Siberian hamsters exhibit decreased body weight and fat after initial exposure to short photoperiods and increased body weight and fat after extended short photoperiod exposure. The purpose of the present experiments was to determine if uniform changes in white adipose tissue (WAT) pad weights and lipid metabolism correspond to these short photoperiod-induced changes in body fat. Carcass lipid content and testes and fat pad weights [retroperitoneal WAT (RWAT), epididymal WAT (EWAT), and inguinal and dorsal subcutaneous WAT, respectively] were decreased in male hamsters relative to their long day counterparts after 6 and 12 wk of short-day exposure. Moreover, EWAT and RWAT weight, EWAT specific lipoprotein lipase activity, and specific and total lipogenesis were disproportionately decreased relative to the subcutaneous fat pads. The changes in fat pad weight and metabolism were generally reversed coincident with the return to a long-day-like reproductive status after prolonged short-day exposure (24 and 30 wk). In a less detailed experiment, female Siberian hamsters had decreased body, fat pad, and uterine weights after 6 wk of short-day exposure; however, no fat pad-specific changes in weight were observed. The results of these experiments demonstrate that short-day-exposed male Siberian hamsters may be a useful model for examining mechanisms underlying fat pad-specific responses. In addition, gender appears to influence the pattern of short-day-induced lipid depletion in this species.
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.
Daily injections of either 0.8 or 3.2 mg norepinephrine (NE)/kg for 2 wk failed to stimulate brown adipose tissue (BAT) growth, GDP binding, or cytochrome-c oxidase activity (COA) in Syrian hamsters (Mesocricetus auratus). However, a single injection of 1.6 mg NE/kg produced a small (23%) but significant acute increase in BAT GDP binding without affecting COA. Thus there is some loss of sensitivity to NE with chronic treatment in Syrian hamsters. Unilateral sympathectomy by surgical denervation of the interscapular BAT (IBAT) resulted in decreased GDP binding and COA in the denervated pad. Chronic NE treatment in hamsters with denervated IBAT only partially reversed the denervation-induced decreases in GDP binding and COA. It therefore appears that NE is not solely responsible for the maintenance and stimulation of thermogenic activity and COA in Syrian hamster BAT. Denervation of IBAT also resulted in elevated levels of lipoprotein lipase (LPL) in this tissue, a surprising finding since brown and white adipose tissue LPL activity were both stimulated by chronic NE treatment. Therefore, although NE has a stimulatory effect on LPL activity, the primary influence of the neural input to IBAT on this enzyme is inhibitory. These data exemplify dramatic differences between rats and hamsters in the mechanisms controlling BAT thermogenesis and white and brown adipose tissue LPL activity.
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