Effects of agouti-related protein, orexin and melanin-concentrating hormone on oxygen consumption in mice

Neuropeptides in the hypothalamus play a pivotal role in the regulation of energy balance. Agouti-related protein (AGRP), orexin and melanin-concentrating hormone (MCH) have been identified in the hypothalamus as orexigenic peptides. This study was undertaken to investigate the effects of AGRP, orexin and MCH on oxygen consumption. Oxygen consumption was determined by an O2/CO2 metabolism measuring system at 22 degrees C. Mice were kept unrestrained in the chamber without food or water during the light cycle, and the oxygen consumption was measured for 2 h after intra-cerebroventricular (ICV) administration. ICV administered AGRP (1 nmol/mouse) significantly decreased oxygen consumption compared to ACSF-treated controls. Orexin (1 nmol/mouse) significantly increased oxygen consumption, while MCH (1 nmol/mouse) had no significant effect compared to ACSF-treated controls. These results suggest that AGRP, orexin and MCH might have different effects on energy expenditure, thereby regulating appetite and body weight.

Anorexia in space and possible etiologies: an overview

Space travelers experience a flight duration-dependent loss in weight and body mass while in a microgravity environment, despite the absence of increased energy expenditure. Anorexia in space can lead to in-flight caloric deficits of 1330 kcal per 70 kg astronaut per day in the presence of abundant food and has a critical effect on endurance and performance. Microgravity, alterations in the light-and-dark cycle, and exposure to radiation energy are the environmental stresses believed to influence appetite, food intake, and gastrointestinal function during space flight. Review of data and recent studies in rodents during microgravity showed a release of stress hormones and complex neuroendocrine and physiologic changes involving the modulation of hypothalamic activity, food intake-related hormones, and cytokines. The shift of dietary preference to carbohydrates, which occurs in astronauts, denotes a stress physiologic response and augments free-plasma tryptophan concentration in the brain, the precursor of the potent anorexic agent, serotonin (5-HT). Alterations of other neuroendocrine mediators, including corticotropin-releasing factor (CRF), coordinate the stress response, leading to a decrease in appetite and gastrointestinal function. Our laboratories used the antiorthostatic tail-suspension technique to successfully mimic some of these anorexia-related stress responses and to directly demonstrate the role of 5-HT in microgravity-related decreased food intake and delayed gastric emptying. Further rodent studies from our laboratories demonstrated the adverse effect of altered dark-and-light cycles on food intake and body weight. Radiation energy, through its documented effects on appetite, probably contributes to the decreased caloric intake by astronauts. Modulation of hypothalamic activity, 5-HT, and CRF play a critical role in anorexia related to microgravity and circadian rhythm alterations. Specific gene knockout mice (e.g., 5-HT or CRF and their respective receptors) may prove fruitful in defining the pathways by which anorexia in space occurs. An understanding of these pathophysiologic problems as they relate to appetite, food intake, gastric emptying and gastrointestinal function, sufficiently to derive successful practical solutions, may lead to a quantitative enhancement of physiologic well-being and performance status, serving as a productive countermeasure in space.

Gastric bypass model in the obese rat to study metabolic mechanisms of weight loss

A rat model replicating gastric bypass with Roux-en-Y (GB) as used in morbidly obese patients, evolved in our laboratory in stages, using the Zucker rat as an obese model (GB) is presented. In the final model, a 20% gastric fundic pouch to limit the gastric reservoir was created using two staple lines (Ethicon). A 4- to 5-mm end-to-side gastrojejunostomy and a 6- to 8-mm jejunojejunostomy, at 10 cm length of the Roux-en-Y limb, placed 16 cm below the ligament of Treitz, was hand sewn to create a limited area of nutrient digestion and absorption. Controls underwent sham operation. Rats were divided into: (i) sham-op ad lib-fed (Control); (ii) GB; and (iii) sham-op pair fed (PF) in two experiments. In Experiment 1, 24 Zuckers (control n = 8; GB n = 8; PF n = 8) were studied to assess the effectiveness of the model for weight loss. In Experiment 2, 24 Zuckers (8/group) were studied to confirm the effects of the operation on weight loss and on metabolic parameters. Boost was given for 4 days starting 24 h postop and then ground chow was given. Daily food intake (FI), meal size (MZ), meal number (MN), and body weight (BW) were measured. Rats were sacrificed on Day 20 in Experiment 1 and on Day 10 in Experiment 2. Serum metabolites and body fat weight were measured. Data were evaluated using Student's t test. Controls steadily gained BW (5.2-6.1 g/day), reaching approximately 500 g. In GB: FI, MZ, MN, BW, glucose, free fatty acids, insulin, and body fat decreased (P < 0.05). In PF: BW, insulin, triglycerides, and body fat decreased. A dependable, reproducible gastric bypass with Roux-en-Y obesity model was developed. This permits the study of biochemical and eventually molecular mechanism(s) of weight loss resulting from the operation.

