Leptin fails to reduce ethanol intake in Marchigian Sardinian alcohol-preferring rats

Leptin, a hormone secreted by the adipocytes and involved in feeding and energy balance control, has been proposed to modulate alcohol craving in mice and humans. This study evaluated whether leptin modulates alcohol intake in Marchigian Sardinian alcohol-preferring (msP) rats. Rats were offered 10% ethanol either 2h per day at the beginning of dark period of the 12:12h light/dark cycle, or 24h per day. Leptin was injected into the lateral ventricle (LV), the third ventricle (3V), or intraperitoneally (IP) once a day, 1h before the onset of the dark period. Neither acute nor chronic (9 days) leptin injections (1 or 8microg per rat) into the LV or 3V modified ethanol intake in male msP rats, offered ethanol 2h per day. Chronic LV injection of leptin (8 or 32 microg per rat in male rats and 8 or 16 microg per rat in female rats for 7 days), or chronic IP injections of leptin (1mg/kg in male rats for 5 days) failed to modify the intake of ethanol, offered 24h per day. Finally, chronic LV leptin injections (8 or 32 microg per rat for 12 days) did not modify ethanol intake in male msP rats, adapted to ad libitum access to ethanol and then tested after a 6-day period of ethanol deprivation. In contrast, in most of these conditions leptin significantly reduced food intake. These data do not support a role for leptin in alcohol intake, preference, or craving in msP rats.     

Combined stimulation of glucagon-like peptide-1 receptor and inhibition of cannabinoid CB1 receptor act synergistically to reduce food intake and body weight in the rat

Pharmacological activation of the glucagon-like peptide-1 (GLP-1) receptor and inhibition of the cannabinoid CB1 receptor were found to reduce food intake and body weight in humans and animals. Since earlier studies revealed that endocannabinoids may interact with other neurotransmitters to affect feeding behavior, we have examined whether a stable GLP-1 agonist, exendin-4 and a CB1 receptor antagonist, AM 251, may reciprocally enhance their inhibitory effects on food consumption in the rat. Additionally, we have tested whether the blockade of the GLP-1 receptor by exendin (9-39) modifies AM 251-dependent effects on energy balance. In a dose-response study, male Wistar rats were injected intraperitoneally with either 1.5-6.0 μg/kg exendin-4, 0.5-2 mg/kg AM 251, 80-320 μg/kg exendin (9-39) or their vehicle and the daily food and water intake as well as body weight changes were monitored two days before and two days after the injection. Exendin-4 at a dose of 3.0 and 6.0 μg/kg and AM 251 at a dose 2 mg/kg decreased significantly 24-hour food intake and body weight. Therefore, in the next study, the effects of lower doses of exendin-4 (1.5 μg/kg) and AM 251 (1.0 mg/kg) administered alone or together on food consumption were compared. As opposed to being injected alone, the co-administration of the two resulted in a marked decrease in both daily food intake and body weight. Exendin (9-39) did not modify the suppressory effect of the highest AM 251 dose on food consumption. Apparently, the effect of AM 251 on the appetite is not mediated by GLP-1. The concomitant stimulation of GLP-1 receptor and blockade of CB1 receptor, however, may act synergistically to inhibit appetite in the rat.     

Effects of maternal leptin treatment during lactation on the body weight and leptin resistance of adult offspring

We investigate whether leptin treatment to lactating rats affects food intake, body weight and leptin serum concentration and its anorectic effect on their adult offspring. Lactating rats were divided into 2 groups: Lep-single injected with recombinant rat leptin (8 microg/100 g of body weight, daily for the last 3 consecutive days of lactation) and control group (C) that received the same volume of saline. After weaning all pups had free access to the control diet, their body weight and food intake were monitored at each 4 days until 180 days of age, when they were tested for its food intake and response to either leptin (0.5 mg/kg body wt, ip) or saline vehicle. The offspring of the leptin-treated dams gained more weight and had higher food intake from day 37 onward (p<0.05), higher amount of retroperitoneal white adipose tissue (RPWAT) (37%, p<0.05) and higher leptin serum concentration (40%, p<0.05) at 180 days of age compared to control group. The food intake at 2, 4, 6 and 24 h was unaffected after acute injection of leptin in these animals, suggesting resistance to the anorectic effect of leptin. The maternal leptin treatment during lactation makes their adult offspring more susceptible to overweight with resistance to the anorectic effect of leptin.     

