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     

Temperature sensitivity of the electric organ discharge waveform in Gymnotus carapo

At the southern boundary of gymnotiform distribution in America. water temperature changes seasonally, and may be an environmental cue for the onset of breeding. In this study, we aim to describe the role of temperature upon electric organ discharge waveform in Gymnotus carapo, order Gymnotiformes, family Gymnotidae, and to analyze its interactions with the effects of steroid hormones. The effects of water temperature within its natural range were explored using different protocols. All fish tested had temperature-sensitive electric organ discharge waveforms: the amplitude of the last head-negative component (V4) decreased as temperature increased. Rate increases elicited by electrical stimulation had similar but smaller effect on waveform. Temperature sensitivity is a peripheral phenomenon that depends on the conductivity of the aquatic media. We found hormonal-dependent changes in the electric organ discharge waveform not previously described in this species. The amplitude and duration of V4 increased after testosterone administration. Both testosterone treatment and acclimation by sustained temperature at 27-28 degrees C (environmental simulation of breeding conditions) induced a decrease in temperature sensitivity. As in the related species Brachyhypopomus pinnicaudatus, our data strongly suggest interactions between temperature sensitivity of the electric organ discharge waveform and sexual maturity that might be crucial for reproduction.     

Variability of the electric organ discharge interval duration in resting Gymnotus carapo

 We recorded the electric organ discharges of resting Gymnotus carapo specimens. We analyzed the time series formed by the sequence of interdischarge intervals. Nonlinear prediction, false nearest neighbor analyses, and comparison between the performance of nonlinear and linear autoregressive models fitted to the data indicated that nonlinear correlations between intervals were absent, or were present to a minor extent only. Following these analyses, we showed that linear autoregressive models with combined Gaussian and shot noise reproduced the variability and correlations of the resting discharge pattern. We discuss the implications of our findings for the mechanisms underlying the timing of electric organ discharge generation. We also argue that autoregressive models can be used to evaluate the changes arising during a wide variety of behaviors, such as the modification in the discharge intervals during interaction between fish pairs. Received: 14 March 2000 / Accepted in revised form: 9 October 2000     

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     

Innervation pattern and electric organ discharge waveform in Gymnotus carapo (Teleostei; Gymnotiformes)

The electrogenic organ (EO) of Gymnotus carapo has two main portions: a posterior region consisting of four bilaterally arranged electrocyte rows; and an anterior portion composed of only two. The lateral row (LR) of the anterior portion contains doubly innervated electrocytes with axon terminals from different nerves on their rostral and caudal faces. The LR is continuous with the most dorsal row of the caudal region. This row also contains doubly innervated electrocytes. The medial row (MR) electrocytes of the anterior region and ventral rows of the caudal region are exclusively caudally innervated. All caudal faces of the anterior or abdominal region are supplied by two nerves which originate from spinal roots VIII to XXI. Roots I to VII give origin to pure rostral nerves whose electromotor axons terminate on the rostral surfaces of the first seven LR electrocytes. A given doubly innervated electrocyte is supplied on its caudal face by a nerve originating several segments (usually seven) posterior to the spinal root supplying its rostral face. Transections of the spinal cord at the level of root VIII isolate the activity of the rostral surfaces of the first electrocytes. The EO discharge (EOD) then appears as a head negative deflection which arises from abdominally located electrocytes. Its monophasic character reveals that the activity remains restricted to the rostral electrocyte surfaces. Damage of the abdominal portion of the EO abolishes the first negative deflection of the normal pulse. Transections of the spinal cord at the level of root XXI isolate the activity of the whole abdominal portion of the EO. Since both doubly and singly innervated electrocytes remain active, the EOD appears biphasic. Comparative studies have shown that the EOD of Hypopomus sp. lacks any early negative wave and correspondingly all its electrocytes are exclusively caudally innervated.     

