Inhibition of gastric emptying by bombesin-like peptides is dependent upon cholecystokinin-A receptor activation

Ileal interposition attenuates the satiety responses evoked by cholecystokinin-8 and -33

One of the possible mechanisms by which the weight-reducing surgical procedure ileal interposition (II) works is by increasing circulating levels of lower gut peptides that reduce food intake, such as glucagon like peptide-1 and peptide YY. However, since this surgery involves both lower and upper gut segments, we tested the hypothesis that II alters the satiety responses evoked by the classic upper gut peptide cholecystokinin (CCK). To test this hypothesis, we determined meal size (MS), intermeal interval (IMI) and satiety ratio (SR) evoked by CCK-8 and -33 (0, 1, 3, 5nmol/kg, i.p.) in two groups of rats, II and sham-operated. CCK-8 and -33 reduced MS more in the sham group than in the II group; CCK-33 prolonged IMI in the sham group and increased SR in both groups. Reduction of cumulative food intake by CCK-8 in II rats was blocked by devazepide, a CCK(1) receptor antagonist. In addition, as previously reported, we found that II resulted in a slight reduction in body weight compared to sham-operated rats. Based on these observations, we conclude that ileal interposition attenuates the satiety responses of CCK. Therefore, it is unlikely that this peptide plays a significant role in reduction of body weight by this surgery.

Caudal brainstem processing is sufficient for behavioral, sympathetic, and parasympathetic responses driven by peripheral and hindbrain glucagon-like-peptide-1 receptor stimulation

The effects of peripheral glucagon like peptide-1 receptor (GLP-1R) stimulation on feeding, gastric emptying, and energetic responses involve vagal transmission and central nervous system processing. Despite a lack of studies aimed at determining which central nervous system regions are critical for the GLP-1R response production, hypothalamic/forebrain processing is regarded as essential for these effects. Here the contribution of the caudal brainstem to the control of food intake, core temperature, heart rate, and gastric emptying responses generated by peripheral delivery of the GLP-1R agonist, exendin-4 (Ex-4), was assessed by comparing responses of chronic supracollicular decerebrate (CD) rats to those of pair-fed intact control rats. Responses driven by hindbrain intracerebroventricular (fourth i.c.v) delivery of Ex-4 were also evaluated. Intraperitoneal Ex-4 (1.2 and 3.0 microg/kg) suppressed glucose intake in both CD rats (5.0+/-1.2 and 4.4+/-1.1 ml ingested) and controls (9.4+/-1.5 and 7.7+/-0.8 ml ingested), compared with intakes after vehicle injections (13.1+/-2.5 and 13.2+/-1.7 ml ingested, respectively). Hindbrain ventricular Ex-4 (0.3 microg) also suppressed food intake in CD rats (4.7+/-0.6 ml ingested) and controls (11.0+/-2.9 ml ingested), compared with vehicle intakes (9.3+/-2.1 and 19.3+/-4.3 ml ingested, respectively). Intraperitoneal Ex-4 (0.12, 1.2, 2.4 microg/kg) reduced gastric emptying rates in a dose-related manner similarly for both CD and control rats. Hypothermia followed ip and fourth i.c.v Ex-4 in awake, behaving controls (0.6 and 1.0 C average suppression) and CD rats (1.5 and 2.5 C average suppression). Intraperitoneal Ex-4 triggered tachycardia in both control and CD rats. Results demonstrate that caudal brainstem processing is sufficient for mediating the suppression of intake, core temperature, and gastric emptying rates as well as tachycardia triggered by peripheral GLP-1R activation and also hindbrain-delivered ligand. Contrary to the literature, hypothalamic/forebrain processing and forebrain-caudal brainstem communication is not required for the observed responses.

Effects of oral preload, CCK or bombesin administration on short term food intake of melanocortin 4-receptor knockout (MC4RKO) mice

