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O Macadar

Publications and source records attributed to O Macadar.

At least 19 recordsLinked to original sources
PubMed2007

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].

Animals↗
PubMed2002

High-frequency stimulation of the subthalamic nucleus silences subthalamic neurons: a possible cellular mechanism in Parkinson's disease.

The subthalamic nucleus participates in the control of movement and is considered a surgical target in the treatment of parkinsonian symptoms. Using the rat brain in vitro slice technique we show that sustained high-frequency (>100 Hz) electrical stimulation (i.e., 'tetanic stimulation') of the nucleus, as used in humans to treat Parkinson's disease, silenced subthalamic neurons. Two main cell types were identified. 'Tonic cells' (68%) showed delayed inward rectification, fired continuously, switched to bursting and stopped firing when strongly depolarized with injected current. Tetanic stimulation of the nucleus induced a steady depolarization (approximately 18 mV) that triggered action potentials at a high rate followed by bursts and finally (approximately 25 s) totally silenced tonic cells. The control tonic activity was recovered rapidly (<10 s) after ending stimulation. 'Phasic cells' (25%) discharged a single initial brief burst of action potentials both when depolarized by prolonged current injection and tetanic stimulation and did not show inward rectification. An infrequent cell type called 'phasic-tonic' (7%) showed a mixed discharge. We suggest that the silencing effect of tetanic stimulation is not a frequency-dependent presynaptic depression and could result from the gradual inactivation of Na+-mediated action potentials. These findings suggest that the remission of parkinsonian symptoms by treatment with high-frequency electrical stimulation of the subthalamic nucleus in humans may primarily reside on its capacity to suppress the action potential activity of subthalamic neurons.

Action Potentials↗
PubMed2001

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.

Acclimatization↗
PubMed2001

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.

Animals↗
PubMed1999

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.

Animals↗
PubMed1998

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.

Air↗
PubMed1995

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.

Animals↗
PubMed1987

Limbic epilepsy induced in the rat by dendrotoxin, a polypeptide isolated from the green mamba (Dendroaspis angusticeps) venom.

Dendrotoxin was isolated from green mamba (Dendroaspis angusticeps) venom and its effects on motor behavior and cortical and subcortical bioelectrical activity were studied in the rat. In chronic experiments, free moving rats injected i.p. with dendrotoxin, presented motor behavior similar to that described in rat amygdaloid epilepsy and bioelectrical signs of epilepsy beginning at the amygdala were observed. In acute experiments, rats anaesthetized with urethane were intracerebrally or intracerebroventricularly injected with dendrotoxin, which produced bioelectrical signs of epilepsy. Following intracerebroventricular injection, signs of epileptic discharge were first observed at the dorsal hippocampus. When dendrotoxin was microinjected in the amygdala or the hippocampus, the seizures appeared at the injection sites with a shorter latency and the bioelectrical epileptic signs lasted longer than when injections were given in non-limbic structures, such as the globus pallidus or the mesencephalic reticular formation. Dendrotoxin is a very powerful toxin that acts effectively at the level of the limbic system.

Amygdala↗
PubMed1986

A spike generator mechanism model simulates utricular afferents response to sinusoidal vibrations.

Using a model of spike generator mechanism (SGM) with a variable threshold we simulate the responses of utricular afferents to sinusoidal vibrations. It reproduces the phase locking characteristics (bifurcations diagrams) and the stimulus frequency firing rate relationships of different types of utricular afferents. We estimate the model parameters selecting the values which best fit the experimental results and we compare them with those from basic mechanisms involved in utricular codification.

Afferent Pathways↗
PubMed1984

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.(ABSTRACT TRUNCATED AT 250 WORDS)

Abdominal Muscles↗
PubMed1984

Mechanisms of sensory adaptation in the isolated utricle.

The occurrence of receptor adaptation in utricular afferent fibers is now widely recognized. The experiments reported here explored the basic mechanisms of adaptation at the level of the receptor organ. Spike discharges from single utricular afferent fibers were recorded in isolated labyrinths of an elasmobranch, during three types of stimulation: (a) tilts in the gravity field, (b) vibrations, and (c) electrical polarization delivered through the nerve filaments from which recordings were also made. Experimental evidence supported the conclusion that polarization affects the discharge by acting at the level of the spike triggering mechanism, the point of the afferent fiber at which impulses normally arise. Three types of afferent fibers have been described: Types I and II fire spontaneously and show phasic-tonic responses to tilts. Type III fibers do not have spontaneous activity and respond to tilts in a phasic manner. Adaptation to polarizing currents was observed in all afferent fibers. Type II fibers adapted slowly to vibrations whereas types I and III afferent fibers did not. The functional processes situated near the spike triggering site of the sensory axon is referred to as neural whereas those occurring at earlier stages of transduction are called preneural. Adaptation to tilts exhibited two successive components: an early, fast phase and a late, slow one. Our results suggested that these phases can be related to the mechanisms of preneural and neural adaptation, respectively. Because the time course of adaptation to polarizing currents was similar in different afferent fibers, we concluded that preneural adaptation was the origin of the differences among afferent fibers that allowed their classification into phasic, phasic-tonic, and tonic groups. No attempts were made to separate the influence of mechanical coupling and transduction in the production of preneural adaptation.

