N-body Models of Rotating Globular Clusters

We have studied the dynamical evolution of rotating globular clusters with direct $N$-body models. Our initial models are rotating King models; we obtained results for both equal-mass systems and systems composed out of two mass components. Previous investigations using a Fokker-Planck solver have revealed that rotation has a noticeable influence on stellar systems like globular clusters, which evolve by two-body relaxation. In particular, it accelerates their dynamical evolution through the gravogyro instability. We have validated the occurence of the gravogyro instability with direct $N$-body models. In the case of systems composed out of two mass components, mass segregation takes place, which competes with the rotation in the acceleration of the core collapse. The "accelerating" effect of rotation has not been detected in our isolated two-mass $N$-body models. Last, but not least, we have looked at rotating $N$-body models in a tidal field within the tidal approximation. It turns out that rotation increases the escape rate significantly. A difference between retrograde and prograde rotating star clusters occurs with respect to the orbit of the star cluster around the Galaxy, which is due to the presence of a ``third integral'' and chaotic scattering, respectively.Comment: 16 pages, 17 figures, accepted by MNRA

A Component of Retinal Light Adaptation Mediated by the Thyroid Hormone Cascade

Analysis with DNA-microrrays and real time PCR show that several genes involved in the thyroid hormone cascade, such as deiodinase 2 and 3 (Dio2 and Dio3) are differentially regulated by the circadian clock and by changes of the ambient light. The expression level of Dio2 in adult rats (2–3 months of age) kept continuously in darkness is modulated by the circadian clock and is up-regulated by 2 fold at midday. When the diurnal ambient light was on, the expression level of Dio2 increased by 4–8 fold and a consequent increase of the related protein was detected around the nuclei of retinal photoreceptors and of neurons in inner and outer nuclear layers. The expression level of Dio3 had a different temporal pattern and was down-regulated by diurnal light. Our results suggest that DIO2 and DIO3 have a role not only in the developing retina but also in the adult retina and are powerfully regulated by light. As the thyroid hormone is a ligand-inducible transcription factor controlling the expression of several target genes, the transcriptional activation of Dio2 could be a novel genomic component of light adaptation

Developmental changes of cone opsin expression but not retinal morphology in the hypothyroid Pax8 knockout mouse

PURPOSE. The effects of postnatal hypothyroidism on retinal development and spatial patterning of cone opsin expression were studied in Pax8-deficient mice. Pax8(-/-) mice are incapable of synthesizing thyroxine and serve as a model for congenital hypothyroidism. METHODS. Pax8(-/-), Pax8(-/-), and Pax8(-/-) littermates were studied. Serum thyroid hormone levels, body weight, and eye size were measured. Retinal cell-type-specific antibodies were used on frozen sections to examine the postnatal development of the major retinal cell classes and of retinal structure. The expression of short-wavelength-sensitive (S) and middle-to-long-wavelength-sensitive (M) cone opsins was assessed with opsin antibodies on retinal sections and whole retinas. The pattern of S opsin mRNA was assessed by in situ hybridization. RESULTS. In Pax8(-/-) mice, S opsin was upregulated in all cones, whereas M opsin was downregulated throughout the retina, the wild-type dorsoventral gradients of S and M opsin expression were absent. Otherwise, Pax8(-/-) mice showed no overt mutant phenotype in eye size, gross retinal anatomy, and the time-course of structural differentiation of retinal photoreceptors, horizontal cells, bipolars, amacrines, ganglion cells, and Muller glia cells. CONCLUSIONS. Pax8(-/-) mice show a pattern of cone opsin expression that differs substantially from the wild-type pattern, but exhibit no apparent alterations in general retinal development. The finding that a postnatal decrease in serum thyroid hormone yields changes in postnatal cone opsin expression is consistent with a ligand-dependent role of thyroid hormone receptor beta 2 in S opsin repression and M opsin activation. (Invest Ophthalmol Vis Sci. 2010;51:1719-1727) DOI:10.1167/iovs.09-359