Ghrelin

BACKGROUND

The gastrointestinal peptide hormone ghrelin was discovered in 1999 as the endogenous ligand of the growth hormone secretagogue receptor. Increasing evidence supports more complicated and nuanced roles for the hormone, which go beyond the regulation of systemic energy metabolism.

SCOPE OF REVIEW

In this review, we discuss the diverse biological functions of ghrelin, the regulation of its secretion, and address questions that still remain 15 years after its discovery.

MAJOR CONCLUSIONS

In recent years, ghrelin has been found to have a plethora of central and peripheral actions in distinct areas including learning and memory, gut motility and gastric acid secretion, sleep/wake rhythm, reward seeking behavior, taste sensation and glucose metabolism.

Physiology: does gut hormone PYY3-36 decrease food intake in rodents?

Batterham et al. report that the gut peptide hormone PYY3-36 decreases food intake and body-weight gain in rodents, a discovery that has been heralded as potentially offering a new therapy for obesity. However, we have been unable to replicate their results. Although the reasons for this discrepancy remain undetermined, an effective anti-obesity drug ultimately must produce its effects across a range of situations. The fact that the findings of Batterham et al. cannot easily be replicated calls into question the potential value of an anti-obesity approach that is based on administration of PYY3-36.

Chronic central infusion of cocaine- and amphetamine-regulated transcript (CART 55-102): effects on body weight homeostasis in lean and high-fat-fed obese rats

BACKGROUND

Cocaine- and amphetamine-regulated transcript (CART) is expressed within hypothalamic nuclei implicated in the regulation of feeding behaviour. It is up-regulated by leptin, and CART-derived peptides acutely inhibit food intake.

OBJECTIVE

The present study was designed to assess the long-term effects of central CART administration on food intake, body weight, plasma levels of glucose, insulin, leptin, free fatty acids and triglycerides, and on fuel utilisation in normal and high-fat-fed obese rats.

DESIGN

Normal and high-fat-fed obese rats were cannulated intracerebroventricularly (i.c.v.) and infused for 6 days with CART (55-102) or its vehicle. At day 4, animals were placed in an indirect calorimeter for a 24 h period during which the respiratory quotient and the energy expenditure were determined hourly.

RESULTS

In both normal and obese animals, the chronic i.c.v. infusion of CART (55-102) had marked, sustained inhibitory effects on food intake and body weight gain that were accompanied by decreases in plasma insulin and leptin levels. Using indirect calorimetry, it was observed that CART infusion promoted an increase in lipid oxidation in normal and in obese animals, although this increase reached statistical significance only in the obese group. The hypothalamic CART mRNA expression was found to be higher in obese rats (displaying hyperleptinaemia) than in normal animals.

CONCLUSION

The data together show that chronic i.c.v. CART infusion is effective in inhibiting food intake, favouring lipid oxidation and limiting fat storage, both in normal and high-fat-diet-induced obese rats. The CART pathway thus seems to be an important determinant of body weight homeostasis in normal animals as well as in a model of nutritionally induced obesity.

Rodent obesity models: an overview

Animal obesity models differ widely in type and extent of obesity. They are either based on environmental factors (eg. high fat diet induced obesity), spontaneous mutants (i.e. ob/ob mice), genetically engineered animals (eg. mice with melanocortin receptor subtype-4 gene disruption (knock-out)) or mechanical intervention (eg. chemical lesion of the ventromedial hypothalamus). This article reviews available rodent models to study obesity and attempts to highlight the greatest utility for each model.

Weight gain decreases elevated plasma ghrelin concentrations of patients with anorexia nervosa

OBJECTIVE

Ghrelin is a new gastric hormone that has been identified as an endogenous ligand for the growth hormone (GH) secretagogue receptor subtype 1a (GHS-R1a). Ghrelin administration however not only stimulates GH secretion but also induces adiposity in rodents by increasing food intake and decreasing fat utilization. We hypothesized that impaired ghrelin secretion in anorexia nervosa may be involved in the pathogenesis of this eating disorder. To examine this hypothesis and to further investigate the role for ghrelin in regulating energy homeostasis, we analyzed circulating ghrelin levels in patients with anorexia nervosa and examined possible correlations with clinical parameters before and after weight gain.

METHODS

Plasma ghrelin levels were measured in overnight fasting plasma samples from 36 female patients with anorexia nervosa (age: 25.0+/-1.2 years, BMI: 15.2+/-0.2 kg/m(2)) before and after weight gain following psychotherapeutic treatment intervention in a psychosomatic institution. Plasma ghrelin levels were also measured in fasting plasma samples from 24 age-matched female controls (31+/-1.4 years, BMI: 22.9+/-0.45 kg/m(2)). For quantification of ghrelin levels a commercially available radioimmunoassay (Phoenix Pharmaceuticals, USA) was used.

RESULTS

Fasting plasma ghrelin levels in anorectic patients were significantly higher (1057+/-95 pg/ml) than in normal age-matched female controls (514+/-63 pg/ml n=24, P=0.02). Therapeutic intervention in a psychosomatic institution caused an BMI increase of 14% (P<0.001) leading to a significant decrease in circulating ghrelin levels of 25%, (P=0.001). A significant negative correlation between Deltaghrelin and DeltaBMI was observed (correlation coefficient: -0.47, P=0.005, n=36).

CONCLUSION

We show for the first time that fasting plasma levels of the novel appetite-modulating hormone ghrelin are elevated in anorexia nervosa and return to normal levels after partial weight recovery. These observations suggest the possible existence of ghrelin resistance in cachectic states such as caused by eating disorders. Future studies are necessary to investigate putative mechanisms of ghrelin resistance such as a possible impairment of intracellular ghrelin receptor signaling in pathophysiological states presenting with cachexia.

Absence of cocaine- and amphetamine-regulated transcript results in obesity in mice fed a high caloric diet

Cart (cocaine- and amphetamine-regulated transcript) was first identified to be a major brain mRNA up-regulated by cocaine and amphetamine. The CART protein has been established as a satiety factor closely associated with the action of leptin. To assess CART's role as an anorexigenic signal, we have generated CART-deficient mice by gene targeting. On a high fat diet, CART-deficient and female heterozygous mice, but not male heterozygous mice, showed statistically significant increases in weekly food consumption, body weight, and fat mass compared with their wild-type littermates. Furthermore, CART-deficient and female heterozygous mice were significantly heavier when fed a high fat diet than on a regular chow diet at 17 wk of age and at the 14th wk of the feeding studies. However, wild-type or male heterozygous mice showed no weight variations attributable to caloric contents of the diet at that age. Contrary to the obese phenotypes shown in MC4R-, proopiomelanocortin-, or leptin-deficient mice, our results showed that CART deficiency predisposed mice to become obese on a calorically dense diet. The results also show that CART may not be a major anorectic signal compared with proopiomelanocortin or leptin in the regulation of energy homeostasis.

Plasma ghrelin concentration and energy balance: overfeeding and negative energy balance studies in twins

Central (intracerebral ventral) and peripheral (subcutaneous and intraperitoneal) administration of ghrelin causes obesity in rodents by increasing food intake and decreasing fat oxidation. Recent studies in humans have shown that plasma ghrelin concentration was inversely related to body fat and was lower in Pima Indians, a population susceptible to obesity. Whether ghrelin plays a role in the etiology of obesity in humans is unknown. We, therefore, measured plasma ghrelin concentration before and after two interventions in monozygotic twins previously studied at Laval University, Quebec City. Twelve pairs of monozygotic twins were overfed by 84,000 kcal over a 100-day period, whereas another seven pairs of monozygotic twins were submitted to a 53,000 kcal negative energy balance induced by exercise over a 93-day period. At baseline, for all the subjects, plasma ghrelin concentration was negatively correlated with body mass and body fatness (r varying from 0.36 to 0.45). The intraclass coefficient for the twin resemblance (r(I) = 0.75; p = 0.006) indicated that plasma ghrelin concentration is a familial trait. In response to the 100-day intervention, plasma ghrelin exhibited a non-significant decrease of 61 +/- 30 fmol/l (p = 0.18) with overfeeding and a non-significant increase of 58 +/- 34 fmol/l (p = 0.17) with negative energy balance. However, there was no relationship between baseline plasma ghrelin concentration and the magnitude of body weight change in both interventions. These first experimental data under "clamped energy balance conditions" do not provide evidence that plasma ghrelin is involved in the etiology of human obesity. However, studies in free-living individuals are needed to clarify this question.

