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L L Pavlik

Publications and source records attributed to L L Pavlik.

At least 19 recordsLinked to original sources
PubMed2005

Distribution of calcium ions in the mixed synapses of Mauthner neurons in the goldfish in normal conditions, in exhaustion, and in conditions of adaptation to exhaustion.

The aim of this study was to investigate the structure of large myelinated club terminals of Mauthner neurons (MN) in the goldfish at different levels of functional activity and the distribution within these synapses of calcium ions as assessed using a modified pyroantimonate method. In intact preparations, calcium pyroantimonate precipitates were not seen in gap junctions (GJ) or desmosome-like contacts (DLC). Fibrillar bridges in DLC clefts were not contrasted. After natural stimulation, which induces long-term adaptation in MN, GJ showed electron-dense precipitates lining the whole cleft. Granules and clumps of precipitate were also seen in DLC clefts, with intense deposition on bridges. Increases in calcium ion concentrations to and above the levels detectable by the pyroantimonate method are known to block electrotonic transmission; filamentous actin is known to conduct the electrotonic signal as a cation current. The staining of DLC bridges with calcium pyroantimonate is therefore evidence for an association between calcium ions and actin molecules, as DLC bridges consist of actin, i.e., we have obtained evidence for the functioning of bridges as electrotonic transsynaptic shunts at the moment of fixation. These data lead to the conclusion that DLC in mixed synapses, apart from the known adhesive functions, also have a communication function. This appears in extreme conditions, allowing the synapse to maintain or change its conductivity according to ongoing need.

Acclimatization↗
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↗
PubMed2005

Localization of calcium ions in mixed synapses of Mauthner neurons during exposure to substances altering the conductivity of gap junctions.

The pyroantimonate method was used to study the distribution of calcium ions in the mixed synapses of Mauthner neurons after exposure to substances altering the electrotonic conductivity of these synapses mediated by gap junctions (GJ). Ecdysone, an agent which increases GJ conductivity, produced precipitates of calcium pyroantimonate coating the whole postsynaptic surface of the GJ area, making them strongly asymmetrical. Precipitate granules were also seen to appear in the clefts of desmosome-like contacts (DLC). Chlorpromazine, which decreases GJ conductivity, produced precipitates in GJ clefts and on the pre- and postsynaptic membranes. No precipitate formed in DLC clefts. These results demonstrate that ecdysone acts as an agent selectively increasing GJ conductivity without affecting DLC function. Chlorpromazine had a double action, blocking conduction through both GJ and DLC. Thus, studies of agents altering GJ permeability require consideration of the possibility that they may interact with actin-containing structures also involved in the transport of the electrotonic signal.

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

Ultrastructure of desmosome-like contacts of mixed synapses of Mauthner neurons in long-term potentiation.

Electron microscopic morphometry was used to study the effects of long-term potentiation on the structure of fibrillar "cross-bridges" in the clefts of desmosome-like contacts in mixed synapses in Goldfish Mauthner neurons. These experiments showed that the number of bridges increased as the level of potentiation of electrotonic transmission increased. The structure of bridges changed after potentiation, which did not occur in controls (incubation). Double bridges appeared, which could have an altered (from control) organization within the cleft. The results obtained here and previously suggest that the bridges may be made of actin. Bridges are evidently a channel in which actin is organized as in nanotubules or plasmodesmata, and this may explain the stability of the bridge structure to treatment with cytochalasin and other external damaging factors which we have observed.

Animals↗
PubMed2004

[Substances affecting gap junction conductivity and Mauthner neuron mixed synapses].

The aim of this work was to study the effect of dopamine, 20-hydroxyecdysone and chlorpromazine, drugs changing the conductance of gap junctions (GJs), on the ultrastructure of goldfish Mauthner neurons mixed synapses. It was shown that desmosome-like contacts (DLCs) were the presumable targets for dopamine, that increased the electrotonic conductance of mixed synapses. Their hypertrophy, as well as an increase in the number of bridges in their clefts suggests that neuronal actin is involved in the mechanism of dopamine-induced increase of electrotonic conductance. This assumption is further supported by the transformation of extracted monomeric muscular actin into polymeric actin in the presence of dopamine. On the contrary, GJs were shown to be damaged by dopamine treatment. Ecdysone, which is also known to increase the conductance of GJs, changed their structure, increasing their number in the zone of apposition and reducing the profile length, but practically not affecting the DLC structure. It also does not interact with isolated actin in vitro. Therefore, the mechanism of ecdysone action is mainly associated with GJ function. Chlorpromazine, which reduces the conductance of GJs, was shown to damage, partially or completely, DLC admembranous fibrillar material, thus preventing actin polymerization, as shown by in vitro experiments, but had no effect on GJs. Therefore, the mechanism of its action, appears to be based on the changes in the state of neuronal actin.

Actins↗
PubMed2004

[Calcium ions localization in mixed synapses of mauthner cells after exposure to substances changing the gap junction conductivity].

Using the pyroantimonate method, the distribution of calcium ions in Mauthner cell mixed synapses was studied following their treatment with substances changing gap junction (GJ) synapse electrotonic conductivity. Ecdyson, the drug known to increase GJ conductivity, induced the appearance of calcium pyroantimonate precipitates covering the whole postsynaptic area of GJs, making them highly asymmetric. Formation of precipitate grains was also observed in the cleft of desmosome-like contacts (DLCs). Chlorpromazine, substance that decreases GJ conductivity, induced the formation of precipitates in GJ cleft and on both pre- and postsynaptic sides of their surfaces. No precipitates were seen in DLC cleft. These results show that ecdyson acts as a substance selectively increasing the GJ conductivity with no effect on DLC function. Chlorpromazine has dual effect, by blocking the communication through both GJ and DLC. Thus, using chemicals that change GJ permeability, the possibility of their interaction with actin-containing structures which also participate in the electrotonic signal transfer, should be taken into consideration.

