Effects of different intermittent peptide YY (3-36) dosing strategies on food intake, body weight, and adiposity in diet-induced obese rats

Chronic administration of anorexigenic substances to experimental animals by injections or continuous infusion typically produces either no effect or a transient reduction in food intake and body weight. Our aim here was to identify an intermittent dosing strategy for intraperitoneal infusion of peptide YY(3-36) [PYY(3-36)] that produces a sustained reduction in daily food intake and adiposity in diet-induced obese rats. Rats (665+/-10 g body wt, 166+/-7 g body fat) with intraperitoneal catheters tethered to infusion swivels had free access to a high-fat diet. Vehicle-treated rats (n=23) had relatively stable food intake, body weight, and adiposity during the 9-wk test period. None of 15 PYY(3-36) dosing regimens administered in succession to a second group of rats (n=22) produced a sustained 15-25% reduction in daily food intake for >5 days, although body weight and adiposity were reduced across the 9-wk period by 12% (594+/-15 vs. 672+/-15 g) and 43% (96+/-7 vs. 169+/-9 g), respectively. The declining inhibitory effect of PYY(3-36) on daily food intake when the interinfusion interval was >or=3 h appeared to be due in part to an increase in food intake between infusions. The declining inhibitory effect of PYY(3-36) on daily food intake when the interinfusion interval was <3 h suggested possible receptor downregulation and tolerance to frequent PYY(3-36) administration; however, food intake significantly increased when PYY(3-36) treatments were discontinued for 1 day following apparent loss in treatment efficacies. Together, these results demonstrate the development of a potent homeostatic response to increase food intake when PYY(3-36) reduces food intake and energy reserves in diet-induced obese rats.     

Chronic injection of pansomatostatin agonist ODT8-SST differentially modulates food intake and decreases body weight gain in lean and diet-induced obese rats

The aim of this study was to investigate the central actions of the stable pansomatostatin peptide agonist, ODT8-SST on body weight. ODT8-SST or vehicle was acutely (1μg/rat) injected or chronically infused (5μg/rat/d, 14d) intracerebroventricularly and daily food intake, body weight and composition were monitored. In lean rats, neither acute nor chronic ODT8-SST influenced daily food intake while body weight was reduced by 2.2% after acute injection and there was a 14g reduction of body weight gain after 14d compared to vehicle (p<0.01). In diet-induced obese (DIO) rats, chronic ODT8-SST increased cumulative 2-week food intake compared to vehicle (+14%, p<0.05) and also blunted body weight change (-11g, p<0.05). ODT8-SST for 14d reduced lean mass (-22g and -25g respectively, p<0.001) and total water (-19g and -22g respectively, p<0.001) in lean and DIO rats and increased fat mass in DIO (+16g, p<0.001) but not lean rats (+1g, p>0.05) compared to vehicle. In DIO rats, ODT8-SST reduced ambulatory (-27%/24h, p<0.05) and fine movements (-38%, p<0.01) which was associated with an increased positive energy balance compared to vehicle (+50g, p<0.01). Chronic central somatostatin receptor activation in lean rats reduces body weight gain and lean mass independently of food intake which is likely related to growth hormone inhibition. In DIO rats, ODT8-SST reduces lean mass but promotes food intake and fat mass, indicating differential responsiveness to somatostatin under obese conditions.     

Effects of intermittent intraperitoneal infusion of salmon calcitonin on food intake and adiposity in obese rats

Chronic administration of anorexigenic substances to experimental animals by injections or continuous infusion typically produces no effect or a transient reduction in daily food intake and body weight. Our aim was to identify an intermittent dosing strategy for intraperitoneal infusion of salmon calcitonin (sCT), a homolog of amylin that produces a sustained 25-35% reduction in daily food intake and adiposity in diet-induced obese rats. Rats (649 +/- 10 g body wt, 27 +/- 1% body fat), with intraperitoneal catheters tethered to infusion swivels, had free access to a 45% fat diet. Food intake, body weight, and adiposity during the 7-wk test period were relatively stable in the vehicle-treated rats (n = 16). None of 10 sCT dosing regimens administered in succession to a second group of rats (n = 18) produced a sustained 25-35% reduction in daily food intake for >5 days, although body weight and adiposity were reduced by 9% (587 +/- 12 vs. 651 +/- 14 g) and 22% (20.6 +/- 1.2 vs. 26.5 +/- 1.1%), respectively, across the 7-wk period. The declining inhibitory effect of sCT on daily food intake with the 6-h interinfusion interval appeared to be due in part to an increase in food intake between infusions. The declining inhibitory effect of sCT on daily food intake with the 2- to 3-h interinfusion interval suggested possible receptor downregulation and tolerance to frequent sCT administration; however, food intake increased dramatically when sCT was discontinued for 1 day after apparent loss of treatment efficacy. Together, these results demonstrate the activation of a potent homeostatic response to increase food intake when sCT reduces food intake and energy reserves in diet-induced obese rats.     