Interoceptive "satiety" signals produced by leptin and CCK

The present studies assessed the extent to which the adiposity signal leptin and the brain-gut hormone cholecystokinin (CCK), administered alone or in combination, give rise to interoceptive sensory cues like those that are produced by a low (1h) level of food deprivation. Rats were trained with cues arising from 1 to 24-h food deprivation as discriminative stimuli. For one group, 24-h food deprivation predicted the delivery of sucrose pellets, whereas 1-h food deprivation did not. Another group received the reversed deprivation level-sucrose contingency. After asymptotic performance was achieved, the effects of leptin and CCK on food intake and on discrimination performance were tested under 24-h food deprivation. In Experiment 1a, leptin administered into the third cerebroventricle (i3vt) at 3.5 or 7.0 microg doses had little effect, compared to saline on food intake or discriminative responding. In Experiment 1b, leptin (7.0 microg, i3vt) combined with CCK-8 (2 microg/kg, i.p.) reduced food intake significantly, but the findings indicated that CCK-8 alone produces interoceptive discriminative cues more like those produced by 1- than 24-h food deprivation. Experiment 2a tested rats with i.p. leptin (0.3 and 0.5mg/kg). Although neither dose suppressed intake, the 0.3mg/kg dose produced interoceptive cues like 1-h food deprivation. Experiment 2b tested two doses of CCK-8 (2 and 4 mg/kg, i.p.) and found significant intake suppression and generalization of discrimination with both doses of CCK-8. These findings suggest a role for both leptin and CCK in the production of sensory consequences that correspond to "satiety".     

Effects of leptin replacement alone and with exendin-4 on food intake and weight regain in weight-reduced diet-induced obese rats

Weight loss in obese humans produces a relative leptin deficiency, which is postulated to activate potent orexigenic and energy conservation mechanisms to restrict weight loss and promote weight regain. Here we determined whether leptin replacement alone or with GLP-1 receptor agonist exendin-4 attenuates weight regain or promotes greater weight loss in weight-reduced diet-induced obese (DIO) rats. Forty percent restriction in daily intake of a high-fat diet in DIO rats for 4 wk reduced body weight by 12%, body fat by 29%, and plasma leptin by 67% and normalized leptin sensitivity. When food restriction ended, body weight, body fat, and plasma leptin increased rapidly. Daily administration of leptin [3-h intraperitoneal (ip) infusions (4 nmol·kg(-1)·h(-1))] at onset and end of dark period for 3 wk did not attenuate hyperphagia and weight regain, nor did it affect mean daily meal sizes or meal numbers. Exendin-4 (50 pmol·kg(-1)·h(-1)) infusions during the same intervals prevented postrestriction hyperphagia and weight regain by normalizing meal size. Coadministration of leptin and exendin-4 did not reduce body weight more than exendin-4 alone. Instead, leptin began to attenuate the inhibitory effects of exendin-4 on food intake, meal size, and weight regain by the end of the second week of administration. Plasma leptin in rats receiving leptin was sevenfold greater than in rats receiving vehicle and 17-fold greater than in rats receiving exendin-4. Together, these results do not support the hypothesis that leptin replacement alone or with exendin-4 attenuates weight regain or promotes greater weight loss in weight-reduced DIO rats.     

