Henry S. Koopmans

Cannabinoid (CB)1 receptor antagonist, AM 251, causes a sustained reduction of daily food intake in the rat

Cannabinoid (CB)(1) receptors are present throughout the nervous system, including several areas implicated in the control of food intake. Central and peripheral administration of CB(1) agonists increase food intake while CB(1) receptor antagonists reduce food intake. However, in some previous studies, tolerance to the anorectic effects of CB(1) antagonists develops within days. To further delineate the role of endogenous cannabinoid signaling in energy intake, we studied the effects of the CB(1) antagonist AM 251 (1.25, 2.5 and 5 mg/kg ip), the anandamide membrane transporter inhibitor VDM 11 (10 mg/kg ip), and the CB(1) agonists anandamide (1 mg/kg ip), and methanandamide (1 mg/kg ip), on food intake. A single administration of the CB(1) antagonist AM 251 significantly reduced food intake for a total of 6 days (P<.05). Reductions in food intake brought about by AM 251 were accompanied by reductions in weight gain for 6 days (P<.05). Contrary to expectations, VDM 11 did not increase food intake in this study. Anandamide was also unable to increase food intake; however, the more stable agonist methanandamide significantly increased food intake 3 h after administration (P<.05). These results support the role of CB(1) receptor antagonists in the treatment of obesity and suggest that the anorectic effect of AM 251 may last longer than previously reported.

The effects of ileal transposition and jejunoileal bypass on food intake and GI hormone levels in rats

Male Wistar rats received three different types of small intestinal surgery. Two groups of rats had either 10 or 20 cm of lower ileum transposed to mid-duodenum. A third comparison group of rats had 85% jejunoileal bypass. All three experimental groups showed a sustained post-operative reduction in food intake and a change in body weight gain. Measurements made 36 days after surgery showed that all experimental groups had a large increase in basal and meal-stimulated enteroglucagon. The total-integrated plasma levels of gastrin, GIP, insulin and blood glucose were significantly reduced. At sacrifice, there were large increases in the wet weight of the small intestine and pancreas. These changes were probably due to the chronic stimulation of the lower ileum with nutrient-rich chyme and may be due to the release of ileal hormones.

AM 251 produces sustained reductions in food intake and body weight that are resistant to tolerance and conditioned taste aversion

The cannabinoid 1 (CB1) receptor has been implicated in the regulation of food intake. Here, we examine the effect of the CB1 receptor antagonist AM 251 on food intake and body weight over a prolonged period. Further, we examine whether AM 251 produces conditioned taste aversion (CTA) and if sustained antagonism at central receptors contributes to its anorectic effect. The effect of AM 251 of food intake and body weight was examined in daily (1 mg kg−1) and 5‐day (5 mg kg−1) dosing schedules. Matching reductions in food intake and body weight were observed in both paradigms. A single administration of AM 251 (5 mg kg−1) significantly reduced food intake for 4 days. Tolerance to the anorectic effects of AM 251 did not develop in either dosing strategy. Active avoidance of AM 251 (3; 5 mg kg−1, i.p.) was examined using a CTA assay. Rats showed no evidence of CTA associated with AM 251. We investigated the sustained effect of AM 251 (5 mg kg−1, i.p.) on CB1 receptors in the hypothalamus using Δ9‐tetrahydrocannabinol (8 mg kg−1, i.p.) induced hypothermia. AM 251 initially blocked hypothermia, but this effect was not seen 2 or 4 days later. The results demonstrate that smaller, or infrequent, administrations of AM 251 can produce sustained reductions in food intake and body weight in rat. Reductions in food intake were sustained longer than AM 251 antagonized the effects of a CB1 receptor agonist in the hypothalamus, and occurred independently of CTA.