Preproghrelin-derived peptide, obestatin, fails to influence food intake in lean or obese rodents

Objectives: Obestatin has been initially characterized as a new peptide derived from the ghrelin precursor, which suppresses food intake and inhibits the orexigenic and prokinetic actions of ghrelin when injected peripherally or centrally in lean mice. However, reproducing these data remains controversial. Reasons for the disparity may be the use of different doses, routes, and animal models. We aimed to investigate the effects of peripheral and intracisternal (IC) injection of obestatin on feeding, gastric motility, and blood glucose in rats as well as in diet‐induced obese (DIO) mice. Research Methods and Procedures: Food intake and gastric emptying of a semi‐liquid caloric meal were measured after intraperitoneal (IP) injection of obestatin in rats and DIO mice. Gastric phasic motility and blood glucose were monitored in urethane‐anesthetized rats after IC or intravenous (IV) injection of obestatin. Results: Obestatin injected intraperitoneally at doses ranging from 0.1 to 3 mg/kg influenced neither acute food intake nor gastric emptying in rats. Obestatin injected intravenously at 0.3 or 3 mg/kg and IC at 7.5 or 30 µg/rat modified neither fasted gastric phasic motility nor blood glucose levels, while ghrelin (30 µg/kg, IV) increased and vagotomy suppressed gastric motility, and an oligosomatostatin analog (3 µg/rat, IC) decreased blood glucose. Obestatin, injected intraperitoneally (0.3 mg/kg) in DIO mice, did not alter feeding response to a fast, while urocortin 1 (10 µg/kg, IP) induced a 73.3% inhibition at 2 hours. Discussion: Our data demonstrate that peripheral administration of obestatin did not modify food intake in rats or obese mice or gastric motor function in rats.     

Reduced central leptin sensitivity in rats with diet-induced obesity

On low-fat chow diet, rats prone to diet-induced obesity (DIO) have increased arcuate nucleus neuropeptide Y (NPY) expression but similar leptin levels compared with diet-resistant (DR) rats (19). Here, body weight and leptin levels rose in DIO rats, and they defended their higher body weight after only 1 wk on a 31% fat high-energy (HE) diet. However, DIO NPY expression did not fall to DR levels until 4 wk when plasma leptin was 168% of DR levels. When switched to chow, DIO rats lost carcass fat (18). By 10 wk, leptin levels fell to 148% and NPY expression again rose to 150% of DR levels. During 4 wk of food restriction, DIO leptin fell by approximately 50% while NPY increased by 30%. While both returned to control levels by 8 wk, DIO rats still regained all lost weight when fed ad libitum. Finally, the anorexic effect of intracerebroventricular leptin (10 microg) was inversely correlated with subsequent 3-wk weight gain on HE diet. Thus NPY expression and food intake are less sensitive to the leptin's suppressive effects in DIO rats. While this may predispose them to develop DIO, it does not fully explain their defense of a higher body weight on HE diet.     

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.     

