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 24 h were comparable, and so chose 12 h deprivation for the subsequent studies. In the second, 12 h 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 12 h fast. Mice were tested repeatedly with CCK-8 (2, 6, or 18 μg/kg ip) or BN (2, 4 or 8 μg/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.     

Increased short-term food satiation and sensitivity to cholecystokinin in neurotrophin-4 knock-in mice

Neurotrophin-4 (NT-4) knockout mice exhibited decreased innervation of the small intestine by vagal intraganglionic laminar endings (IGLEs) and reduced food satiation. Recent findings suggested this innervation was increased in NT-4 knock-in (NT-4KI) mice. Therefore, to further investigate the relationship between intestinal IGLEs and satiation, meal patterns were characterized using solid and liquid diets, and cholecystokinin (CCK) effects on 30-min solid diet intake were examined in NT-4KI and wild-type mice. NT-4KI mice consuming the solid diet exhibited reduced meal size, suggesting increased satiation. However, compensation occurred through increased meal frequency, maintaining daily food intake and body weight gain similar to controls. Mutants fed the liquid diet displayed a decrease in intake rate, again implying increased satiation, but meal duration increased, which led to an increase in meal size. This was compensated for by decreased meal frequency, resulting in similar daily food intake and weight gain as controls. Importantly, these alterations in NT-4KI mice were opposite, or different, from those of NT-4 knockout mice, further supporting the hypothesis that they are specific to vagal afferent signaling. CCK suppressed short-term intake in mutants and controls, but the mutants exhibited larger suppressions at lower doses, implying they were more sensitive to CCK. Moreover, devazepide prevented this suppression, indicating this increased sensitivity was mediated by CCK-1 receptors. These results suggest that the NT-4 gene knock-in, probably involving increased intestinal IGLE innervation, altered short-term feeding, in particular by enhancing satiation and sensitivity to CCK, whereas long-term control of daily intake and body weight was unaffected.     

The role of the stomach in the control of appetite and the secretion of satiation peptides

It is widely accepted that gastric parameters such as gastric distention provide a direct negative feedback signal to inhibit eating; moreover, gastric and intestinal signals have been reported to synergize to promote satiation. However, there are few human data exploring the potential interaction effects of gastric and intestinal signals in the short-term control of appetite and the secretion of satiation peptides. We performed experiments in healthy subjects receiving either a rapid intragastric load or a continuous intraduodenal infusion of glucose or a mixed liquid meal. Intraduodenal infusions (3 kcal/min) were at rates comparable with the duodenal delivery of these nutrients under physiological conditions. Intraduodenal infusions of glucose elicited only weak effects on appetite and the secretion of glucagon-like peptide-1 (GLP-1) and peptide YY (PYY). In contrast, identical amounts of glucose delivered intragastrically markedly suppressed appetite (P < 0.05) paralleled by greatly increased plasma levels of GLP-1 and PYY (≤3-fold, P < 0.05). Administration of the mixed liquid meal showed a comparable phenomenon. In contrast to GLP-1 and PYY, plasma ghrelin was suppressed to a similar degree with both intragastric and intraduodenal nutrients. Our data confirm that the stomach is an important element in the short-term control of appetite and suggest that gastric and intestinal signals interact to mediate early fullness and satiation potentially by increased GLP-1 and PYY secretions.     

Behavioral components of high-fat diet hyperphagia: meal size and postprandial satiety

Previously, rats fed a high-fat liquid diet (HF) ad libitum consumed more kilocalories and had greater weight gain than rats fed a liquid high-carbohydrate diet (HC) of equivalent energy density (Warwick, Z. S., and H. P. Weingarten. Am. J. Physiol. Regulatory Integrative Comp. Physiol. 269: R30-R37, 1995). The present series of experiments sought to clarify the behavioral expression of HF hyperphagia by comparing HF and HC with regard to meal size and magnitude of postingestive satiety effect. Meal size of HF was greater than HC at 2.3 kcal/ml and also when diets were formulated at 1.15 kcal/ml. In a preload-test meal paradigm, an orally consumed HF preload was less satiating than a calorically equivalent HC preload across a range of preload volumes and intermeal intervals. Sensory-specific satiety was ruled out as an explanation of the relatively greater intake of test meal after an HF preload meal; an intragastrically delivered HF preload was less satiating than intragastric HC. Furthermore, a fat (corn oil emulsion) preload was less satiating than a carbohydrate (sucrose) preload when an evaporated milk test meal was used. These findings indicate that hyperphagia on an HF diet is expressed in increased meal size and decreased intermeal interval.     