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

[The ultrastructure of desmosome-like contacts in mixed synapses of Mauthner cells followed long-term potentiation].

Using electron microscopic morphometry, the effect of long-term potentiation on the structure of transverse fibrillar bridges localized in a cleft of desmosome-like contacts in mixed synapses of goldfish Mauthner neurons, was studied. The number of bridges was shown to increase with the augmentation of the potentiation level of electrotonic conductivity. In contrast to control (incubation), the structure of bridges was also changed after potentiation. The paired bridges appeared that could have an arrangement within the cleft different from that one in control. On the basis of the present results in conjunction with earlier findings, the possible actin nature of these bridges was suggested. The bridge seems to represent a channel containing actin, arranged in a pattern similar to that one in nanotubules or plasmodesmas, what can, possibly, explain the stability of bridge structure observed after application of [symbol: see text] D and other extrinsic damaging factors.

Animals↗
PubMed2003

[Calcium ions distribution in mixed synapses of the mauthner neurons of goldfish in the norm, fatigue, and adaptation state].

The aim of this investigation was to study the structure of giant myelinated club-shaped terminals (afferent mixed synapses) of goldfish Mauthner (M-) cells in different functional states and to demonstrate calcium ion localization in them using modified pyroantimonate method. It was shown that in intact preparations calcium pyroantimonate precipitate was detected neither in gap junctions (GJ) nor in desmosome-like junctions (DLJ). The fibrillar bridges within DLJ cleft were not contrasted. After natural stimulation, which elaborated a long-term adaptation of M-cells, electron dense precipitate was found in GJ, lining all the cleft. Simultaneously fine granules and aggregates of precipitate appeared in DLJ gap and were intensely deposited over the bridges. It is known that the increase of calcium ion concentration up to and above the level demonstrable by pyroantimonate method blocks the electrotonic coupling and that filamentous actin is able to conduct electrotonic signal as a cationic current. Therefore calcium pyroantimonate staining of DLJ bridges, which were earlier shown to contain actin, indicates the association of calcium ions with filamentous actin, i.e. the functioning of bridges as transsynaptic electrotonic shunts at a moment of fixation. The data obtained allow to make a conclusion that DLJ in mixed synapses have not only a known adhesive function, but also a communicative one. The latter is manifested in extreme conditions, thus permitting synapse to maintain or change their conductivity in accordance with environmental demands.

Adaptation, Physiological↗
PubMed2002

Three-dimensional (3-D) structures formed by immortalized human fibroblast cells in simulated microgravity.

3-D structures were obtained at rotatory cultivation of CH immortalyzed human fibroblasts attached to glass microcarrier beads. The morphology of cells from these cultures was studied by scanning electron microscopy. A number of structural alterations in fibrillar filopodia of CH cells were revealed as compared with cells grown in stationary monolayer cultures, namely, smaller length, uneven caliber, the presence of curvatures, and disturbed branching pattern. Filopodia displayed unusual formations: protuberance-like and "mammoth's tusk"-like off-shoots, foamy spreadings in distal segments, and spiral windings of filopodia. The susceptibility of CH cells morphology to mechanical environment makes them a promising model for gravitational biology studies.

Journal Article↗
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

Study of F-actin interaction with planar and liposomal bilayer phospholipid membranes.

Interaction of the cytoskeletal protein F-actin with planar bilayer lipid membrane (BLM) induced formation of single ionic channels in both NaCl and KCl bathing solutions. We also recorded noiselike high-currentjumps with a mean conductivity of approximately 160 pS, which might represent the simultaneous opening and closing of several channels of lower conductivity. The ratio of cation to anion permeabilities (Pc/Pa) of the BLM with many channels in KCl was 26 +/- 2. Freeze-fracture electron microscopy revealed fibrillar-like structures on the hydrophobic surfaces of liposomal membranes. We also observed some structural features giving evidence for the penetration of F-actin fibers through an artificial phospholipid membrane. We suggest that the F-actin/lipids complexes can transmit electric signals in synaptic and other intercellular contacts.

Actins↗
PubMed1999

[Effect of cytochalasin D on the structure of mixed synapses and their electrotonic conductivity].

By methods of qualitative and quantitative electronmicroscopy the ultrastructure of mixed synapses of the goldfish Mauthner cell (MC) and their ability to strengthen electrotonic transmission after application of cytochalasin D, a highly specific inhibitor of actin polymerization. On the background of cytochalasin action tetranization of afferent rootlets of acoustic nerve, terminated with mixed synapses on the MC lateral dendrite, failed to induce any long-term potentiation of electronic coupling. On the contrary, a long-term depression of electrotonic transmission was evoked in this case. On the ultrastructureal level such a depression of synaptic conductivity was seen to correlate with a decrease in the number and total length per synaptic apposition of the desmosome-like contacts, known as actin-containing structures, as compared with unstimulated control preparations. In addition, experimental preparations were characterized with unusual quantity of asymmetric desmosome-like contacts, hemidesmosome, whose number during a long-term depression became 4 fold higher than in the control synapses. The data obtained support our recent suggestion on the role of filamentous actin in induction and long-term maintenance of enhanced electrotonic conductivity at mixed synapses.

Animals↗
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

[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↗