Circulating ghrelin levels are decreased in human obesity

Ghrelin is a novel endogenous natural ligand for the growth hormone (GH) secretagogue receptor that has recently been isolated from the rat stomach. Ghrelin administration stimulates GH secretion but also causes weight gain by increasing food intake and reducing fat utilization in rodents. To investigate the possible involvement of ghrelin in the pathogenesis of human obesity, we measured body composition (by dual X-ray absorption) as well as fasting plasma ghrelin concentrations (radioimmunoassay) in 15 Caucasians (8 men and 7 women, 31+/-9 years of age, 92+/-24 kg body wt, and 29+/-10% body fat, mean +/- SD) and 15 Pima Indians (8 men and 7 women, 33+/-5 years of age, 97+/-29 kg body wt, and 30+/-8% body fat). Fasting plasma ghrelin was negatively correlated with percent body fat (r = -0.45; P = 0.01), fasting insulin (r = -0.45; P = 0.01) and leptin (r = -0.38; P = 0.03) concentrations. Plasma ghrelin concentration was decreased in obese Caucasians as compared with lean Caucasians (P < 0.01). Also, fasting plasma ghrelin was lower in Pima Indians, a population with a very high prevalence of obesity, compared with Caucasians (87+/-28 vs. 129+/-34 fmol/ml; P < 0.01). This result did not change after adjustment for fasting plasma insulin concentration. There was no correlation between fasting plasma ghrelin and height. Prospective clinical studies are now needed to establish the role of ghrelin in the pathogenesis of human obesity.

Harnessing the power of the group for latency-aged sexual abuse victims

For children who have been sexually abused, group therapy is often a preferred treatment modality. Although much consideration has been given to what structured activities, goals, and objectives should be included in groups for sexually abused children, limited attention has been paid to the dynamics and processes inherent in the group. This article follows the dynamic unfolding of a short-term structured group for latency-aged, female sexual abuse victims. Special emphasis is placed on listening to the messages underlying children's actions and responding to the message rather than suppressing or limiting the behavior. It is argued that by attending to group dynamics and processes in a theme-focused group, structured activities become more effective and the power of the group can be harnessed to foster opportunities for healing that otherwise might go unrecognized.

Increased weight gain after ovariectomy is not a consequence of leptin resistance

The positive correlation between leptin and body fat mass has caused some investigators to speculate that leptin resistance contributes to obesity. Loss of ovarian function in human and rat is associated with increased fat mass gain and increased circulating leptin levels. To study whether ovariectomy produces leptin resistance, Sprague-Dawley female rats were ovariectomized or sham operated and injected with leptin for 35 days. Ovariectomy (OVX) produced hyperphagia and increased gain in both lean and fat mass. Daily leptin injections initially decreased food intake significantly, but feeding gradually increased to a stable level by day 16 and remained at that level for the duration of study. Body composition analysis indicated that chronic injection of leptin to OVX rats dramatically decreased (P < 0.05) fat mass [30 +/- 2 (SE) g, vehicle, to 3 +/- 1 g, leptin]. Using indirect calorimetry, we observed that OVX did not change energy expenditure or total level of fuel utilization. Leptin administration increased fat utilization and prevented reduction in calorie expenditure that is typically associated with food restriction. Leptin treatment to OVX rats decreased plasma triglyceride, free fatty acid, and insulin concentrations, whereas glucose concentration was normal. Withdrawal of leptin triggered hyperphagia, indicating that leptin biology remained throughout the duration of the chronic treatment. The same dose of leptin produced qualitatively similar data in sham-operated rats. Thus we concluded that the loss of ovarian function in rats is not associated with a change in leptin sensitivity.

Ghrelin induces adiposity in rodents

The discovery of the peptide hormone ghrelin, an endogenous ligand for the growth hormone secretagogue (GHS) receptor, yielded the surprising result that the principal site of ghrelin synthesis is the stomach and not the hypothalamus. Although ghrelin is likely to regulate pituitary growth hormone (GH) secretion along with GH-releasing hormone and somatostatin, GHS receptors have also been identified on hypothalamic neurons and in the brainstem. Apart from potential paracrine effects, ghrelin may thus offer an endocrine link between stomach, hypothalamus and pituitary, suggesting an involvement in regulation of energy balance. Here we show that peripheral daily administration of ghrelin caused weight gain by reducing fat utilization in mice and rats. Intracerebroventricular administration of ghrelin generated a dose-dependent increase in food intake and body weight. Rat serum ghrelin concentrations were increased by fasting and were reduced by re-feeding or oral glucose administration, but not by water ingestion. We propose that ghrelin, in addition to its role in regulating GH secretion, signals the hypothalamus when an increase in metabolic efficiency is necessary.

Chronic leptin administration promotes lipid utilization until fat mass is greatly reduced and preserves lean mass of normal female rats

Leptin is a hormone synthesized and secreted from adipose tissue. To study the physiologic effects of chronic leptin treatment, normal adult female Sprague-Dawley rats were injected subcutaneously for 35 days. Twice daily injections (250 microgram/day, b.i.d.) resulted in a significant (P<0.05) decrease in food intake that was maintained for 10 days before gradually returning to control level by day 21. Leptin decreased body weight by a maximum of 12% of the initial body weight on day 22 and remained reduced for the duration of the treatment. After 35 days of treatment, visible peritoneal adipose tissue was not detected. Body composition analysis showed that chronic injection of leptin resulted in a dramatic decrease in fat content (28+/-2 to 4+/-2 g, P<0.05; mean+/-SEM) while the lean content remained unchanged. Rats pair-fed to the leptin-treated group but treated with vehicle had the same body composition (23+/-3 g fat mass) as that measured for the ad libitum fed controls. Using indirect calorimetry we observed that leptin decreased respiratory quotient and thus increased fat oxidation. Leptin also prevented energy expenditure reduction typically associated with food restriction. Leptin treatment for 35 days decreased plasma triglyceride (0.75+/-0.07 to 0.30+/-0.03 mM, P<0.05), free fatty acid (0.56+/-0.06 to 0.32+/-0.04 mM) and insulin (3.2+/-0.5 to 1. 4+/-0.4 ng/ml, P<0.05) concentrations despite the fact that food intake was normalized by day 35. Withdrawal of leptin triggered hyperphagia indicating that leptin biology remained throughout the duration of the chronic treatment. These data suggest that leptin reduces fat mass by initially decreasing appetite and by maintaining enhanced fat utilization even when food intake has returned to that of vehicle-treated control.

Pharmacological characterization of (125)I-1229U91 binding to Y1 and Y4 neuropeptide Y/Peptide YY receptors

1229U91 (GW1229 or GR231118) [lle,Glu,Pro,Dpr,Tyr, Arg,Leu,Arg, Tyr-NH(2))2 cyclic (2,4'),(2'4)-diamide] has been reported by several research groups to be a potent antagonist at the Y1 neuropeptide Y (NPY) receptor subtype. However, 1229U91 also displaces (125)I-peptide YY (PYY) with high affinity from the Y4 subtype. Previously, we reported that 1229U91 had full agonist properties for the Y4 receptor. To characterize the pharmacological properties of 1229U91 directly, we had it radioiodinated with the chloromine-T method. (125)I-1229U91 bound to cell lines expressing the human Y1 and Y4 receptors with high affinity. The K(d) and B(max) for (125)I-1229U91 binding to Y1 were 14.9 pM and 1458 fmol/mg protein, respectively. The Y4 receptor bound (125)I-1229U91 with a K(d) of 12.5 pM and a B(max) of 1442 fmol/mg protein. When competing (125)I-1229U91 binding from Y1 and Y4 receptors, a similar rank order of potency was observed: 1229U91 > [Leu(31),Pro(34)]-NPY >/= [Leu(31),Pro(34)]-PYY > PYY >/= NPY > NPY(2-36) > PYY(3-36). Pancreatic polypeptide (PP) potently displaced (125)I-1229U91 from the Y4 receptor, but displayed little affinity for Y1. In autoradiographic studies with rat brain sections, (125)I-1229U91 bound with a distribution similar to that reported for the Y1 receptor when localized with (125)I-[Leu(31),Pro(34)]-PYY. Brain regions exhibiting binding sites for (125)I-PP were not detected with this radioligand. Those include the interpeduncular nucleus and the periventricular nucleus of the hypothalamus. Furthermore, (125)I-labeled rat PP was not displaced from these areas with 10 nM 1229U91. Thus, (125)I-1229U91 is a high affinity Y1 and Y4 radioligand and binds with a distribution in the rat brain consistent with the localization of the Y1 receptor.

