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].

Analysis of behavior-related excitatory inputs to a central pacemaker nucleus in a weakly electric fish.

Gymnotid electric fish explore their environment and communicate with conspecifics by means of rhythmic electric organ discharges. The neural command for each electric organ discharge arises from activity of a medullary pacemaker nucleus composed of two neuronal types: pacemaker and relay cells. During different behaviors as in courtship, exploration and agonistic interactions, these species display specific electric organ discharge frequency and/or waveform modulations. The neural bases of these modulations have been explained in terms of segregation of inputs to pacemaker or relay cells, as well as differential activation of the glutamate receptors of these cells. One of the most conspicuous electric organ discharge frequency modulations in Gymnotus carapo results from the activation of Mauthner cells, a pair of reticulospinal neurons that are involved in the organization of sensory-evoked escape responses in teleost fish. The activation of Mauthner cells in these animals produces a prolonged increase in electric organ discharge rate, whose neural mechanisms involves the activation of both N-methyl-D-aspartate (NMDA) and metabotropic glutamatergic receptors of pacemaker cells. Here we provide evidence which indicates that pacemaker cells are the only cellular target of the synaptic inputs responsible for the Mauthner cell initiated electric organ discharge modulation at the medullary pacemaker nucleus. Additionally, although pacemaker cells express both NMDA and non-NMDA ionotropic receptors, we found that non-NMDA receptors are not involved in this synaptic action which suggests that NMDA and non-NMDA receptor subtypes are not co-localized at the subsynaptic membrane. NMDA receptor activation of pacemaker cells seems to be an efficient neural strategy to produce long-lasting enhancements of the fish sampling capability during Mauthner cell-initiated motor behaviors.

[Central modulation of a sensory system by a motor command. One intention with two results]. / Modulación central de un sistema sensorial por un comando motor. Una intención con dos resultados.

INTRODUCTION AND DEVELOPMENT: Neuronal mechanisms that underlie diverse sensory motor integration processes (SMI) are essential for the motor control and determine the general organization of the nervous system. Spinal cord, sensory relay nucleus of brainstem and thalamus as well as higher motor control structures are some of the levels, of increasing complexity, at which several processes of SMI occurs during the execution of a motor act. The mechanisms that underlie SMI strategies operating at higher hierarchical levels of motor control are poorly understood. Escape response in teleosts fish is an advantageous experimental model for the analysis of the neural basis of behavior and of the mechanisms and functional consequences of diverse strategies of ISM. We describe several levels of ISM that operate in the neural system that organize this response in most teleosts and we deal with a detailed description of a novel strategy that occurs in Gymnotus carapo, a South American weakly electric fish. In this species, the activation of the Mauthner cell, a command neuron for the initial phase of escape, produces a powerful modulation of the sensory system responsible for active electrorreception, its main sensory modality. CONCLUSION: The neural basis of behavior, even those relatively simple, exhibit several strategies of complex SMI that determine its performance and whose cellular mechanisms begin to be unraveled.