Prandial lactate infusion inhibits spontaneous feeding in rats

To investigate the acute effects of lactate on spontaneous feeding, we infused lactate in the hepatic portal vein (0.5, 1.0, and 1.5 mmol lactate/meal) or in the vena cava (1.0 and 1.5 mmol lactate/meal) of ad libitum-fed rats during their first spontaneous nocturnal meal. Infusions (5 min, 0.1 ml/min) were remotely controlled, and a computerized feeding system recorded meal patterns. In separate crossover tests, meal size decreased independent of the infusion route after 1.0 and 1.5 mmol but not after 0.5 mmol lactate. The subsequent intermeal interval (IMI) tended to decrease only after vena cava infusion of 1.0 mmol lactate. The size of the second nocturnal meal increased after the 1.0 mmol lactate infusion. Hepatic portal infusion of 1.5 mmol lactate increased the satiety ratio [subsequent IMI (min)/meal size (g)] by 175%, which was higher than the insignificant 43% increase after vena cava infusion. Hepatic portal infusion of 1.5 mmol lactate also increased systemic plasma lactate but not glucose concentration at 1 min after the end of infusion. The results are consistent with the idea that meal-induced increases in circulating lactate play a role in the control of meal size (satiation). Moreover, the results suggest that lactate also contributes to postprandial satiety and that the liver is involved in this effect. The exact mechanisms of lactate's inhibitory effects on feeding and the site(s) where lactate acts to terminate meals remain to be identified.     

Gastrin releasing peptide-29 evokes feeding responses in the rat

In mammals, gastrin releasing peptide (GRP) 10 and 27 reduce food intake. In the current work, we test the hypothesis that GRP-29, the large molecular form of GRP in the rat, also evokes feeding responses consistent with a possible role in satiety. Here, we measured three feeding responses, size of first meal, intermeal interval (IMI, time between first and second meal) and satiety ratio (SR, satiation period for every unit of food consumed in the first meal), in overnight food deprived rats following GRP-10, 27 or 29 (0, 0.3, 1.0, 2.1, 4.1, 10.3, 17.2 nmol/kg) intraperitoneally and presentation of a 10% sucrose test diet. GRP-29 and GRP-27 reduced the size of the first meal, prolonged IMI and increased SR, but GRP-10 failed to exhibit similar feeding responses. The order of potency was GRP-29 = GRP-27 > GRP-10. The current data support a role for GRP-29 in the short-term regulation of food intake.     

The feeding responses evoked by endogenous cholecystokinin are regulated by different gastrointestinal sites

The current study tested the hypothesis that cholecystokinin (CCK) A receptor (CCKAR) in areas supplied by the celiac artery (CA), stomach and upper duodenum, and the cranial mesenteric artery (CMA), small and parts of the large intestine, is necessary for reduction of meal size, prolongation of the intermeal interval (time between first and second meal) and increased satiety ratio (intermeal interval/meal size or amount of food consumed during any given unit of time) by the non-nutrient stimulator of endogenous CCK release camostat. Consistent with our previous findings camostat reduced meal size, prolonged the intermeal interval and increased the satiety ratio. Here, we report that blocking CCKAR in the area supplied by the celiac artery attenuated reduction of meal size by camostat more so than the cranial mesenteric artery route. Blocking CCKAR in the area supplied by the cranial mesenteric artery attenuated prolongation of the intermeal interval length and increased satiety ratio by camostat more so than the celiac artery route. Blocking CCKAR in the areas supplied by the femoral artery (control) failed to alter the feeding responses evoked by camostat. These results support the hypothesis that CCKAR in the area supplied by the CA is necessary for reduction of meal size by camostat whereas CCKAR in the area supplied by the CMA is necessary for prolongation of the intermeal interval and increased satiety ratio by this substance. Our results demonstrate that meal size and intermeal interval length by camostat are regulated through different gastrointestinal sites.     

