cholecystokinin decreases food intake in rats

Cholecystokinin Decreases Food Intake in Rats’ James Gibbs,2 Robert C. Young, and Gerard P. Smith From the Edward W. Bourne Behavioral Research Laboratory, Department of Psychiatry, New York Hospital-Come11 Medical Center, Westchester Division, White Plains, New York

Abstract Partially purified cholecystokinin (CCK) was injected intraperitoneally into fasted rats prior to food presentation. The hormone produced a large dose-related suppression of intake of solid and liquid diets. Identical doses of the synthetic terminal octapeptide of cholecystokinin produced identical results. An effective dose of CCK did not suppress drinking after water deprivation. Treated animals did not appear ill and were not hyper- thermic; neither CCK nor the octapeptide produced learning of a taste aversion in bait-shyness tests. The effect of CCK is not a property of all gut hormones, since injections of secretin did not affect feeding. These studies raise the possibility that CCK plays an inhibitory role in the short-term control of feeding behavior.

An unidentified blood-born factor appears to act as a ‘‘satiety signal” (Davis, Campbell, Gallagher, & Zurakov, 1971). Gut hormones released by ingested food might play such a role in the short-term control of food intake. This idea is not a new one. In 1937, MacLagan reported decreased feeding in rabbits following injections of a crude extract of canine intestine. The extract was called enterogastrone because it was thought to contain the substance which mediated the inhibition of gastric secretion observed after a fatty meal (Kosaka & Lim, 1930). More recently, crude intestinal extracts also inhibited food intake in rats (Glick & Mayer, 1968; Ugolev, 1960). Using a partially

‘The authors thank Frank P. Brooks, Alan N. Epstein, William T. Lhamon, Paul R. McHugh, and Eliot Stellar for their interest and encouragement. Joseph Antin and Jonathan Holt gave expert technical assistance. This research was supported by Na- tional Institutes of Health Career Development Award 7K04 NS38601 and Grant NS08042 to Gerard P. Smith. *Requests for reprints should be sent to James Gibbs, Edward W. Bourne Behavioral Research Laboratory, Department of Psychiatry, New York Hospital-Come11 Medi- cal Center. Westchester Division, 21 Bloomingdale Road, White Plains, New York 10605. ’Ivy dog unit of CCK (Ivy & Janacek, 1959): “that amount of vasodilantin-free cholecystokinin which when dissolved in normal saline solution and injected intra- venously in the dog during 10-15 seconds causes within 1-5 minutes a rise in intra-gall-bladder pressure of 1 cm. of bile.” Adapted from the Journal of’Comparative and Physiological Psychology, 1973;84(3): 488495. Copyright 0 1997 by the American Psychological Association. Reprinted with per- mission.

purified preparation of porcine enterogastrone, Schally, Redding, Lucien, and Meyer (1967) reduced food intake of mice; injections of secretin or pancreatic glucagon were not effective.

Cholecystokinin (CCK), the other available intestinal hormone, has enterogastrone activity in the dog (Johnson & Grossman, 1970; Nakajima & Magee, 1970). Glick, Thom- as, and Mayer (1 97 1) reported that CCK failed to affect food intake in rats. We report here, however, that CCK significantly decreased food intake in rats under different experimental conditions. Part of this work has been reported previously in the form of a preliminary communication (Gibbs, Young, & Smith, 1972).

Experiment 1 Method

The subjects were 29 adult male Sprague-Dawley rats (Hormone Assay, Chicago, Illinois) weighing 450-550 gm. They were housed and tested in individual cages on an artificial 12-hr. light cycle and were maintained on Purina Rat Chow pellets and tap water.

The rats were deprived of food for 5 % hr. (10 a.m.- 3:30 p.m.) each day. Fifteen minutes prior to presentation of a weighed amount of Purina pellets, the rats were injected intraperitoneally (ip) with 1 ml. of either .15 M saline or partially purified porcine CCK (10% W/W, GIH Research Unit, Karolinska Institutet, Stockholm, Sweden) in doses of 2.5, 5 , 10, 20, or 40 Ivy dog U/kg body weight dissolved in .15 M ~ a l i n e . ~ Food consumption was calculated by sub- tracting the weight of the remaining pellets and crumbs from the initial weight at 30-min. intervals for 150 min. Tap water was available throughout the test period.

Experiments were designed in a crossover pattern; food consumption of each rat after CCK administration was compared with consumption after saline administration on the preceding or following day. Statistical comparisons were made with a matched-pairs t test.