Distribution of serotonin 5-hydroxytriptamine 1B (5-HT(1B)) receptors in the normal rat hypothalamus

This is the study first attempting to evaluate distribution of neurons expressing serotonin 5-hydroxytriptamine 1B (5-HT(1B)) receptors in hypothalamus by using immunocytochemistry. The 5-HT(1B)-immunoreactive neurons were widely distributed in hypothalamus. Accumulations of 5-HT(1B) neurons occurred in magnocellular nuclei, supraoptic nucleus, paraventricular nucleus (dorsolateral part) and accessory perifornical, circular and retrochiasmatic nuclei. Magnocellular neurons manifested an intense immunostaining suggesting a high level of 5-HT(1B) receptors. Large and middle-sized neurons with different 5-HT(1B) staining patterns were scattered throughout lateral hypothalamus, periventricular nucleus and lateral preoptic area. Immunofluorescent double-labeling revealed a great overlapping of the distribution 5-HT(1B) neurons and dense network of neuropeptide Y-immunoreactive fibers in paraventricular, supraoptic and arcuate nuclei. The potential functional significance of 5-HT(1B) receptors in the 5-HT control of endocrine functions and feeding are discussed.

Central mechanisms involved with catabolism

PURPOSE OF REVIEW

Catabolism conjures up an end-metabolic process in which muscle and fat tissue are broken down into their constituent parts to provide nutrients for the body, secondary to a noxious stimulus that prevents the organism from adequately nourishing itself. However, catabolism is a primary event, initiated in the brain in response to perceived or real stresses or noxious stimuli, which has a secondary effect of inhibiting food intake and consequently the break down of skeletal muscle and adipose tissues to provide nutrients for the body to survive.

RECENT FINDINGS

This is achieved via a cascade of neurohormonal monoaminergic and peptidergic mediators in the central nervous system, invoking the cortex, the limbic system and the hypothalamus. Among the most detailed mediators studied are corticotropin-releasing factor and serotonin which, via the hypothalamic-pituitary-adrenal axis and the sympathetic and parasympathetic nervous system, stimulate catecholamines and cortisol and inhibit anabolic hormones, insulin, leptin, ghrelin, including neuropeptide Y and other neuropeptides, among them the paracrine-acting cytokines. Simultaneously, there occurs stimulation of the counter-regulatory hormones cortisol, glucagon and the melanocortin family of neuropeptides.

SUMMARY

The net effect is anorexia, with the inhibition of food intake, body weight loss, delayed gastric emptying and functions, the stimulation of gluconeogenesis, glycogenolysis and ketogenesis as sources of metabolic fuel, which if unabated leads ultimately to cachexia. The use of antagonists and the removal of stress or noxious stimuli experimentally test different pathways of this dynamic metabolic picture. Several studies have demonstrated important progress towards our understanding of the central mechanisms involved in anorexia and weight loss, which we summarize in this review.

Hypothalamic dopaminergic receptor expressions in anorexia of tumor-bearing rats

Our past microdialysis studies in ventromedial hypothalamic nucleus (VMN) and lateral hypothalamic area (LHA) of changes in dopamine concentrations in response to changes in food intake [characterized as feeding pattern (changes in meal number and size)] in anorexia of cancer show abnormal presynaptic dopaminergic neurotransmission. To determine postsynaptic receptor status, studies were done in tumor-bearing (TB) and non-tumor-bearing (NTB) free-feeding control rats while continuously measuring their food intake via a rat eater meter. When TB rats developed anorexia, TB and control rats were killed, and postsynaptic D(1)- and D(2)-receptor mRNA expression in LHA and VMN were measured via RT-PCR. At anorexia, food intake decreased initially by a decrease in meal number, whereas a concurrent increase in meal size occurred for 24 h in an attempt to maintain food intake constant. Then meal size also decreased. At this time, D(1)- and D(2)-receptor mRNA expressions in LHA and VMN of TB vs. controls were significantly upregulated. Verification of D(1)- or D(2)-receptor changes to changes in meal number and size at anorexia was made by injection of intra-VMN or -LHA dopaminergic receptor antagonists. Intra-VMN D(1)-receptor antagonist (SCH-23390) in TB rats decreased food intake mainly via a decrease in meal size. Intra-VMN D(2)-receptor antagonist (sulpiride) in TB rats increased food intake via an increase in meal number and in NTB free-feeding rats by an increase in meal size. Intra-LHA D(1)-receptor antagonist in TB rats had no effect on food intake or feeding pattern. Intra-LHA D(2)-receptor antagonist in TB and in NTB free-feeding rats increased food intake via an increase in meal number. Our data provide evidence that postsynaptic dopaminergic receptor subtypes in the hypothalamus are involved in the regulation of meal size, meal number, and thus food intake in anorectic TB rats.

