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Am J Physiol Gastrointest Liver Physiol 291: G156-G162, 2006. First published March 30, 2006; doi:10.1152/ajpgi.00569.2005
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NEUROREGULATION AND MOTILITY

Targeted disruption of the murine CCK1 receptor gene reduces intestinal lipid-induced feedback inhibition of gastric function

K. L. Whited,1 D. Thao,2 K. C. Kent Lloyd,1,2 A. S. Kopin,3 and H. E. Raybould1

1Department of Anatomy, Physiology, and Cell Biology and 2Center for Comparative Medicine, School of Veterinary Medicine, University of California, Davis, California; and 3Department of Medicine, Center for Gastroenterology Research on Absorptive and Secretory Processes, Tupper Research Institute, Tufts-New England Medical Center, Tufts University School of Medicine, Boston, Massachusetts

Submitted 19 December 2005 ; accepted in final form 21 March 2006


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Cholecystokinin (CCK), acting at CCK1 receptors (CCK1Rs) on intestinal vagal afferent terminals, has been implicated in the control of gastrointestinal function and food intake. Using CCK1R–/– mice, we tested the hypothesis that lipid-induced activation of the vagal afferent pathway and intestinal feedback of gastric function is CCK1R dependent. In anesthetized CCK1R+/+ ("wild type") mice, meal-stimulated gastric acid secretion was inhibited by intestinal lipid infusion; this was abolished in CCK1R–/– mice. Gastric emptying of whole egg, measured by nuclear scintigraphy in awake mice, was significantly faster in CCK1R–/– than CCK1R+/+ mice. Gastric emptying of chow was significantly slowed in response to administration of CCK-8 (22 pmol) in CCK1R+/+ but not CCK1R–/– mice. Activation of the vagal afferent pathway was measured by immunohistochemical localization of Fos protein in the nucleus of the solitary tract (NTS; a region where vagal afferents terminate). CCK-8 (22 pmol ip) increased neuronal Fos expression in the NTS of fasted CCK1R+/+ mice; CCK-induced Fos expression was reduced by 97% in CCK1R–/– compared with CCK1R+/+ mice. Intralipid (0.2 ml of 20% Intralipid and 0.04 g lipid), but not saline, gavage increased Fos expression in the NTS of fasted CCK1R+/+ mice; lipid-induced Fos expression was decreased by 47% in CCK1R–/– compared with CCK1R+/+mice. We conclude that intestinal lipid activates the vagal afferent pathway, decreases gastric acid secretion, and delays gastric emptying via a CCK1R-dependent mechanism. Thus, despite a relatively normal phenotype, intestinal feedback in response to lipid is severely impaired in these mice.

gastric emptying; gastric acid secretion; cholecystokinin; vagal afferent; nucleus of the solitary tract

CHOLECYSTOKININ (CCK) is a peptide shown to exert a number of biological actions within the proximal gastrointestinal tract that together maximize the efficiency of nutrient intake and absorption, including effects on gastric motility (24), gastric emptying (23, 25), gastric acid secretion (17), pancreatic secretion (12), gallbladder contraction (13), and control of food intake (20). Intraluminal products of lipid digestion stimulate the release of CCK from intestinal enteroendocrine cells (10, 13, 18). Postprandially released CCK, in turn, acts on CCK1 receptors (CCK1Rs) on vagal afferent nerve terminals in the intestinal mucosa (2), which, in turn, results in stimulation of neurons in the nucleus of the solitary tract (NTS) (4, 6, 19), a brain region involved in the regulation of proximal gastrointestinal function and food intake.

Studies using acute administration of CCK1R antagonists suggest that this CCK-dependent pathway mediates, at least in part, lipid-induced inhibition of gastric function (9, 15, 17, 31) and short-term reduction in food intake and satiety (20, 26, 27). Despite the pharmacological evidence demonstrating that blockade of the CCK1R can alter postprandial gastrointestinal function and regulation of food intake, the CCK1R knockout mouse displays a remarkably normal phenotype (11). CCK1R knockout mice show identical growth curves and daily food intake when maintained on a normal ad libitum feeding schedule. Pancreatic morphology was similar in the two strains, and glucose homeostasis was unaltered. However, inhibition of food intake in response to exogenous CCK was abolished in the receptor knockout mice. CCK1R knockout mice have larger gallbladder volumes, decreased small intestinal transit times, and a higher susceptibility to develop gallstones (30).

