Biomedical subjects

O Fehér

Publications and source records attributed to O Fehér.

At least 19 recordsLinked to original sources

PubMed1996

Neuronal plasticity induced by neonatal monocular (and binocular) enucleation.

Monocular (ME) and binocular enucleation has become a useful experimental tool for analyzing the mechanisms of neural plasticity. ME when performed during an early postnatal period (up to 15 days after birth) initiates a series of adaptive reactions in the visual (and other sensory) system(s) which tend to compensate for the lost sensory capacity. Extirpation of one eye (usually the right) destroys afferents to both lateral geniculate bodies dorsal nucleus (CGLd) and superior colliculi (CS), being severely impaired by the degeneration of retino-geniculate and collicular synapses. The sprouting of retinogeniculate fibers coming from the remaining eye replaces these synapses in both CGLds. Ipsilateral representation of the remaining eye (usually of minor significance) becomes extended in the left CGLd and consequently in the left visual area, just as in the superior colliculi. A similar but somewhat smaller extension takes place in the contralateral CGLd and visual cortex. The strengthening of commissural connections results in a remarkable extension of callosally connected stripes and patches in both hemispheres. After ME in the critical period, the control over behavior is taken over by the remaining eye. Its power of resolution is improved because of the higher survival of (mainly ipsilaterally projecting) ganglion cells. Therefore, both hemispheres are still available for storing visual information. In ME rats the learning of visual tasks requires both hemispheres, but relearning is still possible after extirpation of the contralateral one. The possible two main mechanisms of adaptive plastic changes are: (i) replacement of degenerated synapses by sprouting collaterals of ingrowing foreign fibers, and (ii) neurons having morphologically intact but inactive synapses establishing connections with afferent fibers other than the usual. The same mechanism is seen operating in cross-modal adaptive reactions as well.

Animals↗

PubMed1996

A computer model of the visual cortex.

A three-layered simulation of the visual cortex was constructed, receiving inputs from 252 retinal photoreceptors. Lateral geniculate body was taken as a simple relay station. In the cortical module afferent neurons with circular receptive fields, orientation selective simple cells, tuned complex cells, excentricity neurons and different kinds of inhibitory neurons were included. The model reproduces neuronal and network responses recorded in experiments and some sensory illusions, of which one is here demonstrated.

Afferent Pathways↗

PubMed1996

A computer model of processes of binocular vision in the cerebral cortex.

Fundamentals and options of modeling cortical processes of binocular vision are dealt with in this paper. Physiologically characterized cell types of the visual cortex are modelled and used as basis of a network implementing a simulation of the binocular vision. An attempt is made to establish correlations between typical activity patterns of visual cortical cells and the network incorporating them.

Animals↗

PubMed1994

Neonatal monocular enucleation-induced cross-modal effects observed in the cortex of adult rat.

The cortices of neonatally enucleated rats were explored for somatosensory responses with special reference to an extension into the occipital cortex. Monocular enucleation was performed on rats at birth. The animals were raised and from the age of three months the activity evoked by either electric stimulation of the vibrissa pad or bending of the vibrissae was tested in the contralateral cortex by electric recording and autoradiography. It was found that early enucleation caused an expansion of the somatosensory responses, among others into the visual area. Neurons responsive to visual and somatosensory stimuli were demonstrated in the anterior part of the primary and secondary visual areas, contralateral to the enucleation. Electrophysiological and autoradiographic studies unambiguously proved that early enucleation exerted a significant cross-modal effect on the somatosensory responsive area.

Aging↗

PubMed1994

Rapid synthesis of a glycine-rich protein in the cerebral cortex of the rat during activity.

A glycine-rich protein was previously discovered (Rojik and Fehér, 1976) in the nervous system, which proved to be neuron-specific and in its appearance highly dependent on the actual intensity of neural activity. It was named by us neuragin. Time relations and regulation of neuragin synthesis were examined by the glycine labelling method in the cerebral cortex of the rat. During activation of the somato-sensory area, 2 min stimulation sufficed to provoke the appearance of neuragin in the neurons. The newly synthesised protein persisted in the nerve cells at least for 6 hours after stimulation. Phorbol-12-monoacetate strongly enhanced, sphingosin depressed the synthesis of neuragin. It is concluded, that neuragin is an immediate early gene product, similar to proteins encoded by genes c-fos and c-jun.

