Uridine as a Regulator of Functional and Ultrastructural Changes in the Brain of Rats in a Model of 6-OHDA-Induced Parkinson's Disease

Using a model of Parkinson's disease (PD) induced by the bilateral injection of neurotoxin 6-hydroxydopamine (6-OHDA) into rat brain substantia nigra (SN), we showed uridine to exert a protective effect associated with activation of the mitochondrial ATP-dependent potassium (mitoK-ATP) channel. Injection of 4 µg neurotoxin evoked a 70% decrease in the time the experimental animal spent on the rod in the RotaRod test, an increase in the amount of lipid peroxides in blood serum and cerebral-cortex mitochondria and the rate of reactive oxygen species formation, and a decrease in Ca2+ retention in mitochondria. Herewith, lymphocytes featured an increase in the activity of lactate dehydrogenase, a cytosolic enzyme of glycolysis, without changes in succinate-dehydrogenase activity. Structural changes occurring in the SN and striatum manifested themselves in the destruction of mitochondria, degeneration of neurons and synapses, and stratification of myelin sheaths in them. Subcutaneous injections of 30 µg/kg uridine for 22 days restored the neurotoxin-induced changes in these parameters to levels close to the control. 5-Hydroxydecanoate (5 mg/kg), a specific mitoK-ATP channel inhibitor, eliminated the beneficial effect of uridine for almost all characteristics tested, indicating the involvement of the mitoK-ATP channel in the protective effect of uridine. The mechanism of the protective effect of uridine and its therapeutic applications for the prevention and treatment of PD are discussed.

Autophagy in Neurons of the Prefrontal Cortex and Hippocampus of Rats after Trimethyltin Chloride Intoxication

Ultrastructural studies of the hippocampus and the prefrontal cortex of rats were performed 7, 30, and 50 days after their damage by neurotoxicant trimethyltin chloride (TMT). Significant damage to neurons was observed in both brain structures. In the hippocampus, a large number of autophagosomes (0.9±0.1 per μm²) appeared in the soma of neurons, dendrites, and axons in 7 days after intoxication. In addition, we observed the appearance of hyperchromic neurons with abnormal structure of mitochondria. In the prefrontal cortex, damaged neurons also contained autophagosomes, but their number was significantly lower (0.3±0.1 per μm²). The number of autophagosomes decreased with increasing the time after TMT administration: 30 days after injection, the content of autophagosomes in the hippocampus was 0.10±0.01 per μm², while in the prefrontal cortex, autophagosomes were no longer found. We hypothesized that autophagy in the hippocampus was not effective enough to prevent neuronal death caused by the neurotoxicant.

The Effect of S-15176 Difumarate Salt on Ultrastructure and Functions of Liver Mitochondria of C57BL/6 Mice with Streptozotocin/High-Fat Diet-Induced Type 2 Diabetes

Simple Summary

Type II diabetes mellitus (T2DM) is one of the most common diseases, which currently represents a major medical and social problem due to the chronic course, high rates of disability and mortality among patients. Mitochondria of the liver and other vital organs are one of the main targets of T2DM at the intracellular level. The pathological changes in the structure of mitochondria, hyperproduction of reactive oxygen species by the organelles, disorders in mitochondrial transport systems and ATP synthesis are now widely recognized as important factors in the development of diabetes. Therefore, treatment strategies to attenuate mitochondrial injury may result in cellular reprogramming and alleviation of the diabetes-related pathological complications. The aim of present work was to investigate the possible protective effect of S-15176, a potent derivative of the anti-ischemic agent trimetazidine, against mitochondrial damage in the liver of mice with experimental T2DM. The data indicate that S-15176 attenuates mitochondrial dysfunction and ultrastructural abnormalities in the liver of T2DM mice. The mechanisms underlying the protective effect of S-15176 are related to the stimulation of mitochondrial biogenesis and the inhibition of lipid peroxidation in the organelles. One may assume that the compound acts as a mitochondria-targeted metabolic reprogramming agent in T2DM.

