Psychopharmacology
Psychopharmacology (1989) 97:443447
9 Springer-Verlag 1989
Food intake in baboons: effects of diazepam Richard W. Foltin, Marian W. Fischman, and Maryanne F. Byrne Division of Behavioral Biology, Department of Psychiatry and Behavioral Sciences, The Johns Hopkins University School of Medicine, 600 North Wolfe Street, Houck E-2, Baltimore, MD 21205, USA Abstract. Food intake of four adult male baboons (Papio c. anubis) was monitored during daily experimental sessions lasting 22 h. Food was available under a two-component operant schedule. Following completion of the first "procurement component" response requirement, access to food, i.e., a meal, became available under the second "consumption component" during which each response produced a 1-g food pellet. After a 10-min interval in which no response occurred, the consumption component was terminated. The effects of diazepam (DZP: 0.12-4.0 mg/kg) were determined by having the baboons drink a dose on Tuesdays and Fridays 45-60 min before the daily session. DZP produced dose-dependent increases in food intake in three of the four baboons. DZP increased the size of the first two meals and total duration of eating, but had no effect on eating rate or the number of meals within a session. The effect of DZP on the topography of feeding of baboons differs from previous reports on the effects of benzodiazepines on the topography of feeding in rodents. This suggests that species or procedural differences influence the effects of DZP on food intake. Key words: Feeding behavior - Meal patterns - Free-feeding Baboons - Benzodiazepines - Diazepam - Anxiolytics
Bezodiazepines (BZPs) reliably increase food intake in a variety of species including the mouse (e.g., Soubrie et al. 1976), rat (e.g., Bainbridge 1968), cat (e.g., Fratta et al. 1976), beagle dog (Brown et al. 1981), horse (Brown et al. 1976), pigeon (Cooper and Posada-Andrews 1979) and rhesus monkey (e.g., Foltin and Schuster 1983). This increase in food intake is a result of the direct appetitive action of BZPs, rather than an indirect consequence of the anxiolytic effects of these drugs (Cooper 1980; Berridge and Treit 1986). Recently, in addition to recording total consumption, many investigators have begun analyzing the effects of drugs on the patterning of food intake (e.g., Blundell and Latham 1982; Leibowitz et al. 1986). Although BZPs have been consistently reported to decrease latency to initiate feeding and increase the total duration of feeding (e.g., Cooper and Francis 1979; Cooper and Yerbury 1986), the literature is less consistent with respect to the effects of BZPs on meal frequency and local eating rate. Both increases
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(Cooper 1980) and an absence of changes in meal frequency (Brown et al. 1981; Cooper and Yerbury 1986) have been reported following BZP administration. Local eating rate has been reported to increase (Foltin et al. 1985), decrease (Cooper and Francis 1979) and remain unchanged (Cooper and Yerbury 1986) following BZP administration. All of the above studies have used short sessions lasting between 10 min and 2 h. Unfortunately, the few studies (Feldman and Smith 1978; Johnson 1978; Cooper and Posada-Andrews 1979) that measured food intake in longer 4-h sessions did not analyze the effects of benzodiazepines on the pattern of feeding. Thus, the effects of benzodiazepines on 24-h intake have not been assessed. In the present experiment, the effects of diazepam (DZP) on food intake of free-feeding baboons were determined.
