Formation, maintenance, and functional uncoupling of connections between identified Helisoma neurons in situ

Previous work with identified Helisoma neurons has characterized an array of neuroplastic responses to axotomy that include the generation of new neuritic outgrowth, the reinnervation of target organs, and the formation of new electrical synapses. These responses are not random, but rather occur in a precise, predictable manner under a variety of culture conditions. The present investigation demonstrates that specific identified neurons display similar neuroplastic "behavior" within the living animal. In response to in situ nerve crushes, neurons B4 and B5 generate new neuritic outgrowth, neuron B4 functionally reinnervates the salivary glands, and new electrical synapses form between the left and right neurons B5. The in situ paradigm employed in the present experiments made it possible to examine responses to axotomy over longer periods than in earlier studies with organ cultures. New B5R-B5L connections, previously found to be stable over the short term in culture, gradually decreased in strength in situ, and the cells effectively uncoupled by 8 weeks after axotomy. This uncoupling did not depend upon target reinnervation and occurred in the continued presence of neurites in the buccal commissure. It is suggested that the stability of new connections is related to whether the connection previously existed in the unperturbed nervous system. The similarities between the ability of identified neurons to grow and to form synaptic connections in situ and in culture suggests that neurons are endowed with a specific program of regenerative responses that can be expressed reliably in a wide variety of environmental conditions.     

Frequency-dependent and cell-specific effects of electrical activity on growth cone movements of cultured Helisoma neurons

The present experiments address the question of how stimulation parameters, which evoke action potentials in neuronal cell bodies, influence growth cone movements of different identified neurons. The motility of growth cones of Helisoma buccal neurons B19 and B4 was monitored while somata were stimulated simultaneously via an intracellular microelectrode. The findings show that the responses of growth cones of B19 and B4 contain components that are common as well as unique to each neuron. Whereas rates of growth cone advance were suppressed in a graded fashion by stimulus frequencies beyond a threshold of 2 s-1 for both neurons, B4 was more sensitive to electrical stimulation and exhibited a new response, namely, growth rates were enhanced during the poststimulation recovery period after stimulation at specific frequencies. Thus, electrical activity can result in enhancement as well as in inhibition of growth cone movement. Changes in number of filopodia on B19 and B4 were graded also, with B4 again displaying greater sensitivity. The frequency dependence of filopodia compared to growth rate changes was different and suggests a possible dissociation between filopodial activity and growth cone motility. Patterned electrical activity produced effects similar to constant stimulation for B19 growth cones, whereas it decreased the threshold frequency and eliminated the growth enhancement effect for B4. Taken together, these data demonstrate that the quantitative features of electrical activity as well as intrinsic properties of neurons both determine the growth cone response to changes in neuronal activity.     

Selective fasciculation as a mechanism for the formation of specific chemical connections between Aplysia neurons in vitro

Selective fasciculation of growth cones along preestablished axon pathways expressing matching or complementary adhesion molecules is thought to be an important strategy in axon guidance. Growth cone inhibiting factors also appear to influence pathfinding decisions. We have used identified Aplysia neurons in vitro to explore the hypothesis that similar mechanisms could be involved in target selection. Co-cultures of L10 neurons with RB neuron targets or R2 neurons with RUQ neuron targets reliably formed chemical connections. In contrast, co-cultures of L10 with RUQ targets usually failed to form detectable chemical connections unless cell–cell contact was forced during plating by intertwining the major axons. These data suggested that differences in the ability to form cell–cell contacts might underlie the observed synaptic specificity. This notion was supported when fluorescent dye fills of L10 and R2 revealed a positive correlation between the amount of target contact and the frequency of synapse formation: L10–RUQ cultures showed much less target contact than L10–RB or R2–RUQ cultures. To examine the cellular mechanisms of these differences in target contact, presynaptic growth cones were observed as they interacted with target processes. L10–RUQ cultures showed much less fasciculation and more avoidance behavior compared to L10–RB and R2–RUQ cultures. This initial specificity suggested that the differences in amount of target contact arose through selective fasciculation and avoidance rather than through selective elimination after indiscriminate fasciculation. Selective fasciculation and avoidance might, therefore, aid in target selection by regulating the amount of contact between presynaptic processes and potential target cells. © 1993 John Wiley & Sons, Inc.     

The Extended Branch-Arrow Model of the formation of retino-tectal connections

This paper presents XBAM (the Extended Branch-Arrow Model), a new model of the development of the retino-tectal topographic mapping as observed in frog, toad, and goldfish visual systems. The updating process employed by XBAM is distributed in nature and depends upon interactions between branches of retinal fibers, the branches and the boundaries of the tectum and grafts, and the branches and the tectal surface. Results of computer simulation of the model are related to experimental data obtained from tectal and retinal graft and lesion studies, and comparisons are also made with other models.     

The cellular mechanisms and regulation of metastasis formation

Data accumulating in biochemistry, molecular and cell biology, and experimental oncology indicate that metastasis, which is the formation of secondary tumor growth foci during cancer progression, is a highly determinate and regulated process. The process includes the emergence of a metastatic cell population possessing special properties to allow cell dissemination and seeding in distant organs and the formation of a specific microenvironment in target organs. Among other changes, metastatic cells display plasticity, being capable of switching their motility to the most efficient mode depending on the properties of surrounding tissues; start expressing specific surface receptors to ensure their migration to target organs; and acquire certain features of stem cells, being capable of surviving and reproducing in an alien microenvironment. A specific niche, known as the premetastatic niche, develops in the target organ in a strong coordination with the above changes to stimulate the initiation and growth of a prospective metastasis. The review considers the recent findings related to the mechanisms that regulate the emergence of the metastatic cell population, the formation of premetastatic niches, and the coordination of the two processes.     

