Dynamics and stimulus-dependence of pacemaker control during behavioral modulations in the weakly electric fish,Apteronotus

1. Weakly electric fish generate around their bodies low-amplitude, AC electric fields which are used both for the detection of objects and intraspecific communication. The types of modulation in this signal of which the high-frequency wave-type gymnotiform, Apteronotus, is capable are relatively few and stereotyped. Chief among these is the chirp, a signal used in courtship and agonistic displays. Chirps are brief and rapid accelerations in the normally highly regular electric organ discharge (EOD) frequency. 2. Chirping can be elicited artificially in these animals by the use of a stimulus regime identical to that typically used to elicit another behavior, the jamming avoidance response (JAR). The neuronal basis for the JAR, a much slower and lesser alteration in EOD frequency, is well understood. Examination of the stimulus features which induce chirping show that, like the JAR, there is a region of frequency differences between the fish's EOD and the interfering signal that maximally elicits the response. Moreover, the response is sex-specific with regard to the sign of the frequency difference, with females chirping preferentially on the positive and most males on the negative Df. These features imply that the sensory mechanisms involved in the triggering of these communicatory behaviors are fundamentally similar to those explicated for the JAR. 3. Additionally, two other modulatory behaviors of unknown significance are described. The first is a non-selective rise in EOD frequency associated with a JAR stimulus, occurring regardless of the sign of the Df. This modulation shares many characteristics with the JAR. The second behavior, which we have termed a 'yodel', is distinct from and kinetically intermediate to chirping and the JAR. Moreover, unlike the other studied electromotor behaviors it is generally produced only after the termination of the eliciting stimulus.     

How lesioning the nucleus praeeminentialis affects electrolocation behavior in the weakly electric fish, Apteronotus leptorhynchus

The ability to orient to and track moving electrolocation targets was assessed in normal Apteronotus leptorhynchus and in those with unilateral lesions of the nucleus praeeminentialis dorsalis.

1. Each fish was trained to hover between two vertical metal rods and track their movement. Two aspects of this behavior were measured: a) the hovering position of the fish relative to stationary rods; b) the latency between the onset of rod motion and the fish's tracking response. Control fish hovered midway between stationary rods, while lesioned fish hovered closer to the rod ipsilateral to the lesion. Response latency varied negatively with rod diameter in both sets of fish, and lesioned fish exhibited shorter latencies than control fish. While the response latencies of control fish were shortest when their starting position was midway between the rods, lesioned animals' latencies were shortest when they hovered closer to the rod ipsilateral to their lesion.
2. Control fish responded to the approach of a single metal ball to either side of the body with nearly equal latencies and fish-to-object distances. After lesioning, response latency increased and fish-to-object distance decreased for approaches to the side ipsilateral to the lesion; opposite changes occurred for contralateral approaches.

     

Mauthner cell-evoked synaptic actions on pacemaker medullary neurons of a weakly electric fish

The present study was designed to examine the synaptic events in neurons of the pacemaker nucleus of Gymnotus carapo during the increase in rate of the electric organ discharge following activation of Mauthner cells. Pacemaker and relay cells were investigated using intracellular recordings which were performed under two different conditions: (1) with the pacemaker nucleus spontaneously discharging and (2) after its activity was abolished by anesthesia. Mauthner axon activation induced an increase in the rate of pacemaker cell discharges. This response was accompanied by an increase in the slope of the pacemaker potential (up to 110%) and a depolarization of these cells. The discharges of relay cells followed one to one those of pacemaker cells. In contrast to that observed in pacemaker cells, only brief depolarizing antidromic effects could be evoked in relay cells after Mauthner axon activation. In quiescent pacemaker cells, Mauthner cell activation induced a prolonged (up to 500 ms) depolarizing potential with an average amplitude of 1.92 ± 0.82 mV; its latency was 4.43 ± 1.14 ms. Our data indicate that, within the pacemaker nucleus, the population of pacemaker cells is the only target for Mauthner cell-evoked, short-latency excitatory synaptic actions. Accepted: 1 March 1997     

Testosterone modulates female chirping behavior in the weakly electric fish,Apteronotus leptorhynchus

