Cholecystokinin antagonist and lipid intake as a function of caloric density and familiarity

Physiology&Behavior,Vol. 51, pp. 1057-1060, 1992

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Cholecystokinin Antagonist and Lipid Intake as a Function of Caloric Density and Familiarity J. A. D E U T S C H , 1 D O N G - Y I C H E N A N D M A G D A L E N A

ROCIO CARRERA-ALBERRO

Department of Psychology, University of California, San Diego, La Jolla, CA 92093 Received 13 S e p t e m b e r 199 1 DEUTSCH, J. A., D.-Y. CHEN AND M. R. CARRERA-ALBERRO. Cholecystokinin antagonist and lipid intake as a function of caloric density andfamiliarity. PHYSIOL BEHAV 51(5) 1057-1060, 1992.--The effect of treatment with the cholecystokinin antagonist L364,718 on intake of different dilutions of corn oil emulsion was tested under two levels of familiarity with the oil emulsion. No increase in intake was observed. To see if the CCK antagonist was effective under our conditions, exogenous CCK was administered under the same conditions. A complete suppression of the large reduction produced by CCK on intake was found. Cholecystokinin antagonist

Food intake

Lipid intake

SOME work (17) suggests that endogenous cholecystokinin secreted by the duodenum is not involved in mediating hunger satiety. Ifcholecystokinin is involved in satiety then other routes of possible mediation of satiety should be explored. That cholecystokinin may be involved in signalling satiety but not through duodenal secretion was made plausible by the studies (7,20) that report that the CCK antagonist lorglumide significantly attenuates the suppression of sham feeding induced by duodenal infusions of lipids and oleic acid. It seems unlikely that this effect of the CCK antagonist is mediated by its action on CCK secreted by the duodenum and more likely that it is acting on some neural pathway in which CCK is involved as transmitter. Studies (10) have shown that in the rat, only protein, but not amino acids, carbohydrates or fats stimulate cholecystokinin secretion when introduced into the duodenum. It has been further demonstrated (13) that CCK regulates digestive enzyme gene expression in the pancreas, controlling the amounts of trypsin and chymotrypsin available. The pancreas changes the amount and composition of its secretion depending on the diet. A major regulatory influence on such adaptation has been shown to be cholecystokinin (5,13,18). Pancreatic proteases in the duodenum suppress CCK release. This, in its turn, reduces the gene expression in the pancreas that ultimately would produce trypsin or chymotrypsin, the digestive enzymes that break down protein. On the other hand, the presence of protein that has not been broken down stimulates CCK release (4-6,9). In this way an increase in protein intake over the average that has occurred in previous meals and not the absolute quantity of protein leads to a release of CCK. It is thus difficult to see how the monitoring of CCK level secreted by the duodenum could be useful in controlling food intake. That CCK secreted by the duodenum does not play a role in regulation of food intake is further supported by the results

' Requests for reprints should be addressed to J. A. Deutsch.

1057

Cholecystokinin

L364,718

in (17) that elevation of plasma CCK six to ninefold through the intraduodenal infusion of soybean trypsin inhibitor does not decrease food intake. It, therefore, seems unlikely that duodenal secretion of CCK is involved in satiety. That an alternate role for CCK in producing satiety may exist is suggested by the studies (7,20) that found that a CCK antagonist attenuates the reduction in sham eating produced by lipids in the duodenum. We attempted to take this line of investigation further by using a more specific CCK antagonist while the rat ingested oil normally. We designed an experiment in which we varied nutrient density and familiarity of a diet that would not stimulate CCK release. This was done by offering varying dilutions of corn oil. Corn oil was emulsified with water in varying proportions. According to Liddle et al. (10) duodenal secretion of CCK should not occur except with protein. EXPERIMENT 1 In this experiment we tested the effect of the cholecystokinin antagonist L364,718 on the intake of different dilutions of oil emulsion.

Method Subjects. Thirty naive Sprague-Dawley rats with weights ranging from 400 to 500 grams served as subjects. They were housed in individual suspended cages with wire mesh bottoms on a 12:12 light:dark cycle and maintained on laboratory rat chow (Wayne Labs) and water, both ad libitum until the onset of experiment. Apparatus. Training and testing were conducted in the rats' home cages. During the training and testing sessions, oil emul-

C H O L E C Y S T O K I N I N A N T A G O N I S T A N D LIPIDS

1059 TABLE 1 EXPERIMENT 2 OIL EMULSIONCONSUMPTION(g)

35 A

w E

30

c= tm O~

25

eO *-' O.

