Jacqueline S. Duncan

Effects of fasting and refeeding on ob gene expression in white adipose tissue of lean and obese (ob/ob) mice

A 33‐mer antisense oligonucleotide has been utilized as a probe for the rapid chemiluminescence‐based detection of ob (obese) mRNA. Expression of the ob gene was evident in several white adipose tissue depots of mice (epididymal, highest; subcutaneous and omental, lowest), but not in other organs. Fasting (24 h) induced a substantial fall in ob mRNA in the epididymal fat of lean mice, which was rapidly reversed on refeeding, responses consistent with the putative role of ob in energy balance. Fasting had no effect, however, on ob mRNA levels in obese (ob/ob) mice.

Developmental changes in hypothalamic leptin receptor: relationship with the postnatal leptin surge and energy balance neuropeptides in the postnatal rat

In the adult brain, leptin regulates energy homeostasis primarily via hypothalamic circuitry that affects food intake and energy expenditure. Evidence from rodent models has demonstrated that during early postnatal life, leptin is relatively ineffective in modulating these pathways, despite the high circulating levels and the presence of leptin receptors within the central nervous system. Furthermore, in recent years, a neurotrophic role for leptin in the establishment of energy balance circuits has emerged. The precise way in which leptin exerts these effects, and the site of leptin action, is unclear. To provide a detailed description of the development of energy balance systems in the postnatal rat in relation to leptin concentrations during this time, endogenous leptin levels were measured, along with gene expression of leptin receptors and energy balance neuropeptides in the medial basal hypothalamus, using in situ hybridization. Expression of leptin receptors and both orexigenic and anorexigenic neuropeptides increased in the arcuate nucleus during the early postnatal period. At postnatal day 4 (P4), we detected dense leptin receptor expression in ependymal cells of the third ventricle (3V), which showed a dramatic reduction over the first postnatal weeks, coinciding with marked morphological changes in this region. An acute leptin challenge robustly induced suppressor of cytokine signaling-3 expression in the 3V of P4 but not P14 animals, revealing a clear change in the location of leptin action over this period. These findings suggest that the neurotrophic actions of leptin may involve signaling at the 3V during a restricted period of postnatal development.

Hyperleptinaemia in mice induced by administration of the tyrosine hydroxylase inhibitor α-methyl-p-tyrosine

α‐methyl‐p‐tyrosine (αMPT), an inhibitor of tyrosine hydroxylase, was administered to mice to block noradrenaline synthesis. Ten hours after injection of αMPT there was a 6‐fold increase in plasma leptin. The level of ob mRNA in epididymal white adipose tissue was also increased, but UCP1 mRNA in brown fat fell. In contrast to lean mice, ob mRNA in white fat of ob/ob mice was not increased by αMPT. αMPT raised plasma leptin in fasted as well as fed mice. Hyperleptinaemia was attenuated by treatment with a β3‐adrenoceptor agonist. Inhibition of noradrenaline synthesis leads to the rapid induction of hyperleptinaemia; it is suggested that sympathetic tone plays a pivotal role in regulating leptin production.

Repletion of TNFα or leptin in calorically restricted mice suppresses post-restriction hyperphagia

SUMMARY The causes of post-restriction hyperphagia (PRH) represent a target for drug-based therapies to prevent obesity. However, the factors causing PRH are poorly understood. We show that, in mice, the extent of PRH was independent of the time under restriction, but depended on its severity, suggesting that PRH was driven by signals from altered body composition. Signals related to fat mass were important drivers. Circulating levels of leptin and TNFα were significantly depleted following caloric restriction (CR). We experimentally repleted their levels to match those of controls, and found that in both treatment groups the level of PRH was significantly blunted. These data establish a role for TNFα and leptin in the non-pathological regulation of energy homeostasis. Signals from adipose tissue, including but not limited to leptin and TNFα, regulate PRH and might be targets for therapies that support people engaged in CR to reduce obesity.

Effect of photoperiod on mitochondrial GDP binding and adenylate cyclase activity in brown adipose tissue of djungarian hamsters

Experiments were designed to investigate the involvement of brown adipose tissue (BAT) thermogenesis in the weight loss exhibited by Djungarian hamsters (Phodopus sungorus campbelli) in response to a short photoperiod. Significant decreases in body weight preceded reductions in food intake, suggesting a photoperiod-induced change in energy expenditure. Sixteen weeks exposure to short photoperiod resulted in large decreases in body weight and interscapular BAT mass that were accompanied by an increase in the thermogenic activity of BAT (estimated by mitochondrial GDP binding). However, exposure to short photoperiod for 8 weeks, that induced smaller but significant reductions in body weight, was without effect on the BAT parameters measured. This suggests that increased BAT thermogenesis is unlikely to initiate, or contribute to, the early stages of photoperiod-induced weight loss. In addition, short photoperiod failed to induce any change in the specific activity or sensitivity of adenylate cyclase in BAT membranes, in contrast to the downregulation of catecholamine-stimulated cAMP production observed in BAT following cold exposure.

A spontaneous binge-like eating model in mice using unpredictable once weekly access to palatable diets

Many pre-clinical models of binge-like eating involve predictable, scheduled, access to a palatable diet high in fat (HF), where access may be preceded by anticipatory behaviour. Here, to introduce spontaneity into the binge-type consumption of palatable diets, mice were allowed 2 h access on a random day once per week and at a random time within an 8 h window either side of the transition from dark phase to light phase. Despite normal intake of a stock diet prior to unpredictable access to HF diet, mice immediately initiated a substantial eating episode when presented with HF diet. Following this consumption, compensatory hypophagia was observed relative to stock diet-fed controls, and cumulative energy intakes converged. There were no effects of HF diet on body weight or body composition over a 12-week period. Binge-like consumption was also observed on unpredictable access to the complete liquid diet, chocolate Ensure, but not with a 10% sucrose solution. Binge-like responses to unpredictable access to HF diet or Ensure were similar in male and female mice, although there were effects of sex on caloric consumption from stock diet in the compensatory period following palatable diet intake, with higher intakes in females. The timing of the 2h access period relative to light phase transition affected intake of palatable diets, but less robustly than the equivalent effect on stock diet intake during the same timed periods - the diurnal patterning of energy intake was diet sensitive. The large spontaneous binge-like consumption on unpredictable access to either solid or liquid palatable diets in mice of either sex offers the potential to combine these attributes with other manipulations where a developing obesity is part of the binge-like eating phenotype.