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Biomedical subjects

N R Tiras

Publications and source records attributed to N R Tiras.

At least 19 recordsLinked to original sources
PubMed2006

Correlation between the sizes of Mauthner neurons and the preference of goldfish to turn to the right or left.

Three-dimensional computer reconstruction working from serial histological sections was used to study the morphology of the right and left Mauthner neurons (MN) in goldfish fry showing marked preferences to turn stably to the right or left in a narrow water channel or showing no asymmetry in their choice of side during turns. Visually, fish with left-sided motor asymmetry had larger MN on the right side, while fish with right-sided motor asymmetry had larger MN on the left side. Fish with symmetrical turns to the right and left showed no differences in MN size. Quantitative assessment of the MN of fish with preferences for turns to one side or the other revealed significant differences in the sizes of the somatic part, the axon hillock, and the axons of neurons located on the contralateral side of the medulla oblongata. Analysis of the statistical relationships between the functional (motor) asymmetry of fish and the morphological asymmetry of the somatic parts of MN in the same fish revealed a stable correlation (0.69) between these measures. Given that MN initiate unilateral turns of the body in free movement, the data obtained here lead to the conclusion that the larger neuron is more frequently activated in natural conditions as compared with the smaller, contralateral, neuron.

Animals↗
PubMed2005

The structure of mixed synapses in Mauthner neurons during exposure to substances altering gap junction conductivity.

The aim of the present work was to study the effects of dopamine, ecdysone, and chlorpromazine, substances which alter the conductivity of gap junctions (GJ), on the ultrastructure of mixed synapses in goldfish Mauthner neurons. These studies showed that dopamine, which increased the electrical conductivity of mixed synapses, appeared to target desmosome-like contacts (DLC). Hypertrophy of DLC, along with increases in the numbers of bridges within their clefts, showed that the mechanism by which dopamine increased electrical conductivity involved neuronal actin. This was indicated by the transformation of isolated monomeric muscle actin into polymerized actin in the presence of dopamine. Conversely, GJ were degraded by dopamine. Ecdysone, which also increased GJ conductivity, altered GJ structure, increasing the numbers of GJ at the attachment zone and decreasing the sectional length. but had virtually no effect on DLC structure. Ecdysone also showed no interaction with DLC in in vitro conditions. The mechanism of action of ecdysone is thus associated primarily with GJ function. Chlorpromazine, which decreased GJ conductivity, partially or completely degraded the fibrillar juxtamembrane material of DLC, preventing actin polymerization, with corresponding in vitro effects, but produced no changes in GJ. The mechanism of its action therefore appears to be based on changes in the state of neuronal actin.

Animals↗
PubMed2004

Involvement of actin in the electrotonic conductivity of mixed synapses in Mauthner neurons in the goldfish.

The effects of phalloidin, a preperation which highly specifically and selectively polymerizes actin and which binds to actin, on the electrotonic conductivity and structure of mixed synapses were studied in goldfish Mauthner neurons (MN). These experiments showed that paired subthreshold electrical stimulation of the afferent input in the presence of phalloidin led to increases in the amplitude of MN responses to the second stimulus by an average of 80%. In controls, this amplitude increased by only 10% and only when suprathreshold stimuli were used, while subthreshold stimuli were ineffective. We regard these results as demonstrating increases in the conductivity of mixed synapses, this being induced by polymerization of actin. At the ultrastructural level, application of phalloidin to MN and their mixed synapses induced increases in the sizes and numbers of actin-containing desmosome-like contacts, and in the numbers of fibrillar bridges in the clefts of these contacts. Use of colloidal gold as a label for phalloidin demonstrated that bridges were made of actin. The interdependent morphofunctional changes seen in mixed synapses provide grounds for suggesting a role for actin in the conduction of the electrotonic signal through mixed synapses. The structural substrate for this process may be provided by bridges in the clefts of desmosome-like contacts.

Actins↗
PubMed2004

Morphofunctional changes in incubated Mauthner neurons in goldfish treated with peptides from scorpion venom.

Electron microscopy with negative contrast showed that direct interaction of one of the peptide fractions of scorpion venom with monomeric chromatographically pure actin led to polymerization of actin, transforming it from the globular form to the fibrillar form. The effects of prolonged orthodromic stimulation on the evoked electrical activity and ultrastructure of Mauthner neurons (MN) were studied in incubated slices of goldfish medulla oblongata in the presence of this actin-polymerizing venom fraction. Peptides in this fraction were found to stabilize the amplitude of the electrical response of MN to exhaustion and to protect the ultrastructure of afferent chemical synapses and the neurons themselves from damage induced by stimulation. Enhancements in morphofunctional resistance were accompanied by stabilization of actin-containing specialized synaptic structures--desmosome-like contacts. The data obtained here provide evidence that peptides of this fraction of scorpion venom have direct actions on the actin component of the MN cytoskeleton and demonstrate potential for its use as a pharmacological tool able to penetrate living cells with value for studying the role of actin in the mechanisms of adaptation and memory.

