Melanocortin receptors mediate leptin effects on feeding and body weight but not adipose apoptosis

The melanocortin (MC) system is a known downstream mediator of leptin signaling in the brain; thus, activation of MC receptors by melanotan II (MTII), a MC3/4 receptor agonist, was hypothesized to increase adipose apoptosis, a phenomenon seen after leptin treatments. To test this hypothesis, male Sprague-Dawley rats received pretreatments of intracerebroventricular injections of artificial cerebrospinal fluid (aCSF, 5 microl) or SHU9119 (1.0 nmol/5 microl), an MC3/4 receptor antagonist. One hour later, aCSF (5 microl), leptin (10 microg/5 microl), or MTII (0.1 nmol/5 microl) was injected intracerebroventricularly in the aCSF-pretreated groups, and either leptin (10 microg/5 microl) or MTII (0.1 nmol/5 microl) was injected intracerebroventricularly in SHU9119-pretreated groups. Each pair of treatments was given once daily for four successive days. Body weight (BW), food intake (FI), and body temperature (BT) were measured daily at 4- and 24-h intervals. SHU9119 completely prevented the decrease in FI and BW caused by either MTII or leptin. Muscle mass remained unchanged regardless of treatment, but both leptin and MTII significantly reduced mass of inguinal (iWAT), retroperitoneal (rWAT), and epididymal (eWAT) white adipose tissues (P<.05). SHU9119 prevented the decrease in mass of intrascapular brown fat, iWAT, and rWAT (P<.05). Leptin, but not MTII, increased DNA fragmentation in eWAT (P<.05), but SHU9119 pretreatment had no effect on leptin-induced apoptosis. Thus, although the MC receptors in the brain are involved in mediating actions of leptin on FI, fat mass, and BW, leptin-induced adipose apoptosis is regulated independently of MC receptors.     

Repeated fasting/refeeding elevates plasma leptin without increasing fat mass in rats

It is known that repeated fasting and refeeding increase capacity of fat storage in adipose tissue as an adaptive response to fasting. However, the amount of weight gain in fasted/refed animals falls behind the control level in most rodent studies. Leptin, an adipocyte-derived hormone that impacts on energy homeostasis, may be up-regulated by repeated cycles of fasting and refeeding. In this study, we investigated the adaptive response of leptin to repeated cycles of 1-day fasting and 1-day refeeding for 42 days in rats. The repeated fasting and refeeding (RFR) rats gained less body weight than the controls. Daily food intake of the RFR rats was decreased after Day 16 and remained suppressed. Circulating leptin levels of the RFR rats were significantly elevated at Day 35 compared with the controls and at Day 44 compared with the controls and pair-fed (PF) rats. Leptin mRNA levels of these rats were also significantly increased in retroperitoneal white adipose tissue (RT-WAT) compared with the controls and PF rats. Moreover, hypothalamic proopiomelanocortic (POMC) gene expression was augmented in the RFR rats compared with the controls and PF rats. However, there was no statistical difference in percent visceral fat mass among the experimental groups. Lipoprotein lipase (LPL) mRNA levels of RFR rats were significantly increased in RT-WAT compared with the controls and PF rats. These data indicated that leptin was up-regulated in response to chronic repeated fasting and refeeding cycles without a concomitant increase in adiposity, and the augmented leptin levels were associated with an increase in POMC gene expression, reduced food intake, and diminished body weight gain.     

Central melanocortin receptor agonist reduces spontaneous and scheduled meal size but does not augment duodenal preload-induced feeding inhibition

