Functional and motor neuronal analysis of defensive siphon responses in {\it Aplysia californica}

One of the central goals of neuroscience research is to determine how networks of neurons control and modify behavior. One of the most influential model systems for this kind of analysis is the siphon and gill withdrawal reflex of the marine mollusc A. californica. In response to tactile stimulation, the siphon displays 3 different responses: (1) a posterior pointing and leveling (flaring) of the siphon in response to tail stimulation (the siphon T response), (2) constriction and anterior pointing to head stimulation (the siphon H response) and (3) constriction and withdrawal between the animal's parapodia (the siphon S response). The siphon S response is pseudoconditioned by a noxious tail stimulus to resemble the siphon T response. Behavioral and combined behavioral/intracellular studies were conducted to determine the motor neuronal control of these behaviors and to search for mechanisms of siphon response transformation following pseudoconditioning. The present studies have found that the flaring component of pseudoconditioned siphon S responses occurs during mantle pumping (MP) triggered by noxious tail stimulation. Siphon stimulation also triggers MP, as recorded in neurons of the Interneuron II pattern generator which commands MP. The 4 LF$\rm\sb{SB}$ siphon motor neurons (SMNs) were found necessary and sufficient for the siphon T response, while SMNs RD$\rm\sb S$ and LD$\rm\sb{S1}$ were found necessary and sufficient for the siphon H response. Following pseudoconditioning, there is an increase in the number of evoked spikes to the test stimulus for the LF$\rm\sb{SB}$ cells and a decreased number for RD$\rm\sb S.$ Siphon flaring occurring during the pseudoconditioned response correlates with increased LF$\rm\sb{SB}$ activity during triggered MP cycles. This suggests that psuedoconditioning is in part due to reconfiguration of the motor outputs of the Interneuron II network. These results suggest that these defensive responses are controlled and patterned by a well-defined, finite set of motor neurons and interneurons (Interneuron II) that are dedicated to specific behavioral functions, but also have parallel distributed properties. ^

Anatomical organization of the memory underlying sensitization in the siphon-withdrawal reflex circuit of {\it Aplysia californica}

Previous studies have shown that short-term sensitization of the Aplysia siphon-withdrawal reflex circuit results in multiple sites of change in synaptic efficacy. In this dissertation I have used a realistic modeling approach (using an integrate-and-fire scheme), in conjunction with electrophysiological experiments, to evaluate the contribution of each site of plasticity to the sensitized response.^ This dissertation contains a detailed description of methodology for the construction of the model circuit, consisting of the LFS motor neurons and ten interneurons known to convey excitatory input to them. The model replicates closely the natural motor neuron firing response to a brief tactile stimulus.^ The various circuit elements have different roles for producing circuit output. For example, the sensory connections onto the motor neuron are important for the production of the phasic response, while the polysynaptic interneuronal connections are important for producing the tonic response.^ The multiple sites of plasticity that produce changes in circuit output also have specialized roles. Presynaptic facilitation of the sensory neuron to LFS connection enhances only the phasic component of the motor neuron firing response. The sensory neuron to interneuron connections primarily enhance the tonic component of the motor neuron firing response. Also, the L29 posttetanic potentiation and the L30 presynaptic inhibition primarily enhance the tonic component of the motor neuron firing response. Finally, the information content at the various sites of plasticity can shift with changes in stimulus intensity. This suggests that while the sites of plasticity encoding memory are fixed, the information content at these sites can be dynamic, shifting in anatomical location with changes in the intensity of the test stimulus.^ These sites of plasticity also produce specific changes in the behavioral response. Sensory-LFS plasticity selectively increases the amplitude of the behavioral response, and has no effect on the duration of the behavioral response. Interneuronal plasticity (L29 and L30) affects both the amplitude and duration of the behavioral response. Other sensory plasticity also affect both the amplitude and duration of the behavioral response, presumably by increasing the recruitment of the interneurons, which provide all of the effect on duration of the behavioral response. ^

The role of sensory neuron outgrowth in long-term sensitization of the tail-elicited tail-siphon withdrawal reflex of Aplysia