L364 + CCK Veh. + CCK

20 15

E

5

O 0 L364+CCK

Veh.+CCK

FIG. 2. The comparison of 15-rain oil consumption between L364 + CCK-treated animals, represented by white bar, and Veh. +CCK-treated animals, represented by diagonal slash bar, during the test.

from an averaged consumption of 13.15 grams in the training session to 2.79 grams in the testing session, was revealed after the injection of CCK in the absence of the injection of L364,718 in the other group, t(14) = 7.92, p < 0.01. The consumption of oil emulsion in the first 15 min, second 15 min, and last 30 m i n is shown in Table 1. It can be seen that as time went on, the rats with vehicle and CCK injections increased their intake gradually, whereas the rats in the other group decreased, indicating that exogenous C C K was reduced in effectiveness as time progressed. EXPERIMENT 3 As no effect of CCK inhibition on intake was found in Experiment 1, it was hypothesized that a higher level of training or familiarity with the nutrient dilutions might reveal an effect.

Method Subjects. Fourteen male Sprague-Dawley rats (315-335 g) were housed and tested in individual wire mesh cages and main-

E

Second 15 min

Last 30 min

12.44 -+ 3.46 2.79 ___3.07

6.06 - 2.23 4.46 _ 2.85

4.05 --- 3.05 6.06 -+ 3.20

IIII

10

O

First 15 min

tained on a 12:12 light:dark cycle. Tap water was available ad lib. Rat chow (Wayne Labs) was presented for 1 h the first 12 days, and 2 h the next days, beginning at 1600. Training. For the next 16 days the rats were trained to drink a Mazola corn oil/water emulsion for an additional hour starting at 1000. The solution was available at dilutions of 1:1, !:2, 1:4, and 1:8, emulsified with 1% lecithin, in standard glass water bottles. Presenting one dilution per day, the ratios were ordered in four different sequences in a Latin square fashion, to control for carryover and order effects. Testing. Two groups of seven rats were then assigned to one of two conditions: vehicle-vehicle (V-V), and vehicle-antagonist (V-A). The initial dilution sequence was repeated, and rats were injected once immediately before they were presented with dilutions 1:2 and 1:8, eight times throughout the entire sequence. Drugs. Same as in Experiment 1. The a m o u n t of emulsion drank was measured 30 and 60 minutes after its presentation by weighing the bottles.

Results While dilution of the oil emulsion increased intake up to an asymptotic value, the cholecystokinin inhibitor did not increase intake at the two values (1:2, 1:8) at which it was administered as shown in Fig. 4. The cholecystokinin inhibitor seemed actually to depress intake at the 1:2 ratio of oil to water. The group injected with the cholecystokinin inhibitor drank 19.37 ml (SEM _+ 2.78) and the vehicle control group drank 23.1 ml (SEM _+ 0.88). This difference opposite to that expected is significant, t(12 df) = 3.13, p < 0.01.

35

35-

30

30

25

2S

-!-I-

20 C

2O

o

15

E ~= lO

10

o ~0

O

5

5 0

!

I'! L364+CCK Group

Veh.÷CCK Group

FIG. 3. The comparisons of first 15-min oil consumption in training session (white bars) and testing session (diagonal slash bars) for L364 + CCK-treated group and vehicle + CCK-treated group.

!:2

1:4

1:8

FIG. 4. 30-rain oil consumption in Experiment 3. White bar: Showing the consumption in the sequences when saline (l:l and 1:4) and vehicle ( 1:2 and 1:8) were injected. Diagonal slash bar: Results in the sequences when saline was injected. Hatched bar: Showing the results in the sequences when L364,716 was injected.

1060

DEIJTSCH, C H E N A N D ( ' A R R E R A - , M B E R R ( I DISCUSSION

Treatment with cholecystokinin antagonists has produced increases in food intake according to some reports (13,8,12,15,16,19,20). The present study reports another discrepant result. For it appears that the results of studies that show an increase of sham feeding upon CCK inhibitor administration when lipids are inserted into the duodenum, do not generalize to the normal feeding (7,20). There could be various reasons for such a discrepancy. For instance, the above workers used a less specific CCK antagonist. However, it is unlikely that differences in caloric density of the nutrient or its familiarity to the animal can be invoked to explain the discrepancy reported here. Under the range of conditions utilized in our experiments, the administra-

tion of a specific CCK antagonist does not produce tile increase in food intake that would be predicted by the hypothesis thal cholecystokinin signals satiety. It is interesting to note that the C C K antagonist here reduces the effect of exogenously administered CCK while ha~ing no effect on normal food intake. A similar effect has been obtained by a drug with no known antagonism to CCK, the antiemetic trimethobenzamide (21 ). This antielnetic produces no increase in food intake in the normal case where no CCK has been administered. On the other hand, food intake of an animal given C C K rises when the antiemetic is administered also. This suggests that the reduction in intake seen after the exogenous administration of CCK seen in this experiment is not necessarily due to satiety, but could more plausibly be interpreted as due to malaise.

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