Hepato-vagal pathway associated with nicotine's anorectic effect in the rat

Nicotine reduces appetite and body weight. Because the hepato-portal area senses different types of nutrients that transmit signals via vagal afferent nerves to the hypothalamus to modify food intake and feeding pattern, we investigated the effect of nicotine on a hepato-vagal-hypothalamic pathway. Low doses of nicotine (< 10 ng) injected into portal vein (i.p.v.) decreased, while high doses of nicotine increased (> or = 10 ng) electrophysiological activity of hepatic vagal afferents. Stimulatory effect of high dose of nicotine on vagal hepatic afferents was blocked by a prior i.p.v. injection of curare (30 microg) or hexamethonium (1 mg). Furthermore, activities of gastric vagal and adrenal sympathetic efferents were suppressed by low-dose, but stimulated by high-dose i.p.v. nicotine. These reflex effects did not occur in hepatic vagotomized rats. Results of experiments demonstrate that in addition to nicotine's anorectic effect being mediated via a direct central action, nicotine also acts peripherally via hepatic vagal afferents from sensors of nicotine in the hepato-portal region.

Nicotine alters the usual reciprocity between meal size and meal number in female rat

Tobacco smoking reduces appetite and body weight (BW). Cessation of smoking leads to hyperphagia and weight gain. Daily food intake (FI) is a function of meal number (MN) and meal size (MZ), i.e., FI=MNxMZ. Under normal conditions, the female Fischer rat has a periodic reciprocal fluctuation between MZ and MN corresponding to phase of estrous cycle. Wide fluctuations between MZ and MN compensate each other to keep FI constant. Nicotine (5 mg/kg BW/day) was infused via osmotic minipump for 7 days. Controls received saline. FI, MZ, and MN were measured by an Automated Computerized Rat Eater Meter. Nicotine significantly decreased BW and FI via a decrease in MZ without compensatory increase of MN. Nicotine cessation led to hyperphagia, normalizing BW loss via an increase in MZ, which exceeded a compensatory decrease in MN. Nicotine significantly prolonged the estrous cycle by an extension of proestrous phase. Nicotine significantly lengthened the intermeal interval (IMI), delaying the start of the next meal and simultaneously decreasing subsequent MZ. Stopping nicotine led to normalization of IMI and MZ. Data show that nicotine alters the usual reciprocal regulation between MZ and MN and leads to a prolongation of the estrous cycle.

Gender differences in tumor-induced anorectic feeding pattern in Fischer-344 rats

Gender differences of feeding pattern in normal male and female rats are well recognized. Differences in gender-related feeding patterns have also been established following a variety of experimental manipulations, such as hypothalamic lesions, nicotine infusion, and total parenteral nutrition administration. Anorexia is a common feature during tumor growth. The present study examined whether the feeding indices constituting the feeding patterns differed with the development of cancer anorexia in male and female rats. Sixteen male and 15 female Fischer-344 rats had their food intake (FI) and feeding indices, meal number (MN) and meal size (MZ), continuously measured by a computerized rat eater meter. Viable methylcholanthrene (MCA) sarcoma cells (10(6)) were inoculated subcutaneously in 10 male (M-TB) and 8 female (F-TB) Fischer rats, while the rest were controls and received an equal volume of vehicle. Tumor-bearing (TB) rats became anorectic by Day 18, when the weight of the tumor was approximately 8% of the total body weight (BW). A notable decrease in BW was observed in both M-TB and F-TB. A decrease in FI resulted from different feeding indices between male and female rats. In male rats, lower FI was due to a decrease in both MN and MZ. In female rats, lower FI was solely due to a decrease in MN. The data show that gender differences in feeding patterns, which are an external manifestation of biochemical changes in the brain, occur following development of cancer-related anorexia suggesting that besides other factors, cancer anorexia is also influenced by sex-related hormones.