Relationship Between Hunger-Satiety Feelings and Various Metabolic Parameters in Women with Obesity During Controlled Weight Loss

Objective : Satiety plays an important role in weight control. The meaning of fasting hormone levels and satiety feelings, and how post-absorptive changes after meals high in carbohydrate regulate appetite remains to be demonstrated. Research Methods and Procedures : Prospective metabolic study with 25 non-diabetic obese women at the Energy Metabolism Research Unit of the Department of Nutrition Sciences, University of Alabama at Birmingham. We analyzed fasting and postprandial ratings of hunger-satiety and values of various metabolic parameters (serum glucose and insulin, plasma cholecystokinin, respiratory quotient) during controlled weight loss. The postprandial measures were assessed following a test meal providing 320 kcal and yielding a food quotient of 0.89. Results : In the fasting state, there was no correlation between hunger-satiety ratings and any of the measured metabolic parameters. Under postprandial conditions, satiety was positively related to glucose (p = 0.002) and insulin (p = 0.002) responses to the test meal. In multivariate analysis including glucose, insulin, cholecystokinin, hunger-satiety ratings and respiratory quotient, insulin was the only independent predictor of satiety in the postprandial state. Discussion : These data suggest an association between the endogenous insulin response and feelings of postprandial satiety. Insulin's satiation properties, which could well be mediated by other hormones, may represent a primary factor of food intake regulation after meals relatively high in carbohydrate.     

Link between intestinal CD36 ligand binding and satiety induced by a high protein diet in mice

CD36 is a ubiquitous membrane glycoprotein that binds long-chain fatty acids. The presence of a functional CD36 is required for the induction of satiety by a lipid load and its role as a lipid receptor driving cellular signal has recently been demonstrated. Our project aimed to further explore the role of intestinal CD36 in the regulation of food intake. Duodenal infusions of vehicle or sulfo-N-succinimidyl-oleate (SSO) was performed prior to acute infusions of saline or Intralipid (IL) in mice. Infusion of minute quantities of IL induced a decrease in food intake (FI) compared to saline. Infusion of SSO had the same effect but no additive inhibitory effect was observed in presence of IL. No IL- or SSO-mediated satiety occurred in CD36-null mice. To determine whether the CD36-mediated hypophagic effect of lipids was maintained in animals fed a satietogen diet, mice were subjected to a High-Protein diet (HPD). Concomitantly with the satiety effect, a rise in intestinal CD36 gene expression was observed. No satiety effect occurred in CD36-null mice. HPD-fed WT mice showed a diminished FI compared to control mice, after saline duodenal infusion. But there was no further decrease after lipid infusion. The lipid-induced decrease in FI observed on control mice was accompanied by a rise in jejunal oleylethanolamide (OEA). Its level was higher in HPD-fed mice than in controls after saline infusion and was not changed by lipids. Overall, we demonstrate that lipid binding to intestinal CD36 is sufficient to produce a satiety effect. Moreover, it could participate in the satiety effect induced by HPD. Intestine can modulate FI by several mechanisms including an increase in OEA production and CD36 gene expression. Furthermore, intestine of mice adapted to HPD have a diminished capacity to modulate their food intake in response to dietary lipids.     

Synergism by individual macronutrients explains the marked early GLP-1 and islet hormone responses to mixed meal challenge in mice

Apart from glucose, proteins and lipids also stimulate incretin and islet hormone secretion. However, the glucoregulatory effect of macronutrients in combination is poorly understood. We therefore developed an oral mixed meal model in mice to 1) explore the glucagon-like peptide-1 (GLP-1) and islet hormone responses to mixed meal versus isocaloric glucose, and 2) characterize the relative contribution of individual macronutrients to these responses. Anesthetized C57BL/6J female mice were orally gavaged with 1) a mixed meal (0.285kcal; glucose, whey protein and peanut oil; 60/20/20% kcal) versus an isocaloric glucose load (0.285kcal), and 2) a mixed meal (0.285kcal) versus glucose, whey protein or peanut oil administered individually in their mixed meal caloric quantity, i.e., 0.171, 0.055 and 0.055kcal, respectively. Plasma was analyzed for glucose, insulin and intact GLP-1 before and during oral challenges. Plasma glucose was lower after mixed meal versus after isocaloric glucose ingestion. In spite of this, the peak insulin response (P=0.02), the peak intact GLP-1 levels (P=0.006) and the estimated β-cell function (P=0.005) were higher. Furthermore, the peak insulin (P=0.004) and intact GLP-1 (P=0.006) levels were higher after mixed meal ingestion than the sum of responses to individual macronutrients. Compared to glucose alone, we conclude that there is a marked early insulin response to mixed meal ingestion, which emanates from a synergistic, rather than an additive, effect of the individual macronutrients in the mixed meal and is in part likely caused by increased levels of GLP-1.     