Changes in leptin expression are not associated with corresponding changes in CCAAT/enhancer binding protein-alpha

C/EBP-alpha binds a C/EBP consensus site in the leptin promoter and activates transcription in vitro. We assessed adipose tissue expression of C/EBP-alpha, leptin and beta-actin in Sprague Dawley rats under conditions that modulate leptin mRNA abundance in order to study the relationship between leptin and C/EBP-alpha expression patterns. During acute fasting, which decreased the level of leptin and beta-actin mRNA, C/EBP-alpha mRNA expression was unaltered. In leptin-treated and pair-fed animals, C/EBP-alpha mRNA was unaltered compared to ad libitum fed controls, while leptin and beta-actin mRNA expression was again decreased. These results indicate that changes in the level of leptin gene expression are not directly associated with changes in the level of C/EBP-alpha abundance.

The effect of exercise on leptin concentration in healthy men and in type 1 diabetic patients

PURPOSE

Leptin is a recently discovered hormone that appears as a regulator of energy balance. It is important to know whether leptin concentrations are changed under conditions of altered energy homeostasis. Consequently, we examined the effects of exercise with fasting and exercise with feeding on circulating leptin concentrations in healthy men and in type 1 diabetic patients with normal body weight and well controlled diabetes.

METHODS

Leptin concentrations were determined with radioimmunoassay.

RESULTS

During a 3-h cycle ergometer exercise with fasting, leptin decreased by 42% (P < 0.01) in nine healthy men and by 23% (P = 0.05) in eight male type 1 diabetic patients. Leptin fell equally by 12% (P < 0.03) both in nine healthy men and in eight male type 1 diabetic patients who were studied as a resting control group. The absolute fall in leptin in healthy men was similar in the exercise and resting control groups (0.8 +/- 0.1 microgram.L-1 vs 0.8 +/- 0.2 microgram.L-1). However, due to lower leptin concentration before the exercise, the relative decrease (42%) was greater than during the resting control study (12%, P < 0.005). This difference was not seen in the diabetic patients. Fasting leptin concentration correlated positively with BMI (r = 0.75, P < 0.001) and fasting insulin (r = 0.71, P < 0.01) in healthy men as well as with insulin level (r = 0.54, p < 0.05) in type 1 diabetic patients. When exercise was performed with feeding, and this was associated with a significant rise in serum cortisol level (marathon run, 14 healthy men and 7 type 1 diabetic patients), leptin concentration did not change significantly.

CONCLUSIONS

1) During morning hours, leptin decreases both in healthy men and in type 1 diabetic patients, reflecting a diurnal variation of leptin concentration and the effect of fasting on leptin concentration. 2) The fall in leptin during morning hours is augmented by physical exercise in healthy men. 3) If exercise is performed with feeding and associated with a rise in serum cortisol level, leptin concentration remains unchanged. These data suggest that although exercise may reduce circulating leptin levels, the effect is small and can be counterbalanced by feeding or a rise in serum cortisol concentration.

A comparative survey of beliefs about "normal" childhood sexual behaviors

OBJECTIVE

This study sought to collect data on what adults believe constitutes normal childhood sexual behaviors, and how variables, such as role, gender, and life experience might contribute to the formation of one's beliefs.

METHODS

A survey describing 20 different scenarios of children under the age of 13 interacting with themselves or other children in a sexual manner was administered to four groups of adults: sexual abuse experts; therapists involved in a sexual abuse training program; medical students attending a human sexuality program; and group facilitators of the human sexuality program.

RESULTS

Behaviors that involved oral, vaginal, or anal penetration were judged by a majority of adults to be abnormal sexual behaviors in children under 13 years of age. Professionals working with sexually abused children rated certain sexual behaviors as more abnormal than adults participating in a human sexuality course. Both sexual abuse trainees and facilitators of the human sexuality course showed more directional biases than other groups, with trainees always rating behaviors in the direction of abnormal and facilitators always rating behaviors in the direction of normal. Females also judged many of the sexual behaviors to be more abnormal than males.

CONCLUSIONS

Role and gender significantly influence what adults believe constitutes normal and abnormal childhood sexual behavior.

Radioimmunoassay of rat leptin: sexual dimorphism reversed from humans

Adipose tissue secretes leptin, which interacts with receptors in the hypothalamus. In rodent models of obesity, leptin increases metabolism and decreases food intake, which helps to maintain normal body composition. Accurate and precise methods to quantitate circulating leptin concentrations are needed for physiological studies. We developed an RIA to measure leptin in rat plasma, serum, or adipocyte culture fluids. The working range of the assay, defined by the detection limit and the highest calibrator, was 0.5-50 micrograms/L. Recovery of 1.6-11.6 micrograms/L leptin added to serum was 92-103%. The rat leptin RIA correlated well with a previously developed mouse RIA when rat plasma was assayed with both methods (r = 0.94), but the mouse leptin assay underestimated rat leptin in plasma. Within- and between-run CVs were 2.4% to 5.7%. Plasma leptin concentrations correlated directly with percentage of body fat, and correlation improved when the results were separated by gender (r = 0.796, P < 0.001 for males; r = 0.710, P < 0.001 for females). Leptin concentrations were generally higher in male rats than in females; plasma leptin increased 0.60 microgram/L for each percentage of increase in body fat for males but only 0.22 microgram/L for females. We conclude that rat serum/plasma leptin concentrations are accurately and precisely measured with this new RIA.

Leptin inhibits in vitro hypothalamic somatostatin secretion and somatostatin mRNA levels

Leptin, the product of the ob gene, is a recently discovered hormone secreted by adipocytes that regulates food intake and energy expenditure. The site of action of leptin is likely to be the hypothalamus, since this area is important in the control of food intake and leptin receptor mRNA is particularly abundant in this area. In order to further unravel the mechanisms by which leptin acts, we have studied the effect of leptin on in vitro somatostatin synthesis and secretion. Leptin administration to fetal rat neurones in monolayer culture led to a time dependent decrease in basal somatostatin secretion and somatostatin mRNA levels, the maximal effect being observed with 6x10(-8) M leptin after 24 h incubation. Furthermore, leptin completely blunted 10(-7) M Neuropeptide Y-induced increase in somatostatin secretion and somatostatin mRNA levels as well as 10(-3) M (Bu)2-cAMP and 10(-6) M A23187-induced somatostatin secretion. Finally, leptin (3x10(-8) M M) also inhibited low glucose (1.1 mM) induced-somatostatin secretion in perifused adult hypothalami. This data indicates that leptin can influence the neuroendocrine system by regulating hypothalamic somatostatin gene expression.

Plasma leptin levels in healthy children and adolescents: dependence on body mass index, body fat mass, gender, pubertal stage, and testosterone

Leptin, the product of the ob gene, is thought to play a key role in the regulation of body fat mass. Beyond this function, it appears to be an integral component of various hypothalamo-pituitary-endocrine feedback loops. Because childhood and puberty are periods of major metabolic and endocrine changes, leptin levels and various hormonal parameters were investigated in a large cohort of healthy children and adolescents (312 males, 401 females, age 5.8-19.9 yr). For this purpose, a specific and sensitive RIA was developed that allowed the accurate measurement of low leptin levels in young lean children. With this assay, leptin proved to be a comparatively stable protein under common conditions of blood sampling and storage. Leptin levels increased in girls with age (r = 0.47, P < 0.0001), but decreased in boys (r = -0.34, P < 0.0001). An analysis according to pubertal stage showed a steady increase in girls between 2.51 micrograms/L (median) at Tanner stage 1 to 6.24 micrograms/L at Tanner stage 5. In boys, leptin levels were highest at Tanner stage 2 (2.19 micrograms/L) and declined thereafter to 0.71 microgram/L at Tanner stage 5. A strong exponential relationship was observed for leptin levels with body mass index (BMI) and percentage body fat as determined by bioelectric impedance measurements in a subgroup of subjects. This relationship was similar between boys and girls at Tanner stages 1 and 2. In boys, there was a significant decline of leptin at a given BMI with further progression of puberty that was much less pronounced in girls. Although the relative increase of leptin with BMI and percent body fat was the same in both genders, the absolute values at a given BMI or percent body fat were significantly lower in boys in late puberty and in adolescents. In boys, but not in girls, there was an inverse correlation with testosterone concentrations (r = -0.43, P < 0.0001), which explained 10.5% of the variation of leptin levels in a multiple regression model. Since BMI proved to be the major influencing variable, reference ranges were constructed using a best-fit regression line of the form leptin = a*e(b*BMI) and stratifying ranges according to gender and pubertal stage. In conclusion, these data suggest that 1) plasma leptin levels increase in girls and decrease in boys after Tanner stage 2 as the pubertal development proceeds; 2) they show a significant gender difference especially in late puberty and adolescence, even after adjustment for BMI or percent body fat; 3) the lower levels in males may be explained at least in part by a suppressive effect of androgens; 4) reference ranges with BMI as the independent variable should be stratified according to gender and pubertal stage.