The weakly electric fish, Apteronotus leptorhynchus, produces a wave-like electric organ discharge (EOD) utilized for electrolocation and communication. Both sexes communicate by emitting chirps: transient increases in EOD frequency. In males, chirping behavior and the jamming avoidance response (JAR) can be evoked by an artificial EOD stimulus delivered to the water at frequencies 1–10 Hz below the animal's own EOD. In contrast, females rarely chirp in response to this stimulus even though they show consistent JARs. To investigate whether this behavioral difference is hormone dependent, we implanted females with testosterone (T) and monitored their chirping activity over a 5 week period. Our findings indicate that elevations in blood levels of T cause an enhancement of chirping behavior and a lowering of basal EOD frequency in females. Elevated blood levels of T also appear to modulate the quality of chirps produced by hormone treated females. The effects of T on female chirping behavior and basal EOD frequency appear specific, since the magnitude of the JAR was not affected by the hormonal treatment. These findings suggest that seasonal changes in circulating concentrations of T may regulate behavioral changes in female chirping behavior and basal EOD frequency.Abbreviations DHT dihydrotestosterone - E estradiol - EOD elecdric organ discharge - GSI gonadal size index - JAR jamming avoidance response - PPn prepacemaker nucleus - T testosterone     

Electric interactions through chirping behavior in the weakly electric fish, Apteronotus leptorhynchus

The weakly electric fish Apteronotus leptorhynchus produces wave-like electric organ discharges distinguished by a high degree of regularity. Transient amplitude and frequency modulations (“chirps”) can be evoked in males by stimulation with the electric field of a conspecific. During these interactions, the males examined in this study produced six types of chirps, including two novel ones. Stimulation of a test fish with a conspecific at various distances showed that two electrically interacting fish must be within 10 cm of each other to evoke chirping behavior in the neighboring fish. The chirp rate of all but one chirp type elicited by the neighboring fish was found to be negatively correlated with the absolute value of the frequency difference between the two interacting fish, but independent of the sign of this difference. Correlation analysis of the instantaneous rates of chirp occurrence revealed two modes of interactions characterized by reciprocal stimulation and reciprocal inhibition. Further analysis of the temporal relationship between the chirps generated by the two fish during electric interactions showed that the chirps generated by one individual follow the chirps of the other with a short latency of approximately 500–1000 ms. We hypothesize that this “echo response” serves a communicatory function.     

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     

Effect of temperature on spinal cord regeneration in the weakly electric fish, Apteronotus leptorhynchus

Temperature manipulation has been shown to significantly affect recovery after spinal cord injury in various mammalian model systems. Little has been known thus far about the impact of temperature on structural and functional recovery after central nervous system lesions in regeneration-competent, poikilotherm organisms. In the present study, we addressed this aspect using an established model of adult spinal cord regeneration, the weakly electric teleost fish Apteronotus leptorhynchus. We observed an overall beneficial effect of increased temperature on both structural and behavioral recovery after amputation of the caudal spinal cord. Fish kept at 30°C recovered the amplitude of the electric organ discharge at more than twice the rate observed in fish kept at 22°C, within the first 20 days post-injury. This improved recovery was supported by increased cell proliferation and decreased apoptosis levels in fish kept at 30°C. The high temperature appeared to have a direct inhibitory effect on apoptosis and to lead to a compression of the duration of the wave of post-lesion apoptosis. The latter effect was presumably induced through the acceleration of the metabolic rate, a phenomenon also supported by the observation that re-growth of the tail was significantly increased in fish kept at 30°C.     

Enzyme reaction rate studies in electromotor neurons of the weakly electric fish Apteronotus leptorhynchus

A histochemical analysis of reaction rates of a series of enzymes was performed in electromotor neurons of the weakly electric fish Apteronotus leptorhynchus. These neurons were selected because of their functional homogeneity. The high metabolic activity of these cells as well as their large size facilitate cytophotometric analysis in cryostat sections. Sections were incubated for the activity of hexokinase, glucose-6-phosphate dehydrogenase, succinate dehydrogenase, NADPH dehydrogenase, NADPH ferrihaemoprotein reductase and beta-hydroxybutyrate dehydrogenase. All media contained polyvinyl alcohol as tissue stabilizer and Nitro BT as final electron acceptor. Measurements were performed with a Vickers M85a cytophotometer. Linear relationships between the specific formation of formazan (test minus control reaction) and incubation time were obtained for all enzymes although some reactions showed an initial lag phase or an intercept with the ordinate. The relatively high activities of hexokinase, succinate dehydrogenase and the extremely low activity of hydroxybutyrate dehydrogenase indicate that energy is mainly supplied by glycolysis. Glucose-6-phosphate dehydrogenase showed a high activity whereas NADPH reductase and dehydrogenase activity were low in electromotor neurons, indicating that the NADPH generated is largely used for biosynthesis. Despite their synchronous firing pattern activity, electromotor neurons showed a considerable heterogeneity with respect to their metabolic activity.     