NMDA receptor blockade attenuates CCK-induced reduction of real feeding but not sham feeding

Systemic injection of MK-801, a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptor ion channels, increases meal size and delays satiation. We examined whether MK-801 increases food intake by directly interfering with actions of cholecystokinin (CCK). Prior administration of MK-801 (100 microg/kg ip) reversed the inhibitory effects of CCK-8 (2 and 4 microg/kg ip) on real feeding of both liquid and solid foods. MK-801 alone did not alter 30-min sham intake of 15% sucrose compared with intake after saline. Furthermore, while CCK-8 (2 or 4 microg/kg ip) reduced sham intake, this reduction was not attenuated by MK-801 pretreatment. To ascertain whether MK-801 attenuation of CCK-induced reduction of real feeding was associated with attenuated inhibition of gastric emptying, we tested the effect of MK-801 pretreatment on CCK-induced inhibition of gastric emptying of 5-ml saline loads. Ten-minute gastric emptying was accelerated after MK-801 (3.9 +/- 0.2 ml) compared with saline vehicle (2.72 +/- 0.2 ml). CCK-8 (0.5 microg/kg ip) reduced 10-min emptying to 1.36 +/- 0.3 ml. Pretreatment with MK-801 did not significantly attenuate CCK-8-induced reduction of gastric emptying (0.9 +/- 0.4 ml). This series of experiments demonstrates that blockade of NMDA ion channels reverses inhibition of real feeding by CCK. However, neither inhibition of sham feeding nor inhibition of gastric emptying by CCK is attenuated by MK-801. Therefore, increased food intake after NMDA receptor blockade is not caused by a direct interference with CCK-induced satiation. Rather, increased real feeding, either in the presence or absence of CCK, depends on blockade of NMDA receptor participation in other post-oral feedback signals such as gastric sensation or gastric tone.     

The TRPA1 Agonist,Methyl Syringate Suppresses Food Intake and Gastric Emptying

Transient receptor potential channel ankryn 1 (TRPA1) expressed in the gastrointestinal tract is associated with gastric motility, gastric emptying, and food intake. In this study, we investigated the effects of methyl syringate, a specific and selective TRPA1 agonist, on food intake, gastric emptying, and gut hormone levels in imprinting control region (ICR) mice. The administration of methyl syringate suppressed cumulative food intake and gastric emptying. In addition, treatment with ruthenium red (RR), a general cation channel blocker, and HC-030031, a selective TRPA1 antagonist, inhibited methyl syringate-induced reduction of food intake and delayed gastric emptying in ICR mice. Methyl syringate also increased plasma peptide YY (PYY) levels, but not glucagon-like peptide-1 (GLP-1) levels. The elevation in PYY was blocked by treatment with RR and HC-030031. The present findings indicate that methyl syringate regulates food intake and gastric emptying through a TRPA1-mediated pathway and, by extension, can contribute to weight suppression.     

Gastric electrical stimulation parameter dependently alters ventral medial hypothalamic activity and feeding in obese rats

Gastric electrical stimulation (GES) has been used to treat obesity with unclear mechanisms and limited parameter ranges. This study explores effects of GES parameters on ventral medial hypothalamic (VMH) activity, feeding, and body weight in diet-induced obese (DIO) rats. For experiment 1, discharge rates were recorded in 39 gastric distension-responsive (GD-R) neurons in 12 DIO rats. Basal rates were compared with rates under GES using varied pulse amplitudes, widths, frequencies, and train-on times. For experiment 2, a crossover experiment in 16 DIO rats measured food intake and weight effects of GES pulse width, the parameter with the steepest neuronal response gradient in experiment 1. Treatments were sham and 0.5-, 2.0-, and 5.0-ms pulse GES. In experiment 1, 11 of 13 GES parameter sets tested produced significantly (P < 0.05) altered discharge rates of GD-R neurons. Increases in pulse amplitude (P < 0.05) and width (P < 0.0001) produced significant upward linear trends in response over the range tested, with the trend being strongest for pulse width. In experiment 2, over 4 days of 0.5-, 2.0-, and 5.0-ms GES treatment, food intake was 9.6% (P < 0.05), 21.0% (P < 0.0001), and 47.3% (P < 0.0001) lower than under sham-GES, whereas body weight changes were 0.7 (P = 0.48), 2.2 (P < 0.05), and 3.5 (P < 0.002) percentage points lower, respectively. We concluded that GES pulse width increases had the largest effect on VMH neuronal activity, and these effects were paralleled by pulse width-dependent reductions in food intake and body weight. Lengthening pulse width beyond the range used in prior clinical studies may be critical to making GES a viable obesity treatment.     