Central melanocortin (MC) receptor agonists inhibit food intake and may be downstream mediators of the effects of central leptin, which (1) reduces food intake by selectively decreasing meal size and (2) augments the feeding-inhibitory effects of gastrointestinal food stimuli. Central administration of the MC-3/4 receptor (MC-3/4R) agonist, MTII, inhibits feeding in rats, but its effects on meal pattern and potential interactions with gastrointestinal controls of food intake remain unclear. We examined meal patterns and intake in male Sprague-Dawley rats following central intracerebroventricular administration of MTII (0.01-1.0 nmol) in two situations: (1) during daytime 60-min scheduled access to liquid glucose (12.5%) in combination with a duodenal preload of 12.5% glucose or physiological saline (4.4 ml/10 min), and (2) during subsequent overnight access to 45 mg of solid chow pellets. Both duodenal glucose preloads and MTII reduced subsequent glucose intake. However, no dose of MTII augmented the reductions in food intake produced by duodenal glucose alone. During overnight access to pelleted chow, the 0.1- and 1.0-nmol doses of MTII reduced food intake, meal size, meal duration, and body weight, and increased the satiety ratio (duration of intermeal interval/preceding meal size) but did not change meal frequency. The present data (1) demonstrate that MTII, like leptin, reduces food intake by a selective reduction in meal size and not meal frequency, and (2) suggest that MTII increases the feeding-inhibitory potency of negative feedback signals critical to the control of meal size during spontaneous chow access, but not scheduled access to palatable liquid nutrient solutions.     

Feeding response to melanocortin agonist predicts preference for and obesity from a high-fat diet

Overconsumption and increased selection of high fat (HF) foods contribute to the development of common obesity. Because the hypothalamic melanocortin (MC) system plays an integral role in the regulation of food intake and dietary choice, we tested the hypothesis that proneness (-P) or resistance (-R) to dietary-induced obesity (DIO) may be due to differences in MC function. We found that prior to developing obesity and while still maintained on chow, acute, central administration of MTII, an MC agonist, produced a greater anorectic response in DIO-P rats than in DIO-R rats. However, after only 5 days of exclusive HF feeding, the DIO-R rats had significantly greater suppression of intake after MTII treatment than they did when maintained on chow. In addition, the DIO-P rats were much less responsive to MTII treatment than the DIO-R rats after only 5 days of the HF diet. In fact, MTII-induced anorexia during HF feeding correlated negatively with body weight gained on the HF diet. These results suggest that the voluntary decrease of HF feeding in DIO-R rats may be mediated by increased endogenous MC signaling, a signal likely compromised in DIO-P rats. Differences in MC regulation may also explain the observed preference for HF over a lower fat food choice in DIO-P rats. Finally, the results indicate that responses to exogenous MC challenge can be used to predict proneness or resistance to DIO.     

Neuronal histamine decreases fat accumulation and up-regulates UCP family in db/db obese mice

Neuronal histamine decreases fat accumulation and up-regulates UCP family in db/db obese mice     

Effects of long-term ingestion of aspartame on hypothalamic neuropeptide Y, plasma leptin and body weight gain and composition

The aim of this study was to determine the effects of the chronic ingestion of aspartame (ASP) on brain neuropeptide Y (NPY) concentrations, plasma hormones, food intake and body fat. Two groups of male Long-Evans rats, fed on a control (C) well-balanced diet, had to drink either a 0.1% ASP solution or water for a period of 14 weeks starting at weaning. Food intake and body weight were weekly recorded. At the end of the experiment, fat pads were sampled, leptin and insulin were measured in the plasma and NPY in several microdissected brain areas. Substituting ASP for water led to lower body weight (-8%; P<.004) and lower fat depot weight (-20%; P<.01) with no differences in energy intake or plasma insulin concentrations. Plasma leptin was significantly reduced by 34% (P<.05). Leptin concentrations were well-correlated with final body weight (r=.47; P<.025) and fat pad mass (r=.53; P<.01). NPY concentrations were 23% lower (P<.03) in the arcuate nucleus of ASP rats with no differences in other brain areas. The beneficial effects on body composition could be related to the decreased effects of NPY on lipid and energy metabolism, independently of insulin. The reasons for the NPY decrease (regulatory or toxicological) are not obvious. The constitutive amino acids of the ASP molecule might participate in the NPY regulation.     