Hormonal and body size correlates of electrocommunication behavior during dyadic interactions in a weakly electric fish, Apteronotus leptorhynchus

Brown ghost knife fish, Apteronotus leptorhynchus, produce sexually dimorphic, androgen-sensitive electrocommunication signals termed chirps. The androgen regulation of chirping has been studied previously by administering exogenous androgens to females and measuring the chirping response to artificial electrical signals. The present study examined the production of chirps during dyadic interactions of fish and correlated chirp rate with endogenous levels of one particular androgen, 11-ketotestosterone (11KT). Eight males and four females were exposed to short-term (5-min) interactions in both same-sex and opposite-sex dyads. Twenty-four hours after all behavioral tests, fish were bled for determination of plasma 11KT levels. Males and females differed in both their production of chirps and their ability to elicit chirps from other fish: males chirped about 20-30 times more often than females and elicited 2-4 times as many chirps as females. Among males, chirp rate was correlated positively with plasma 11KT, electric organ discharge frequency, and body size. Combined with results from experimental manipulation of androgen levels, these results support the hypothesis that endogenous 11KT levels influence electrocommunication behavior during interactions between two male fish.     

Spontaneous modulations of the electric organ discharge in the weakly electric fish, Apteronotus leptorhynchus: a biophysical and behavioral analysis

Brown ghosts, Apteronotus leptorhynchus, are weakly electric gymnotiform fish whose wave-like electric organ discharges are distinguished by their enormous degree of regularity. Despite this constancy, two major types of transient electric organ discharge modulations occur: gradual frequency rises, which are characterized by a relatively fast increase in electric organ discharge frequency and a slow return to baseline frequency; and chirps, brief and complex frequency and amplitude modulations. Although in spontaneously generated gradual frequency rises both duration and amount of the frequency increase are highly variable, no distinct subtypes appear to exist. This contrasts with spontaneously generated chirps which could be divided into four "natural" subtypes based on duration, amount of frequency increase and amplitude reduction, and time-course of the frequency change. Under non-evoked conditions, gradual frequency rises and chirps occur rather rarely. External stimulation with an electrical sine wave mimicking the electric field of a neighboring fish leads to a dramatic increase in the rate of chirping not only during the 30 s of stimulation, but also in the period immediately following the stimulation. The rate of occurrence of gradual frequency rises is, however, unaffected by such a stimulation regime.     

Differential production of chirping behavior evoked by electrical stimulation of the weakly electric fish, Apteronotus leptorhynchus

Aperonotus leptorhynchus (Gymnotiformes) produces wave-like electric organ discharges distinguished by a high degree of constancy. Transient frequency and amplitude modulations of these discharges occur both spontaneously and during social interactions, which can be mimicked by external electrical stimulation. The so-called chirps can be divided into four different types. Independent of the type of chirp produced under spontaneous conditions, the fish generate only significant numbers of type-2 chirps under evoked conditions. The rate of production of chirps of this type is largely determined by the frequency relative to the fish's frequency and signal intensity. Frequencies of + 10 Hz of the fish's own discharge frequency most effectively elicit chirps. Type-2 chirps can also be evoked through stimulation at or near the higher harmonic frequencies of the fish's frequency, but the chirp rate decreases with increasing number of the higher harmonic component. Over a certain range, the rate of production of type-2 chirps increases with increasing stimulus intensity. At very high intensities the generation of type-2 chirps is accompanied by the production of a novel type of electrical signal ("abrupt frequency rise") characterized by a frequency increase of approximately 20 Hz and high repetition rates of roughly 10 s(-1). We hypothesize that the different types of electric modulations subserve different behavioral functions.     

An in vitro physiological preparation of a vertebrate communicatory behavior: chirping in the weakly electric fish, Apteronotus

