No description available.

Considerable research has been done to improve hydrate formation rate. One of the ideas is to introduce mechanical mixing which later tend to complicate the design and operation of the hydrate formation processes. Another approach is to add surfactant (promoter) that will improve the hydrate formation rate and also its storage capacity to be closer to the maximum hydrate storage capacity. Surfactant is widely known as a substance that can lower the surface or interfacial tension of the water when it is dissolved in it. Surfactants are known to increase gas hydrate formation rate, increase storage capacity of hydrates and also decrease induction time. However, the role that surfactant plays in hydrate crystal formation is not well understood. Therefore, understanding of the mechanism through morphology studies is one of the important aspects to be studied so that optimal industrial processes can be designed. In the present study the effect of three commercially available anionic surfactants which differ in its alkyl chain length on the formation/dissociation of hydrate from a gas mixture of 90.5 % methane – 9.5% propane mixture was investigated. The surfactants used were sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate (STS), and sodium hexadecyl sulfate (SHS). Memory water was used and the experiments for SDS were carried out at three different degrees of under-cooling and three different surfactant concentrations. In addition, the effect of the surfactant on storage capacity of gas into hydrate was assessed. The morphology of the growing crystals and the gas consumption were observed during the experiments. The results show that branches of porous fibre-like crystals are formed instead of dendritic crystals in the absence of any additive. In addition, extensive hydrate crystal growth on the crystallizer walls is observed. Also a “mushy” hydrate instead of a thin crystal film appears at the gas/water interface. Finally, the addition of SDS with concentration range between 242ppm – 2200ppm (ΔT =13.10C) was found to increase the mole consumption for hydrate formation by 14.3 – 18.7 times. This increase is related to the change in hydrate morphology whereby a more porous hydrate forms with enhanced water/gas contacts. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

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After dendrite transection, two primary injury current pathways may acount for cell death: (1) the lesion current at the site of injury and (2) the voltage sensitive calcium channels along the dendrite. Lesions were made with a laser microbeam in mouse spinal monolayer cell cultures. Polylysine was tried as a positively charged "molecular bandage" to block the lesion current. The calcium channel blockers, verapamil and nifedipine, were used to reduce the calcium channel current. Control toxicity curves were obtained for all three compounds. The results show that neither verapamil, nifedipine, nor polylysine (MW: 3,300) protect nerve cells after dendrite amputation 100 ptm from the soma. The data also indicate that these compounds do not slow the process of cell death after such physical trauma.

No description available.

