Voltage-gated potassium conductances in Gymnotus electrocytes(AB).

Electrocytes are muscle-derived cells that generate the electric organ discharge (EOD) in most gymnotiform fish. We used an in vitro preparation to determine if the complex EOD of Gymnotus carapo was related to the membrane properties of electrocytes. We discovered that in addition to the three Na(+)-mediated conductances described in a recent paper [Sierra F, Comas V, Buño W, Macadar O (2005) Sodium-dependent plateau potentials in electrocytes of the electric fish Gymnotus carapo. J Comp Physiol A 191:1-11] there were four K(+)-dependent conductances. Membrane depolarization activated a delayed rectifier (I(K)) and an A-type (I(A)) current. I(A) displayed fast voltage-dependent activation-inactivation kinetics, was blocked by 4-aminopyridine (1 mM) and played a major role in action potential (AP) repolarization. Its voltage dependence and kinetics shape the brief AP that typifies Gymnotus electrocytes. The I(K) activated by depolarization contributed less to AP repolarization. Membrane hyperpolarization uncovered two inward rectifiers (IR1 and IR2) with voltage dependence and kinetics that correspond to the complex "hyperpolarizing responses" (HRs) described under current-clamp. IR1 shows "instantaneous" activation, is blocked by Ba(2+) and Cs(+) and displays a voltage and time dependent inactivation that matches the hyperpolarizing phase of the HR. The activation of IR2 is slower and at more negative potentials than IR1 and is resistant to Ba(2+) and Cs(+). This current fits the depolarizing phase of the HR. The EOD waveform of Gymnotus carapo is more complex than that of other gymnotiform fish species, the complexity originates in the voltage responses generated through the interactions of three Na(+) and four K(+) voltage- and time-dependent conductances although the innervation pattern also contributes [Trujillo-Cenóz O, Echagüe JA (1989) Waveform generation of the electric organ discharge in Gymnotus carapo. I. Morphology and innervation of the electric organ. J Comp Physiol A 165:343-351].

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.

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.

Origin and propagation of spontaneous electrographic sharp waves in the in vitro turtle brain: a model of neuronal synchronization.

OBJECTIVES: Neuronal synchronization is a basic feature in the generation of epileptiform discharges. Spontaneous large sharp waves (LSWs) can be recorded in the turtle brain in vitro, indicating the synchronous activation of large neuronal populations. The aim of this study was to analyze the spatial and temporal distribution of LSWs within the brain; the participation of glutamate in LSWs generation was also investigated. METHODS: Extracellular field potentials were recorded in vivo (n = 4) and in vitro (n = 36). LSWs were recorded from cerebral cortex, optic tectum, and thalamus. RESULTS: LSWs were recorded from cerebral cortex, optic tectum and thalamus. No LSWs were observed in cerebellum and brain stem. In some experiments, LSWs could be recorded only from medial cortex. Latency studies demonstrated that, within each hemisphere, medial cortex led the generation of LSWs; in addition, isolated medial cortex could sustain LSWs. Intracortical laminar field potentials in medial cortex indicated that LSWs generate mainly in the molecular layer, probably at pyramidal cell dendrites. Pharmacological experiments demonstrated that NMDA and non-NMDA glutamate receptors are involved in LWSs generation. CONCLUSIONS: These results suggest that turtle medial cortex is the pacemaker area for LSWs generation and it can be a useful model to study cellular and circuital mechanisms of neuronal synchronization.

The electric organ discharge of Brachyhypopomus pinnicaudatus. The effects of environmental variables on waveform generation.

The electric organ discharge of Brachyhypopomus pinnicaudatus was studied by recording (1) the discharge field potentials in water at different conductivities and temperatures and (2) the spatiotemporal pattern of electromotive forces of the equivalent source. An early deflection, head positive (P wave), and a late deflection, head negative (N wave), are the major components of the discharge, however a striking double positive peak is generated at the abdominal level. Comparisons of this species with other pulse gymnotids provide evidence for common patterns of organization of the electrogenic system: (1) There is a head-to-tail activation wave along the fish; (2) the electromotive force increases exponentially from head to tail, but it is differentially attenuated by the passive tissues in male and females; (3) the abdominal region generates a complex species-specific waveform, whereas the tail discharge is similar across species. In B. pinnicaudatus the electric organ discharge waveform is sensitive to endocrine and environmental stimuli. The effect of seasonal sex differences on electrogenic and passive tissue, the changes in impedance matching between the fish's body and the environment, and the modulation of membrane properties by temperature, are able to modify the EOD waveform. Since these factors change during the breeding season, their appropriate combination might be crucial for reproduction.

Computational model of the jamming avoidance response in the electric fish Gymnotus carapo.

The unperturbed electric organ discharges of Gymnotus carapo fish are highly periodic with interpulse intervals around 40 ms. The 'jamming avoidance response' happens when fish interact and is a transitory interval shortening in the fish with the faster discharge that decreases the likelihood that pulses from it and the other fish will coincide. Our model's basic components match certain experimentally demonstrated facts. First, recurrence equations reproduce the periodic unperturbed discharges. Secondly, when an isolated pulse arrives, the two intervals following that with the pulse are shortened by amounts that decrease up to a minimum and then increase. Simulations demonstrated that this model reproduces satisfactorily the jamming avoidance response. Two additional conditions were demonstrably necessary: (i) one was that every pulse arrive within the hot cophase window (and not elsewhere); (ii) the other condition was that, along successive pulses, the respective cophases decrease by moderate amounts. In short, we conclude that the fish's jamming avoidance response can involve the simple computational rules implied by the proposed equations.

N-type Ca2+ channels mediate transmitter release at the electromotoneuron-electrocyte synapses of the weakly electric fish Gymnotus carapo.

The effects of omega-conotoxin-GVIA (omega-CgTX) on synaptic transmission were studied in the electromotoneuron-electrocyte synapses of the electric organ (EO) of the weakly electric fish Gymnotus carapo. omega-CgTX selectively and irreversibly blocked excitatory postsynaptic potentials (EPSPs) in a dose dependent-manner. The toxin had no effect on: (a) resting postsynaptic membrane potential and conductance; (b) postsynaptic action potentials elicited by depolarizing transmembrane current pulses; (c) the action potential conduction in the presynaptic fiber; (d) acetylcholine (ACh)-induced postsynaptic responses. Nifedipine - a selective dihydropyridine antagonist of the L-type voltage-dependent Ca2+ channels (VDCCs) - did not affect synaptic transmission. Transmission was also undisturbed by the peptide omega-Agatoxin (omega-Aga-IVA), the low molecular weight polyamine, funnel-web toxin (FTX) - both included in the venom of the spider Agelenopsis aperta - and its synthetic analog sFTX, all selective blockers of P-type VDCCs. Since omega-CgTX irreversibly blocks the N-type VDCCs, we conclude that presynaptic N-type VDCCs mediate transmitter release at electromotoneuron terminals. The VDCCs involved in fish peripheral electromotoneuron-electrocyte presynaptic transmitter release are therefore similar to those in amphibian, reptilian and avian peripheral synapses, but differ from mammalian and invertebrate motoneuron terminals.