Dissertations / Theses on the topic 'Nervous system – Invertebrates'

Les inclusions des cellules gliales et des neurones des ganglions cérébroïdes de crépidule et cérépleuraux de la moule ont été examinés. Une première approche a permis de distinguer les granules de neurosécretion de type neuroendocrine des inclusions pigmentées. L'ultrastructure de ces inclusions permet de les caracteriser avec précision. L'étude biochimique révèle la présence de carotènes et de différentes xanthophylles. On précise enfin le rôle de ces inclusions dans des conditions d'hypoxie et d'anoxie du milieu

Au niveau d'une synapse neuro-neuronale d'aplysie, la concentration présynaptique d'acétylcholine (ACH) contrôle le nombre de quanta libérés par un stimulus donné sans que la taille du quantum d'ACH soit modifiée. Par l'utilisation de l'hemicholinium-3 (hc-3), un bloquant connu du transport de choline, une facilitation de la libération d'ACH est mise en évidence, alors qu'intracellulairement, l'hc-3 bloque une des étapes du mécanisme de libération de l'ACH. Les quanta restant de taille constante, il en est déduit que seuls les quanta satures sont libérés.

Most organisms, from cyanobacteria to humans are equipped with circadian clocks. These endogenous and self-sustained pacemakers allow organisms to adapt their physiology and behavior to daily environmental variations, and to anticipate them. The circadian clock is synchronized by environmental cues (i.e. light and temperature fluctuations). The fruit fly, Drosophila melanogaster, is well established as a model for the study of circadian rhythms. Molecular mechanisms of the Drosophilacircadian clock are conserved in mammals. Using genetic screens, several essential clock proteins (PER, TIM, CLK, CYC, DBT, SGG and CK-II) were identified in flies. Homologs of most of these proteins are also involved in generating mammalian circadian rhythms. In addition, there are only six neuronal groups in the adult fly brain (comprising about 75 pairs of cells) that express high levels of clock genes. The simplicity of this system is ideal for the study of the neural circuitry underlying behavior. The first half of this dissertation focuses on a genetic screen designed to identify novel genes involved in the circadian light input pathway. The screen was based on previous observations that a mutation in the circadian photoreceptor CRYPTOCHROME (CRY) allows flies to remain rhythmic in constant light (LL), while wild type flies are usually arrhythmic under this condition. 2000 genes were overexpressed and those that showed a rhythmic behavior in LL (like crymutants) were isolated. The candidate genes isolated in the screen present a wide variety of biological functions. These include genes involved in protein degradation, signaling pathways, regulation of transcription, and even a pacemaker gene. In this dissertation, I describe work done in order to validate and characterize such candidates. The second part of this dissertation focuses on identifying the pacemaker neurons that drive circadian rhythms in constant light (LL) when the pacemaker gene period is overexpressed. We found that a subset of pacemaker neurons, the DN1s, is responsible for driving rhythms in constant light. This attractive finding reveals a novel role for the DN1s in driving behavioral rhythms under constant conditions and suggests a mechanism for seasonal adaptation in Drosophila.

La premiere partie est consacree a l'approche neurochimique chez la drosophile. Est decrite une methode rapide d'estimation des metabolites du tryptophane et de leur evolution en fonction de "l'experience" de l'insecte. Les donnees de l'approche pharmacologique soulignent que l'ingestion de milieux nutritifs chimiquement definis et de composition variable, est susceptible de retentir sur le taux de ces metabolites. La localisation de ces neurones serotoninergiques par immunoperoxydase et l'etude des recepteurs membranaires a la serotonine et aux opiaces font l'objet de la deuxieme partie. Enfin les repercutions de l'ingestion de milieux nutritifs definis sur le comportement sexuel sont etudiees

P. Brassicae présente une diapause nymphale facultative induite par les photopériodes courtes. Aucune modification expérimentale du contexte hormonal (ecdysteroïdes et hormones juvéniles) au dernier stade larvaire ne permet de changer la programmation du développement déterminée par la photopériode. On peut provoquer la reprise du développement des chrysalides diapausantes par un séjour au froid ou par injection d'ecdysone. Les hormones exogènes stimulent la reprise du développement par deux mécanismes: une action morphogénétique directe sur les tissus cibles et une néosynthese d'ecdysone.