Effects of Exendin‐4 Alone and With Peptide YY3–36 on Food Intake and Body Weight in Diet‐Induced Obese Rats

Significant weight loss following Roux‐en‐Y gastric bypass surgery (RYGB) in obese humans correlates with enhanced secretion of anorexigenic gut hormones glucagon‐like peptide‐1 (GLP‐1) and peptide YY_3–36 (PYY_3–36). Our aim here was to identify a dosing strategy for intraperitoneal (IP) infusion of GLP‐1 homologue exendin‐4 alone and with PYY_3–36 that produces a sustained reduction in daily food intake and body weight in diet‐induced obese (DIO) rats. We tested 12 exendin‐4 strategies over 10 weeks. Exendin‐4 infused during the first and last 3 h of the dark period at 15–20 pmol/h (0.15 nmol/kg/day) produced a sustained 24 ± 1% reduction in daily food intake for 17 days, and decreased body weight by 7%. In a separate group of DIO rats, none of seven dosing strategies combining exendin‐4 and PYY_3–36 produced a similar reduction in daily food intake for >10 days. The subsequent decline in efficacies of exendin‐4 alone and with PYY_3–36 on food intake and body weight in each experiment suggested possible receptor downregulation and tolerance to treatments. However, when treatments were discontinued for 1 day following losses in efficacies, daily food intake significantly increased. Together, these results demonstrate that (i) intermittent IP infusion of a low dose of exendin‐4 can produce a relatively prolonged reduction in daily food intake and body weight in DIO rats, (ii) co‐infusion of exendin‐4 and PYY_3–36 does not further prolong this response, and (iii) activation of an orexigenic mechanism gradually occurs to counteract the inhibitory effects of exendin‐4 alone and with PYY_3–36 on food intake and body weight.     

Exendin-4, but not dipeptidyl peptidase IV inhibition, increases small intestinal mass in GK rats

Long-term treatment with dipeptidyl peptidase IV inhibitors (DPPIV-I) or glucagon-like peptide (GLP)-1 analogs may potentially affect intestinal growth by down- or upregulating the intestinotrophic hormone GLP-2. This study compared the intestinotrophic effects of 12-wk administration of vehicle, exendin-4 (Ex-4; 5 nmol/kg bid sc), or DPPIV-I (NN-7201, 10 mg/kg qd orally) in GK rats. Some animals were observed additionally for 9 wk after the end of treatment. Both treatments lowered glycated hemoglobin A1c at wk 12 vs. control (Ex-4, -0.8%; DPPIV-I, -0.4%). Body weight was reduced by Ex-4 compared with control (361 +/- 4 vs. 399 +/- 5 g; P < 0.001) because of reduced food intake, whereas neither parameter was affected by DPPIV-I. Linear bone growth was unaffected by either treatment. After treatment end, food intake in Ex-4 animals increased, and, by wk 21, body weight was identical in all groups. The small intestine of Ex-4-treated animals was larger at wk 12 compared with control (length, 135.6 +/- 1.6 vs. 124.5 +/- 2.3 cm, P < 0.001; absolute weight, 8.4 +/- 0.2 vs. 6.4 +/- 0.4 g, P < 0.001), being most pronounced proximally, where the absolute cross-sectional area related to body weight increased by 24% because of increased mucosal thickness. These effects were reversible, and 9 wk after the end of treatment, no differences between Ex-4 and control were apparent. Plasma GLP-2 concentrations were unaltered by either treatment, and Ex-4 had no agonistic or antagonistic effects on the transfected GLP-2 receptor. DPPIV-I had no intestinal effects. In conclusion, the continued presence of Ex-4 is necessary to maintain weight loss in GK rats. Effective antihyperglycemic treatment with Ex-4 increases intestinal mass reversibly, whereas DPPIV-I lacks intestinal effects.     