Abstract

S-15176, a potent derivative of the anti-ischemic agent trimetazidine, was reported to have multiple effects on the metabolism of mitochondria. In the present work, the effect of S-15176 (1.5 mg/kg/day i.p.) on the ultrastructure and functions of liver mitochondria of C57BL/6 mice with type 2 diabetes mellitus (T2DM) induced by a high-fat diet combined with a low-dose streptozotocin injection was examined. An electron microscopy study showed that T2DM induced mitochondrial swelling and a reduction in the number of liver mitochondria. The number of mtDNA copies in the liver in T2DM decreased. The expression of Drp1 slightly increased, and that of Mfn2 and Opa1 somewhat decreased. The treatment of diabetic animals with S-15176 prevented the mitochondrial swelling, normalized the average mitochondrial size, and significantly decreased the content of the key marker of lipid peroxidation malondialdehyde in liver mitochondria. In S-15176-treated T2DM mice, a two-fold increase in the expression of the PGC-1α and a slight decrease in Drp 1 expression in the liver were observed. The respiratory control ratio, the level of mtDNA, and the number of liver mitochondria of S-15176-treated diabetic mice tended to restore. S-15176 did not affect the decrease in expression of Parkin and Opa1 in the liver of diabetic animals, but slightly suppressed the expression of these proteins in the control. The modulatory effect of S-15176 on dysfunction of liver mitochondria in T2DM can be related to the stimulation of mitochondrial biogenesis and the inhibition of lipid peroxidation in the organelles.

Effect of TGF-beta1 on long-term synaptic plasticity and distribution of AMPA receptors in the CA1 field of the hippocampus

Using the methods of electrophysiology and immunohistochemistry, the effect of the transforming factor beta-1(TGF-β1), an anti-inflammatory cytokine, on the long-term post-tetanic potentiation (LTP) in CA1 field hippocampal slices and the distribution of the GluR1 subunit of the AMPA receptor has been studied. It was shown that TGF-β1 at a concentration of 10 ng/ml did not significantly affect the initial stage of LTP and substantially changed the distribution of synaptic AMPA receptors in response to tetanic stimulation. Twenty five minutes after the tetanization, the main pool of AMPA receptors (90%) was due to the postsynaptic density (PSD). By contrast, LTP in the presence of TGF-β1 was accompanied by less pronounced changes in the distribution of AMPA receptors. Their localization in both pre- and postsynaptic regions remained nearly the same as that in the control. It may be suggested that the normal distribution of AMPA receptors in spinous synapses promotes the stabilization of potentiated synapses, thereby retaining LTP for longer terms.

NMDA and GABA receptor presence in rat heart mitochondria

The purpose of this study is to demonstrate the presence of three more receptors in mitochondria. Two N-methyl-d-aspartate receptor (NMDAR) subunits (NR1 and NR2B) are found by protein immunoblot and immunogold labeling in mitochondria fraction isolated from rat heart. These data allow supposing NMDAR presence and functioning in the inner mitochondrial membrane. There are no signs of receptor presence obtained in heart tissue lysate, that indicates the receptor localization exactly in mitochondria. The possible receptor functions discussed are its participation in calcium transport and in excitation-metabolism coupling. Besides, preliminary evidence is obtained of GABA_A and GABA_B receptors presence in heart mitochondria. One can surmise their role in metabolism regulation and their possible co-operation with NMDAR just as in the nervous system.

On the regulative role of the glutamate receptor in mitochondria

The purpose of this work was to study the regulative role of the glutamate receptor found earlier in the brain mitochondria. In the present work a glutamate-dependent signaling system with similar features was detected in mitochondria of the heart. The glutamate-dependent signaling system in the heart mitochondria was shown to be suppressed by γ-aminobutyric acid (GABA). The GABA receptor presence in the heart mitochondria was shown by golding with the use of antibodies to α- and β-subunits of the receptor. The activity of glutamate receptor was assessed according to the rate of synthesis of hydrogen peroxide. The glutamate receptor in mitochondria could be activated only under conditions of hypoxic stress, which in model experiments was imitated by blocking Complex I by rotenone or fatty acids. The glutamate signal in mitochondria was shown to be calcium- and potential-dependent and the activation of the glutamate cascade was shown to be accompanied by production of hydrogen peroxide. It was discovered that H2O2 synthesis involves two complexes of the mitochondrial electron transfer system - succinate dehydrogenase (SDH) and fatty acid dehydrogenase (ETF:QO). Thus, functions of the glutamate signaling system are associated with the system of respiration-glycolysis switching (the Pasteur-Crabtree) under conditions of hypoxia.