There is strong evidence that CCK plays a major role in the postprandial regulation of gastric emptying and gastric acid secretion via activation of the vagal afferent pathway. However, evaluation of postprandial gastrointestinal function has not been performed in CCK1R knockout mice. The aim of the present study was to determine whether there was a deficit in the ability to detect lipid in the intestine of CCK1R knockout mice, leading to loss of lipid-induced activation of the vagal afferent pathway and intestinal feedback of gastric function. The specific aims were to determine whether 1) inhibition of gastric emptying and gastric acid secretion in response to intestinal lipid and 2) lipid-induced activation of the vagal afferent pathway are altered in CCK1R knockout mice.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Animals

Experiments were performed using male CCK1R–/– knockout mice (11) and 129S6/SvEv wild-type mice (hereafter referred to as CCK1R+/+ mice; Taconic, Oxnard, CA). Mice were of initial weight 18–20 g (6–10 wk of age) and were maintained on a standard laboratory diet (Purina Laboratory Chow). Mice were fasted overnight but allowed water ad libitum prior to all experimental procedures. The experiments were performed in accordance with protocols approved by the Institutional Animal Use and Care Committee.

Measurement of Gastric Acid Secretion

The gastric phase of acid secretion was measured in the mouse using an intragastric meal stimulation model as described previously (32). Mice (CCK1R+/+ and CCK1R–/–, n = 5 mice/group) were anesthetized initially with tribromoethanol (Avertin, 12.5 mg/ml, 250 mg/kg ip; Sigma, St. Louis, MO) for induction and maintained with thiobutabarbital (Inactin, 1 mg/ml, 50 mg/kg sc; Sigma). Following tracheotomy, a cannula [polyethylene (PE)-90] was inserted into the trachea. Through a midline abdominal incision, the pylorus was isolated and tied off. A double-lumen gastric cannula, fashioned from PE-10 tubing within a larger length of PE-160, was inserted orally into the stomach to simultaneously perfuse and collect gastric perfusate. Another cannula (PE-50) was inserted and fixed in place in the proximal duodenum immediately distal to the ligated pylorus for perfusion of lipid. After the abdominal incision was sutured closed, the preparation was allowed to stabilize for 15 min, after which 0.9% saline was infused continuously (4 ml/h) into the stomach through the gastric cannula. Basal gastric acid secretion was recorded for 45 min at 15-min intervals from time (t)–45 to t0 (min). From t15 to t150, 8% peptone (Becton-Dickinson, Franklin Lakes, NJ) was infused continuously (4 ml/h) into the stomach to simulate the gastric phase of a meal. From t90 to t150, 6% lipid (0.78 ml/h, 120 mg; Fresenius Kabi) was infused into the duodenum. From t165 to t195, the intragastric peptone infusion was replaced with 0.9% saline to return to basal levels of gastric acid secretion. All 15-min collections of gastric acid perfusates were back titrated to a pH of 7.0 with 0.001 M NaOH using an autoburette (model ABU 901; Radiometer, Copenhagen, Denmark) attached to a pH meter (model 290, Radiometer). Following completion of the experiment, mice were euthanized while still under anesthesia via CO2 fixation and cervical dislocation.

Gastric Emptying of Food

Following an overnight fast, CCK1R+/+ and CCK1R–/– mice (n = 5 mice group) were allowed to free-feed on standard laboratory diet. The food was removed after 60 min, and the mice received 0.05 ml of either 0.9% saline or CCK (22 pmol ip, Sigma). Two hours following treatment, the mice were anesthetized with pentobarbital sodium (50 mg/ml, 100 mg/kg ip; Western Medical Supply, Arcadia, CA), and their stomachs were isolated and removed. The weight of the full stomach and the weight of the emptied stomach were recorded. Stomach volume was calculated by subtracting the weight of the empty stomach from the weight of the full stomach.