Electric organ discharge frequency modulation evoked by water vibration in Gymnotus carapo

• 1.1. The electric organ discharge (EOD) frequency modulations evoked by brief water vibration were analysed in the pulse-type fish Gymnotus carapo.
• 2.2. The response consisted of a transient increase of the EOD frequency at short latency (30 msec). Response profiles were characteristic of the specimen and relatively independent on stimulus intensity.
• 3.3. Conversely, they were dependent on stimulation sequence, showing a rapid decrement along successive stimuli and high temporal discrimination.
• 4.4. The brief latencies indicate a relatively simple neural circuit.
• 5.5. The response may be an electrolocation enhancement strategy for the detection of moving objects based on “sampling” the periphery at a higher frequency.

     

Analysis of the jamming avoidance response in the electric fish Gymnotus carapo

The electric fish Gymnotus carapo emits brief (4 ms) electric pulses separated by much longer intervals of high regularity (coefficient of variation 0.01–0.02). Two main changes in the firing patterns of electric organ discharge were observed when two fish were placed together. (1) All fish pairs showed an increase in the frequency difference between the two fish, in comparison with the value observed in isolated fish, prior to the interaction. This change increased the number of beats per second between both discharge trains, i.e., the number of times per second that the higher rate discharge sweeps the lower rate one when displayed on an oscilloscope. (2) When the sweeping velocity fell below 2–3 sweeps/s, transient frequency increases were also observed in the electric organ discharge of the higher rate fish when they were about to discharge simultaneously. The contribution to jamming avoidance of these two changes was analyzed by comparing recordings of behavioral interactions with simulations produced by a computational model. The jamming effect of the firing of a conspecific located in the same tank was evaluated by counting the number of coincidences between both trains (occurrence of discharges of the two fish within 2 ms of one another). The number of coincidences was evaluated as a function of the sweeping velocity in both simulations (with and without transient frequency increases) and real fish. As the sweeping velocity increased, single coincidences increased slightly in simulations without transient frequency increases, whereas the successive coincidences (coincidences repeated in successive discharges) decreased abruptly. The simulation including transient frequency increases eliminated the successive coincidences and decreased the single ones. Only when the sweeping velocity was less than 2–3 sweeps/s, did transient frequency increases improve the coincidence-avoiding performance of the simulation. The number of coincidences observed in natural behavioral interactions for the different sweeping velocities coincided with the distributions obtained with the simulations. As successive coincidences are known to be more detrimental for electrolocation than single ones, the increase in the sweeping velocity may be considered a jamming avoidance strategy in Gymnotus carapo, in addition to the already described transient frequency increases. Received: 2 June 1998 / Accepted in revised form: 18 November 1998     

Electric organ activation in Gymnotus carapo: Spinal origin and peripheral mechanisms

A new technique of multiple-air-gap recording was developed to study the EO activation process in Gymnotus carapo. Using this technique, the spatiotemporal pattern of electromotive force generation was investigated in normal and spinal-lesioned animals.Our data indicate that the EOD may be considered as the result of the sequential activation of 3 defined portions of the EO: the abdominal portion (included in the rostral 25% of the fish body), the central portion (comprising the intermediate 50% of the fish body) and the tail portion (the caudal 25% of the fish body). The EOD generated at each portion is characterized by: 1) timing respect to the pacemaker nucleus discharge, 2) speed of progression within the region, 3) waveform, and 4) magnitude.Spinal sections demonstrated that EMNs serving relatively small portions of the EO are widely distributed (convergence) and that surgical exclusion of relatively small portions of the spinal cord diminishes the amplitude of the EOD along an extended portion of the EO (divergence).Abbreviations EMF electromotive force - EMN electromotor-neurons - EO electric organ - EOD electric organ discharge - PMNFP pacemaker nucleus field potential - PEN posterior electromotor nerve - PNA peripheral neural activity     

Interference with the GABAergic system in the dorsolateral telencephalon and modulation of the electric organ discharge frequency in the weakly electric fish Gymnotus carapo

