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Increase in ethanol consumption in rats due to caloric deficit
Alcohol, Vol 1, pp 413-415, 1984 ©Ankho International Inc Pnnted m the U S A
0741-8329/84 $3 00 + .00
BRIEF COMMUNICATION
Increase in Ethanol Consumption in Rats Due to Caloric Deficit ALEXANDER
STIGLICK AND IAN WOODWORTH
Addtctton Research Foundation o f Ontario, 33 Russell Street, Toronto, Canada M 5 S 2S1 and D e p a r t m e n t o f Pharmacology, University o f Toronto, Toronto, Canada M 5 S 1A8 R e c e i v e d 16 J u l y 1984 STIGLICK, A. AND I WOODWORTH Increase in ethanol consurnptton tn rats due to caloric defictt ALCOHOL 1(5) 413-415, 1984 --Naive food-restricted and food-satmted rats were given a choice between ethanol (8%, 16%, or 32%) and water for 22 hours/day over 14 days. On all days and at all concentrations, retakes of ethanol were significantly higher in the food-restncted ammals_ Doses consumed by these animals were highest when 32% ethanol was used, with a mean daily retake 0f6.83 g/kg Preference scores, calculated as the percent of total fired intake as ethanol, were also much higher in the food-restricted animals. These findings demonstrate that the calonc value of ethanol may be an important factor m ethanol self-administration, but they do not rule out the possible ~mportance of pharmacological effects Ethanol self-admlnlstrat~on
Food restriction
Rat
T H E R E is a controvery over whether ethanol is consumed by laboratory animals for Its calonc value or for its pharmacological effects. Advocates of the " c a l o n c hypothesis" maintain that the reinforcing properties of ethanol are derived mainly or solely from its caloric value (e g , [2,5]) Considerable evidence has recently accumulated, however, to suggest that ethanol is consumed by animals for the pharmacological effect (for a review see [4]) This question is of obvious importance for attempts to produce an experimental animal model of alcoholism If the caloric properties of ethanol are important in the voluntary consumption of the drug, then food restriction should enhance ethanol drinking. Although this effect has been reported previously, the animals usually were not ethanol-naive, and typically only one concentration of ethanol was used (e.g., [1, 3, 6]). Ethanol drinking can be affected profoundly by prior experience with the drug, and also by the ethanol concentration [4]. The present study circumvents these problems by using ethanol-naive animals and three concentrations of the drug, In a two-bottle choice paradigm
of 430 g. During this period the animals were handled and weighed at least twice per week. They were then randomly assigned to either a satiated group (n=30) or a foodrestricted group (n=30), after which body weights were measured daily. The satiated rats were maintained on ad lib food and water. The food-restricted group continued to have free access to water, but food was limited to 1-4 standard lab chow pellets per day, to reduce body weights to 80% of that of the satiated animals. After attaining this weight, the rats entered the "ethanol drinking phase," during which the body weights of the food-restricted rats were kept at 80% of the mean weight of the satiated antmals, by daily adjustment of the food ration For the ethanol drinking phase the two groups were further subdivided randomly into three groups each, of equal mean weight (n=10 per group). Each group had access to either 8%, 16%, or 32% ethanol (w/v, in tap water) for 14 days, with water available concurrently The ethanol and water solutions were provided in two Richter tubes on each rat's cage. The tubes were placed on the cage for 22 hours per day, and the tube positions were alternated daily. N e w solutions were prepared each day. The dally consumptions of ethanol and water were measured and corrected for evaporation and spillage. The data were analyzed by 3-way analysis of variance, wlth,~two grouping factors (Food Condition, Ethanol Concentration), and one repeated measures factor (Days). Simple effect tests [7] were used for significant interactions
METHOD The subjects were 60 naive male Long-Evans rats, weighing approximately 300 g upon arrival They were exposed to continuous room light (i.e., with no dark cycle), to sunulate the lighting conditions of a 24 hr/day operant study conducted in this laboratory (P Beardsley, I. Woodworth, A. Stlglick, H Kalant, unpubhshed study). The rats were housed individually, and given free access to both food and water for seven weeks, until they had attained a mean weight
RESULTS At all three ethanol concentrations, the volumes ingested were substantially higher in the food-restricted than in the
413