Interactions between neurons and glia occupy a central role in many aspects of development, maintenance, and function of the central nervous system (CNS). A fundamental event in CNS development is the elaboration of two distinct neuronal processes, axons and dendrites. The overall aim of this research was to characterize the interactions between central nervous system neurons and astroglial cells that regulate dendrite growth from cerebral cortical neurons. Embryonic (E18) mouse cerebral cortical neurons were cocultured with early postnatal (P4) rat astroglia derived from cerebral cortex, retina, olfactory bulb, mesencephalon, striatum and spinal cord. Axon and dendrite outgrowth from isolated neurons was quantified using morphological and double-labeling immunohistochemical techniques at 18 hours and 1, 3 and 5 days in vitro. Neurons initially extended the same number of neurites, regardless of the source of glial monolayer; however, astroglial cells differed in their ability to maintain primary dendrites. Homotypic cortical astroglia maintained the greatest number of primary dendrites. Astroglia derived from the olfactory bulb and retina maintained intermediate numbers of dendrites, whereas only a small number of primary dendrites were maintained by astroglia derived from striatum, spinal cord or mesencephalon. Initially longer axons were observed from neurons grown on astroglia that did not maintain dendrite number. After 5 days in vitro, axon growth was similar on the various monolayers, total primary dendrite outgrowth, however, was nearly threefold greater on astroglia derived from the cortex, retina and olfactory bulb than on astroglia derived from mesencephalon, striatum or spinal cord. This effect was principally on the number of primary dendrites rather than the elongation of individual dendrites and was independent of neuron survival. Similar morphological differences were observed after 5 days in vitro when cortical neurons were grown on polylysine in either a noncontact coculture system where astroglia continuously conditioned the culture medium or in astroglial conditioned medium. Preliminary biochemical analysis of the medium conditioned by cortical astroglia using heat and trypsin degradation, ultracentrifugation, dialysis, and heparin affinity chromatography suggested that a heparin binding protein with a molecular weight between 10 and 100kDa may be responsible for astroglial mediated dendrite growth. Neurons that were grown in medium conditioned by either mesencephalic or cortical astroglia for the first 24 hours followed by culture medium from astroglia of the alternate source for 4 days in vitro, confirmed that astroglia maintained, rather than initiated, the outgrowth of the primary dendritic arbor. In the next series of experiments, E18 mouse cortical neurons were cocultured with neonatal (P4) or mature (P12) rat astroglia derived from cortex and mesencephalon or astroglia derived from P4 and P12 lesioned cortex. After 5 days in vitro, the maturational age of astroglia did not appear to alter the extent of primary dendrite growth; instead dendrite growth reflected the region of the CNS from which the astroglia were derived. By contrast, a reduced ability to support axon growth from mouse cortical neurons in culture was observed on astroglia derived from mature rat cortex or mesencephalon. Reactive astroglia demonstrated similar neurite supporting characteristics to mature astroglia and were able to maintain dendrite growth, principally primary dendrite number. Axon elongation, however, was reduced on both neonatal and mature reactive astroglia. Neuron survival did not correlate with the ability of the various astroglia to support process outgrowth. Collectively these results indicate: 1) neuron-glial interactions are critical for the regulation of process outgrowth from embryonic cortical neurons in vitro, 2) axon and dendrite growth appear to be differently controlled by astroglia, 3) CNS astroglia demonstrate regional differences in maintaining, but not initiating growth of the primary dendritic arbor, 4) this effect may be due, in part, to release of a diffusible heparin binding protein factor, and 5) mature and reactive astroglia support primary dendrite, but limited axon growth. We propose therefore that the local astroglial environment maintains primary dendrite growth from neurons until synaptic contacts can be established. A mechanism that maintains the primary dendritic arbor and allows separate regulation of axon and dendrite growth, prior to the arrival of afferents, may be critical for establishing appropriate and specific synaptic connections. These findings have important implications in understanding development and function of the mammalian central nervous system and may lead to novel strategies for intervention in acute and chronic neurological disorders.

In the mammalian brain, information is believed to be encoded at the cellular level through alterations in synaptic weights. Furthermore, changes in synaptic strength are correlated with structural changes at dendritic spines, such as growth and shrinkage, which may serve to shape inputs into functional domains and increase the computational power of neurons. Neuroanatomical alterations in dendritic spines have been described in humans with intellectual disability, further supporting the relationship between neuronal structure and function. Fragile X Syndrome (FXS) is the most common single-gene neurodevelopmental disorder, and a hallmark feature of this disorder is the increased density of long spines in several brain regions including the hippocampus. Identification of FXS spines as filopodia-like has led to the theory that these spines are immature, and that altered spine development underlies the cognitive dysfunction in this disorder. However, the functional capacity of the long spines observed in FXS is not well understood. For my thesis work, I used two photon imaging, glutamate uncaging and electrophysiology to perform a high-resolution characterization of dendritic spine structure, function, and plasticity in the hippocampus of the FXS mouse model in order to determine what gives rise to these alterations and how this contributes to the observed neuronal dysfunction in this disorder. From my dissertation research, I find that while Fmr1 KO neurons have region-specific alterations in both dendrite and spine morphology, the functional responses of single synapses in FXS mutant neurons are grossly normal. FXS spines respond proportionally to increased levels of glutamate release, and the linear relationship between structure and function is preserved at these synapses. In addition, structural plasticity, both growth and shrinkage, at single inputs is similar in magnitude to control neurons following synaptic potentiation and depression, respectively. However, upon more detailed examination of structural plasticity, either at single or multiple inputs, I find several deficits. First, following structural plasticity, I observe aberrant heterosynaptic plasticity in Fmr1 KO neurons, where unstimulated mutant spines located in close proximity to activated spines become significantly larger compared to neighboring spines in control neurons, which showed no significant change in size. Next, competition for mGluR-LTD does not occur in Fmr1 KO neurons, leading to an increase in spines that undergo spine shrinkage. I conclude from this work that while spine morphology is altered in FXS, spines develop with functional synapses that have the capacity to express bidirectional forms of structural plasticity. However, these spines undergo abnormal structural plasticity across stimulated inputs, leading to the expression of aberrant heterosynaptic structural plasticity. As activity is integrated across a dendritic branch, such excess plasticity observed in Fmr1 KO neurons could contribute to the altered spine morphology as well as cognitive dysfunction observed in FXS.