We investigated whether either heterozygous (HET) or homozygous (knockout, KO) disruption of the melanocortin type 4 receptor (MC4R) gene alters post ingestive responsiveness of mice. Specifically, we tested the hypothesis that hyperphagia in MC4RKO mice might be due to a deficit in processes that sustain intermeal intervals (satiety) and/or processes that terminate ongoing episodes of eating (satiation). To test satiety, mice drank an oral preload and then we monitored intake of a subsequent liquid diet test meal. To test satiation, we examined the effect of exogenous administration of cholecystokinin (CCK) and bombesin (BN) on the size of a liquid diet meal. Experiment 1 was comprised of two studies. In the first, we determined that the intake of all three genotypes following fasts of either 6, 12, or 24h were comparable, and so chose 12h deprivation for the subsequent studies. In the second, 12h fasted mice were allowed to consume a fixed preload, approximately 50% of their expected mean intake and, following delays of either 30 or 60 min, were allowed to consume to satiation. Compared with no preload, the preload significantly reduced meal size comparably in all three genotypes. The reduction in intake was greater when the test meal was presented 30 compared with 60 min after the preload, again with no genotype differences in this decay of satiety. In experiment 2, we administered either CCK or BN and examined suppression of meal size after a 12h fast. Mice were tested repeatedly with CCK-8 (2, 6, or 18 microg/kg ip) or BN (2, 4 or 8 microg/kg ip) with vehicle injection days intervening. The 30 min intakes of HET and KO mice were suppressed more than those of WT following either CCK or BN. These experiments suggest that diminished responsiveness to nutrients or gut satiety hormones is not responsible for hyperphagia in MC4RKO mice.

Bombesin, but not amylin, blocks the orexigenic effect of peripheral ghrelin

The interaction between ghrelin and bombesin or amylin administered intraperitoneally on food intake and brain neuronal activity was assessed by Fos-like immunoreactivity (FLI) in nonfasted rats. Ghrelin (13 microg/kg ip) increased food intake compared with the vehicle group when measured at 30 min (g/kg: 3.66 +/- 0.80 vs. 1.68 +/- 0.42, P < 0.0087). Bombesin (8 microg/kg) injected intraperitoneally with ghrelin (13 microg/kg) blocked the orexigenic effect of ghrelin (1.18 +/- 0.41 g/kg, P < 0.0002). Bombesin alone (4 and 8 microg/kg ip) exerted a dose-related nonsignificant reduction of food intake (g/kg: 1.08 +/- 0.44, P > 0.45 and 0.55 +/- 0.34, P > 0.16, respectively). By contrast, ghrelin-induced stimulation of food intake (g/kg: 3.96 +/- 0.56 g/kg vs. vehicle 0.82 +/- 0.59, P < 0.004) was not altered by amylin (1 and 5 microg/kg ip) (g/kg: 4.37 +/- 1.12, P > 0.69, and 3.01 +/- 0.78, respectively, P > 0.37). Ghrelin increased the number of FLI-positive neurons/section in the arcuate nucleus (ARC) compared with vehicle (median: 42 vs. 19, P < 0.008). Bombesin alone (4 and 8 microg/kg ip) did not induce FLI neurons in the paraventricular nucleus of the hypothalamus (PVN) and coadministered with ghrelin did not alter ghrelin-induced FLI in the ARC. However, bombesin (8 microg/kg) with ghrelin significantly increased neuronal activity in the PVN approximately threefold compared with vehicle and approximately 1.5-fold compared with the ghrelin group. Bombesin (8 microg/kg) with ghrelin injected intraperitoneally induced Fos expression in 22.4 +/- 0.8% of CRF-immunoreactive neurons in the PVN. These results suggest that peripheral bombesin, unlike amylin, inhibits peripheral ghrelin induced food intake and enhances activation of CRF neurons in the PVN.

Intracerebroventricular administration of MK-801 increases food intake through mechanisms independent of gastric emptying

Systemic or hindbrain administration of MK-801, a noncompetitive N-methyl-D-aspartate receptor antagonist, increases meal size. To examine whether MK-801 enhances intake by increasing gastric emptying, we administered MK-801 (2.0 microg/3.0 microl) into the fourth ventricle [intracerebroventricular (ICV)] and measured feeding and gastric emptying of 5-ml NaCl or 15% sucrose loads. In a parallel experiment, we examined food intake and gastric emptying following intraperitoneal (IP) injection of MK-801 (100 microg/kg). MK-801, either IP or ICV, increased 30-min sucrose intake compared with control (12.3 +/- 0.7 vs. 9.8 +/- 0.5 and 16.6 +/- 2.0 vs. 10.7 +/- 0.7 ml, for IP and ICV administration, respectively). Also, IP MK-801 increased 5-min gastric emptying of NaCl (4.13 +/- 0.1 ml emptied) and sucrose (3.11 +/- 0.1 ml emptied) compared with control (3.75 +/- 0.2 and 2.28 +/- 0.1 ml emptied for NaCl and sucrose loads, respectively). In contrast, ICV MK-801 did not alter NaCl emptying (3.82 +/- 0.1 ml emptied) compared with control (3.82 +/- 0.3 ml emptied) and actually reduced gastric emptying of sucrose (2.1 +/- 0.2 and 2.94 +/- 0.1 ml emptied, for MK and vehicle, respectively). These data confirm previous results that systemic as well as hindbrain injection of MK-801 increases food intake. However, because ICV MK-801 failed to increase gastric emptying, these results indicate that MK-801 increases food intake through mechanisms independent of altered gastric emptying.

