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Long-term alteration of dopaminergic neurotransmission is known to modulate the D2/D3 receptor expression in the brain. The modulation can occur as a response to pathological processes or pharmacological intervention. The receptor density can be monitored by in vivo positron emission tomography (PET) of [11C] raclopride. To obtain accurate measurements of receptor-ligand interaction, it is essential to estimate binding parameters at true (if transient) equilibrium of bound and unbound ligand quantities. We designed this study as a comparison of two quantitative approaches to transient equilibrium, the TRansient EquilibriuM BoLus Estimation (TREMBLE) method and the Transient Equilibrium Model (TEM) method, to determine binding parameters at transient equilibrium with bolus injection of the radioligand. The data demonstrates that TREMBLE unlike TEM identified the time at which equilibrium existed. TREMBLE revealed that equilibrium prevailed at one or more times after bolus injection and identified differences of receptor density among regions such as putamen and caudate nucleus. We demonstrated that TREMBLE is a quantitative approach suitable for the study of pathophysiological conditions of certain types of neurotransmission the brain. [ABSTRACT FROM AUTHOR]

The finite-element method of solid mechanics was applied to calculate the three-dimensional structure of closed circular DNA modeled as an elastic rod. The four models represent a straight rod, two rods containing either one or two20 degree bends and a circular O-ring. The results show that bent DNA converts twist and writhe much more efficiently than unbent DNA, and the presence of bends causes nonuniformities in the local twist increment. These features are important in the understanding of the biological effects of DNA looping.

Studies on closed circular DNA molecules have made possible the quantification of topological linking changes in DNA. The surface linking number, SLk, define surface wrapping while the winding number, phi, and helical repeat, h, define local winding. These variables are used to characterize the population average linking change, delta Lk, which is applied to the analysis of DNA-protein complex structure.