Animals↗
PubMed2003

[Involvement of actin in electrotonic conductivity in the mixed synapses of goldfish Mauthner neurons].

The effect of highly specific and selective actin-polymerizing and labelling agent, phalloidin, on electrotonic conductivity and structure of the mixed synapses of goldfish Mauthner neurons (MN) was studied. It was shown that the paired subthreshold electrostimulation of afferent input against a background of phalloidin application resulted in the average 80% increase of the amplitude of MN response to the second stimulus. In control group it increased by only 10% and was observed only after suprathreshold stimulation, while subthreshold stimuli were ineffective. We interpret these data as the manifestation of increased conductivity of the mixed synapses, induced by actin polymerization. At the ultrastructural level, phalloidin application at MN and their mixed synapses increased the size and number of actin-containing desmosome-like junctions, as well as the number of fibrillar bridges crossing their cleft. Using the phalloidin-colloid gold marker, the actin nature of these bridges was demonstrated. Interdependent morpho-functional changes found in the mixed synapses, provide the indication of actin involvement in the conduction of electrotonic signal through the mixed synapse. The bridges crossing the cleft of desmosome-like junction could be the structural substrate of this process.

Actins↗
PubMed2003

[Effect of polypeptides from scorpion venom on morpho-functional changes of the incubated Mauthner neurons in goldfish].

Using the electron microscopical method of negative staining it was shown that one of the fractions obtained from scorpion venom directly interacts with monomeric chromatographically pure actin causing its polymerization and transformation from globular into fibrillar form. The effect of long-term orthodromic stimulation on evoked electric activity and on the ultrastructure of Mauthner neurons (MN) in the goldfish hindbrain slices incubated in vitro was studied after the application of this venom fraction. The peptides of this fraction were shown to stabilize the amplitude of MN evoked electric responses to fatigue and to protect the ultrastructure of afferent chemical synapses and MN themselves from stimulation-induced injuries. Increase in morpho-functional resistivity is accompanied by the stabilization of specialized synaptic structures--actin-containing desmosome-like junctions. These data suggest the direct effect of peptides from scorpion venom fraction on actin component of MN cytoskeleton and show the perspective for their application as pharmacological tools capable of penetration into the living cells that may be used for investigation of the role of actin in the mechanisms of adaptation and memory.

Actins↗
PubMed2001

[Structural differences between the desmosome-like contacts in the chemical and afferent synapses of Mauthner neurons in the goldfish].

Comparative ultrastructural investigation of the desmosome-like contacts at chemical and mixed afferent synapses of the goldfish Mauthner neurons was carried out. It was revealed that these contacts at mixed synapses differed from those at chemical ones by thin transverse fibrillar bridges which cross the gap and connect two adjoining membranes of the junction. We suppose that these crossbridges together with gap junctional connexons may serve as a substrate for electronic coupling at mixed synapses demonstrated earlier.

Afferent Pathways↗
PubMed2000

[Desmosome-like contacts of Mauthner neurons as targets of scorpion venom].

Desmosome-like contacts (DLC) in afferent chemical synapses of the Mauthner cells (MC) were investigated after application of low and high molecular mass peptide fractions 6 and 9, correspondingly, from the Central Asiatic black scorpion Orthochirus scrobiculosus. Besides, the DLC were examined in condition of a training induced morpho-functional stability of the MC (adaptation) mediated by transformation of actin monomers into polymers. In addition, the structure of DLC was studied after cytochalasin application which disrupts F-actin. Fraction 6 was shown to increase the length of DLC and osmiophily of fibrous material. Similar changes in DLC were caused by adaptation. Fraction 9 decreased the osmiophily of the fibrous material, made DLC asymmetric, but did not influence their length. Similar changes in DLC were seen also after cytochalasin D application. Taking into account our previous data on the role of F-actin in the MC functioning, which were obtained following specific pharmacological treatments, the similarity of ultrastructural changes in DLC after both adaptation and fraction 6 application, on the one hand, and after both cytochalasin D and fraction 9 application, on the other one, enabled us to suggest that these fractions may contain peptides able to exert influence of the actin cytoskeleton.

