Andrew G. Swick

Leptin is a potent stimulator of bone growth in ob/ob mice

Leptin, the product of the obese gene, is a circulating hormone secreted primarily from adipocytes. The lack of leptin in ob/ob mice, who are homozygous for the obese gene, results in hyperglycemia, hyperinsulinemia, hyperphagia, obesity, infertility, decreased brain size and decreased stature. To this end, we investigated the role of leptin as a hormonal regulator of bone growth. Leptin administration led to a significant increase in femoral length, total body bone area, bone mineral content and bone density in ob/ob mice as compared to vehicle treated controls. The increase in total body bone mass was a result of an increase in both trabecular and cortical bone mass. These results suggest that the decreased stature of the ob/ob mouse is due to a developmental defect that is readily reversible upon leptin administration. Our demonstration that the signalling or long form (Ob-Rb) of the leptin receptor is present in both primary adult osteoblasts and chondrocytes suggests that the growth promoting effects of leptin could be direct. In summary, these results indicate a novel role for leptin in skeletal bone growth and development.

Leptin alters metabolic rates before acquisition of its anorectic effect in developing neonatal mice

Leptin inhibits food intake and increases metabolic rates in adult mice. Neonatal mice need to maximize food intake and also maintain high thermoregulatory metabolic rates to optimize survival, suggesting that leptin may function differentially in neonatal versus adult animals. The efficacy of exogenous leptin to alter these two physiological functions during development was thus examined in C57BL/6J lean (+/+ or ob/+) and ob/ ob(leptin-deficient) mice. Intraperitoneal leptin administration (1 mg/kg body wt) to lean and ob/ obpups from 7 to 10 days of age did not affect milk intake, oxygen consumption, body weight, or epididymal fat pad weights. Intracerebroventricular injection of 1 μg leptin to 9-day-old pups also failed to influence milk intake or oxygen consumption. Because neither lean nor ob/ obpups responded to exogenous leptin, high endogenous plasma leptin concentrations per se in these lean mice do not explain their resistance to leptin. Leptin administered intracerebroventricularly also failed to alter milk/food intakes of 17-day-old pups but markedly increased oxygen consumption of these older mice. By 28 days of age, intracerebroventricular leptin inhibited food intake. The well-defined actions of leptin to reduce food intake and enhance metabolic rates do not develop synchronously. The ability of leptin to accelerate metabolic rates is acquired early in life and independent of its anorectic action, which may promote survival of neonates.Leptin inhibits food intake and increases metabolic rates in adult mice. Neonatal mice need to maximize food intake and also maintain high thermoregulatory metabolic rates to optimize survival, suggesting that leptin may function differentially in neonatal versus adult animals. The efficacy of exogenous leptin to alter these two physiological functions during development was thus examined in C57BL/6J lean (+/+ or ob/+) and ob/ob (leptin-deficient) mice. Intraperitoneal leptin administration (1 mg/kg body wt) to lean and ob/ob pups from 7 to 10 days of age did not affect milk intake, oxygen consumption, body weight, or epididymal fat pad weights. Intracerebroventricular injection of 1 microg leptin to 9-day-old pups also failed to influence milk intake or oxygen consumption. Because neither lean nor ob/ob pups responded to exogenous leptin, high endogenous plasma leptin concentrations per se in these lean mice do not explain their resistance to leptin. Leptin administered intracerebroventricularly also failed to alter milk/food intakes of 17-day-old pups but markedly increased oxygen consumption of these older mice. By 28 days of age, intracerebroventricular leptin inhibited food intake. The well-defined actions of leptin to reduce food intake and enhance metabolic rates do not develop synchronously. The ability of leptin to accelerate metabolic rates is acquired early in life and independent of its anorectic action, which may promote survival of neonates.

CP-809,101, a selective 5-HT2C agonist, shows activity in animal models of antipsychotic activity

CP-809,101 is a potent, functionally selective 5-HT(2C) agonist that displays approximately 100% efficacy in vitro. The aim of the present studies was to assess the efficacy of a selective 5-HT(2C) agonist in animal models predictive of antipsychotic-like efficacy and side-effect liability. Similar to currently available antipsychotic drugs, CP-809,101 dose-dependently inhibited conditioned avoidance responding (CAR, ED(50)=4.8 mg/kg, sc). The efficacy of CP-809,101 in CAR was completely antagonized by the concurrent administration of the 5-HT(2C) receptor antagonist, SB-224,282. CP-809,101 antagonized both PCP- and d-amphetamine-induced hyperactivity with ED(50) values of 2.4 and 2.9 mg/kg (sc), respectively and also reversed an apomorphine induced-deficit in prepulse inhibition. At doses up to 56 mg/kg, CP-809,101 did not produce catalepsy. Thus, the present results demonstrate that the 5-HT(2C) agonist, CP-809,101, has a pharmacological profile similar to that of the atypical antipsychotics with low extrapyramidal symptom liability. CP-809,101 was inactive in two animal models of antidepressant-like activity, the forced swim test and learned helplessness. However, CP-809,101 was active in novel object recognition, an animal model of cognitive function. These data suggest that 5-HT(2C) agonists may be a novel approach in the treatment of psychosis as well as for the improvement of cognitive dysfunction associated with schizophrenia.