Neuronal outgrowth has been proposed in many systems as a mechanism underlying memory storage. For example, sensory neuron outgrowth is widely accepted as an underlying mechanism of long-term sensitization of defensive withdrawal reflexes in Aplysia. The hypothesis is that learning leads to outgrowth and consequently to the formation of new synapses, which in turn strengthen the neural circuit underlying the behavior. However, key experiments to test this hypothesis have never been performed. ^ Four days of sensitization training leads to outgrowth of siphon sensory neurons mediating the siphon-gill withdrawal response in Aplysia . We found that a similar training protocol produced robust outgrowth in tail sensory neurons mediating the tail siphon withdrawal reflex. In contrast, 1 day of training, which effectively induces long-term behavioral sensitization and synaptic facilitation, was not associated with neuronal outgrowth. Further examination of the effect of behavioral training protocols on sensory neuron outgrowth indicated that this structural modification is associated only with the most persistent forms of sensitization, and that the induction of these changes is dependent on the spacing of the training trials over multiple days. Therefore, we suggest that neuronal outgrowth is not a universal mechanism underlying long-term sensitization, but is involved only in the most persistent forms of the memory. ^ Sensory neuron outgrowth presumably contributes to long-term sensitization through formation of new synapses with follower motor neurons, but this hypothesis has never been directly tested. The contribution of outgrowth to long-term sensitization was assessed using confocal microscopy to examine sites of contact between physiologically connected pairs of sensory and motor neurons. Following 4 days of training, the strength of both the behavior and sensorimotor synapse and the number of appositions with follower neurons was enhanced only on the trained side of the animal. In contrast, outgrowth was induced on both sides of the animal, indicating that although sensory neuron outgrowth does appear to contribute to sensitization through the formation of new synapses, outgrowth alone is not sufficient to account for the effects of sensitization. This indicates that key regulatory steps are downstream from outgrowth, possibly in the targeting of new processes and activation of new synapses. ^

Nicotinamide nucleotide and adenylate concentrations in mante, siphon and gill tissue of old and young Laternula elliptica individuals under control and experimental conditions

Future oceans are predicted to contain less oxygen than at present. This is because oxygen is less soluble in warmer water and predicted stratification will reduce mixing. Hypoxia in marine environments is thus likely to become more widespread in marine environments and understanding species-responses is important to predicting future impacts on biodiversity. This study used a tractable model, the Antarctic clam, Laternula elliptica, which can live for 36 years, and has a well-characterized ecology and physiology to understand responses to hypoxia and how the effect varied with age. Younger animals had a higher condition index, higher adenylate energy charge and transcriptional profiling indicated that they were physically active in their response to hypoxia, whereas older animals were more sedentary, with higher levels of oxidative damage and apoptosis in the gills. These effects could be attributed, in part, to age-related tissue scaling; older animals had proportionally less contractile muscle mass and smaller gills and foot compared with younger animals, with consequential effects on the whole-animal physiological response. The data here emphasize the importance of including age effects, as large mature individuals appear to be less able to resist hypoxic conditions and this is the size range that is the major contributor to future generations. Thus, the increased prevalence of hypoxia in future oceans may have marked effects on benthic organisms' abilities to persist and this is especially so for long-lived species when predicting responses to environmental perturbation.

Tratado de las fuentes intermitentes, y de la causa de sus fluxos, y supresiones : de la naturaleza, y uso de el siphon, o cantimplora, y si este tuvo parte en el artificio de los organis hydraulicos, y en algunos oraculos de la gentilidad : Dialogo entre el doctor D. Pedro A. y el Lic. D. Juan B. /

Mode of access: Internet.

