[Osphradial chemosensory organ as a probable trigger of the cardiac system adaptive reaction to the effect of heavy metals in aquatic mollusks].

The responses of osphradium in the fresh-water mollusk Viviparus sp. and single osphradial neurons in the pond snail Lymnaea stagnalis to L-glutamine and L-asparagine as well as the changes in these responses under the effect of heavy metals (Cu, Cd) were recorded electrophysiologically. The functional connections of osphradium with the identified neurons of the central pattern generator of respiratory movements and cardiac activity as well as the modification of these connections under the effect of short-term application of HgCl2 solution on the snail's osphradium were investigated. The cardiac rhythm in the mollusk Littorina littorea under the effect of Cu ions was registered non-invasively in long-lasting experiments. The dose-dependent short-term effects of heavy metals changes after osphradium injury were revealed. The implication of osphradium in adaptive reactions of the cardiac system in aquatic mollusks to the environmental heavy metal pollution is suggested. The dependence of cardiac rhythm on the degree of accumulation of copper ions in the mollusk tissues was detected. The results obtained are essential for unraveling neural mechanisms and pathways allowing heavy metals to affect the functional state of hydrobionts, particularly, the cardiac activity frequency characteristics of which are widely used as informative biomarkers to assess physiological condition of aquatic invertebrates.

[Probable structural functional evolution of sensory surface of osphradia of aquatic prosobranchian molluscs].

At present, in ecotoxicological studies, as biomarkers there are used physiological reactions of invertebrates, based on diverse reflex. The primary chain of the reflex is chemo-, mechano-, and osmoreceptors. The structures are exposed on the surface of body and mantle cavity. Earlier, a hypothesis was put forward, which suggested that the polymodal osphradial organ of the pond snail might participate in adaptive reactions of aquatic molluscs to toxicants. The known homology of osphradial structures allows spreading this suggestion on marine representatives of various subclasses of Mollusca, although diversity of structure, of ways of nutrition, and multiplicity of aquatic molluscs can impede interpretation of future ecotoxicological studies. To elucidate this issue, we carried out the comparative electron microscopy study of osphradial organs in representatives of various families of Prosobranchia (Mollusca, Gastropoda). By ultrastructural parameters in the osphradial organs, five sensory cellular complexes (SCC) have been revealed. A probable connection is demonstrated of these cellular complexes with the known chemo-, mechano-, and osmoreceptor modalities. Structure of the complexes become more complicated in the process of evolution of gastropods and depends on the way of nutrition of molluscs. Thus, the primitively built osphradium of the herbivorous mollusc Viviparus sp. is a polymodal receptor and initial prototype for further morphophysiological constructions. Osphradium of littorine is the next chain of evolitionary transformations and combines in itself the osmo- and chemosensory SCC. In osphradia of the new, ctenidial type the total receptor surface increases and the ultrastructural specialization of cellular complexes occurs, which promotes the appearance in them of the sarcophagous way of nutrition. For predator marine molluscs actively searching for their preys by odor, there is identified an additional superficial sensory cellular complex. It is located on lateral surfaces and on ventral edge of petals of osphradia, near zones of cilium supportive cells. These seem to be mechanoreceptor structures tracing direction and rate of flow of liquid along osphradium. A connection of SCC with the certain modality of outer nutrition and with the way of nutrition is suggested.

[Cytoplasmic syncytial interneuronal connection in molluscs].

The absolute criteria developed by the authors have been presented; they allow revealing cytoplasmic syncytial connections between processes of nerve cells in vivo and in vitro at the light microscopy level by using classical methods and time lapse videoshooting in the phase contrast. With aid of electron microscopy, metastable membrane contacts and their perforations, cytoplasmic syncytial interneuronal pores, and fusion of nerve processes are demonstrated. In the culture of isolated molluscan neurons, the process of formation of syncytial connection between processes of the same neuron or of different neurons is reproduced. Processes of one neuron, which have syncytial connection with another neuron, are shown to remain viable after death of its neuronal soma. The cytoplasmic varicosities formed on processes of one neuron are able to overcome the place of syncytial contact with processes of another neuron and to move to the body of the latter. A hypothesis is put forward that the cytoplasmic syncytial connection between nerve processes is formed under the condition of the absence of their glial sheaths.