β3-Adrenergic regulation of leptin, food intake, and adiposity is impaired with age

With age, there are increases in adiposity, leptin mRNA in white adipose tissue (WAT), and serum leptin levels in rats. Beta3-adrenergic agonists are anti-obesity agents in rodents, and activation of beta3-adrenergic receptors (beta3AR) mediates an acute decrease in food consumption, increased thermogenesis in brown adipose tissue (BAT), increased lipolysis in WAT, and suppression of leptin gene expression and serum leptin levels. Because beta3AR signal transduction is impaired with age in BAT and WAT, beta3AR-mediated regulation of leptin, feeding behavior, and adiposity may also be impaired with age. To test this hypothesis, we infused young (6 month) and old (24 month) F344 x BN rats with the beta3AR agonist CL316,243 (1 mg kg(-1) day(-1)) using osmotic minipumps for 7 days. Food intake, body mass, adiposity, and serum leptin, as well as leptin, uncoupling protein 1 (UCP1), and lipoprotein lipase (LPL) mRNA in BAT or WAT were determined. In young rats, CL316,243 infusion reduced body mass and adiposity, suppressed serum leptin and leptin mRNA levels in WAT, induced UCP1 in WAT, and increased UCP1 and LPL mRNA levels in BAT. Moreover, treatment with CL316,243 was associated with a marked but transient suppression of food intake. Most of the responses elicited in the old rats with CL316,243 treatment were qualitatively similar to those in the young rats, but blunted in magnitude. Beta3AR activation of adenylyl cyclase was also reduced in the aged rats. These data suggest that, although CL316,243 is an effective agent in aged rats, the maximum responses are reduced, suggesting that the impaired beta3AR signal transduction with age is a major determining factor in the reduced effectiveness of CL316,243 in older rats.     

Aged-obese rats exhibit robust responses to a melanocortin agonist and antagonist despite leptin resistance

To address whether defective melanocortin activation is one element of leptin resistance with age, we infused centrally the melanocortin agonist, MTII and antagonist, SHU9119 in young and old rats. Food intake, energy expenditure, adiposity, BAT UCP1, and leptin expression in white fat as well as hypothalamic expressions of MC3R, MC4R, POMC, AgRP and NPY were assessed. The MTII-evoked anorexia was transient whereas the SHU9119-induced hyperphagia was sustained in young and old. MTII elevated oxygen consumption in both ages. The oxygen consumption waned gradually in young but increased continuously in aged following MTII infusion. The MTII-mediated induction in BAT UCP1 was similarly robust in both ages as was the SHU9119-mediated suppression in UCP1. POMC and MC3/4 receptor expressions were unaltered with age. These findings demonstrate the effectiveness of MTII to bypass leptin resistance in aged-obese rats. The equally strong orexigenic response to SHU9119 coupled with unaltered POMC expression and food intake in the young versus old suggest that melanocortin tone is unchanged with age despite impaired melanocortin activation by leptin.     

MSG intake suppresses weight gain, fat deposition, and plasma leptin levels in male Sprague-Dawley rats

Monosodium l-glutamate (MSG), an umami taste substance, may be a key molecule coupled to a food intake signaling pathway, possibly mediated through a specific l-glutamate (GLU) sensing mechanism in the gastrointestinal tract. Here we investigated the effect of the spontaneous ingestion of a 1% MSG solution and water on food intake and body weight in male Sprague-Dawley rats fed diets of varying caloric density, fat and carbohydrate contents. Fat mass and lean mass in the abdomen, blood pressure, and several blood metabolic markers were also measured. Rats given free access to MSG and water showed a high preference (93-97%) for the MSG solution, regardless of the diet they consumed. Rats ingesting MSG had a significantly smaller weight gain, reduced abdominal fat mass, and lower plasma leptin levels, compared to rats ingesting water alone. Naso-anal length, lean mass, food and energy intakes, blood pressure, blood glucose, and plasma levels of insulin, triglyceride, total cholesterol, albumin, and GLU were not influenced by the ingestion of the MSG solution. These same effects were observed in a study of adult rats. Together, these results suggest that MSG ingestion reduces weight gain, body fat mass, and plasma leptin levels. Moreover, these changes are likely to be mediated by increased energy expenditure, not reduced energy intake or delayed development. Conceivably, these effects of MSG might be mediated via gut GLU receptors functionally linked to afferent branches of the vagus nerve in the gut, or the afferent sensory nerves in the oral cavity.     