Leptin inhibition of the hypothalamic-pituitary-adrenal axis in response to stress

Leptin is a newly identified protein hormone that is synthesized and secreted by adipose tissue. Absence of the mature hormone is responsible for the obese phenotype of ob/ob mice. The hypothalamic-pituitary-adrenal axis (HPAA) is activated in ob/ob mice, and chronic administration of leptin to ob/ob mice decreases plasma corticosterone levels, suggesting that the adipose hormone is capable of inhibiting the HPAA. The aim of this study was to determine whether leptin feeds back acutely to inhibit the HPAA of normal mice and rats. Male C57BL mice were injected ip with 100 microl saline and 2 or 4 microg/g BW mouse leptin in saline vehicle, and 4 h later they were subjected to 2 h of restraint stress by taping the hind limbs together or no stress. Hind leg restraint stimulated the HPAA as measured by significant (P < 0.05) elevation of both ACTH and corticosterone. Pretreatment with recombinant mouse leptin blocked the stress-mediated stimulation of both plasma hormones. To determine whether this inhibition was exerted at the hypothalamic level through inhibition of CRH, we studied leptin action on isolated rat hypothalami perifused with Krebs-Ringer buffer containing glucose (5.5 mM). CRH secretion was stimulated by decreasing the glucose concentration of the buffer to 1.1 mM. A surge of CRH was released over a 2-h period (basal integrated release was 14.4 +/- 1.6 pg/2 h, n = 5 and increased to 34.7 +/- 3.1 pg/2 h, n = 14). This response was blocked by mouse leptin in a dose-dependent manner (integrated stimulated CRH secretion was 30.6 +/- 2.5 pg/2 h, n = 5; 20.5 +/- 3.6 pg/2 h, n = 7; 15.3 +/- 4.3 pg/2 h, n = 3 for 1 nM, 3 nM and 30 nM, respectively). Leptin did not alter secretion of ACTH from rat primary cultured pituitary cells. These data demonstrate that leptin can inhibit hypothalamic CRH release, either directly or indirectly through another hypothalamic neuropeptide such as neuropeptide-Y. Dysfunctional leptin, insufficient leptin levels, or leptin resistance should each result in a partial open loop, thereby accounting for elevated glucocorticoid levels that accompany and contribute to many obese phenotypes. Leptin's ability to inhibit CRH release is the likely explanation for its ability to inhibit activation of the HPAA in response to stress.

Leptin and thermogenesis in humans

We examined changes in serum leptin concentrations and thermogenesis in 12 healthy men (39 +/- 2 years, BMI 22.8 +/- 0.3 kg m-2) during a 4 h euglycaemic hyperinsulinaemia performed on a control day and after a day of competitive marathon runs. As compared with the control day, after the marathon, baseline FFA concentration (543 +/- 0.73 vs. 955 +/- 96 mumol L-1, P < 0.05), lipid oxidation rate (0.8 +/- 0.1 vs. 1.2 +/- 0.1 mg kg-1 min-1, P < 0.01) and energy expenditure (5.2 +/- 0.1 vs. 5.5 +/- 0.1 kJ min-1, P < 0.01) were elevated. Baseline serum leptin concentrations were similar on the control and postexercise days and increased during insulin infusion by 35-45% on both days (P < or = 0.01). Baseline serum leptin concentration correlated directly with serum insulin (r = 0.65, P < 0.05) and cortisol (r = 0.64, P < 0.05), and inversely with serum growth hormone concentration (r = -0.66, P < 0.05). In the postexercise study, the rise in energy expenditure during insulin clamp correlated with serum leptin concentration as determined before (r = 0.61, P < 0.05) or at the end of insulin infusion (r = 0.55, P < 0.05). Thus, in healthy individuals with normal body weight: (1) hyperinsulinaemia increases serum leptin concentrations; (2) a rise in energy expenditure correlates with serum leptin concentration. These data suggest that leptin is involved in the regulation of energy expenditure in humans.

Serum leptin concentration and fuel homeostasis in healthy man

We studied the interrelationship between the obese gene product serum leptin, insulin and counter-regulatory hormone concentrations and glycogen synthesis in 26 healthy men. A 4-h euglycaemic insulin clamp with muscle biopsies was performed after a resting control day in 26 subjects, and in 14 of them also after heavy, glycogen-depleting (32%, P < 0.01) exercise. Serum leptin concentrations were at baseline 34% (0.67 +/- 0.18 vs. 1.03 +/- 0.13 ng L-1, P < 0.05) lower after the exercise, and rose during hyperinsulinaemia by 56% (to 1.38 +/- 0.19 ng L-1, P < 0.001) and 34% (to 1.05 +/- 0.20 ng L-1, P < 0.01) after the post-exercise and control studies respectively. Basal serum leptin concentration correlated positively with body mass index (r = 0.42, P < 0.05), serum cortisol concentration (r = 0.70, P < 0.001) and the rise in muscle glycogen content during the clamp (r = 0.43, P < 0.05) and inversely with serum growth hormone concentration (r = -0.43, P < 0.05). There was a positive correlation between serum leptin after hyperinsulinaemia and serum insulin concentration during the hyperinsulinaemia (r = 0.42, P < 0.05). After exercise, basal serum leptin level correlated with serum triglyceride concentration (r = 0.82, P < 0.001) and after hyperinsulinaemi serum leptin correlated positively with muscle glycogen content (r = 0.56, P < 0.05). It was concluded that serum leptin concentrations correlate directly with serum insulin, cortisol and triglyceride and inversely with growth hormone concentrations. They are decreased by glycogen-depleting exercise and increase during hyperinsulinaemic clamp. These data suggest that leptin is associated with factors regulating fuel homeostasis and its hormonal control in man.

Leptin synthesis is resistant to acute effects of insulin in insulin-dependent diabetes mellitus patients

Insulin stimulates ob gene expression and increases serum leptin concentrations in mice and in noninsulin-dependent diabetes mellitus patients. Obese women have higher ob gene messenger ribonucleic acid levels than obese men, suggesting that sex hormones are involved in the regulation of leptin synthesis. We studied the relationship among leptin, insulin, and testosterone in 15 men with insulin-dependent diabetes mellitus (IDDM; age, 29 +/- 2 yr; body mass index, 22.7 +/- 0.5 kg/m2; body fat, 9.5 +/- 1.0%; insulin dose, 44 +/- 4 U/day; hemoglobin A1c, 8.1 +/- 0.3%; diabetes duration, 12.7 +/- 2.0 yr) and 15 healthy control subjects (age, 27 +/- 1 yr; body mass index, 22.6 +/- 0.4 kg/m2; body fat, 9.6 +/- 0.5%) in the fasting state. In addition, the effect of a 4-h euglycemic hyperinsulinemia (approximately 600 pmol/L) on the plasma leptin concentration was determined. The fasting leptin concentration was negatively correlated to plasma testosterone (r = -0.55; P < 0.05) in IDDM patients. The fasting plasma leptin level rose 25% in healthy subjects (from 1.0 +/- 0.2 to 1.3 +/- 0.3 ng/mL; P < 0.05). The leptin levels were higher in IDDM subjects (P < 0.01) and remained unchanged (2.7 +/- 0.2 vs. 2.7 +/- 0.2 ng/mL) during hyperinsulinemia. We reached the following conclusions. 1) In nonobese IDDM patients, leptin synthesis is resistant to the acute effect of insulin. 2) Serum testosterone may contribute to the regulation of leptin synthesis in IDDM patients.

Ultradian oscillations of leptin secretion in humans

We have recently demonstrated the nocturnal increase in leptin secretion in humans. In the present study we have examined the pulsatile pattern of leptin secretion using two different experimental protocols. The first protocol utilized blood samples withdrawn at 30 minute intervals immediately after meals, at 1 hour intervals between meals, and at 2 hour intervals during the night from 4 lean, 11 obese, and 5 obese NIDDM subjects. Analysis of circulating leptin levels by ULTRA algorithmic program and using matched intra-assay coefficient of variations demonstrated 1 to 7 ultradian oscillations with a mean of 3.25 +/- 0.36 (SEM) pulses per 24 hour period (period: 10.0 +/- 1.5 hours; mean relative amplitude: 0.52 +/- 0.06, n = 20). Significant positive correlations were observed for changes in absolute amplitude with body mass index (p < 0.025) and fasting leptin levels (< 0.0001). In the second series of experiments utilizing 15 minute blood sampling from 10 overnight fasted obese subjects (BMI 35.9 +/- 2.0 kg/m2), ultradian oscillations for leptin were more frequent, i.e., 2 to 7 oscillations (4.20 +/- 0.59), over a 12 hour duration (period: 3.44 +/- 0.49; mean relative amplitude: 0.28 +/- 0.03). The number of oscillations over a 12 hour period correlated significantly with BMI (p < 0.001), fasting leptin levels (p < 0.01), and absolute amplitude (p < 0.005) in a 15 minute sampling protocol. In summary, similar to other hormones, ultradian oscillations of leptin are observed in humans, although the physiological significance in relation to obesity or feeding behavior is not yet understood.