LPS inhibits fasted plasma ghrelin levels in rats: role of IL-1 and PGs and functional implications

LPS injected intraperitoneally decreases fasted plasma levels of ghrelin at 3 h postinjection in rats. We characterized the inhibitory action of LPS on plasma ghrelin and whether exogenous ghrelin restores LPS-induced suppression of food intake and gastric emptying in fasted rats. Plasma ghrelin and insulin and blood glucose were measured after intraperitoneal injection of LPS, intravenous injection of IL-1beta and urocortin 1, and in response to LPS under conditions of blockade of IL-1 or CRF receptors by subcutaneous injection of IL-1 receptor antagonist (IL-1Ra) or astressin B, respectively, and prostaglandin (PG) synthesis by intraperitoneal indomethacin. Food intake and gastric emptying were measured after intravenous injection of ghrelin at 5 h postintraperitoneal LPS injection. LPS inhibited the elevated fasted plasma ghrelin levels by 47.6 +/- 4.9%, 58.9 +/- 3.3%, 74.4 +/- 2.7%, and 48.9 +/- 8.7% at 2, 3, 5, and 7 h postinjection, respectively, and values returned to preinjection levels at 24 h. Insulin levels were negatively correlated to those of ghrelin, whereas there was no significant correlation between glucose and ghrelin. IL-1Ra and indomethacin prevented the first 3-h decline in ghrelin levels induced by LPS, whereas astressin B did not. IL-1beta inhibited plasma ghrelin levels, whereas urocortin 1 had no influence. Ghrelin injected intravenously prevented an LPS-induced 87% reduction of gastric emptying and 61% reduction of food intake. These data showed that IL-1 and PG pathways are part of the early mechanisms by which LPS suppresses fasted plasma ghrelin and that exogenous ghrelin can normalize LPS-induced-altered digestive functions.     

Fixed feeding potentiates interdigestive gastric motor activity in rats: importance of eating habits for maintaining interdigestive MMC

Endogenous ghrelin regulates the occurrence of interdigestive gastric phase III-like contractions in rats. However, the fasted motor pattern is not as regular and potent in humans and dogs. We hypothesize that eating habits play an important role in maintaining a regular interdigestive gastric contractions. We studied the effect of fixed-feeding regimen on interdigestive gastric contractions and plasma acyl ghrelin levels. The fixed-fed rats were trained to the assigned meal feeding regimen, once daily at 12:00 PM to 4:00 PM for 14 days. Free-fed rats were maintained with free access to food. As ghrelin regulates gastric emptying as well, solid gastric emptying was also studied in fixed-fed rats and free-fed rats. In free-fed rats, two of six rats did not show interdigestive gastric phase III-like contractions. In contrast, phase III-like contractions were observed in all rats 14 days after starting the fixed-feeding regimen. The maximal amplitude of phase III-like contractions significantly increased from 8.4 +/- 0.6 to 16.3 +/- 1.8 g (n = 6, P < 0.05) 14 days after the start of the fixed feeding. Fasted and postprandial plasma ghrelin levels were significantly increased after 14 days of fixed feeding. Solid gastric emptying was significantly accelerated in fixed-fed rats (72.1 +/- 4.2%) compared with that of free-fed rats (58.7 +/- 2.7%, n = 6, P < 0.05). Our present findings suggest that fixed feeding increases plasma ghrelin levels, potent interdigestive contractions, and acceleration of gastric emptying.     

Ghrelin accelerates gastric emptying via early manifestation of antro-pyloric coordination in conscious rats

Ghrelin is known to enhance gastric motility and accelerate gastric emptying of liquid and solid food in rats. As solid gastric emptying is regulated by the coordinated motor pattern between the antrum and pylorus (antro-pyloric coordination), we studied the correlation between solid gastric emptying and antro-pyloric coordination in response to ghrelin. Rats were given 1.5 g of solid food after a 24-h fasting. Immediately after the ingestion, ghrelin (0.4-8.0 microg/kg) or saline was administered by intraperitoneal (i.p.) injection. Ninety minutes after the feeding, rats were euthanized and gastric content was removed to calculate gastric emptying. To evaluate the antro-pyloric coordination, strain gauge transducers were sutured on the antrum and pylorus. The incidence of postprandial antro-pyloric coordination was compared between ghrelin-and saline-injected rats. In saline-injected rats, gastric emptying was 58.3+/-3.7% (n=6). Ghrelin (4.0-8.0 microg/kg), accelerated gastric emptying. Maximum effect was obtained by ghrelin (4.0 microg/kg), which significantly accelerated gastric emptying to 77.4+/-3.7% (n=6, p<0.05). The number of antro-pyloric coordination 20-40 min after feeding was significantly increased in ghrelin-injected rats, compared to that of saline-injected rats (n=4, p<0.05). It is suggested that enhanced antro-pyloric coordination play an important role in accelerated solid gastric emptying induced by ghrelin.     