Attenuated satiation response to intestinal nutrients in rats that do not express CCK-A receptors.

Pharmacological experiments suggest that satiation associated with intestinal infusion of several nutrients is mediated by CCK-A receptors. Otsuka Long-Evans Tokushima Fatty, (OLETF), rats do not express CCK-A receptors and are insensitive to the satiation-producing effects of exogenous CCK. To further evaluate the role of CCK-A receptors in satiation by intestinal nutrient infusion, we examined intake of solid (pelleted rat chow) or liquid (12.5% glucose) food intake, following intestinal infusions of fats (oleic acid or fat emulsion), sugars (maltotriose or glucose), or peptone in OLETF rats and Long Evans Tokushima Otsuka control rats (LETO). Intestinal infusion of glucose or maltotriose reduced solid food intake more in LETO than in OLETF rats from 30 min through 4 h post infusion. Reduction of solid food intake by intestinal infusions of fat or peptone did not differ between OLETF and LETO rats during the first 30 min post infusion, but reduction of intake by these infusates was attenuated in OLETF rats over the ensuing 4h post infusion. Intestinal infusion of glucose, oleate, fat emulsion and peptone reduced 30-min intake of 12.5% glucose more in LETO than OLETF rats. Furthermore, pretreatment with the CCK-A receptor antagonist, devazepide, attenuated intestinal nutrient-induced reduction of food intake only in LETO, but not OLETF rats. Our results confirm pharmacological results, indicating that CCK-A receptors participate in satiation by nutrients that elevate plasma CCK concentrations, as well as by nutrients that do not stimulate secretion of endocrine CCK. In addition, our results indicate: 1) that OLETF rats have deficits in the satiation response to a variety of intestinal nutrient infusions; 2) that the temporal pattern for CCK-A receptor participation in satiation by intestinal nutrients is different during ingestion of liquid and solid foods and 3) that intestinal nutrients provide some satiation signals that are CCK-A receptor mediated and some that are not.     

Cholecystokinin-33 inhibits meal size and prolongs the subsequent intermeal interval

There are various forms of the satiety gut-brain peptide cholecystokinin (CCK), a short, widely utilized form or CCK-8, and a long, putatively more effective form or CCK-33. The issue of which of these forms is a more effective satiety peptide is not resolved. Here, we compared the satiety responses, including the sizes of the first three meals (MS) and intermeal intervals (IMI) as well as their calculated satiety ratios (SR), evoked by both peptides. CCK-8 and 33 (1, 3 and 5 nmol/kg, i.p) reduced the size of the first meal similarly, only CCK-33 prolonged the first IMI and increased SR and both peptides failed to affect second and third MS and IMI. As such, CCK-33 is a more effective satiety peptide than CCK-8. The current results confirm previous findings which showed that both peptides reduce food intake by inhibiting meal size, whereas only CCK-33 reduces food intake by prolonging the intermeal interval.     

Leptin-induced satiation mediated by abdominal vagal afferents

Leptin is a hormone secreted into the systemic blood primarily by white adipose tissue. However, leptin also is synthesized and stored by cells in the gastric mucosa. Because gastric mucosal leptin is secreted in response to ingestion of a meal, we hypothesized that it might contribute to satiation (meal termination) by acting on gastrointestinal vagal afferent neurons. To test whether leptin is capable of acutely reducing short-term food intake, we measured consumption of a liquid meal (15% sucrose) following low-dose leptin administration via the celiac artery, which perfuses the upper gastrointestinal tract. Leptin (1, 3, 10 mug) was infused via a chronically implanted, nonocclusive celiac arterial catheter or via a jugular vein catheter with its tip in the right cardiac atrium. Fifteen percent sucrose intake was then measured for 30 min. We found that leptin dose dependently inhibited sucrose intake when infused through the celiac catheter but not when infused into the general circulation via a jugular catheter. Plasma leptin concentrations in the general circulation following celiac arterial or jugular leptin infusions were not significantly different. Celiac arterial leptin infusion did not reduce meal size in vagotomized or capsaicin-treated rats. Finally, we also found that reduction of meal size by celiac leptin infusion was markedly enhanced when coinfused with cholecystokinin, a gastrointestinal satiety peptide whose action depends on vagal afferent neurons. Our results support the hypothesis that leptin contributes to satiation by a mechanism dependent on gastrointestinal vagal afferent innervation of the upper gastrointestinal tract.     

