The relationships among percentage body weight loss, circulating free fatty acids and consummatory behavior in rats

Physiology and Behavior. Vol. 5, pp. 301-309. Pergamon Press, 1970. Printed in Great Britain

The Relationships among Percentage Body Weight Loss, Circulating Free Fatty Acids and Consummatory Behavior in Rats' D O N W. W A L K E R 2 A N D N. R. R E M L E Y

Department of Psychology, Texas Christian University, Fort Worth, Texas, U.S.A. (Received 7 July 1969)

WALKER,D. W. AND N. R. REMLEY. Relationships among percentage body weight loss, circulatingfree fatty acids and consummatory behavior in rats. PHYSIOL.BEHAV. 5(3) 301--309, 1970.--Two experiments were done to determine the empirical relationships among percentage body weight loss, circulating free fatty acids (FFA) and consummatory behavior in rats. In Experiment 1, FFA concentration was found to increase with food deprivation. Both FFA and food consumption per unit time were found to decrease together as the duration of a post-deprivation food consumption test was increased. In Experiment 2, F F A again decreased but rate of bar pressing on a 2 min VI schedule increased as duration of post-deprivation testing increased. In Experiment 1, no statistically significant differences in either food consumption or FFA measures was found between animals deprived to 10 or 20 per cent of original body weight. In Experiment 2, however, both rate of bar pressing and FFA measures were found to vary as a function of 10 per cent body weight loss. The inconsistent relationship between antecedent deprivation conditions and the two response measures was thus reflected in the likewise inconsistent relationship between the two response measures and one index (FFA) of the metabolic condition of the animal. The results are discussed in terms of a lipostatic mechanism which proposes that decreases in circulating FFA concentration may be correlated with satiety and increases in circulating F F A concentrations may be correlated with hunger. Free fatty acids Satiety

Hunger

Food consumption

SOME MOTIVATIONALpsychologists [13, 30-32] have been discouraged with food consumption as a behavioral measure of hunger since its relationship to measures of deprivation conditions is distinctly different from that of other responses. F o r example, many investigators have reported that the rate of bar pressing for food reinforcement increases as food deprivation increases [9, 11, 30, 36]. It has also been reported that the amount of food consumed increases only up to a point as deprivation increases. With further increases in food deprivation the amount of food consumed decreases [4, 5, 30, 31, 36]. The studies using bar pressing as the behavioral measure have employed schedules of reinforcement, such as variable interval or high fixed ratio, which produce large numbers of responses for small amounts of food. Accordingly, bar pressing for food should produce quite different effects on the organism that should ad lib food consumption, since ad lib food consumptionrequires less work (and time) to supply more food to the organism. When an animal is deprived of food, a number of related physiological events occur. There are changes in the nutritional state of the body's cells, changes in the blood chemistry,

Weight loss

Lipostatic hypothesis

Energy regulation

changes in the central nervous system and changes in a host of metabollic processes. Since different response systems obviously also have different physiological effects on the organism, then any definitive statement about hunger motivation must indicate which, or what combination, of the physiological changes are related to both the antecedent deprivation condition and to the behavioral responses of interest. In order to assess the relationships among all three of these variables, a measure indicative of the organism's physiological status is required. Many investigators have thus been interested in the physiological correlates of hunger. The number of experimental investigations and the number of proposed mechanisms have been numerous. Of all the proposed mechanisms, three have received the majority of the experimental consideration. The three are: the "thermostatic" hypothesis of Brobeck [6, 7], the "glucostatic" hypothesis of Mayer [27-29], and the lipostatic hypothesis of Kennedy [22-24]. First, the thermostatic hypothesis proposed by Brobeck is, in general terms, that animals eat to keep warm and stop eating to prevent hyperthermia. Although temperature may

1Based in part on a dissertation by Don W. Walker in partial fulfillment of the requirements for the Ph. D. and supported by a research grant awarded to N. R. Remley by the Texas Christian University Research Foundation (Grant No. Ps 6781). 2Present address: Postdoctoral Research Fellow, Center for Neurobiological Sciences, College of Medicine, University of Florida, Gainesville, Florida, 32601, U.S.A. 301

