Two experiments examined the effects of gonadectomy, photoperiod, and melatonin treatment on food intake, body weight, and carcass composition in male and female Siberian hamsters (Phodopus sungorus sungorus). Gonadectomy caused small decreases in body weight in both sexes but did not affect food intake. In males the weight reduction was due to small (nonsignificant) decreases in all carcass components. Ovariectomy caused a significant reduction in carcass lipid but did not affect lean body mass. A short photoperiod (8 h light, 16 h dark, LD 8:6) caused significant weight losses in all animals except ovariectomized females. Daily melatonin injections (12.5 micrograms sc, 3 h before lights-out) also reduced body weight in males and females housed in a long photoperiod (LD 16:8). The photoperiod- and melatonin-induced weight losses were almost entirely due to decreases in carcass lipid; lean body mass was unaffected by either treatment. Although short photoperiods and melatonin treatments decreased food intake, these changes were preceded by a significant loss of body weight. Thus photoperiod induced changes in food intake may be a consequence of the changes in body weight, rather than vice versa. These findings indicate that photoperiod and melatonin have important effects on adipose tissue mass in both Siberian and Syrian hamsters, even though one species gains weight (Syrian) and the other loses weight (Siberian) in short photoperiods.
Adult male voles were maintained for 10 wk in long or short photoperiods (14 or 10 h of light/day). A third group of animals housed in the long photoperiod was implanted with capsules containing melatonin. Body weight and food intake were measured weekly; various tissues were weighed and analyzed at the time of autopsy. After 10 wk, voles in the short photoperiod weighed 20% less and consumed 30% less food than those housed in the long photoperiod. Total body water and lean body mass were reduced in the short-day animals, although the size of the brown adipose tissue was not affected. White adipose tissue lipoprotein lipase (LPL) activity was markedly reduced in the short-day voles who also manifested gonadal regression and suppression of spermatogenesis. Melatonin mimicked the effects of short photoperiods on LPL activity and on lean body mass; other parameters for melatonin-treated animals were intermediate between those of untreated long- and short-day voles. We hypothesize that winter weight losses experienced by meadow voles in the field are mediated by decreases in the duration of the daily photophase and that the reduction in body mass permits overwintering voles to reduce their energy requirements and the amount of time devoted to foraging. At least part of the seasonal decline in body mass appears due to a decrease in gonadal hormone secretion.
This experiment examined the effects of diet and photoperiod on food intake, body weight, and brown adipose tissue (BAT) activity in female Siberian hamsters (Phodopus sungorus sungorus). BAT function was assessed by measuring both the sympathetic nervous system activity of BAT [estimated by the rate of norepinephrine (NE) turnover] and BAT thermogenic activity (estimated by GDP binding to BAT mitochondria). Nineteen weeks of high-fat feeding in long photoperiod [16:8 light-dark cycle (LD)] caused a 20% increase in food intake but did not affect body weight. Both NE turnover rate and GDP binding in interscapular BAT (IBAT) were increased four- to eightfold relative to that from chow-fed controls. Thus it appears that in Siberian hamsters BAT can serve the same energy-dissipating function during diet-induced overeating previously established in rats and mice. Nineteen-week exposure to a short photoperiod (LD 8:16) produced a reduction in body weight but did not affect food intake. Both NE turnover rate and GDP binding in IBAT were increased two- to fourfold relative to that from long-photoperiod controls. Thus it appears that in Siberian hamsters the photoperiod-induced improvements in thermogenic capacity are mediated via the same mechanisms as are cold- or diet-induced thermogenesis.
Non-shivering thermogenesis is an energy demanding process that primarily occurs in brown and beige adipose tissue. Beyond regulating body temperature, these thermogenic adipocytes regulate systemic glucose and lipid homeostasis. Historically, research on thermogenic adipocytes has focused on glycolytic metabolism due to the discovery of active brown adipose tissue in adult humans through glucose uptake imaging. The importance of lipids in non-shivering thermogenesis has more recently been appreciated. Uptake of circulating lipids into thermogenic adipocytes is necessary for body temperature regulation and whole-body lipid homeostasis. A wide array of circulating lipids contribute to thermogenic potential including free fatty acids, triglycerides, and acylcarnitines. This review will summarize the mechanisms and regulation of lipid uptake into brown adipose tissue including protein-mediated uptake, lipoprotein lipase activity, endocytosis, vesicle packaging, and lipid chaperones. We will also address existing gaps in knowledge for cold induced lipid uptake into thermogenic adipose tissue.
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