Studies of chemoreceptor perception in mollusks.

The formation and operation of the peripheral nervous system can be observed and studied in mollusks using as an example the primitive chemoreceptor organ, the osphradium, which is connected to the visceral arch of the CNS and analyzes the physicochemical properties of water in the mantle cavity. Nerve recordings showed that the osphradium is sensitive to excess osmotic pressure, sodium chloride, and amino acids. In addition, the osphradium responds to the quality of the water in which the animal is living. The osphradium of the pond snail retains its ancient multisensory function, uniting the perception of various chemical and physical stimuli. Patch clamp recordings at fixed potential or current were used to study membrane currents in identified ganglion and receptor cells, associated with increases in the concentrations of Na+ and L-aspartate in the solution bathing the osphradium. The influx current appears to be a sodium and/or calcium current, and is not blocked by tetraethylammonium, while the efflux current is a potassium current, as has been shown for the taste cells of vertebrates.

Influence of HgCl2 on the osphradial multisensory system of Lymnaea stagnalis L.

The osphradial multisensory system of Lymnaea stagnalis L. (Pulmonata, Basommatophora) was used to demonstrate the modulation of chemosensory information both at periphery and central nervous system (CNS) following heavy metal treatments. A semi-intact preparation including osphradium, CNS and the right inner parietal nerve (r.i.p.n.) connecting them was used to record both extracellular activity of nerve and intracellular activity of central neurons receiving information from osphradium. The ion currents of osphradium were recorded using patch-clamp method. The changes in nerve and neuronal activity were expressed by averaging of firing frequency and interspike intervals. The chemosensory function of osphradium was shown by application of L-aspartate, urea, saccharose and stagnant water to its surface. The central neurons reacting to the stimulation ofosphradium were located to visceral, right parietal, pedal and cerebral ganglia of Lymnaea. Both the acute and chronic treatments with HgCl2 damaged the sensory function of osphradium traced on the flow of information from periphery to central neurons. At the same time, mercury chloride modified the synaptic connections of respiratory pattern generators as well as the Ca- and K-dependent ion currents of osphradial neurons. The results proved the multisensory role of osphradium sensing the alterations in the environment and its usefulness in monitoring the effects of pollutants at various level of regulation from chemosensory epithelium to CNS.

[Study of the chemoreception in mollusca]. / Issledovanie khemoretseptornogo vospriiatiia u molliuskov.

Recording of the mollusc osphardium nerve activity revealed the osphardium sensitivity to hyperosmotic pressure, sodium chloride, and aminoacidosis. The osphardium was found to detect quality of the water where the animals were kept. Pont snail osphradium preserves ancestors' multisensority which unites perception of different chemical and physical signals. Patch-clamp method revealed membrane currents in certain ganglia and receptor cells which are sensitive to Na+ and L-aspartate increase in the solution around the osphardium. Inward components of these currents are, probably, of the sodium and/or calcium nature, whereas the outward components--of the potassium one.

The electrical responses of osphradial nerve and central neurons to chemicals stimulation of Lymnaea osphradium.

The osphradial sensory system of the mollusc Lymnaea stagnalis was studied by electrophysiological methods. Bioelectrical activity of the osphradial nerve appeared to be dependent on concentrations of D2, mannit, urea and NaC1 in surrounding media. Studies on impulse activities of axons of the osphradial nerve disclose diverse units. Each of them has its own range of response, often overlapping with the others. Several large neurons and cell clusters, answering to the adequate osphradial stimulation, were identified in the viscero-parietal complex. This response is expressed as excitation, inhibition and redistribution of impulse patterns. The participation of the osphradial sensory system in the regulation of vegetative functions of Lymnaea is discussed.

[Structure and synaptic organization of the osphradium of the lamellibranch mollusk Unio pectorum]. / Stroenie i sinapticheskaia organizatsiia osfradiia plastinchatozhabernogo molliuska Unio pectorum.