1. An in vitro preparation of the medullary pacemaker nucleus of the weakly electric fish Apteronotus leptorhynchus was studied which fires regularly and synchronously at the fish's characteristic frequency of electric organ discharge (EOD). Upon bipolar stimulation of tissue regions through which pass prepacemaker nucleus afferents to the pacemaker, a brief, transient increase in discharge frequency ensued at short-latency (Fig. 1A). 2. Intracellular recordings revealed that the acceleration was accompanied by a depolarization and decline in action potential amplitude. The magnitude of these changes was both phase- (Fig. 5) and amplitude-dependent, with the latter showing an evident threshold effect (Figs. 4 and 12). The response was reversibly blocked by high Mg2+ saline (Fig. 1B), and the magnitude of the accelerations showed marked facilitation during repeated stimulation (Fig. 6). 3. Optical and histological identification allowed characteristically different responses in the intracellular recordings to be attributed to the two cell types of the pacemaker nucleus: pacemaker and relay cells (Figs. 2 and 3). Similar responses have been observed at these respective recording locations in the intact animal during chirping (Dye and Heiligenberg 1987). 4. Simultaneous recordings of pairs of cells revealed a transient change in the phase relationship of firing during the accelerations which was most marked between relay and pacemaker cells (Fig. 7). These dual recordings also revealed that the relay cells depolarize and accelerate more than pacemaker cells (Fig. 10), suggesting that they are the principal effectors of this behavioral modulation. 5. Trains of pulses additionally elicited a long-lasting frequency elevation which occurred at a slightly higher threshold than the brief accelerations. This slow frequency change relaxed back to baseline following a biexponential time course which closely resembled that of a distinct behavior seen in intact fish, termed 'yodeling' (Dye 1987).     

Electrocommunication behaviour and non invasively-measured androgen changes following induced seasonal breeding in the weakly electric fish, Apteronotus leptorhynchus

Androgens are known to be involved in reproductive behaviours including courtship and aggression. According to the Challenge Hypothesis, androgen activity upregulates male reproductive behaviour seasonally and also modulates short term adaptation of these behaviours in response to social context. In the weakly electric fish, Apteronotus leptorhynchus, 11-ketotestosterone (11-KT) has been previously implicated in the regulation of electrocommunication behaviours that are believed to have roles in both aggression and courtship. Changes in male 11-KT levels were quantified using a non-invasive measurement technique alongside changes in electrocommunication behaviour following environmental cues that simulated the onset of the breeding season. Males showed an increase in mean electric organ discharge frequency (EODf), which is consistent with earlier results showing a female preference for high EODf. A subset of males with high initial EODfs showed increases in both 11-KT and EODf, which provides support for an EODf-based dominance hierarchy in this species. Males housed in social conditions and exposed to breeding conditioning also showed higher overall electric organ discharge frequencies and 11-KT compared to males housed in isolation. Evidence is presented that another type of electrocommunication signal previously implicated in courtship may also serve as an inter-male signal of submission. Our results are consistent with earlier observations that electrocommunication signals produced during inter-male aggression serve in deterring attacks, and their pattern of production further suggested the formation of a dominance hierarchy.     

The control of pacemaker modulations for social communication in the weakly electric fish Sternopygus

Summary Nearly sinusoidal electric organ discharges (EODs) of the weakly electric fish Sternopygus, occur at a regular rate within a range from 50 to 200 Hz and are commanded by a medullary pacemaker nucleus (Pn). During courtship and aggression, the rate of EODs is modulated as smooth EOD-frequency rises or brief EOD-interruptions (Hopkins 1974b). The present study examines the control of such modulations. Rises were elicited by L-glutamate stimulation of the diencephalic prepacemaker nucleus, the only previously known source of input to the Pn. We demonstrate an additional input to the Pn, the sublemniscal prepacemaker nucleus (SPPn). L-glutamate stimulation of this area caused EOD-interruptions.The Pn contains electrotonically coupled pacemaker cells which generate the rhythm of the EODs, as well as relay cells which transmit the command pulse to the spinal motor neurons that innervate the electric organ. Pacemaker cells recorded intracellularly during EOD-interruptions continued firing at their regular frequency but with slightly increased jitter. Relay cells, on the other hand, were strongly depolarized and fired spikelets at a greatly increased frequency during EOD-interruptions. Thus EOD-interruptions were caused by SPPn input to relay cells that caused their massive depolarization, blocking the normal input from pacemaker cells without greatly affecting pacemaker cell firing characteristics.Application to the Pn of an antagonist to NMDA-type glutamate receptors blocked EOD-frequency rises and EOD-interruptions. Antagonists to quisqualate/ kainate receptor-types were ineffective.Abbreviations EOD Electric Organ Discharge - JAR Jamming Avoidance Response - Pn pacemaker nucleus - PPn diencephalic prepacemaker nucleus - SPPn sublemniscal prepacemaker nucleus     

Development of a sexual dimorphism in a central pattern generator driving a rhythmic behavior: The role of glia‐mediated potassium buffering in the pacemaker nucleus of the weakly electric fish Apteronotus leptorhynchus