Interaction of serotonin and cholecystokinin in the lateral parabrachial nucleus to control sodium intake

Serotonin [5-hydroxytryptamine (5-HT)] and CCK injected into the lateral parabrachial nucleus (LPBN) inhibit NaCl and water intake. In this study, we investigated interactions between 5-HT and CCK into the LPBN to control water and NaCl intake. Male Holtzman rats with cannulas implanted bilaterally in the LPBN were treated with furosemide + captopril to induce water and NaCl intake. Bilateral LPBN injections of high doses of the 5-HT antagonist methysergide (4 microg) or the CCK antagonist proglumide (50 microg), alone or combined, produced similar increases in water and 1.8% NaCl intake. Low doses of methysergide (0.5 microg) + proglumide (20 microg) produced greater increases in NaCl intake than when they were injected alone. The 5-HT(2a/2c) agonist 2,5-dimetoxy-4-iodoamphetamine hydrobromide (DOI; 5 microg) into the LPBN reduced water and NaCl intake. After proglumide (50 microg) + DOI treatment, the intake was not different from vehicle treatment. CCK-8 (1 microg) alone produced no effect. CCK-8 combined with methysergide (4 microg) reduced the effect of methysergide on NaCl intake. The data suggest that functional interactions between 5-HT and CCK in the LPBN may be important for exerting inhibitory control of NaCl intake.     

Effects of leptin and cholecystokinin in rats with a null mutation of the leptin receptor Lepr(fak)

The Koletsky ("corpulent) obese rat is homozygous for an autosomal recessive mutation of the leptin receptor (Lepr) that results in hyperphagia, obesity, and hyperlipidemia. Unlike the Lepr mutation that characterizes the fatty Zucker rat (Lepr(fa)), the Koletsky mutation (Lepr(fak)) is null. Because the Lepr(fak) mutation is null, exogenous leptin should have no effect on body weight or food intake in fa(k)/fa(k) rats. We confirmed that prediction: murine leptin, administered into the third ventricle for 5 consecutive days, did not affect daily food intake or body weight in fa(k)/fa(k) rats but produced dose-related inhibitions of food intake and body weight in +/+ and +/fa(k) rats. Although fa(k)/fa(k) rats did not respond to leptin, their response to CCK-8 (4 microg/kg ip) injected before 30-min test meals of 10% sucrose was not different from that of +/+ or +/fa(k) rats. These results demonstrate that the fa(k)/fa(k) rat is a good model in which to analyze the controls of food intake, energy expenditure, and energy storage in the absence of leptin effects.     

Peptides that regulate food intake: glucagon-like peptide 1-(7-36) amide acts at lateral and medial hypothalamic sites to suppress feeding in rats

Glucagon-like peptide 1-(7-36) amide (GLP-1) potently inhibits rat feeding behavior after central administration. Because third ventricular injection of GLP-1 appeared to be less effective than lateral ventricular injection, we have reexamined this issue. In addition, we attempted to identify brain regions other than the paraventricular nucleus of the hypothalamus that are sensitive toward GLP-1-induced feeding suppression. Finally, we examined the local role of endogenous GLP-1 by specific GLP-1 receptor blockade. After lateral ventricular injection, GLP-1 significantly inhibited food intake of 24-h-fasted rats in a dose-dependent fashion with a minimal effective dose of 1 microg. After third ventricular injection, GLP-1 (1 microg) was similarly effective in suppressing food intake, which extends previous findings. Intracerebral microinjections of GLP-1 significantly suppressed food intake in the lateral (LH), dorsomedial (DMH), and ventromedial hypothalamus (VMH), but not in the medial nucleus of the amygdala. The minimal effective dose of GLP-1 was 0.3 microg at LH sites and 1 microg at DMH or VMH sites. LH microinjections of exendin-(9-39) amide, a GLP-1 receptor antagonist, at 1 or 2.5 microg did not alter feeding behavior in 24-h-fasted rats. In satiated animals, however, a single LH injection of 1 microg exendin-(9-39) amide significantly augmented food intake, but only during the first 20 min (0.6 vs. 0.1 g). With three repeated injections of 2.5 microg exendin-(9-39) amide every 20 min, 1-h food intake was significantly increased by 300%. These data strongly support and extend the concept of GLP-1 as a physiological regulator of food intake in the hypothalamus.     