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Days FIG 1 Mean dally ethanol v o l u m e s c o n s u m e d by previously ethanol-naive rats over 14-day ethanol drinking phase Body weights of food-restricted animals were held at 80% of the weights of satiated rats Animals had access to ethanol and water m R~chter tubes for 22 hours/day (a) 8% ethanol (w/v), (b) 16% ethanol, (c) 32% ethanol, 0 , food-restricted, Z2, food-satiated ( n = 1 0 for each group) Vertical bars indicate the largest S E M for each group
2
4
6
Days FIG 2 Mean dally ethanol doses Ig/kg) in food-restricted and roodsatiated rats during 14-day ethanol drinking phase (a) 8% Iw/vl ethanol, (b) 16~ ethanol, (c) 32% ethanol, @, food-restricted, food-satiated ( n - 10 per group)
ETHANOL DRINKING AND FOOD RESTRICTION satiated animals (significant main effect of F o o d F(1,54)= 19.91, p < 0 001), especially during the first few days (significant F o o d × Days interaction F(13,702)=3 36, p<0.01). At each concentration, there was no day on which satiated rats drank as m u c h ethanol as food-restricted animals (Fig. 1) T w o other effects are apparent from Fig 1. First, the volume c o n s u m e d was inversely related to the alcohol concentratlon, o v e r the range of 8%, to 32% (significant main effect of Concentration F(2,54)=6 85, p < 0 002). Second, there was a greater ~'practlce'" effect at the 8% concentration than at the two higher concentratuons (significant Concentration × Days interaction F(26,702)=2.48, p < 0 . 0 1 ) In other words, o v e r 14 days the animals learned to accept 8% ethanol better than rats provided with higher concentrations. T h e s e two effects occurred to about the same extent In food-restricted as in satiated animals (3-way interaction was not significant) Because the food-restricted rats weighed 20% less than the satmted animals, the food-related difference in ethanol intake was e v e n more apparent when e x p r e s s e d in g/kg/day (dose) As shown in Fig. 2, the food-restricted rats c o n s u m e d much larger doses than satiated animals (significant main effect of Food: F(1,54)-25 48, p < 0 00001) In addition, the dose c o n s u m e d by food-restricted rats increased sharply as ethanol concentration was raised from 8% to 32% (sngnlficant Food × Concentratnon interaction' F(2,54)=4.29, p < 0 02), with a mean daily retake of 6 83 (_+0.92) g/kg at 32%_ This increase in dose did not o c c u r in the satiated animals F ( < I ) A preference index for ethanol was also calculated for each rat, as the percent of total fluid taken as ethanol_ The mean preference scores in the satiated rats were 20.0% (_+6_6), 7 9r/c (+2 2), and 5 8% (-+2.1), with 8, 16, and 32% ethanol, respectively The corresponding values for the food-restricted animals were 34.6% (_+8 4), 32 5% (_+5 0), and 25 I% (-+3 6) H e n c e , the food-restricted subjects drank a significantly larger percentage of fluid as ethanol at all concentrations (significant main effect of Food F(1,54)=37 5, p < 0 00001). D[SCUSSION
The present findings d e m o n s t r a t e that food restriction greatly increases ethanol c o n s u m p t i o n by the rat, both in absolute volume and m dose (g/kg/day) The mean alcohol preference scores did not attain 50% at any of the ethanol concentratuons tested, but at all three concentrations the food-restricted animals showed significantly higher preference than the satiated rats This indicates that food restrlc-
415 tlon was able to o v e r c o m e , at least in part, the innate aversion of the rat for ethanol, e v e n at very hLgh concentrations. Although absolute v o l u m e o f intake decreased progressively with increasing concentration, the dose (g/kg/day) c o n s u m e d by food-restricted subjects increased sharply as concentration increased. Very similar findings were obtained in operant studies of ethanol consumption (P Beardsley, I. W o o d w o r t h , A. Stigllck, H Kalant, unpublished observations, [4]), in which food restriction was an essential c o m p o nent Such findings appear to support the " c a l o r i c h y p o t h e s i s , " but a number of arguments can be raised against the conclusion that the extra alcohol consumption by food-restricted animals is motivated only by the caloric value of the ethanol For example, this does not account for the fact that the food-restricted animals took in a higher g/kg dose of the 32% ethanol than of the 16% or 8% Since they were all weightreduced to the same degree, they might have been expected to take in the same dose, regardless of concentration. Furthermore, one cannot assume that the food-restricted and satiated animals differed only in body weight and in level of hunger It is possible that food restriction resulted in some e n d o c n n e or neurophyslologlcal change that affected alcohol preference by some m e c h a n i s m related to the pharmacological effect of ethanol This suggestion is supported by the observation that in our operant study (P