Effects of vagal and splanchnic section on food intake, weight, serum leptin and hypothalamic neuropeptide Y in rat

Truncal vagotomy can cause reduced food intake and weight loss in humans and laboratory animals. In order to investigate some of the factors that might contribute to this effect, we studied changes in ingestive behaviour, whole body and organ weights, serum leptin and hypothalamic neuropeptide Y in rats with bilateral vagal section, bilateral splanchnic nerve section and combined vagotomy plus splanchnectomy. Pyloromyotomy was combined with vagotomy to lessen effects of vagotomy on gastric emptying. Animals with vagotomy or vagotomy plus splanchnectomy lost weight and decreased their daily food intake relative to animals with splanchnectomy alone, rats with bilateral sham exposure of one or both nerve, or rats with pyloromyotomy alone. Serum leptin and white fat mass, 4 weeks after vagotomy, were about 20% of the values in the sham-operated animals at this time. No effect for splanchnic nerve section alone was observed. Pyloromyotomy caused no reduction in weight or fat mass, but reduced serum leptin. Following vagotomy with or without splanchnic nerve section, neuropeptide Y was elevated in the arcuate nucleus relative to values for the other four groups. Changes in neuropeptide Y were inversely correlated with levels of serum leptin. It is concluded that the effect of vagotomy could be due to the loss of a feeding signal carried by vagal afferent neurons, or to changed humoral signals, for example, increased production of a satiety hormone. However, it cannot be attributed to signals that reduce feeding (for example, gastric distension) reaching the central nervous system via the splanchnic nerves. The changes were sufficient to cause weight loss even though serum leptin was decreased, a change that would be expected to increase food intake.

Identification of intestinofugal neurons projecting to the coeliac and superior mesenteric ganglia in the rat.

Intestinofugal neurons are parts of the afferent limbs of inhibitory intestino-intestinal reflexes. These neurons have been mapped in guinea-pigs, where they have a gradient of increasing frequency of occurrence from oral to anal, but not in other species. In the present work in the rat, a species that is more amenable to physiological study than the guinea-pig, we have used retrograde tracing to map the distribution of the cell bodies of intestinofugal neurons projecting to the coeliac-superior mesenteric ganglion complex. Labelled nerve cells were found in the myenteric, but not the submucosal plexus. They were mono-axonal neurons, most with Dogiel type I morphology, and were immunoreactive for choline acetyltransferase, implying that they are cholinergic, which is consistent with functional studies. The cells increased in number per unit area from the stomach, through the small intestine, to the caecum. The results are consistent with physiological studies that reveal distal to proximal inhibitory reflexes that are more potent from distal compared to proximal sites.

The Effects of Glucose or Lipid Infused Intravenously or Intragastrically on Voluntary Food Intake in the Rat

Glucose or lipid was infused intravenously (IV) or intragastrically (IG) 30 min before and also during the 17 h when rats were fed both a high-carbohydrate diet and a high-fat diet. Three-day infusions of 28.1 kcal of glucose reduced daily food intake by 19.7 +/- 1.9 kcal/day, representing an oral intake reduction equivalent to 70% of each calorie infused. Infusions of 28.1 kcal of lipid reduced baseline food intake by 11.2 +/- 2.7 kcal/day or 40% of each calorie infused (p < 0.0005). Furthermore, infusions of nutrient IG reduced baseline food intake by 17.6 +/- 2.1 kcal/day or 63% of each calorie infused, and infusions of nutrient IV reduced baseline food intake by 13.7 +/- 2.6 kcal/day or 49% of each calorie infused (p < 0.05). Also, glucose infusions (1.0 kcal/40 min) reduced 10-min food intake from saline baseline levels by 1.1 +/- 0.5 kcal, but lipid infusions had no effect. Infused glucose is more effective than lipid in inhibiting short-term intake, daily food intake, and intake of high-carbohydrate diet, and IG infusion is more effective than IV infusion in inhibiting daily food intake.