High fat maintenance diet attenuates hindbrain neuronal response to CCK

Rats maintained on a high fat diet reduce their food intake less in response to exogenous cholecystokinin (CCK) than rats maintained on a low fat diet. In addition, inhibition of gastric emptying by CCK is markedly attenuated in rats maintained on a high fat diet. Both inhibition of food intake and gastric emptying by CCK are mediated by sensory fibers in the vagus nerve. These fibers terminate on dorsal hindbrain neurons of the nucleus of the solitary tract and area postrema. To determine whether diet-induced changes in the control of feeding and gastric emptying are accompanied by altered vagal sensory responsiveness, we examined dorsal hindbrain expression of Fos-like immunoreactivity (Fos-li) following intraperitoneal CCK injection of rats maintained on high fat or low fat diets. Following CCK, there were numerous Fos-li nuclei in the area postrema and in the commissural and medial subnuclei of the nucleus of the solitary tract of rats maintained on a low fat diet. However, Fos-li was absent or rare in the brains of rats maintained on a high fat diet. These data suggest that the vagal sensory response to exogenous CCK is reduced in rats maintained on a high fat diet. Our results also are consistent with our previous findings that CCK-induced reduction of food intake and gastric emptying are both attenuated in rats maintained on a high fat diet. In addition our results support the hypothesis that attenuation of CCK-induced inhibition of food intake and gastric emptying may be due to diet-induced diminution of vagal CCK responsiveness.     

Intravenous infusion of glucagon-like peptide-1 potently inhibits food intake, sham feeding, and gastric emptying in rats

Glucagon-like peptide-1(7-36)-amide (GLP-1) is postulated to act as a hormonal signal from gut to brain to inhibit food intake and gastric emptying. A mixed-nutrient meal produces a 2 to 3-h increase in plasma GLP-1. We determined the effects of intravenous infusions of GLP-1 on food intake, sham feeding, and gastric emptying in rats to assess whether GLP-1 inhibits food intake, in part, by slowing gastric emptying. A 3-h intravenous infusion of GLP-1 (0.5-170 pmol.kg(-1).min(-1)) at dark onset dose-dependently inhibited food intake in rats that were normally fed with a potency (mean effective dose) and efficacy (maximal % inhibition) of 23 pmol.kg(-1).min(-1) and 82%, respectively. Similar total doses of GLP-1 administered over a 15-min period were less potent and effective. In gastric emptying experiments, GLP-1 (1.7-50 pmol.kg(-1).min(-1)) dose-dependently inhibited gastric emptying of saline and ingested chow with potencies of 18 and 6 pmol.kg(-1).min(-1) and maximal inhibitions of 74 and 83%, respectively. In sham-feeding experiments, GLP-1 (5-50 pmol.kg(-1).min(-1)) dose-dependently reduced 15% aqueous sucrose intake in a similar manner when gastric cannulas were closed (real feeding) and open (sham feeding). These results demonstrate that intravenous infusions of GLP-1 dose-dependently inhibit food intake, sham feeding, and gastric emptying with a similar potency and efficacy. Thus GLP-1 may inhibit food intake in part by reducing gastric emptying, yet can also inhibit food intake independently of its action to reduce gastric emptying. It remains to be determined whether intravenous doses of GLP-1 that reproduce postprandial increases in plasma GLP-1 are sufficient to inhibit food intake and 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.     

Gastric emptying and Ca2+ and K+ channels of circular smooth muscle cells in diet‐induced obese prone and resistant rats

Objective:

Accelerated gastric emptying that precipitates hunger and frequent eating could be a potential factor in the development of obesity. The aim of this study was to study gastric emptying in diet‐induced obese‐prone (DIO‐P) and DIO‐resistant (DIO‐R) rats and explore possible differences in electrical properties of calcium (Ca²⁺) and potassium (K⁺) channels of antral circular smooth muscle cells (SMCs).

Design and Methods:

Whole‐cell patch‐clamp technique was used to measure Ca²⁺ and K⁺ currents in single SMCs. Gastric emptying was evaluated 90 min after the ingestion of a solid meal.