The functional role of the dorsal portions of dorsolateral telencephalon in modulating the stable electric organ discharge was determined by microinjection of an agonist or antagonist GABAergic drug in the awake weak electric fish, Gymnotus carapo. The dorsolateral telencephalon, which is interconnected with the preglomerular complex and the dorsocentral telencephalic area was microinjected through a guide cannula previously implanted, with different doses of bicuculline, muscimol and saline. Microinjection of bicuculline into the dorsolateral telencephalon induced a complex response consisting of increase, decrease and abrupt interruptions in the frequency of electric organ discharges and an increase in motor activity. Motor activity and modulations in the electric organ discharge are dose dependent. The somatic, but not the electric, effect is abolished under anesthesia by urethane, suggesting that the two responses are parallel but unrelated in terms of occurrence. These data, together with former neuroanatomical findings by this laboratory, suggest two parallel pathways by which the blockage of GABAA receptors in the dorsolateral telencephalon causes modulations in the firing of the medullary pacemaker nucleus. A possible route for the motor effect through reticular projection from the torus semicircularis dorsalis is discussed.     

Serotonergic control of electric organ discharge in Gymnotus carapo. Role of 5-HT2A/2c receptor subtypes

Resting frequency of the EOD and amplitude of the frequency changes induced by different stimulus modalities (novelty responses) were measured in Gymnotus carapo before and after pharmacological modulation of serotonergic transmission and in control groups. Stimulation of serotonergic transmission induced a decrease of resting frequency and the appearance of spontaneous frequency bursts. The amplitude of the transient change in discharge frequency induced by photic, acoustic, mechanical and electric stimuli was significantly enhanced after serotonergic stimulation. These effects were also produced by a selective ligand of 5-HT_2A/2c receptor subtypes. Selective blockade of the same receptors prevented the agonist effect and induced opposite changes, thus suggesting the existence of a tonic serotonergic control.     

Waveform generation of the electric organ discharge inGymnotus carapo

Summary The electric organ (EO) ofGymnotus carapo was studied using different neurohistological techniques including conventional electron microscopy. The electric tissue extends along the fish body from the pectoral girdle to the tip of the tail, constituting a single, undivided organ. However, taking into account the number, arrangement, and innervation of the electrocytes, it is possible to divide the EO into three different portions. The more rostral portion is included within the ventral wall of the abdominal cavity. It consists of singly and doubly innervated electrocytes arranged in two rows at each side of the midline. Innervation of this zone is supplied by the first 5–7 segmental nerves and by the anterior electromotor nerves. Segmental nerves terminate on the rostral faces of doubly innervated electrocytes; axons stemming from the anterior electromotor nerves end on the caudal faces of both doubly and singly innervated electrocytes. There is an intermediate body-tail region in which the electrocytes are arranged in four dorsoventral tubes (tubes 1 to 4) on each side of the midline. In this zone, doubly innervated electrocytes (confined within tube 1) coexist together with singly innervated ones, receiving nerve terminals on their caudal faces (tubes 2, 3, and 4). The innervation characteristics appear modified at more distal portions of the tail where the doubly innervated electrocytes of tube 1 are replaced by singly innervated units. The most distal portion of the EO (approximately its terminal 30%) consists of numerous, homogeneously innervated electrocytes with nerve endings distributed exclusively on their caudal faces. Nerve supply to the intermediate and distal regions derives from the posterior electromotor nerves (PENs) which appear as well-defined anatomical entities beyond the level of metamere XXVII. At the bodytail and more distal regions the innervation pattern of the EO is particularly complex. Thin nerve trunks arise from the PENs and project ventrally toward the electrocyte tubes. Before reaching the electric tissue the electromotor axons branch frequently. Our anatomical studies indicate that the EO is heterogeneous, a feature consistent with most recent electrophysiological and biophysical experiments.Abbreviations AEN anterior electromotor nerve - EMN electromotoneurons - EO electric organ - EOD electric organ discharge - LLN lateral line nerve - PEN posterior electromotor nerve     