Evidence of free and bound leptin in human circulation. Studies in lean and obese subjects and during short-term fasting

Little is known about leptin's interaction with other circulating proteins which could be important for its biological effects. Sephadex G-100 gel filtration elution profiles of 125I-leptin-serum complex demonstrated 125I-leptin eluting in significant proportion associated with macromolecules. The 125I-leptin binding to circulating macromolecules was specific, reversible, and displaceable with unlabeled leptin (ED50: 0.73 +/- 0.09 nM, mean +/- SEM, n = 3). Several putative leptin binding proteins were detected by leptin-affinity chromatography of which either 80- or 100-kD proteins could be the soluble leptin receptor as approximately 10% of the bound 125I-leptin was immunoprecipitable with leptin receptor antibodies. Significantly higher (P < 0.001) proportions of total leptin circulate in the bound form in lean (46.5 +/- 6.6%) compared with obese (21.4 +/- 3.4%) subjects. In lean subjects with 21% or less body fat, 60-98% of the total leptin was in the bound form. Short-term fasting significantly decreased basal leptin levels in three lean (P < 0.0005) and three obese (P < 0.005) subjects while refeeding restored it to basal levels. The effects of fasting on free leptin levels were more pronounced in lean subjects (basal vs. 24-h fasting: 19.6 +/- 1.9 vs. 1.3 +/- 0.4 ng/ml) compared with those in obese subjects (28.3 +/- 9.8 vs. 14.7 +/- 5.3). No significant (P > 0.05) decrease was observed in bound leptin in either group. These studies suggest that in obese individuals the majority of leptin circulates in free form, presumably bioactive protein, and thus obese subjects are resistant to free leptin. In lean subjects with relatively low adipose tissue, the majority of circulating leptin is in the bound form and thus may not be available to brain receptors for its inhibitory effects on food intake both under normal and food deprivation states.

Nocturnal rise of leptin in lean, obese, and non-insulin-dependent diabetes mellitus subjects

We studied 24-h profiles of circulating leptin levels using a sensitive and specific RIA in lean controls and obese subjects with or without non-insulin-dependent diabetes mellitus (NIDDM) during normal routine activity. Serum leptin levels were significantly higher in obese (41.7 +/- 9.0 ng/ml; n = 11) and obese NIDDM (30.8 +/- 6.7; n = 9) subjects compared with those in lean controls (12.0 +/- 4.4, n = 6). In all the three groups, serum leptin levels were highest between midnight and early morning hours and lowest around noon to midafternoon. The nocturnal rise in leptin levels was significant when data were analyzed by ANOVA (lean: F = 3.17, P < 0.0001, n = 4; obese: F = 2.02, P < 0.005, n = 11; and obese NIDDM: F = 4.9, P < 0.0001, n = 5). The average circadian amplitude between acrophase and nadir was 75.6% in lean, 51.7%, in obese and 60.7% in obese NIDDM groups, respectively. No significant correlations (P > 0.05) were observed between circulating levels of leptin and either insulin or glucose levels in any of the 20 subjects studied for 24-h profiles. The nocturnal rise in leptin observed in the present study resembles those reported for prolactin, thyroid-stimulating hormone, and free fatty acids. We speculate that the nocturnal rise in leptin could have an effect in suppressing appetite during the night while sleeping.

Serum immunoreactive-leptin concentrations in normal-weight and obese humans

BACKGROUND

Leptin, the product of the ob gene, is a hormone secreted by adipocytes. Animals with mutations in the ob gene are obese and lose weight when given leptin, but little is known about the physiologic actions of leptin in humans.

METHODS

Using a newly developed radioimmunoassay, wer measured serum concentrations of leptin in 136 normal-weight subjects and 139 obese subjects (body-mass index, > or = 27.3 for men and > or = 27.8 for women; the body-mass index was defined as the weight in kilograms divided by the square of the height in meters). The measurements were repeated in seven obese subjects after weight loss and during maintenance of the lower weight. The ob messenger RNA (mRNA) content of adipocytes was determined in 27 normal-weight and 27 obese subjects.

RESULTS

The mean (+/- SD) serum leptin concentrations were 31.3 +/- 24.1 ng per milliliter in the obese subjects and 7.5 +/- 9.3 ng per milliliter in the normal-weight subjects (P < 0.001). There was a strong positive correlation between serum leptin concentrations and the percentage of body fat (r = 0.85, P < 0.001). The ob mRNA content of adipocytes was about twice as high in the obese subjects as in the normal-weight subjects (P < 0.001) and was correlated with the percentage of body fat (r = 0.68, P < 0.001) in the 54 subjects in whom it was measured. In the seven obese subjects studied after weight loss, both serum leptin concentrations and ob mRNA content of adipocytes declined, but these measures increased again during the maintenance of the lower weight.

CONCLUSIONS

Serum leptin concentrations are correlated with the percentage of body fat, suggesting that most obese persons are insensitive to endogenous leptin production.

Characterization of growth hormone-releasing hormone (GHRH) binding to cloned porcine GHRH receptor

To study structure-activity relationships of growth hormone-releasing hormone (GHRH), a competitive binding assay was developed using cloned porcine adenopituitary GHRH receptors expressed in human kidney 293 cells. Specific binding of [His1, 125I-Tyr10,Nle27]hGHRH(1-32)-NH2 increased linearly with protein concentration (10-45 micrograms protein/ tube). Binding reached equilibrium after 90 min at 30 degrees C and remained constant for at least 240 min. Binding was reversible to one class of high-affinity sites (Kd = 1.04 +/- 0.19 nM, Bmax = 3.9 +/- 0.53 pmol/mg protein). Binding was selective with a rank order of affinity (IC50) for porcine GHRH (2.8 +/- 0.51 nM), rat GHRH (3.1 +/- 0.69 nM), [N-Ac-Tyr1, D-Arg2]hGHRH(3-29)-NH2 (3.9 +/- 0.58 nM), and [D-Thr7]GHRH(1-29)-NH2 (189.7 +/- 14.3 nM), consistent with their binding to a GHRH receptor. Nonhydrolyzable guanine nucleotides inhibited binding. These data describe a selective and reliable method for a competitive GHRH binding assay that for the first time utilizes rapid filtration to terminate the binding assay.

Synthesis of PCR-derived, single-stranded DNA probes suitable for in situ hybridization

We report the novel synthesis of polymerase chain reaction (PCR)-derived single-stranded DNA (ssDNA) probes and their subsequent application in in situ hybridizations. Serial transverse sections of an 11.5-day postcoitum mouse embryo were hybridized to a 33P-ssDNA, 33P-RNA, or 35S-RNA probe corresponding to the same 181-bp sequence in the myogenin cDNA. Signal obtained using 33P-ssDNA was more intense than that using 33P-RNA probe, while signal/noise ratios obtained with both 33P-probes were far superior to those obtained with 35S-probe. Digoxigenin-labeled chicken growth hormone (GH) ssDNA gave slightly more intense signal than did digoxigenin-labeled chicken GH RNA when hybridized to chicken pituitary sections. 32P-ssDNA probes were found to be suitable for Northern blot hybridization. Advantages of using ssDNA probes for in situ hybridization include: (1) The ssDNA technique is rapid and simple. There was no need to clone a DNA template into a special RNA vector or order special T7-containing PCR primers. ssDNA probes can be synthesized in less than 1 day using any primers which currently exist in a laboratory (optimal probe length for in situ hybridization is between 50 and 200 bp). (2) In three separate in situ experiments, ssDNA probes yielded more intense signal than RNA probes. (3) ssDNA probes are potentially more stable than RNA probes. (4) Since the RNAse rinse is eliminated, posthybridization rinses are shortened when hybridizing with ssDNA probes. The ssDNA probes produced by this protocol can be labeled with a variety of different isotopes (both radioactive and nonradioactive), and are excellent probes for use in in situ hybridizations.