[ULTRASTRUCTURE OF MOTOR NEURONS AND SYNAPSES IN THE OCULOMOTOR NERVE NUCLEI IN MICE]

Motor neuron and synapse ultrastructure in the somatic area of oculomotor nerve nuclei (cranial nerve III nuclei) was studied in C57 Black/6 mice. It was shown that that axodendritic and axosomatic synapses were characterized by rounded subjunctional bodies, located at the postsynaptic side of the synaptic contact at some distance from postsynaptic density. At the site of the subjunctional densities, the synaptic gap was expanded to 30 µm. In the same synapse, the synaptic gap could be reduced twice or more. Axosomatic and axodendritic synapses occured on spines. On the soma and dendrites of motoneurons, en passant type of synapses were found. No gap junctions were observed.

[Serotoninergic synapses on the ventral dendrite of the mauthner neuron (ultrastructural study with immunogold labeling)]

Using immunogold labeling, excitatory serotoninergic synapses of both chemical and mixed types, were found on the ventral dendrite (VD) of goldfish Mauthner neuron (MN).They are characterized by the presence of several mitochondria in the bouton and by an obligatory desmosome-like contact (DLC) besides the active zone (AZ). Their AZs were commonly found to make contact with the unlabeled chemical crested synapses, which, in turn, directly interacted with VD. These synapses were practically devoid of mitochondria and had no DLCs, thus allowing to identify them as the inhibitory ones. This "two-level" organization of excitatory serotoninergic and inhibitory synapses appears to be related to the reciprocal mechanism of the regulation of MN functional activity by visual input.

[Dopamine as a possible substance for oncotherapy and for quantitative evaluation of cytosolic G-actin]

Viability, histology and ultrastructure of normal cells and cells of different degrees of malignancy after interaction with dopamine as well as the ability of these cells and isolated G-actin in model experiments to stain by Falck technique were studied. It is shown that dopamine, virtually having no effect on the viability of the "normal" non-tumorigenic transformed cells, noticeably reduces cell viability of slightly tumorigenic cells, causes a significant reduction in viability of attachable cancerous cells and a very significant decrease in cell viability of cancerous cells growing in suspension. The intensity of fluorescence of the cytosole in cells treated with dopamine, has been very high and varied in different cultures, and that of isolated actin directly depended on its concentration. Common to all cell morphological feature of damage from the action of dopamine and the putative substrate of fluorescence was actodopamine filaments network strands (identified on the structure and size), which appears in the cytosole loci, where they were absent in control. The data show that dopamine can be used as an oncotherapeutic remedy and diagnostic tool interacting with G-actin as a cellular target.

[Morphological changes of THP-1 tumor cells exposed to dopamine in vitro]

The effect of dopamine (DA) on the viability and morphology of cultured tumor THP-1 cells (human acute monocytic leukemia) was studied. DA in concentration of 10(-5) M had virtually no effect on the culture, while in concentration of 10(-4) M to 10(-3) M it stopped the growth and caused a sharp increase in cell death after 24 and 48 hours. Incubation with DA reduced the cell diameter, progressively increased their vacuolization and intensity of fluorescence after treatment by Falck method. Electron microscopical study has shown that cells exposed for 1 day to DA in the concentrations starting with 10(-4) M, demonstrated smoothing of their surface with the disappearance of microvilli and clasmatosis vesicles, actin filaments perforating the plasma membrane, the emergence of an increasingly dense network of filaments in the cytosole and karyoplasm and, finally, apoptotic cell death. It is suggested that the oncotherapeutic cellular target for DA is a cytosolic G-actin, which at a certain DA concentration, turns into filaments that damage the cells, break the cell cycle and cause cell death.

