Spatial distribution of the medullary command signal within the electric organ of Gymnotus carapo

The pacemaker nucleus of Gymnotus carapo contains two types of neurons: pacemaker cells which set up the frequency of the electric organ discharge (EOD) and relay cells which convey the command signal to the spinal cord. Direct activation of a single relay cell provides enough excitation to discharge a pool of spinal electromotor neurons and electrocytes, generating a small EOD (unit EOD). Different relay cells generate unit EODs of variable size and waveform, indicating the involvement of different groups of electrocytes. A special technique of EOD recording (multiple air-gap) was combined with intracellular stimulation of relay cells to study the spatial distribution within the electric organ (EO) of the command signal arising from different relay cells. Three types of relay cells could be identified: type I commanding the rostral 10% of the EO, type II which distribute their command all along the EO and type III driving the caudal 30%. Waveform analysis of unit EODs indicates that doubly innervated electrocytes which are the most relevant for attaining the specific EOD waveform, receive a favored command from the pacemaker nucleus.Abbreviations CV conduction velocity - EMF electromotive force - EMN electromotor neuron - EO electric organ - EOD electric organ discharge - PN pacemaker nucleus - uEOD unit electric organ discharge     

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.     

Behavioral evidence of a latency code for stimulus intensity in mormyrid electric fish

Mormryid electric fish (Gnathonemus petersii) respond to novel stimuli with an increase in the rate of the electric organ discharge (EOD). These novelty responses were used to measure the fish's ability to detect small changes in the amplitude and latency of an electrosensory stimulus. Responses were evoked in curarized fish in which the EOD was blocked but in which the EOD motor command continued to be emitted. An artificial EOD was provided to the fish at latencies of 2.4 to 14.4 ms following the EOD motor command.Novelty responses were evoked in response to transient changes in artificial EOD amplitude as small as 1% of baseline amplitude, and in latency as small as 0.1 ms. Changes in latency were effective only at baseline delays of less than 12.4 ms.The sensitivity to small changes in latency supports the hypothesis that latency is used as a code for stimulus intensity in the active electrolocation system of mormyrid fish. The results also indicate that a corollary discharge signal associated with the EOD motor command is used to measure latency.Abbreviations EOD electric organ discharge - ELL electrosensory lateral line lobe - epsp excitatory post synaptic potential     

From distributed sensory processing to discrete motor representations in the diencephalon of the electric fish,Eigenmannia

Summary During their jamming avoidance response (JAR), weakly electric fish of the genusEigenmannia shift their electric organ discharge (EOD) frequency away from a similar EOD frequency of a neighboring fish. The behavioral rules and neural substrates for stimulus recognition and motor control of the JAR have been extensively studied (see review by Heiligenberg 1986). The diencephalic nucleus electrosensorius (nE) links sensory processing within the torus semicircularis and optic tectum with the mesencephalic prepacemaker nucleus which, in turn, modulates the medullary pacemaker nucleus and hence the EOD frequency. Two separate areas within the nE responsible for JAR-related EOD frequency rises and frequency falls, respectively, were identified by iontophoresis of the excitatory amino acid L-glutamate. Bilateral lesion of the areas causing EOD frequency rises resulted in elimination of JAR-related frequency rises above a baseline frequency obtained in the absence of a jamming stimulus. Similarly, bilateral lesion of the areas causing frequency falls resulted in a loss of JAR-related frequency falls below the baseline frequency. Whether these areas are also responsible for non-JAR-related frequency shifts is not known. The strength of response and spatial extent of the areas causing frequency shifts varied among fish and also varied in individual fish, reflecting the strength of JAR-related frequency shifts and the balance of activities in frequency-rise and frequency-fall areas. Local application of bicuculline-methiodide or GABA demonstrated a tonic inhibitory input to each area and suggests a reciprocal inhibitory interaction between the two ipsilateral areas, possibly accounting for much of the individual plasticity.The nE thus is a site for neuronal transformation from distributed, topographically organized processing within the laminated structures of the torus and tectum to discrete cell clusters which control antagonistic motor responses.Abbreviations EOD electric organ discharge - JAR jamming avoidance response - Df difference frequency between jamming signal and the fish's own EOD - nE nucleus electrosensorius - PPn prepacemaker nucleus     