Animals↗

PubMed1993

Deep sensibility of the mystacial pad in the rat and its cortical representation.

The cortical representation of the rat's mystacial pad was examined with the aid of evoked field potentials and recording of single cell activity. Mechanical bending of the vibrissae activated the well-known area within the somato-sensory cortex. Electrical stimulation of the mystacial pad with inserted needle electrodes, bi- and monopolarly, caused a widespread activation extending practically to the whole exposed cortex, including visual, acoustic and motor areas (MSS potentials). The evoked field potentials were accompanied by well-recordable unit activity, mainly in the upper 1000 microns of the cortical depth. Capsaicin, injected into the mystacial pad on the 8th-10th postnatal day heavily impaired the MSS potentials as recorded at 2 months of age, and only moderately acted on the mechanically evoked potentials. So did also the acutely injected capsaicin. Peak latency of the MSS potentials seemed to be in correlation with the distance from the punctum maximum. The latencies of unit potentials, however, did not show such dependence, they were between 8 and 10 ms. MSS potentials are thought to represent cortical projection mainly of thermo- and nociceptive fibers, which play an important role in the early postnatal life.

Animals↗

PubMed1993

Modification in primary visual cortical activity of rat induced by neonatal monocular enucleation, an electrophysiological and autoradiographic study.

Mononuclear enucleation was performed in rats at birth. The animals were raised and from the age of 3 months the evoked activity was tested in the contralateral visual cortex both by mapping of evoked potentials and autoradiography. It was found that monocular enucleation changed the distribution of evoked activity characteristically. The focus of activity shifted laterally and restricted itself to the binocular part of the primary visual cortex (Oc1B), while, in the medial part of it (Oc1M) hardly any evoked activity or labelled neurons were found

Animals↗

PubMed1993

A computer model of the convulsive membrane.

A computer model of the convulsive membrane was built up by combining the H-H equations of normal ionic currents with a pathological current flowing through calcium channels and carrying sodium ions. Thus any form of paroxysmal depolarization shift could be simulated, by varying two rate constants and potassium conductance. The simulated PDSs are initiated by a minimal conductance pulse and terminate spontaneously according to the inherent rules of the model system. The program is written in Turbo-Pascal language and can be run on any PC.

Calcium Channels↗

PubMed1992

Neonatal enucleation induces correlated modification in sensory responsive areas and pial angioarchitecture of the parietal and occipital cortex of albino rats.

This study was carried out to investigate whether correlations existing in normal adult rats (Ambach et al., '86) between functional characteristics of neocortical areas and their pial angioarchitecture can be correspondingly modified under pathological conditions. The right eyes of albino rats were enucleated on the 1st, 8th, 15th and 30th day after birth, respectively. At the age of 3 to 4 months, the responsiveness of the parieto-occipital cortex to sensory stimuli was studied in enucleated animals and age matched controls. After the mapping of visually and somatosensorily evoked potentials, the vascular system was filled with dye. Monocular enucleation at birth induced bilateral modifications in sensory responsiveness and corresponding changes in pial angioarchitecture, especially in the venous drainage fields. In comparison with the controls, a considerable expansion was observed in the overlapping zone between visually and somatosensorily responsive areas. In contrast, borders of the visual cortex toward the auditory and retrosplenial areas were essentially stable. Corresponding changes were found in the pial distribution patterns of cerebral veins but not of arteries. The major effect of neonatal enucleation on angioarchitecture was a change in the subdivision of the parieto-occipital veins drainage fields. This was due to a significant enlargement of the anterior accessory occipital (O3) vein, which compressed the drainage fields of the parietal and occipital veins and completely separated them from one another. The results suggest that during ontogenesis: (1) alterations in the formation of sensory input may interfere with neocortical angiogenesis, especially the structuring of veins, (2) after monocular enucleation this influence is prominent in parietal and occipital cerebral veins, and (3) these angiogenetic processes are vulnerable only during the perinatal and early postnatal period.