Results The extract containing CCK inhibited food intake

(Table 1). The effect was large-a 50% suppression at 30

284 OBESITY RESEARCH Vol. 5 No. 3 May 1997

Classics in Obesity

min. by the 40 U/kg dose. The magnitude of the inhibition was dose related over the range of doses tested, and the lowest effective dose of CCK was 5 U/kg.

The inhibition occurred within the first 30 min. of the test period; furthermore, an analysis of the amounts eaten during each 30-min. interval (Figure 1) demonstrates that the effect of CCK was in fact limited to this first 30 min. even with the largest dose (40 U/kg) of the hormone. The CCK-injected rats compensated for their early deficit in food intake by eating more than saline-injected controls late in the test period. By the time of the 150-min. measurement, only rats which had received the largest dose of CCK still had a significant decrease of cumulative food intake (Table 1).

Experiment 2 The inhibition of feeding produced by the 10% prepa-

ration of CCK could have been due to impurities in the extract and not the hormone itself. We tested this possibility by using 2 purified compounds: (a) the synthetic C-terminal octapeptide of CCK (SQ 19,844, the gift of M. Ondetti, Squibb Laboratories, New Jersey) and (b) caerulein (the gift of A. Anastasi, Farmitalia, Milan, Italy). Caerulein is a decapeptide extracted from the skin of the frog (Hyla cae- rulea). The C-terminal heptapeptide amides of caerulein and CCK differ by only 1 amino acid. Anastasi, Bernardi, Bertaccini, Bosisio, de Castiglione, Erspamer, Goffredo, and Impicciatore (1968) reported that caerulein was 15 times as potent by weight as CCK in stimulating pancreatic volume flow in the dog, and this factor was used to calculate doses.

-;i Time 0-Mmtn 10-bOnin OD-Wnln W !Dmn 1 2 0 l j O n t n

n. 29 29 ?9 29 20 ?Q 19 ?Q 29 29 - Figure 1: Consumption of pellets (X in gm. & SE, during 30-min. intervals following intraperitoneal injection of saline (light bars) or 40 U/kg of cholecystokinin (CCK) dissolved in saline (dark bars). (CCK significantly inhibited food intake only for the 0-30 min. interval; double asterisk indicates p < .001.)

Method The 28 subjects for the octapeptide tests were from the

same group used in Experiment 1. They were injected with 1 ml. of either .15 M saline or octapeptide of CCK in doses of 2.5, 20, or 40 Ivy dog U/kg body weight dissolved in .15 M saline.

Twelve adult male Sprague-Dawley rats (Hormone As- say, Chicago, Illinois) weighing 450-550 gm. were subjects for the caerulein tests. They were injected with either .15 M saline or caerulein in amounts of . l , .4, 3, and 1.2 p.g/kg

TABLE 1

CHOLECYSTOFININ (EXPERIMENT 1) OR SYNTHETIC C-TKI~MIHAL OCTAPRPTIDE OF CHOLKCYSTOMNIN (EXPERIMENT 2)

CUMUL.4TIVE FOOD C O S S U M P T I O N (AS PERCKNT.4QE OF CONTROL) OF RATS AFTER TIK\TMI.:HT IVITH

- Treatment dose

(in IVY dog U/kg)

2.6, CCK

10.0, CCK 20.0, CCK

40.0, CCK

2.6, octapeptide 6.0, CCK

a . 0 , ortaprptide

40.0, octapeptide

Time alter food presentation (in min.)

I 30

94 98 74'. **** ,a***, **** la**** 74**** a**** 51****

60 ' I 94

103 go**** 72+*** 74**** 75**** (;5**** 70****

87 105 92 73**** 77****

(#**** 75****

81****

97 99 95 91 !)O n7 I 3 84**** "( ****

-- _._____ _______- -_____ Note. hlcaii rontrol iiitnkes for all tests ranged 5.4-i.7 gm. at 150 min. wid wcre nirtLwrcd on t hc day

immedit~tely preceding or lollowiiig a treatniciit day. Ewh pcrcentsge 'R" obtirilied from H iiiinimiini of lo rats.

p < .O5, signitirnntly dilferent from 10 U/kg perreiitage at 30 Ii i i i i .

'** p < .01, nignifcantly different from 40 U/kg perrentage at 30 niin. ** p < .02, sigiiitic*a~rtly diflc*rent froin ntrliue rontrols, matrhed-pairs t t ~ t ~ , 2-1 niled.