Measurement of Gastric Emptying by Nuclear Scintigraphy

This method was described in detail in a previous publication (33). Briefly, during the experimental session, uncooked whole egg was radioactively labeled with 15 MBq (0.4 mCu) of 99mTc-labeled mebrofenin (Amersham Health, Sacramento, CA) per 25 ml whole egg and cooked in a microwave oven. The sample was weighed, and the radioactivity of the sample was measured using a gamma camera. Mice were allowed to freely feed on cooked whole egg for 5 min. The initial radioactivity of the fed mouse was measured to permit calculation of amount of the diet consumed by the mouse. Awake mice were immediately placed in the restraints, and we obtained a dynamic series of images of the mice continuously for 60 min and then again at 120–125 min. The mice were imaged using a Technicare Omega 500 gamma camera equipped with a high-resolution parallel hole collimator, and Nuclear Mac 5.2.1 software was used. Image analysis was facilitated using custom software developed using MATLAB 6.5. From the dynamic acquisition, a series of images were generated that covered multiple mice in the field of view. From these images, small rectangular regions were selected to segment the image so that the data from a single mouse could be displayed. Beginning with the first single mouse image, a circular region of interest was then manually positioned over each image that the user determined was free of motion in a location that the user decided included only the stomach. If an image was skipped because of motion, the data was not used. The sum of the pixel values in each region along with the time after feeding that the image was acquired were recorded in a table. These count rates were then corrected for the physical decay of technetium 99m, and gastric half-emptying time (t1/2; in min) was calculated.

Immunohistochemistry: c-Fos Protein Expression in the NTS

Following treatment (see Experimental Protocols; Effect of intestinal lipid on Fos protein expression in the NTS), mice were anesthetized with pentobarbital sodium (50 mg/ml, 100 mg/kg ip; Western Medical Supply) and transcardially perfused with 20 ml of heparinized 0.9% saline (0.1 ml heparin/100 ml saline) followed by 25 ml of 4% paraformaldehyde (Sigma). The brain stem was removed and postfixed in 4% paraformaldehyde for 1 h. The tissue was transferred to a 25% sucrose solution with 1% sodium azide (Sigma) for 2 h at room temperature and then refrigerated at 4°C overnight. Tissue samples were cut at ~40 µm on a cryostat. Sections were incubated for 1 h in 2% goat serum in PBS (Chemicon, Temecula, CA). Samples were incubated in primary antibody (1:2,000 rabbit anti-Fos; Santa Cruz Biotechnology, Santa Cruz, CA) for 2 h, followed by incubation with the secondary antibody (1:200 Alexa 488 goat anti-rabbit; Molecular Probes, Eugene, OR) for 2 h. Tissue was thoroughly washed between each incubation period.

Images were taken on a Provis microscope and analyzed using Paint Shop Pro, edition 7. A stereotaxic mouse brain atlas was used to determine the location of the NTS in each section of tissue (22). Representative sections were chosen to represent regions of the NTS (bregma, –8.00 to –6.48 mm). A region of interest was drawn around the NTS, and all activated neurons in the NTS region of interest were counted. Neurons were determined to be immunopositive (above threshold) by their color and size. A total of six representative sections across the NTS were analyzed per mouse, and these values were used in subsequent statistical analyses.

Measurement of Plasma Triglyceride

Fasted mice or fasted mice gavaged with 0.2 ml of 20% Intralipid (15-min pretreatment, 0.04 g lipid) were anesthetized with pentobarbital sodium (50 mg/ml, 100 mg/kg ip; Western Medical Supply). The abdominal cavity was opened, and the inferior vena cava was isolated. Using a heparinized syringe, we collected blood from the inferior vena cava and centrifuged it; the plasma was separated and stored at –80°C. Plasma triglyceride (TAG) levels were assessed using a TAG kit (Sigma).

CCK Immunohistochemistry

Fed mice (CCK1R+/+ and CCK1R–/–, n = 3 mice/group) were anesthetized with pentobarbital sodium (50 mg/ml, 100 mg/kg ip; Western Medical Supply) and transcardially perfused with 20 ml of heparinized 0.9% saline (0.1 ml heparin/100 ml saline) followed by 25 ml of 4% paraformaldehyde (Sigma). The duodenum was removed and postfixed in 4% paraformaldehyde for 1 h. The tissue was transferred to a 25% sucrose solution with 1% sodium azide (Sigma) for 2 h at room temperature and then refrigerated overnight. Sections were cut at 12 µm (longitudinal gut) or 20 µm (transverse gut) using a cryostat. Sections were incubated for 1 h in 2% goat serum in PBS (Chemicon) and then incubated in primary antibody (1:5,000 rabbit CCK; AB1972, Chemicon) for 3 h, followed by incubation with the secondary antibody (1:200 Alexa 488 goat anti-rabbit, Molecular Probes) for 2 h. Tissue was thoroughly washed between each incubation period. Images were taken on a Provis microscope, and a representative image was selected from each mouse strain (CCK1R+/+ and CCK1R–/–) for both transverse and longitudinal sections of the duodenum to compare endocrine cell morphology.