GAT-1 regulates both tonic and phasic GABA(A) receptor-mediated inhibition in the cerebral cortex

γ-Aminobutyric acid 1 (GAT-1) is the most copiously expressed GABA transporter; we studied its role in phasic and tonic inhibition in the neocortex using GAT-1 knockout (KO) mice. Immunoblotting and immunocytochemical studies showed that GAT-2 and GAT-3 levels in KOs were unchanged and that GAT-3 was not redistributed in KOs. Moreover, the expression of GAD65/67 was increased, whereas that of GABA or VGAT was unchanged. Microdialysis studies showed that in KOs spontaneous extracellular release of GABA and glutamate was comparable in WT and KO mice, whereas KCl-evoked output of GABA, but not of glutamate, was significantly increased in KOs. Recordings from layer II/III pyramids revealed a significant increase in GABA_AR-mediated tonic conductance in KO mice. The frequency, amplitude and kinetics of spontaneous inhibitory post-synaptic currents (IPSCs) were unchanged, whereas the decay time of evoked IPSCs was significantly prolonged in KO mice. In KO mice, high frequency stimulation of GABAergic terminals induced large GABA_AR-mediated inward currents associated with a reduction in amplitude and decay time of IPSCs evoked immediately after the train. The recovery process was slower in KO than in WT mice. These studies show that in the cerebral cortex of GAT-1 KO mice GAT-3 is not redistributed and GADs are adaptively changed and indicate that GAT-1 has a prominent role in both tonic and phasic GABA_AR-mediated inhibition, in particular during sustained neuronal activity.     

Adipose-specific overexpression of GLUT4 reverses insulin resistance and diabetes in mice lacking GLUT4 selectively in muscle

Adipose tissue plays an important role in glucose homeostasis and affects insulin sensitivity in other tissues. In obesity and type 2 diabetes, glucose transporter 4 (GLUT4) is downregulated in adipose tissue, and glucose transport is also impaired in muscle. To determine whether overexpression of GLUT4 selectively in adipose tissue could prevent insulin resistance when glucose transport is impaired in muscle, we bred muscle GLUT4 knockout (MG4KO) mice to mice overexpressing GLUT4 in adipose tissue (AG4Tg). Overexpression of GLUT4 in fat not only normalized the fasting hyperglycemia and glucose intolerance in MG4KO mice, but it reduced these parameters to below normal levels. Glucose infusion rate during a euglycemic clamp study was reduced 46% in MG4KO compared with controls and was restored to control levels in AG4Tg-MG4KO. Similarly, insulin action to suppress hepatic glucose production was impaired in MG4KO mice and was restored to control levels in AG4Tg-MG4KO. 2-deoxyglucose uptake during the clamp was increased approximately twofold in white adipose tissue but remained reduced in skeletal muscle of AG4Tg-MG4KO mice. AG4Tg and AG4Tg-MG4KO mice have a slight increase in fat mass, a twofold elevation in serum free fatty acids, an approximately 50% increase in serum leptin, and a 50% decrease in serum adiponectin. In MG4KO mice, serum resistin is increased 34% and GLUT4 overexpression in fat reverses this. Overexpression of GLUT4 in fat also reverses the enhanced clearance of an oral lipid load in MG4KO mice. Thus overexpression of GLUT4 in fat reverses whole body insulin resistance in MG4KO mice without restoring glucose transport in muscle. This effect occurs even though AG4Tg-MG4KO mice have increased fat mass and low adiponectin and is associated with normalization of elevated resistin levels.     

Pancreatic glucagon fails to inhibit sham feeding in the rat

Rats were equipped with chronic gastric cannulas, and the effects of intraperitoneal injections of pancreatic glucagon on sham feeding, with cannulas open, and real feeding, with cannulas closed, were compared. Glucagon (100–2,500 μg/kg) suppressed cumulative food intake during real feeding tests 9–33%, but had no effect during sham feeding. Despite their increased food intakes, sham feeding rats took discrete meals terminated by the behavioral satiety sequence. In addition to not affecting total intake, glucagon failed to affect meal size, latency to rest, or intermeal interval during sham feeding. To investigate the possiblity that glucagon did not inhibit sham feeding because it did not elicit hyperglycemia, we measured hepatic vein blood glucose after glucagon injections in sham feeding rats: glucagon elicited marked hyperglycemia during sham feeding. Therefore, the absence of glucagon's satiety effect in sham feeding is not due to the lack of hepatic glycogenolysis and hyperglycemia. These results suggest that some mechanism other than hyperglycemia which normally accompanies food ingestion is necessary for glucagon to have a satiety effect.     