Neuronal networks produce reliable functional output throughout the lifespan of an animal despite ceaseless molecular turnover and a constantly changing environment. The cellular and molecular mechanisms underlying the ability of these networks to maintain functional stability remain poorly understood. Central pattern generating circuits produce a stable, predictable rhythm, making them ideal candidates for studying mechanisms of activity maintenance. By identifying and characterizing the regulators of activity in small neuronal circuits, we not only obtain a clearer understanding of how neural activity is generated, but also arm ourselves with knowledge that may eventually be used to improve medical care for patients whose normal nervous system activity has been disrupted through trauma or disease. We utilize the pattern-generating pyloric circuit in the crustacean stomatogastric nervous system to investigate the general scientific question: How are specific aspects of rhythmic activity regulated in a small neuronal network? The first aim of this thesis poses this question in the context of a single neuron. We used a single-compartment model neuron database to investigate whether co-regulation of ionic conductances supports the maintenance of spike phase in rhythmically bursting “pacemaker” neurons. The second aim of the project extends the question to a network context. Through a combination of computational and electrophysiology studies, we investigated how the intrinsic membrane conductances of the pacemaker neuron influence its response to synaptic input within the framework of the Phase Resetting Curve (PRC). The third aim of the project further extends the question to a systems-level context. We examined how ambient temperatures affect the stability of the pyloric rhythm in the intact, behaving animal. The results of this work have furthered our understanding of the principles underlying the long-term stability of neuronal network function.

High temporal resolution of vision relies on the rapid kinetics of the photoresponse in the light-sensing photoreceptor neurons. It is well known that the rapid recovery of photoreceptor membrane potential at the end of light stimulation depends on timely deactivation of the visual transduction cascade within photoreceptors. Whether any extrinsic factor contributes to the termination speed of the photoresponse is unknown. In this thesis, using Drosophilaas a model system, I show that a feedback circuit mediated by both neurons and glia in the visual neuropile lamina is required for rapid repolarization of the photoreceptor at the end of the light response. In the first part of my thesis work, I provide evidence that lamina epithelial glia, the major glia in the visual neuropile, is involved in a retrograde regulation that is critical for rapid repolarization of the photoreceptor at the end of light stimulation. I identified the gene affected in a slrp (slow receptor potential) mutant that is defective in photoreceptor response termination, and found it needs to be expressed in both neurons and epithelial glia to rescue the mutant phenotype. The gene product SLRP, an ADAM (a disintegrin and metalloprotease) protein, is localized in a special structure of epithelial glia, gnarl, and is required for gnarl formation. This glial function of SLRP is independent of the metalloprotease activity. In the second part of my thesis work, I demonstrate that glutamatergic transmission from lamina intrinsic interneurons, the amacrine cells, to the epithelial glia is required for the rapid repolarization of photoreceptors at the end of the light response. From an RNAi-based screen, I identified a vesicular glutamate transporter (vGluT) in amacrine cells as an indispensable factor for the rapid repolarization of the photoreceptor, suggesting a critical role of glutamatergic transmission from amacrine cells in this retrograde regulation. Further, I found that loss of a glutamate-gated chloride channel GluCl phenocopies vGluT downregulation. Cell specific knockdown indicates that GluCl functions in both neurons and glia. In the lamina, a FLAG-tagged GluCl colocalized with the SLRP protein in the gnarl-like structures, and this localization pattern of GluCl depends on SLRP, suggesting that lamina epithelial glia receive glutamatergic input from amacrine cells through GluCl at the site of gnarl. Since the amacrine cell itself is innervated by photoreceptors, these observations suggest that a photoreceptor — amacrine cell — epithelial glia — photoreceptor feedback loop facilitates rapid repolarization of photoreceptors at the end of the light response. In summary, my thesis research has revealed a feedback regulation mechanism that helps to achieve rapid kinetics of photoreceptor response. This visual regulation contributes to the temporal resolution of the visual system, and may be important for vision during movement and for motion detection. In addition, this work may also advance our understanding of glial function, and change our concept about the effect of glutamatergic transmission.