Me~o~ Animals and apparatus. Four adult male baboons (Papio cynocephalus anubis), ranging in weight from 27.3 to 40.9 kg were housed in standard primate cages (approximately 0.94 x 1.21 m z 1.52 m high for the three larger baboons, and 0.82 • 0.94 m • 1.2 m high for the smallest baboon). The light-dark cycle was controlled by natural light. Chewable vitamins (Goldline, Ft. Lauderdale, FL) and a piece of fresh fruit (80-100 kcal) were given daily. Water was continuously available. Due to the necessity of sedating baboons in order to determine body weight, animals were weighed only at the start and end of the experiment. Attached to the front of each cage was a panel consisting of a food hopper, two stimulus lights, a Lindsley lever (Gerbrands, Arlington, MA), and a pellet dispenser (BRS-LVE model PDC-005, Beltsville, MD). All schedule contingencies were programmed using an Apple Ile computer located in an adjacent room. Feeding schedule. Food was available 22 h/day, from 11:00 A.M. to 9:00 A.M. the following morning. The remaining 2 h of the day were used for cage and animal maintenance. Illumination of a red stimulus light indicated the availability of the initial component of the two-component schedule of food delivery. This "procurement" component required completion of a fixed number of responses. Upon completion of the ratio requirement, the red stimulus light was extinguished, and a green stimulus light was illuminated to indicate the availability of the second component of the
444 food delivery schedule. During this "consumption" component, each lever pull resulted in the delivery of a single 1-g banana-flavored pellet (3.1 kcal/g, Bio-Serv, Frenchtown, NJ) into the food hopper. After a 10-min interval in which no responses occurred, the consumption component was terminated, the green light extinguished, and the red light illuminated. All pellets earned during each consumption component were defined as occurring within a single meal. In order to gain access to another meal, the baboon was required to complete the ratio requirement of the procurement component again. Initially, ten responses on the lever were required to complete the ratio requirement of the procurement component (FR 10: FR l). This response requirement was in effect until responding stabilized (less than 10% variation in the number of meals and less than 20% variation in food intake for 3 consecutive days). The procurement component response requirement was then systematically increased for each baboon until the number of meals stabilized between two and three per session. This resulted in different procurement response requirements among baboons. The response requirement was 100 responses for R-82, 200 responses for A-33 and 0-02, and 400 responses for V-3. Although differences in response requirements were required to equate meal numbers across baboons, these procedures engendered similar patterns, inter-meal intervals and intake during meals across baboons (Foltin and Fischman 1988).
Procedure. Diazepam (0.12-4.0 mg/kg, courtesy of Hoffman LaRoche, Nutley, NJ) was suspended in 75 ml dilute orange flavored (Tang, General Foods Co., White Plains, NY, 90 kcal) or fruit punch flavored (Giant, Giant Foods Co., Washington, DC, 45 kcal) solution containing a suspending agent (Suspending Agent K, 2 mg/ml concentration, Bio-Serv, Frenchtown, NJ). Doses were administered 45 60 rain prior to the start of the daily session on Tuesdays and Fridays, assuming that food intake on the previous two days was stable. A dose of 1.0 or 2.0 mg/kg was tested first in each baboon, while subsequent dose order varied among baboons with each dose tested once. Throughout the experiment, on Mondays, Wednesdays, and Thursdays, baboons were occasionally given the suspending agent in flavored solutions without any drug. Data analysis. Response rate and inter-response times (IRTs) were recorded during both procurement and consumption components. Inter-response times for the procurement component were summarized in five 1.0-s bins with an additional bin for IRTs longer than 5 s. Responses during each quarter of the consumption component were summarized in five 2.0-s bins with an additional bin for IRTs > 10 s. Thus, it was possible to compare pattern of IRTs as a function of the quarter of a meal. Data analysis was accomplished using linear regression (Systat Inc., Evanston, IL). Effects were considered statistically significant if P < 0.05. Results
Mean pellet intake for the three sessions prior to the start of the determination of the DZP dose-response function was 324.3_+20.8 g (mean___SEM) for 0-02, 324.3_+32,4 g for V-3, 258.0_+7.8 g for R-82, and 369.0_+18.8 g for A-33, with 70-100% of total intake occuring in the first 8 h of
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1. Eight-hour and 22-h pellet intake expressed as per cent of baseline, for individual boboons, as a function of dose of DZP. Upper left panel 0-02. Lower left panel V-3. Upper right panel R-82. Lower right panel A-33. - n - 8 h; =- 22 h