Vulnerability of synaptosomes from apoE knock-out mice to structural and oxidative modifications induced by A beta(1-40): implications for Alzheimer's disease

Apolipoprotein E (apoE) plays an important role in the response to central nervous system injury. The e4 allele of apoE and amyloid beta-peptide (Abeta) are associated with Alzheimer's disease (AD) and may be central to the pathogenesis of this disorder. Recent studies demonstrate evidence for neurodegeneration and increased lipid peroxidation in transgenic mice lacking apoE (KO). In the current study, synaptosomes were prepared from apoE KO mice to determine the role of apoE in synaptic membrane structure and to determine susceptibility to oxidative damage by Abeta(1-40). ApoE KO mice exhibited structural modifications to lipid and protein components of synaptosomal membranes as determined by electron paramagnetic resonance in conjunction with lipid- and protein- specific spin labels. Incubation with 5 microM Abeta(1-40) resulted in more severe oxidative modifications to proteins and lipids in apoE KO synaptosomes as measured by protein carbonyls, an index of protein oxidation, and TBARs and protein-bound 4-hydroxynonenal (HNE), markers of lipid oxidation. Together, these data support a role for apoE in the modulation of oxidative injury and in the maintenance of synaptic integrity and are discussed with reference to alterations in AD brain.     

The stomach, cholecystokinin, and satiety

The stomach of the rhesus monkey empties liquids in a fashion that varies with the character of the solutions. Physiological saline empties exponentially. Glucose solutions empty biphasically--rapidly for the first minutes, then slowly and proportionately to glucose concentration to deliver glucose calories through the pylorus at a regulated rate (0.4 kcal/min). This prolonged and regulated second phase of gastric emptying depends on intestinal inhibition of the stomach. Cholecystokinin (CCK), a hormone released by food in the intestine, is an inhibitor of gastric emptying. In vitro receptor autoradiography demonstrates CCK receptors to be clustered on the circular muscle of the pylorus. Exogenous CCK, in doses that inhibit gastric emptying, will reduce food intake only if combined with an infusion of saline in the stomach. These observations indicate how gastric distension can be a means for provoking satiety. The variably sustained distension produced by the stomach's slow, calorically regulated emptying could prolong intermeal intervals and thus permit high-calorie meals to inhibit further caloric intake over time. CCK, by directly inhibiting gastric emptying during a meal, could promote gastric distension and so restrict the duration and size of individual meals.     

Ultrasonic vocalizations induced by sex and amphetamine in M2, M4, M5 muscarinic and D2 dopamine receptor knockout mice

Adult mice communicate by emitting ultrasonic vocalizations (USVs) during the appetitive phases of sexual behavior. However, little is known about the genes important in controlling call production. Here, we study the induction and regulation of USVs in muscarinic and dopaminergic receptor knockout (KO) mice as well as wild-type controls during sexual behavior. Female mouse urine, but not female rat or human urine, induced USVs in male mice, whereas male urine did not induce USVs in females. Direct contact of males with females is required for eliciting high level of USVs in males. USVs (25 to120 kHz) were emitted only by males, suggesting positive state; however human-audible squeaks were produced only by females, implying negative state during male-female pairing. USVs were divided into flat and frequency-modulated calls. Male USVs often changed from continuous to broken frequency-modulated calls after initiation of mounting. In M2 KO mice, USVs were lost in about 70-80% of the mice, correlating with a loss of sexual interaction. In M5 KO mice, mean USVs were reduced by almost 80% even though sexual interaction was vigorous. In D2 KOs, the duration of USVs was extended by 20%. In M4 KOs, no significant differences were observed. Amphetamine dose-dependently induced USVs in wild-type males (most at 0.5 mg/kg i.p.), but did not elicit USVs in M5 KO or female mice. These studies suggest that M2 and M5 muscarinic receptors are needed for male USV production during male-female interactions, likely via their roles in dopamine activation. These findings are important for the understanding of the neural substrates for positive affect.