Spinal mechanisms of electric organ discharge synchronization in Gymnotus carapo

Summary The duration of the electric organ discharge (EOD) in Gymnotus carapo is brief and independent of fish size. Spinal mechanisms involved in electrocyte synchronization were explored by recording spontaneous action potentials of single fibers from the electromotor bulbospinal tract (EBST). Using the field potential of the medullary electromotor nucleus (MEN) as a temporal reference we calculated the orthodromic conduction velocity (CV) of these fibers (range: 10.7–91 m/s).The CVs (in m/s) of fibers recorded at the same level of the spinal cord were significantly different in small and large fish; this difference disappeared when CV were expressed as percentage of body length/ms. Plotting these values against conduction distance (also in %) showed that low CV fibers predominate in the rostral cord while only fast fibers are found at distal levels. Moreover, antidromic stimulation of the distal cord was only effective on high CV fibers. The orthodromic CVs in the distal portion of the recorded fibers were calculated by collision experiments; no significant differences were found between proximal and distal portions.The spatial distribution of CV values within the EBST is proposed to play the main role in synchronizing the electromotoneurons' activity along the spinal cord.Abbreviations EOD electric organ discharge - EO electric organ - EBST electromotor bulbospinal tract - MEN medullary electromotor nucleus - CV conduction velocity - EMN electromotoneuron     

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     

Distinct mechanisms of modulation in a neuronal oscillator generate different social signals in the electric fishHypopomus

Summary The medullary pacemaker nucleus of the gymnotiform electric fish,Hypopomus, is a relatively simple neuronal oscillator which contains pacemaker cells and relay cells. The pacemaker cells generate a regular discharge cycle and drive the relay cells which trigger pulse-like electric organ discharges (EODs). The diencephalic prepacemaker nucleus (PPN) projects to the pacemaker nucleus and modulates its activity to generate a variety of specific discharge patterns which serve as communicatory signals (Figs. 2 and 3).While inducing such signals by microiontophoresis of L-glutamate to the region of the PPN (Fig. 4) of curarized animals, we monitored the activity of neurons in the pacemaker nucleus intracellularly. We found that pacemaker cells and relay cells were affected differently in a manner specific to the type of EOD modulation (Figs. 5–10). The normal sequence of pacemaker cell and relay cell firing was maintained during gradual rises and falls in discharge rate. Both types of cells ceased to fire during interruptions following a decline in discharge rate. During sudden interruptions, however, relay cells were steadily depolarized, while pacemaker cells continued to fire regularly. Short and rapid barrages of EODs, called chirps, were generated through direct and synchronous activation of the relay cells whose action potentials invaded pacemaker cells antidromically and interfered with their otherwise regular firing pattern.Abbreviations EOD electric organ discharge - HRP horseradish peroxidase - NMDA N-Methyl-D-Aspartate - PPN prepacemaker nucleus     

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     

The electric image in weakly electric fish: I. A data-based model of waveform generation inGymnotus carapo

Understanding how electrosensory images are generated and perceived in actively electrolocating fish requires the study of the characteristics of fish bodies as electric sources. This paper presents a model ofGymnotus carapo based on measurements of the electromotive force generated by the electric organ and the impedance of the passive tissues. A good agreement between simulated and experimentally recorded transcutaneous currents was obtained. Passive structures participate in the transformation of the electromotive force pattern into transcutaneous current profiles. These spatial filtering properties of the fish's body were investigated using the model. The shape of the transcutaneous current profiles depends on tissue resistance and on the geometry and size of the fish. Skin impedance was mainly resistive. The effect of skin resistance on the spatial filtering properties of the fish's body was theoretically analyzed.The model results show that generators in the abdominal and central regions produce most of the currents through the head. This suggests that the electric organ discharge (EOD), generated in the abdominal and central regions is critical for active electrolocation. In addition, the well-synchronized EOD components generated all along the fish produce large potentials in the far field. These components are probably involved in long-distance electrocommunication.Preliminary results of this work were published as a symposium abstract.     