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the session. Figure 1 compares pellet intake, expressed as per cent of baseline, during the first 8 h and the entire 22 h of the daily session, as a function of dose of DZP for each baboon individually. The DZP dose-response function had an inverted U-shape in 0-02 and V-3. In 0-02 (upper left panel) daily intake increased to 160% of baseline following 1.0 mg/kg and total daily intake increased only to 120% of baseline following 2.0 mg/kg. Due to this decrease in effect following 2.0 mg/kg, 0-02 was not given 4.0 mg/kg. DZP (0.25 mg/kg) increased 8-h intake in V-3 to 125% of baseline (lower left panel), but increases in 22-h food intake of V-3 were not evident at any dose, while following 2.0 mg/kg food intake was abolished. Data on the descending portion of the DZP dose-response function for these two baboons were not included in the statistical analyses. In the remaining two baboons (R-82: upper right panel; A-33 : lower right panel), food intake increased with increasing DZP dose. Further reference to the effects of the highest dose of DZP will reflect only the data collected in these two baboons. When data were combined for analysis, DZP significantly increased intake during the first 8 h of the session [t(22)= 1.72, P (one-tail)< 0.05], and during the entire session [t(22) = 1.71, P (one-tail) < 0.05]. Under baseline conditions, latency to the first meal was 22.5• min. There was no consistent relationship between DZP dose and latency to the first meal. Latency to the first meal included the time between the onset of the session and the first response in the initial procurement component as well as the time to complete the response requirement during that procurement component. Response rate under baseline conditions during the first procurement component was 0.49 + 0.11 responses/s. DZP had no significant effect on response rate during the first procurement component. The proportion of responses in each IRT bin during the first procurement component were not distributed evenly [F(5, 110)=442.44, P<0.001]. Approximately 80% of the responses were separated by less than 1 s (bin 1), and 10% of the responses were separated by
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Fig. 2. A Mean consumption rate (responses/s), i.e., response rate, during the first meal as a function of dose of DZP. B Mean duration of the first meal as a function of dose of DZP. C Mean number of meals as a function of dose of DZP. D Mean weight of pellets consumed during the second meal as a function of dose of DZP. E Mean interval between the first and second meals, i.e., consumption components, as a function of dose of DZP. F Mean total duration of feeding as a function of dose of DZP. Only two baboons were tested with 4.0 mg/kg diazepam. Error bars indicate SEMs
greater than 5 s (bin 6). DZP did not have a significant effect on the distribution of responses during the first procurement component. Under baseline conditions, the average first meal consisted of 140.8 + 30.6 g. DZP significantly increased the size of the first meal up to 270 g following 4.0 mg/kg [t(22)= 3.30, P (one-tail)< 0.04]. In contrast to the increase in meal size, DZP had no significant effect on the rate of responding during the first consumption component (Fig. 2: panel A). Although the average duration of the first meal following each dose of DZP was larger than under baseline conditions, this effect was not statistically significant (Fig. 2: panel B). Analysis of the IRT distributions of responses as a function of quarter of the meal following DZP administration indicate a significant effect of IRT bin [F(5, 10)= 62,15, P < 0.001], with the greatest proportion of responses separated by less than 2 s (bin 1), and the next greatest proportion of responses separated by more than 10 s (bin 6). In addition, there was a significant quarter of the meal by bin interaction [F(15, 330) = 2.46, P < 0.002]. Under baseline conditions, as the quarter of the meal increased, the proportion of responses separated by less than 2 s decreased, and the proportion of responses separated by more than 10 s increased. DZP did not have a significant effect on the distribution of responses within the first meal.
Discussion
The results of the present study indicate clearly that oral DZP administration increased food intake in non-deprived baboons. The maximal increase in food intake was larger than that observed previously in rhesus monkeys (Foltin and Schuster 1983; Foltin et al. 1985). In these earlier studies, access to food was limited to daily 2-h sessions, and mean increases of up to 150% of baseline were observed following 4.0 mg/kg DZP (Foltin and Schuster 1983). In the present study, where food was available 22 h per day, mean increases of up to 287% of baseline were obtained. In one additional study using primates (Delgado et al. 1976), increases in food intake were not observed following oral DZP doses up to 10 mg/kg. However, in that study it was possible that the sedative effects of the drug interfered with the complex response requirement to obtain food. Although food intake increased in all four baboons after at least one dose of DZP, the dose-response function for DZP had an inverted U-shape in two baboons, and there were substantial individual differences in sensitivity to the effects of DZP on food intake. A previous study in which complete dose-response functions were determined in rats for the effect of a variety of BZPs indicated that these drugs produced inverted U-shaped dose-response functions on sweetened milk intake (Poschel 1971). Verification of the inverted U-shaped dose-response function would have been obtained by testing higher doses in the two baboons who had the largest increases in food intake. However, report of a death of a rhesus monkey (Foltin and Schuster 1983) following aspiration of vomitus after DZP suggests that testing higher doses was contraindicated. It is interesting to speculate about the cause of the differences in the magnitude of the effect of DZP on food intake. The baboon that had the smallest increase in food intake following DZP was the animal with the largest procurement ratio, while, conversely, the baboon that had the largest increase in food intake following DZP was the animal with the smallest procurement ratio. This suggests that the differ-