Changes in Differential Functional Magnetic Resonance Signals in the Rodent Brain Elicited by Mixed-Nutrient or Protein-Enriched Meals

BACKGROUND & AIMS The hypothalamus and brain stem have important roles in regulating food intake; the roles of other nonhomeostatic centers in detecting nutrient content of ingested food have been poorly characterized. We used blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) to map brain regions that are responsive to intragastric infusion of isocaloric amounts of a mixed nutrient or protein, and assessed the role of blood glucose in the observed BOLD signal changes. METHODS Brain images were acquired, using a 9.4 T MRI system, from anesthetized rats during intragastric infusion of saline (n = 7), or 12 kcal of a mixed nutrient (n = 13) or protein (n = 6). Nutrient-induced changes in blood parameters and the effects of intravenous infusion of saline or glucose (n = 5/treatment) on BOLD fMRI signal changes were also evaluated. Intragastric nutrient infusion reduced the BOLD fMRI signal intensity in homeostatic (hypothalamus, nucleus tractus solitarius) and nonhomeostatic (thalamus, hippocampus, caudate putamen, cerebral cortex, cerebellum) centers; these effects were mimicked qualitatively by intravenous glucose. In contrast to a mixed meal, protein load reduced the BOLD fMRI signal in the amygdala. BOLD fMRI signal changes were inversely correlated with circulating concentrations of amylin, insulin, peptide YY, and glucagon-like peptide-1. CONCLUSIONS The caloric content of a meal is signaled from the gut to the brain and affects activity in homeostatic and non-homeostatic centers; blood glucose concentrations have an important role. The satiety effects of protein are associated with activity changes specifically in the amygdala.

Effect of intravenous nutrient infusions on food intake in rats.

To assess the effect of gut signals on food intake two types of nutrients were infused intravenously for 17.5 hours in 17 hour fed rats. In the first experiment a solution of 25% d-glucose and 4.25% amino acids (Travasol) was infused at levels of 26 and 52 kcal/day for two consecutive four-day periods. During infusion periods, food intake was reduced from saline baseline levels by 18.9 +/- 1.7 and 34.8 +/- 1.8 kcal/day, respectively. This represents an oral intake reduction of approximately 70% of the infused calories. In contrast, food intake was reduced 17.4 +/- 1.7 kcal/day below saline baseline levels when 40 kcal of Nutralipid were infused. The reduction in food intake was only 43% of the lipid calories infused. These results indicate that infusions of glucose and amino acids are more effective than infusion of fats in inhibiting daily food intake, that gut signals associated with absorption of fat provide important satiety signals and that removal of fat from the bloodstream has relatively little effect on daily food intake.

Internal signals cause large changes in food intake in one-way crossed intestines rats.

Fifteen pairs of parabiotic rats had either a surgical operation in which a 15 or 30 cm segment of upper small intestine was disconnected from the digestive tract of one rat in the pair and reconnected to the transected duodenum of its partner or had control surgery. Food eaten by one rat in the pair went to the rats own stomach, traveled through 5 cm of its upper duodenum and then crossed into the isolated segment of the partners small intestine. After traversing the 15 or 30 cm isolated segment, the remaining unabsorbed food crossed back into the lower duodenum of the rat that fed. Food eaten by the partner went through its own digestive tract, but bypassed the isolated segment of its own upper small intestine. The operation produced a large and sustained change in food intake of both rats in a pair. For the rats with 30 cm crossed segments, the rat that lost intestinal chyme into its partner ate 3.6 times as much food as did its partner for a period of many months. At sacrifice, the rats that ate more, weighed less and had less body fat. These large changes in food intake may be caused by internal changes associated with changes in the amount of food absorbed into each rat or by differential stimulation of the lower digestive tracts. The results clearly show that there is a major internal control mechanism for the amount of food eaten.

The effect of non-nutritive sweeteners on body weight in rats

Artificial sweeteners are used to provide a sweet taste to a food while removing the calories associated with sugar. The importance of non-nutritive sweeteners (NNS) for the control of body weight has never been proved. In this long-term study, 81 rats fed ad libitum on chow and water were given either an 11% sucrose solution, a solution artificially sweetened with saccharin and aspartame or served as controls. Over an 8-week period, the sucrose rats gained considerable weight while the NNS rats showed the same weight gain as controls. When the sweetened solutions were switched, obese sucrose rats lost weight during the next 8 weeks while rats previously on NNS gained weight rapidly. The results show that substitution of artificial sweeteners for sugars prevents weight gain and promotes weight loss in rats.