Aims/hypothesis. Ageing is associated with insulin and leptin resistance in mammals. These alterations might be caused by the increased adiposity associated with ageing, by ageing alone or both. We studied whether leptin resistance occurs at the central level in the Wistar rat and we aimed to discriminate between the effects of ageing from those of the increased adiposity associated with ageing. Methods. Leptin was infused intracerebroventricularly at a constant rate in young adult, old and old Wistar rats fasted for 3 months, using osmotic pumps. The effects on body weight, daily food intake, Lee index, adiposity and serum leptin values were analysed. The effect of food restriction on the expression of the long form of leptin receptor in the hypothalamus was also studied. Results. Leptin decreased daily food intake and body weight in young and old Wistar rats. With a dose of 10 µg/day similar responses were obtained in young and old rats but with a dose of 0.2 µg/day, only young rats showed decreases in these parameters. Foodrestriction in old rats lowered adiposity and serum leptin to values close to those of young rats, recovered responsiveness to i.c.v. administration of leptin at the dose of 0.2 µg/day and increased leptin receptor expression in the hypothalamus. Conclusion/interpretation. Our data show that old Wistar rats have a decreased response to leptin at the central level. Food-restriction recovers leptin responsiveness and increases leptin receptor in the hypothalamus suggesting that adiposity plays a key role in the development of leptin resistance associated with ageing. [Diabetologia (2002[Diabetologia ( ) 45:997-1003 Keywords Leptin, leptin-resistance, food-restriction, ageing, Wistar rat.

Leptin modulates glucose homeostasis by acting as an insulin-sensitizing factor in most insulin target tissues. Nevertheless, insulin-dependent glucose uptake in white adipose tissue decreases after in vivo treatment with leptin. Moreover, elevated leptin concentrations inhibit insulin metabolic effects in adipocytes. Here we studied both, direct and centrally mediated effects of leptin on insulin signaling in rat adipocytes. Adipocyte incubation with low leptin concentrations did not modify the insulin stimulation of mitogen-activated protein kinase (MAPK). However, at elevated concentrations, leptin impaired insulin-stimulated MAPK activity, glycogen synthase kinase (GSK)3␤ phosphorylation, and insulin receptor tyrosine phosphorylation without altering vanadate stimulation. An increase of suppressor of cytokine signaling-3 protein was also observed. Central administration of leptin decreased insulin effects on adipocyte MAPK and GSK3␤ phosphorylation. In insulin-resistant aged rats with hyperleptinemia and central leptin resistance, insulin poorly stimulated MAPK and central leptin infusion did not further deteriorate adipocyte insulin responsiveness. Food restriction increased MAPK stimulation by insulin and restored the ability of centrally infused leptin to attenuate adipocyte insulin signaling in aged rats. We conclude that leptin can modulate, in an inhibitory manner, adipocyte insulin signaling by two different ways: as an autocrine signal and, indirectly, through neuroendocrine pathways. These mechanisms may be of relevance in situations of hyperleptinemia, such as aging and/or obesity. Diabetes 53

L EPTIN, A CYTOKINE produced mainly by the white adipose tissue (WAT), is actively involved in the control of body weight and food intake. Dysregulations of leptin actions are associated with obesity, insulin resistance, and type 2 diabetes. These facts point to leptin and its actions as targets of study to clarify these metabolic disorders and identify potential therapeutic strategies. Several reports have shown that leptin regulates energy homeostasis by controlling peripheral lipid metabolism, and leptin administration reduces triacylglyceride (TAG) stores and promotes fatty acid (FA) oxidation in lean and adipose tissues (1-7). Therefore, it has been postulated that one of the roles of leptin is to reduce lipid accumulation in nonadipose tissues, preventing lipotoxicity (8).In the liver an acute iv leptin infusion decreases liver TAG secretion, increases hepatic FA oxidation and ketogenesis, and, as a result, decreases liver TAG levels (9). However, an acute intracerebroventricular (icv) leptin administration does not decrease liver TAG levels (9 -11). On the other hand, chronic icv leptin treatment decreases TAG content in liver and plasma compared with ad libitum fed controls (12), suggesting that leptin, acting at central level, plays an important role depleting TAG levels in this tissue.In WAT, the effect of leptin on lipid metabolism has not been fully characterized and remains controversial. Thus, adenovirus-induced hyperleptinemia increases the expression of enzymes of FA oxidation such as acyl-coenzyme A oxidase and carnitine palmitoyl transferase (CPT)-1, and depletes the TAG without a concomitant increase in the levels of circulating free FAs (3). This suggests that leptin favored intracellular FA oxidation in adipocytes. Furthermore, treatment of isolated rat adipocytes with leptin up-regulates the expression of acyl-coenzyme A oxidase, CPT-1, uncoupling protein 2, and peroxisome proliferator activated receptor (PPAR) ␣, all of which are involved in lipid oxidation (13). In addition, in vivo (14) and in vitro (15) studies indicated a paracrine/autocrine stimulation of lipolysis in rodent adi-

Renal PTHrP was rapidly and transiently increased in rats with folic acid-induced acute renal failure, featuring as an early response gene. In addition, changes in ACE and Ang II expression were also found in these animals. PTHrP induces a mitogenic response in folic acid-damaged renal tubular cells both in vivo and in vitro. Our results support the notion that PTHrP up-regulation participates in the regenerative process in this model of acute renal failure and is a common event associated with the mechanisms of renal injury and repair.

Leptin interacts with specific receptors in hypothalamic nuclei and modulates energy balance. Growing evidence has shown the association of obesity and hyperleptinaemia with non-insulin-dependent diabetes mellitus and insulin resistance. The aged Wistar rat shows peripheral insulin resistance in the absence of obesity and alterations of glucose homeostasis. However, it is not known whether, in these animals, the leptin action is altered. Here we studied the effect of ageing on plasma leptin concentration and the ability of hypothalamic nuclei to capture i.c.v.-injected digoxigenin-labelled leptin. Our data indicate that 24-month-old animals are hyperleptinaemic. However, daily food intake was greater in old animals, suggesting that they are leptin resistant. Leptin uptake in the hypothalamus was reduced in old rats. This uptake was a receptor-mediated process as demonstrated by displacement. Leptin accumulation in hypothalamic nuclei was partially colocalized with neuropeptide Y fibres. Immunohistochemical and western blot analyses showed a lower amount of the long form of leptin receptors in the hypothalamus of aged rats. Analysis by RT-PCR also demonstrated a decreased expression of leptin receptor mRNA in old animals. We conclude that the lower leptin uptake may be explained, at least in part, by a decreased amount of receptors in hypothalamic neurones of the aged rats.