446 ences in sensitivity to the appetitive effects of DZP may be related to the response requirement for food delivery. The range of responses to DZP indicates that the operant schedule under which food is delivered may interact significantly with the appetitive effects of DZP, even to the point where increases in food intake are obliterated. DZP increased food intake by increasing the duration of time spent responding during the consumption components, ie., feeding, without affecting rate of responding, or the frequency of bouts of eating. The size and duration of both the first and second meals of the session were increased following DZP. The majority of the previous studies on the effects of BZPs on feeding topography have analyzed food intake during sessions lasting less than 30 min, or changes in food intake during the first 5-10 min of longer sessions in animals that have been food deprived. Under conditions involving food deprivation and short sessions, BZPs have consistently been reported to decrease the latency to initiate feeding and increase duration of feeding in rats (Cooper and Francis 1979; Cooper and Webb 1984). The results of previous studies have been less consistent with respect to the effect of BZPs on meal frequency and local rate of eating. Benzodiazepines have been reported to increase meal frequency in rats (Cooper 1980) and have no effect on meal frequency in dogs (Brown et al. 1981). With respect to local eating rate, BZPs have been reported to increase rate in rhesus monkeys (Foltin et al. 1985), and decrease rate in rats (Cooper and Francis 1979). Benzodiazepine administration increased the amount of food consumed over a given period of time in non-deprived cats, but not above the amount of food consumed over the same period of time in deprived cats (Fratta et ai. 1976; Mereu et al. 1976). Finally, in the only study that evaluated the effects of a BZP on the pattern of food intake in nondeprived rats, midazolam increased the duration of feeding without affecting local eating rate or meal frequency (Cooper and Yerbury 1986). The absence of changes in response rate following DZP administration in non-deprived baboons replicated the response to midazolam in non-deprived rats. Thus, the effect of BZPs on response rates and local rate of eating may be dependent on the deprivation conditions of the organism. The specificity of the DZP effect can be analyzed by comparing drug effects between procurement and consumption components. If a drug had no effect on feeding per se, but disrupted food intake due to a non-specific behavioral effect, the rate of responding and the pattern of responding in both the procurement and consumption components would be disrupted. If a drug specifically changed the pattern of responding in the consumption component, but not the procurement component, it could be argued that the change in pattern was a specific consequence of an interaction between food intake and drug. Since DZP had no systematic effects on response rate and pattern during either the procurement or the consumption components, the results indicate that the increases in food intake associated with DZP administration were not due to non-specific increases in rate of responding. Recent evidence suggests that it may be possible to differentiate the effects of drugs on the initiation, i.e., "hunger," and termination of feeding, i.e., "satiation" (e.g., Blundell 1977; Leibowitz and Shor-Posner 1986). Increases in latency to initiate feeding are presumed evidence for an effect on hunger, while reductions in meal size, and
increase in inter-meal intervals are presumed evidene for an effect on satiation. A decrease in latency coupled with an increase in meal size, as previously reported in deprived rats (Cooper and Francis 1979; Cooper and Yerbury 1986), supports the hypothesis that BZPs specifically enhance" appetite" (Cooper 1980), but an increase in meal size without a decrease in latency, as reported here, may be indicative of increased appetite, decreased ability to satiate, or both. The present finding of increased meal size, and inter-meal interval coupled with the absence of changes in meal frequency or response rate during consumption components are difficult to ascribe to a single mechanism. In addition, the differences in the effects of DZP on feeding in deprived and non-deprived animals adds another complicating factor. It appears that the effect of BZPs on food intake are the result of a complex interaction between drug administration, deprivation conditions, the operant schedule controlling food delivery, and normal systems affecting initiation and termination of feeding. Further research using non-deprived subjects comparing the effects of BZPs and other manipulations known to affect eating behavior, e.g., the administration of a cholecystokinin antagonist, will be useful in partitioning the relative influence of the large variety of factors affecting normal feeding.
Acknowledgements. This research was supported by DA-4130 from The National Institute on Drug Abuse. The assistance of Nondita Bhaduri, Richard Wurster, and Drs. Thomas Kelly, Herbert Barry III, and Joseph Brady is gratefully acknowledged. Diazepam was kindly provided by Dr. P.F. Sorter of Hoffman La-Roche, Nutley, NJ. A preliminary report of these findings was presented at the 1988 meeting of The Behavioral Pharmacology Society.
References
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Received February 25, 1988 / Final version Sepember 22, 1988