细菌生物膜的形成与调控机制

细菌通过自身合成的水合多聚物粘附在固体表面,以固着的方式生长从而形成生物膜,细菌生物膜的形成涉及到几个明显的阶段,包括起始的附着、细胞与细胞之间的吸附与增殖、生物膜的成熟、及最后细菌的脱离等四个阶段,生物膜的形成增加了细菌对抗生素的抗性以及帮助细菌逃逸寄主的免疫攻击等,从而引起临床上持续性的慢性感染等各种问题;生物膜结构非常复杂,除了细菌分泌的各种胞外多糖,胞外蛋白质外,最新的研究表明,DNA也是生物膜的一个重要成分.针对近年来的最新文献报道分别对生物膜的形成、结构以及调控机制等进行综述.     

Involvement of actin and Na+-K+ ATPase in urine formation of the freshwater pulmonate Helisoma

The site and process of urine formation in the renopericardial system of Helisoma have been investigated. Osmotic pressure and protein content of hemolymph from the heart, pericardial fluid from the pericardial cavity, prourine from the kidney sac, and urine from the ureter have been determined. Osmotic pressure is equal in hemolymph, pericardial fluid, and prourine, but less in urine. Protein content is similar in hemolymph and pericardial fluid, but much less in prourine and urine. Hemoglobin molecules are present in hemolymph and pericardial fluid but not in prourine. It is suggested that in Helisoma the kidney sac is the site of prourine formation, and prourine is an ultrafiltrate of hemolymph. The kidney epithelial cells contain 6- to 7-nm microfilaments which react with heavy meromyosin producing unidirectional arrowheads. Numerous actin filaments are present in the vicinity of the lateral cell membranes and basal processes. It is possible that the actin filaments regulate the extracellular spaces for prourine passage. It is postulated that the actin-rich kidney epithelium may generate hydrostatic pressure for ultrafiltration. Na⁺-K⁺ ATPase is located on the luminal side of the kidney epithelium, which may regulate intracellular fluid level of the kidney epithelial cells, and thereby regulate their cell volume. Thus Na⁺-K⁺ ATPase may be involved in the regulation of extracellular spaces in kidney epithelial cells. The enzyme may participate in the production of hyposmotic urine.     

The mechanisms in NO-dependent regulation of electrical and mechanical activity in smooth muscles

The role of nitric oxide (NO) and its implication in intracellular and intercellular signaling pathways attract an attention of many research teams up to now. Away of its signaling functions. NO is considered as one of the key molecules in maintenance of balance between the physiological and pathological processes due to cytoprotective and cytotoxic functions of this molecule. In this regard, elucidation of the NO-dependent mechanisms, involved into the physiological processes and pathophysiological reactions, remains an urgent problem of conntemporary biology and medicine. Analysis of obtained results establishes a relative contribution of electro- and pharmaco-mechanical coupling mechanisms in NO-dependent regulation of smooth muscle cels (SMC) functions. The authors show that elevation of intracellular Ca2+ concentration by biologically active substances promotes relaxing effect of NO through both voltage-dependent and -independent intracellular mechanisms of calcium redistribution. Namely the peculiarities of considered mechanisms in each certain type of SMCs cause the final direction of alterations in contractility and membrane potential. It has been shown that voltage-dependent effects of NO are mediated by suppression of calcium and/or sodium components and modulation of Ca2+ -dependent and ATP-seisitive potassium components of SMC membrane permeability, Voltage-independent NO control of mechanical smooth muscles activity mainly is mediated by 1) modulation of protein kinase C (PK-C) branch of calcium signaling system, 2) ratio of cyclic nucleotides intracellular concentrations (cGMP/cAMP), and 3) directional mode of electrosilent Na+, K+, 2Cl- -cotransport. Our results show that the features of the myogenic effects of NO are caused by the peculiarities of PK-C operation in SMC.     

Role of chemical signalling in release of motor programs during embryogenesis of freshwater snails Lymnaea stagnalis and Helisoma trivolvis

We have earlier found that freshwater pond snails Helisoma trivolvis and Lymnaea stagnalis, when reared under conditions of starvation, release chemical signals that reversibly suppress larval development of conspecific embryos. Here, we report that (i) these signals are not strictly conspecific and affect also embryos of a closely related species, which occupies a similar environmental niche; (ii) besides the development of embryos, the signals also affect the release of main motor programs, such as locomotion, feeding, and cardiac activity; (iii) action of the signals is bidirectional: they retard the development and release of motor programs at the early larval stages (trochophore to veliger) and accelerate them at later stages (late veliger to hatching). A possible adaptive significance of the described phenomena is discussed.     

Metallic taste from electrical and chemical stimulation

A series of three experiments investigated the nature of metallictaste reports after stimulation with solutions of metal saltsand after stimulation with metals and electric currents. Tostimulate with electricity, a device was fabricated consistingof a small battery affixed to a plastic handle with the anodeside exposed for placement on the tongue or oral tissues. Intensityof taste from metals and batteries was dependent upon the voltageand was more robust in areas dense in fungiform papillae. Metallictaste was reported from stimulation with ferrous sulfate solutions,from metals and from electric stimuli. However, reports of metallictaste were more frequent when the word ‘metallic’was presented embedded in a list of choices, as opposed to simplefree-choice labeling. Intensity decreased for ferrous sulfatewhen the nose was occluded, consistent with a decrease in retronasalsmell, as previously reported. Intensity of taste evoked bycopper metal, bimetallic stimuli (zinc/copper) or small batteries(1.5–3 V) was not affected by nasal occlusion. This differencesuggests two distinct mechanisms for evocation of metallic tastereports, one dependent upon retronasal smell and a second mediatedby oral chemoreceptors.