Leptin inhibits food-deprivation-induced increases in food intake and food hoarding

Food deprivation stimulates foraging and hoarding and to a much lesser extent, food intake in Siberian hamsters. Leptin, the anorexigenic hormone secreted primarily from adipocytes, may act in the periphery, the brain, or both to inhibit these ingestive behaviors. Therefore, we tested whether leptin given either intracerebroventricularly or intraperitoneally, would block food deprivation-induced increases in food hoarding, foraging, and intake in animals with differing foraging requirements. Hamsters were trained in a running wheel-based food delivery foraging system coupled with simulated burrow housing. We determined the effects of food deprivation and several peripheral doses of leptin on plasma leptin concentrations. Hamsters were then food deprived for 48 h and given leptin (0, 10, 40, or 80 microg ip), and additional hamsters were food deprived for 48 h and given leptin (0, 1.25, 2.5, or 5.0 microg icv). Foraging, food intake, and hoarding were measured postinjection. Food deprivation stimulated food hoarding to a greater degree and duration than food intake. In animals with a foraging requirement, intracerebroventricular leptin almost completely blocked food deprivation-induced increased food hoarding and intake, but increased foraging. Peripheral leptin treatment was most effective in a sedentary control group, completely inhibiting food deprivation-induced increased food hoarding and intake at the two highest doses, and did not affect foraging at any dose. Thus, the ability of leptin to inhibit food deprivation-induced increases in ingestive behaviors differs based on foraging effort (energy expenditure) and the route of administration of leptin administration.     

Gastroprotection and control of food intake by leptin. Comparison with cholecystokinin and prostaglandins.

Circulating peptide leptin which is the product of the ob gene is known to provide feedback information on the size of fat stores to central OB-receptors that control food intake. Recently, leptin messenger RNA and leptin protein have been detected in gastric epithelium and leptin was found to be released by CCK into circulation but the physiological role of this gastric leptin remains unknown. As CCK has been reported to protect gastric mucosa against various noxious agents, we designed the study to determine the influence of leptin and CCK on the gastroprotection and the control of food intake and to compare them with classic gastroprotective substance, prostaglandin E2, in rats with acute gastric mucosal lesions induced by topical application of 75% ethanol. Four series of Wistar rats (A, B, C and D) were used to determine; A) the effects of various doses of leptin (0.1-10 microg/kg) given intraperitoneally (i.p.) on ethanol-induced gastric lesions, gastric blood flow (GBF) and plasma levels of immunoreactive leptin; B) the effects of various doses of CCK-8 (0.1-10 microg/kg i.p.) on ethanol-induced gastric lesions, GBF and plasma levels of leptin; C) the effects of various doses of PGE2 (12.5--100 microg/kg) given intragastrically (i.g.) on ethanol-induced gastric lesions and GBF and D) the influence of leptin, CCK and PGE2 on the intake of liquid meal in rats. Rats were anesthetized with ether 1 h after i.g. administration of 75% ethanol to measure the GBF using H2-gas clearance technique and blood samples were withdrawn for the measurement of plasma leptin levels by radioimmunoassay (RIA). Food intake was assessed in separate group of rats fasted 18 h and then fed with liquid caloric meal. Leptin, CCK and PGE2 reduced dose-dependently gastric lesions induced by 75% ethanol, the dose reducing these lesions by 50% (ED50) being, respectively, 1 microg/kg, 5 microg/kg and 20 microg/kg. The protective effects of leptin, CCK-8 and PGE2 were accompanied by significant attenuation of the fall of the GBF caused by ethanol. Leptin and CCK reduced also dose-dependently the food intake while PGE2 was not effective. Leptin and CCK resulted a dose-dependent increment in the plasma leptin levels. We conclude that: 1) exogenous leptin and CCK, causing similar increments in plasma immunoreactive leptin levels, protect dose-dependently gastric mucosa against the damage provoked by 75% ethanol; 2) Leptin and CCK afford similar gastroprotective activity to that attained with PGE2 but unlike PGE2 were highly effective in the reduction in food intake and 3) the protective effects of leptin, CCK and PGE2 were accompanied by significant increase of GBF suggesting that the protection afforded by these substances are mediated, at least in part, by gastric hyperemia.     