Petri Net Siphon Analysis and Network Centrality Measures for Identifying Combination Therapies in Signaling Pathways

Aberrant behavior of biological signaling pathways has been implicated in diseases such as cancers. Therapies have been developed to target proteins in these networks in the hope of curing the illness or bringing about remission. However, identifying targets for drug inhibition that exhibit good therapeutic index has proven to be challenging since signaling pathways have a large number of components and many interconnections such as feedback, crosstalk, and divergence. Unfortunately, some characteristics of these pathways such as redundancy, feedback, and drug resistance reduce the efficacy of single drug target therapy and necessitate the employment of more than one drug to target multiple nodes in the system. However, choosing multiple targets with high therapeutic index poses more challenges since the combinatorial search space could be huge. To cope with the complexity of these systems, computational tools such as ordinary differential equations have been used to successfully model some of these pathways. Regrettably, for building these models, experimentally-measured initial concentrations of the components and rates of reactions are needed which are difficult to obtain, and in very large networks, they may not be available at the moment. Fortunately, there exist other modeling tools, though not as powerful as ordinary differential equations, which do not need the rates and initial conditions to model signaling pathways. Petri net and graph theory are among these tools. In this thesis, we introduce a methodology based on Petri net siphon analysis and graph network centrality measures for identifying prospective targets for single and multiple drug therapies. In this methodology, first, potential targets are identified in the Petri net model of a signaling pathway using siphon analysis. Then, the graph-theoretic centrality measures are employed to prioritize the candidate targets. Also, an algorithm is developed to check whether the candidate targets are able to disable the intended outputs in the graph model of the system or not. We implement structural and dynamical models of ErbB1-Ras-MAPK pathways and use them to assess and evaluate this methodology. The identified drug-targets, single and multiple, correspond to clinically relevant drugs. Overall, the results suggest that this methodology, using siphons and centrality measures, shows promise in identifying and ranking drugs. Since this methodology only uses the structural information of the signaling pathways and does not need initial conditions and dynamical rates, it can be utilized in larger networks.

Redescription of Hiatella meridionalis d'Orbigny, 1846 (Mollusca, Bivalvia, Hiatellidae) from Argentina

The redescription of Hiatella meridionalis (d’Orbigny, 1846) is provided as first attempt to improve the systematics of the genus in the regions of Atlantic and western Pacific. This reanalysis is based on specimens collected in the vicinity of the type localities and is based on detailed morphology of samples that some researches consider a single, wide ranging species. From the morphological characters, the more interesting are: a high quantity of papillae at incurrent siphon; the retractor muscles of siphon divided in two bundles; the small size of the palps; the muscular ring in the stomach; and the zigzag fashion of the short intestinal loops. These characters distinguish the species from the other hiatellids so far examined. Type material of the species was examined, by first time illustrated, and the lectotype is designated.

Caval collapse during cardiopulmonary bypass: a reproducible bench model.

OBJECTIVES: During open heart surgery, so-called atrial chatter, a phenomenon due to right atria and/or caval collapse, is frequently observed. Collapse of the cava axis during cardiopulmonary bypass (CPB) limits venous drainage and may result downstream in reduced pump flow on (lack of volume) and upstream in increased after-load (stagnation), which in turn may both result in reduced or even inadequate end-organ perfusion. The goal of this study was to reproduce venous collapse in the flow bench. METHODS: In accordance with literature for venous anatomy, a caval tree system is designed (polyethylene, thickness 0.061 mm), which receives venous inflow from nine afferent veins. With water as medium and a preload of 4.4 mmHg, the system has an outflow of 4500 ml/min (Scenario A). After the insertion of a percutaneous venous cannula (23-Fr), the venous model is continuously served by the afferent branches in a venous test bench and venous drainage is augmented with a centrifugal pump (Scenario B). RESULTS: With gravity drainage (siphon: A), spontaneously reversible atrial chatter can be generated in reproducible fashion. Slight reduction in the outflow diameter allows for generation of continuous flow. With augmentation (B), irreversible collapse of the artificial vena cava occurs in reproducible fashion at a given pump speed of 2300 ± 50 RPM and a pump inlet pressure of -112 mmHg. Furthermore, bubbles form at the cannula tip despite the fact that the entire system is immersed in water and air from the environment cannot enter the system. This phenomenon is also known as cavitation and should be avoided because of local damage of both formed blood elements and endothelium, as well embolization. CONCLUSIONS: This caval model provides a realistic picture for the limitations of flow due to spontaneously reversible atrial chatter vs irreversible venous collapse for a given negative pressure during CPB. Temporary interruption of negative pressure in the venous line can allow for recovery of venous drainage. This know-how can be used not only for testing different cannula designs, but also for further optimizing perfusion strategies.