Central pattern generators play a critical role in the neural control of rhythmic behaviors. One of their characteristic features is the ability to modulate the oscillatory output. An important yet little‐studied type of modulation involves the generation of oscillations that are sexually dimorphic in frequency. In the weakly electric fish Apteronotus leptorhynchus, the pacemaker nucleus serves as a central pattern generator that drives the electric organ discharge of the fish in a one‐to‐one fashion. Males discharge at higher frequencies than females—a sexual dimorphism that develops under the influence of steroid hormones. The two principal neurons that constitute the oscillatory network of the pacemaker nucleus are the pacemaker and relay cells. Whereas the number and size of the pacemaker and relay cells are sexually monomorphic, pronounced sex‐dependent differences exist in the morphology, and subcellular properties of astrocytes, which form a syncytium closely associated with these neurons. In females, compared to males, the astrocytic syncytium covers a larger area surrounding the pacemaker and relay cells and exhibits higher levels of expression of connexin‐43 expression. The latter indicates a strong gap‐junction coupling of the individual cells within the syncytium. It is hypothesized that these sex‐specific differences result in an increased capacity for buffering of extracellular potassium ions, thereby lowering the potassium equilibrium potential, which, in turn, leads to a decrease in the oscillation frequency. This hypothesis has received strong support from simulations based on computational models of individual neurons and the whole neural network of the pacemaker nucleus.     

Pharmacological characterization of ionic currents that regulate high-frequency spontaneous activity of electromotor neurons in the weakly electric fish, Apteronotus leptorhynchus

The neural circuit that controls the electric organ discharge (EOD) of the brown ghost knifefish (Apteronotus leptorhynchus) contains two spontaneous oscillators. Both pacemaker neurons in the medulla and electromotor neurons (EMNs) in the spinal cord fire spontaneously at frequencies of 500-1,000 Hz to control the EOD. These neurons continue to fire in vitro at frequencies that are highly correlated with in vivo EOD frequency. Previous studies used channel blocking drugs to pharmacologically characterize ionic currents that control high-frequency firing in pacemaker neurons. The goal of the present study was to use similar techniques to investigate ionic currents in EMNs, the other type of spontaneously active neuron in the electromotor circuit. As in pacemaker neurons, high-frequency firing of EMNs was regulated primarily by tetrodotoxin-sensitive sodium currents and by potassium currents that were sensitive to 4-aminopyridine and kappaA-conotoxin SIVA, but resistant to tetraethylammonium. EMNs, however, differed from pacemaker neurons in their sensitivity to some channel blocking drugs. Alpha-dendrotoxin, which blocks a subset of Kv1 potassium channels, increased firing rates in EMNs, but not pacemaker neurons; and the sodium channel blocker muO-conotoxin MrVIA, which reduced firing rates of pacemaker neurons, had no effect on EMNs. These results suggest that similar, but not identical, ionic currents regulate high-frequency firing in EMNs and pacemaker neurons. The differences in the ionic currents expressed in pacemaker neurons and EMNs might be related to differences in the morphology, connectivity, or function of these two cell types.     

Pharmacological characterization of ionic currents that regulate the pacemaker rhythm in a weakly electric fish

Electric organ discharge (EOD) frequency in the brown ghost knifefish (Apteronotus leptorhynchus) is sexually dimorphic, steroid-regulated, and determined by the discharge rates of neurons in the medullary pacemaker nucleus (Pn). We pharmacologically characterized ionic currents that regulate the firing frequency of Pn neurons to determine which currents contribute to spontaneous oscillations of these neurons and to identify putative targets of steroid action in regulating sexually dimorphic EOD frequency. Tetrodotoxin (TTX) initially reduced spike frequency, and then reduced spike amplitude and stopped pacemaker activity. The sodium channel blocker muO-conotoxin MrVIA also reduced spike frequency, but did not affect spike amplitude or production. Two potassium channel blockers, 4-aminopyridine (4AP) and kappaA-conotoxin SIVA, increased pacemaker firing rates by approximately 20% and then stopped pacemaker firing. Other potassium channel blockers (tetraethylammonium, cesium, alpha-dendrotoxin, and agitoxin-2) did not affect the pacemaker rhythm. The nonspecific calcium channel blockers nickel and cadmium reduced pacemaker firing rates by approximately 15-20%. Specific blockers of L-, N-, P-, and Q-type calcium currents, however, were ineffective. These results indicate that at least three ionic currents-a TTX- and muO-conotoxin MrVIA-sensitive sodium current; a 4AP- and kappaA-conotoxin SIVA-sensitive potassium current; and a T- or R-type calcium current-contribute to the pacemaker rhythm. The pharmacological profiles of these currents are similar to those of currents that are known to regulate firing rates in other spontaneously oscillating neural circuits.     

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.     

Distribution of Kv1-like potassium channels in the electromotor and electrosensory systems of the weakly electric fish Apteronotus 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.