The electric organ discharges of the gymnotiform fishes: I. Apteronotus leptorhynchus

We present high temporal and spatial resolution maps in 3-dimensions of the electric field vector generated by the weakly electric fish, Apteronotus leptorhynchus. The waveforms and harmonic composition of the electric organ discharge (EOD) are variable around the fish but highly stable over long times at any position. We examine the role of harmonics on the temporal and spatial characteristics of the EOD, such as the slew rate and rostral-to-caudal propagation. We also explore the radial symmetry of the fish's field. There are major differences in the direction of the electric field vector at the head and caudal body. In the caudal part of the fish, the electric field vector rotates during the EOD cycle. However, rostral of the pectoral fin, the field magnitude and sign oscillate while maintaining relatively constant orientation. We discuss possible functional ramifications of these electric field patterns to electrolocation, communication, and electrogenesis.Abbreviations EOD electric organ discharge - EO electric organ - RMS root mean square - ADC analog-to-digital converter     

Interruption of pacemaker signals is mediated by GABAergic inhibition of the pacemaker nucleus in the African electric fish <Emphasis Type="Italic">Gymnarchus niloticus</Emphasis>

The wave-type African weakly electric fish Gymnarchus niloticus produces electric organ discharges (EODs) from an electric organ in the tail that is driven by a pacemaker complex in the medulla, which consists of a pacemaker nucleus, two lateral relay nuclei and a medial relay nucleus. The prepacemaker nucleus (PPn) in the area of the dorsal posterior nucleus of the thalamus projects exclusively to the pacemaker nucleus and is responsible for EOD interruption behavior. The goal of the present study is to test the existence of inhibition of the pacemaker nucleus by the PPn. Immunohistochemical results showed clear anti-GABA immunoreactive labeling of fibers and terminals in the pacemaker nucleus, but no apparent anti-glycine immunoreactivity anywhere in the pacemaker complex. GABA injection into the pacemaker nucleus could induce EOD interruptions that are comparable to the interruptions induced by glutamate injection into the PPn. Application of the GABA_A receptor blocker bicuculline methiodide reversibly eliminated the effects of stimulation of the PPn. Thus the EOD interruption behavior in Gymnarchus is mediated through GABAergic inhibition of the pacemaker nucleus by the PPn.     

Androgens alter electric organ discharge pulse duration despite stability in electric organ discharge frequency.

Weakly electric fish in the genus Sternopygus emit a sinusoidal, individually distinct, and sexually dimorphic electric organ discharge (EOD) that is used in electrolocation and communication. Systemically applied androgens decrease EOD frequency, which is set by a medullary pacemaker nucleus, and increase pulse duration, which is determined by the cells of the electric organ (the electrocytes), in a coordinated fashion. One possibility is that androgens broaden the EOD pulse duration by acting on the pacemaker neurons, thereby effecting a change in pacemaker firing frequency, and that the change in EOD pulse duration is due to an activity-dependent process. To determine whether androgens can alter pulse duration despite a stable pacemaker nucleus firing frequency, we implanted small doses of dihydrotestosterone in the electric organ. We found that androgen implants increased EOD pulse duration, but did not influence EOD frequency. In addition, using immunocytochemistry, we found that electrocytes label positively with an androgen receptor antibody. While it is not known on which cells androgens act directly, together these experiments suggest that they likely act on the electrocytes to increase EOD pulse duration. Since pulse duration is determined by electrocyte action potential duration and ionic current kinetics, androgens may therefore play a causative role in influencing individual variation and sexual dimorphism in electrocyte electrical excitability, an important component of electrocommunicatory behavior.     