Results:

Solid gastric emptying in the DIO‐P rats was significantly faster compared with that in the DIO‐R rats. The peak amplitude of L‐type Ca²⁺ current (I_Ba,L) at 10 mV in DIO‐P rats was greater than that in DIO‐R rats without alternation of the current–voltage curve and voltage‐dependent activation and inactivation. The half‐maximal inactivation voltage of transient outward K⁺ current (I_Kto) was more depolarized (～4 mV) in DIO‐P rats compared with that in DIO‐R rats. No difference was found in the current density or recovery kinetics of I_Kto between two groups. The current density of delayed rectifier K⁺ current (I_Kdr), which was sensitive to tetraethylammonium chloride but not 4‐aminopyridine, was lower in DIO‐P rats than that in DIO‐R rats.

Conclusion:

The accelerated gastric emptying in DIO‐P rats might be attributed to a higher density of I_Ba,L, depolarizing shift of inactivation curve of I_Kto and lower density of I_Kdr observed in the antral SMCs of DIO‐P rats.     

Reduced anorexic effects of insulin in obesity-prone rats fed a moderate-fat diet

Rats prone to develop diet-induced obesity (DIO) have reduced central sensitivity to many metabolic and hormonal signals involved in energy homeostasis. High-fat diets produce similar defects in diet-resistant (DR) rats. To test the hypothesis that genotype and diet exposure would similarly affect central insulin signaling, we assessed the anorectic effects of 8 mU third ventricular (iv3t) insulin before and after 4 wk intake of a 31% fat, high-energy (HE) diet intake in outbred (OutB) rats. Rats were retrospectively designated as DR or DIO by their low or high weight gains on HE diet. Before the HE diet, iv3t insulin reduced 4-h and 24-h chow intake by 53% and 69% in DR rats but by only 17% and 27% in DIO rats, respectively. Also, the anorectic response to iv3t insulin in OutB rats was inversely correlated (r = 0.72, P = 0.002) with subsequent 4-wk weight gain on the HE diet. Similarly, in selectively bred (SB) chow-fed DR rats, 8 mU iv3t insulin reduced 4-h and 24-h intake by 21% and 22%, respectively, but had no significant effect in SB DIO rats. Four-week HE diet intake reduced 4-h and 24-h insulin-induced anorexia by 45% in OutB DR rats and completely abolished it in SB DR rats. Reduced insulin responsiveness was unassociated with differences in arcuate nucleus insulin receptor mRNA expression between DIO and DR rats or between rats fed chow or HE diet. These data suggest that DIO rats have a preexisting reduction in central insulin signaling, which might contribute to their becoming obese on the HE diet. However, since the HE diet reduced central insulin sensitivity in DR rats but did not make them obese, it is likely that other brain areas are involved in insulin's anorectic action or that other pathways contribute to the development and maintenance of obesity.     

Repeated ICV administration of oxyntomodulin causes a greater reduction in body weight gain than in pair-fed rats

Oxyntomodulin (OXM) is a product of proglucagon processing in the intestine and the central nervous system. We reported that intracerebroventricular (ICV) and intranuclear administration of OXM caused an inhibition of food intake in rats (Dakin CL, Gunn I, Small CJ, Edwards CM, Hay DL, Smith DM, Ghatei MA, and Bloom SR. Endocrinology 142: 4244-4250, 2001). In this study, we investigated the effect of twice-daily ICV administration of OXM, 1 nmol, for 7 days. A pair-fed control was included. These animals were restricted to the food intake of the OXM group but injected twice daily with saline. OXM-treated animals gained significantly less weight than either control group (day 8: OXM, 12.2 +/- 1.9 g vs. pair fed, 21.0 +/- 2.1 g; P < 0.005). OXM treatment caused a reduction in epididymal white adipose tissue (OXM, 1.13 +/- 0.03 g vs. pair fed, 1.29 +/- 0.04 g; P < 0.05) and interscapular brown adipose tissue (OXM, 0.15 +/- 0.01 g vs. pair fed, 0.18 +/- 0.01 g; P < 0.05) and increased core temperature compared with saline control, suggestive of enhanced energy expenditure. The food restriction-induced suppression in plasma TSH, seen in the pair-fed group, was prevented by OXM, potentially via increased release of hypothalamic TRH. In summary, ICV OXM causes reduced body weight gain and body adiposity following chronic administration.