[Morphological bases of dopamine effect on HEP-2 tumor cells viability]

The purpose of the present investigation was to study the morpho-functional organization of a classical object of cytological research - cultured HEp-2 tumor cells, using dopamine as a penetrating agent, inducing the polymerization of cytosolic actin. It was demonstrated that dopamine introduced into the incubation medium reduced viability and caused morphological disturbances of cultured HEp-2 cells; these effects were proportional to dopamine concentrations (1.0 x 10(-4) M to 1.0 x 10(-3) M) and exposure duration (2 to 3 days). These cells, according to ultrastructural data, underwent fusion and lysis because of the appearance of actin filaments network in the loci of globular actin prevalence in control cells. Dopamine receptors had no effect on cytotoxic effect of dopamine. This was indicated by fluorescent microscopical evidence of dopamine penetration into experimental cells in the presence of haloperidol, as well as destruction of HEp-2 cells under the action of pyrimidinethione, similar to dopamine by characteristics, but lacking its own receptors. It is suggested that cytoplasmic target for dopamine is globular actin and that induced polymerization of this cytoskeletal protein caused injury to tumour cells.

[Cytoskeletal regulation of the cellular function by dopamine]

The interaction of dopamine with model membranes, isolated G-actin, and living cells, such as Mauthner neurons and fibroblast-like BHK-21 cells has been studied. It was found that in vitro dopamine passes through the phospholipid membrane and directly polymerizes G-actin due to incorporation into threads as their integral part. In in vivo conditions, it penetrates inside the cell and induces the appearance of a network of actin filaments in loci rich in globular actin. The data suggest that there exists a mechanism of dopamine interaction with living cells, which is based on direct polymerization of cytosolic G-actin as its cellular target. The reorganization of the actin cytoskeleton leads to changes in the morphofunctional status of cells.

[Localization in a cell of a protein forming the ATP-dependent potassium-selective channels in the bilayer lipid membrane. An ultrastructural study]

The localization in the cell of the protein forming the ATP-dependent potassium-selective channels in the bilayer lipid membrane has been studied. The electron microscope investigation of rat liver and heart tissue sections after their incubation with Abs against this protein and the visualization of the protein with secondary Abs conjugated with colloid gold were carried out. Colloid gold particles were observed both in mitochondrial membranes and in membranes of endoplasmic and sarcoplasmic reticulum. In heart mitochondria, these particles were significantly greater than in liver mitochondria. The localization of the channel protein both in mitochondria and reticulum, as well as the structural similarity between the mitochondrial channel and the precursor of calreticulin suggest that the channel protein belongs to the family of calreticulins. The possible function of the protein as a channel subunit of the mitochondrial ATP-dependent potassium channel is discussed.

[Cytochemical and ultrastructural characteristic of BHK-21 cells exposed to dopamine]

BHK-21 cells were incubated in a medium containing dopamine (DA) and then their catecholamine content evaluated by using the Falck cytochemical method. The significant intensification of cell fluorescence as compared to that one in control preparations was detected; this effect was proportional to DA concentration and exposure duration and was more pronounced in cells in suspension than in those attached to the substrate. Simultaneous ultrastructural investigation has shown that an increased intensity of the cytoplasm fluorescence correlated with the appearance of the dense network of fibrils that were morphologically identified as F-actin microfilaments. Prior blockade of dopaminergic receptors by haloperidol did not change the following DA effect both on the fluorescence intensity and cell ultrastructure. The data obtained suggest that DA chronically acting on the living cells was able to penetrate into the cytoplasm, causing actin polymerization and incorporating into the newly formed actin cytoskeleton. Structurally, this may be manifested by cytoskeleton and its derivative hypertrophy, that could have a substantial effect on general morphology of the cell.