Waveform generation in Rhamphichthys rostratus (L.) (Teleostei,Gymnotiformes)

Rhamphichthys rostratus (L.) emits brief pulses (2 ms) repeated very regularly at 50 Hz. The electric organ shows a heterogeneous distribution of the electrocyte tubes and the occurrence of three electrocyte types (caudally innervated, rostrally innervated and marginallycaudally innervated). In the sub-opercular region the electric organ consists of a pair of tubes containing only caudally innervated electrocytes. At the abdominal region the EO consists of three pairs of tubes. Each pair contains one of the described electrocyte types. The number of electrocyte tubes increases toward the tail to reach nine or ten pairs in the most caudal segments. In the intermediate region most tubes contain doubly innervated electrocytes except the ventral pair that contains caudally innervated electrocytes. The caudal 25% contains exclusively caudally innervated electrocytes. The electric organ discharge consists of five wave components (V1 to V5). Electrophysiological data are consistent with the hypothesis that V1 results from the activity of the rostral faces of rostrally innervated electrocytes. V2 results from the activities of rostral faces of marginally-caudally innervated electrocytes while V3 results from the activities of caudal faces of most electrocytes. Curarization experiments demonstrated that V4 and V5 result from action potential invasion and are not directly elicited by neural activity.Abbreviations AEN1 anterior electromotor nerve 1 - AEN2 anterior electromotor nerve 2 - BMB boraxic methylene blue - CIE caudally innervated electrocytes - EMF electromotive force - EO electric organ - EOD electric organ discharge - I current amplitude - MCIE marginally-caudally innervated electrocytes - MT medial tubes - PEN posterior electromotor nerve - R _n internal impedance - RIE rostrally innervated electrocytes - Rl load resistor - SAT short abdominal tubes - V voltage amplitude     

An internal current source yields immunity of electrosensory information processing to unusually strong jamming in electric fish

Summary The electric organ of a fish represents an internal current source, and the largely isopotential nature of the body interior warrants that the current associated with the fish's electric organ discharges (EODs) recruits all electroreceptors on the fish's body surface evenly. Currents associated with the EODs of a neighbor, however, will not penetrate all portions of the fish's body surface equally and will barely affect regions where the neighbor's current flows tangentially to the skin surface. The computational mechanisms of the jamming avoidance response (JAR) in Eigenmannia exploit the uneven effects of a neighbor's EOD current to calculate the correct frequency difference between the two interfering EOD signals even if the amplitude of a neighbor's signal surpasses that of the fish's own signal by orders of magnitude. The particular geometry of the fish's own EOD current thus yields some immunity against the potentially confusing effects of unusually strong interfering EOD currents of neighbors.Abbreviations DF frequency difference - ELL electrosensory lateral line lobe - EOD electric organ discharge - JAR jamming avoidance response     

Sexual and seasonal plasticity in the emission of social electric signals. Behavioral approach and neural bases

Behavior in electric fish includes modulations of a stereotyped electric organ discharge (EOD) in addition to locomotor displays. Gymnotiformes can modulate the EOD rate to produce signals that participate in different behaviors. We studied the reproductive behavior of Brachyhypopomus pinnicaudatus both in the wild and laboratory settings. During the breeding season, fish produce sexually dimorphic social electric signals (SES): males emit three types of chirps (distinguished by their duration and internal structure), and accelerations, whereas females interrupt their EOD. Since these SES imply EOD frequency modulations, the pacemaker nucleus (PN) is involved in their generation and constitutes the main target organ to explore seasonal and sexual plasticity of the CNS. The PN has two types of neurons, pacemakers and relays, which receive modulatory inputs from pre-pacemaker structures. These neurons show an anisotropic rostro-caudal and dorso-ventral distribution that is paralleled by different field potential waveforms in distinct portions of the PN. In vivo glutamate injections in different areas of the PN provoke different kinds of EOD rate modulations. Ventral injections produce chirp-like responses in breeding males and EOD interruptions in breeding females, whereas dorsal injections provoke EOD frequency rises in both sexes. In the non-breeding season, males and females respond with interruptions when stimulated ventrally and frequency rises when injected dorsally. Our results show that changes of glutamate effects in the PN could explain the seasonal and sexual differences in the generation of SES. By means of behavioral recordings both in the wild and in laboratory settings, and by electrophysiological and pharmacological experiments, we have identified sexual and seasonal plasticity of the CNS and explored its underlying mechanisms.     

