Postnatal leptin surge and regulation of circadian rhythm of leptin by feeding. Implications for energy homeostasis and neuroendocrine function

Leptin responsiveness and gene dosage for leptin receptor mutation (fa) in newborn rats

To determine the degree to which the leptin receptor mutation (fa) influences the responsiveness to leptin during the first postnatal week, we injected recombinant leptin (600 pmol. g-1. day-1 sc from day 1 to day 7) into wild-type (+/+), heterozygous (+/fa), and fatty (fa/fa) rat pups. Growth and final body fat content of these leptin-treated pups were compared with those of saline-treated littermates of the same genotype. The body mass of the leptin-treated +/+ pups, but not that of the +/fa and fa/fa pups, increased more slowly than that of their respective controls, and fat content at day 7 was reduced by 37% in +/+ pups, by 22% in +/fa pups, but not at all in fa/fa pups. Plasma leptin remained excessively high throughout the day under this treatment, but a 30-fold lower leptin dose, causing only moderate changes of plasma leptin, still reduced the body fat of +/+ pups significantly. We conclude that leptin participates in the control of even the earliest stages of fat deposition and that the response to supraphysiological doses of leptin is markedly reduced in 1-wk-old pups with one fa allele and absent in pups with two fa alleles.

Physiological perspectives on leptin as a regulator of reproduction: role in timing puberty

How nutrition regulates reproductive activity remains a major unsolved question of reproductive biology. Reducing the level of nutrition during adulthood can lead to infertility, primarily through reduction of GnRH secretion. Inquiry about such a mechanism has its roots in the search for cues timing the onset of fertility, because the tempo of sexual maturation is much more closely associated with body growth than with chronological age. Growth depends on the quantity and quality of food intake. When food availability is low, small, short-lived species with high metabolism and reduced growth may not even attain puberty before they die. In longer-lived species, puberty is delayed for months or even years until more food becomes available. To appreciate fully how the pubertal progression is timed will require understanding how peripheral signals relating information about energy metabolism are sensed by the brain and how such information is routed through pathways controlling GnRH secretion. Here, we provide some background and physiologic perspective on the question of whether the fat-derived hormone leptin is the unique peripheral signal, is an important signal, is but one of a constellation of signals, or is not a signal timing puberty.

Leptin's actions on the reproductive axis: perspectives and mechanisms

Energy availability influences reproductive fitness. The activity of the reproductive axis is sensitive to the adequacy of nutrition and the stores of metabolic reserves. The adipocyte-derived hormone leptin is postulated to reflect the state of nutrition and energy reserves and serve as a metabolic gate to the reproductive system. Genetically obese ob/ob mice (lacking endogenous leptin) are infertile, and treatment of these animals with exogenous leptin stimulates the activity of the reproductive endocrine system and induces fertility in both sexes. Severely food-restricted animals have reduced circulating levels of leptin, which are associated with markedly reduced secretion of the gonadotropins, LH, and FSH. Treatment of food-restricted mice, rats, sheep, and monkeys with exogenous leptin reverses the diet-induced inhibition of gonadotropin secretion. Leptin has also been suggested to have a role in timing the onset of puberty in several species, although evidence that leptin is the primary metabolic signal for initiating the onset of puberty in any species is controversial. Notwithstanding this debate, it is undisputed for all species studied to date that adequate levels of leptin in the circulation are essential (but not sufficient) for pubertal progression and that leptin treatment can reverse the delay in sexual maturation caused by food restriction. Double-label in situ hybridization studies in the brain of the mouse, rat, and monkey have revealed that hypothalamic neurons expressing proopiomelanocortin and neuropeptide Y coexpress the leptin receptor, whereas no evidence has been adduced that GnRH neurons express this receptor. Together, these observations suggest that leptin is a metabolic signal to the neuroendocrine reproductive system and that under conditions of inadequate energy reserves, low leptin levels act as a metabolic "gate" to inhibit the activity of the neuroendocrine reproductive axis in both sexes.

Mice lacking melanin-concentrating hormone are hypophagic and lean

Feeding is influenced by hypothalamic neuropeptides that promote (orexigenic peptides) or inhibit feeding. Of these, neuropeptide Y (NPY) in the arcuate nucleus and melanin-concentrating hormone (MCH) and orexins/hypocretins in the lateral hypothalamus have received attention because their expression is increased during fasting and because they promote feeding when administered centrally. Surprisingly, absence of the orexigenic neuropeptide NPY fails to alter feeding or body weight in normal mice. As deficiency of a single component of the pathway that limits food intake (such as leptin or receptors for melanocortin-4) causes obesity, it has been suggested that orexigenic signals are more redundant than those limiting food intake. To define further the physiological role of MCH and to test the redundancy of orexigenic signals, we generated mice carrying a targeted deletion of the MCH gene. MCH-deficient mice have reduced body weight and leanness due to hypophagia (reduced feeding) and an inappropriately increased metabolic rate, despite their reduced amounts of both leptin and arcuate nucleus pro-opiomelanocortin messenger RNA. Our results show that MCH is a critical regulator of feeding and energy balance which acts downstream of leptin and the melanocortin system, and that deletion of a gene encoding a single orexigenic peptide can result in leanness.

Unraveling the central nervous system pathways underlying responses to leptin

Here we summarize recent progress in the biology of leptin, concentrating on its central nervous system (CNS) actions. The product of the ob gene, leptin is a circulating hormone produced by white adipose tissue that has potent effects on feeding behavior, thermogenesis and neuroendocrine responses. Leptin regulates energy homeostasis, as its absence in rodents and humans causes severe obesity. We consider the physiological mechanisms underlying leptin action, along with several novel hypothalamic neuropeptides that affect food intake and body weight. The molecular causes of several other obesity syndromes are discussed to illuminate how the CNS regulates body weight. We describe neural circuits that are downstream of leptin receptors and propose a model linking populations of leptin-responsive neurons with effector neurons underlying leptin's endocrine, autonomic and behavioral effects.

