Neuroanatomical and electrophysiological studies have shown that hypothalamic POMC neurons are targets of the adipostatic hormone leptin. However, the physiological relevance of leptin signaling in these neurons has not yet been directly tested. Here, using the Cre/loxP system, we critically test the functional importance of leptin action on POMC neurons by deleting leptin receptors specifically from these cells in mice. Mice lacking leptin signaling in POMC neurons are mildly obese, hyperleptinemic, and have altered expression of hypothalamic neuropeptides. In summary, leptin receptors on POMC neurons are required but not solely responsible for leptin's regulation of body weight homeostasis.

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The effects of diet and adiposity have been implicated in disturbances of female reproductive function. In an effort to better elucidate the relationship between obesity and female fertility, we analyzed the effect of increasing dietary fat content on body composition, insulin sensitivity, and pregnancy rates in two common inbred mouse strains, DBA/2J and C57BL/6J. After 16 wk, females of both strains on the high fat diet developed glucose intolerance and insulin resistance, but only the female DBA/2J mice developed dietary-induced obesity and hyperleptinemia. The high fat diet was associated with more than a 60% decrease in natural pregnancy rates of female DBA/2J mice, whereas the fertility of female C57BL/6J mice was unaffected. Despite developing a similar degree of obesity, insulin resistance, and hyperleptinemia, male DBA/2J mice did not manifest diminished fertility. Obese female DBA/2J mice achieved normal ovulatory responses and pregnancy rates after exogenous gonadotropin stimulation, suggesting their fertility defect to be central in origin. Real-time PCR quantification of hypothalamic cDNA revealed a 100% up-regulation of neuropeptide Y and a 50% suppression of GnRH expression accompanied by a 95% attenuation of leptin receptor type B expression in obese female DBA/2J mice. These findings suggest that obesity-associated hyperleptinemia, and not insulin resistance or increased dietary fat per se, gradually induces central leptin resistance, increases hypothalamic neuropeptide Y-ergic tone, and ultimately causes hypothalamic hypogonadism. The data establish high fat-fed female DBA/2J mice as a wild-type murine model of obesity-related infertility.

alpha-Melanocyte-stimulating hormone (alpha-MSH) is a hypothalamic neuropeptide proposed to play a key role in energy homeostasis. To investigate the behavioral, metabolic, and hypothalamic responses to chronic central alpha-MSH administration, alpha-MSH was infused continuously into the third cerebral ventricle of rats for 6 days. Chronic alpha-MSH infusion reduced cumulative food intake by 10.7% (P < 0.05 vs. saline) and body weight by 4.3% (P < 0.01 vs. saline), which in turn lowered plasma insulin levels by 29.3% (P < 0.05 vs. saline). However, alpha-MSH did not cause adipose-specific wasting nor did it alter hypothalamic neuropeptide mRNA levels. Central alpha-MSH infusion acutely activated neurons in forebrain areas such as the hypothalamic paraventricular nucleus, as measured by a 254% increase in c-Fos-like immunoreactivity (P < 0.01 vs. saline), as well as satiety pathways in the hindbrain. Our findings suggest that, although an increase of central melanocortin receptor signaling acutely reduces food intake and body weight, its anorectic potency wanes during chronic infusion and causes only a modest decrease of body weight.

Although central leptin signaling appears to play a major role in the regulation of food intake and energy metabolism, the physiological role of peripheral leptin signaling and its relative contribution to whole-body energy metabolism remain unclear. To address this question, we created a mouse model (Cre-Tam mice) with an intact leptin receptor in the brain but a near-complete deletion of the signaling domain of leptin receptor in liver, adipose tissue, and small intestine using a tamoxifen (Tam)-inducible Cre-LoxP system. Cre-Tam mice developed marked hyperleptinemia (ϳ4-fold; P < 0.01) associated with 2.3-fold increase (P < 0.05) in posttranscriptional production of leptin. Whereas this is consistent with the disruption of a negative feedback regulation of leptin production in adipose tissue, there were no discernable changes in energy balance, thermoregulation, and insulin sensitivity. Hypothalamic levels of phosphorylated signal transducer and activator of transcription 3, neuropeptide expression, and food intake were not changed despite hyperleptinemia. The percentage of plasma-bound leptin was markedly increased (90.1-96 vs. 41.8 -74.7%; P < 0.05), but plasma-free leptin concentrations remained unaltered in Cre-Tam mice. We conclude from these results that 1) the relative contribution to whole-body energy metabolism from peripheral leptin signaling is insignificant in vivo, 2) leptin signaling in adipocyte constitutes a distinct short-loop negative feedback regulation of leptin production that is independent of tissue metabolic status, and 3) perturbation of peripheral leptin signaling alone, although increasing leptin production, may not be sufficient to alter the effective plasma levels of leptin because of the counter-regulatory increase in the level of leptin binding protein(s). (Endocrinology 148: 3987-3997, 2007) L EPTIN IS PRODUCED predominantly in adipocytes and plays a pivotal role in regulating food intake, energy expenditure, and glucose homeostasis (1-4). The effects of leptin on energy balance and metabolism are mediated by its cognate receptor (LEPR), which is a member of the class I cytokine receptor superfamily (5). Several splicing variants of LEPR, including the membrane-bound isoforms (Ra to Rd) and a soluble isoform (Re), have been identified (5-7). Rb, the only isoform capable of signaling via the Janus kinases and signal transducer and activator of transcription (STAT) pathway, is responsible for mediating the metabolic effects of leptin (6,8). LEPR is expressed in many tissues, and the Rb isoform is expressed at high levels in the hypothalamus and other regions of the brain and at substantially lower levels in many peripheral tissues, including adipose tissues, skeletal muscle, adrenal glands, pancreatic islets, liver, kidney, lymph nodes, and gonads (5,6,8).LEPR in the central nervous system (CNS), through a complex neuroendocrine network and autonomic nervous system that are yet to be fully delineated, appears to play a dominant role over that in peripheral tissues in mediating th...