Experimental Protocols

Effect of intestinal lipid on gastric emptying. CCK1R+/+ (n = 6) and CCK1R–/– (n = 6) mice were used for these experiments. Mice were acclimated to restraint and to the experimental diets during training sessions two to three times a week for 2 wk prior to the first experimental session. The mice were offered ~1 g of labeled egg product and allowed to freely feed for 5 min prior to the imaging session.

Effect of intestinal lipid on Fos protein expression in NTS. CCK1R+/+ (n = 3 mice/treatment group) and CCK1R–/– (n = 3 mice/treatment group) mice were used for these experiments. Fasted mice were gavaged with 0.2 ml of 20% Intralipid (0.04 g lipid, Fresenius Kabi) or 0.9% saline or treated with CCK (22 pmol ip, Sigma). The dose of 0.2 ml of Intralipid was used because it contains approximately the same amount of lipid as found in the average amount of whole egg consumed by the mice (0.04 g TAG in 0.2 ml Intralipid vs. 0.038 g TAG in 0.35 mg whole egg). After 120 min, mice were anesthetized with pentobarbital sodium (100 mg/kg ip, Western Medical Supply) and transcardially perfused with fixative, permitting tissue removal.

Statistical Analysis

Scintigraphy. Decay corrected counts vs. time were analyzed using nonlinear regression and fit to a one-phase exponential decay curve, and the t1/2 was calculated. Significant differences in t1/2 emptying between treatment groups were calculated using a one-way ANOVA followed by a Bonferroni multiple comparison test. P < 0.05 was taken as significantly different. The t1/2 values are means ± SE.

Fos protein expression in the NTS. Significant differences in the number of Fos-positive neurons between treatment groups were calculated using a one-way ANOVA followed by a Bonferroni multiple- comparison test. P < 0.05 was taken as significantly different. The number of Fos-positive neurons is expressed as means ± SE.

Gastric acid secretion, gastric emptying of chow, and plasma TAG levels. Significant differences between treatment groups were calculated using a nonpaired Student’s t-test. P < 0.05 was taken as significantly different. Values are means ± SE and expressed as milliequivalents of gastric acid secretion per minute per milliliter, grams of food consumed, or milligram plasma TAG per milliliter.


   RESULTS
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 GRANTS
 REFERENCES
 
Lipid-Induced Inhibition of Gastric Acid Secretion

There was no significant difference in basal gastric acid secretion between CCK1R+/+ and CCK1R–/– mice [0.24 ± 0.04 vs. 0.17 ± 0.06 meq·min–1·ml–1, respectively, not significant (NS), n = 5]. Intragastric perfusion with peptone stimulated gastric acid secretion in both CCK1R+/+ and CCK1R–/– mice, and there was no significant difference between the two groups of mice (Fig. 1). Intestinal perfusion with lipid significantly inhibited peptone-stimulated gastric acid secretion in CCK1R+/+ mice by 70% (P < 0.05). However, in CCK1R–/– mice, intestinal lipid had no significant effect on peptone-stimulated gastric acid secretion (Fig. 1); the mean lipid-induced inhibition of gastric acid secretion was significantly different between the two groups of mice (0.16 ± 0.03 vs. 0.004 ± 0.03 meq·min–1·ml–1 in CCK1R+/+ and CCK1R –/– mice, respectively, P < 0.005).


Figure 1
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  		Fig. 1. Lipid-induced inhibition of peptone-stimulated gastric acid secretion is abolished in CCK1 receptor (CCK1R) knockout mice. A: time course of change in gastric acid secretion in individual CCK1R wild-type (CCK1R+/+) and knockout (CCK1R–/–) mice. Gastric acid secretion was continuously measured in anesthetized mice in response to intragastric perfusion with peptone (t0–t150) and duodenal perfusion with Intralipid (t90–t150). In both the wild-type mouse and knockout mouse, intragastric peptone stimulates gastric acid secretion, but duodenal lipid infusion inhibited meal-stimulated gastric acid secretion only in the wild-type control. B: mean change in gastric acid secretion in CCK1R knockout and wild-type controls following intragastric peptone and intragastric peptone together with intestinal lipid perfusion (n = 5 mice/group, *P < 0.05).