Animals↗

PubMed1992

Modification induced in visual cortical activity of rat by neonatal monocular enucleation.

Monocular enucleation was performed on rats at birth. The animals were raised and from the age of 3 months the evoked activity was tested in the contralateral visual cortex both by mapping of evoked potentials and by autoradiography. It was found that monocular enucleation changed the distribution of the evoked activity characteristically. The focus of activity shifted laterally and was restricted to the binocular part of the primary visual cortex, while hardly any evoked activity or labelled neurons were found in its medial part.

Animals↗

PubMed1990

Properties of the slow inward current induced by pentylenetetrazol in Helix neurons.

The slow inward current (SIC) induced by 50 mM pentylenetetrazol was studied on central neurons of Helix pomatia in view of its ionic dependence and sensitivity to channel blockers in the presence of 30 mM TEA. A single electrode voltage clamp was used to measure currents evoked by ramp pulses and voltage steps. Sodium withdrawal had variable effects on this current while TTX had no influence on it. Inorganic and organic calcium channel blockers, on the contrary, always produced a partial or total block of the SIC. It is concluded that the slow inward current is mediated by Ca channels impaired by PTZ. Various ions--Na, Ca, and even Tris--may participate in it in variable proportions.

Action Potentials↗

PubMed1988

The effect of pentylenetetrazol on the metacerebral neuron of Helix pomatia.

The effects of Pentylenetetrazol (PTZ) on the metacerebral giant cell (MCC) of the snail, Helix pomatia were studied. Actions on membrane resistance, time constant, resting and action potentials, outward and inward ionic currents were examined. Superfusion with PTZ in concentrations of 25 to 50 mmol/l, induced a gradually evolving convulsive state, which could be studied by intracellular recording from the MCCs. In the pre-convulsive state an acceleration of the spontaneous activity developed and was followed by paroxysmal depolarization shifts (PDSs), in the convulsive phase. PTZ prolonged the membrane time constant by about 10 percent, but this could not be traced back to alterations in membrane resistance or capacity. The resting membrane potential was not significantly altered; the action potentials were prolonged by slowing down of both the rising and decaying phases. The outward potassium currents were repressed by PTZ in a voltage dependent manner. The decrease of the IA current became more pronounced at increasingly positive command pulses, while IK was relieved from depression especially at longer pulse durations. Inward currents were isolated with the aid of suppression of outward currents by 50 mmol/l TEA. Under these conditions sodium currents, measured in calcium deficient Ringer solution were moderately depressed, while the calcium currents, examined during sodium-free superfusion, were mildly enhanced by PTZ. It is concluded that PTZ effects on ionic conductances, on membrane parameters, on the resting potential and ionic currents explain only modifications of spike potentials occurring in the convulsive state and do not account for the PDS, the central phenomenon of the convulsive electrographic activity, at least in this thoroughly examined type of neuron.

Action Potentials↗

PubMed1987

Interactions of acoustic and somatosensory evoked responses in a polysensory cortex of the cat.

Interactions of acoustic and somatosensory evoked potentials were studied in the anterior suprasylvian gyrus of the cat. The interactions showed dynamic changes and were susceptible to different kinds of influences. The interactions could be influenced by synchronous activation of the acoustic and somatosensory inputs with 2 Hz frequency, or by elevating the stimulus frequency. Interactions could be influenced by amphetamine and gamma-glutamyl-taurine, drugs known as capable of influencing the arousal level of the brain. The antagonists of amphetamine prevented this effect. Drugs acting on the cortical GABA-ergic system proved also to be decisive in the interactions of evoked potentials of different origins. In some experiments unit activity was recorded parallel with evoked potentials.

Acoustic Stimulation↗

PubMed1987

The ionic mechanism of the pentylenetetrazol convulsions.

The ionic dependence and the nature of conductance was examined at slowly inactivating inward current in metacerebral giant cells of Helix pomatia, induced by 50 mM pentylenetetrazol. Ramp and square wave depolarizations in voltage clamp mode revealed, that withdrawal of sodium ions prevented this current to flow. While TTX was ineffective, Mn, Co and Ni-ions and verapamil blocked the current. It is concluded that PTZ, especially in presence of TEA impairs calcium channels, which loose their specificity and transmit sodium ions, with very slow kinetics.