Restoration of juvenile baseline growth hormone secretion with preservation of the ultradian growth hormone rhythm by continuous delivery of growth hormone-releasing factor

The ability of continuously delivered GH-releasing factor (GRF) to enhance GH secretion while maintaining the normal ultradian GH rhythm was investigated. Synthetic human GH-releasing factor (hGRF(1-44)NH2) was continuously infused for 4 days by means of i.v. catheters to 11-week-old broiler chickens. At this age, overall endogenous GH secretion is low, and baseline GH is barely detectable. Six birds per treatment received vehicle (control), 0.324 mg hGRF(1-44)NH2/kg body weight per day (low dose) or 3.24 mg hGRF(1-44)NH2/kg body weight per day (high dose). After 4 days of GRF conditioning, concurrent with continued GRF infusion, serial blood samples were removed via atrial catheters at 15-min intervals for 6 h and GH plasma profiles determined. High dose GRF significantly increased GH plasma concentrations over tenfold compared with controls; however, most of this increase reflected an increase in basal GH, which was reinstated to juvenile baseline levels. Augmentation of pulse amplitude above this increased baseline was not proportionately as high, and failed to reach juvenile levels. The ultradian rhythm of GH was not altered by continuous GRF administration. Both low and high dose GRF treatments resulted in significant enlargement of the anterior pituitary gland. Total pituitary GH mRNA levels, although elevated over twofold by GRF treatment, were not significantly different from controls. Measures of plasma GH magnitude (overall and baseline mean, and peak amplitude) were significantly correlated with pituitary GH mRNA for control birds, but were not correlated for GRF treatments. Feed intake was markedly depressed (33%) on the high dose GRF treatment, in conjunction with total inhibition of body weight gain over the 4-day period of administration. Longitudinal bone growth and width of the epiphyseal growth plate were also significantly reduced by high dose GRF treatment, probably reflecting the reduced level of nutrient intake, despite high circulating concentrations of GH.

Comparative antitumor effects of hormonal ablation, estrogen agonist, estrogen cytotoxic derivative, and antiestrogen in the PAIII rat prostatic adenocarcinoma

The effects of hormonal ablation, estrogen, estrogen-derived cytotoxic agent, and estrogen antagonist therapies used clinically were evaluated on in vitro colony formation, in vivo growth, and lymphatic and pulmonary metastasis of the PAIII tumor. Ventral prostatic and seminal vesicle weights were evaluated in the same animals to assess androgen-related responses. Estradiol, estramustine phosphate, and testosterone had no effects on PAIII colony formation in vitro. Castration, hypophysectomy, estradiol benzoate, and estramustine phosphate treatment of PAIII-bearing Lobund Wistar rats produced significant (P less than 0.05) regression of male accessory sex organs. Of these treatments, only hypophysectomy had significant (P less than 0.05) inhibitory effects on primary PAIII growth and lymphatic and pulmonary metastasis. LY117018 [6-hydroxy-2-(p-hydroxyphenyl)benzo(b)thien-3-yl-p-2-(l-pyrrolidin yl)ethoxy phenyl ketone] has antiestrogenic activity but produces no significant agonist responses. LY117018 had no effect upon PAIII colony formation in vitro. Following s.c. implantation of PAIII cells, LY117018 (2.0, 10.0, or 20.0 mg/kg s.c.) had no effect on primary tumor growth in the tail. In vitro LY117018 administration produced marked antimetastatic effects. In a dose-dependent manner, LY117018 inhibited PAIII metastasis to the gluteal (97%) and iliac lymph nodes (88%) (P less than 0.05 for both). LY117018 also maximally inhibited pulmonary metastasis by 86% (P less than 0.05). Maximal regression of 42% for ventral prostatic and 35% for seminal vesicle weights were also seen after LY117018 administration (P less than 0.05 for both). Co-administration of estradiol benzoate had no antagonistic effect upon the antitumor responses produced by LY117018. The mechanism of action of LY117018 is not known. The failure of estradiol benzoate to affect PAIII growth and metastasis supports the contention that the responses to LY117018 are not attributable to simple antagonism of estrogen action. LY117018 may be exerting its antitumor effects through autocrine, paracrine, or endocrine mechanisms. LY117018 represents a class of agents with potential utility in treating metastatic cancer of the prostate.

Production and biological activity of hybrid growth hormone-releasing hormone propeptides

Growth hormone-releasing hormone (GHRH), a hypothalamic hormone that stimulates the synthesis and release of growth hormone (GH) from anterior pituitary cells, has been previously produced by synthetic peptide chemistry and recombinant DNA procedures. GHRH is capable of stimulating growth as well as eliciting other anabolic effects on animals and thus may have potential applications in agriculture and human medicine. However, economical production of GHRH by recombinant DNA process has been difficult since GHRH is degraded rapidly by endogenous E. coli proteases. We report here an efficient process to produce hybrid GHRH analogs of higher molecular weight. These hybrid GHRH propeptides (proGHRH) are comprised of an analog of GHRH (44 aa) and the human GHRH carboxy-terminal peptide (33 aa). In E. coli K-12 RV308, the expression levels of the proGHRH analogs were estimated to be 10% of the total cellular protein. An in vitro assay to measure the release of rat growth hormone by GHRH analogs using crude E. coli lysates was also developed. This assay showed that the proGHRH analogs produced in E. coli efficiently stimulated GH release from rat anterior pituitary cells. One proGHRH analog, [alao]-proGHRH, was purified ans shown to efficiently elevate plasma GH levels in wether lambs. Our data indicate that the hybrid proGHRH peptides, unlike other hormone propeptides such as proinsulin, are remarkably bioactive.

Sexual dimorphic porcine pituitary response to growth hormone-releasing hormone

The aim of this study was to compare growth hormone (GH) response of barrows and gilts to porcine growth hormone-releasing hormone (pGRH) at the pituitary level. Anterior pituitary cells from barrows and gilts responded to pGRH in a dose-dependent manner. The median effective pGRH concentration (EC50) which stimulated GH release from cells of barrows was greater (P less than .05) than that for cells obtained from intact female siblings. Maximal pGRH mediated GH secretion from barrows was not different (P greater than 0.05) than that from gilt stimulated cells. These data demonstrate that somatotrophs of growing peripubertal gilts are more responsive to pGRH stimulation than are cells from their castrated male siblings. This difference could be caused by castration of the neonatal male.

Solid-phase synthesis and biological properties of psi [CH2NH] pseudopeptide analogues of a highly potent somatostatin octapeptide

A series of psi [CH2NH] pseudopeptide analogues of a potent somatostatin octapeptide analogue, H-D-Phe-Cys-Try-D-Trp-Lys-Val-Cys-Thr-NH2, were synthesized by using a newly developed solid-phase method for direct introduction of the CH2NH peptide bond isostere. Analogues were obtained in normal yields via a combination of sodium cyanoborohydride mediated reductive alkylation of resin-bound peptide amine with a tert-butoxycarbonyl amino acid aldehyde for introduction of a CH2NH bond and normal solid-phase peptide chemistry. A racemization test on a model pseudodipeptide indicated that no more than 6% racemization took place during the aldehyde preparation and alkylation steps. Analogues were examined for their ability to inhibit growth hormone release in vivo in sodium pentobarbital anesthetized rats and in vitro from cultured rat anterior pituitary cells. Analogues modified at the amide carbonyl of Cys2 and Lys5 showed the highest in vivo activity (20 and 8 times more potent than SRIF, respectively) and in vitro activity (0.17 and 0.67 times as active as SRIF). This compared to in vivo and in vitro potencies of 8000% and 100%, respectively, for the parent analogue. Modification of the other cyclic ring amide carbonyls of Tyr3, D-Trp4, and Val6 or the N- or C-terminal amide carbonyl of D-Phe1 or Cys7 gave analogues with considerably lower in vitro or in vivo potencies. The results appear to offer support for a proposed type II beta-turn solution conformation centered on the D-Trp-Lys portion of SRIF octapeptide analogues, resulting in possible hydrogen-bonding interactions between Tyr3 and Val6 and D-Phe1 and Thr8 residues in the parent peptide. These would then be disrupted by methylene replacement of the carbonyl groups with concomitant loss of biological activity as was observed.

