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Notes: Ultrathin (001) Si films bonded onto (001) Si wafers, inducing grain boundaries with twist angles varying from 0.5° to 12°, were studied by transmission electron microscopy. A great structural difference between low (ψ〈5°) and high (ψ〉6°) twist angles was observed. In low twist angle grain boundaries, "twist interfacial dislocations" are dissociated and produce rough interfaces with no oxide precipitates. It is the opposite in high-angle grain boundaries: there is no dissociation, the interfaces are smoother but contain oxide precipitates. These differences are not attributed to the thin thickness of one grain, but to the large atomic differences between high- and low-angle twist grain boundaries, which is not the case for tilt grain boundaries. © 2000 American Institute of Physics.

Notes: Cocaine and amphetamine-regulated transcript (CART) mRNA and immunoreactivity are expressed abundantly in the hypothalamus. Central administration of various fragments of this neuropeptide decreases food intake in rodents. To find out whether CART might play a role in the physiological regulation of energy balance, we used in situ hybridization to investigate whether CART mRNA abundance changed in two chronic obese/fat versus lean states and after acute dietary restriction. In the first study, mice were treated with goldthioglucose to destroy glucose-responsive neurones in the ventromedial hypothalamus. This produced hyperphagia and obesity: 7 weeks after treatment, those receiving goldthioglucose weighed 70% more than the controls. CART mRNA abundance in the arcuate nucleus of goldthioglucose-treated mice was decreased by 71% compared to levels in the control mice, but CART expression was unaffected in the dorsolateral hypothalamus. In the second study, male Siberian hamsters were exposed to short days to induce a physiological winter response in which body weight decreases as fat reserves are catabolized, and food intake correspondingly declines. After 8 weeks in short days, body weight had declined by 18% relative to controls maintained in long days in a summer fat state. CART mRNA levels did not differ significantly between the two groups in any hypothalamic areas. In the third study, male Siberian hamsters, either in long days or after 12 weeks exposure to short days to induce weight loss, were subject to a 48-h period of fasting. Although photoperiod per se did not affect CART expression, fasting produced a significant decrease in CART mRNA in the arcuate nucleus of hamsters in both the long- and short-day state. We conclude that CART-producing cells are involved in energy homeostasis: the marked decrease in CART expression in the arcuate nucleus in goldthioglucose-lesioned mice may contribute to the development of obesity, and the decrease following acute dietary restriction in hamsters may reflect a compensatory mechanism to reduce caloric expenditure, but our results do not indicate that CART is involved in long-term seasonal regulation of body weight.

Notes: Seasonal mammals commonly exhibit robust annual cycles of adiposity, food intake and energy metabolism. These cycles are driven by changes in the external daylength signal, which generates a diurnal melatonin profile and acts on neuroendocrine pathways. The white adipose tissue hormone leptin reflects overall adiposity in seasonal mammals, and consequently undergoes significant seasonal fluctuations in secretion. The seasonally breeding Siberian (Djungarian) hamster is a convenient laboratory model to study the effect of a seasonal time-keeping clock on energy metabolism, appetite regulation and the control of adiposity. We have shown that administration of exogenous leptin at physiological doses induces significant loss of adipose tissue for short-day housed winter-like hamsters in which endogenous adipose tissue and leptin concentrations are already low. By contrast, long-day housed hamsters with high adipose tissue reserves are refractory to the effects of leptin. This phenomenon of seasonal leptin resistance appears to be a general feature of other seasonally breeding mammals, and may reflect the operation of an annual timer controlling leptin uptake and/or action on central nervous system signal transduction pathways. The mobilization of fat by leptin in short-day housed hamsters is not associated with changes in expression in either anorexic or anabolic peptides expressed in leptin-receptor rich structures in the arcuate region of the hypothalamus, and suggests that leptin may target other structures. These data contrast with studies, which show that homeostatic mechanisms in response to feed-restriction induce changes in hypothalamic peptides in a similar manner to nonphotoperiodic species. Thus, the long-term seasonal regulation of body weight set point and leptin feedback may operate through separate pathways to those responsible for acute responses to food restriction.