Energy metabolism and expression of uncoupling proteins 1, 2, and 3 after 21 days of recovery from intracerebroventricular mouse leptin in rats

Animals tend to maintain a lower body weight for an extended period after leptin administration has ended. This may be due to an enhancement of metabolic rate that persists after treatment withdrawal. Our objectives were to determine the period of leptin influence, when injected intracerebroventricularly (icv), on food intake, body weight, and energy expenditure. Additionally, the relationship between expressions of UCP1, UCP2, and UCP3 in different adipose tissues and heat production (HP) was assessed. Twenty-four adult male Sprague-Dawley rats were injected intracerebroventricularly with either 10 g mouse leptin or 10 l vehicle once per day for 4 days. At 24 h after the last injection, one group was killed while the other was placed in calorimetry chambers and monitored for 21 days of recovery. Leptin-injected rats exhibited an overshoot of food intake and respiratory quotient (RQ) during recovery, but body weight remained significantly lower up to 6 days. HP decreased in both groups over time but remained higher in the leptin group through recovery. However, retained energy (RE) was significantly greater than control for about 8 days. Overall, UCP expression was reduced at the end of recovery in parallel with the decline in HP. Brown adipose tissue (BAT) was the most responsive to leptin administration by dramatically changing UCP1 and UCP3 mRNA levels. Our data show that leptin has extended effects on energy expenditure but relieves control on food intake and RQ after treatment withdrawal. This translated into a reduced positive energy balance that slowed body weight recovery.     

Ovarian modulation of lipoprotein lipase activity in white adipose tissue of ground squirrels

Golden-mantled ground squirrels were ovariectomized (OVX) or Sham-OVX during the weight gain phase of the circannual body weight cycle. Other squirrels, OVX or Sham-OVX during the weight loss phase, were subcutaneously implanted with estradiol-filled or empty Silastic capsules. The mass of several fat pads as well as adipose tissue lipoprotein lipase (LPL) activity were determined at autopsy. Ovariectomy at either phase of the annual cycle was without effect on body weight. However, LPL activity of the parametrial fat pad was increased in OVX as compared to Sham-OVX squirrels. Fourteen days of estradiol treatment during the weight loss phase decreased the mass and LPL activity of the retroperitoneal fat depot but did not affect these parameters in perirenal adipose tissue. Although estradiol exerts different or opposite effects on body mass and food intake of rats and ground squirrels, ovariectomy and estrogen treatment affect LPL activity in a similar fashion in both species.     

The HPA axis modulates the CNS melanocortin control of liver triacylglyceride metabolism

The central melanocortin system regulates lipid metabolism in peripheral tissues such as white adipose tissue. Alterations in the activity of sympathetic nerves connecting hypothalamic cells expressing melanocortin 3/4 receptors (MC3/4R) with white adipocytes have been shown to partly mediate these effects. Interestingly, hypothalamic neurons producing corticotropin-releasing hormone and thyrotropin-releasing hormone co-express MC4R. Therefore we hypothesized that regulation of hypothalamo-pituitary adrenal (HPA) and hypothalamo-pituitary thyroid (HPT) axes activity by the central melanocortin system could contribute to its control of peripheral lipid metabolism. To test this hypothesis, we chronically infused rats intracerebroventricularly (i.c.v.) either with an MC3/4R antagonist (SHU9119), an MC3/4R agonist (MTII) or saline. Rats had been previously adrenalectomized (ADX) and supplemented daily with 1mg/kg corticosterone (s.c.), thyroidectomized (TDX) and supplemented daily with 10 μg/kgL-thyroxin (s.c.), or sham operated (SO). Blockade of MC3/4R signaling with SHU9119 increased food intake and body mass, irrespective of gland surgery. The increase in body mass was accompanied by higher epididymal white adipose tissue (eWAT) weight and higher mRNA content of lipogenic enzymes in eWAT. SHU9119 infusion increased triglyceride content in the liver of SO and TDX rats, but not in those of ADX rats. Concomitantly, mRNA expression of lipogenic enzymes in liver was increased in SO and TDX, but not in ADX rats. We conclude that the HPA and HPT axes do not play an essential role in mediating central melanocortinergic effects on white adipose tissue and liver lipid metabolism. However, while basal hepatic lipid metabolism does not depend on a functional HPA axis, the induction of hepatic lipogenesis due to central melanocortin system blockade does require a functional HPA axis.     