 
Effect of CCK Treatment on Gastric Emptying of Chow

In preliminary experiments, there were no significant differences in the amount of food in the stomachs of the CCK1R+/+ and CCK1R–/– mice at the end of the 60-min feeding period. There were no significant differences in gastric emptying of a chow meal between CCK1R+/+ and CCK1R–/– mice in response to saline (Fig. 2). In contrast, administration of CCK (22 pmol ip) significantly inhibited gastric emptying in CCK1R+/+ mice (P < 0.01, n = 5, Fig. 2). Gastric emptying of chow in response to administration of CCK (22 pmol ip) was significantly different between CCK1R+/+ and CCK1R–/– mice (P < 0.03, n = 5, Fig. 2).


Figure 2
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  		Fig. 2. Gastric emptying of chow is significantly slowed in response to CCK in CCK1R wild-type but not CCK1R knockout mice. Gastric emptying of chow was measured and expressed as grams of food remaining in the stomach following treatment with saline or CCK-8 (22 pmol, n = 5 mice group); *P < 0.01, saline vs. CCK-8; **P < 0.03, CCK1R wild type vs. knockout.

 
Lipid-Induced Inhibition of Gastric Emptying

There was no significant difference between CCK1R+/+ and CCK1R –/– mice in the amount of test diet (whole egg) consumed (0.30 ± 0.14 vs. 0.40 ± 0.08 g, n = 6 mice group, NS). We have previously demonstrated that increasing the amount of lipid in the diet produces a significant decrease in the rate of gastric emptying in C57BL/6J mice (33). In CCK1R+/+ mice, t1/2 was 53 ± 5 min; there was a significant shortening of t1/2 in CCK1R–/– mice to 30 ± 4 min (P < 0.01, n = 6).

Lipid- and CCK-Induced Activation of Neurons in the NTS

In fasted CCK1R+/+ and CCK1R–/– mice gavaged with saline, the number of neurons expressing Fos protein in the NTS was small and not significantly different between the two groups (NS, n = 3 mice/group, Fig. 3). In CCK1R+/+ mice, gavage with Intralipid significantly increased Fos protein expression in the NTS compared with saline gavage (P < 0.0001, n = 3 mice/group, Fig. 3). In CCK1R–/– mice, gavage with Intralipid significantly increased Fos protein expression in the NTS compared with saline gavage (P = 0.001, n = 3 mice/group, Fig. 3). However, Fos protein expression in the NTS in response to Intralipid gavage was significantly attenuated by 47% in CCK1R–/– mice compared with CCK1R+/+ mice (P < 0.001, n = 3 mice/group, Fig. 3).


Figure 3
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  		Fig. 3. Mean data of lipid-induced activation of neurons in the nucleus of the solitary tract (NTS) showing attenuation in CCK1R knockout mice. Activation of Fos expression in the NTS in CCK1R knockout and wild-type control mice with saline (n = 3), Intralipid gavage (n = 3), or CCK administered intraperitoneally (n = 3) is shown. *P < 0.001, CCK1R wild type vs. knockout with Intralipid; **P < 0.0001, CCK1R wild type vs. knockout with CCK-8; ***P < 0.05, CCK1R knockout Intralipid vs. CCK-8.

 
To determine whether the response to CCK was absent in the CCK1R-null mice, we administered exogenous CCK (15-min pretreatment, 22 pmol ip). Fos protein expression in the NTS of CCK1R+/+ mice was significantly increased by CCK compared with saline gavage (P < 0.0001, n = 3 mice/group, Fig. 3). In CCK1R+/+ mice, there was no significant difference in CCK- or Intralipid-induced Fos expression in the NTS (Fig. 3). In CCK1R–/– mice, administration of CCK failed to significantly increase Fos protein expression (NS, n = 3 mice/group, Fig. 3). CCK-induced Fos expression was significantly reduced by 97% in CCK1R–/– vs. CCK1R+/+ mice (P < 0.0001, n = 3 mice/group, Fig. 3).

Plasma TAG Assay

In CCK1R+/+ and CCK1R–/– mice, Intralipid gavage significantly increased plasma TAG compared with fasting TAG levels (P < 0.01 and P < 0.0001, respectively, n = 4–6 mice/group, Table 1). However, there was no significant difference between fasting or lipid-fed TAG levels in CCK1R+/+ and CCK1R–/– mice.