Beardsley, I. W o o d w o r t h , A Stiglick, H Kalant, unpublished study), rats similarly reduced in body weight c o n s u m e d m u c h larger doses of ethanol than in the present work ( 10 g/kg/day with the 32% ethanol solution), e v e n though w a t e r was also available concurrently The caloric hypothesis provides no evident explanation for this M o r e o v e r , prehmlnary w o r k in this laboratory suggests that high ethanol intake in the operant c h a m b e r does not decrease appreciably in the same rats under certain conditions of ad lib feeding Therefore, it seems likely that some interaction between the operant training and the pharmacological effect of ethanol must have contributed to the higher intake In summary, the present w o r k demonstrates that caloric restriction increases ethanol consumption by the rat, but does not rule out the possibility that the increase is due at least in part to the pharmacologncal effects of ethanol as distinct from its caloric contribution It is not yet possible to identify which pharmacologic effects might be responsible ACKNOWLEDGEMENTS The authors wLsh to thank Dr Harold Kalant for help m preparing the DLscusslon We also thank Mr Lonme Currm for drawing the figures and Mrs June Shepperd for typing the manuscript
REFERENCES
1 Black, E L and G L_ Martin Extmctnon of alcohol drmklngin rats following acquisition on a fixed-ratio schedule of reinforcement P~ychonom S~t 29' 152-154, 1972 2 Freed, E X and D Lester Schedule-reduced consumption of ethanol Calories or chemotherapy '~ Phv~tol Beha~ 5: 555-560, 1970 3 Lester, D and L A Greenberg Nutnhon and the etiology of alcoholism The effect of sucrose, sacchann and fat on the selfselection of ethyl alcohol by rats Q J Stud AIcohol 13: 553-560. 1952
4 Mensch, R A Alcohol self-admmlstranon by experimental animals In Re~ear~ h A d v a m es m AI~ ohol and Drug Problem~, vol 8, edLted by R G Smart, F B Glaser, Y Israel, H Kalant, R E Popham, W Schmfdt, H Cappell and E M_ Sellers New York Plenum Press, 1984, m press 5 Sherman, J E_,C F HLckis, A G RLce, R W RusmlakandJ Garcna Preferences and aversions for stimuli paired with ethanol m hungry rats A t o m Learn Behav 11" 101-106, 1983 6 Westerfeld, W W and J Lawrow_ The effect of caloric restrnct~on and thiamin deficLency on the voluntary consumption of alcohol by rats Q J Stud Alcohol 14: 378--384, 1953 7 Wirier, B_ J Stattst~ al Print iple~ in Experimental Design, New York McGraw Hdl, 1971
0741-8329/84 $3 00 + .00
BRIEF COMMUNICATION
Increase in Ethanol Consumption in Rats Due to Caloric Deficit ALEXANDER
STIGLICK AND IAN WOODWORTH
Addtctton Research Foundation o f Ontario, 33 Russell Street, Toronto, Canada M 5 S 2S1 and D e p a r t m e n t o f Pharmacology, University o f Toronto, Toronto, Canada M 5 S 1A8 R e c e i v e d 16 J u l y 1984 STIGLICK, A. AND I WOODWORTH Increase in ethanol consurnptton tn rats due to caloric defictt ALCOHOL 1(5) 413-415, 1984 --Naive food-restricted and food-satmted rats were given a choice between ethanol (8%, 16%, or 32%) and water for 22 hours/day over 14 days. On all days and at all concentrations, retakes of ethanol were significantly higher in the food-restncted ammals_ Doses consumed by these animals were highest when 32% ethanol was used, with a mean daily retake 0f6.83 g/kg Preference scores, calculated as the percent of total fired intake as ethanol, were also much higher in the food-restricted animals. These findings demonstrate that the calonc value of ethanol may be an important factor m ethanol self-administration, but they do not rule out the possible ~mportance of pharmacological effects Ethanol self-admlnlstrat~on
Food restriction
Rat
T H E R E is a controvery over whether ethanol is consumed by laboratory animals for Its calonc value or for its pharmacological effects. Advocates of the " c a l o n c hypothesis" maintain that the reinforcing properties of ethanol are derived mainly or solely from its caloric value (e g , [2,5]) Considerable evidence has recently accumulated, however, to suggest that ethanol is consumed by animals for the pharmacological effect (for a review see [4]) This question is of obvious importance for attempts to produce an experimental animal model of alcoholism If the caloric properties of ethanol are important in the voluntary consumption of the drug, then food restriction should enhance ethanol drinking. Although this effect has been reported previously, the animals usually were not ethanol-naive, and typically only one concentration of ethanol was used (e.g., [1, 3, 6]). Ethanol drinking can be affected profoundly by prior experience with the drug, and also by the ethanol concentration [4]. The present study circumvents these problems by using ethanol-naive animals and three concentrations of the drug, In a two-bottle choice paradigm