**** p < .OW, sigiiitirantly dimeretit froni c l i r ~ i ~ i c r rontrols, niatr~irc~-ptiirs I tests, 1 - t n i ~ .

OBESITY RESEARCH Vol. 5 No. 3 May 1997 285

Classics in Obesity

body weight dissolved in .15 M saline. Other details of these procedures were exactly as described in Experiment 1.

Results Equivalent doses of octapeptide of CCK produced in-

hibitions of feeding equivalent to those caused by the extract containing CCK (Table 1).

Treatment with caerulein also inhibited feeding in a dose-related fashion. Figure 2 compares consumption of food after treatment with CCK, octapeptide, and caerulein.

Experiment 3 It was possible that the decrease in feeding produced by

CCK did not represent a reduced motivation to eat, but was achieved through some interference with the motor acts re- quired for eating solid food. We examined this possibility by testing the effect of CCK on rats eating a liquid diet.

Method The subjects were 8 adult male Sprague-Dawley rats

weighing 450-550 gm. They were maintained on a balanced liquid diet preparation (No. 116 E. C., General Biochemi- cals, Chagrin Falls, Ohio) diluted to 50% strength with water. Tap water was also available.

The rats were deprived of the liquid diet for 17 hr. (6:OO p.m.-1 1:00 a.m.). Fifteen minutes prior to presentation of a measured amount of liquid diet (diluted to 25% strength for the test period) in graduated drinking tubes, animals were injected ip with 1 ml. of either .15 M saline or partially

Figure 2: Consumption of solid food (expressed as percentage of control consumption) during first 30 min. of the test period following intraperitoneal injection of cholecystokinin-CCK-(light bars), octapeptide (dark bars), and caerulein (hatched bars) in various doses. (Caerulein doses were calculated by assuming that caerulein was 15 times as potent by weight as CCK [Anastasi, Bernardi, Bertaccini, Bosisio, de Castiglione, Erspamer, Goffredo, & Impiccia- tore, 1968; A. Anastasi, personal communication, Decem- ber 19711. Mean control intakes ranged 2.84.3 gm. Single and double asterisks indicate statistical differences from saline control at p < .01 and p < .001, respectively.)

purified CCK in doses of .15, .6,2.5, 10, or 38 Ivy dog U/kg body weight dissolved in .15 M saline. Measurements of consumption were made at .5, 15, 30, 60, 90, 120, and 150 min. from the time of diet presentation. Tap water was available throughout the test period. As in Experiment 1, a crossover design was used, so that each animal served as its own control.

Results The CCK inhibited liquid food consumption (Table 2).

As with solid food, the inhibition was dose related. The threshold dose was .6 U/kg, which produced an inhibition during the 30-60 min. interval. Injections of 2.5 U/kg caused an earlier and more marked depression of feeding.

An analysis of the amounts eaten during each interval (Figure 3) showed similarities to the pattern of inhibition seen with solid food: The effect of CCK was limited to the early part of the test period (0-15 min., primarily the 5-15 min. interval), and hormone-injected rats ate more than saline controls late in the test period (60-90 min. interval).

An analysis of cumulative liquid diet consumption re- veals that the effect of CCK was to limit the size of the test meal during the 5-15 min. interval (Figure 4).

Note that both CCK-treated and saline-treated rats ate eagerly when food was presented, consuming approximately equivalent amounts during the first 5 min. Hormone- treated animals then stopped feeding, while controls went on to take a larger initial meal.

Experiment 4 This experiment attempted to determine whether the

inhibition of ingestion produced by CCK was specific for food by testing water consumption following CCK administration.

TABLE 2 CUMMULATIVE LIQUID DIET C o s e r ~ ? ~ i o s (AS PER-

CESTAGE OF COSTROL) OF RATS .;TTER TREAT- J l E S T WITH C H O L E C Y S T O I i I S I S C‘CK) I N

EXPERIMEST 3

Time after food presentark: in min.) (in Dose Ivy ~

dog u’kg’, 5 l j 30

99

Sole . Mean control intakes for sll tests ranged 52.9-61.1 ml. a t 150min. and %’ere nirssured on the day immediately preceding or follaning a treatment day. Each percentage of inhibition n7as ob- tained from 8 rats.

* p < .05, matched-pairs t tests, “tailed. ** p < .01, matched-pairs 1 tests, ?-tailed.