302 be correlated with food consumption, it has been shown that it most likely has its effect indirectly through a metabolic factor [8, 23]. The second proposed mechanism is the glucostatic hypothesis. Mayer has hypothesized that the central nervous system is capable of detecting the availability of glucose by means of hypothalamic glucoreceptors. The hypothesis being that increased availability of glucose corresponds to satiety and decreased availability of glucose to hunger. However, it has been sometimes reported that food consumption does not change following glucose injection in man [3] or in rats [21]. Slow intravenous diffusion of glucose has also been recently found to have no detectable effect on food consumption in rats [1]. Although much evidence has been reported supporting both the thermostatic and glucostatic mechanisms, the majority of it can also be incorporated by an extended version of Kennedy's lipostatic hypothesis. Kennedy [22-24] has suggested that the regulation of food intake is concerned with the prevention of a surplus of energy intake over expenditure, and thus to prevent the deposit of excess fat. He proposes that the hypothalamus is sensitive to some unspecified metabolite(s) reflecting the size of the body's fat stores. Kennedy further hypothesizes that an organism will adjust its food intake and behavior in order to maintain some privileged body weight. Although the lipostatic hypothesis has received significant support it has not been the subject of much experimental interest. This is probably because the hypothesis is not spelled out ill detail and is not stated in a form which allows for deductive testing. However, more recent evidence strongly suggests that a lipostatic mechanism may indeed play a role in the regulation of food consumption. Adipose tissue is an energy bank; deposits are made at mealtime and withdrawals made on demand. The total store of tissue and fluid carbohydrates (glucose and glycogen) is rarely more than 300 cal in an adult male human. The fat stores are then extremely important in an organism's day-today energy balance. Energy is released by the fat depots in the form of free fatty acids ( F F A ) into the blood. Of significance is the fact that F F A can be directly used for energy by non-nervous tissues of the body and serve as the main source of energy for muscular work [15, 39]. There are a number of factors that cause an increase in F F A mobilization from adipose tissue that are of direct importance in consideration of a measure reflecting the deprivation conditions of the organism and relating the measure both to antecedent deprivation conditions and to behavioral responses. Blood F F A levels increase with food deprivation [12, 16, 18, 39, 40], exposure to cold [25, 26] and during exercise [15]. Blood F F A levels decrease with increases in glucose utilization or carbohydrate administration [12, 35, 39, 40], and with increases in blood amino acid concentrations [19]. Bates, Mayer and Nauss [2] report that the amount of F F A mobilized daily is proportional to the size of the fat depots of the animal, so, if the central nervous system were sensitive to some correlate of F F A mobilization or utilization, then body weight and ad lib food consumption should be highly related, as they are. Adipose tissue cells are richly vascularized and directly innervated by sympathetic nerve fibers. Correll [10] found electrical stimulation of nerve fibers innervating rat and rabbit adipose tissue to increase F F A release from the tissue, as compared to non-stimulated tissue. He also found food deprivation to increase this response to stimulation. This

WALKER AND RLMLI '~ finding suggests the possibility of a feedback system laetween adipose tissue metabolism and the central nervous system by means of direct innervation as well as the vascular system. .The possibility of such a feedback system is further supported by some recent reports by Haessler and Crawford [17] and StarT, Crawford and Haessler [37]. These investigators found that fat pads from rats made hyperphagic by ventromedial hypothalamic lesions release only one-fourth as much F F A as does adipose tissue of normal control animals. They also report that the hyperphagic fat tissue contained an abnormal distribution of fatty acids. The relationshit~ between disturbances in fat metabolism and hyperphagia is as yet unspecified, but appears to lend support to a lipostatic mechanism. A meaningful theoretical statement about the relationship between fat metabolism and food consumption, however, requires information about the empirical relationship between food consumption and fat metabolism of normal animals. It appears, then, that F F A concentration in the blood is a leading candidate as a physiological measure reflecting the physiological status of the organism when deprived of food. Since blood F F A levels have also been shown to change with the expenditure of energy, it may also reflect the differential physiological effects of different behavioral response systems. Because F F A concentration in the blood is highly correlated with all three of the other most likely candidates; temperature, glucose availability and size of the fat depots, it may best reflect the metabolic changes occurring during both hunger and satiety. Before any definitive statements can be made, however, several basic questions need to be answered. First, what is the empirical relationship between food deprivation and blood F F A concentrations ? Although it is known that F F A levels increase with food deprivation, no systematic investigation of the relationship has been undertaken. Second, what is the relationship between F F A levels and consummatory behavior ? Third, what differential effects do different consummatory response systems have on F F A levels? The present study was designed as an attempt to answer the above three questions. The study consisted of two experiments. The first experiment was designed to assess the empirical relationships among percentage ad lib weight loss serum F F A concentrations, and post-deprivation food consumption. The second study was identical in design, but the response measure was bar pressing for food on a two minute, variable interval schedule of reinforcement. EXPERIMENT 1 : FOOD CONSUMPTION