CCK inhibits the orexigenic effect of peripheral ghrelin

CCK and ghrelin exert antagonistic effects on ingestive behavior. The aim of the present study was to investigate the interaction between ghrelin and CCK administered peripherally on food intake and neuronal activity in specific hypothalamic and brain stem nuclei, as assessed by c-Fos-like immunoreactivity (c-FLI) in nonfasted rats. Ghrelin (13 microg/kg body wt) injected intraperitoneally significantly increased the cumulative food intake when measured at 30 min and 1 h after injection, compared with the vehicle group (2.9 +/- 1.0 g/kg body wt vs. 1.2 +/- 0.5 g/kg body wt, P < 0.028). Sulfated CCK octapeptide (CCK-8S) (2 or 25 microg/kg body wt) injected simultaneously blocked the orexigenic effect of ghrelin (0.22 +/- 0.13 g/kg body wt, P < 0.001 and 0.33 +/- 0.23 g/kg body wt, P < 0.0008), while injected alone, both doses of CCK-8S exerted a nonsignificant trend to reduce food intake. Ghrelin (13 microg/kg body wt ip) markedly increased the number of c-FLI-positive neurons per section in the arcuate nucleus (ARC) compared with vehicle (median: 31.35 vs. 9.86, P < 0.0001). CCK-8S (2 or 25 microg/kg body wt ip) had no effect on neuronal activity in the ARC, as assessed by c-FLI (median: 5.33 and 11.21 cells per section), but blocked the ghrelin-induced increase of c-fos expression in this area when both peptides were administered simultaneously (median: 13.33 and 12.86 cells per section, respectively). Ghrelin at this dose had no effect on CCK-induced stimulation of c-fos expression in the paraventricular nucleus of the hypothalamus and the nucleus of the solitary tract. These results suggest that CCK abolishes ghrelin-induced food intake through dampening increased ARC neuronal activity.     

Role of leptin in the control of postprandial pancreatic enzyme secretion.

Leptin released by adipocytes has been implicated in the control of food intake but recent detection of specific leptin receptors in the pancreas suggests that this peptide may also play some role in the modulation of pancreatic function. This study was undertaken to examine the effect of exogenous leptin on pancreatic enzyme secretion in vitro using isolated pancreatic acini, or in vivo in conscious rats with chronic pancreatic fistulae. Leptin plasma level was measured by radioimmunoassay following leptin administration to the animals. Intraperitoneal (i.p.) administration of leptin (0.1, 1, 5, 10, 20 or 50 microg/kg), failed to affect significantly basal secretion of pancreatic protein, but markedly reduced that stimulated by feeding. The strongest inhibition has been observed at dose of 10 microg/kg of leptin. Under basal conditions plasma leptin level averaged about 0.15 +/- 0.04 ng/ml and was increased by feeding up to 1.8 +/- 0.4 ng/ml. Administration of leptin dose-dependently augmented this plasma leptin level, reaching about 0.65 +/- 0.04 ng/ml at dose of 10 microg/kg of leptin. This dose of leptin completely abolished increase of pancreatic protein output produced by ordinary feeding, sham feeding or by diversion of pancreatic juice to the exterior. Leptin (10(-10)-10(-7) M) also dose-dependently attenuated caerulein-induced amylase release from isolated pancreatic acini, whereas basal enzyme secretion was unaffected. We conclude that leptin could take a part in the inhibition of postprandial pancreatic secretion and this effect could be related, at least in part, to the direct action of this peptide on pancreatic acini.     

Leptin restores plasma cholesterol, glucose and weight loss induced by IFNalpha treatment

Leptin, an adipokine, a major regulator of food intake, was recently suggested to play a role in immune response. We previously showed that weight reduction following IFNalpha therapy is due, at least in part, to direct induction of adipose tissue apoptosis. We now studied the effect of leptin on IFNalpha treated adipocytes in vitro and in vivo. Diet induced obese C57/B6 mice were treated continually with recombinant (r) IFNalphaA/D + leptin (100 U/g body weight + 10 microg/day, respectably) or leptin (10 microg/day) alone for 8 days. Co-administration of IFNalphaA/D + leptin significantly reduced plasma cholesterol (P<0.001), glucose (P<0.007) and pro-apoptotic protein levels (P<0.05). Additionally, co-administration prevented loss of body weight due to adipocyte apoptosis. Thus, leptin co-administration with IFNalphaA/D decreases some of the side effects of IFNalpha administration such as weight loss, cholesterol and glucose levels.     