Classics in Obesity

"I- -

n . I U 8 8 R U 8 8 S S 8 U Y 8 - Figure 3: Consumption of diluted liquid diet (X in ml. f SE,) during intervals following intraperitoneal injection of saline (light bars) or 10 U/kg of cholecystokinin (CCK) dissolved in saline (dark bars). (CCK significantly inhibited food intake during the 0-5 min. [p < .05] and 5-15 min. [p < .001] intervals. The CCK-treated rats ate significantly more than controls during the 60-90 min. interval [p < .05].)


weighing 450-550 gm. They were housed and maintained as described in Experiment 1. The rats were deprived of water for 12 hr. (1O:OO p.m.-1O:OO a.m.). One-half hour before water presentation, food was removed from the cages for the duration of the test. Fifteen minutes prior to water

-

Figure 4: Cumulative consumption of diluted liquid diet (X in ml. f SE,) following intraperitoneal injection of saline (light bars) or 38 U/kg of cholecystokinin-CCK-(dark bars). (The CCK limited meal size. Intake of hormone- treated rats was significantly depressed during the 5-15 min. and 15-30 min. [p < .001], and the 30-60 min. [p < .05] intervals.)

T

T 25 -

- f 2 0 -

- 15 -

10 -

5-

L

presentation in graduated drinking tubes, rats were injected ip with 1 ml. of either .15 M saline or partially purified CCK in a dose of 20 U/kg body weight dissolved in .15 M saline. Measurements of consumption were made at 5 and 15 min. from the time of water presentation. As in Experiment 1, a crossover design was used.

Results The CCK did not suppress drinking. There were no

significant differences in the amounts drunk during t h e e 5 min. period: Saline-injected rats drank 9.8 f .5 ml. (X f SE,) and CCK-injected rats drank 9.2 f .5 ml. (p > .05, matched-pairs t test, 2-tailed). Hormone-injected rats showed a slight increase in water intake during the 5-15 min. interval, drinking 2.0 f .3 ml., while saline-injected rats drank 1.1 f .3 ml. (p < .005). This 5-15 min. interval is the period when CCK is most effective in suppressing liquid diet consumption (see Experiment 3).

Experiment 5 It is possible that CCK inhibited feeding by making the

rats sick, but this possibility is not likely: (a) Rats ate rapidly following injections of even large doses of CCK, but simply stopped eating sooner (see Figure 4); (b) they maintained control intakes of both solid food and liquid diet during 8 mo. of frequent CCK injections; and (c) they did not have elevated body temperatures following injections of 40 U/kg of CCK. Temperature measurements were made with a rectal probe thermometer under conditions identical to the feeding-test situation. Saline-injected rats had a body temperature of 97.8 f .2"F. (X f SE,), while CCK-injected rats had a temperature of 97.7 f .2"F. (n = 8, p > .05, t test for means of 2 samples, 2-tailed).

In addition, we tried to detect any inapparent distress by attempting to induce learning of a taste aversion with CCK. We employed the experimental design of Nachman (1970), using lithium chloride to make rats ill in temporal association with the novel taste of saccharin, resulting in bait shyness to saccharin on a subsequent presentation.


(Hormone Assay) weighing 450-550 gm. They were housed and maintained as described in Experiment 1. The animals were deprived of water and given a daily 10-min. test of water consumption, measured in graduated drinking tubes, in their home cages. After being trained for 4 days to drink all their daily water during this limited period, rats were presented on Day 5 with a .25% sodium saccharin solution instead of water. One minute following this 10- min. exposure to the novel taste of saccharin, rats were injected with either (a) .15 M NaC1; (b) .15 M LiCl, .6% body weight; (c ) partially purified CCK, 40 U/kg body weight dissolved in .15 M NaC1; or (d) synthetic terminal


Classics in Obesity

octapeptide of CCK, 40 U/kg body weight dissolved in .15 M NaC1. On Days 6 and 7, all rats were provided with water for the usual 10-min. test period and, without interruption, 20 min. of additional drinking time. On Day 8, all rats were again offered the .25% sodium saccharin solution for 10 min. Amounts drunk by each treatment group on each day were statistically compared with a t test for means of 2 samples.

Results Neither CCK nor octapeptide of CCK produced learn-

ing of a taste aversion as measured by the amount of saccharin drunk at the second offering (Table 3).

None of the treatments immediately following the first saccharin offering produced overt symptoms such as diar- rhea. Furthermore, on Test Days 6 and 7, there were no statistically significant differences in amounts of water drunk by groups treated with saline, LiCl, CCK, and octapeptide; thus, the animals were not made chronically ill by the treatments. Nevertheless, rats injected with LiCl on Day 4 displayed a marked aversion for saccharin on Day 8.

