Paul Trayhurn

Localization of leptin receptor mRNA and the long form splice variant (Ob-Rb) in mouse hypothalamus and adjacent brain regions by in situ hybridization

Expression of the leptin receptor gene has been examined in mouse hypothalamus and other brain regions by in situ hybridization. With a probe recognizing all the known splice variants, receptor mRNA was evident in several brain regions (cortex, hippocampus, thalamus), with strong expression in the hypothalamus (arcuate, ventromedial, paraventricular and ventral premammillary nuclei), choroid plexus and leptomeninges. A probe specific to the long splice variant of the leptin receptor (Ob‐Rb), containing the putative intracellular signaling domain, again revealed strong expression in the hypothalamus; there was, however, minimal hybridization to choroid plexus and leptomeninges. These results indicate that the hypothalamus is a key site of leptin action, although other brain regions are also targeted.

Circulating leptin in women: a longitudinal study in the menstrual cycle and during pregnancy

To investigate whether leptin is linked to reproduction, circulating levels were measured longitudinally throughout spontaneous menstrual cycles and during pregnancy in normal women.

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.

Regulation of leptin receptor and NPY gene expression in hypothalamus of leptin-treated obese (ob/ob) and cold-exposed lean mice

Leptin receptor gene expression has been measured in arcuate and ventromedial hypothalamic nuclei. Receptor mRNA in both hypothalamic areas was higher in obese mice than in lean littermates. Twice daily leptin administration for 7 days profoundly affected food intake, reduced leptin receptor mRNA in the arcuate nucleus, and had a similar effect on neuropeptide Y gene expression. A single leptin injection was ineffective. Exposure of lean mice to cold for 24 h caused an induction of leptin receptor and NPY mRNA which was normalized when animals were returned to the warm. Regulation of receptor gene expression may be an important component in the reading of the leptin signal.

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.

Leptin and reproduction

Fertility in mammals requires adequate nutrition and a certain reserve of metabolic fuel (Van Der Spuy, 1985). Individuals experiencing wasting diseases (e.g. insu- lin-dependent diabetes), severe dietary restriction (e.g. anorexics), or who are high-performance athletes (e.g. mara- thon runners or ballet dancers) may have severely impaired reproductive systems (De Souza & Metzger, 1991; Stewart, 1992; Griffen

Regulation of the level of uncoupling protein in brown adipose tissue by insulin requires the mediation of the sympathetic nervous system

The role of the sympathetic nervous system in the regulation by insulin of the level of uncoupling protein in brown adipose tissue has been examined. The amount of uncoupling protein was substantially reduced in streptozotocin‐diabetic rats, while insulin replacement to diabetic animals induced a partial restoration. Unilateral denervation of the interscapular brown fat pads also lowered the amount of uncoupling protein, and in diabetic animals inhibited the stimulation of the level of the protein by insulin replacement. Maintenance of normal uncoupling protein levels requires both insulin and the sympathetic system; regulation of the protein by insulin involves sympathetic mediation.

Brown adipose tissue: from thermal physiology to bioenergetics

Brown adipose tissue is an organ in mammals specialized for the generation of heat. The tissue plays an important role in thermoregulatory heat production (nonshivering thermogenesis), and in nutritional energetics (through the process of diet-induced thermogenesis). Much of the current interest in brown adipose tissue has been catalysed by the postulate (1970’s) that a reduced capacity for thermogenesis underlies the development of obesity. Heat is generated in brown fat by a controlled uncoupling of oxidative phosphorylation, a process regulated by a tissue-specific mitochondrial uncoupling protein,Mr 32–33,000. The immunological identification of uncoupling protein is now used as a biochemical criterion for distinguishing brown fat from white adipose tissue. The gene coding for uncoupling protein has been cloned in several species, and a number of factors regulating the expression of the gene, as well as the amount and activity of the protein itself, have been documented. In addition to its direct role in heat production, brown adipose tissue has some notable general metabolic properties, such as in the conversion of thyroxine to triiodothyronine. An overview of the biology of brown adipose tissue is presented in this article, with an emphasis on some recent developments.

Adaptive changes in the concentration of the mitochondrial ‘uncoupling’ protein in brown adipose tissue of hamsters acclimated at different temperatures

The effect of acclimation at different temperatures on the activity of interscapular brown adipose tissue has been investigated in the hamster, a hibernator. Between 31° and 4°C the cytochrome oxidase activity of the tissue increased 4- to 5-fold, mitochondrial GDP binding per mg of mitochondrial protein doubled, and the amount of uncoupling protein rose from 1.7% to 5.4% of total mitochondrial protein. It is concluded that there are clear adaptive changes induced by temperature in brown adipose tissue of the hamster, but the changes are limited in comparison with those in the mouse.

New insights into the development of obesity: obese genes and the leptin system.

The incidence of obesity is rising rapidly in a number of countries. In the UK, for example, the proportion of adult men and women classified as obese on the basis of a BMI >30 doubled in the decade between 1980 and 1991 (Prentice & Jebb, 1995). Despite the growing problems associated with the epidemic of obesity, current treatment modalities have remained stalled. This is also true of attempts to understand the aetiology of the disorder in terms of the fundamental underlying mechanisms. Much debate in the past has focused on whether the obese are hyperphagic, or have metabolic abnormalities relating to energy expenditure. This has now evolved, however, to the point where the issue is being discussed in terms of whether energy expenditure is falling (because of inactivity) faster than the documented decline in energy consumption in the population (Prentice & Jebb, 1995). New insights into the regulation of energy balance and the fundamental causes of obesity are rapidly emerging following the recent application of molecular genetics. This paper summarizes these developments, focusing particularly on the biology of the system associated with leptin, the product of the ob (obese) gene.