Identification and characterization of retinoic acid-induced morphological and electrophysiological changes in an invertebrate nervous system

The vitamin A metabolite, retinoic acid (RA) is known to play an important role in the development, patterning and regeneration of nervous tissue, both in the embryo and in the adult. Classically, RA is known to mediate the transcription of target genes through the binding and activation ofits nuclear receptors: the retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Recently, mounting evidence from many animal models has implicated a number of RA-mediated effects operating independently of gene transcription, and thus highlights nove~ nongenornic actions of RA. For example, recent work utilizing cultured neurons from the pond snaa Lymnaea stagnalis, has shown that RA can elicit a regenerative response, growth cone turning, independently of "classical" transcriptional activation While this work illustrates a novel regeneration-inducing effect in culture, it is currently -unknown whether RA also induces regeneration in situ. This study has sought to determine RA's regenerative effucts at the morphological and molecular levels by utilizing an in situ approach focusing on a single identified dopaminergic neuron which possesses a known "mapped" morphology within the CNS. These studies show, for the first time in an invertebrate, that RA can increase neurite outgrowth of dopaminergic cells that have undergone a nerve-crush injury. Utilizing Western blot analysis, it was shown that this effect appears to be independent of any changes in whole CNS expression levels of either the RAR or RXR. Additionally, utilizing immunohistochemistry, to examine protein localization, there does not appear to be any obvious changes in the RXR expression level at the crush site. Changes in cell morphology such as neurity extension are known to be modulated by changes in neuronal firing activity. It has been previously shown that exposure to RA over many days can lead to changes in the electrophysiological properties of cultured Lymnaea neurons; however, no studies have investigated whether short-term exposure to RA can elicit electrophysiological changes and/or changes in firing pattern of neurons in Lymnaea or any other species. The studies performed here show, for the first time in any species, that short-tenn treatment with RA can elicit significant changes in the firing properties of both identified dopaminergic neurons and peptidergic neurons. This effect appears to be independent of protein synthesis, activation of protein kinase A or phospholipase C, and calcium influx but is both dose-dependent and isomer-dependent. These studies provide evidence that the RXR, but not RAR, may be involved, and that intracellular calcium concentrations decrease upon RAexposure with a time course, dose-dependency and isomer-dependency that coincide with the RA-induced electrophysiological changes. Taken together, these studies provide important evidence highlighting RA as a multifunctional molecule, inducing morphological, molecular and electrophysiological changes within the CNS, and highlight the many pathways through which RA may operate to elicit its effects.

The Effects of Annealing URu2Si2 on the Resistivity and Meissner Effect

The enigmatic heavy fermion URu2Si2, which is the subject of this thesis, has attracted intensive theoretical and experimental research since 1984 when it was firstly reported by Schlabitz et al. at a conference [1]. The previous bulk property measurements clearly showed that one second order phase transition occurs at the Hidden Order temperature THO ≈ 17.5 K and another second order phase transition, the superconducting transition, occurs at Tc ≈ 1 K. Though twenty eight years have passed, the mechanisms behind these two phase transitions are still not clear to researchers. Perfect crystals do not exist. Different kinds of crystal defects can have considerable effects on the crystalline properties. Some of these defects can be eliminated, and hence the crystalline quality improved, by annealing. Previous publications showed that some bulk properties of URu2Si2 exhibited significant differences between as-grown samples and annealed samples. The present study shows that the annealing of URu2Si2 has some considerable effects on the resistivity and the DC magnetization. The effects of annealing on the resistivity are characterized by examining how the Residual Resistivity Ratio (RRR), the fitting parameters to an expression for the temperature dependence of the resistivity, the temperatures of the local maximum and local minimum of the resistivity at the Hidden Order phase transition and the Hidden Order Transition Width ∆THO change after annealing. The plots of one key fitting parameter, the onset temperature of the Hidden Order transition and ∆THO vs RRR are compared with those of Matsuda et al. [2]. Different media used to mount samples have some impact on how effectively the samples are cooled because the media have different thermal conductivity. The DC magnetization around the superconducting transition is presented for one unannealed sample under fields of 25 Oe and 50 Oe and one annealed sample under fields of 0 Oe and 25 Oe. The DC field dependent magnetization of the annealed Sample1-1 shows a typical field dependence of a Type-II superconductor. The lower critical field Hc1 is relatively high, which may be due to flux pinning by the crystal defects.