A carbohydrate diet rich in sucrose increased insulin and WAT in macronutrient self-selecting rats

In order to evaluate the influence of a carbohydrate (CHO) diet rich in sucrose (37%) on food choice and body composition, Wistar rats received a food selection diet (protein, CHO, fat) from the time of weaning to 13 weeks of age. Three groups of animals were examined: the first received a CHO diet containing 37% sucrose; the second, a diet containing only 10% sucrose; and the third, control group, received a complete standard diet (14% protein, 72% CHO including 10% sucrose). Food intakes and body weight (BW) were recorded four times a week. No differences in total food intake were observed between the two self-selecting groups and the control group, and no differences were observed between the two self-selecting groups in terms of their protein intake (about 45% of the total calorie intake). The latter groups modified their selections during the 10-week period, but the variations were similar. BW gain in the 37% group was lower but the white adipose tissue (WAT)/total BW ratio was significantly higher than those seen in the control and 10% groups. Insulinemia was higher in 37% and control groups. In conclusion, the high preferences for protein and fat were identical, whatever the CHO diet composition. The sucrose level in the diet was an essential factor for the development of hyperinsulinemia, leptin resistance and thus a higher prevalence of obesity. These results confirm the importance of the quality of CHO sources in the diet.     

Norepinephrine turnover in brown and white adipose tissue after partial lipectomy

Total body fat is restored after the surgical removal (i.e., partial lipectomy) of white adipose tissue (WAT), and this is accomplished via increases in the mass of nonexcised WAT pads. The underlying mechanism for this apparent regulation of total body fat is unknown. One possibility is via the sympathetic nervous system (SNS) innervation of WAT and brown adipose tissue (BAT) through the regulation of lipolysis and thermogenesis, respectively. Specifically, decreases in SNS activity might fuel lipectomy-induced body fat compensation through energy saved from decreased BAT thermogenesis and would promote lipid accretion through decreased WAT basal lipolysis. Therefore, we tested whether lipectomy triggered decreases in the SNS drive [as indicated by the norepinephrine turnover (NETO)] to nonexcised WAT or to BAT, at times before the lipectomy-induced fat pad mass compensation was complete. Siberian hamsters received either sham or bilateral epididymal WAT lipectomy, and NETO was measured in the remaining WAT and interscapular BAT (IBAT) before, and 3 and 6 weeks after surgery. Total dissected WAT, and inguinal and retroperitoneal WAT masses were significantly increased following lipectomy, whereas dorsal subcutaneous WAT and IBAT masses, as well as food intake, were unchanged. The only significant change in NETO was a marked decrease (approximately 90%) in IBAT NETO at Week 3 postlipectomy compared with the sham-lipectomized controls. These findings suggest that the lipid accretion of nonexcised WAT pads triggered by lipectomy may be partially fueled by decreased BAT thermogenesis, inasmuch as decreased IBAT NETO reflects decreased BAT heat production.     

TRH decreases food intake and increases water intake and body temperature in rats

Thyrotropin-releasing hormone (TRH) is a key regulator of the hypothalamus-pituitary-thyroid axis, which plays an important role in energy homeostasis and is involved in the regulation of feeding behavior. In the present study, we investigated the effects of acute and chronic TRH treatment on water intake, body temperature and feeding behavior in rats. TRH (0, 4, 16 and 64 mg/kg) was injected subcutaneously twice a day (06:00 and 18:00 h) in rats fed ad libitum. TRH decreased food and water intake in the first few hours (P < .05). There was a small reduction in food intake over the 24-h period, but body weight was not affected (P < .05). When TRH was injected at a dose of 32 mg/kg twice a day (06:00 and 18:00 h) for 5 days, T(3) and T(4) concentrations were increased (P < .05). TRH increased body temperature for 2 h after injection. Water intake was markedly increased (P < .05), but there was no effect on food intake or body weight. These results show that whereas a single injection of TRH decreases short-term food and water intake in rats, repeated daily treatments stimulate water intake but not food intake, and, thus, the increase in water consumption is mediated independently of food intake under these conditions.     