Leptin constrains acetylcholine-induced insulin secretion from pancreatic islets of ob/ob mice.

Hypersecretion of insulin from the pancreas is among the earliest detectable metabolic alterations in some genetically obese animals including the ob/ob mouse and in some obesity-prone humans. Since the primary cause of obesity in the ob/ob mouse is a lack of leptin due to a mutation in the ob gene, we tested the hypothesis that leptin targets a regulatory pathway in pancreatic islets to prevent hypersecretion of insulin. Insulin secretion is regulated by changes in blood glucose, as well as by peptides from the gastrointestinal tract and neurotransmitters that activate the pancreatic islet adenylyl cyclase (e.g., glucagon-like peptide-1) and phospholipase C (PLC) (e.g., acetylcholine) signaling pathways to further potentiate glucose-induced insulin secretion. Effects of leptin on each of these regulatory pathways were thus examined. Leptin did not influence glucose or glucagon-like peptide-1-induced insulin secretion from islets of either ob/ob or lean mice, consistent with earlier findings that these regulatory pathways do not contribute to the early-onset hypersecretion of insulin from islets of ob/ob mice. However, leptin did constrain the enhanced PLC- mediated insulin secretion characteristic of islets from ob/ob mice, without influencing release from islets of lean mice. A specific enhancement in PLC-mediated insulin secretion is the earliest reported developmental alteration in insulin secretion from islets of ob/ob mice, and thus a logical target for leptin action. This action of leptin on PLC-mediated insulin secretion was dose-dependent, rapid-onset (i.e., within 3 min), and reversible. Leptin was equally effective in constraining the enhanced insulin release from islets of ob/ob mice caused by protein kinase C (PKC) activation, a downstream mediator of the PLC signal pathway. One function of leptin in control of body composition is thus to target a PKC-regulated component of the PLC-PKC signaling system within islets to prevent hypersecretion of insulin.

Discovery of N-benzyl-2-[(4S)-4-(1H-indol-3-ylmethyl)-5-oxo-1-phenyl-4,5-dihydro-6H-[1,2,4]triazolo[4,3-a][1,5]benzodiazepin-6-yl]-N-isopropylacetamide, an orally active, gut-selective CCK1 receptor agonist for the potential treatment of obesity.

We describe the design, synthesis, and structure-activity relationships of triazolobenzodiazepinone CCK1 receptor agonists. Analogs in this series demonstrate potent agonist activity as measured by in vitro and in vivo assays for CCK1 agonism. Our efforts resulted in the identification of compound 4a which significantly reduced food intake with minimal systemic exposure in rodents.

Leptin constrains phospholipase C-protein kinase C-induced insulin secretion via a phosphatidylinositol 3-kinase-dependent pathway.

Leptin-deficient Lepob/Lepob mice hypersecrete insulin in response to acetylcholine stimulation of the phospholipase C-protein kinase C (PLC-PKC) pathway, and leptin constrains this hypersecretion. Leptin has been reported to activate phosphatidylinositol 3-kinase (PI 3-K) and subsequently phosphodiesterase (PDE) to impair protein kinase A (PKA)-induced insulin secretion from cultured islets of neonatal rats. We determined if PKA-induced insulin secretion was also hyperresponsive in Islets from Lepob/Lepob mice, and if leptin impaired this pathway in islets from these mice. Additionally, the possible role for PI 3-K and PDE in leptin-induced control of acetylcholine-induced insulin secretion was examined. Stimulation of Insulin secretion with GLP-1, forskolin (an activator of adenylyl cyclase), or IBMX (an inhibitor of PDE) did not cause hypersecretion of insulin from islets of young Lepob/Lepob mice, and leptin did not inhibit GLP-1-induced insulin secretion from islets of these mice. Inhibition of PDE with IBMX also did not block leptin-induced inhibition of acetylcholine-mediated insulin secretion from islets of Lepob/Lepob mice. But, preincubation of islets with wortmannin, an Inhibitor of PI 3-K activity, blocked the ability of leptin to constrain acetylcholine-induced insulin secretion from islets of Lepob/Lepob mice. We conclude that the capacity of the PKA pathway to stimulate insulin secretion is not increased in islets from young Lepob/Lepob mice, and that leptin does not regulate this pathway in islets from mice. Leptin may stimulate PI 3-K to constrain PLC-PKC-induced insulin secretion from Islets of Lepob/Lepob mice.

