other species studied, is associated with altered EOD frequencies and was noted in both maturational EOD changes and in estrogen-induced changes. Thus, tuning plasticity appears to be a general phenomenon which occurs concurrent with a variety of EOD changes.

Evidence for a direct effect of androgens upon electroreceptor tuning

Tuberous electroreceptors of individual wave type weakly electric fish are tuned to the fundamental frequency of that fish's electric organ discharge (EOD). EOD frequency and receptor best frequency (BF) are both lowered following systemic injection of 5-alpha-dihydrotestosterone (DHT). A previous study (Meyer et al. 1984) showed that the effect of DHT on the EOD generating circuitry was independent of an ongoing EOD and suggested that its effect on electroreceptor tuning was indirect, possibly mediated by the electric field. We have continued these studies to determine the factors which influence electroreceptor tuning. Baseline recordings of EOD frequency, receptor oscillations, and single afferent tuning curves were taken. After fish were electrically silenced by spinal cord transection they were injected daily with either DHT or saline or were implanted with either DHT-filled or empty silastic capsules. As previously reported, the EOD frequency (determined from pacemaker nucleus recordings) was lowered in DHT-treated, transected fish and increased in control fish. Similarly, receptor tuning was lowered in the DHT-treated, silenced fish. Oscillation frequencies decreased in both treated and control groups, but significantly more in the hormone group. Single afferent best frequencies were lowered in both DHT groups and raised in their respective control groups. In another series of experiments exogenous electric fields capable of driving receptors in a 1-to-1 phase-locked manner were placed around silenced fish. We were unable to elicit any shift in pacemaker frequency or electroreceptor tuning regardless of stimulus field geometry. Four transected fish were injected with DHT and placed in exogenous electric fields of higher frequency than their original EOD. Even in the presence of a higher frequency electric field, DHT lowered EOD frequency and afferent BF. We conclude that androgens produce effects both on the EOD generating circuitry, probably at the level of the pacemaker nucleus, and on electroreceptors, probably, ultimately, on receptor cell membrane conductances. These effects occur in parallel allowing the two parameters to remain well matched. In contrast to former predictions, exogenous electric fields alone appear unable to shift receptor tuning.

Tonotopic organization in the midbrain of a teleost fish

Recordings from small clusters of units within the auditory midbrain of the Japanese carp, Cyprinus carpio, provide the first evidence for tonotopy within the teleost brain. These findings suggest that place mechanisms of frequency coding and tonotopy, previously thought to have evolved first in amphibians, may be general and ancient features of the vertebrate auditory system.

Diurnal cycle for spontaneous oto-acoustic emission frequency

Spontaneous oto-acoustic emission frequency was measured in two subjects at different times on the same day. A statistically significant frequency decrease during the morning hours was observed for both subjects. Possible mechanisms (ear canal pressure, body temperature, androgen level) for this decrease are discussed.

Postembryonic changes in the peripheral electrosensory system of a weakly electric fish: addition of receptor organs with age

The organization of the peripheral electrosensory system of the cheek was studied in an age-graded series of Sternopygus dariensis in Nissl-stained sections and silver-stained whole mounts of skin. As in other gymnotoids, both ampullary and tuberous electroreceptors are present. Small fish have only one ampullary organ or tuberous organ per axon, and the number of receptor organs per axon increases with age in both ampullary and tuberous systems. Large fish may have up to ten tuberous organs per axon, although the distribution of tuberous organs per axon is bimodal with one peak occurring at a single receptor organ per axon and the other peak shifting upward in relation to the age of the fish. The ampullary system adds receptor organs at a faster rate and a large fish may have 20 ampullary organs per axon. With increasing size, the number of sensory receptor cells in each organ remains constant for both types of electroreceptors. Evidence is presented for addition of new electroreceptor units by de novo production in small fish and increases in the number of organs in existing electroreceptor units by division of previously formed organs in medium-sized and large fish. As the surface area of the skin increases with growth, the density of electroreceptor units decreases and, although new receptor organs are still being added to existing receptor units, no generation of new receptor units occurs in medium-sized to large fish.

Cable theory in neurons with active, linearized membranes

This investigation aims at exploring some of the functional consequences of single neurons containing active, voltage dependent channels for information processing. Assuming that the voltage change in the dendritic tree of these neurons does not exceed a few millivolts, it is possible to linearize the non-linear channel conductance. The membrane can then be described in terms of resistances, capacitances and inductances, as for instance in the small-signal analysis of the squid giant axon. Depending on the channel kinetics and the associated ionic battery the linearization yields two basic types of membrane: a membrane modeled by a collection of resistances and capacitances and membranes containing in addition to these components inductances. Under certain specified conditions the latter type of membrane gives rise to a membrane impedance that displays a prominent maximum at some nonzero resonant frequency fmax. We call this type of membrane quasi-active, setting it apart from the usual passive membrane. We study the linearized behaviour of active channels giving rise to quasi-active membranes in extended neuronal structures and consider several instances where such membranes may subserve neuronal function: 1. The resonant frequency of a quasi-active membrane increases with increasing density of active channels. This might be one of the biophysical mechanisms generating the large range over which hair cells in the vertebrate cochlea display frequency tuning. 2. The voltage recorded from a cable with a quasi-active membrane can be proportional to the temporal derivative of the injected current. 3. We modeled a highly branched dendritic tree (delta-ganglion cell of the cat retina) using a quasi-active membrane. The voltage attenuation from a given synaptic site to the soma decreases with increasing frequency up to the resonant frequency, in sharp contrast to the behaviour of passive membranes.(ABSTRACT TRUNCATED AT 400 WORDS)

Hormonal control of sexual differentiation: changes in electric organ discharge waveform

Males and females of some mormyrid electric fishes generate electrical pulses that differ in waveform and duration. For one such species, testosterone or dihydrotestosterone induces females and immature males to produce the mature male electric organ discharge which is two times the duration of the female or immature discharge. Estradiol has only a weak effect. For a second species where males and females have similar electric organ discharges, testosterone produces no effect. The data suggest that androgens affect the electric organ itself.