Body weight regulation is mediated through several major signaling pathways, some of which have been delineated by positional cloning of spontaneous genetic mutations in mice. Lepr(db/db) mice are obese due to a defect in the signaling portion of the leptin receptor, which has led to extensive study of this highly conserved system over the past several years. We have created an allelic series at Lepr for the further examination of LEPR signaling phenotypes using both the FLP /frt and CRE /loxP systems. By inserting a frt-PGK-neo-frt sequence in Lepr intron 16, we have generated a conditional gene repair Lepr allele ( Lepr-neo) that elicits morbid obesity, diabetes, and infertility in homozygous mice, recapitulating the obesity syndrome of Lepr(db/db) mice. Thus, in vivo excision of the PGK-neo cassette with a FLP recombinase transgene restores the lean and fertile phenotype to Lepr(flox/flox) mice. In the same construct, we have also inserted loxP sites that flank Lepr coding exon 17, a region that encodes a JAK docking site required for STAT3 signaling. CRE-mediated excision of Lepr coding exon 17 from Lepr with a frameshift in subsequent exons results in a syndrome of obesity, diabetes, and infertility in LeprDelta17/Delta17 mice, which is indistinguishable from Lepr(neo/neo) and Lepr(db/db) mice. We conclude that suppression of Lepr gene expression by PGK-neo is phenotypically equivalent to deletion of the Lepr signaling motifs, and therefore the Lepr(neo/neo) mouse may be used to investigate conditional gene repair of Lepr signaling deficiency.

Leptin administration potentiates the satiety response to signals such as cholecystokinin (CCK), that are released from the gut during a meal. To investigate the physiological relevance of this observation, we hypothesized that leptin deficiency, induced by fasting, attenuates the satiety response to CCK. To test this hypothesis, 48-h-fasted or fed rats were injected with i.p. saline or CCK. Fasting blunted the satiety response to 3.0 microg/kg CCK, such that 30-min food intake was suppressed by 65.1% (relative to saline-treated controls) in fasted rats vs. 85.9% in the fed state (P < 0.05). In a subsequent experiment, rats were divided into three groups: 1) vehicle/fed; 2) vehicle/fasted; and 3) leptin-replaced/fasted; and each group received 3.0 microg/kg i.p. CCK. As expected, the satiety response to CCK was attenuated by fasting in vehicle-treated rats (30-min food intake: vehicle/fed, 0.3 +/- 0.1 g; vehicle/fasted, 1.7 +/- 0.4 g; P < 0.01), and this effect was prevented by leptin replacement (0.7 +/- 0.2 g, P < 0.05 vs. vehicle/fasted; P = not significant vs. vehicle/fed). To investigate whether elevated neuropeptide Y (NPY) signaling plays a role in the effect of leptin deficiency to impair the response to CCK, we measured the response to 3.0 microg/kg i.p. CCK after treatment with 7.5 microg intracerebroventricular NPY. We found that both CCK-induced satiety and its ability to increase c-Fos-like-immunoreactivity in key brainstem-feeding centers were attenuated by NPY pretreatment. We conclude that an attenuated response to meal-related satiety signals is triggered by leptin deficiency and may contribute to increased food intake.

In the field of obesity research, two separate lines of study have emerged which explore the mechanism by which food intake is regulated: short-term control of food intake, and the central regulation of energy balance. The former studies the satiety response during consumption of meals, whereby satiety signalling originating in the gut is transduced into a neural signal that modulates satiety pathways in the brainstem. This review describes a neuroanatomically based model in which leptin and insulin signalling in the hypothalamus governs long-term regulation of energy balance via mechanisms that are integrated with satiety hormone signalling in the brainstem. The functional outcome of this integration is a cumulative meal-to-meal regulation of food intake, that over relatively long intervals serves to maintain stable adipose stores. Our model provides a context within which continued investigation of neuroendocrine mechanisms that control food intake and body weight can be explored, and has potential application to our current understanding of clinical obesity and its treatment.