Method Subjects. The animals were 60 naive male Holtzman albino rats of theSprague-Dawley strain, with a mean weight of 323 g. Apparatus. The apparatus used in this experiment consisted of a Cohen constant weight control system and an Estefline,Angus ink writing event recorder. When a cage was used for ad lib feeding, food was delivered on demand. Each time the animal removed a pell~ from the food cup, it was replaced, but another pellet was not delivered until the animal had removed the previous one. The animal's weight, then was free to vary in accordance with the animal's food intake. The animal's weight was read directly from the scale on the cage without disturbing him. The number of pellet deliveries to each cage during each 5 min. interval was automatically recorded on an Eats'lineAngus ink writing event recorder. The amount of water

PERCENTAGE BODY WEIGHT LOSS IN RATS consumed was recorded by use of calibrated burets. The burets were mounted separately from the cage, so that their weight would not affect the scale. Experimental design. The two independent variables used in this experiment were percentage loss of ad lib body weight (10 or 20 per cent) and the length of time the animal was allowed to eat (0, 30, 60 or 90 rain) before determination of blood F F A was made. Two groups of ad lib (or 0 per cent loss) animals were included in addition for purposes of F F A analysis, but not for food consumption. One of these groups was subjected to blood analysis for F F A just after a period of eating (0-post) and the other group just before eating (0-we). There were ten groups, with each group consisting o f 6 animals. The dependent variables were the number of 0.097 g pellets of food consumed during each of the three 30 rain periods and the F F A concentrations in micro-equivalents per liter of serum after 0, 30, 60 or 90 min of post-deprivation ad lib food consumption. Procedure. All animals were placed in the weight control cages on ad lib feeding for a period of 24 hr prior to beginning deprivation to allow adaptation to the environment and to adapt to eating in the cages. The cages were located in a virtually sound-proof experimental r o o m illuminated by a 100 W light bulb. The light remained on 24 hr a day. The temperature was maintained between 74 and 77°F. An exhaust fan ran continuously to provide a masking noise. After the 24 hr of ad lib adaptation given all animals, predeprivation weights were recorded for each animal and the target weight commensurate with each animal's ad lib weight and deprivation condition computed. Each animal was deprived of food until he reached the weight appropriate for the deprivation condition to which he was assigned. Animals assigned to one of the ad lib groups remained on ad lib feeding in the cage until tested. When each animal reached the weight appropriate to his deprivation condition, his cage was adjusted to begin ad lib feeding for the interval of time appropriate to his treatment condition (0, 30, 60 or 90 min). After the animal had been allowed to eat for the appropriate length of time, the animal was removed from the cage, and water consumption and weight gain during the test interval recorded. After the animal was removed, an injection of sodium pentobarbital anesthetic was given intraperitoneally. The animal's chest was opened, exposing the heart. Five ml of blood was then drawn directly from the left ventricle of the heart not more than 5-10 rain after the animal was removed from the cage. The blood was then centrifuged at 5000 r p m for 20 rain in order to separate serum from the red blood cells and proteins. The serum was then put into a capped tube and stored at 4°C until the F F A analysis was done. The F F A analysis was done by a colorimetric technique developed by Duncombe [14] and extended for use on biological fluids by Itaya and Ui [20]. The technique was demonstrated by Itaya and Ui to be a sensitive, precise and reliable technique for F F A determination of rat serum or whole blood.

Results The predetermined alpha level of acceptance was 0.05 for all analyses. The observed alpha level for all results reported to be statistically significant is thus p < 0.05. The results indicate that serum F F A concentrations increase with increasing food deprivation. The four groups included in this analysis, in order of increasing deprivation, were: the

303 0 per cent body weight loss group immediately after consuming a meal (0 per cent-post), the 0 per cent body weight loss group prior to a meal (0 per cent-pre), the 10 per cent body weight loss group receiving no food (10 per cent-0) and the 20 per cent body weight loss group receiving no food (20 per cent-0). The data are graphically illustrated in Fig. 1. Although the overall F ratio is statistically significant, a Newman -Keuls test revealed no statistically significant differences between the 0 per cent groups or between the 10 per cent-0 and 20 per cent-0 groups. The F F A levels of both the 10 per cent-0 and 20 per cent 0 groups, however, were found to be significantly higher than both of the 0 per cent groups.