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  		Table 1. Plasma TAG levels in CCK1R–/– mice and CCK1R+/+ controls under basal conditions and after Intralipid gavage

 
CCK Immunohistochemistry

Representative images chosen for transverse and longitudinal sections of the duodenum were compared to determine if there were any gross morphological differences in endocrine cell expression between CCK1R+/+ (n = 3) and CCK1R–/– mice (n = 3). CCK-expressing endocrine cell distribution and overall morphology appeared to be the same between the two strains of mice (Fig. 4).


Figure 4
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  		Fig. 4. Photomicrographs of mice duodenum showing distribution of CCK-expressing endocrine cells in chow-fed CCK1R wild-type control and knockout mice. a, CCK1R+/+ transverse section; b, CCK1R–/– transverse section; c, CCK1R+/+ longitudinal section; d, CCK1R–/– longitudinal section. Scale bar = 200 µm.

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
The present study evaluated the integrity of the pathway mediating lipid-induced intestinal feedback in CCK1R-null mice. The results show that intestinal lipid-induced inhibition of meal-stimulated gastric acid secretion is abolished and gastric emptying of a lipid-containing meal is markedly accelerated in CCK1R-null mice. Furthermore, activation of the vagal afferent pathway, as evidenced by Fos protein expression in the NTS, induced by oral lipid gavage was attenuated, although not abolished, in CCK1R knockout mice. These data show that activation of vagal afferents by intestinal lipid is markedly impaired in CCK1R–/– mice, suggesting that there is a deficit in the detection of CCK release in these mice and a consequent lack of feedback regulation of gastric function.

The first aim of this study was to determine whether inhibition of gastric motor and secretory function in response to dietary lipid is dependent on the presence of CCK1Rs. There was no difference in either basal or meal-stimulated gastric acid secretion between null and wild-type mice, but inhibition of meal-stimulated gastric acid secretion in response to lipid in the intestine was completely abolished in CCK1R–/– mice. This data confirms previous observations that meal-stimulated gastric acid secretion is not dependent on the CCK1R (16) and that lipid-induced intestinal feedback inhibition of gastric acid secretion is dependent on CCK1Rs on vagal afferent nerve terminals, possibly activating a vagovagal reflex (14, 17). These findings support the hypothesis that lipid-induced activation of intestinal feedback to inhibit gastric acid secretion is CCK1R dependent (14). It is important to note that it is also possible that CCK acts via a humoral pathway to inhibit gastric acid secretion in response to dietary lipid. There are abundant CCK1Rs in the stomach (21, 29), and CCK has been shown to stimulate somatostatin release to inhibit gastric acid secretion (28, 34).

In addition, we determined the effect of the deletion of the CCK1R on regulation of gastric emptying. First, we quantified the gastric emptying in CCK1R-null mice in response to exogenous CCK treatment and found that in CCK1R–/– mice the inhibitory effect of CCK on gastric emptying was abolished. Furthermore, we quantified the gastric emptying rate of a whole egg test meal in CCK1R-null mice. There was no significant difference between the amounts of whole egg test meal consumed between wild-type and knockout mice, but in CCK1R knockout mice the gastric emptying of a meal containing lipid was significantly accelerated. This suggests that the normal intestinal feedback inhibition of gastric emptying induced by a meal is significantly reduced. Because whole egg contains both fat and protein, both of which are reported to release CCK and delay gastric emptying, we are unable to determine whether the deficit is due to lack of ability to detect protein or lipid. However, in a previous publication using this methodology, we demonstrated that the half-emptying time for egg white is not different from that of lipid in the CCK1R knockout mice obtained in the present study (33), suggesting that the deficit is in the detection of lipid. These findings support a significant role of CCK1R in a lipid meal-induced inhibition of gastric emptying.