Experiment 6 This experiment asked 2 questions: First, is the inhibi-

tion of feeding demonstrated with CCK a property of other gut hormones? Secretin, another polypeptide hormone released from the duodenal mucosa as a consequence of food intake, was used for this test. Second, does secretin poten- tiate the action of CCK on feeding? To answer this question, the 2 hormones were administered together in a range of dose combinations and the results compared with the effects of CCK injections alone.

TABLE 3 WILLINGNESS OF RATS TO DRINK SACCHARIX

SOLUTION AT SECOND OFFERING WHEN INJECTIONS OF VARIOUS CHEMICALS

FOLLOWED FIRST OFFERING t ,

Treatment I I Sacchadn intake at I n I gecond offering

(X in ml. f S E M ) I

CCK effect 1 .15 M LiCl F 1 . 7 f .5 CCK" in .15 M NaCl ; 6 ~ 16.5 f 1.5b. * .15 31 NaCl ~ 0 , 14.2 f 1.5*

Octapeptide effect ~I .15 M LiCl 5 j 0.0 f 0.0 Octapeptide" in .15 .\I

.15 11 NaCl 5 18.6 f l .F* NaCl 0 1 16.8 f 2.0ti. *

8 40 Ivy dog U/kg. b Not statistically different from NaC1-injected

* p < ,001, different from LiC1-injected rats, rats.

t test for means of 2 samples, %tailed.

Method The 12 subjects for the secretin tests were fully grown

male rats similar to those used in the previous experiments. They were injected ip on different experimental days with 1 ml. of either (a) .15 M saline; (b) pure natural porcine secretin (4,000 clinical U/mg, GIH Research Unit, Stock- holm, Sweden) in doses of 2.5, 5 , 10, or 40 clinical U/kg body weight dissolved in .I5 M saline; or (c) partially purified CCK in doses of 2.5, 5 , 10, or 40 Ivy dog U/kg body weight.

The same rats were subjects for the combination experiment. They were injected with 1 ml. of either (a) partially purified CCK in doses of 2.5, 10, or 40 Ivy dog U/kg body weight or (b) 1 of the following combinations of CCK (Ivy dog units) plus secretin (clinical units): 2.5 U/kg CCK+10 U/kg secretion; 2.5 U CCK+4O U secretin; 10 U CCK+10 U secretin; or 40 U CCK+10 U secretin. All animals in these procedures were maintained with diluted liquid diet on the deprivation schedule described in Experi- ment 3.

Results Secretin did not significantly affect feeding (Figure 5).

The 10 clinical U/kg dose of secretin produced the largest suppression of feeding (lo%), but this was not statistically significant.

No combination of secretin and CCK produced a suppression of food intake significantly greater than the decrease after CCK in the same dose administered alone.

Discussion This work demonstrates that the gut hormone CCK

suppresses feeding in the rat.

Figure 5: Consumption of diluted liquid diet (expressed as percentage of control consumption) during first 15 min. of the test period following intraperitoneal injection of cholecystokinin (Ivy dog units, dark bars) or secretin (clinical units, hatched bars) in various doses. (Mean control intakes ranged 28.7-35.4 ml. Single and double asterisks indicate statistical differences from saline controls at p < .01 and p < .001, respectively.)


Classics in Obesity

A partially purified preparation of CCK decreased intake of both solid and liquid food (Experiments 1 and 3), but did not decrease water intake (Experiment 4). The suppression of food intake by this preparation was a property of the CCK molecule and was not due to impurities present in the extract, because the synthetic C-terminal octapeptide of CCK and the decapeptide caerulein suppressed food intake (Experiment 2). Both the octapeptide (Rubin, Engel, Drun- gis, Dzelzkalns, Grigas, Waugh, & Yiacas, 1969) and caerulein (Stening & Grossman, 1969a; Stening, Johnson, & Grossman,. 1969; Vagne & Grossman, 1968) have the physiological spectrum of action of the whole CCK molecule.

We also tested the effect of secretin on feeding. Like CCK, secretin is a polypeptide gut hormone released by ingested food. Its chemical structure is different from CCK and, in particular, it does not share the C-terminal heptapeptide sequence of CCK. Secretin failed to suppress food intake (Experiment 6). Despite their strucural differences, secretin potentiates several of the gastrointestinal effects of CCK (Henriksen & Worning, 1967; Spingola, Meyer, & Grossman, 1970; Stening & Grossman, 1969b). Under our experimental conditions, however, secretin did not potenti- ate the inhibition of food intake by CCK (Experiment 6).