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F F A levels were found to decrease significantly after food consumption. Figure 2 represents mean F F A concentrations after 0, 30, 60 or 90 min of access to food for animals under deprivation conditions of 10 or 20 per cent body weight loss. The control F F A range represented in the figure corresponds to the 0 per cent-post and 0 per cent-we groups. The only statistically significant effect in this analysis is the amount of time allowed access to food. This indicates that F F A levels decrease as food consumption increases for both 10 and 20 per cent weight loss conditions. There are no statistically significant differences between the F F A levels of the 10 and 20 per cent weight loss groups either before or after food consumption. Closer inspection of the data reveals that the

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statistically significant decrease in F F A levels reaches asymptote after 30 min of food consumption. The results also show that the amount of food consumed after deprivation decreases during successive 30 min intervals of time (Fig. 3). There are no statistically significant differences, however, in food consumption due to percentage body weight loss during any of the 30 min test intervals. The results also indicate that the total amount of food consumed increases for both 10 and 20 per cent body weight loss as the length of the post-deprivation food consumption test increases from 30-60 min (Fig. 4). The analysis reveals a statistically significant overall effect of test session length on the amount of food consumed. The amount of food consumed during the last 30 min of the 90 min sessions was so minimal that no statistically significant differences were found between the 60 and 90 min groups for either 10 or 20 per cent body weight loss. Again, percentage of body weight loss was not found to have a statistically significant effect on food consumption. EXPERIMENT

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three Grayson-Stadler operant boxes (Model No. E31258-100) and test chambers (Model No. E3125AA-3), associated relay circuitry and a Grayson-Stadler print out counter (Model No. E4600A). The operant boxes were automatically programmed for a continuous reinforcement schedule (CRF) or a variable interval (VI) schedule of reinforcement. Experimental design. The two independent variables in this experiment were percentage of ad lib body weight loss (10 or 20 per cent) and the length of time the animals were allowed access to the bar (0, 30, 60 or 90 min) before determination of serum F F A was made. Six animals were assigned to each of the eight groups. Procedure. The procedure followed in this experiment was identical to that described for Experiment 1, with the following exceptions. When each animal had lost the amount of weight appropriate for its original ad lib body weight and deprivation condition, bar press training was begun for that animal. The first training session consisted of 25 responses on a C R F schedule and 225 responses on a 30 sec VI schedule. On termination of the first training session, the animal was replaced in its weight control cage until the target weight was again reached(approximately 10-12hr). The second training session was then begun. The second training session consisted of 250 responses on a I min VI schedule and 250 responses on a 2 rain VI schedule. After completion of the second training session, the animal was again replaced in its weight control cage and maintained at the appropriate percentage of body weight

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until tested on the 2 min VI schedule 10-12 hr later. Testing consisted of placing the animal in the operant box in which it had been trained and measuring the number of responses during successive 5 min intervals for either 30, 60 or 90 min. The number of responses during each 5 min interval was automatically recorded on the print out counter. The number of reinforcements was automatically counted by the use of electromechanical digital counters. On termination of the test session, the animal was removed from the operant box and a blood sample taken, as described in Experiment 1, for purposes of F F A analysis. The animals assigned to the 0 min groups were trained exactly as the other groups. After reaching the appropriate body weight subsequent to training, however, the 0 min groups were not tested before blood removal. Results The predetermined alpha level of acceptance was 0.05 for all analyses. The observed alpha level for all results reported to be statistically significant is thus p < 0.05. The results of this experiment indicate that serum F F A concentrations decrease as test duration increases for both 10 and 20 per cent body weight loss groups. Figure 5 represents F F A concentration as a function of percentage body weight loss and test duration. There are no statistically significant differences, however, in F F A concentration between the 30, 60 and 90 min groups. F F A concentration thus appears to

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into a threeway analysis. Figure 8 graphically illustrates the mean F F A concentrations as a function of test duration (0, 30, 60 or 90 min), percentage body weight loss (10 or 20 per cent) and type of task (food consumption or bar pressing). The results indicate that F F A levels decrease as test duration is increased as a result of both food consumption and bar pressing. All groups reach asymptotic decrease after 30 rain of testing in either task. This is indicated by the fact that all groups within each task do not show statistically significant differences in F F A concentrations after 30 rain of testing. F F A concentrations are reduced to different asymptotic levels in the two tasks, however. This is indicated by the statistically significant Duration x Task interaction and by the statistically significant differences found between all groups in the bar press task and all groups in the food consumption task after 30 min test duration. It should also be pointed out that the asymptotic decrease in F F A reached by animals in the food consumption task is within the ad lib control range of F F A concentrations represented in Fig. 8. There are no