[The influence of dopamine on the ultrastructure of BHK-21 cells]

The influence of dopamine on the haloperidol of BHK-21 cells being in suspension or attached to substrate was investigated. It was shown that the ultrastructural changes affected mainly the cellular loci enriched by the cytoskeleton actin such as intercellular desmosome-like contacts, microvilli and cortical layer or mesh just beneath the plasmatic membrane. The desmosome-like contacts were hypertrophied, their electron density was increased and fibrilar bridges appeared in specialized contacts. Many microvilli fused with each other and with plasma membrane of the neighboring cells, or, on the contrary, split up. Frequently, the membrane surface between microvilli and particularly their apical parts was seen to be pierced by thin thread, morphologically similar to actin filaments. The cytoplasmatic matrix onto ultrathin sections had blotched appearance and at the ultrastructural level was represented by numerous randomly oriented actin filaments. The effect of dopamine was more pronounced in the BHK-21 cells when being in suspension than in attached to the substrate ones, which presumably occurred due to known lesser differentiation of the cytoskeleton in the formers. Finally, it was established that the preliminary blockade of cellular D2 receptors with haloperidol neither affected the ultrastructure of BHK-21 cells nor prevented the following effect of dopamine. The data obtained suggest the direct interactions of dopamine with the actin cytoskeleton.

Changes in the ultrastructure and function of goldfish Mauthner neurons in the presence of 3,4-dihydro-2(1H)-pyrimidinethione

The behavioral effects of 3,4-dihydro-2(1H)-pyrimidinethione (DPT, a pyrimidine derivative), which is used as a test system for detecting tumor growth, on the ultrastructure and function of Mauthner neurons (MN), were studied in goldfish. Application of DPT to MN was found to lead to increased resistance of neurons to exhaustive stimulation, which was accompanied by increases in the sizes of actin-containing membrane desmosome-like contacts, along with the formation of bundles of actin stress fibers; these effects are similar to those previously reported with dopamine. The similarity of the morphofunctional changes in MN on exposure to an artificial chemical substance for which there are no membrane receptors and dopamine itself suggests that they have trophic effects on the stabilization and polymerization of cytoskeletal actin due to direct penetration into postsynaptic neurons.

[Interaction of dopamine with artificial phospholipid membranes]

The interaction of dopamine (DA) with phospholipid membranes has been investigated. The membrane current in planar bilipid membrane (BLM) modified by amphotericin B in voltage clamp conditions under alternating polarity was shown to symmetrically increase 1.2 times when DA was added outside the BLM. This implies a uniform change of charge on each membrane surface and hence the diffusion of DA within the BLM and its exposure on the internal side. The appearance of single threads and bundles of filaments within the internal liposomal cavities was observed in the ultrastructure of suspended thin-walled liposomes filled with globular actin after the introduction of DA into external solution. This reshaped liposomes into rod-like, spindle-shaped or angular structures. Actin serves as a marker for DA due to its property to polymerize itself under the influence of DA. Thus, the structural reorganization of liposomes manifests the presence of DA inside them and the induction of actin polymerization.

[Changes of the goldfish Mauthner neuron ultrastructure and function under the influence of 3,4-dihydro-2(1H)-pyrimidinethione]

The effects of pyrimidine derivative 3,4-dihydro-2(1H)-pyrimidinethione, (DPT) used as a test-system for detection of tumor growth, on the goldfish Mauthner neurons (MN) ultrastructure and function, as manifested in behavioral changes, were studied. The results of investigations demonstrated that an application of DPT on MN had the effects similar to those of dopamine application, as established earlier, causing the enhancement of MN resistance to fatigue stimulation, accompanied by an increase of the dimensions of the actin containing desmosome-like afferent admembranous synaptic contacts, and formation of the cytoplasmic bundles of actin stress-fibers. Similarity of morpho-functional changes of MN, induced by DPT, an artificial chemical substance, which has no receptors on the neuronal membrane, and by natural neurotransmitter dopamine, allows us to suggest possible trophic stabilizing and polymerizing effects of both substances on cytoskeletal actin due to their direct penetration into postsynaptic neuron.

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.

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.

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.

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.

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.

[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.