Interaction of apolipoprotein AIV with cholecystokinin on the control of food intake

Apolipoprotein AIV (apo AIV) and cholecystokinin (CCK) are peptides that act both peripherally and centrally to reduce food intake by decreasing meal size. The present study examined the effects of intraperitoneally administered bolus doses of recombinant apo AIV, CCK-8, and a combination of subthreshold doses of apo AIV and CCK on 4-h food intake in rats that were fasted overnight. Apo AIV at 100 microg/kg reduced food intake significantly relative to the saline control for 1 h, as did doses of CCK-8 at or above 0.125 microg/kg. Doses of apo AIV (50 microg/kg) or CCK (0.06 microg/kg) alone had no effect on food intake. However, when these subthreshold doses of apo AIV and CCK were administered together, the combination produced a significant inhibition of food intake relative to saline controls (P < 0.001), and the duration of the effect was longer than that caused by the administration of either apo AIV or CCK alone. The satiation effect produced by CCK-8 + apo AIV was attenuated by lorglumide, a CCK1 receptor antagonist. We conclude that, whereas the intraperitoneal administration of doses of either recombinant apo AIV or CCK at or above threshold levels reduces food intake, the coadministration of subthreshold doses of the two peptides is highly satiating and works via CCK1 receptor.     

Estradiol treatment increases CCK-induced c-Fos expression in the brains of ovariectomized rats

The ovarian hormone estradiol reduces meal size and food intake in female rats, at least in part by increasing the satiating potency of CCK. Here we used c-Fos immunohistochemistry to determine whether estradiol increases CCK-induced neuronal activation in several brain regions implicated in the control of feeding. Because the adiposity signals leptin and insulin appear to control feeding in part by increasing the satiating potency of CCK, we also examined whether increased adiposity after ovariectomy influences estradiol's effects on CCK-induced c-Fos expression. Ovariectomized rats were injected subcutaneously with 10 microg 17beta-estradiol benzoate (estradiol) or vehicle once each on Monday and Tuesday for 1 wk (experiment 1) or for 5 wk (experiment 2). Two days after the final injection of estradiol or vehicle, rats were injected intraperitoneally with 4 microg/kg CCK in 1 ml/kg 0.9 M NaCl or with vehicle alone. Rats were perfused 60 min later, and brain tissue was collected and processed for c-Fos immunoreactivity. CCK induced c-Fos expression in the nucleus of the solitary tract (NTS), area postrema (AP), paraventricular nucleus of the hypothalamus (PVN), and central nucleus of the amygdala (CeA) in vehicle- and estradiol-treated ovariectomized rats. Estradiol treatment further increased this response in the caudal, subpostremal, and intermediate NTS, the PVN, and the CeA, but not in the rostral NTS or AP. This action of estradiol was very similar in rats tested before (experiment 1) and after (experiment 2) significant body weight gain, suggesting that adiposity does not modulate CCK-induced c-Fos expression or interact with estradiol's ability to modulate CCK-induced c-Fos expression. These findings suggest that estradiol inhibits meal size and food intake by increasing the central processing of the vagal CCK satiation signal.     