Differential binding of somatostatin agonists to somatostatin receptors in brain and adenohypophysis

In order to improve our understanding of the ligand specificity of somatostatin (SRIF) receptors in pituitary and brain, and to identify analogs that bind to one type exclusively, we compared several new SRIF analogs for competitive binding to pituitary and cerebral cortex membranes. Binding of [125I-Tyr11]SRIF to hypophysis and brain was of high affinity [KD = 0.76 nM (0.2-1.3) and 0.37 nM(0.1-0.8), mean (95% confidence limits)] and was characteristic of binding to one class of sites in both tissues. Competition by several SRIF analogs for such radioligand binding demonstrated that ligand specificity of adenohypophysial receptors was distinctly different from that of cerebral cortex. Two cyclic octapeptides (sequence; see text) bound to pituitary SRIF receptors with high affinity [Ki = 0.85 nM (0.5-1.2) and 0.35 nM (-0.3-0.9)] and were potent inhibitors of GH secretion from primary cultured pituitary cells [EC50 = 0.009 nM (0-0.02) and 0.017 nM (0.01-0.02), respectively]. However, these selective peptides did not compete (Ki much greater than 1 microM) for radioligand binding to brain. Amidation of the C-terminal end appeared to strikingly alter brain SRIF receptor recognition of the substituted ligand. Indeed, such amidation of the parent peptide, (sequence; see text) resulted in a reduced ability to displace labeled ligand from brain sites [Ki = 165.3 nM (47.6-282.9) to 842.2 nM (603.9-1,081)] but did not affect competition for pituitary receptors. Our results indicate that anterior pituitary SRIF receptors (SRIFa) have ligand specificities which are clearly different from those of their brain counterparts (SRIFb).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of N-terminal modifications on the biological potencies of growth hormone releasing factor analogues with varying chain lengths

The excellent retention of biological potencies observed with human growth hormone releasing factor analogues with chains 29-44 amino acid residues long is suddenly lost when further amino acid residues are removed from the C-terminus. For instance, 1-27 and 1-24 exhibited little biological activity (greater than 1%) in vivo and in vitro in the rat. Studies were made to determine whether this was due to conformational changes rather than simply the loss of amino acids needed for direct receptor interactions. These involved the introduction of mild conformational restraint in the N-terminal region by the introduction of D-amino acid residues previously shown to increase the potency of the 1-29 peptide. D-Ala in position 2, responsible for a 40- to 50-fold increase in activity in the 1-29 species, resulted in little increase in the potency of 1-27 or 1-24 sequences. However, N-terminal acetylation, responsible for a 12-fold increase in 1-29 in vivo potency, caused greater than 50-fold increase in 1-27 potency but had little effect on 1-24 potency. Likewise, D-Asn in position 8 was far more effective in increasing the potency of the 1-27 sequence compared to the 1-29 ([D-Asn8]-GRF(1-29)NH2, 220% vs. [D-Asn8, Leu27]-GRF(1-27)NH2, 53%; in vivo]. This differential effect was even more clear in vitro. The highest in vivo potency in the 1-27 series was achieved with [D-Asp3,D-Asn8,Leu27]-GRF(1-27)NH2 (200%); however, this analogue was still far less potent than its 1-29 counterpart (3800%). None of the D-amino acid substitution strategies were effective in increasing 1-22 peptide potencies to detectable levels. The results indicate that the effect of N-terminal substitutions and resulting potencies of the GRFs is very much dependent on chain length, perhaps suggesting that C-terminal amino acids promote conformational effects at the N-terminus and/or vice versa.

Strategies in the design of synthetic agonists and antagonists of growth hormone releasing factor

Previous research on the favorable effects of mild conformational restriction in the N-terminal region of glucagon has led us to carry out analogue studies on the sequence-related 1-12 region of GRF(1-29)NH2. Replacement of each of the first 11 amino acids by its D-isomer in turn gave a total of 5 analogues exhibiting increases in potency. Other analogues containing multiple D-amino acid replacements were also examined and found to be highly potent, for instance: D-Tyr-1,D-Ala-2, 2630; His-1,D-Ala-2, 3440; Ac-His-1,D-Ala-2, 1574; D-Ala-2,Nle-27, 1840; D-Ala-2,D-Asn-8,Nle-27, 1580; D-Ala-2,D-Asp-3,D-Asn-8,Nle-27, 2000; D-Asp-3,D-Asn-8,Nle-27, 3810 (GRF(1-29) = 100%). It is possible that these results with D-isomers reflect the presence of reverse turns (beta-bends) in this region of GRF. Indeed, the qualitative predictive method of Chou and Fasman supports this theory and indicates reverse turns in the 1-5 and 6-10 sequences. Further studies were performed to test this hypothesis by introducing even more rigidity into the N-terminal region via disulfide bond formation between positions normally containing aromatic amino acids. None of the bridged peptides displayed biological activity which suggests that chain folding does not produce any proximity among N-terminal residues. We had shown previously that position 2(Ala) was extremely sensitive to both conformational and side-chain alterations. This observation was extended to analogues containing Sar and Pro, both of which were also inactive on GH release at the doses tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Octapeptide analogs of somatostatin exhibiting greatly enhanced in vivo and in vitro inhibition of growth hormone secretion in the rat

Analogs of a superactive somatostatin (SRIF) octapeptide (code named SMS 201-995 (1)) were synthesized using solid-phase synthetic methodology and assayed for their ability to inhibit growth hormone release from cultured rat anterior pituitary cells and in sodium pentobarbital-anesthetized rats. One analog: (Formula: see text) exhibited greatly enhanced in vitro inhibitory activity (greater than 1,000x) relative to both the parent octapeptide molecule and to the 14 amino acid SRIF molecule. This analog which was also very potent in vivo contains a tyrosine residue and, given its high in vitro activity, may be of investigative importance as a radioiodinated ligand in receptor assays. An octapeptide retro-inverso analog also exhibited significant SRIF-like activity. Several very low activity octapeptide analogs were synthesized and were found to be devoid of SRIF-antagonist activity. A dodecapeptide analog previously shown to be superactive in vivo also demonstrated high in vitro activity.

Structural requirements for the activation of rat anterior pituitary adenylate cyclase by growth hormone-releasing factor (GRF): discovery of (N-Ac-Tyr1, D-Arg2)-GRF(1-29)-NH2 as a GRF antagonist on membranes

The efficacy and potency of 14 GH-releasing factor (GRF) analogs, substituted in position 1 to 7, on adenylate cyclase activation in crude homogenates from rat anterior pituitary were related to those of human pancreatic GRF(1-29)-amide and vasoactive intestinal peptide. Among several D-amino acid substitutions, that in position 2 was the only one to yield a super-agonist [with a Kact (concentration required for half-maximal adenylate cyclase activation) 2 times lower than that of GRF(1-29)-NH2]. By contrast, D-isomer substitution in position 1 and 3 was without effect and D-isomer substitution in position 4, 6, or 7 decreased the affinity of the analog. The N-acetylated analog of GRF was as potent and active as the parent peptide, and the identity of the amino acid in position 2 of (N-Ac-Tyr1)-GRF(1-29)-NH2 proved to be determining for enzyme activation, with D-Phe2 and D-Trp2 derivatives acting as partial agonists and the (N-Ac-Tyr1,D-Arg2) analog being an efficient competitive antagonist of GRF(1-29)-NH2. With use of this antagonist, it was possible to demonstrate that GRF and vasoactive intestinal peptide receptors represent distinct entities in the rat anterior pituitary.

Adenohypophyseal response to hypophysiotropic hormones in male obese Zucker rats

Description of the recessive, homozygote obese Zucker rat (fafa) includes disorders of growth and reproduction. The aim of this study was to compare responsiveness of adenohypophyseal cells, obtained from male fafa rats and from their lean siblings, to growth hormone-releasing factor (GRF) and to luteinizing hormone-releasing hormone (LHRH). Pituitary cells were cultured for 4 days and were then challenged with either GRF-29 (the NH2-terminal 29 amino acid GRF peptide that expresses full biological activity of its parent 44 amino acid molecule) or [D-Trp6]LHRH (LHRH-A, an LHRH agonist). Medium was assayed for growth hormone (GH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) by radioimmunoassay. Dose-response curves were compared using the computer program ALLFIT. The median effective GRF-29 concentration (EC50) computed for hypophyseal cells cultured from lean animals (0.30 +/- 0.01 fM; means +/- SE of 4 experiments) was less (P less than 0.01) than that calculated for cells obtained from fafa rats (15.8 +/- 6.7 fM). In contrast, cells derived from lean littermates required a larger (EC50) concentration of LHRH-A than did gonadotrophs cultured from obese rats [58.2 +/- 1.2 vs. 10.7 +/- 1.2 pM (P less than 0.01) and 59.4 +/- 10.4 vs. 15.7 +/- 7.6 pM (P less than 0.05)] to secrete LH and FSH, respectively. Our data describe an attenuated pituitary response to GRF-29 and an enhanced response to LHRH-A in the fafa.