Metabolic responses to intracerebroventricular leptin and restricted feeding

Leptin is a hormone secreted by adipocytes, which plays an important role in the control of food intake and metabolic processes. In the current study, a dose-dependent relationship was shown between a bolus intracerebroventricular rat recombinant leptin administration and reductions in food intake and body weight in Sprague-Dawley rats. During the 24 h postinjection period, food intake was decreased by 24, 26, and 52% with 0.625, 2.5, and 10 microg of leptin, respectively. Body weight was reduced by 2, 3, and 5% at 24 h after leptin administration at the doses of 0.156, 2.5, and 10 microg, respectively. Furthermore, indirect calorimetry demonstrated that five daily i.c.v. injections of leptin resulted in an increase in heat production per unit of metabolic body size and fat oxidation by approximately 10 and 48%, respectively. In contrast, food-restricted rats that consumed the equivalent amount of food as leptin-treated rats for 5 days decreased their energy expenditure by 10%. Food restriction was found to decrease respiratory quotient in a similar pattern as the leptin administration. When ad lib feeding was resumed, food-restricted rats quickly recovered their normal food intakes, body weights, and metabolism. Conversely, leptin treatment has prolonged effects on body weight resulting from different metabolic responses than food restriction. Leptin not only suppresses food intake, but also enhances energy expenditure to reduce fat depots.     

Intraperitoneal leptin modifies macronutrient choice in self-selecting rats

This study aimed to evaluate the consequences on food intake and body weight (BW) of leptin administration in rats receiving a choice between the three macronutrients. Two studies were performed: during the first, rats received an acute intraperitoneal (IP) leptin administration (1 mg/kg) twice (at 8 and 14 weeks of age), at the beginning of the nocturnal cycle, while during the second, they received a chronic leptin infusion (osmotic minipump, 7 days). The total 24-h food intake after acute leptin injections was reduced by 14% and 17%, respectively. Body weight gain (BWG) after leptin injections was about half that seen on control days. Chronic leptin infusion reduced total intake, affecting mainly protein (P). Fat intake increased slightly since day 2 and became significant on the fourth day. After the leptin infusion, carbohydrate (CHO) eaters (>35% carbohydrate/total energy) significantly reduced the carbohydrate proportion in their total energy intake. There was no difference concerning macronutrient selection by fat eaters (Hfat). Leptin infusion reduced the number of mixed meals on the first day. In addition, the thermogenesis of brown adipose tissue (BAT) was higher in leptin than in control (C) rats. Consequently, leptin injections reduced food intake and BWG and increased thermogenesis, thus acting on the two terms of the energy balance. Moreover, leptin has different effects on macronutrient preferences, dependent upon age (tests 1 and 2) and the type (acute or chronic) of injection. High leptinemia level related to age or to minipump infusion lead to leptin resistance as found in old or obese subjects. It could explain our results.     

Regulation of body weight in humans

The mechanisms involved in body weight regulation in humans include genetic, physiological, and behavioral factors. Stability of body weight and body composition requires that energy intake matches energy expenditure and that nutrient balance is achieved. Human obesity is usually associated with high rates of energy expenditure. In adult individuals, protein and carbohydrate stores vary relatively little, whereas adipose tissue mass may change markedly. A feedback regulatory loop with three distinct steps has been recently identified in rodents: 1) a sensor that monitors the size of adipose tissue mass is represented by the amount of leptin synthesized by adipose cells (a protein encoded by the ob gene) which determines the plasma leptin levels; 2) hypothalamic centers, with specific leptin receptors, which receive and integrate the intensity of the signal; and 3) effector systems that influence the two determinants of energy balance, i.e., energy intake and energy expenditure. With the exception of a few very rare cases, the majority of obese human subjects have high plasma leptin levels that are related to the size of their adipose tissue mass. However, the expected regulatory responses (reduction in food intake and increase in energy expenditure) are not observed in obese individuals. Thus obese humans are resistant to the effect of endogenous leptin, despite unaltered hypothalamic leptin receptors. Whether defects in the leptin signaling cascade play a role in the development of human obesity is a field of great actual interest that needs further research. Present evidences suggest that genetic and environmental factors influence eating behavior of people prone to obesity and that diets that are high in fat or energy dense undermine body weight regulation by promoting an overconsumption of energy relative to need.     