A humoral factor which is proposed as an inhibitory signal regulating meal-taking behavior must meet certain criteria:

1. The proposed signal should be activated as a consequence of feeding. CCK, a polypeptide hormone, is released from the duodenal mucosa by ingested food: Mea- sureable increases in CCK activity in the plasma occur in the anesthetized dog and cat following duodenal perfusion with emulsified fat, long- and short-chain fatty acids, pep- tones, and hydrochloric acid (Berry & Flower, 1971; Ivy & Oldberg, 1928).

If the proposed inhibitory signal were released in pro- portion to the amount of food ingested, this would account for the observation of Le Magnen ande Tallon (1 966) that meal size of rats is correlated with the length of the next intermeal interval but not with the length of the preceding interv a1 .

2. The proposed signal, administered exogenously prior to a meal, should significantly decrease meal size. Experiments 1 and 3, respectively employing solid and liquid foods, clearly fulfill this criterion (see Tables 1 and 2). If the relationship between the signal and feeding inhibition is an orderly one, a dose-response relationship will be ob- tained; the inhibition of feeding caused by CCK was large and dose related in both experiments. The finding that CCK significantly decreases food intake in the rat contrasts with the results of Glick et al. (1971). These authors injected rats intraperitoneally and intraaortically with CCK or secretin and studied the effect of each hormone on the rate of bar pressing for pellets or undiluted liquid diet. They found no effect of secretin and a trend, not statistically significant,

toward decreased bar pressing after CCK. Differences between their experiments and the work reported here include the dose of CCK, length of deprivation, and measure of motivation (operant task vs. consumption). Which of these factors is the important difference is unclear.

3. The proposed inhibitory signal, in order to account for the frequency of meal-taking behavior, should have a relatively rapid onset and brief duration of action. As measured by its ability to contract gall-bladder strips, CCK is released and metabolized rapidly. It appears in the portal circulation of the cat 1-3 min. after introduction of emulsified fat into the duodenum and has a half-life of 10-15 min. (Berry & Flower, 1971). The behavioral effects of CCK seen in Experiment 1 were also rapid and brief Feed- ing was decreased 15-20 min. following injection, and this effect was limited entirely to the 0-30 min. interval of the test period with solid food (see Figure 1) and to the (r15 min. interval with liquid food (see Figure 3).

4. The effect of the proposed signal should not be due to illness. There was no indication that rats were toxic after administration of CCK in the doses used in this study. Ap- pearance, eagerness to eat, maintenance of control food intake over several months, and lack of body temperature elevation after CCK treatment all indicated that the animals were not made ill by injections. However, it was possible that nausea or other discomforts were temporarily present and that feeding was inhibited by such disturbance.

Following Rzbska’s (1953) description of bait shyness, Garcia, Kimeldorf, and Hunt (1961) established that rats will learn to shun novel taste stimuli which are followed by treatments that cause illness. We used this observation in Experiment 5 to search for behavioral evidence of any sub- clinical discomfort caused by CCK. We were unable to demonstrate any conditioned taste aversion.

5. The proposed inhibitory signal or group of signals should be effective in physiological doses. The amount of CCK employed in the lowest effective dose of these experiments appears to be of the same order of magnitude as the available measurements of plasma levels: Berry and Flower ( 197 1) reported maximum concentrations of 10 mU/ml of plasma CCK activity following hydrochloric acid perfusion of the duodenum in the dog and cat; the 2.5 U/kg dose of CCK, which gave a clear suppression of liquid food intake (Table 2), is approximately equivalent to an extracellular fluid concentration of 12.5 mU/ml, assuming instant and homogeneous distribution in an extracellular space equal to 20% body weight.

From the data presented in this study, however, it can- not be concluded that the effect of CCK is a physiological one. It is important to note that the design of Experiments 1 and 3 does not eliminate the release of endogenous signals which suppress feeding. Figure 4 demonstrates this by showing that saline-injected control rats also stopped eating 15-30 min. after feeding began. Exogenous CCK may in-


Classics in Obesity

teract with these endogenous signals (endogenous CCK or other humoral or neural factors) to suppress feeding. Until such signals are known and their effects and interactions controlled, the threshold dose of CCK for the inhibition of feeding must be considered provisional.

Our results with exogenous CCK fulfill 4 of 5 criteria for a humoral inhibitory signal which participates in the short-term control of food intake. Whether these results represent a natural function of the endogenous hormone re- mains to be determined.

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cholecystokinin decreases food intake in rats

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