The second aim of this study was to determine whether activation of the vagal afferent pathway by intestinal lipid is dependent on CCK and CCK1 receptors. Fos protein expression in neurons in the nucleus of the NTS was used as a measure of activation of the vagal afferent pathway (4, 19). Stimulation of neurons induces transcriptional and translational activity of the c-fos oncogene and results in the production of intracellular regulatory factors like Fos protein, the expression of which can be used to trace pathways of neuronal activation. Duodenal perfusion with lipid emulsion and exogenous peripheral CCK administration have both been shown to significantly increase Fos protein expression in the NTS of the rat (35). The CCK1R has been shown to be involved in both CCK- and nutrient-induced Fos protein expression in the NTS (8). In the present study, increased activation of the NTS in response to exogenous CCK was completely abolished in CCK1R knockout mice. Thus these mice do not possess a functional CCK-dependent afferent neuronal pathway. In addition, overall neuronal activation in the NTS in response to intestinal Intralipid was significantly reduced in CCK1R knockout mice compared with wild-type controls. It is interesting to note that, although there was a significant attenuation in neuronal activation in CCK1R–/– mice in response to intestinal Intralipid, the effect of the intraluminal lipid was not abolished. The residual Fos protein expression in response to intestinal lipid in CCK1R–/– mice might be due to gastric distension, which has been shown to activate neurons in the NTS (7). Alternatively, as we have shown that gastric emptying is more rapid in CCK1R–/– mice, this would produce a greater spread of nutrients down the length of the intestine. As a result, it is possible that an increase in exposure of the distal gut to lipid may stimulate the release of PYY, which has also been shown to increase Fos protein expression in the NTS (3) and play a role in lipid-induced inhibition of gastric function. It is also possible that in the CCK1R–/– mice there is a novel, CCK1R-independent pathway by which lipid is detected to alter gastric function. Therefore, it is possible that the residual response to lipid in the CCK1R–/– mice may be the result of CCK-dependent compensatory mechanisms in this knockout model. Prior research has been conducted on the possible existence of a CCK1R-independent pathway for CCK detection. A recent study in CCK1R-deficient Otsuka Long-Evans Tokushima fatty rats demonstrated that CCK-induced Fos protein expression in vagal sensory neurons is mediated by activation of both CCK1 receptors and CCK2 receptors (5). In the current study, activation of neurons in the NTS by exogenous CCK was absent in CCK1R–/– mice. From this, we can assume that a novel CCK receptor is unlikely to be involved in the responses of vagal afferents to lipid in our CCK1R knockout mouse model.

It is also important to consider the possibility that the knockout mouse model used in this study might exhibit a reduction in its ability to digest and absorb dietary lipid and that the reduction in the response to lipid is due to a lack of formation of free fatty acid. A lack of the CCK1R might result in an inability of CCK to activate gallbladder and pancreatic secretion and, therefore, could result in a reduction in the ability of the mouse to digest and absorb lipid. However, we found no significant difference in plasma TAG levels between CCK1R–/– mice and their controls under fasted or fed (Intralipid gavage) conditions, and both strains of mice demonstrated a significant increase in plasma TAG upon lipid feeding compared with saline. It should be noted that measurement of plasma TAG is an indirect index of lipid absorption, because the chylomicrons must first pass through the liver. However, these findings suggest that the knockout animals are able to digest and absorb dietary lipid in a fashion similar to their controls. In the absence of CCK, meal-induced gallbladder contraction and relaxation of the sphincter of Oddi could still occur due to an increase in neural stimuli, such as activation of the efferent vagal pathway. Similarly, in the absence of CCK, meal-induced pancreatic secretion could be mediated by vagal efferent activity via release of acetylcholine and stimulation of muscarinic receptors as well as the gastrointestinal hormone secretin. It is interesting to note that our observations also imply that there are no apparent gross morphological differences in expression of CCK endocrine cells between CCK1R–/– and CCK1R+/+ mice. This would suggest that the CCK1R+/+ and CCK1R–/– mice both possess the ability to produce CCK but that the CCK1R–/– mice are unable to respond to it. However, definitive conclusions cannot be drawn from this information, and a detailed understanding of the ability of these mice to release CCK remains to be determined.

In conclusion, the data show that lipid-induced inhibition of intestinal feedback is markedly attenuated in CCK1R–/– mice. Despite no significant alterations in overall food intake or body weight (11), CCK1R–/– mice exhibit a significant increase in individual meal size (1), and there are major changes in postprandial regulation of gastric function. These findings support the hypothesis that intestinal lipid activates vagal afferents and initiates intestinal feedback via a CCK1R pathway.


   GRANTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
This study was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grants DK-41004 (to H. E. Raybould), DK-45752 (to K. C. K. Lloyd), and DK-46767 (to A. S. Kopin).


   FOOTNOTES
 

Address for reprint requests and other correspondence: H. E. Raybould, 1321 Haring Hall, Vet Med: APC, Univ. of California-Davis School of Veterinary Medicine, Davis, CA 95616 (e-mail: heraybould{at}ucdavis.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

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