[Ultrastructural mechanism of the long-term potentiation of synaptic transmission]

A review. The data concerning the structural changes that accompany long-term potentiation (LTP) of synaptic transmission are analyzed. A bulk of morphological studies is aimed at searching for quantitative and qualitative structural LTP signs and elucidating the involvement of cytoskeleton in their formation. The role of cytoskeletal protein actin in synaptic structural and functional modification is discussed. On the basis of experimental evidence obtained by the authors a proposal is made that actin is involved into the LTP not only as a contractile protein but as a cable which strengthen the electrotonic properties of the synapses.

[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.

[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.

[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.

[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.

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.

[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.

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.

[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.

[Long-term morphofunctional preservation of guinea pig hippocampal slices following short-term treatment with cyclooxygenase inhibitors]

Hippocampal slices treated with cyclooxygenase inhibitor indomethacin for three min during the sectioning (45 min) or aspirin (0.5 min) in long term (up to 5 days) preservation in periodic nocturnal hypothermia were studied morphofunctionally using light microscopy and electrophysiological registration of induced population responses of area CAI to stimulation of Schaffers collaterals. Structural disorders were revealed in control slices as early as the third hour of incubation and they were destroyed following the first hypothermal challenge (24 hrs following preparation). The structure in slices treated with blockers remained more stable as compared to control ones and the activity was registered until d 3 (aspirin) and 5 (indomethacin). Morphological changes were not immediately, followed by electric activity decline. On the whole it may be suggested that essential viability increase occurred due to destructive processes inhibition mediated by short living cyclooxygenase metabolites.

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.

[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.

[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.

[An ultrastructural study of the hippocampal neurons in mice subjected to heat exposure]

The ultrastructure of hippocampal neurons in mice subjected to heat treatment for 40 min at 42 degrees C was studied. Such a treatment has been shown to result in significant changes in the structure of pyramidal neurons. An increased chromatin condensation in the nuclei was found in addition to deep invagination of their envelopes and reduction or fragmentation of the reticulum cisterns. These changes were accompanied by mitochondrial swelling and increased number of clathrine vesicles around the Golgi complex. The heat exposure followed by feeding on vitamins and beta-carotin appeared to diminish chromatin condensation and kept the cisterns invariable.

[Actin study of the synapses of surviving hippocampal slices during long-term potentiation]

A study was made of the synaptic actin ultrastructural localization in the hippocampal slices at long-lasting potentiation of area CA, using myosin subfragment-1 labeling. A specific qualitative ultrastructural sign of the potentiated hippocampal synapses was revealed for the first time - the formation in spines of rodlike bundles of actin filaments resembling the cilia. They penetrate the spine stalks to pass through the spine core towards the postsynaptic densities of active zones. The thinner bridges link the filament bundles with the actin cytoskeleton meshwork, with spine apparatus cisterns and with postsynaptic membranes of the active zones. Besides the increasing density of the presynaptic actin meshwork was shown. The changes in the actin cytoskeleton being taken into consideration, its contractile properties account for some morphofunctional features of the potentiated synapses known before and predict previously unknown ones.

[Detection of giant axospinal synapses in the hippocampus on semithin sections]

A histological study of plastic semithin hippocampal sections after the treatment of myosin with tannic acid and detergent and then with subfragment-1 has shown the neuronal dendrites to be covered with dark spine-like thickenings. Electron microscopic analysis of the same preparations has revealed that the electron dense discrete formations represent synaptic terminals, situated on the dendrites with a definite periodicity. We discuss the use of such a method for the quantitative calculation of synapses in nerve tissue at the combined histological and electron microscopic levels of investigation.

[Cytochemical detection of actin in the structure of the synaptic apparatus of hippocampal field CA3]

Ultrastructural distribution of actin in dendrites, dendritic spines and presynaptic boutons of the hippocampal area CA3 of the guinea pig was investigated using decoration and immunocytochemical methods. The distribution of actin was non-homogeneous in all the parts of neurons. The highest concentration of this contractile protein was revealed in the spine cytoplasm. Here actin forms a dense cytoskeleton meshwork and is present also in postsynaptic densities. An intimate interaction between the spine actin cytoskeleton and the postsynaptic actin densities has been revealed. This feature may indicate the involvement of actin cytoskeleton in the organization and maintenance of dimensions, location and geometry of active zones.