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PERCENTAGE BODY WEIGHT LOSS IN RATS DISCUSSION

It is logical that increases in FFA levels should reach an asymptotic level, and perhaps decrease, at some level of increasing percentage body weight loss. There is a multiplicity of determinants of an organism's circulating FFA concentration at a given point in time. Some of the determinants work in opposition, some in concordance, to homeostatically determine the circulating F F A level commensurate with the organism's energy laalance. Food deprivation decreases glucose availability and glucose utilization, and increases F F A mobilization and F F A utilization lay non-nervous tissue. Increased F F A mobilization should increase circulating F F A as long as F F A utilization is not greater than F F A mobilization. The larger the amount of fat in the body, the higher the F F A concentration should be if other determinants are held constant. It appears reasonable to assume that as the deprivation condition is increased in severity, F F A levels should reach an asymptote, or decrease, after some point when F F A utilization is more rapid than mobilization, or the amount of fat in the body is sufficiently decreased. Gross inspection of the lumbar and perirenal fat pads was done in the present study. The animals subjects deprived to 20 per cent body weight loss appeared to have little or no fat remaining compared to 10 per cent weight loss animals. It is of interest to note that Kennedy [22] found the percentage of fat in the body of normal ad lib rats to range from 12-17 per cent. The possibility exists that although a 20 per cent body weight loss, as compared to a 10 per cent loss, may produce more rapid F F A mobilization, the circulating F F A concentrations may not differ under the two conditions. In Experiment 2 both F F A levels and rate of bar pressing were found to be statistically different as a function of percentage of body weight loss. As with food consumption in Experiment 1, bar pressing in Experiment 2 was found to reduce F F A levels as the duration of testing was increased. Bar pressing, however, was found to reduce F F A levels to a significantly higher asymptotic level than did food consumption. It should be pointed out that the reductions in F F A found during the food consumption and bar pressing tasks may have entirely different etiologies. The food consumption task relative to the bar press task, resulted in less energy expenditure and more energy input. This should result in decreased F F A levels due to increased lipogenesis and decreased lipolysis due to a surplus of energy input over energy expenditure. The bar press task, on the other hand, resulted in more energy expenditure and little energy input relative to the food consumption task. This should result in decreased F F A levels due to increased utiliTation of F F A lay the muscles [15] when the mobilization rate of F F A is probably at asymptote. The response rate in the bar pressing task was higher under conditions of 20 per cent body weight loss than under 10 per cent body weight loss, although all animals received the same amount of food. The animals under 20 per cent body weight loss, then, possibly were utilizing more F F A than 10 per cent weight loss subjects due to greater energy expenditure. If 20 per cent body weight loss results in less percentage of fat in the body compared to a I0 per cent loss, as suspected, then a 20 per cent loss would result in less F F A availability even though F F A mobilization rate was equal or greater than that under 10 per cent weight loss. If F F A utilization is more rapid and F F A availability less under conditions of 20 per cent body weight loss, then circulating F F A levels would be expected to be lower under conditions of 20 per cent body weight loss than under conditions of 10 per cent weight loss.