The relatively few serotonin (5-HT) neurons located in the Dorsal Raphe Nucleus (DRN) give rise to an extensive axonal network modulating a wide-range of brain functions and behaviors. In turn, the DRN receives inputs from several brain regions and therefore exhibits the characteristics of a hub network. While recent technological advancements have provided an unprecedented look at the neurobiology of the DRN, important knowledge gaps remain in understanding how the constellation of synaptic inputs to this region confers 5-HT neurons their unique coding features. As a first step towards characterizing the DRN's input processing strategy, we set out to explore the landscape of dendritic operation operating in DRN 5-HT neurons. Using multi-photon microscopy and in vitro electrophysical recordings, we conducted a morphological and electrophysiological survey of 5-HT neurons where we identified two structurally and morphologically distinct types of glutamatergic synapses both expressing small NMDAR-mediated conductance. Our initial findings provide valuable insights on local rules that govern how synaptic inputs to the DRN are being processed to ultimately confer 5-HT neurons their unique coding features.

Dans les couches II-III du néocortex du rat, les neurones pyramidaux sont connus pour générer des potentiels d'action rétropropagés dans les dendrites. Cela entrainerait l'entrée de calcium dans les dendrites, diminuant le calcium disponible au niveau présynaptique. Sachant que la probabilité de relâche des vésicules de neurotransmetteurs est dépendante de la concentration en calcium périsynaptique, nous avons supposé que les potentiels d'action provoquent une diminution temporaire en [Ca²⁺]₀ autour des neurones qui émettent des potentiels d'action, ce qui affecte la libération de neurotransmetteurs. Ce mémoire présente une vérification de l'effet, sur le taux d'échec synaptique, du potentiel d'action dans le neurone postsynaptique dans des délais de l'ordre des dizaines de millisecondes. Les enregistrements in vitro ont été réalisés en current-clamp avec un ou quatre potentiels d'action et en voltage-clamp avec une ou quatre dépolarisations simulant des potentiels d'action. Les réponses synaptiques ont été examinées de 10ms à 100ms après le ou les potentiels d'action. Nous avons démontré que quatre potentiels d'action dans un neurone postsynaptique diminuent la probabilité de libération de neurotransmetteurs au niveau présynaptique pour des dizaines de millisecondes.

Neurons display exquisite specificity in synaptic connectivity, but we lack a complete under-standing of neuronal connectivity and the rules that govern it. A major impediment to addressing this question lies in the vast diversity of neurons, the small size and large number of synapses formed by any given neuron over a wide territory, and the need to study these connections in intact tissue. We therefore developed an image-based tool to assess synaptic specificity in tissue sections and dissociated culture. We focused on three interneuron subpopulations that target distinct subcellular regions of the post-synaptic cell: soma-targeting basket cells (BCs), axon initial segment (AlS)-targeting chandelier cells (ChCs), and distal dendrite-targeting somatostatin cells (SstCs). Using mouse dissociated cortical culture as a starting point, we built a machine learning (ML) based image processing and analysis pipeline to classify individual presynaptic boutons at scale. Supervised ML classification revealed similar subcellular targeting profiles for these interneuron populations in slice and culture, indicating that targeting is primarily regulated by cell intrinsic programs. We also observed a remarkable target-dependent laminar organization in vivo. An unsupervised ML analysis using the same input data not only identified the same three canonical targeting classes, but also revealed that these classes are comprised of multiple subpopulations. In slice, these synaptic subpopulations displayed distinct laminar or-ganization. In dissociated culture, two soma-targeting synaptic subpopulations mapped to target cells with different cellular profiles. The six dendrite-targeting synaptic subpopulations were found at increasing distances from target soma, suggesting molecularly distinct proximal, medial, and distal dendritic compartments in culture. Tracking subtype targeting across axonal branches of individual neurons indicated that SstCs and BCs utilize distinct targeting strategies in culture that accord with established findings in vivo. In sum, our synaptic analysis pipeline revealed novel synaptic subpopulations in interneurons. Further analysis uncovered novel aspects of interneuron synaptic biology that, remarkably, are retained in culture.