Effects of central or peripheral leptin administration on norepinephrine turnover in defined fat depots

Leptin preserves lean tissue but decreases adipose tissue by increasing lipolysis and/or inhibiting lipogenesis. The sympathetic nervous system (SNS) is a primary regulator of lipolysis, but it is not known if leptin increases norepinephrine turnover (NETO) in white adipose tissue. In this study, we examined the effect of leptin administered either as a chronic physiological dose (40 microg/day for 4 days from ip miniosmotic pumps) or as an acute injection in the third ventricle (1.5 microg injected two times daily for 2 days) on NETO and the size of brown and white fat depots in male Sprague Dawley rats. NETO was determined from the decline in tissue norepinephrine (NE) during 4 h following administration of the NE synthesis inhibitor alpha-methyl-para-tryrosine. The centrally injected leptin-treated animals demonstrated more dramatic reductions in food intake, body weight, and fat pad size and an increase in NETO compared with the peripherally infused animals. Neither route of leptin administration caused a uniform increase in NETO across all fat pads tested, and in both treatment conditions leptin decreased the size of certain fat pads independent of an increase in NETO. Similar discrepancies in white fat NETO were found for rats pair fed to leptin-treated animals. These results demonstrate that leptin acting either centrally or peripherally selectively increases sympathetic outflow to white fat depots and that a leptin-induced change in fat pad weight does not require an increase in NETO.     

Peptide S is a novel potent inhibitor of voluntary and fast-induced food intake in rats

Peptide S (NPS or PEPS) and its cognate receptor have been recently identified both in the central nervous system and in the periphery. NPS/PEPS promotes arousal and has potent anxiolytic-like effects when it is injected centrally in mice. In the present experiment, we tested by different approaches its central effects on feeding behaviour in Long-Evans rats. PEPS at doses of 1 and 10 microg injected in the lateral brain ventricle strongly inhibited by more than 50% chow intake in overnight fasted rats with effects of longer duration with the highest dose (P<0.0001). A similar decrease was observed for the spontaneous intake of a high-energy palatable diet (-48%; P<0.0001). This anorexigenic effect was comparable to that induced by corticotropin-releasing hormone in fasted rats at equimolar doses. However, peptide S did not modify food intake stimulated by neuropeptide Y (NPY) at equimolar doses. It also did not affect the fasting concentrations of important modulators of food intake like leptin, ghrelin, and insulin in circulation. This study therefore showed that peptide S is a new potent anorexigenic agent when centrally injected. Its inhibitory action appears to be independent of the NPY, ghrelin, and leptin pathways. Development of peptide S agonists could constitute a new approach for the treatment of obesity.     

Feeding and temperature responses to intravenous leptin infusion are differential predictors of obesity in rats

Obesity is frequently associated with leptin resistance. The present study investigated whether leptin resistance in rats is present before obesity develops, and thus could underlie obesity induced by 16 wk exposure to a liquid, palatable, high-energy diet (HED). Before HED exposure, male Wistar rats (weighing between 330 and 360 g) received intravenous infusions of 20 microg leptin 2 h before dark (approximately 57 microg/kg rat). Relative to saline infusion, this caused a highly variable effect on food intake (ranging between -94 and +129%), with food intake suppression that appeared negatively correlated with HED-induced increases in body weight gain, caloric intake, adiposity, and plasma leptin levels. In contrast, leptin's thermogenic response was positively correlated to body weight gain linked to weights of viscera, but not to adiposity. Before HED exposure, leptin unexpectedly increased food intake in some rats (fi+, n = 8), whereas others displayed the normal reduction in food intake (fi-, n = 7). HED-exposed fi+ rats had higher plasma leptin levels, retroperitoneal fat pad weight, HED intake, and body weight gain than fi- and chow-fed rats. These parameters were also higher in HED-exposed fi-rats relative to chow rats, except for plasma leptin concentrations. It is concluded that leptin's reduced efficacy to suppress food intake could predict obesity on an HED. An unexpected orexigenic effect of leptin might potentially contribute to this as well.     