Intra-supraoptic nucleus sulpiride improves anorexia in tumor-bearing rats

Previous studies suggest that the dopaminergic system in the supraoptic nucleus (SON) is involved not only in the water balance control but also in the food intake regulation. Since we reported that an injection of the D2 receptor antagonist, sulpiride, into specific hypothalamic nuclei (e.g. the LHA, or the VMN) increases food intake in anorectic tumor-bearing rats, as well as in normal rats, we hypothesized that an injection of sulpiride into the SON would also improve cancer anorexia. Sulpiride injection (4 microg/0.5 microl) into bilateral SON of anorectic tumor-bearing male rats significantly improved food intake via increases in both meal size and meal number. These data suggest that pharmacological manipulation of the hypothalamic dopaminergic system is feasible in amelioration of cancer anorexia.

Role of hypothalamic monoamines in nicotine-induced anorexia in menopausal rats

BACKGROUND

Nicotine reduces body weight by reducing appetite. Estradiol modulates food intake. Menopause or ovariectomy (Ovx) increases food intake and body weight. Nicotine and estradiol individually influence hypothalamic dopamine (DA) and serotonin (5-HT), whose interaction influences food intake and body weight. We investigated whether lower weight gain in menopausal smokers is mediated via changes in hypothalamic DA/5-HT.

METHODS

Ovx or sham-operated female rats had 2 microdialysis guide cannulas simultaneously implanted in ipsilateral ventromedial nucleus of hypothalamus (VMN) and contralateral lateral hypothalamic area (LHA). Rats were divided into 4 groups and received a continuous subcutaneous infusion of nicotine or saline Ovx and sham. DA and 5-HT in LHA and VMN were measured by in vivo microdialysis.

RESULTS

Nicotine infusion decreased food intake and body weight in Ovx and sham groups. Increase in LHA-DA and VMN-5-HT in sham group occurred with nicotine, whereas an increase in VMN-DA in Ovx groups with and without nicotine and VMN-5-HT in Ovx group with nicotine was observed.

CONCLUSIONS

In the presence of estradiol (ovary intact sham-operated rats), nicotine lowers food intake and body weight via increased LHA-DA and VMN-5-HT. In menopause (Ovx rats), nicotine lowers food intake and body weight only via increased VMN-DA and 5-HT. Data show that lower weight gain is mediated via changes in hypothalamic monoamines, primarily via ventromedial hypothalamus.

Feeding behavior during sialodacryoadenitis viral infection in rats

Sialodacryoadenitis (SDA) is a highly contagious common viral infection in rats, akin to mumps in humans. Anorexia occurs during such viral infection. But the pattern of the decrease in food intake (a decrease in either meal size and meal number or both) during spontaneous viral infection has not been previously characterized. We observed the onset of anorexia and an abnormal feeding pattern during an opportunistic SDA viral infection in our rat colony. We thus studied seven male rats. Before the viral infection there was a positive association between food intake and meal number (P<.05). After infection food intake decreased by 68%. This occurred via a significant decrease in meal size (by 69%) (P<.05); and a nonsignificant decrease in meal number (P=.71). This pattern of decreased food intake is similar to that occurring during indomethacin-induced ulcerative ileitis, where we previously measured an increase in plasma tumor-necrosis factor (TNF)-alpha. Anorexia in response to bacterial lipopolysaccharide administration, which is also linked to plasma TNF-alpha, is however, caused only via a decrease in meal number. The differences in the decrease in the feeding pattern between the SDA viral and a bacterial infection suggest that factors other than TNF-alpha alone play a significant role in the mechanism of anorexia during a viral infection.