Molecular evidence from ascidians for the evolutionary origin of vertebrate cranial sensory placodes

Cranial sensory placodes are specialised areas of the head ectoderm of vertebrate embryos that contribute to the formation of the cranial sense organs and associated ganglia. Placodes are often considered a vertebrate innovation, and their evolution has been hypothesised as one key adaptation underlying the evolution of active predation by primitive vertebrates. Here, we review recent molecular evidence pertinent to understanding the evolutionary origin of placodes. The development of vertebrate placodes is regulated by numerous genes, including members of the Pax, Six, Eya, Fox, Phox, Neurogenin and Pou gene families. In the sea squirt Ciona intestinalis (a basal chordate and close relative of the vertebrates), orthologues of these genes are deployed in the development of the oral and atrial siphons, structures used for filter feeding by the sessile adult. Our interpretation of these findings is that vertebrate placodes and sea squirt siphon primordia have evolved from the same patches of specialised ectoderm present in the common ancestor of the chordates.

Molecular evidence from Ciona intestinalis for the evolutionary origin of vertebrate sensory placodes

Cranial sensory placodes are focused areas of the head ectoderm of vertebrates that contribute to the development of the cranial sense organs and their associated ganglia. Placodes have long been considered a key character of vertebrates, and their evolution is proposed to have been essential for the evolution of an active predatory lifestyle by early vertebrates. Despite their importance for understanding vertebrate origins, the evolutionary origin of placodes has remained obscure. Here, we use a panel of molecular markers from the Six, Eya, Pax, Dach, FoxI, COE and POUIV gene families to examine the tunicate Ciona intestinalis for evidence of structures homologous to vertebrate placodes. Our results identify two domains of Ciona ectoderm that are marked by the genetic cascade that regulates vertebrate placode formation. The first is just anterior to the brain, and we suggest this territory is equivalent to the olfactoty/adenohypophyseal placodes of vertebrates. The second is a bilateral domain adjacent to the posterior brain and includes cells fated to form the atrium and atrial siphon of adult Ciona. We show this bares most similarity to placodes fated to form the vertebrate acoustico-lateralis system. We interpret these data as support for the hypothesis that sensory placodes did not arise de novo in vertebrates, but evolved froth pre-existing specialised areas of ectoderm that contributed to sensory organs in the common ancestor of vertebrate and tunicates. Published by Elsevier Inc.

Objective characterization of the course of the parasellar internal carotid artery using mathematical tools

Background Along the internal carotid artery (ICA), atherosclerotic plaques are often located in its cavernous sinus (parasellar) segments (pICA). Studies indicate that the incidence of pre-atherosclerotic lesions is linked with the complexity of the pICA; however, the pICA shape was never objectively characterized. Our study aims at providing objective mathematical characterizations of the pICA shape. Methods and results Three-dimensional (3D) computer models, reconstructed from contrast enhanced computed tomography (CT) data of 30 randomly selected patients (60 pICAs) were analyzed with modern visualization software and new mathematical algorithms. As objective measures for the pICA shape complexity, we provide calculations of curvature energy, torsion energy, and total complexity of 3D skeletons of the pICA lumen. We further measured the posterior knee of the so-called ""carotid siphon"" with a virtual goniometer and performed correlations between the objective mathematical calculations and the subjective angle measurements. Conclusions Firstly, our study provides mathematical characterizations of the pICA shape, which can serve as objective reference data for analyzing connections between pICA shape complexity and vascular diseases. Secondly, we provide an objective method for creating Such data. Thirdly, we evaluate the usefulness of subjective goniometric measurements of the angle of the posterior knee of the carotid siphon.