The jamming avoidance response ofEigenmannia: properties of a diencephalic link between sensory processing and motor output

Summary Brain regions participating in the control ofEigenmannia's electric organ discharge frequency were localized by electrical microstimulation and anatomically identified by means of horseradish peroxidase deposition. A diencephalic region was found which, when stimulated, caused electric organ discharge (EOD) frequency increases of similar magnitude and time course as the frequency increases seen during the jamming avoidance response. Single unit recordings from this region revealed one cell type which preferentially responded to stimuli that cause the acceleration phase of the jamming avoidance response (electric organ discharge frequency increase). A second cell type responded preferentially to stimuli which cause EOD frequency decrease, and both cell types were tuned to stimuli which evoked maximal jamming avoidance behaviors.The results of the horseradish peroxidase experiments showed that the recording and stimulation sites correspond to the previously described nucleus electrosensorius. Our results confirm the earlier finding that this nucleus receives output from the torus semicircularis and we also found that the N. electrosensorius projects to the mesencephalic prepacemaker nucleus. The prepacemaker projects to the medullary pacemaker nucleus which generates the commands that evoke electric organ discharges.The anatomical and physiological results described here establish this diencephalic region as a link between the major sensory processing region for the jamming avoidance response, the torus semicircularis, and a mesencephalic pre-motor region, the prepacemaker nucleus.Abbreviations AM amplitude modulation - DF Delta F - ELLL electrosensory lateral line lobe - EOD electric organ discharge - JAR jamming avoidance response - NE nucleus electrosensorius - PPN prepacemaker nucleus - PN pacemaker nucleus     

The development of the Jamming Avoidance Response (JAR) in Eigenmannia: An innate behavior indeed

Summary In its Jamming Avoidance Response (JAR), the gymnotiform electric fish Eigenmannia shifts its electric organ discharge (EOD) frequency away from similar interfering frequencies. Continual behavioral measurements were carried out in 164 juvenile fish until a correct JAR emerged. Sixty-four of these fish were raised in complete isolation, the remainder in a community of their siblings. A correct JAR emerged in fish of 1.2–1.6 cm in body length, corresponding to a developmental age of 24–32 days. In 6 of 164 fish, the emergence of a correct JAR followed an interim appearance of an incorrect JAR, which involved frequency shifts in the direction opposite to those of a correct JAR. The fish raised in isolation developed the same forms of behavior and showed the same sequence in their appearance as did socially raised fish. This indicates that the JAR and its developmental schedule are innate. The appearance of an incorrect JAR suggests initial errors or incompleteness in the wiring of central nervous connections. A correct JAR ultimately emerged even if a stimulus regimen was offered that rewarded frequency shifts in the direction opposite to those of a correct JAR. This indicates that the development of the JAR is immune to experimental alterations of sensory experience.Abbreviations Df frequency difference between a jamming stimulus and fish's EOD - ELL electrosensory lateral line lobe - EO electric organ - EOD electric organ discharge - JAR Jamming Avoidance Response - nE nucleus electrosensorius - nE subnucleus of nE, causing drop of EOD frequency - nE subnucleus of nE, causing rise of EOD frequency - Pn pacemaker nucleus - PPn prepacemaker nucleus     

Interruption of pacemaker signals by a diencephalic nucleus in the African electric fish, Gymnarchus niloticus

The African electric fish Gymnarchus niloticus rhythmically emits electric organ discharges (EODs) for communication and navigation. The EODs are generated by the electric organ in the tail in response to the command signals from the medullary pacemaker complex, which consists of a pacemaker nucleus (PN), two lateral relay nuclei (LRN) and a medial relay nucleus (MRN). The premotor structure and its modulatory influences on the pacemaker complex have been investigated in this paper. A bilateral prepacemaker nucleus (PPn) was found in the area of the dorsal posterior nucleus (DP) of the thalamus by retrograde labeling from the PN. No retrogradely labeled neurons outside the pacemaker complex were found after tracer injection into the LRN or MRN. Accordingly, anterogradely labeled terminal fibers from PPn neurons were found only in the PN. Iontophoresis of l-glutamate into the region of the PPn induced EOD interruptions. Despite the exclusive projection of the PPn neurons to the PN, extracellular and intracellular recordings showed that PN neurons continue their firing while MRN neurons ceased their firing during EOD interruption. This mode of EOD interruption differs from those found in any other weakly electric fishes in which EOD cessation mechanisms have been known.     

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

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

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.