The osphradial organ has been studied in Lamellibranchia--Unio pectorum--by means of scanning and transmissive electron microscopy. On the surface of the distal part of this hemosensory organ there is a distinct division into zones. The central part of the osphradial torus is occupied by the receptory zone, formed predominantly by supporting cells with microvilli and by peripheral processes of the subepithelial receptory cells. The lateral surfaces are occupied with ciliar areas of the ciliar supporting cells. In the receptory zone two types of the peripheral processes of the receptory cells are identified; they differ by the number of kinocilia and by ultrastructural organization of the apical part. Axon-like processes of the receptory cells interact with axons and dendrites of the ganglionic cells, forming axo-axonal, axodendritic and axosomatic synapses. The facts revealed demonstrate a high level of specialization of the osphradial receptory surface, connected with polymodality of this organ.

[Ultrastructure of the osphradium of Viviparus sp. (Mollusca, Prosobranchia)]. / Ul'trastruktura osfradiia zhivorodki Viviparus sp. (Mollusca, Prosobranchia).

By means of scanning and transmissive electron microscopy, osphradial structure has been studied in the freshwater herbivorous mollusc Viviparus sp. Ciliary supporting and microvillous cells are described; they do not form any specific for the osphradium zones on its surface. The number of the villi, belonging to one supporting cell, is within 20-40. Peripheral processes of the receptory subepithelial cells have microvilli and 1-2 kinocilia on their apical surface. Central processes of the primary sensitive receptor cells go into the neuropil of the osphradial ganglion, that is situated in the center of the osphradial torus. Among uni- and multipolar ganglionic cells typical neurosecretory cells with electron opaque granules 200 nm in diameter are revealed. Release of neurosecretion takes place in the neuropil and in the connective tissue of the osphradial ganglion. A suggestion is made on modulating role of neuropeptides in the osphradial ganglion, that enables to change sensitivity of this receptor organ depending on physiological state of the animal and on the environment.

[Ultrastructure of the osphradium of Siphonaria grisea (Mollusca, Pulmonata)]. / Ul'trastruktura osfradiia Siphonaria grisea (Mollusca, Pulmonata).

By means of scanning and transmissive electron microscopic methods osphradium of Siphonaria grisea has been studied. The osphradium of the animal is presented as a small torulus formed by supporting ciliated cells. Among them bodies of receptory cells are situated; they are of smaller size and decorated with a bundle of microvilli. Central processes of the receptory cells, penetrating through a thick layer of the connective tissue, reach the subepithelial neural trunk. Concentration of the neural cells in the periphery of the trunk is small, and in the central area no chemical synapses are revealed. The data presented demonstrate a primitive structure of the osphradial chemoreceptory organ in Siphonaria.

[Ultrastructure of the surface of the osphradium of the anterior gilled mollusk Murex saxatilis L]. / Ul'trastruktura poverkhnosti osfradiia perednezhabernogo molliuska Murex saxatilis L.

The Murex saxatilis L. (Mollusca, Prosobranchia) osphradium is of the most complex "ctenidial" type and is situated immediately behind the siphon that brings water to the mantel cavity. By means of scanning and transmissive electron microscopical investigation, three zones have been revealed on the surface of the osphradium petals: secretory, intermediate and receptory. The secretory zone occupies the lateral part of the petal and is formed with 1-2 layers of cells. The intermediate zone, as a narrow stria, is situated between the secretory and the receptory zones. Here, together with the secretory cells, the ciliary cells are present. The receptory zone occupies nearly the whole surface of the petal. It is much thicker and is formed of pseudostratified epithelium where 5-7 raws of nuclei can be counted. Besides the cell types mentioned, there are defined bipolar receptory cells in it. The apical surface of the receptory cells has up to 200 cilia, and fine peripheral processes--1-2 cilia. Presence of the complex receptory zone makes it possible to suggest certain differentiation of the stimuli already at the peripheral level of the sensory system.

[Structure and cellular organization of the osphradium of Limnea stagnalis L]. / Stroenie i kletochnaia organizatsiia osfradiuma Limnea stagnalis L

Light microscopy was used to study the structure and cellular organization of the osphradial organ of the pulmonary mollusque L. stagnalis. The osphradium unites the epithelial canal and the ganglion consisting of two cell populations. On the internal surface of the V-shaped osphradial canal there are three zones of cells: secretory, villous and epithelial. The villous zone of the canal is related with sensory bipolar and multipolar neurons of the ganglion. The irritation percepted by these cells seems to be transferred through numerous zones of neuropile to large unipolar neurons of the ganglion cortical layer.