Food intake in rhesus monkeys following central administration of orexins.

Orexin A has been reported to stimulate food intake in rats while orexin B does not. The purpose of this study was to determine the role of orexin A or orexin B administration on food intake in adult, male rhesus monkeys. Food intake was measured at 2 and 8 h after the morning feeding following central injections of vehicle, orexin A (10, or 20 microg) or orexin B (10, 30, or 100 microg). When compared to vehicle injections, the 10 and 20 microg doses of orexin A decreased food intake at 2 h post-dose by 45% and 64%, respectively. Eight-hour food intake was decreased at only the 20 microg orexin A dose. Orexin B at all doses and time points did not alter food intake when compared to vehicle. These results indicate that orexin A exhibits anorectic activity while orexin B does not affect food intake in the rhesus monkey.

Leptin potentiates the anti-obesity effects of rimonabant

We hypothesized that a combination of low doses of rimonabant and leptin would markedly reduce body weight through the modulation of neuronal activity within the hypothalamus. To this end, high fat diet-induced obese rats were randomized to receive either leptin (0.5mg/kg subcutaneously), rimonabant (3mg/kg), the combination of both, or vehicle, daily for a duration of 2 weeks. A subset of rats was pair-fed to the combination-treated animals and received either vehicle or leptin. At the end of the weight loss phase, leptin treatment was maintained for 7 days while rimonabant was discontinued to assess changes in body weight during the rebound phase. The combination of rimonabant and leptin resulted in a marked inhibition of food intake and a profound reduction in body weight that was greater than achieved with either leptin or rimonabant alone. Treatment with leptin during the rebound phase inhibited compensatory increases in body weight associated with restitution of ad libitum feeding in previously pair-fed rats. Moreover, leptin partially blunted the rebound in food intake and body weight associated with cessation of rimonabant therapy.To investigate the effect of the combination on neuronal firing in the rat hypothalamus, single unit activity was recorded from brain slices containing the ventromedial and arcuate nuclei. The combination of rimonabant and leptin synergistically increased and decreased neuronal firing in the ventromedial and arcuate nuclei, respectively. Overall, these data demonstrate profound anti-obesity effects of combining cannabinoid type 1 receptor antagonists and leptin.

Identification and characterization of a leptin-responsive neuroblastoma cell line

The adipocyte-derived hormone leptin plays a critical role in a variety of physiological and pathological actions. As such the determination of leptin signal transduction pathways are important both for understanding the molecular mechanisms of leptin action and for identifying sites for possible therapeutic intervention. Since the hypothalamus is the primary site of leptin action, we sought to identify a neuronal-derived human cell line containing the long form of the leptin receptor (OBRb). To this end, we screened several neuroblastoma cell lines and isolated a sub-line of SH-SY5Y cells, which we designated as SH-OBRb, for further studies. We characterized the transduction pathways induced by leptin in SH-OBRb cells and demonstrated that OBRb mediates tyrosine phosphorylation of STAT3, phosphorylation of ERK1/2, but not SAPK/JNK and p38 MAPK, in a dose and time dependent fashion. In addition, Akt appears to be phosphorylated in the basal state and to be insensitive to further activation by leptin. In summary, we have isolated a unique cell line that can be utilized as a model for use in the study of leptin action and molecular mechanisms.

Leptin acts peripherally to limit meal-induced increases in plasma insulin concentrations in mice: a brief communication.

Leptin inhibits food intake and lowers plasma insulin concentrations. This study was designed to determine whether leptin acts independent of food-intake regulation to affect meal-induced increases in plasma insulin concentrations. Leptin-deficient, Lepob/Lepob mice were administered 1 μg leptin intracerebroventricularly (ICV) or intraperitoneally. Food intake and plasma insulin concentrations of mice administered leptin ICV before a meal were lower, as expected, than were intakes and plasma insulin concentrations of mice administered vehicle ICV. However when food intake was controlled, meal-induced increases in plasma insulin were unaffected by ICV administration of leptin. Intraperitoneal administration of 1 μg leptin before a meal lowered meal-induced increases in plasma insulin concentrations without influencing the size of the meal. We conclude that plasma leptin concentrations can affect meal-induced insulin secretion independent of the central nervous system actions of leptin associated with food-intake regulation.