Synchronized release of dopamine and serotonin in the medial and lateral hypothalamus of rats

A positive linear correlation between dopamine and serotonin release was found in the ventromedial hypothalamus and in the lateral hypothalamic area in fasting rats and in fed rats during intermeal intervals. Dopamine release in the ventromedial hypothalamus positively correlated with dopamine and serotonin release in the lateral hypothalamic area, which occurred only during intermeal intervals and was non-significant during the meal consumption periods or during fasting. Meal size correlated significantly only with a decrease in serotonin release in the lateral hypothalamic area. The study was designed to evaluate the relationship between dopamine and serotonin release in these hypothalamic areas and their dependence on feeding status. Microdialysis was performed simultaneously via two probes, one in the ventromedial hypothalamus and the other in the contralateral lateral hypothalamic area, of freely moving male lean Zucker rats over 24h with preserved light and dark phase, either with ad libitum access to food and water, or when no food was available. Dopamine and serotonin concentrations were measured by high-performance liquid chromatography with electrochemical detection in 20-min dialysis samples. Time-series analysis was applied to determine linear correlations between monoamines and in relation to food intake. Data showed that release of dopamine and serotonin is synchronized within the ventromedial hypothalamus and lateral hypothalamic area, particularly in the dark phase and when no food was ingested. However, synchronized release of monoamines between these nuclei occurred only during intermeal intervals: the periods of satiety. These findings suggest a tight relationship between dopaminergic and serotonergic systems of the lateral hypothalamic area and ventromedial hypothalamus, which is influenced by the feeding state and which may be involved in maintaining the balance within and between the centers of the parasympathetic and sympathetic nervous systems. The data also illustate that food intake is coupled unequivocally to the release of dopamine and serotonin in the hypothalamus, suggesting it as a mechanism of activation of postsynaptic neurons associated with new metabolic status.

Comparison of early feeding pattern dynamics in female and male rats after reversible ventromedial nucleus of hypothalamus block

BACKGROUND

In the hypothalamus, a number of interconnected foci, including the ventromedial nucleus of hypothalamus (VMN), interact to control food intake (FI). FI is a function of meal number (MN) and meal size (MZ). Because gender differences exist in feeding patterns, we aimed at investigating the role of the VMN in determining the relationship of MZ and MN in female and male rats.

METHODS

FI and feeding patterns of 10 female and 12 male Fischer-344 rats were studied after VMN block, achieved via stereotaxically located intra-VMN microinjection of the neuronal blocker, colchicine (0.32 microgram dissolved in 50 nL of isotonic injectate and instilled on each side into the VMN).

RESULTS

After colchicine injection in normal female rats, an immediate and significant increase in FI occurs as a result of the following: 1) increased MZ in dark and light phases and 2) increased light phase MN with consequent loss of the normal diurnal cycle in FI. Recovery of feeding cycle and normal vaginal smear pattern occurred by study's end. In normal male rats, VMN block resulted in the following: 1) an increase in FI resulting from increased MN occurring predominantly during the light phase, thereby 2) disrupting the usual light/dark feeding cycle.

CONCLUSIONS

Sexual differences in regulation of FI occur after temporary reversible VMN block: in female rats, MZ is more sensitive to experimental modulation, whereas in male rats it is MN.

Hypothalamic dopamine and serotonin in the regulation of food intake

Because daily food intake is the product of the size of a meal and the frequency of meals ingested, the characteristic of meal size to meal number during a 24-h light-dark cycle constitutes an identifiable pattern specific to normal states and obesity and that occurs during early cancer anorexia. An understanding of simultaneous changes in meal size and meal number (constituting a change in feeding patterns) as opposed to an understanding of only food intake provides a more insightful dynamic picture reflecting integrated behavior. We have correlated this to simultaneous changes in dopamine and serotonin concentrations and to their postsynaptic receptors, focusing simultaneously on two discrete hypothalamic food-intake-related nuclei, in response to the ingestion of food. The relation between concentrations of dopamine and serotonin limited to the lateral hypothalamic area (LHA) and the ventromedial nucleus (VMN) as they relate to the influence of meal size and meal number during the hyperphagia of obesity and anorexia of cancer as measured in our experiments are discussed. Based on these data, conceptual models are proposed concerning: 1) an "afferent-efferent neurotransmitter unit," with facilitatory or inhibitory neuropeptide properties to generate an appropriate neuroendocrine and neuronal response that ultimately modifies food intake; 2) initiation and termination of a meal, thereby determining the number and size of a meal under normal conditions; and 3) a schema integrating the onset mechanism of cancer anorexia. Nicotine is used as a tool to further explore the relation of meal size to meal number, with a focus on simultaneous changes in dopamine and serotonin concentrations in the LHA and VMN with the onset of acute anorexia of nicotine infusion and acute hyperphagia of nicotine cessation. Data concerning the role of sex-related hormones on dopamine and serotonin with regard to the LHA and VMN in relation to the modulation of food intake are also presented.