5-HT3 receptors participate in CCK-induced suppression of food intake by delaying gastric emptying

Serotonin type 3 (5-HT(3)) receptors have been shown to participate in the negative-feedback control of food intake. We previously reported that cholecystokinin (CCK)-induced suppression of food intake is partly mediated through 5-HT(3) receptors when rats were tested on a preferred liquid diet, but whether such an effect occurs when they are tested on a solid maintenance diet is unknown. In the present study, we examined the effects of ondansetron, a selective 5-HT(3) antagonist, on CCK-induced suppression of solid chow intake. Intraperitoneal administration of ondansetron significantly attenuated 30- and 60-min CCK-induced reduction of food intake, with suppression being completely reversed by 120 min. It is not known whether 5-HT(3) receptors directly mediate CCK-induced satiation or whether their participation depends on CCK acting as part of a feedback cascade to inhibit ongoing intake. Because CCK-induced inhibition of sham feeding does not depend on additive gastric/postgastric-feedback signals, we examined the ability of ondansetron to reverse CCK-induced satiation in sham-feeding rats. Ondansetron did not attenuate reduction of sham feeding by CCK, suggesting that ondansetron does not directly antagonize CCK-satiation signals. CCK suppresses real feeding through a delay in gastric emptying. Ondansetron could attenuate CCK-induced reduction of food intake by reversing CCK-induced inhibition of gastric emptying. We found that blockade of 5-HT(3) receptors attenuates CCK-induced inhibition of gastric emptying of a solid meal, as well as saline and glucose loads. We conclude that 5-HT(3) receptors mediate CCK-induced satiation through indirect mechanisms as part of a feedback cascade involving inhibition of gastric emptying.

Peptide signals regulating food intake and energy homeostasis

The adiposity hormone leptin has been shown to decrease food intake and body weight by acting on neuropeptide circuits in the hypothalamus. However, it is not clear how this primary hypothalamic action of leptin is translated into a change in food intake. We hypothesize that the behavioral effect of leptin ultimately involves the integration of neuronal responses in the forebrain with those in the nucleus tractus solitarius in the caudal brainstem, where ingestive behavior signals are received from the gastrointestinal system and the blood. One example is the peptide cholecystokinin, which is released from the gut following ingestion of a meal and acts via vagal afferent nerve fibers to activate medial nucleus tractus solitarius neurons and thereby decrease meal size. While it is established that leptin acts in the arcuate nucleus in the hypothalamus to stimulate anorexigenic neurons that inhibit food intake while simulataneously inhibiting orexigenic neurons that increase food intake, the mechanisms linking these effects with regions of the caudal brainstem that integrate cues related to meal termination are unclear. Based on an increasing body of supportive data, we hypothesize that this integration involves a pathway comprising descending projections from neurons from the paraventricular nucleus to neurons within the nucleus tractus solitarius that are activated by meal-related satiety factors. Leptin's anorexic effect comprises primarily decreased meal size, and at subthreshold doses for eliciting an effect on food intake, leptin intensifies the satiety response to circulating cholecystokinin. The location of neurons subserving the effects of intracerebroventricular administration of leptin and intraperitoneal injection of cholecystokinin on food intake has been identified by analysis of Fos expression. These studies reveal a distribution that includes the paraventricular nucleus and regions within the caudal brainstem, with the medial nucleus tractus solitarius having the most pronounced Fos expression in response to leptin and cholecystokinin, and support the hypothesis that the long-term adiposity signal leptin and the short-term satiety signal cholecystokinin act in concert to maintain body weight homeostasis.

Bombesin and its family of peptides: prospects for the treatment of obesity

Bombesin, its family of bombesin-like peptides, and many other peptides/hormones modulate biological and behavioral functions in animals. Among the wide variety of functions influenced by bombesin/bombesin-like peptides, the most prominent may be their role in feeding-related behavior. Over many years, intensive psychopharmacological studies have addressed the mechanisms by which these peptides induce feeding suppression, and the results suggest the applicability of bombesin/bombesin-like peptides for the treatment of eating disorders and/or obesity in humans. Recent studies using gene-knockout mice also shed new light on the relationship between bombesin/bombesin-like peptides and feeding behavior. In addition, genetic analyses of the possible links between bombesin/bombesin-like peptides/receptors and human obesity have also been undertaken. Here, we briefly review the literature pertaining to the relationship between bombesin/bombesin-like peptides and feeding behavior-with particular attention to human subjects-and discuss the pharmacotherapeutic potential of bombesin/bombesin-like peptides with regard to obesity.