Animals↗
PubMed1999

Ultrastructure of Mauthner cells in fish adapted to long-duration vestibular stimulation and the effect of ethanol.

Adaptation or resistance of fish Mauthner cells (M-cells) to long duration (2 h) vestibular stimulation (LDS) was produced by daily brief and gradually increasing vestibular stimulation (training). The LDS resistance was accompanied by an increase in the number of desmosome-like junctions in the afferent axosomatic synapses. F-actin, the main component of desmosome-like contacts, has been suggested to be responsible for the increased resistance of M-cells to LDS. The purpose of the present study was to investigate the capacity of M-cells to adapt to LDS under the influence of ethanol, which alters the content of F-actin in cells. The experiments were carried out in goldfish fry. Vestibular stimulation (training and LDS) was performed in special drums that were rotated in two planes. The training time was increased from 1 min on day 1 to 30 min on day 30. For ethanol exposure, fish were immersed daily in a 2% ethanol solution for 20 min. To assess the level of resistance to LDS, motor activity indicating the functional state of M-cells was evaluated before and after LDS. The results show that exposure to ethanol reduces the resistance to LDS in both untrained and trained fish. Electron microscopic data demonstrated some structural changes in the synaptic endings located on M-cell soma in ethanol-exposed fish. Wrapping of boutons by cytoplasmic outgrowths and myelin-like structures was observed. Morphometric analysis revealed that exposure to ethanol without training decreases the number of desmosome-like contacts, probably due to ethanol-induced depolymerization of cytoskeletal actin. Ethanol exposure also partly suppressed the increase in the number of desmosome-like contacts that occurs as a result of training. In ethanol-treated trained fish, however, a concomitant increase in the length of desmosome-like contacts was observed. As training alone leads to the formation of additional desmosome-like contacts of standard length, it is possible that although a sufficient amount of such structures cannot be formed in the M-cells of ethanol-exposed trained fish, the existing contacts can be elongated. Thus, possibly changes of the actin state are involved in the adaptation of M-cells to LDS.

Actins↗
PubMed1998

In vitro long-term potentiation of electrotonic responses of goldfish mauthner cells is accompanied by ultrastructural changes at afferent mixed synapses.

The potentiated afferent mixed synapses of the Mauthner cells of fry and adult goldfish in stumps of the medulla oblongata incubated long-term in vitro were studied by electrophysiological and electron microscopic methods. It was shown that brief high-frequency stimulation of posterior branches of the eighth nerve induced a long-term potentiation of electrotonic transmission at large and small mixed club endings. It was about 135% upon subthreshold stimulation and about 200% upon suprathreshold stimulation. The ultrastructural analysis of ultrathin sections of potentiated mixed synaptic endings revealed an increase in the dimensions of desmosome-like contacts which was proportional to the degree of potentiation, about 135% or 200%, depending on the type of stimulation. The dimensions of gap junctions remained unchanged. The dimensions of active zones at potentiated synapses were reduced two-fold as compared with their unpotentiated counterparts, irrespective of the type of stimulation. Considering that desmosome-like contacts consist predominantly of F-actin, a molecule which possesses electroconductivity, it can be assumed that this cytoskeletal protein is involved in the process of potentiation. The increase in the synapse electrical conductivity can be mediated either directly, by shunting the synaptic junction with polymer actin filaments in the region of desmosome-like contacts, or indirectly, via the interaction of actin with gap junction connections situated nearby.

Animals↗
PubMed1998

[Ultrastructural changes in afferent mixed synapses in long-term potentiation of electrotonic responses in Mauthner neurons of goldfish medulla oblongata].

Ultrastructural synaptic features of long-term potentiation (double increase of amplitude) of electrotonic responses of the goldfish Mauthner neurons (MN) in conditions of prolonged in vitro incubation of medulla oblongata fragments were studied. Potentiation was induced by tetanization of the 8th nerve posterior roots, axonal terminals of which end on distal segments of MN lateral dendrite as piniform mixed synapses and increase their electrotonic conductivity. It was established that potentiated synapses vary from control ones in 1) greater length of synaptic contact; 20 greater (30% higher) ratio of desmosome like structures (DLS); 3) greater area of the DLS individual and total sections (two-fold increase); 4) two-fold decrease of total active zone square. Gap junction (GJ) parameters did not change. Taking an account of the proven actin origin of DLS, reports on actin filaments electron conductivity, close localization of DLS and GJ in the synaptic contact and correlative increase of synapse electric conductivity and DLS parameters. DLS contribution to electrotonic transmission is suggested, direct or mediated by actin interaction with nearby localized GJ connexons.