307 Taken as a whole, the above logic would suggest the possibility of an interaction between percentage body weight loss and energy expenditure in determining the concentration of circulating FFA. This interaction would be expected to be evident in the bar pressing task, but absent in the food consumption task. The fact that bar pressing rate was found to be higher, and F F A levels lower, under 20 per cent weight loss as compared to a 10 per cent loss may be a result of such an interaction. If an interaction exists between antecedent deprivation conditions and energy expenditure required by different response systems as hypothesized here, then it is premature to dispense with food consumption as a measure of hunger motivation as suggested by others [13, 30--32]. Such an interaction between deprivation conditions and consequent response systems would predict inconsistent functional relationships between different behavioral response measures and antecedent deprivation conditions. It should be pointed out that the asymptotic decrease found in F F A levels during the bar press task reaches a level significantly higher than ad lib (0 per cent body weight) controls. The decrease in F F A levels found in the food consumption task, on the other hand reaches a level well within control levels. The behavioral responses in the two tasks correspond to the above relationships. In the food consumption task, consumption of food ceased shortly after F F A levels reached control F F A levels. In the bar pressing task responding not only did not cease during the duration of testing measured, but actually increased. F F A concentration, then may be at least correlated with satiation. Recently, Adair, Miller and Booth [1] found that continuous, slow intravenous infusion of glucose had no effect on food consumption in rats, but infusion of a complete liquid diet produced marked hypophagia. In further experiments in which controlled infusion of six nutritive substances was undertaken, they concluded that the factor primarily responsible for the hypophagia was an elevation of blood amino acid concentrations. As previously mentioned, increases in blood amino acid concentration are highly correlated with decreases in blood F F A concentration [19]. Thus the hypophagia could have also been due to decreased F F A concentrations. Adair et al. [1 ] also report that two rats infused with fatty acids showed a probable increase in food consumption and an above normal weight gain. The combined results of the present study and the Adair et al. study [1] lead the present authors to suggest that increases in blood F F A concentrations may be correlated with hunger and decreases in blood F F A concentrations may be correlated with satiety. This hypothesis is also apparently supported by a recent article by Steffens [38] published after preparation of this manuscript. Steffens measured blood glucose and F F A concentrations periodically before, during, and after meals in both ad lib and 24 hr deprived rats by use of a chronic intracardial catheter. Glucose decreased and F F A increased as a result of deprivation. Blood glucose increased to a level above control level and F F A decreased to control level as a result of a meal. Several comments concerning this study [38] are in order. First, Steffens stated that F F A levels in the anesthetized rat differ markedly from normal rats. He must have been referring to chronic anesthetization, and not acute anesthetization, since the F F A concentrations we report in the present study are quite comparable to Steffens' own results and results of others for both ad lib [15, 20] and deprived [20] conditions. Second, Steffens' results are hard to interpret since he replaced blood taken for analysis with the transfusion of whole rat blood of unknown glucose and F F A concentrations. The transfusion

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could serve either to increase or decrease the concentration of glucose and F F A in subsequent samples depending on the concentrations in the transfused blood relative to the concentration of the endogenous blood. Some of the above possible conclusions are based on, as yet, speculative hypotheses• Before any definite conclusions are drawn, more parametric data must be forthcoming. It is possible, however, that F F A may fit Kennedy's [22-24] lipostatic hypothesis as a metabolite important in regulation of food consumption and body weight or more generally, energy regulation• The results of the relationship between percentage body weight loss and consummatory behavior found in this investigation are generally similar to those of previous investigators. No differences were found in amount of food consumed as a function of percentage body weight loss (10 or 20 per cent) during the 90 mi~ of post-deprivation food consumption sampled. When the results of the present investigation and those of previous investigations are examined together, it appears that the results critically depend on the duration of the test interval. For example, Moll [33, 34], using a 5 min test, found rats deprived of 20 per cent body weight ate significantly less food than did rats deprived of 10 per cent body weight. Dufort and Wright [13] found no differences in food consumption during a 2 hr test among rats deprived of food for 24, 36, 48, 72, 96, 144, 168 or 192 hr. In the present study, the mean hours of food deprivation required to reduce rats to a 10 per cent body weight loss was 27.78 hr, and to a 20 per cent body weight loss, 79.55 hr. Thus, the duration of the test, the

deprivation conditions of comparable groups, and the vesult~ of Dufort and Wright's and the present investigator's arc similar. Using a test duration of 24 hr, Remley [36] recorded the amount of food consumption at 6 hr intervals and found food consumption to be less for rats deprived of 20 per cent body weight compared to 10 per cent. The rate of bar pressing on a 2 min ~ariable interval schedulc of reinforcement was found in Experiment 2 to increase as percentage of body weight loss was increased from 10 to 20 per cent. This result is commensurate with previous research by a number of investigators [9, 11, 3l, 36]. All of the above investigators used either a long variable interval schedule or a high fixed ratio schedule. Both of these schedules of reinforcement provide relatively high energy expenditures and relatively small energy inputs. There is need for similar studies using continuous reinforcement schedules which provide more energy input for less energy expenditure, tt is hypothesized that the results of such an investigation would be more commensurate with studies using food consumption tests. Most investigators interested in hunger motivation have not considered factors other than food intake to be important in the energy regulation of the organism. As Kennedy [24] states, there is no a priori reason why energy balance should be maintained by control of appetite alone, since it depends as much on energy expenditure as on energy input. Any complete integration of hunger motivation must include representation of the importance of not only food consumption, but also energy expenditure and energy conservation.

REFERENCES

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