A field potential analysis of the electromotor system in Gymnotus carapo

To investigate the synchronization mechanisms operating in the electromotor system, electric organ discharge related field potentials of neural origin were recorded in intact fish. Components corresponding to the relay nucleus, the bulbar-spinal electromotor tract, the electromotoneurons and the peripheral nerves were identified. Delays between components were used to estimate the following intervals: (1) the conduction time along the cord (central conduction interval), (2) the interval between the local activation of the tract and the electromotoneuron firing within a restricted portion of the cord (coupling interval) and (3) the conduction time along the peripheral axons plus the time taken by synaptic activation of the electrocytes (peripheral interval). While central conduction interval increases with the distance from the pacemaker, the coupling interval diminishes. Peripheral interval also diminishes from rostral to caudal targets in the electric organ. It is concluded that the electrocyte synchronization, resulting from matching the three above-defined intervals, is achieved by a cascade of synergetic mechanisms.Abbreviations EBST electromotor bulbar-spinal tract - EMN electromotoneuron - EO electric organ - EOD electric organ discharge - PEN posterior electromotor nerve - PNA peripheral nerve activity - SFP skin surface field potentials     

Mauthner cell-initiated abrupt increase of the electric organ discharge in the weakly electric fishGymnotus carapo

Stimulation of the spinal cord of the electric fish Gymnotus carapo, evoked an abrupt increase in the discharge rate of the electric organ. At the maximum of this response, the rate increased an average of 26 ± 11.8%. The duration of the response was 4.9 ± 2.12 s; its latency was 10.4 ± 1.1 ms. Activation of the Mauthner axon played a decisive role in this phenomenon as indicated by the following: (1) recordings from the axon cap of the Mauthner cell demonstrated that the response was evoked if the Mauthner axon was antidromically activated and (2) a response that was similar to that produced by spinal cord stimulation, was elicited by intracellular stimulation of either Mauthner cell. Stimulation of the eighth nerve could also increase the discharge rate of the electric organ. The effect was greater if a Mauthner cell action potential was elicited. The findings described in the present report, indicate the existence of a functional connection between the Mauthner cell and the electromotor system in Gymnotus carapo. This connection may function to enhance the electrolocative sampling of the environment during Mauthner-cell mediated behaviors. This is a novel function for the Mauthner cell.Abbreviations EHP extrinsic hyperpolarizing potential - EOD electric organ discharge - M-AIR Mauthner initiated abrupt increase in rate - M-cell Mauthner cell - M-axon Mauthner axon - PM pacemaker nucleus - PM-cell pacemaker cell - PPn prepacemaker nucleus - SPPn sublemniscal prepacemaker nucleus     

Comparative studies of the aggressive behaviour of two gymnotid electric fish (Gymnotus carapo and Hypopomus artedi).

Electrical and motor behaviour was studied during aggressive interactions in two species of electric fish (Gymnotus carapo and Hypopomus artedi). Paired comparison methods were used to obtain the data, and analysis of the results was undertaken at three different levels: (1) The repertoire of behaviour (motor and electrical); (2) The changes in relative frequency of components with increasing exposure to conspecifics; (3) Sequential analysis of patterns of behaviour occuring both within and between individuals. The results of the three analyses showed: (1) A great similarity in repertoire although a much higher rate of interaction for Gymnotus. The relative importance of the "off" and "burst" signals was reversed for the two species. A prolonged orientation component in Hypopomus was found to be related to the occurrence of discharge synchrony. (2) Habituation of intense components in favour of electrical signalling in both species. (3) Important differences in both the length and structure of sequences.     

Novelty response in the weakly electric fish Gymnotus carapo: seasonal differences and the participation of the telencephalon in its modulation

The arousal or novelty response (increased frequency of electric organ discharge in the presence of an electric stimulus delivered by a pair of electrodes placed in the water of the experimental chamber) and its habituation were studied in Gymnotus carapo, a weak field electric fish, in different seasons (September and March) and in animals with extensive or restricted telencephalic lesions. The novelty response was reduced but not abolished by repeated stimuli at fixed intervals in normal animals both in September (pre-mating season) and in March (beginning of the dry season). The magnitude of the novelty response did not differ between sham-operated animals and animals with extensive (detelencephalization) or restricted telencephalic lesions (dorsocentral area) within their respective groups (September or March). The novelty response to the first applied stimulus was significantly greater in normal, sham-operated, and derelencephalated animals in September compared to March, although baseline firing rate did not differ. The maximum extent of response reduction occurred after the 18th stimulus in September for all three experimental groups, whereas in the 4 groups tested in March the maximum reduction occurred between the 16th and 72nd stimulus, indicating that in March there is a greater fluctuation in the arousal level.