of 430 g. During this period the animals were handled and weighed at least twice per week. They were then randomly assigned to either a satiated group (n=30) or a foodrestricted group (n=30), after which body weights were measured daily. The satiated rats were maintained on ad lib food and water. The food-restricted group continued to have free access to water, but food was limited to 1-4 standard lab chow pellets per day, to reduce body weights to 80% of that of the satiated animals. After attaining this weight, the rats entered the "ethanol drinking phase," during which the body weights of the food-restricted rats were kept at 80% of the mean weight of the satiated antmals, by daily adjustment of the food ration For the ethanol drinking phase the two groups were further subdivided randomly into three groups each, of equal mean weight (n=10 per group). Each group had access to either 8%, 16%, or 32% ethanol (w/v, in tap water) for 14 days, with water available concurrently The ethanol and water solutions were provided in two Richter tubes on each rat's cage. The tubes were placed on the cage for 22 hours per day, and the tube positions were alternated daily. N e w solutions were prepared each day. The dally consumptions of ethanol and water were measured and corrected for evaporation and spillage. The data were analyzed by 3-way analysis of variance, wlth,~two grouping factors (Food Condition, Ethanol Concentration), and one repeated measures factor (Days). Simple effect tests [7] were used for significant interactions
METHOD The subjects were 60 naive male Long-Evans rats, weighing approximately 300 g upon arrival They were exposed to continuous room light (i.e., with no dark cycle), to sunulate the lighting conditions of a 24 hr/day operant study conducted in this laboratory (P Beardsley, I. Woodworth, A. Stlglick, H Kalant, unpubhshed study). The rats were housed individually, and given free access to both food and water for seven weeks, until they had attained a mean weight
RESULTS At all three ethanol concentrations, the volumes ingested were substantially higher in the food-restricted than in the
413
414
STIGLICK
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Restricted
Satialed
Satiated
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Days FIG 1 Mean dally ethanol v o l u m e s c o n s u m e d by previously ethanol-naive rats over 14-day ethanol drinking phase Body weights of food-restricted animals were held at 80% of the weights of satiated rats Animals had access to ethanol and water m R~chter tubes for 22 hours/day (a) 8% ethanol (w/v), (b) 16% ethanol, (c) 32% ethanol, 0 , food-restricted, Z2, food-satiated ( n = 1 0 for each group) Vertical bars indicate the largest S E M for each group
2
4
6
Days FIG 2 Mean dally ethanol doses Ig/kg) in food-restricted and roodsatiated rats during 14-day ethanol drinking phase (a) 8% Iw/vl ethanol, (b) 16~ ethanol, (c) 32% ethanol, @, food-restricted, food-satiated ( n - 10 per group)
ETHANOL DRINKING AND FOOD RESTRICTION satiated animals (significant main effect of F o o d F(1,54)= 19.91, p < 0 001), especially during the first few days (significant F o o d × Days interaction F(13,702)=3 36, p<0.01). At each concentration, there was no day on which satiated rats drank as m u c h ethanol as food-restricted animals (Fig. 1) T w o other effects are apparent from Fig 1. First, the volume c o n s u m e d was inversely related to the alcohol concentratlon, o v e r the range of 8%, to 32% (significant main effect of Concentration F(2,54)=6 85, p < 0 002). Second, there was a greater ~'practlce'" effect at the 8% concentration than at the two higher concentratuons (significant Concentration × Days interaction F(26,702)=2.48, p < 0 . 0 1 ) In other words, o v e r 14 days the animals learned to accept 8% ethanol better than rats provided with higher concentrations. T h e s e two effects occurred to about the same extent In food-restricted as in satiated animals (3-way interaction was not significant) Because the food-restricted rats weighed 20% less than the satmted animals, the food-related difference in ethanol intake was e v e n more apparent when e x p r e s s e d in g/kg/day (dose) As shown in Fig. 2, the food-restricted rats c o n s u m e d much larger doses than satiated animals (significant main effect of Food: F(1,54)-25 48, p < 0 00001) In addition, the dose c o n s u m e d by food-restricted rats increased sharply as ethanol concentration was raised from 8% to 32% (sngnlficant Food × Concentratnon interaction' F(2,54)=4.29, p < 0 02), with a mean daily retake of 6 83 (_+0.92) g/kg at 32%_ This increase in dose did not o c c u r in the satiated animals F ( < I ) A preference index for ethanol was also calculated for each rat, as the percent of total fluid taken as ethanol_ The mean preference scores in the satiated rats were 20.0% (_+6_6), 7 9r/c (+2 2), and 5 8% (-+2.1), with 8, 16, and 32% ethanol, respectively The corresponding values for the food-restricted animals were 34.6% (_+8 4), 32 5% (_+5 0), and 25 I% (-+3 6) H e n c e , the food-restricted subjects drank a significantly larger percentage of fluid as ethanol at all concentrations (significant main effect of Food F(1,54)=37 5, p < 0 00001). D[SCUSSION