Acute and chronic leptin effect upon in vivo and in vitro rat thyroid iodide uptake

Leptin has stimulatory effects on the hypothalamic-pituitary-thyroid axis and on deiodinases activities. Here, we evaluated the effect of leptin injection upon in vivo and in vitro thyroid 125I uptake (RAIU). We designed two experiments: acute leptin (LepA) with a single dose of leptin (8 microg/100 g BW/sc), and chronic leptin (LepC), injected with the same dose of LepA, once a day, for 6 days. In parallel, control groups were saline-injected. For in vivo study, part of the animals were injected with 125I (3700 Bq) and killed after 15 or 120 min. In vivo thyroid RAIU was not changed in LepA animals. However, LepC animals showed higher in vivo thyroid RAIU (15 min:+130% and 120 min:+72%; p<0.05). For in vitro study, the other animals were killed and their thyroids were incubated with 125I. Thyroids explants from LepA and LepC groups presented lower thyroid 125I content (-32% and -29% p<0.05, respectively). The amount of our data suggest that, in vitro, leptin causes a direct inhibition of the rat thyroid RAIU, but in vivo, the effect of leptin was different according to the treatment period, which indicates that other indirect mechanisms are involved in the in vivo leptin chronic stimulation of the thyroid gland.     

Estrogen and leptin have differential effects on FSH and LH release in female rats

Prior experiments have shown that the adipocyte hormone leptin can advance puberty in mice. We hypothesized that it would also stimulate gonadotrophin secretion in adults. Since the secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) is drastically affected by estrogen, we hypothesized that leptin might have different actions dependent on the dose of estrogen. Consequently in these experiments, we tested the effect of injection of leptin into the third cerebral ventricle of ovariectomized animals injected with either the oil diluent, 10 microg or 50 microg of estradiol benzoate 72 hr prior to the experiment. The animals were ovariectomized 3-4 weeks prior to implantation of a cannula into the third ventricle 1 week before the experiments. The day after implantation of an external jugular catheter, blood samples (0. 3 ml) were collected just before and every 10 min for 2 hr after 3V injection of 5 microl of diluent or 10 microg of leptin. Both doses of estradiol benzoate equally decreased plasma LH concentrations and pulse amplitude, but there was a graded decrease in pulse frequency. In contrast, only the 50-microg dose of estradiol benzoate significantly decreased mean plasma FSH concentrations without significantly changing other parameters of FSH release. The number of LH pulses alone and pulses of both hormones together decreased as the dose of estrogen was increased, whereas the number of pulses of FSH alone significantly increased with the higher dose of estradiol benzoate, demonstrating differential control of LH and FSH secretion by estrogen, consistent with alterations in release of luteinizing hormone releasing hormone (LHRH) and the putative FSH-releasing factor (FSHRF), respectively. The effects of intraventricularly injected leptin were drastically altered by increasing doses of estradiol benzoate. There was no significant effect of intraventricular injection of leptin (10 microg) on the various parameters of either FSH or LH secretion in ovariectomized, oil-injected rats, whereas in those injected with 10 microg of estradiol benzoate there was an increase in the first hr in mean plasma concentration, area under the curve, pulse amplitude, and maximum increase of LH above the starting value (Deltamax) on comparison with the results in the diluent-injected animals in which there was no alteration of these parameters during the 2 hr following injection. The pattern of FSH release was opposite to that of LH and had a different time-course. In the diluent-injected animals, probably because of the stress of injection and frequent blood sampling, there was an initial significant decline in plasma FSH at 20 min after injection, followed by a progressive increase with a significant elevation above the control values at 110 and 120 min. In the leptin-injected animals, mean plasma FSH was nearly constant during the entire experiment, coupled with a significant decrease below values in diluent-injected rats, beginning at 30 min after injection and progressing to a maximal difference at 120 min. Area under the curve, pulse amplitude, and Deltamax of FSH was also decreased in the second hour compared to values in diluent-injected rats. In contrast to the stimulatory effects of intraventricular injection of leptin on pulsatile LH release manifest during the first hour after injection, there was a diametrically opposite, delayed significant decrease in pulsatile FSH release. This differential effect of leptin on FSH and LH release was consistent with differential effects of leptin on LHRH and FSHRF release. Finally, the higher dose of E2 (50 microg) suppressed release of both FSH and LH, but there was little effect of leptin under these conditions, the only effect being a slight (P < 0.04) increase in pulse amplitude of LH in this group of rats. The results indicate that the central effects of leptin on gonadotropin release are strongly dependent on plasma estradiol levels. These effects are consistent w