Effects of intracerebroventricularly administered leptin on protein selection in the rat

The effect of centrally administered rat leptin on selection of 5 and 30% protein diets was investigated in male Sprague-Dawley rats with indwelling i.c.v. cannulas. Leptin (0 vs 2.5 microg/day) was administered for 4 consecutive days, followed by an 8-day withdrawal period. Total intake was reduced to approximately 50% of that in the vehicle injected group during each day following leptin administration. Intake of both the 5 and 30% diets was reduced. Vehicle-treated rats selected a 13-15% CP diet. Diet selection in leptin-treated rats was not different during the first day, but on Days 2-4, leptin-treated rats selected a 10% CP diet. Intake began to normalize within 24-48 h after the last treatment, and was not different by Day 3 of the withdrawal period. Body weight was reduced by leptin treatment, and despite the normalization of food intake, did not recover during the withdrawal period. Rats were sacrificed at the end of the 8-day withdrawal period. Despite the reduction in body and carcass weights, liver, kidney, heart, and soleus muscle weights were not different between control and leptin-treated groups when expressed on an absolute or relative basis. However, epididymal and retroperitoneal fat pad weights were still reduced 56 and 78%, respectively, in rats that had been previously treated with leptin for 4 days and then not treated for 8 days. In addition, circulating T3 levels remained elevated in rats that had been treated with leptin. Centrally administered leptin has little effect on muscle mass, but had potent effects on intake of nonobese rats and a sustained effect on adipose tissue mass, thyroid hormone status, and body weight after withdrawal. Results from rats selecting between diets varying in protein content suggest that leptin may cause avoidance of protein.     

Hypothalamic implants of dilute estradiol fail to reduce feeding in ovariectomized rats

To investigate further the site where estradiol (E(2)) inhibits food intake, we tested the effects on feeding of subcutaneous and intrahypothalamic implants of 10% E(2) benzoate in cholesterol (CHOL) or CHOL alone. E(2) was implanted subcutaneously in Silastic tubes, and intrahypothalamically via bilateral 29-gauge cannulas into the paraventricular nucleus (PVN) or the medial preoptic area (MPA) of ovariectomized (OVX) Sprague-Dawley and Long-Evans rats. Three-day implant periods followed 3-day baseline periods. Rats were allowed ad libitum access to chow and tap water, and food intake and body weight were measured each day. Subcutaneous 10% E(2) implants in Sprague-Dawley rats reduced food intake 21% on Day 2 and 34% on Day 3 (P's<.01) and decreased 3-day body weight gain 11 g (P<.05). In contrast, 10% E(2) implants in the PVN of Sprague-Dawley rats did not change food intake or body weight. Implants of 10% or 20% E(2) in the MPA also failed to decrease food intake. MPA implants of 10% E(2) decreased body weight gain 8 g (P<.05), but MPA implants of 20% E(2) decreased weight gain only 4 g (P>.05). To determine whether the strain of rat affected our negative results on food intake, we implanted 10% E(2) into the PVN of Long-Evans rats. Again, PVN E(2) did not decrease food intake significantly in comparison to the pretest baseline. PVN E(2) did, however, decrease body weight gain 5 g and decreased food intake 6% compared to rats with implants of CHOL (both P<.05), but these effects appeared to be due to an increase in feeding in the CHOL group in comparison to their baseline. Finally, CHOL and E(2) implants did not impair the responsivity of the PVN because acute implants of norepinephrine (NE) into the PVN of E(2)- or CHOL-treated Long-Evans rats significantly increased food intake. Our results do not support the hypothesis that E(2)'s actions in either the PVN or the MPA are sufficient to account for its inhibitory effects on feeding.