Animals↗

PubMed1987

Application of the glycine labelling method to the cerebellum, hippocampus and spinal cord.

3H-glycine was applied to the cat cerebellar cortex under resting conditions and during inferior olive stimulation which activated the climbing fiber system on a restricted area. Electric recording was made. The autoradiograms showed, that under resting condition labelled glycine was incorporated mainly in granule, Golgi and basket cells and only a few Purkinje and stellate cells were active. Also cerebellar glomeruli remained without labelling. On climbing fiber stimulation Purkinje cells became activated singly and grouped, also Golgi and stellate cells increased in number. Granule cells were totally inhibited. 3H-glycine, when applied to the rat hippocampus, the dentate gyrus, CA1 and CA4 fields showed labelling at low frequency stimulation. When 400 Hz high frequency stimulation periods were interposed, long-term potentiation ensued. The overall labelling of each hippocampal region was intensified significantly, indicating that glycine incorporation may be a sign not only of excitation but also of long-term potentiation. 3H-glycine was applied to frog spinal cord during rest and dorsal root stimulation. Interneurons and motor neurons excited by the afferent fibers showed intensive glycine uptake. It is concluded that the glycine labelling method is suitable for detecting neural excitation in the structures dealt with in this paper.

Animals↗

PubMed1986

Promotion by sodium bromide of functional synapse formation from foreign nerves in the superior cervical ganglion of adult rat with intact preganglionic nerve supply.

The possible effect of sodium bromide (NaBr) (a substance with known inhibitory action on synaptic transmission) was studied on synapse formation with foreign nerves, implanted into the superior cervical ganglion of adult rats. It was found that in spite of the presence of preganglionic nerve supply, both implanted nerves (n.XII and n.X, respectively) were enabled to establish functional synapses with the principal ganglion cells in NaBr-treated animals. In contrast, synapse formation was almost absent in ganglia of sodium chloride drinking (control) rats with intact preganglionic nerve supply. This effect of NaBr is considered to be analogous to that of GABA, whose promoting action on synaptogenesis in adult rat superior cervical ganglion has been previously described.

Animals↗

PubMed1986

Spatial correlation between sensory regions and the drainage fields of pial veins in rat cerebral cortex.

Visual and somatosensory evoked potentials were mapped in the cerebral cortex of adult rats and, after filling the cerebral arteries and veins with dye, the mappings were then compared to the distribution of pial veins. A close relationship was found between the position, size and shape of the occipital venous drainage field and the distribution of visual evoked potentials with high amplitudes and short latencies. Accordingly, such potentials evoked by stimulation of the forepaw and tailroot were confined to the fronto-parietal drainage field. In the case of individual variations in the expansion and shape of sensory areas, the medial and lateral borders of the occipital drainage field and the medial border of the fronto-parietal drainage field covaried. Only at the common border between these two drainage fields, visual evoked potentials with small amplitudes and long latencies extended into the parietal drainage field and overlapped with somatosensory evoked potentials. This overlapping area corresponds in position to the anterior part of the peristriate cortex. A comparison between the vascular organization and cytoarchitectonic maps of the rat cortex indicates that other parts of the characteristic pattern of venous drainage fields may also correlate with the cytoarchitectonic and functional organization of the cerebral cortex. These observations suggest that during morphogenesis the formation of sensory projections to the cerebral cortex may interact with the angiogenesis, mainly with the development of veins.

Animals↗

PubMed1986

Sensory interactive zones in the rat cerebral cortex.

The acoustic, somatosensory and visual areas of the rat cerebral cortex were mapped in view of overlapping regions, giving polysensory evoked responses. In case of each pair of modalities overlapping areas were found between the cortical representations. The evoked potentials of two different modalities show occlusive and facilitatory interactions in these overlapping areas. In the "core" region of these areas, points with especially high values of occlusion are concentrated while at the periphery weak occlusion or even facilitation is characteristic. It is supposed that the polysensory areas interposed between primary sensory fields represent not only areas with mixed responsiveness but also stage for real physiological interactions between different sensory systems.

Animals↗