Afferent Pathways↗
PubMed1998

[Central Asiatic black scorpion venom protect Mauthner neurons from the damaging action of prolonged stimulation].

The effect of long-term natural stimulation resulting in prolonged fatigue and structural disorders as well as the black scorpion poison influence on these processes were studied in gold fish Mauthner neurons (MN). The poison previously applied to MN significantly protects their function and structure from the stimulatory effect and subsequently allows the neurons to recover more fast. Appearance of numerous desmosome-like (actin-containing) contacts and proliferation of subsurface cisterns, the accumulators of calcium ions was noted immediately after the poison action and stimulation and one day later in afferent synapse ultrastructure. The poison interaction with neuronal actin, mediated by alteration of calcium-accumulating systems is probably one of the mechanisms of its protective effect.

Animals↗
PubMed1998

[Experimentally induced actin depolymerization disrupts the adaptive state of a neuron].

The state of filament actin in Mauthner neuron (MN) of gold-fish adapted to long-term natural stimulation was investigated after introduction of actin-depolymerizing cytochalasin D. Cytochalasin was demonstrated to cause almost complete disappearance of long actin bundles that were characteristic for the cytoplasm of adapted fish and were absent from MN of intact fish. Polymeric actin supporting the cytoskeleton integrity was suggested to be involved in mechanisms underlying the increase of MN resistance extreme loads.

Actins↗
PubMed1996

[The ultrastructure of Mauthner's neurons in the surviving medulla oblongata of goldfish].

Ultrastructure of Mauthner neurons (MN) was studied in fragments of myelencephalon incubated for 0.5-1.5 hours obtained from immature gold fish. Incubation terms increase along with destruction extent and reorganization of cytoplasmic organelles and nucleus with structure of MN afferent synaptic apparatus less disturbed. Specialized synaptic contacts-active zones, cleft contacts and desmosome-like structure are well retained. The data obtained indicate MN viability during the incubation terms studied, which allows to use surviving grafts with MN as a model for studying ultrastructural bases of certain functional changes of neurons and other neurobiological problems.

Animals↗
PubMed1992

[The structure of the reticulum of Mauthner's neurons in tadpoles of the clawed toad grown under increased gravitational force].

The ultrastructure of the Mauthner cells (M-cells) and the behaviour of Xenopus laevis tadpoles, reared from eggs under increased gravity (2.9 g) which changes the activity of an afferent vestibular input, were investigated. Besides, a study was made of tadpoles after the hindbrain ablation at earlier embryonal stages which significantly altered the microenvironment of M-cells. It is shown that experimental treatments enhance the proliferation of endoplasmic reticulum and its derivatives, so called subsurface cisterns, in the subsynaptic areas. Some structural changes of the synaptic active zones and the cytoskeleton of M-cells were also noticed. It is assumed that the development of the endoplasmic reticulum promotes an intense removal of calcium ions from subsynaptic areas. The plasticity of the endoplasmic reticulum together with other ultrastructural changes apparently stipulate the adaptation of neurons to changed conditions of functioning.

Animals↗
PubMed1991

Alterations in the cytoskeleton of the goldfish Mauthner cells under various pharmacological treatments.

The ultrastructural changes in the cytoskeleton of the goldfish Mauthner cells (M-cells) at various functional states induced by intracerebral microinjections of biologically active substances were studied. Under the action of kainic acid, a structural analog of the excitatory neurotransmitter of glutamate, the density of the cytoplasmic matrix increased. Cytotoxin II from the cobra toxin, which blocks acetylcholine transmission, had an opposite effect upon the M-cell cytoskeleton. Simultaneously, in some areas of the neural cytoplasm strands of an electron-dense material of various shapes appeared. They had an unique structure which did not resemble any known cytoskeleton element. The molecular composition of the strands is unknown, but similar strands appeared after injections of phalloidin or cytochalasin B, both disturbing the microfilamental component of the cytoskeleton. Decoration with myosin subfragment-1 revealed actin in intact M-cells which was organized as crossing loose filaments and bundles of parallel fibers. The morphology of the fiber bundles resembles the helical part of the strands appearing after the treatment with phalloidin, cytochalasin B, or cytotoxin II. It is suggested that the cytoplasmic matrix of M-cells is a dynamic system which responds to the functional changes by thickening or loosening of its cytoskeletal elements or by formation of new structures.

Acoustic Stimulation↗