The present findings d e m o n s t r a t e that food restriction greatly increases ethanol c o n s u m p t i o n by the rat, both in absolute volume and m dose (g/kg/day) The mean alcohol preference scores did not attain 50% at any of the ethanol concentratuons tested, but at all three concentrations the food-restricted animals showed significantly higher preference than the satiated rats This indicates that food restrlc-
415 tlon was able to o v e r c o m e , at least in part, the innate aversion of the rat for ethanol, e v e n at very hLgh concentrations. Although absolute v o l u m e o f intake decreased progressively with increasing concentration, the dose (g/kg/day) c o n s u m e d by food-restricted subjects increased sharply as concentration increased. Very similar findings were obtained in operant studies of ethanol consumption (P Beardsley, I. W o o d w o r t h , A. Stigllck, H Kalant, unpublished observations, [4]), in which food restriction was an essential c o m p o nent Such findings appear to support the " c a l o r i c h y p o t h e s i s , " but a number of arguments can be raised against the conclusion that the extra alcohol consumption by food-restricted animals is motivated only by the caloric value of the ethanol For example, this does not account for the fact that the food-restricted animals took in a higher g/kg dose of the 32% ethanol than of the 16% or 8% Since they were all weightreduced to the same degree, they might have been expected to take in the same dose, regardless of concentration. Furthermore, one cannot assume that the food-restricted and satiated animals differed only in body weight and in level of hunger It is possible that food restriction resulted in some e n d o c n n e or neurophyslologlcal change that affected alcohol preference by some m e c h a n i s m related to the pharmacological effect of ethanol This suggestion is supported by the observation that in our operant study (P Beardsley, I. W o o d w o r t h , A Stiglick, H Kalant, unpublished study), rats similarly reduced in body weight c o n s u m e d m u c h larger doses of ethanol than in the present work ( 10 g/kg/day with the 32% ethanol solution), e v e n though w a t e r was also available concurrently The caloric hypothesis provides no evident explanation for this M o r e o v e r , prehmlnary w o r k in this laboratory suggests that high ethanol intake in the operant c h a m b e r does not decrease appreciably in the same rats under certain conditions of ad lib feeding Therefore, it seems likely that some interaction between the operant training and the pharmacological effect of ethanol must have contributed to the higher intake In summary, the present w o r k demonstrates that caloric restriction increases ethanol consumption by the rat, but does not rule out the possibility that the increase is due at least in part to the pharmacologncal effects of ethanol as distinct from its caloric contribution It is not yet possible to identify which pharmacologic effects might be responsible ACKNOWLEDGEMENTS The authors wLsh to thank Dr Harold Kalant for help m preparing the DLscusslon We also thank Mr Lonme Currm for drawing the figures and Mrs June Shepperd for typing the manuscript
REFERENCES
1 Black, E L and G L_ Martin Extmctnon of alcohol drmklngin rats following acquisition on a fixed-ratio schedule of reinforcement P~ychonom S~t 29' 152-154, 1972 2 Freed, E X and D Lester Schedule-reduced consumption of ethanol Calories or chemotherapy '~ Phv~tol Beha~ 5: 555-560, 1970 3 Lester, D and L A Greenberg Nutnhon and the etiology of alcoholism The effect of sucrose, sacchann and fat on the selfselection of ethyl alcohol by rats Q J Stud AIcohol 13: 553-560. 1952
4 Mensch, R A Alcohol self-admmlstranon by experimental animals In Re~ear~ h A d v a m es m AI~ ohol and Drug Problem~, vol 8, edLted by R G Smart, F B Glaser, Y Israel, H Kalant, R E Popham, W Schmfdt, H Cappell and E M_ Sellers New York Plenum Press, 1984, m press 5 Sherman, J E_,C F HLckis, A G RLce, R W RusmlakandJ Garcna Preferences and aversions for stimuli paired with ethanol m hungry rats A t o m Learn Behav 11" 101-106, 1983 6 Westerfeld, W W and J Lawrow_ The effect of caloric restrnct~on and thiamin deficLency on the voluntary consumption of alcohol by rats Q J Stud Alcohol 14: 378--384, 1953 7 Wirier, B_ J Stattst~ al Print iple~ in Experimental Design, New York McGraw Hdl, 1971