Increased fat intake during lactation modifies hypothalamic-pituitary-adrenal responsiveness in developing rat pups: a possible role for leptin

High fat feeding reportedly enhances hypothalamus-pituitary-adrenal (HPA) responses to stress in adult rats. The present study tested whether elevated fat intake during suckling could have short and/or long lasting consequences on HPA regulation in the offspring. Mothers were fed either a control (C; 5% fat) or high fat (HF; 20% fat) diet during the last week of gestation and throughout lactation. After weaning (day 21), pups from C and HF mothers were fed a chow diet. Offspring from both C- and HF-fed mothers were tested for ACTH and corticosterone responses to stress on postnatal days 10 and 35. We found that HF feeding produced higher lipid levels in the milk of HF compared with C lactating rat dams and that offspring of these mothers had significantly increased retroperitoneal fat pad weight and relative adipose mass on day 21 as well as elevated plasma leptin levels on days 10 and 21 of age. After weaning, pups from the HF mothers had lower plasma leptin levels than those from C mothers. Maternal dietary fat affected HPA responsiveness in the offspring in an age-related manner. Neonatal pups (day 10) from the HF mothers exhibited a reduction in the ACTH and corticosterone responses to ether stress. However, in 35-day-old offspring from HF-fed dams, stress-induced ACTH secretion was increased compared with that in pups from the C-fed mothers. These results demonstrate that maternal diet and increased fat intake through the milk are important regulators of HPA responsiveness in neonates and prepubertal rats. During neonatal life, the blunted stress responsiveness seen with elevated fat intake and the resulting high leptin levels might protect the pups from excessive HPA activation. After removal of the maternal dietary influence and reduced leptin levels, enhanced ACTH stress responses are observed as in adult rats fed a HF diet. Because of the inverse relationship between plasma levels of leptin and HPA responses in pups, the possibility exists that the effects of the HF diet on stress responsiveness are mediated by changes in leptin exposure during development.

Hypothalamic ventromedial nuclei amplify circadian rhythms: do they contain a food-entrained endogenous oscillator?

Several endogenous oscillators determine circadian rhythms. One, light-entrained, is in the suprachiasmatic nuclei (SCN), the others, food-entrained, are in unknown sites. To determine how the hypothalamic ventromedial nuclei (VMN) and feeding affect rhythms, we compared nocturnally active rats fed either ad libitum or for 2 hr/d during light [restricted feeding (RF)] and either with or without colchicine-induced disruption of VMN. We measured rhythms in temperature, locomotor activity, feeding, drinking, corticosterone, and the numbers of cells expressing c-Fos in light/dark in hypothalamic nuclei, the suprachiasmatic nuclei, and two major SCN targets, the subparaventricular zone (sPVNz) and paraventricular thalamus (pvTHAL). c-Fos cells were always light > dark in SCN, whereas the VMN and sPVNz lacked light/dark differences except after RF and RF plus VMN disruption, respectively. Controls fed ad libitum had high-amplitude rhythms and, generally, c-Fos cells dark > light. In RF controls, a c-Fos pattern dark > light occurred in VMN; generally, c-Fos cell numbers increased elsewhere maintaining dark > light. By contrast, levels of corticosterone peaked before food. In rats fed ad libitum, VMN with colchicine markedly reduced rhythm amplitudes, not phase. c-Fos patterns were abolished except in pvTHAL and SCN. In RF, VMN disruption blocked corticosterone and light/dark c-Fos patterns in all nuclei but produced a pattern in the sPVNz like SCN. We conclude that VMN amplify rhythmic output from the SCN, and the RF-induced rhythm in VMN enhances c-Fos activity driven by the SCN. The VMN may contain a food-entrained oscillator, and the sPVNz may integrate output from several oscillators.

Hypothalamic ventromedial nuclei amplify circadian rhythms: do they contain a food-entrained endogenous oscillator?

Several endogenous oscillators determine circadian rhythms. One, light-entrained, is in the suprachiasmatic nuclei (SCN), the others, food-entrained, are in unknown sites. To determine how the hypothalamic ventromedial nuclei (VMN) and feeding affect rhythms, we compared nocturnally active rats fed either ad libitum or for 2 hr/d during light [restricted feeding (RF)] and either with or without colchicine-induced disruption of VMN. We measured rhythms in temperature, locomotor activity, feeding, drinking, corticosterone, and the numbers of cells expressing c-Fos in light/dark in hypothalamic nuclei, the suprachiasmatic nuclei, and two major SCN targets, the subparaventricular zone (sPVNz) and paraventricular thalamus (pvTHAL). c-Fos cells were always light > dark in SCN, whereas the VMN and sPVNz lacked light/dark differences except after RF and RF plus VMN disruption, respectively. Controls fed ad libitum had high-amplitude rhythms and, generally, c-Fos cells dark > light. In RF controls, a c-Fos pattern dark > light occurred in VMN; generally, c-Fos cell numbers increased elsewhere maintaining dark > light. By contrast, levels of corticosterone peaked before food. In rats fed ad libitum, VMN with colchicine markedly reduced rhythm amplitudes, not phase. c-Fos patterns were abolished except in pvTHAL and SCN. In RF, VMN disruption blocked corticosterone and light/dark c-Fos patterns in all nuclei but produced a pattern in the sPVNz like SCN. We conclude that VMN amplify rhythmic output from the SCN, and the RF-induced rhythm in VMN enhances c-Fos activity driven by the SCN. The VMN may contain a food-entrained oscillator, and the sPVNz may integrate output from several oscillators.