Prolonged inhibition of growth hormone secretion by peripheral injection of bombesin is mediated by somatostatin in the rat

Peripherally injected bombesin inhibits GH secretion in conscious, freely moving rats and in sodium pentobarbital-anesthetized rats. This inhibition of GH secretion is unusually prolonged, lasting up to 90 min after a single ip injection. The duration of inhibition of GH secretion by bombesin is greater than that observed for somatostatin (SRIF) in the same bioassay. The inhibition of GH release occurs concomitantly with stimulation of gastrin release and is independent of stimulatory effects of bombesin on plasma glucose. The structurally related mammalian gastrin-releasing peptide also inhibits GH secretion in the pentobarbital-anesthetized rat after peripheral injection. Peripherally administered bombesin blocks GH-releasing factor stimulation of GH secretion. Prior treatment of pentobarbital-anesthetized rats with SRIF-specific anti-serum blocks the inhibitory effect of bombesin on GH secretion. No effect of bombesin on GH secretion was observed in primary cultures of rat anterior pituitary cells. These data suggest that peripherally administered bombesin stimulates SRIF secretion, most probably of hypothalamic origin, which, in turn, inhibits pituitary secretion of GH. This sensitivity of the hypothalamus to a peripherally rather than centrally administered peptide has important mechanistic and therapeutic implications.

An extremely sensitive in vitro model for elucidating structure-activity relationships of growth hormone-releasing factor analogs

An improved rat anterior pituitary primary cell culture technique for studying GH-releasing activity of human pancreatic GH-releasing factor (hpGRF) and its analogs is described. Male pituitaries, dispersed by a combination of trypsin digestion and mechanical agitation, were plated at a density of 200,000 cells per well and cultured for 4 days. The attached cells were then stimulated with synthetic hpGRF which was comprised of the first 29 residues of the larger, originally isolated forms and which was amidated at the C-terminal (hpGRF-29). Analogs of hpGRF-29 which were modified in positions 1, 2, 3, or 7, and other secretagogues were similarly tested. Medium was collected after 3 h, and secreted hormone was measured by RIA. The cells were extremely sensitive to hpGRF-29 stimulation, and this effect was specific. The minimal effective dose of hpGRF-29 was an unprecedented 0.4 X 10(-15)M. No stimulation of LH, FSH, or PRL by hpGRF-29 was observed. Bombesin and vasoactive intestinal peptide were ineffective in stimulating GH release. [D-Trp6]LHRH (a potent LHRH agonist), also did not release GH but did stimulate secretion of LH and FSH at doses ranging from 0.4 X 10(-10)M to 1.0 X 10(-9)M. Responses of the cells to hpGRF-29 analogs were characterized by distinct heterologous dose-response curves. [D-Ala2]hpGRF-29 was 50 times more active than its parent 29-amino-acid peptide. [D-Thr7]hpGRF-29, another analog that differed from hpGRF-29 by the insertion of a D-isomer for the naturally occurring L-residue, was about 10,000 times less effective in stimulating GH secretion than was hpGRF-29 itself. Potencies of these and other analogs with respect to GH release in vitro were similar to those estimated in vivo. Thus, this primary cell culture provides an extremely sensitive, selective, and reproducible system for studying hpGRF structure-activity relationships. Further, such tremendous sensitivity to hpGRF can provide a system to study changes in pituitary sensitivity to hpGRF during different physiological states.

Administration of human pancreatic growth hormone-releasing factor (GRF) analogs enhances responsiveness of cultured rat pituitary cells to GRF

The aim of this study was to investigate whether anterior pituitary responsiveness to human pancreatic growth hormone-releasing factor containing 29 amino acids (GRF-29) can be modulated by GRF-29 itself. Male rats were injected (sc) daily for 3 days with 50 ug of GRF-29, or were treated twice daily for 14 days with 5 ug of [D-Ala-2]-GRF-29 (a potent GRF agonist). Control animals were injected with saline. After the last injection, pituitaries were removed, dispersed, cultured for 96 h and then challenged with either GRF-29 or [D-Trp-6]-LHRH (a LHRH agonist). Cultured cells from analog-treated rats were more responsive to GRF-29 stimulation than were cells obtained from controls. In contrast, neither treatment altered the response to [D-Trp-6]-LHRH. These studies indicate that periodic administration of GRF analogs can increase hypophyseal GRF responsiveness. Such control may be an important component in the physiological regulation of GH secretion and has important implications for potential therapeutic uses of GRF analogs.

Prolactin secretion by cultured anterior pituitary cells: influence of culture conditions and endocrine status of the pituitary donor

The characteristics of prolactin (PRL) secretion by cultured anterior pituitary cells in the presence and absence of catecholamines were studied. PRL secretion was markedly influenced by culture conditions such as cell density, culture duration and length of short-term incubation. Dopamine (DA) inhibited PRL release in a dose-dependent manner within a physiological range, and this inhibition was reversed by the stereospecific DA receptor antagonist (+)-butaclamol. In contrast, the inhibition of PRL secretion by norepinephrine (NE) required much higher doses and lacked specificity. Cells obtained from male donors had the lowest basal PRL secretion and were the least responsive to DA inhibition, whereas those obtained from females in late pregnancy had the highest basal PRL secretion and were the most sensitive to DA.

Increase in pituitary dopaminergic receptors after monosodium glutamate treatment

We investigated whether a decrease in arcuate nucleus dopamine (DA) levels resulting from neonatal treatment with monosodium glutamate (MSG) affects the anterior pituitary DA receptors in adult male rats. MSG treatment resulted in a significant reduction in medial basal hypothalamic (MBH) DA levels, no change in its norepinephrine (NE) and epinephrine (E) concentrations, and a marked increase in circulating prolactin (PRL). Scatchard analyses of DA binding characteristics to anterior pituitary membranes using [3H]spiperone revealed linear plots, suggesting a single class of high-affinity, low-capacity binding sites. The DA binding capacity was significantly higher in MSG-treated rats than in controls with no change in affinity. The data indicate that anterior pituitary DA receptors change in accordance with altered physiological conditions. The increase in the number of DA receptors following destruction of the arcuate nucleus is probably a direct effect of reduced DA levels reaching the anterior pituitary gland.

Dopaminergic receptors in the rat anterior pituitary change during the estrous cycle

We investigated whether rat anterior pituitary dopaminergic receptors change in affinity and/or number of binding sites (Bmax) during the estrous cycle and whether they correlate with circulating PRL levels. Dopamine receptors were quantitated in partially purified pituitary membranes by equilibrium binding using [3H]spiperone. AT 0900 h of diestrous day 2 and 0900 h of proestrus, Bmax [316.5 +/- 7.7 and 297.9 +/- 7.3 fmol/mg protein (mean +/- SE), respectively] and plasma PRL levels (18.2 +/- 4.2 and 25.0 +/- 7.3 ng/ml, respectively) were low. Bmax remained low (305.6 +/- 22.1 fmol/mg) at 1300 h of proestrus, but PRL increased 5-fold. At 1700 h of proestrus, Bmax significantly increased to 453.3 +/- 27.2 fmol/mg, coincident with a preovulatory PRL surge (241.5 +/- 45.4 ng/ml). By 0900 h of estrus, Bmax decreased to 389.5 +/- 20.1 fmol/mg and did not return to basal levels until diestrous day 2. PRL also decreased by 0900 h of estrus to 106.6 +/- 13.0 ng/ml and progressively fell to basal level by 0900 h of diestrous day 1. Receptor affinity was unchanged throughout the cycle (range, 0.08-0.15 nM). These data indicate that the number of dopaminergic binding sides changes significantly during the estrous cycle, with the sharpest increase in the afternoon of proestrus. Since a complex relationship is apparent between alterations in dopaminergic receptor number and plasma PRL levels, we postulate that these receptors may be regulated by dopamine, ovarian steroids, and/or PRL itself. Such a change in the number of dopaminergic binding sites may e an important component in pituitary responsiveness to dopamine inhibition of PRL secretion throughout the estrous cycle.

Spatial self-reference systems and shortest-route behavior in toddlers

2 experiments were conducted concerning the development of spatial orientation during the second year of life. Both experiments were focused on oriented search for a hidden target object in the absence of landmarks, which can be accomplished by relating one's movements to knowledge of a target's location. In experiment 1, 18-month-olds were tested to examine the precision with which they use information for the direction and magnitude of self-movement to keep track of the target location. Although the toddlers' search behavior was imprecise, the results showed that they appropriately modulated their search behavior according to the directions and magnitudes of their previous movements away from the hidden target. Experiment 2 was designed to determine whether toddlers can go beyond the information directly experienced in previous routes of travel to infer the shortest route to a hidden target. The results indicated that the 18-month-old and the highly selected 14-month-old subjects can perform spatial inferences of this type. The mechanisms through which these spatial abilities develop are discussed.