Acoustic Stimulation↗
PubMed1983

Effects of intraventricular curarimimetics on hippocampal electrical activity.

The effects on the hippocampal electroencephalogram (EEG) of intraventricular injections of the nicotinic ligand alpha-Naja naja toxin, and of d-tubocurarine, were studied in rats immobilized with gallamine or anesthetized with urethane. The EEG recordings were taped and processed off-line to calculate power spectra, autocorrelation functions, and averages. In addition, the times at which spike-and-wave complexes appeared were identified and autocorrelation histograms and cross correlations (with the EEG) were made. Naja toxin and d-tubocurarine provoked a 3.5- to 5-Hz theta rhythm in both hippocampi. Higher doses elicited rhythmic epileptic spike and wave complexes which appeared at a preferred phase of theta rhythm. Atropine and medial septal lesions blocked the rhythm and disrupted the rhythmicity of epileptiform activity. We conclude that different neural subsystems sustain the theta rhythm and epileptiform spikes, and discuss the possible mechanisms involved.

Animals↗
PubMed1979

Statoacoustic properties of utricular afferents.

1. We classified the utricular afferents on the basis of their spontaneous acitivity and responses to tilts and vibrations. 2. Type I afferents fire spontaneously in a regular pattern; their responses to tilts consist of a phasic-tonic change in firing rate. They may respond to vibrations by increasing or decreasing their rate and show no adaptation. 3. The spontaneous activity and the responses to tilts of type II are similar to those observed in type I afferents. The differences become apparent when the preparation is subjected to a vibrational stimulus, since type II neurons increase their firing rate regardless of the stimulus frequency and show adaptation. 4. Type III neurons have no spontaneous activity. They respond to tilts by firing during the transition from one position to the other. They respond to a vibrational stimulus with maintained firing and show no adaptation. 5. We studied the dynamic responses of each type of neuron. We used sensitivity curves for the study of type III afferents and proposed a statistical method to define gain curves for the study of the other types. 6. The gain curves generated by type I neurons reach their maximum at frequencies of stimulation close to the spontaneous rate of firing. 7. In the gain curves of type II afferents the maximum corresponds to frequencies higher than their spontaneous activity. 8. Sensitivity curves and gain curves give similar results for type III fibers. The sensitivity curves of these afferents were classified into four subtypes. 9. We studied the responses of the three types of afferents to bursts of sinusoidal vibrations. 10. We concluded that the properties of types I and II fibers are fit to carry information about movements and position of the head, but also transmit acoustical information. Type III fibers are more adapted to provide information about acoustical stimuli, but can also convey information about head movements.

Acoustic Stimulation↗
PubMed1978

Multivalued stimulus-response relation in isolated elasmobranch utricles.

The relation between a maintained spatial orientation and the corresponding fully adapted discharge rate was multivalued in all the afferents tonically sensitive to maintained spatial orientation observed in isolated utricles of Rhinobates productus. The spread of rate values was of the order of changes produced by natural tilts. The occurrence of multivaluedness in isolated receptors indicated that peripheral issues are sufficient. Two factors contributed: firstly, the side from which the orientation had been reached (i.e. "hysteresis"): higher adapted rates occurred when the preceding orientation was characterized by lower rates and when the corresponding transition caused acceleration; secondly, "spontaneous" rate variations, some of which resembled markedly, and interacted with, the effects of tilts. It was not possible to identify the basic mechanisms underlying these factors. The multivaluedness in the coding of maintained position, because of its constancy and magnitude, cannot be ignored. It, as well as the sensitivity to fast transients, must be taken into account in utricular models, in evaluations of information transmission, and in psychophysical explorations.

Action Potentials↗
PubMed1975

Response of the elasmobranch utricle to maintained spatial orientation, transitions and jitter.

1. The spike discharges of single first order afferents from the utricle were recorded in the isolated head of the guitarfish and tested for responses to maintained spatial orientation, to transitions and to a small positional jitter representing natural perturbations. Sensitivity to maintained orientation is referred to as "tonic", and to transitions and jitter as "phasic". 2. Most responsive cells were either phasically, or phasically and tonically sensitive. A few were exclusively tonic. Tonic responsiveness implied that maintained orientation was associated with a stationary discharge which differed from one position to another; it sometimes differed also from one station to another at the same position. Transitions from one position to another evoked a rate change that later adapted to the level of the tonic response. Opposite transitions evoked rate changes in the opposite sense. The phasic rate change was usually larger for transitions that increased the rate. Many units were non-responsive. The prevalence of phasic over tonic sensitivity is stressed, and the remarkable heterogeneity of utricular afferents confirms that the macula is not uniform, probably coding a wide variety of head accelerations. 3. The jitter increased the ongoing scatter of intervals and binrates, changing, complicating, or abolishing their periodicity. The jitter could influence the effects of maintained orientation, increasing, decreasing, inverting or even revealing directional sensitivity. It could also force previously independent units into an orientation-dependent correlation; hence, between-cell correlation is potentially useful in coding of spatiel orientation. Naturally occurring perturbations may sonstitute a significant issue of normal operation. 4. Certain afferents from the horizontal semicircular canal showed a slow tonic response to maintained spatial orientation.

Action Potentials↗